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Volume 94-B, Issue SUPP_XLIV October 2012 The International Society for Computer Assisted Orthopaedic Surgery (CAOS)

L. Fieten J. Eschweiler K. Kabir S. Gravius T. Randau K. Radermacher

Cup position planning for total hip replacement (THR) is a complex task which is influenced by several factors. Whereas aspects like appropriate implant fixation and bone stock preservation are rather evaluated according to intra-operative findings, functional analyses using biomechanical hip models can rely on pre-operative imaging. Due to the wide availability and cost-efficiency of X-ray imaging technology along with the common restriction of biomechanical evaluations to the frontal plane, pre-operative imaging for such purposes is usually limited to AP radiographs. One example is biomechanical optimisation based on the so-called BLB score, which has already been introduced into clinical practice. In this approach, the assumed suitability of potential hip centres of rotation (CORs) is presented to the surgeon by applying colour-coding within the pre-operative AP radiograph. However, to realise the plan, the surgeon has to transfer the 2D positions presented in the radiograph into the 3D surgical site.

We developed a CT-based simulation tool allowing for the generation of 3D bone surface models as well as standardised digitally reconstructed radiographs (DRRs). Within a 3D view, the cup, which is represented as a hemisphere, can freely be shifted in the coronal plane. The 2D point corresponding to the COR defined by the hemisphere is then automatically computed.

In our study, four CT datasets of hips with large bony defects were used. After segmentation 3D bone surface models were generated. These bone surface models were aligned on the basis of the pelvic coordinate system [3], and standardised AP DRRs were computed. BLB score evaluation in intact hips assumes that the central beam passes through the centroid of both hip CORs. As only the contra-lateral hip COR was available due to the defects, a virtual ipsi-lateral COR was obtained by mirroring the contra-lateral hip across the mid-sagittal plane.

Twelve surgeons (divided into two groups of six each according to their experience) had the task to shift the cup such that its 3D position would best match a predefined 2D target position, which was close to the virtual ipsi-lateral COR and displayed as a cross within the standardised DRR. However, the current 2D position corresponding to the current 3D position was not revealed during positioning. Once the user was satisfied with the 3D position, the corresponding 2D position was recorded.

The following results were obtained (mean ± SD across six surgeons of the respective group) for the four patients:

x-error, more experienced: 2.0 ± 6.1; −3.0 ± 5.9; 4.1 ± 4.8; 2.1 ± 5.2; x-error, less experienced: 4.3 ± 4.2; −3.1 ± 1.8; 1.9 ± 4.0; 5.2 ± 4.1; |x-error|, more experienced: 5.2 ± 3.0; 5.4 ± 3.2; 5.5 ± 2.7; 4.3 ± 3.0;|x-error|, less experienced: 4.3 ± 4.2; 3.1 ± 1.8; 3.3 ± 2.7; 5.7 ± 3.3; y-error, more experienced: 12.0 ± 9.1; 0.3 ± 4.3; 6.2 ± 6.6; 1.9 ± 3.2;

y-error, less experienced: 6.1 ± 3.1; 0.8 ± 4.0; 2.4 ± 5.5; 1.4 ± 4.1;|y-error|, more experienced: 12.0 ± 9.1; 3.2 ± 2.6; 6.2 ± 6.6; 3.0 ± 1.9;|y-error|, less experienced: 6.1 ± 3.1; 3.4 ± 1.6; 4.6 ± 3.3; 3.2 ± 2.6;total error, more experienced: 13.5 ± 8.9; 6.6 ± 3.5; 9.8 ± 4.1; 5.4 ± 3.4;total error, less experienced: 8.5 ± 2.7; 4.9 ± 1.5; 6.5 ± 2.5; 6.7 ± 3.8.

Our experimental results show that mental 2D/3D matching for cup positioning in pelvises with bony defects is a difficult task, and that mental 2D/3D matching cannot be expected to yield the correct 3D cup positions corresponding to positions predefined in radiographs. The largest errors were found in the patient with the lowest image quality suggesting that image quality plays an important role. On contrary, experience was not found to be an important factor.

We believe that in clinical practice mental 2D/3D matching between pre-operative radiographs and the surgical site without the help of 3D imaging or special tools would be more difficult than the task given in this study because only small portions of the pelvis would be exposed. Furthermore, as additional aspects of cup positioning would need to be taken into consideration simultaneously, the mental load could be expected to be higher. We conclude that in hips with large bony defects cup positioning based on pre-operative radiographs is highly unreliable without additional computer-assistance or intra-operative imaging. If pre-operative radiographs are needed for functional analyses, combination with 3D image data seems attractive: Firstly, 3D images can easily be used for navigation; secondly, they allow for the generation of highly standardised views, which is essential for comparability across multiple patients.

Future studies relying on more datasets with a wider range of defects could also investigate whether cranio-caudal or medio-lateral positioning errors prevail. This is an interesting question since the BLB score usually is much more specific in the medio-lateral direction than in the cranio-caudal direction, implying that correct 2D/3D matching for the cranio-caudal direction appears less important. In the current study involving only four hips, however, no clear tendency could be observed.

This work has been funded in part by the German Ministry for Education and Research (BMBF) in the framework of the orthoMIT project under grant No. BMBF 01EQ0802/BMBF 01IBE02C.

N. Taki N. Mitsugi Y. Mochida Y. Akamatsu H. Kobayashi T. Saito

The efficacy of an imageless navigation system in acetabular cup orientation during total hip arthroplasty (THA) is well known. We evaluated the accuracy of placement of the cup and stem and leg extension length with the imageless navigation system.

Radiographic evaluation was performed in 69 consecutive patients (75 joints) who underwent primary THA from January 2009 to December 2010. Evaluation of the cup inclination angle (CI), cup anteversion angle (CA), leg extension length (L) and stem anteversion angle (SA) was carried out. SA was evaluated in 21 patients who underwent CT scan after surgery. The accuracy of imageless navigation system was evaluated by comparison of the navigation values obtained during surgery with the radiographic or CT measured values.

Good correlation was found between the navigation values and the radiographic or CT measured values in CI (P<0.001, r2 = 0.579), CA (P<0.001, r2 = 0.607), L (P<0.001, r2 = 0.775), and SA (P<0.001, r2 = 0.834). The mean absolute difference between navigation and radiograph or CT was 3.3 degrees (range 0.1 to 9.9 degrees) in CI, 4.6 degrees (0.1 to 11.4 degrees) in CA, 3.2mm (0.7 to 8.3) in L, and 3.6 degrees (0.1 to 10 degrees) in SA.

The results of this study demonstrated that imageless navigation shows good accuracy not only in cup implantation angle but also in leg extension length and in stem anteversion angle according to radiographic and CT evaluation. We conclude that imageless navigation is a useful tool for performing accurate surgery for THA.

M. A. Yaffe B.W. McCoy S. Greene M. Luo M. Cayo S.D. Stulberg

Computer-assisted surgery (CAS) is a tool developed to allow accurate limb and implant alignment in total knee arthroplasty [TKA]. The strength of the technology is that it allows the surgeon to assess soft tissue balance and ligament laxity in flexion and extension. The accuracy of this ligament balancing technology depends upon an accurate determination of femoral component size. This size is established with intraoperative surface registration techniques. Customised instrumentation (CI) is a measured resection technique in which component size is established on preoperative 3D MRI reconstructions. The purpose of this study is to determine how these two computer-based technologies compare with regard to the accuracy with which femoral component size is established in TKA.

67 TKA were performed using CI and 30 TKA were performed using CAS by a single surgeon. CI-predicted and CAS-predicted femoral component size were compared to actual component selection. The process by which CI and CAS perform an anatomic registration was evaluated.

The CI and CAS systems accurately predicted surgeon-selected femoral component size in 89% and 43% of cases, respectively (p<0.001). The discrepancy between predicted and actual femoral component size with CI and CAS was 0.1 and 0.8 sizes, respectively (p<0.001). The maximum deviation between predicted and actual femoral component size was greater in CAS than in PMI (three sizes versus one size, respectively). The anterior cortex cut was flush in 92% of CI cases. The rotation profile was consistent with Whiteside's line in 95% of CI cases.

The CI system was both more accurate and more precise than the CAS navigation system in predicting femoral component size in TKA. CI utilises preoperative MR imaging to generate femoral component sizing based on optimizing medial-lateral fit with a measured posterior femoral bone resection. CAS utilises surface registration techniques based on anatomic site registration that may be subject to intraoperative measurement error due to difficult visualization (femoral epicondyles), inherent subjectivity (Whiteside's line) or anatomic variation (hypoplastic posterior condyles). CI bases implant sizing solely on reproducing an anatomical fit and a measured resection technique, whereas CAS attempts to balance an anatomic fit with optimal soft tissue balancing. In this study, the surgeon's final component selection was more likely to be in accordance with the CI rather than the CAS sizing algorithm. The CI system was capable of accurate femoral component placement in TKA.

This study suggests that intraoperative surface registration may not be as accurate as preoperative 3D MRI reconstructions for establishing optimal femoral component sizing. Surgeons using intraoperative navigation based surface registration need to be aware of this when making femoral component size selection, establishing ligament balance, and determining femoral rotation.

V.K Singh R. Trehan Y. Kamat R. Varkey R. Raghavan A. Adhikari

Computer navigated Total Knee Arthroplasty is routinely performed with gratifying results. New navigation software is now designed to help surgeons balance soft tissues in Total Knee Arthroplasty (TKA). The aim of our study was to compare functional scores at two years between two different techniques of knee balancing.

A prospective randomized control study was conducted between February 2007 and February 2008 involving 52 patients. Two different techniques of knee balancing were used namely, measured resection and gap balancing technique. Each group had 26 patients. Oxford and Knee society scores were done at two years to understand if one technique was better than other.

Oxford and Knee Society Scores improved significantly in both the groups but gap balancing technique achieved slightly better functional scores which were not significant on statistical analysis.

Computer assisted measured resection and gap balancing techniques in TKA reliably improves functional scores postoperatively. Either of the techniques if performed correctly with appropriate patient selection will have satisfactory outcomes.

V.K Singh R. Trehan Y. Kamat R. Varkey R. Raghavan A. Adhikari

Total Knee Arthroplasty (TKA) in obese patients has been under rigorous scrutiny due concerns of less satisfying results and increased risk of perioperative complications. We conducted a prospective study to observe functional scores between obese and non obese patients at two years after mini-robot computer assisted TKA. Average stay, time for wound to be dry and perioperative complications were also compared.

A prospective study was conducted between February 2007 and February 2008 involving 50 patients. Two different groups of 25 each were made on the basis of body mass index (BMI). Oxford and Knee society scores were obtained at two years to observe difference in functional scores between these groups.

Rate of post operative complications or hospital stay was comparable between the two groups. Oxford and Knee society scores improved significantly in both the groups postoperatively. Obese patients had better Oxford and Knee society scores, which were not statistically significant.

There is no difference in early functional outcome and complications between obese and non obese patients after navigated TKA. Navigated TKA in obese patients help precise component placement with appropriate soft tissue balancing leading to improved results.

I. Südhoff K. Reising B. Mollard P. Helwig

The palpation of the controlateral iliac spinae remains a major hurdle to the success of navigation in lateral position. Several studies are seeking for alternative landmarks to compute the anterior pelvic plane (APP). Up to now, none of those methods have been used in clinical routine. Ultrasound navigation offers a great potential to identify new bony landmarks. The tubercles of the lower lumbar spine and the symphysis can easily be imaged. Those points define a sagittal plane, that can be used as a symmetry plane to compute a virtual controlateral spinae from the acquired colateral spinae. A virtual pelvic plane can then be computed. The objective of this study was to check the accuracy and reproducibility of this virtual anterior pelvic plane.

6 hips (3 left, 3 right) from 4 cadavers (mean BMI 22,6; range 19,5–26,7) embalmed with glycerol and alcohol were used for this study. All anatomic landmarks were acquired with the OrthoPilot® Ultrasound navigation system. One experienced surgeon acquired the reference APP with the cadavers lying supine. The cadavers were then placed in lateral position. Two experienced surgeons acquired 6 times following landmarks: 3 lower lumbar tubercles, 3 sacral tubercles (see Figure 1), the posterior spines, the symphysis and the colateral iliac spine. Several sagittal planes were computed using all points (least square plane) and all possible combinations between one symphysis point, one lower lumbar tubercle point (L5, L4 or L3), and one sacral tubercle point (S2 or S1). The angular error of the resulting virtual APP to the reference APP was computed. For each cadaver, an error map was computed to visualize the error of the virtual APP with respect to the height of the used sacral and lumbar tubercles along the spine.

The reference APP was acquired with a good reproducibility: the deviation between each acquisition to the mean of all acquisitions was smaller than 1° (except for cadaver 2 right side, the deviation reached 2 ° in the frontal plane).

As some sacral and lumbar points were mixed during the acquisition, the line joining the posterior spines was used to separate the sacral from the lumbar points. The mean errors and standard deviations were comparable between operators. The least square plane computed with all points strongly depended on the cadaver positioning : for the same cadaver, the mean error reached 0°on the left side and 8° on the right side.

More constant results were obtained by using a combination of 3 points. 5 outliers were identified and removed as they clearly corresponded to erroneous acquisitions on bad quality images. After having removed those outliers, the mean error ranged between 2° and 5° and the standard deviation between 1° and 3°. The best combination of points was a point on the symphysis, the lowest sacral tubercle (S2) and the lowest lumbar tubercle (L5).

This study shows that the symphysis, the lower lumbar and sacral tubercles can be used to define a sagittal plane and thereby define a virtual anterior pelvic plane. Outliers should be suppressed by taking special care to the image quality and by adding a guided ultrasound functionality: visualizing the resulting sagittal plane on the ultrasound picture would enable the surgeon to easily control the accuracy of his acquired plane. The next steps consist in checking the feasibility in a clinical set-up.

J. Van den Broeck R. Wirix-Speetjens J. Vander Sloten

In recent years 3D preoperative planning has become increasingly popular with orthopaedic surgeons. One technique that has shown to be successful in transferring this preoperative plan to the operating room is based on surgical templates that guide various surgical instruments. Such a patient-specific template is designed using both the 3D reconstructed anatomy and the preoperative plan and is then typically produced via additive manufacturing technology. The combination of a preoperative plan and a surgical template has the potential to result in a more accurate procedure than an unguided one, when the following three criteria are met: the template needs to achieve a stable fit on the surgical field, it needs to fit in a unique position, and the surgeon needs to be able to determine the correct, planned position during the surgery. When the template fails one of these conditions, it can be used incorrectly. Consequently the process could result in an inaccurate outcome.

This research focuses on modelling the stability of a surgical template on bone. The relationship between the contact surface of the template and the resulting stability is investigated with a focus on methods to quantify the template stability. The model calculates a quality score on the designed contact surface, which reflects the likelihood of positioning the template on the bone in a stable position. The model used in this study has been experimentally validated to verify its ability to provide a reliable indication of the template stability. This was analysed using finite element analysis where multiple templates and support models with different contact surface shapes were created. The application of forces and moments in varying directions was simulated. Stability is then defined as the ability of a template to resist an applied force or moment. The displacements of the templates were computed and analysed. The results show a minimal displacement of less than 0.01 mm and a maximal displacement larger than 10 mm. The former is considered to be a very stable template design; the latter to be very unstable and hence, would result in an insecure contact.

The geometry of the contact surface had a clear influence on the template stability. Overall, the coverage of curvature variations improved the stability of the template. The displacements of the different finite element simulations were used as criterion for ranking the tested template designs according to their stability on their corresponding model surface. This ranking is then compared to that resulting from the quality score of the stability model. Both rankings showed a similar trend. This evaluation phase thus indicates that the developed stability model can be used to predict the stability of a surgical template during the preoperative design process.

M. Kraus C. Riepl A. Jones F. Gebhard H. Schöll

Fractures of the femoral head are a challenging problem. The most often performed head preserving procedure worldwide is closed reduction and insertion of cannulated screws under fluoroscopic control. The use of navigation is still experimental in general trauma since rigid reference markers must be attached to all fragments. The examined system (Surgix®, Tel Aviv, Israel) is a fluoroscopy based image analysing system. It consists of a workstation and X-ray opaque markers in surgical tools. When the tool is visible in a C-arm shot a trajectory is displayed as additional layer in the image to serve as guidance for the surgeon.

Forty synthetic femurs (Synbone®, Malans, Switzerland) were used and placed inside foam to simulate the soft tissue of the thigh. The models were equipped with 4.5mm radio-opaque markers at the fovea capitis femoris as target point. The aim was to bring the tip of a K-wire as close as possible to the target point entering the bone at the lateral base of the greater trochanter in a center-center position. Twenty were done under image guidance and 20 were operated the conventional way. Outcome measures included the accuracy (the distance between the tip of the wire and the target in a CT), the number of guide wire insertions, procedure duration, radiation exposure and learning curve.

In the image guided group optimal guide wire placement was accomplished on first pass in 65% of the cases as compared to 5% in the conventional group (p = < 0.0001). The average number of trial and error was significantly lower in the guided group (1.7 vs. 5.8, p = < 0.0001). Consequently the average duration of the guided procedure was significantly shorter (p = 0.0008) along with radiation exposure time reduced by over 70% (p = 0.0002). The guidance system hit averaged 5.8 mm off target as compared to 5.3 mm for the freehand method (p = 0.3319).

Image based guidance significantly shortened the procedure, reduced the radiation exposure and the number of trials without changing the surgeons workflow and can be used in trauma cases were reference marker based navigation is not applicable.

M. Haimerl L. Dohmen S. Gneiting E. Sendtner M. Wörner R. Springorum J. Grifka T. Renkawitz

There is a complex interaction among acetabular component position and the orientation of the femoral component in determining the maximum, impingement-free prosthetic range of motion (ROM) in total hip arthroplasty (THA). Regarding restrictions in ROM, femoral antetorsion is one of the most important parameters. But, ROM is also influenced by parameters like the deviation between the femoral shaft and the mechanical axis in a sagittal projection. This deviation is best described as “Femoral Tilt” (FT). This study analysis the incidence of FT in clinical practice and its consequences on post-operative ROM. Based on these results, the effects of changes in FT on ROM-based cup optimisation are assessed by a using a virtual ROM analysis.

For studying the incidence of FT, 40 (16 male, 24 female) postoperative computerised tomography (CT) scans were analysed using a 3D CT planning software. The implant models were superimposed onto the image data to determine their exact position. The anatomical orientations were determined by planning anatomical landmarks and coordinate directions (i.e. mechanical axis, posterior condyle axis). Descriptive statistics were calculated for FT. Effects of changes in FT and CCD on ROM were analysed by calculating zones of compliance. FT was varied between 2.1° and 9.3° for 135°.

The overall range of post-operative values for femoral tilt was 5.7° ± 1.8° (mean ± standard deviation, minimum 1.7°, maximum 10.2°). The zone of compliance significantly depended on FT (difference more than 200%). The optimum cup position changed from 35° radiographic inclination/30° anteversion to 39°/30° when FT was increased from 2.1° to 9.3°.

Within this study, it was demonstrated that FT has a significant effect on postoperative ROM in THAs. First of all, it was shown that clinically FT values lie in a range between 2.1° and 9.3° (95% CI), where we used a long-shaft stem type with a relatively low possibility to influence sagittal tilt angles. FT may significantly change zones of compliance up to 200% as well as optimised cup positions. Thus, standard combined anteversion formulas, which were proposed in the literature to implement femur first approaches for THA, do only particularly address an optimisation of post-operative ROM. Instead, a sophisticated virtual ROM analysis based on a navigated femur-first approach would enable accurate ROM estimations as parameters like FT are hard to be assessed intra-operatively.

A. Mofidi B. Lu M.S. Goddard M.A. Conditt G.G. Poehling R.H. Jinnah

The knee is one of the most commonly affected joints in osteoarthritis. Unicompartmental knee replacement (UKA) was developed to address patients with this disease in only one compartment. The conventional knee arthroplasty jigs, while usually being accurate, may result in the prosthesis being inserted in an undesired alignment which may lead to poor post-operative outcomes. Common modes of failure in UKA include edge loading due to incorrect sizing or positioning, development of disease in the other compartment due to over-stuffing or over-correction and early loosening or stress fractures due to inaccurate bone cuts.

Computer navigation and robotically assisted unicompartmental knee replacement were introduced in order to improve the surgical accuracy of both the femoral and tibial bone cuts. The aim of this study was to assess accuracy and reliability of robotic assisted, unicondylar knee surgery in producing reported bony alignment.

Two hundred and twenty consecutive patients with a mean age of 64 + 11 years who underwent successful medial robotic assisted unicondylar knee surgery performed by two senior total joint arthroplasty surgeons were identified retrospectively. The mean body mass index of the cohort was 33.5 + 8 kg/m2 with a minimum follow-up of 6 months (range: 6–18 months). Femoral and tibial sagittal and coronal alignments as well as the posterior slope of the tibial component were measured in the post-operative radiographs. These measurements were compared with the equivalent measurements collected during intra-operative period by the navigation to study the reliability and accuracy of femoral and tibial cuts. Radiographic evaluation was independently conducted by two observers.

There was an average difference of 2.2 to 3.6 degrees between the intra-operatively planned and post-operative radiological equivalent measurements. For the femur, mean varus/valgus angulation was 2.8 + 2.5 degrees with 83% of those measured within 5% of planned. For the tibia mean varus/valgus angulation was 2.4 + 1.9 degrees with 93% within 5% of planned resection. There was minimal inter-observer variability between radiographic measurements. There were no infections in the evaluated group at the time of radiographic examination.

Alignment for unicondylar knee arthroplasty is important for implant survival and is a more difficult procedure to instrument as it is a minimally invasive surgery. Assuming appropriate planning, robotically assisted surgery in unicondylar knee replacement will result in reliably accurate positioning of component and reduce early component failures caused by malpositioning. A mismatch between pre-planning and post-operative radiography is often caused by poor cementing technique of the prosthesis rather than incorrect bony cuts. Addressing these factors can lead to greater success and improved outcomes for patients.

S. Schumann L.P. Nolte G. Zheng

Tracked B-mode ultrasound (US) potentially provides a non-invasive and radiation-free alternative to percutaneous pointer digitization for intra-operative determination of the anterior pelvis plane (APP). However, most of the published approaches demand a direct access to the corresponding landmarks, which can only be presumed for surgical approaches with the patient in supine position. In order to avoid any change of the clinical routine for total hip arthroplasties (THAs), we propose a new method to determine the pelvic orientation, which could be performed in lateral position.

Our proposed method is based on the acquisition of ultrasound images of the ipsilateral hemi-pelvis, namely the posterior superior iliac spines (PSISs) and iliac crest region. The US images are tracked by a navigation system and further processed to extract three-dimensional point clouds. As only one side of the pelvis is accessible, we estimate the symmetry plane (midsagittal plane) of the pelvis based on additionally digitized bilateral anterior superior iliac spine (ASIS) landmarks. This symmetry plane is further used to mirror the ipsilateral US-derived points to the contralateral side of the pelvis and to register and instantiate a pelvic SSM constructed from 30 CT-scans.

The proposed registration method was evaluated using two plastic pelvis models and two cadaveric pelvises together with special custom-made silicone phantoms to simulate the missing soft-tissue. In each trial, the required data were collected with the pelvis rigidly fixed in lateral decubitus position together with ground truth APP landmarks. A registration error of 3.48° ± 1.10° was found for the anteversion angle, while the inclination angle could be reconstructed with a mean error of 1.26° ± 1.62°.

The performed in-vitro experiments showed reasonably good results, taking the sparsity of the input point clouds into consideration.

X. Kang W.P. Yau Y. Otake R.H. Taylor

Anterior Cruciate Ligament (ACL) rupture is one of the commonest injuries in sports medicine. However, the rates of the reported graft re-rupture range from 2–10%, leading to around 3000 to 10000 revision ACL reconstructions in United States per annum. Inaccurate tunnel positions are considered to be one of the commonest reasons leading to failure and subsequent revision surgery. Additionally, there remains no consensus of the optimal position for ACL reconstructions.

The positions of the bone tunnels in patients receiving ACL reconstruction are traditionally assessed using X-rays. It is well known that conventional X-ray is not a precise tool in assessing tunnel positions. Thus, there is a recent trend in using three-dimensional (3D) CT. However, routine CT carries a major disadvantage in terms of significant radiation hazard. In addition, it is both inconvenient and expensive to use CT as a regular assessment tools during the follow-up.

The goal of the present work is to develop a novel 2D-3D registration method using single X-ray image and a surface model. By performing such registration for two post-operative X-rays, we can further calculate the 3D tunnel positions after ACL reconstructions. Our framework consists of five parts: (1) a surface model of the knee, (2) a 2D-3D registration algorithm, (3) a 3D tunnel position calculation, (4) a graphic user interface (GUI), and (5) a semi-transparency rendering. Among them, the crucial part is our 2D-3D registration method that estimates the relative position of the knee model in the imaging coordinate system. Once registered, the 3D position of an ACL tunnel in the knee model is calculated from the imaging geometry. The only interaction required is to mark the ACL tunnels on the X-rays through the GUI.

We propose two 2D-3D registration methods. One is a contour-based method that uses pure geometric information. Most methods in this category accomplish the registration by extracting contours in X-rays, establishing their correspondences on the 3D model, and calculating the registration parameters. Unlike these methods, which need point-to-point correspondences, our method optimises the registration parameters in a statistical inference framework without giving or establishing point-to-point correspondences. Due to the use of the statistical inference, our method is robust to the spurs and broken contours that automatically extracted by the contour detector.

The second method takes into account both the geometric shape of the object and the intensity property (intensity changes) of the image, where the intensity changes can be detected via image gradients. The use of gradient is based on the interpretation that two images are considered similar, if intensity changes occur at the same locations. The angles between the image gradients and the projected surface normals were used as a distance measure. The summation of the measures for all projected model points gives us the gradient term, which we multiply the contour-based measurement. Multiplication is preferred over addition because addition of the terms would require both terms to be normalised.

To evaluate the feasibility of our methods, a simulation study was conducted using Digitally Reconstructed Radiographs (DRR) of a sawbone underwent a single-bundle ACL reconstruction performed by an experienced orthopedic surgeon. The real position of the bone tunnel entry point was obtained using the CT images, which were acquired using a custom-made well-calibrated cone-beam CT. The knee model was built by downsampling and smoothing the high-resolution CT reconstructions. It is important in our experiments to make the model different from the original reconstruction since this simulates the condition in which patient's CT is unavailable. Two DRRs generated from approximately anteroposterior and lateral viewpoints were used. For each DRR, 50 trials of 2D-3D registration were carried out for the femoral part using 50 different initialisations, which were randomly selected from the values independently and uniformly distributed within ±10 degrees and ±10 mm of the ground-truth.

Compared with the ground-truth established using the CT images, our single image contour-based method achieved accurate estimations in rotations and in-plane translations, which were (−0.67±1.38, −0.98±0.84, −0.42±0.71) degrees and (0.11±0.26, −0.06±1.20) mm for the anteroposterior image, and (−0.78±0.76, −0.37±0.87, 0.70±0.88) degrees and (−0.14±0.22, 0.31±0.71) mm for the lateral one, respectively. The same experiments were also performed using the second method. However, it did not produce desirable results in our experiments. The tunnel entry point was then calculated using the averaged registration result of our contour-based method. The entry point of the tunnel was obtained with high accuracy of 1.25 mm distance error from the real position of the entry point.

For the 2D-3D registration, the estimated off-plane translations showed relatively low accuracy. It is well known that the depth can be difficult to be accurately estimated using one single image. As the result showed, the accuracy in rotations and in-plane translations is more important for ACL tunnel position estimation in our framework. As for the image gradient, it is too sensitive to the small perturbation caused by image noises. A more robust way of integrating the gradient information into our contour-based method is required. We propose a novel approach for estimating the 3D position of bone tunnels in ACL reconstruction using two post-operative X-rays. It was tested in a sawbone study using DRRs. The most significant advantage of our approach is to potentially eliminate the necessity of acquiring a patient's CT. The success in developing and validating the proposed workflow will allow convenient and precise assessment of tunnel positions in ACL reconstruction with minimal risk of radiation hazard.

M.C. Müller P. Belei M. de la Fuente M. Strake K. Kabir C. Burger K. Radermacher D. C. Wirtz

Pertrochanteric femoral fractures are common and intramedullary nailing with a proximal femoral nail (PFNA®) is an accepted method for the surgical treatment. Accurate guide wire and subsequent hardware placement in the femoral neck is believed to be essential in order to avoid mechanical failure. Malpositioned implants may lead to rotational or angular malalignment or “cut out” in the femoral neck. Hip and knee arthritis might be a potential long-term consequence. The conventional technique might require multiple guidewire passes, and relies heavily on fluoroscopy.

A computer-assisted surgical planning and navigation system based on 2D-fluoroscopy was developed in-house as an intraoperative guidance system for navigated guide wire placement in the femoral neck and head. To support the image acquisition process, the surgeon is supported by a so-called “zero-dose C-arm navigation” module. This tool enables a virtual radiation-free preview of the X-ray images of the femoral neck and head. The aim of this study was to compare PFNA® insertion using this system to conventional implantation technique. We hypothesised that guide wire and subsequent implant placement using our software decreases radiation exposure to the minimum of two images and reduces the number of drilling attempts. Furthermore, accuracy of implant placement in comparison to the conventional method might be improved and operation time shortened.

We used 24 identical intact left femoral Sawbones® to simulate reduced pertrochanteric femoral fractures. First, we performed placement of the PFNA® into 12 Sawbones using the conventional fluoroscopic technique (group 1). Secondly, we performed placement of the PFNA® into 12 Sawbones guided by the computer-assisted surgical planning software (group 2). In each group, we first performed open and secondly minimal-invasive intramedullary nailing in six sawbones each. For minimal-invasive guide wire placement, a surgical drape imitated soft tissue coverage. Conventional and navigated technique used a C-arm fluoroscope (Siemens IsoC 3D®, Erlangen, Germany) in conventional 2D mode. Guidewire and subsequent blade placement in the femoral neck was evaluated. We documented: 1: the number of fluoroscopic images; 2: the total number of drilling attempts; 3: implant placement accuracy (3.1. Tip apex distance (TAD); 3.2. visible penetrations of the femoral neck and head; 3.3. blade-corticalis bone distance in the anteroposterior and lateral plane) and the 4: operation time.

The number of fluoroscopic single shots taken to achieve an acceptable PFNA®-blade position was reduced significantly with computer-assistance by 71.5% (p<0.001) in the open and by 72,4% (p<0.001) in the minimally invasive technique. In each operation two X-rays for final documentation were taken. The average number of drilling attempts for the computer-guided system was significantly (p<0.05) less than that of the conventional technique in the minimally invasive procedure. The average number of drilling attempts showed no difference between the computer-assisted and conventional techniques in the open procedure. Accuracy of implant placement showed no difference between the computer-assisted and the conventional group. Computer assistance significantly increased the mean operation time for fixation of pertrochanteric femoral fractures with a PFNA® by 79.8% (p<0.001) in the open technique and by 54.4% (p<0.001) in the minimally invasive technique.

Use of our computer-guided system for fixation of pertrochanteric femoral fractures by a PFNA® decreases the number of fluoroscopic single shots and of suboptimal guide wire passes while maintaining blade placement accuracy that is equivalent to the conventional technique. Computer-assisted surgery with our system increases the operation time and has just been tested in non-fractured sawbones. Although these results are promising, additional studies including fractured sawbones and cadaver models with extension of the navigation process to all steps of PFNA® introduction and with the goal of reducing the operation time are indispensable before integration of this navigation system into the clinical workflow.

K.C. Wong S.M. Kumta L.F. Tse W.K. Ng K.S. Lee

CT and MRI scans are complementary preoperative imaging investigations for planning complex musculoskeletal bone tumours resection and reconstruction. Conventionally, tumour surgeons analyse two-dimensional (2-D) imaging information, mentally integrate and formulate a three-dimensional (3-D) surgical plan. Difficulties are anticipated with increase in case complexity and distorted surgical anatomy. Incorporating computer technology to aid in this surgical planning and executing the intended resection may improve precision. Although computer-assisted surgery has been widely used in cranial biopsies and tumour resection, only small case series using CT-based navigation are recently reported in the field of musculoskeletal tumor surgery. We investigated the results of CT/MRI image fusion for Computer Assisted Tumor Surgery (CATS) with the help of a navigation system.

We studied 21 patients with 22 musculoskeletal tumours who underwent CATS from March 2006 to July 2009. A commercially available CT-based spine navigation system (Stryker Navigation; CT spine) was used. Of the 22 patients, 10 were males, 11 were females, and the mean age was 32 years at the time of surgery (range, 6–80 years). Five tumours were located in the pelvis, seven sacrum, eight femurs, and two tibia. The primary diagnosis was primary bone tumours in 16 (3 benign, 13 sarcoma) and metastatic carcinoma in four. The minimum follow-up was 17 months (average, 35.5 months; range, 17–52 months). Preoperative CT and MRI scan of each patient were performed. Axial CT slices of 0.0625mm or 1.25mm thickness and various sequences of MR images in Digital Imaging and Communications in Medicine (DICOM) format were obtained. CT and MR images for 22 cases were fused using the navigation software. All the reconstructed 2-D and 3-D images were used for preoperative surgical planning. The plane of tumour resection was defined and marked using multiple virtual screws sited along the margin of the planned resection. We also integrated the computer-aided design (CAD) data of custom-made prostheses in the final navigation resection planning for eight cases.

All tumour resections could be carried out as planned under navigation guidance. Navigation software enabled surgeons to examine all fused image datasets (CT/MRI scans) together in two spatial and three spatial dimensions. It allowed easier understanding of the exact anatomical tumor location and relationship with surrounding structures. Intraoperatively, image guidance with the help of fusion images, provided precise visual orientation, easy identification of tumor extent, neural structures and intended resection planes in all cases. The mean time for preoperative navigation planning was 1.85 hours (1 to 3.8). The mean time for intraoperative navigation procedures was 29.6 minutes (13 to 60). The time increased with case complexity but lessened with practice. The mean registration error was 0.47mm (0.31 to 0.8). The virtual preoperative images matched well with the patients' operative anatomy. A postoperative superficial wound infection developed in one patient with sacral chordoma that resolved with antibiotic whereas a wound infection in another with sacral osteosarcoma required surgical debridement and antibiotic. After a mean follow-up of 35.5 months (17–52 months), five patients died of distant metastases. Three out of four patients with local recurrence had tumors at sacral region. Three of them were soft tissue tumour recurrence. The mean functional MSTS score in patients with limb salvage surgery was 28.3 (23 to 30). All patients (except one) with limb sparing surgery and prosthetic reconstruction could walk without aids.

Multimodal image fusion yields hybrid images that combine the key characteristics of each image technique. Back conversion of custom prosthesis in CAD to DICOM format allowed fusion with navigation resection planning and prosthesis reconstruction in musculoskeletal tumours. CATS with image fusion offers advanced preoperative 3-D surgical planning and supports surgeons with precise intraoperative visualisation and identification of intended resection for pelvic, sacral tumors. It enables surgeons to reliably perform joint sparing intercalated tumor resection and accurately fit CAD custom-made prostheses for the resulting skeletal defect.

S.U. Sasaki S.S. Daher A.J. Hernandez R.F.M. Albuquerque M.U. Resende R. Queiroz H. Moscovich

The purpose of the study was to compare prospectively and randomly two ACL reconstruction single bundle techniques, one referred to as traditional and the other referred to as anatomical, where the coronal angulation of the femoral tunnel aimed a more horizontal position at 2 and 10 o'clock. Orthopilot® System (Aesculap, Tuttlingen, Germany) was used to assist tunnel positioning in order to obtain and register translational and rotational stability.

Eighteen patients (14 men and 4 women), average age 33.8 years (range 18 to 49), with isolated ACL lesion were randomized in two groups, A (Conventional) and B (Anatomical). All patients were submitted to ACL navigated arthroscopic reconstruction with quadruple hamstrings grafts. Anteromedial portal access for femoral tunnel drilling was used in all patients. The tibial and femoral tunnels drillings were monitored by the Aesculap® Orthopilot Navigation System. In Group A, the femoral tunnel positioning aimed isometricity. In Group B, femoral tunnel was drilled at 25% of Blumensaat's line length from the posterior cortex, and 30° orientation in coronal plane. Initial and final Maximum Anterior tibial Displacement (MATD), Internal Tibial Rotation (ITR) and External Tibial Rotation (ETR) at 30° knee flexion data were recorded intra operatively by the navigation system.

No horizontal or rotational stability differences were found for MATD (p = 0.68), ITR (p = 014) and ETR (0.13). This study did not support the hypothesis that a more anatomical positioning leads to better rotational or anterior stability.

E.J. Smith H.A. Al-Sanawi B. Gammon P.J. St. John D.R. Pichora R.E. Ellis

Primary internal fixation of uncomplicated scaphoid fractures is growing in popularity due to its advantages over conventional cast fixation. Performing the procedure percutaneously reduces the risk of infection and soft tissue damage, but can be tricky because of the small size and complex three-dimensional (3D) shape of this bone. Computer-assisted navigation has been an invaluable tool in other pin insertion procedures.

This in-vitro study aimed to evaluate two different rendering techniques for our navigation interface: (i) 3D volume rendering of the CBCT image to show digitally-reconstructed radiographs of the anatomy, and (ii) volume-slicing, analogous to CT-images.

As the shape of the scaphoid is highly variable, a plastic model of the wrist was constructed in order to provide consistency that would not be possible in a cadaver-based study. The plastic model featured a removable scaphoid such that a new one was replaced between trials. Three surgeons each performed eight trials using each of the two navigated techniques (yielding a total of 48 trials for analysis). Central placement of scaphoid fixation has been linked with mechanical stability and improved clinical outcomes, thus the surgical goal was to place a K-wire to maximise both depth from the surface and length of the drill path. The wire was drilled through the scaphoid, from distal to proximal, allowing for post-trial analysis of the drill path. A ceiling-mounted OptoTrak Certus camera (Northern Digital Inc., Canada) and a floor-mounted isocentric 3D CBCT C-arm (Innova 4100, GE Healthcare, France) permitted a registration transformation between the tracking and imaging systems to be computed preoperatively, before each trial, using a custom calibration device. Optical local coordinate reference bodies were attached to the wrist model and a custom drill guide for tracking with the Certus camera. During each trial, a 3D spin image of the wrist model was acquired, and rendered according to the technique under study.

For 3D volume rendering, the spin image was rendered as a digitally-reconstructed radiograph (DRR) that could be rotated in three dimensions. In the planning phase, the surgeon positioned a desired drill path on the images. Anterior-posterior and lateral views of the 3D volume rendering were used for navigation during the drilling phase. The real-time orientation of the drill guide was shown relative to these images and the plan on an overhead.

For volume-sliced (VS) navigation, the spin image was volume-rendered and sliced along the principal planes (axial, coronal, sagittal) for planning. A slider interface allowed the surgeon to scroll through the slices in each of the planes, as if they were looking at individual CT slices. Once the desired drill path was positioned, the volume-sliced views were reconfigured to show slices along the oblique planes of the planned path for navigation.

Following all trials, model scaphoids with wire intact were imaged using CT with a slice thickness of 0.625 mm. The CT series were segmented and used to construct 3D digital models of the wire and drilled scaphoid. Algorithms were developed to determine the minimum distance from the centerline of the wire and the scaphoid surface, and to compute the length of the drill path. Screw breach should be avoided as it disrupts the articular surface and may lead to a sequela of cartilage deterioration and osteoarthritic changes. The shortest distance measure was extrapolated to assess whether a standard fixation screw (Accutrak Mini, 1.78 mm radius) would have breached the scaphoid surface. There were three screw breaches noted in the 3D DRR trials, while only one occurred using volume-slicing. The minimum distance from the centerline of the wire to the scaphoid surface can also be thought of as a “safe zone” for screw breach. Although no difference in the mean distance (μ) was noted between groups (μDRR = 2.3 mm, μVS = 2.2 mm), the standard deviation (σ) was significantly higher for the DRR trials (σDRR = 0.50 mm, σVS = 0.37 mm, p < 0.1), suggesting a higher reliability of central placement using VS for navigation. In contrast, the length of the drill paths were significantly longer for the DRR trials (μ = 28.7 mm, σ = 0.66 mm) than for VS-navigation (μ = 28.3 mm, σ = 0.62 mm) at p < 0.1.

The surgical goal was to pick a path that maximised both the length of the path, as well as the minimum distance from the scaphoid surface. Algorithms were developed to find the paths that would maximise: (i) the length and (ii) the distance from the surface of the model scaphoid used in this study. The maximum possible length was 29.8mm (with a minimum distance of 2.2mm from the scaphoid surface), and the maximum distance was 3.3mm (with a length of 27.5mm). Therefore, the set of optimal drill paths had length > 27.5 mm, and distance > 2.8 mm. Of the DRR-navigated trials, 11 were below the minimum optimal depth, and only one trial was below the optimal length; 13 of the 24 trials (54%) were of both optimal length and depth. Of the VS-navigated trials, nine were below the minimal optimal distance, and four were below the minimum optimal length; 11 out of 24 trials (46%) were within both the optimal length and depth.

From this comparative study, we conclude that VS-navigation was superior in locating a central location for the fixation wire, while DRRs were superior in maximising the depth of the drill path. Thus, we propose a hybrid interface, incorporating both volume-slicing and DRRs, in order to maximise the effectiveness of navigation for percutaneous scaphoid pinning.

H. Schöll A. Jones M. Mentzel F. Gebhard M. Kraus

Computer assisted surgery (CAS) is used in trauma surgery to reduce radiation and improve accuracy but it is time consuming. Some trials for navigation in small bone fractures were made, but they are still experimental. One major problem is the fixation of the dynamic reference base for navigation. We evaluated the benefit of a new image based guidance-system (Surgix®, Tel Aviv, Israel) for fracture treatment in scaphoid bones compared to the conventional method without navigation. The system consists of a workstation and surgical devices with embedded radio opaque markers. These markers as well as the object of interest must be on the same C-arm shot. If a tool is detected in an image by the attached workstation additional information such as trajectories are displayed in the original fluoroscopic image to serve the surgeon as aiming device. The system needs no referencing and no change of the workflow.

For this study 20 synthetic hand models (Synbone®, Malans, Switzerland) were randomised in two groups. Aim of this study was a central guide-wire placement in the scaphoid bone, which was blindly measured by using postoperative CT-scans. Significant distinctions related to the duration of surgery, emission of radiation, radiation dose, and trials of guide-wire positioning were observed.

By using the system the surgery duration was with 50 % shortened (p = 0.0054) compared to the conventional group. One reason might be the significant reduction of trials to achieve a central guide-wire placement in the bone (p = 0.0032). Consequently the radiation exposure for the surgeon and the patient could be shortened by reduction of radiation emission (p = 0.0014) and radiation dose (p = 0.0019).

By using the imaged based guidance system a reduction of surgery duration, radiation exposure for the patient and the surgeon can be achieved. By a reduced number of trials for achieving a central guide-wire position the risk of weakening the bone structure can be minimised as well by using the system. The system seems helpful where navigation is not applicable up to now. The surgical workflow does not have to be chanced.

H. Bou-Sleiman L.-P. Nolte M. Reyes

Bone fixation plates are routinely used in corrective and reconstructive interventions. Design of such implants must take into consideration not only good surface fit, but also reduced intra-operative bending and twisting of the implant itself. This process increases mechanical stresses within the implant and affects its durability and the functional outcome of the surgery. Wound exposure and anaesthesia times are also reduced. Current population-based designs consider the average shape of a target bone as a template to pre-shape the implant. Other studies try to enhance the average design by optimising surface metrics in a statistical shape space. This could ensure a low mean distance between the implant and any bone in the population, but does not reduce neither the maximum possible distances nor directly the mechanical forces needed to fit the implant to the specific patient. We propose a population-based study that considers the bending and torsion forces as metrics to be minimised for the design of enhanced fixation plates. Our aim is to minimise the necessary intra-operative deformations of the plates.

In our approach, we first propose to represent a fixation plate by dividing it into discrete sections lengthwise and fitting a plane to each section. The number of sections depends on the size of the implant and anatomical location. It should be small enough to capture the anatomical curvatures, but large enough not to be affected by local noise in the surface.

Surface patches corresponding to common locations for plate fixations are extracted from 200 segmented computed tomography (CT) images. In this work, distal lateral femoral patches are considered. A statistical shape model of the patches is then computed and a large population of 2,197 instances is generated, evenly covering the natural statistical variation within the initial population. These instances are considered as both bone surfaces and potential new designs of the contact surface of the fixation plate.

The key formulation of our solution is to examine the effect of deforming each section of the implant on the rest of the sections and compute the amount of bending and torsion needed to shape one patch to another.

Each instance of the population is fitted to all others and the maximum bending and torsion angles are recorded. A similar process was applied for the mean of the population. The goal is to pick from the population the shape that simultaneously minimises the bending and torsion angles.

The maximum required bending was reduced from 25.3® to 19.3® (24.72% reduction), whereas the torsion component was reduced from 12.4® to 6.2® (50% reduction).

The method proposed in this abstract enhances the current state-of-the-art in orthopaedic implant design by considering the mechanical deformations applied to the implant during the surgery. The obtained results are promising and indicate a noticeable improvement over the standard pre-contouring to the population mean. We plan to further validate the method and as a future outlook, we intend to test the approach in real surgical scenarios.

J.R. Smith P.J. Rowe M. Blyth B. Jones

The aim of this study was to determine the influence of electromagnetic (EM) navigation in total knee arthroplasty (TKA) on post operative function.

In this double blinded randomised control trial, patients with osteoarthritis either received TKA using conventional techniques (n = 49) or EM navigation (iNav Portable Navigation System, Zimmer Orthopaedics) (n = 52). All of the patients were reviewed in the Outcomes Clinic at 3 and 12 months. At 12 months post operation the patients completed an objective biomechanical functional assessment using flexible electrogoniometers, which recorded dynamic knee kinematics during daily activities. Knee joint flexion and extension moments were recorded at the 12 month post operation assessment. The functional assessment included validated questionnaires (Oxford Knee Score, American Knee Society Score, WOMAC Score and Short Form SF-36 Score). All patients underwent CT scanning of the implanted prosthesis to assess component alignment.

Improved alignment was recorded in the navigated group. However there was no significantly significant difference between the two surgical groups in terms of the subjective questionnaire scores. The biomechanical assessment showed no statistically significant differences in the maximum, minimum or excursion knee joint angles between the two surgical groups during the 12 daily functional tasks. However, significant differences were reported in level and slope walking activities during pre-swing phase (at around 60% of the gait cycle). The navigated group had significantly higher knee joint angles during pre swing suggesting a more vigorous push off into swing phase and a more ‘normal’ gait cycle. The two surgical groups were sub divided into males and females for the strength test. The female navigated group recorded a significantly greater hamstring (p = 0.03) and quadriceps (p = 0.003) moment. There was no significant difference in hamstring or quadriceps moments between the navigated and conventional male groups.

The knee kinematics and moment data suggests that the navigated group had an improved functional outcome. However the difference in the post-operation function of the two groups remains minimal despite the better alignment achieved using navigation.

V. Ferrari P. Parchi S. Condino M. Carbone A. Baluganti M. Ferrari F. Mosca M. Lisanti

Pedicle screws fixation to stabilise lumbar spinal fusion is the gold standard for posterior stabilisation. Pedicle screws are today positioned in free hand or under fluoroscopic guidance with an error from 20% up to 40–50%, which can determine the inefficacy of treatment or severe damages to close neurologic structures. Surgical navigation drastically increases screws placement accuracy. However its clinical application is limited due to cost reasons and troubles related to the presence of a localiser in the OR and the need to perform a registration procedure before surgery. An alternative image guided approach is the use of patient specific templates similar to the ones used for dental implants or knee prosthesis. Until now, the proposed solutions allow to guide the drill, and in some cases, as templates fit completely around vertebra, they require the complete removal of soft tissues on a large portion of the spine, so increasing intervention invasiveness. To reduce the soft tissue demolition, some authors proposed a fitting based on small “V shape” contact points, but these solutions can determine instability of the template and the reacting of wrong stable positions.

In our solution, after spine CT acquisition, each vertebra is segmented using a modified version of ITK-SNAP software, on which the surgeon plans screws positioning and finally the template is designed around the chosen trajectories, using a tool which allows to insert cylinders (full or empty) in the segmented images. Each template, printed in ABS, contains two hollow cylinders, to guide the screws, and multiple contact points on the bone surface, for template stabilisation.

We made an in-vitro evaluation on synthetic spine models (by Sawbones) to study different template designs. During this first step an ongoing redesign allowed to obtain an optimal template stability and an easy template positioning to minimise the intervention invasiveness. A first contact point, which fits on the sides of the spinous process, is used to simplify template alignment. The other 4 contact points, which consists of cylinders (diameter 5 mm), fit exactly on spine surface in correspondence to the vertebra's lamina and articular processes to stabilise the template in an unique position. Templates can be used to guide not only the drill, but also Kirschner wires, to guide cannulated screws. After the Kirschner wires insertion the template can be dismounted for its removal (the direction of the kirschner wires are not parallel).

After the definitive template design an ex-vivo animal test on 2 porcine specimens has been conducted to evaluate template performance in presence of soft-tissue in place. The specimens have been scanned with CT, we realised a total of 14 templates and we performed the insertion of 28 Kirschner wires. We evaluated that after the soft tissue dissection and the bone exposure, the template can be easily positioned in the right unique position, with no additional tissue removal compared to the traditional approach, requiring just removal of the soft tissue under the small contact points using an electric cutter. The surgeon evaluated (and corrected) some wrong stable template positions when not all the contact points were in contact with the bone surface. The post-op evaluation was made with a CT scan that showed 1 cortical pedicle violation (3.5%) (grade II according to the FU classification).

M.I. Chaudary E.T. Davis

Mal-positioning of the acetabular component is associated with increased dislocation rate, increased wear and component impingement. Navigation provides real time feedback to the surgeon and allows the accurate position of implants. Compared to conventional techniques of total hip replacement; use of the imageless navigation system has shown to improve accuracy of implant positioning.

When impacting uncemented acetabular components under navigation, there is often a deviation from the planned abduction and anteversion measurement due to deflection of the implant in the reamed cavity. Although there exists the ability to navigate the reaming of the acetabular cavity; this is not widely performed. The ability to ream the acetabular cavity in the exact orientation of the planned acetabular component may provide some theoretical advantage on the final acetabular position. The purpose of this study was to compare the effect of navigated Vs free hand acetabulum reaming on achieving the planned orientation of acetabular component.

In a retrospective study we reviewed two groups of patients who underwent computer navigated placement of the acetabular component with reference to the anterior pelvic plane. We used an imageless computer navigation system for all cases (Brainlab, Munich). All procedures were performed by single surgeon (ETD) through a standard posterior approach. The patients were divided into two groups depending on the availability of the navigated reamer. In the first group (n = 57), acetabulum reaming was done under navigation and in the second group (n = 37) a non-navigated reamer was used. The acetabular cavity was reamed “line to line” or under reamed by 1 or 2mm. Intra-operative acetabular abduction and anteversion angles were planned using navigation at the discretion of the surgeon. Results of planned acetabular abduction and anteversion angles were compared with intra-operative verification using the navigation system.

In the navigated reamer group, the mean error from the planned to verified abduction angle was 1.7 degrees (SD 2.1 degrees) and in the non-navigated reamer group the mean error was 2 degrees (SD 2.6 degrees). In the navigated reamer group, the mean error from the planned to verified anteversion angle was 0.5 degrees (SD 2.8), and in the non-navigated reamer group the mean error was 0.1 degrees (SD 1.6). There was no statistically significant difference in the mean error between the navigated and non-navigated reaming groups for abduction angle (p = 0.54) or anteversion angle (p = 0.24). There was no statistical difference between the mean acetabular component size in the navigated (mean 53mm) and non-navigated (53mm) reamer groups (p = 0.8). There was no statistical difference in the mean difference in reamer size and the acetabular component size in the navigated (0.8mm) and non-navigated reamer groups (0.8mm, p = 0.52).

This study appears to show that performing reaming of the acetabular cavity under navigation does not improve the final orientation of the acetabular component when compared to using conventional non-navigated reamers. However, this study only considered the abduction and anteversion orientation of the component. The move to a range of movement or kinematic orientation of the acetabular component in hip arthroplasty requires control over the off-set of the acetabular component which may be more easily achieved when the reaming is performed under navigation. This study used a conventional posterior approach rather than a minimal incision technique, where the use of navigated reaming may also provide some theoretical advantage when visibility is limited. Further study is required in these two areas.

There appears to be a slightly higher standard deviation for the anteversion measurement in the navigated reamer group when compared to the non navigated reamer group, although this is not significant. It is difficult to account for this as it appears to be opposite of what one would predict. One explanation for this may come in the difference in the angled geometry of the navigated reamer when compared to the straight non navigated reamer. The angled reamer can be more difficult to control forming a cavity in the correct orientation but with the possibility for the cavity to not been perfectly hemispherical.

When using navigation to insert the acetabular component in a planned abduction and anteversion position during hip arthroplasty through a standard incision, navigating the reaming of the acetabular component does not appear to provide any advantage over the use of conventional non-navigated reamers in the final acetabular orientation.

J. Victor A. Premanathan L. Keppler P. Deprez J. Bellemans

Eight consecutive patients with significant malalignment of the lower limb were included in the study. Pre-operative CT scans of the affected limb and the normal contra-lateral side were obtained and 3D models of the patient's anatomy were created, using dedicated software. The healthy contralateral limb was mirrored and geometrically matched to the distal femur or proximal tibia of the healthy side. A virtual opening wedge correction of the affected bone was used to match the geometry of the healthy contralateral bone. Standard lower limb axes measurements confirmed correction of the alignment. Based on the virtual plan, surgical guides were designed to perform the planar osteotomy and achieve the planned wedge opening and hinge axis orientation. The osteotomy was fixed with locking plates and screws. Post-operative assessment included planar X-rays, CT-scan and full leg standing X-rays.

One three-planar, three bi-planar and four single-plane osteotomies were performed. Maximum weightbearing mechanical femoro-tibial coronal malalignment varied between 7° varus and 14° valgus (mean 7.6°, SD 3.1). Corrective angles varied from 7°–15°(coronal), 0°–13°(sagittal) and 0°–23°(horizontal). The maximum deviation between the planned pre-operative wedge angle and the executed post-operative wedge angle was 1° in the coronal, sagittal and horizontal plane. The desired mechanical femorotibial axis on full-leg standing X-rays was achieved in 6 patients. Two patients were undercorrected by 1° and 2° respectively.


3D planning and guided correction of multi-planar deformity of femur or tibia is a feasible and accurate novel technique.

D. Saragaglia M. Blaysat N. Mercier M. Grimaldi

Double level osteotomy (DLO) for severe genu varum is not a common technique. We performed our first computer-assisted double level osteotomy (CADLO) in March 2001 and we published our preliminary results in 2005 and 2007. The rationale to perform this procedure is to avoid oblique joint line in order to have less difficulty in case of revision to a total knee arthroplasty (TKA). The goal of this paper is to present the results of 37 cases operated on between August 2001 and January 2010.

The series was composed of 35 patients (two bilateral), nine females and 26 males, aged from 39 to 64 years old (mean age: 50.5 +/− 7.5). We operated on 20 right knees and 17 left ones. The mean BMI was 29.3 +/− 4.3 for a mean height of 1.71 m and a mean weight of 85.8 kg. The functional status was evaluated according to the LYSHÖLM and TEGNER score. The mean score was of 42.4 +/− 8.9 points (22–69). According to modified AHLBÄCK criteria we operated on seven stage 2, 22 stage 3, five stage 4 and two stage 5. We measured HKA (Hip-Knee-Ankle) angle using RAMADIER's protocol and we also measured the femoral mechanical axis (FMA) and the tibial mechanical axis (TMA) to pose the right indication. These measures were respectively: 168° +/− 3.4° (159°–172°), 87.5° +/− 2.1 (83°–91°) for the FMA and 83.7° +/− 2.6° (78°–88°) for the TMA.

The inclusion criteria were a patient younger than 65 years old with a severe varus deformity (more than 8° − HKA angle ≤ to 172°) and a FMA at 91° or less. All the osteotomies were navigated using the ORTHOPILOT® device (B-BRAUN-AESCULAP, TUTTLINGEN, GERMANY). The procedure was performed as follows: after inserting the rigid-bodies and calibrating the lower leg, we did first the femoral closing wedge osteotomy (from 4 to 7 mm) which was fixed by a an AO T-Plate, and secondly, after checking the residual varus, the high tibial opening wedge osteotomy using a BIOSORB® wedge (Tricalcium phosphate) and a plate (AO T-plate or C-plate). The goals of the osteotomy were to achieve an HKA angle of 182° +/− 2° and a TMA angle of 90° +/− 2°.

The functional results were evaluated using the LYSHÖLM-TEGNER score and the KOOS score. The patients answered the questionnaire at revision or by phone, and the radiological results were assessed by plain radiographs and standing long leg X-Rays between three and six months postoperatively.

We had no complication in this series but one case of recurrence of the deformity related to an impaction of the femoral osteotomy on the medial side. Two patients were lost to follow-up after removing of the plates (24 months) but were included in the results because the file was complete at that date. All the patients were assessed at a mean follow-up of 43 +/− 27 months (12–108). The mean LYSHÖLM-TEGNER score was 78.7 +/− 7.5 points (59–91) and the mean KOOS score was 94.9 +/− 3.3 points (89–100). Thirty-five patients were satisfied (18) or very satisfied (17) of the result. Only two were poorly satisfied. Regarding the radiological results, if we exclude the patient who had a loss of correction, the goals were reached in 32 cases (89%) for the HKA angle and in 31 cases (86%) for the TMA with only one case at 93°. The mean angles were: 181.97° +/− 1,89° (177°–185°) for HKA, 89.86° +/− 1,85° (85°–93°) for TMA and 93.05° +/− 2.3° (89°–99°) for FMA. At that mid-term follow-up no patient had revision to a total knee arthroplasty.

DLO is a very demanding technique. Navigation can improve the accuracy of the correction compared to non computer-assisted osteotomies. The functional results are satisfying and the satisfaction of the patients is very high. Despite the difficulty of the procedure, complications are, in our hands, very rare. We recommend DLO for severe genu varum deformity in young patients to avoid oblique joint line, which will be difficult to revise to TKA.

K. Tokunaga K Watanabe

Total hip arthroplasty (THA) using minimally invasive surgeries (MIS) now become popular operative procedures. It is not easy to understand geometric information of pelvis and femur in the restricted operative fields during MIS-THA. Recently, THA in supine position comes into the limelight again to place acetabular cups in an optimum position because we can minimise the intra-operative pelvic motion during THA in supine position. To verify the usefulness of supine position, we measured the angels of acetabular trial cups intra-operatively using the CT-based navigation system. The trial cup positions were placed according to a conventional acetabular cup alignment guide. We compared the angles of acetabular trial cups between supine and lateral positions through the same MIS antero-lateral (AL) surgical approach.

Thirty eight hips underwent THA in lateral position (the AL group; average age: 63.9 years old, female: 29 cases, 33 hips, male: 5 cases, 5 hips) and 40 hips underwent THA in supine position (the AL Supine group; average age: 62.2 years old, female 40 cases, 40 hips) were subjected in this study. The single surgeon (the first author) performed all surgeries. We used the Roettinger's modified Watson-Jones approach in both groups. The pelvic registration for navigation was carried out using the CT-fluoro matching procedure with VectorVision Hip (BrainLAB, Germany). After acetabular reaming, the acetabular trial cups were placed into the reamed acetabulum to be at 45 degrees of operative inclination (OI) and at 20 degrees of operative anteversion (OA) using a conventional acetabular cup alignment guide. These angles of the trial cups were measured intra-operatively using the CT-based navigation system, VectorVision Hip. After removing the acetabular trial cup, the acetabular cups were placed using the navigation system. Trilogy cups (Zimmer, USA) and AMS HA shells (JMM, Japan) were used in this study.

The average angles of OI were 45.7 degrees (SD 5.5 degrees) in the AL group and 46.3 degrees (SD 4.6 degrees) in the AL Supine group. The average angles of OA were 30.0 degrees (SD 13.5 degrees) in the AL group and 23.5 degrees (SD 8.2 degrees) in the AL Supine group. The hip numbers whose errors were less than 10 degrees were 13 hips in the AL group and 26 hips in the AL Supine group, respectively. There was significant difference in hip numbers whose errors of angles were less than 10 degrees between the AL and Supine groups. The hip numbers whose errors were less than 5 degrees were 7 hips in the AL group and only 6 hips in the AL Supine group, respectively. There was no significant difference in hip numbers whose errors of angles were less than 5 degrees between the AL and Supine groups. The error values of OI were less than 10 degrees except one hip in both groups. However, the error values of 25 hips in the AL group were more than 10 degrees.

In lateral position, the pelvis easily rotated when the affected lower extremity was extended, externally rotated, and adducted during the femoral preparation in the AL group, which resulted in malalignment of acetabular OA. In contrast, most hips could be set with the error values less than 10 degrees in the AL Supine position because the pelvis could be stabilised on the operative table. In addition, landmarks, such as bilateral antero-superior iliac spines and the symphysis pubis, were palpable in supine position. However, the hips with error values less than 5 degrees were only 6 out of 40 hips even though in supine position. Using MIS techniques, we can provide more stable hip joint just after surgery since the muscles surrounding hip joints can be preserved. We have to place acetabular cups in an optimum position to achieve wide range of hip motion to prevent dislocation and to provide limitation-free daily activities for patients. These data suggests that we should use more accurate guide systems for acetabular cup replacement such as navigation systems, patient specific templates, and patient specific mechanical instruments to place acetabular cups in an optimum position.

S.S. Hung P.L. Yen M.Y. Lee G.F. Tseng

Clinical assessment of elbow deformity in children at present is mainly based on physical examination and plain X-ray images, which may be inaccurate if the elbow is not in fully supination; furthermore, the rotational deformity is even harder to be determined by such methods. Morrey suggested that the axis of rotation of the elbow joint can be simplified to a single axis. Based on such assumption, we are proposing a method to assess elbow deformity using rotational axis of the joint, and an optimized calculation algorithm is presented.

The rotation axis of elbow in respective to the upper arm can be obtained from the motion tract of markers placed at the forearm. Cadaver study was done, in which three skeletal motion trackers were placed over both the anterior aspect of humerus, as well as distal ulna. Osteotomy was created at the supracondylar region of humerus through lateral approach, and the bone fragments were stabilized with a set of external skeletal fixator, leaving the soft tissue intact. The amount of deformity was created manually by adjusting the position of the distal fragment via skeletal fixator. Ultrasound 3D motion tracking system from Zebris® was used in this study, and the program was developed under the Matlab environment. Cycles of passive elbow flexion/extension motion were carried out for each set of deformity. The data were initially transformed to humerus coordinate, and since the upper arm was not absolutely stationary, its influence on the measured position of ulna was adjusted. With this adjusted data, a best fit plane that would include most of the ulna positions (MU) within a minimal distance was obtained. The rotation axis was calculated as the normal vector to this plane, and the carrying angle could subsequently be assessed according to the relationship between this axis and the x-axis on the xy-plane as well as on the xz-plane.

Fresh frozen cadaver study was conducted in the Medical Simulation Center at Tzu-Chi University. After adjustment of the raw data to eliminate the influence of humerus motion, the ulna motion could be narrowed down from a band of 10mm to 3mm, with a significant smaller standard deviation. The rotation axis was obtained by the normal vector to the best fit plane. Two different approaches were attempted to find the plane. In the first method, the plane was obtained via least square method from the adjusted ulna positions, and the second method found the plane via RANSAC method. Calculations were repeated several times for each method, and the results showed a variation of 5 degrees in the first method and about 2 degrees in the second method.

Rotational axis can be used to define the 3-dimensional deformity of elbow joint; however, it is difficult to obtain such axis accurately due to hypermobility and multi-directional motion of the shoulder joint. In this study, we have developed another method to assess the elbow deformity using motion analysis system instead of the conventional image studies, and this may be applicable clinically if the facility becomes more accessible in the future. (This research was supported by the project TCRD-TPE-99-30 granted by the Buddhist Tzu-Chi General Hospital, Taipei Branch).

O. Lubovsky O. Safran D. Axelrod E. Peleg C. Whyne

Fractures of the clavicle are relatively common, occurring mostly in younger patients and have historically been managed non-operatively. Recent studies have shown an advantage to surgical reduction and stabilisation of clavicle fractures with significant displacement. Currently, fracture displacement is measured using simple anterior-posterior two-dimensional x-rays of the clavicle. Since displacement can occur in all three-dimensions, however, evaluation of the amount displacement can be difficult and inaccurate. The purpose of this study was to determine the view that provides the most accurate assessment.

Nine CT scans of acute displaced clavicle fractures were analysed with AmiraDEV5.2.2 Imaging software. Measurements for degrees of shortening and fracture displacement of the fracture clavicle were taken. Using a segmentation and manipulation module (ITK toolkit), five digitally reconstructed radiographs (DRRs) mimicking antero-posterior x-rays were created for every CT, with each differing by projection angle (ranging from 20° upwards tilt to 20° downwards tilt). Measurements were taken on each DRR using landmarks of entire clavicle length, distance from vertebrae to fracture (medial fragment length), distance from fracture to acromium (lateral fragment length), and horizontal shortening, and then compared to the true measurement obtained from the original CT.

For all 9 samples, after comparing the measurements of clavicle fracture displacement in each 2D image, we found that an AP view with a 20° downward tilt yielded displacement measurements closest to the 3D (“gold standard”) measurements. The results agree with previous data collected from cadaveric specimens using physical X-ray film images. DDRs enable creation of multiple standard AP radiographs from which accurate tilt can be measured. The large deviation in measurements on different DRR projections motivates consideration of standardising X-ray projections. A uniform procedure would allow one to correctly evaluate the displacement of clavicular fractures if fracture displacement information is to be utilized in motivating surgical decision-making.

R. Strachan P. Konala F. Iranpour M. Prime T. Amirthanayagam A. Amis

Anatomical referencing, component positioning, limb alignments and correction of mechanical axes are essential first steps in successful computer assisted navigation. However, apart from basic gap balancing and quantification of ranges of motion, routine navigation technique usually fails to use the full potential of the registered information. Enhanced dynamic assessment using an upgraded navigation system (Brainlab V. 2.2) is now capable of producing enhanced ‘range of motion’ analysis, ‘tracking curves’ and ‘contact point observations’.

‘Range of motion analysis’ was performed simultaneously for both tibio-femoral and patella-femoral joints. Other dynamic information including epicondylar axis motion, valgus and varus alignments, antero-posterior tibio-femoral shifts, as well as flexion and extension gaps were simultaneously stored as a series of ‘tracking curves’ throughout a full range of motion. Simultaneous tracking values for both tibiofemoral and patellofemoral motion was also obtained after performing registration of the prosthetic trochlea. However, there seems to be little point in carrying out such observations without fully assessing joint stability by applying controlled force to the prosthetic joint.

Therefore, in order to fully assess ‘potential envelopes of motion’, observations have been made using a set of standardised simple dynamic tests during insertion and after final positioning of trial components. Also, such tests have been carried out before and after any necessary ligament balancing. Firstly, the lower leg was placed in neutral alignment and the knee put through a flexion-extension cycle. Secondly the test was repeated but with the lower leg being placed into varus and internal rotation. The third test was performed with the lower leg in valgus and external rotation. Force applied was up to the point where resistance occurred without any gross elastic deformation of capsule or ligament in a manner typical of any surgeon assessing the stability of the construct. Also a passive technique of using gravity to ‘Drop-Test’ the limb into flexion and extension gave useful information regarding potential problems such as blocks to extension, over-stuffing of the extensor mechanism and tightness of the flexion gap. All the definitive tests were performed after temporary medial capsular closure.

Ten total knee arthroplasties have been studied using this technique with particular reference to the patterns of instability found before, during and after adjustments to component positioning and ligament balancing. Marked intra-operative variation in the stability characteristics of the trial implanted joints has been quantified before correction. These corrections have been analysed in terms of change in translations, rotations and contact points induced by any such adjustments to components and ligament. Certain major typical patterns of instability have begun to be identified including excessive rotational and translational movements. Instability to valgus and external rotational stress was found in two cases and to varus and internal rotational stress in one case before correction. In particular, surprising amounts of edge loading in mid-flexion under stress testing has been identified and corrective measures carried out. Reductions in paradoxical tibio-femoral antero-posterior motion were also observed. Global instability and conversely tightness were also observed in early stages of surgery. Adjustments to component sizes, rotations, tibial slope angles and insert thickness were found to be necessary to optimise range of motion and stability characterisitics on an almost case-by-case basis. Two cases were identified where use of more congruent or stabilised components was necessary. Observation of quite marked loss of contact between tibia and femur was seen on the lateral side of the knee in deep flexion in several cases. Patellar tracking was also being observed during such dynamic tests and in two cases staged partial lateral retinacular releases were carried out to centre patellar tracking on the prosthetic trochlea.

Although numbers in this case series are small, it has been possible to begin to observe, classify and quantify patterns of instability intra-operatively using simple stress tests. Such enhanced intra-operative information may in future make it possible to create algorithms for logical and precise adjustments to ligaments and components in order to optimise range of motion, contact areas and stability in TKR.

M. Takemoto M. Neo S. Fujibayashi T. Okamoto E. Ota T. Sakamoto T. Nakamura

The accuracy of pedicle screw placement is essential for successful spinal reconstructive surgery. The authors of several previous studies have described the use of image-based navigational templates for pedicle screw placement. These are designed based on a pre-operative computed tomographic (CT) image that fits into a unique position on an individual's bone, and holes are carefully designed to guide the drill or the pedicle probe through a pre-planned trajectory. The current study was conducted to optimise navigational template design and establish its designing method for safe and accurate pedicle screw placement.

Thin-section CT scans were obtained from 10 spine surgery patients including 7 patients with adolescent idiopathic scoliosis (AIS) and three with thoracic ossification of the posterior longitudinal ligament (OPLL). The CT image data were transferred to the commercially available image-processing software and were used to reconstruct a three-dimensional (3D) model of the bony structures and plan pedicle screw placement. These data were transferred to the 3D-CAD software for the design of the template. Care was taken in designing the template so that the best intraoperative handling would be achieved by choosing several round contact surfaces on the visualised posterior vertebral bony structure, such as transverse process, spinous process and lamina. These contact surfaces and holes to guide the drill or the pedicle probe were then connected by a curved pipe. STL format files for the bony models with planned pedicle screw holes and individual templates were prepared for rapid prototype fabrication of the physical models. The bony models were made using gypsum-based 3D printer and individual templates were fabricated by a selective laser melting machine using commercially pure titanium powder. Pedicle screw trajectory of the bony model, adaptation and stability of the template on the bony model, and screw hole orientation of the template were evaluated using physical models. Custom-made titanium templates with adequate adaptation and stability in addition to proper orientation of the screw holes were sterilised by autoclave and evaluated during surgery.

During segmentation, reproducibility of transverse and spinous processes were inferior to the lamina and considered inadequate to select as contact surfaces. A template design with more bone contact area might enhance the stability of the template on the bone but it is susceptible to intervening soft tissue and geometric inaccuracy of the template. In the bony model evaluation, the stability and adaptation of the templates were sufficient with few small round contact surfaces on each lamina; thus, a large contact surface was not necessary. In clinical patients, proper fit for positioning the template was easily found manually during the operation and 141/142 screws were inserted accurately with 1 insignificant pedicle wall breach in AIS patient.

This study provides a useful design concept for the development and introduction of custom-fit navigational template for placing pedicle screws easily and safely.

M. Malzdorf

The primary objective of implanting a total knee prosthesis is to release the patient from pain and to improve the joint mobility at the same time. This leads to an increased quality of life that is optimally kept for the patient's residual lifespan. Joint mobility and stability requires an intra-operative soft-tissue balancing. To reach the goal of a correct implant positioning and well-balanced ligaments two different operative procedures can be used: the so-called “Femur-first”-technique and the “Tibia-first” technique. Since now more than ten years the CT-free navigation is established as a routine procedure in TKA. Studies investigating this innovative technique have shown to lead to a higher precision regarding implant positioning and leg alignment. The present study compares navigated “Femur-first”-technique and “ Tibia-first”-technique. We hypothesised that, due to its better soft-tissue balance, the tibia first technique (T) would allow a flexion improvement of 10° compared to the femur first technique (F).

Between February 21, 2008, and October 10, 2009, 116 consecutive patients were implanted a Columbus® non-constrained total knee replacement (Aesculap®, Tuttlingen, Germany) using navigation; they were examined before the operation and 1 year after. The TKAs were performed by 3 surgeons experienced in knee replacement surgery. We used the femur first technique (F) in 63 patients, the tibia first technique (T) in 53 patients. We performed the final flexion measurement one year after the operation using a Goniometer and evaluated standing full-length radiographs. In addition, we took standard varus and valgus stress radiographs to evaluate the stability of the collateral ligaments and determine the relative position of the implants to one another. Finally, to compare the two patient groups, we used the following pain and function scores: Knee Society Score (KSS), Oxford Score, Knee Injury and Osteoarthritic Outcome Score (KOOS), Short Form 36 (SF 36), Tegner Lysholm Score.

Concerning maximal flexion as the main parameter, we did not find any significant difference between the F and T groups (maximal flexion in group F: 113.4± 9,8° and in group T: 113.5± 8.4°; p = 0.963); thus we could not confirm our hypothesis. Radiological evaluation of the stability of the collateral ligaments did not reveal any significant difference between the two groups both in the medial and lateral joint cavity (lateral collateral ligament in group F: 3.4± 1.4°, and in group T: 3.9± 1.7°; p = 0.850, and medial collateral ligament in group F: 4.0± 1.4°, and in group T: 4.1± 1.7°; p = 0.086). Concerning the mechanical axis on the standing full-length radiograph as part of the 1-year results, no significant difference was found between the two groups (p = 0.089). Likewise, the pain and function scores did not show any difference between the two groups.

Concerning operating time (OP time) and outliers exceeding 3° of varus/valgus deviation from the ideal mechanical axis, trends were identifiable. The number of outliers tended to be higher in the F group, the OP time in group T seemed longer.

As a conclusion, we can say that both the tibia first and the femur first techniques yield good clinical and radiological results in combination with navigation. In terms of function and patient satisfaction, we did not find any significant difference.

G. Ee H.N. Pang H.Z. Chong M.H. Tan N.N. Lo P.L. Chin S.L. Chia S.J. Yeo

Restoration of the native joint line in total knee arthroplasty is important in restoring ligamentous balance and normal knee kinematics. Failure to achieve this could lead to reduced range of motion, patellofemoral maltracking and suboptimal outcomes. The purpose of this study was to analyse the clinical and functional outcome of patients who demonstrated joint line changes after computer-assisted (CAS) total knee arthroplasty (TKA).

A prospective study was conducted for 168 patients (168 knees) who underwent CAS TKA by two surgeons at a single institution with an average follow-up of two years. The final change in joint line was calculated from the verified tibial resection, distal and posterior femoral cuts. Group A patients had joint line changes of less than 4mm and Group B patients had joint line changes of more than 4mm. Postoperative Oxford scores, Knee scores, Function scores and SF-36 scores were obtained at six months, one year and two years post-TKA. The final range of motion and the mechanical alignment were documented.

There was significant linear correlation between joint line changes and Oxford scores (p = 0.05) and Function scores (p = 0.05) at six months and Oxford scores alone at two years with increasing joint line changes having poorer outcome scores. Group A compared to Group B patients have better outcomes in terms of Oxford scores (mean 20 vs 27, p = 0.0003), Function scores (mean 69 vs 59, p = 0.03), SF-1 (mean 63 vs 50, p = 0.03), SF-2 (mean 66 vs 43, p = 0.05), SF-5 (mean 75 vs 63, p = 0.04), SF-6 (mean 84 vs 59, p = 0.003), SF-7 (mean 96 vs 83, p = 0.02), SF-8 (mean 84 vs 73, p = 0.006) and total SF-36 scores (mean 603 vs 487, P = 0.003), at six months, and Oxford scores (mean 18 vs 23, p = 0.0007) at two years.

In this study, CAS is a useful intra-operative tool for assessing the final joint line in TKA. Outliers in joint line changes of ≥ 4 mm are associated with poorer clinical outcome scores.

S. Hakki K. Pedersen H. Bui W. Webster M. Osman H. Rodriguez

As much as there is interest in mini-invasive surgery (MIS) total hip arthroplasty (THA), there is controversy ranging from a more advantageous to a potentially dangerous alternative to classic THA. The purpose of this study is to compare the results of 130 cementless, standard stem navigated primary THA with data collected retrospectively from 2005 to 2010 (64 classic Hardinge approach (HAL) and 66 MIS intermuscular anterolateral approach by the senior surgeon). Data include: operative time, perioperative bleeding, length of surgery, hospital stay, patient's satisfaction and pain perception. The alignment values at six months to a maximum of 60 months provided necessary statistical information for clinical and radiological comparison of the two groups.

Significant differences were found between the two groups with MIS being superior with respect to less surgical time (p = 0.029) and achieving quicker rehab goals with shorter hospital stay (p <0.001). Pain perception was less in MIS group with a higher satisfaction score (p <0.001). Although both groups have the potential of nerve injury to tensor fascia muscle, it's only the classic HAL that jeopardises the glutei nerve supply.

MIS approach to navigated THA seems to be an acceptable alternative with some advantages to Classic HAL.

L. Dohmen M. Haimerl S. Gneiting M. Schubert C. Buchele E. Sendtner M. Wörner R. Springorum T. Renkawitz

Limited postoperative range-of-motion (ROM) can lead to patient dissatisfaction and dislocation in total hip arthroplasties (THAs). To avoid this, femur first approaches have been developed which optimise particular aspects of ROM by using a virtual analysis of ROM. This study analysis whether it is possible to accurately assess ROM based on an intra-operative acquisition of anatomical structures by using an image-free navigation system. It compares the outcome of a collision detection algorithm when using 3d models from computerised tomography (CT) scans on the one side and intra-operatively acquired 3D models on the other side within a cadaver study. It focuses on peri-acetabular impingements.

During the cadaver session 14 hips (7 cadavers) were treated surgically by using press-fit implants. 3D models of the pelvis and femora were generated based on segmented pre-operative CT data sets. Intra-operative data acquisition was performed by using a CT-free navigation software. Beside standard landmarks, points at the acetabular rim and femoral resection plane were acquired. For assessing ROM, a 3D model of the pelvis was generated. The information about the femoral resection plane was directly entered into the collision detection algorithm. 3D Computer Aided Design (CAD) models provided by the implant manufacturer were used for the implants. Based on this setup, the ROM values for flexion (FLEX), external rotation at 0° flexion (EXT), and internal rotation at 90° flexion (INTROT90) were compared. Differences within intended ROM were considered relevant, since the goal was to enable the prevention of clinically relevant ROM limitations.

The average difference between the CT based and navigation data based ROM analysis was 2.13° ± 3.11° for FLEX, 3.33° ± 5.51° for EXT, and 1.6° ± 3.66° INTROT90. The values reduce to 1.58° ± 2.78° (FLEX) and 0.91° ± 3.77° (INTROT90) when only ROM values within the intended ROM are considered. For EXT all ROM values lied above the threshold for intended ROM. Thus, no relevant differences were found for this motion direction.

In this study, a real-time collision detection based approach was developed and evaluated, which allows to virtually detect prosthetic and bony impingements. It was shown that ROM can be assessed accurately based on an image-free navigation technique. This information can be used intra-operatively to adjust the position of the implants and thus avoid postoperative ROM limitations. In particular, it enables a comprehensive femur first approach which allows us to optimise the post-operative results regarding functional parameters like ROM.

E.M. Suero C. Plaskos P.L. Dixon A.D. Pearle

Long-term implant survivorship in total knee arthroplasty (TKA) depends on the alignment of the tibial and femoral components, as well as on the mechanical alignment of the leg. Computer navigation improves component and limb alignment in TKA compared to the manual technique. However, its use is often associated with an increase in surgical time. We aimed to evaluate the use of adjustable cutting blocks (ACB) in navigated TKA. We hypothesised that the use of ACB would (1) improve tibial and femoral component positioning; (2) improve postoperative mechanical leg alignment; and (3) decrease tourniquet time, when compared to conventional cutting blocks (CCB).

This was a retrospective cohort study of 94 navigated primary TKA. Patients were classified into two groups according to whether the surgery had been performed using ACB or CCB. There were sixty-four patients in the CCB group and 30 patients in the ACB group. Charts were reviewed to obtain the following data: age, gender, body mass index (BMI), tourniquet time and operated side. Pre- and postoperative standing full-leg radiographs and lateral radiographs were reviewed.

Mean coronal femoral alignment for the CCB group was 0.8® varus (SD = 1.95®) and for the ACB group it was 1.1® varus (SD = 1.5®) (P = 0.12). Mean coronal tibial alignment for the CCB group was 0.1® valgus (SD = 1.3®) and for the ACB group it was 0.5® varus (SD = 1.01) (P = 0.15). Sagittal tibial alignment was a mean 0.5® of anterior slope (SD = 2.9®) for the CCB group and 0.7® anterior slope (SD = 2.5®) for the ACB group (P = 0.38).

Preoperatively, the CCB group had a mean mechanical alignment of 1.8® varus (SD = 9.6®), while the ACB group had a mean 1.8® varus (SD = 9.37®) (P = 0.88). After surgery, mechanical leg alignment for the CCB group improved to a mean 0.7® varus (SD = 2.7®) (P = 0.0001), while the ACB group improved to 1.8® varus (SD = 1.7®) (P<0.0001). There was significantly less variability in postoperative mechanical alignment in the ACB group (P = 0.0091).

Mean tourniquet time for the CCB group was 91 minutes (SD = 17.7 minutes). The ACB group a mean tourniquet time of 76 minutes (SD = 16.7 minutes) (P = 0.01). In the multiple linear regression model, the use of an ACB reduced tourniquet time by 16.8 minutes (P = 0.001).

Adjustable cutting blocks for TKA significantly reduced postoperative mechanical alignment variability and tourniquet time compared to conventional navigated instrumentation, while providing equal or better component alignment.

N. Nakamura T. Murase K. Tsuda N. Sugano D. Iwana M. Kitada H. Kawakami

We developed a custom-made template for corrective femoral osteotomy during THA in a patient with a previous Schanz osteotomy.

A seventy-year-old woman presented to our clinic with a chief complaint of right hip, left knee and left ankle pain with marked limp. She had undergone Schanz osteotomy of the left femur because of high dislocation of the left hip when she was 20 years old. After right THA was performed, we decided to perform left THA with corrective femoral osteotomy. A custom-made osteotomy template was designed and manufactured with use of CT data. During surgery, we placed the template on the bone surface, cut the bone through a slit on the template, and corrected the deformity as preoperatively simulated. Two years after surgery, she had no pain in any joints, could walk more than one hour without limp. Japanese Orthopedic Association hip score were 100 points for both hips.

THA in patients with previous Schanz osteotomy was reported to be technically demanding and the rate of complications was high. In 2008, Murase T et al. developed a system, including a 3D computer simulation program and a custom-made template to corrective osteotomy of malunited fractures of the upper extremity. We applied the system to corrective femoral osteotomy during THA in a patient with a previous Schanz osteotomy. The surgical procedure was technically easy and accurate osteotomy brought the patient to acquire good alignment of lower extremities with good clinical results.

T. Sakai J. Koyanagi T. Yamazaki T. Watanabe N. Sugano H. Yoshikawa K. Sugamoto

The in vivo kinematics of squatting after total hip arthroplasty (THA) has remained unclear. The purpose of the present study was to elucidate range of motion (ROM) of the hip joint and the incidence of prosthetic impingement during heels-down squatting after THA.

23 primary cementless THAs using a computed tomography-based navigation system (CT-HIP, Stryker Navigation, Freiberg, Germany) were investigated using fluoroscopy. An acetabular component with concavities around the rim (TriAD HA PSL, Stryker Orthopaedics, Mahwah, NJ) and a femoral component with reduced neck geometry (CentPiller, Stryker Orthopaedics), which provided a large oscillation angle, were used. The femoral head size was 28mm (8 hips), 32mm (10 hips), and 36mm (5 hips). Post-operative analysis was performed within 6 months in 6 hips, and at 6 months to 2 years in 17 hips. Successive hip motion during heels-down squatting was recorded as serial digital radiographic images in a DICOM format using a flat panel detector. The coordinate system of the acetabular and femoral components based on the neutral standing position was defined. The images of the hip joint were matched to three-dimensional computer aided design models of the acetabular and femoral components using a two-dimensional to three-dimensional (2D/3D) registration technique. In the previous computer simulation study of THA, the root mean square errors of rotation was less than 1.3°, and that of translation was less than 2.3 mm.

We estimated changes in the relative angle of the femoral component to the acetabular component, which represented the hip ROM, and investigated the incidence of prosthetic impingement during squatting. We also estimated changes in the flexion angle of the acetabular component, which represented the pelvic posterior tilting angle (PA), and the flexion angle of the femoral component, which represented the femoral flexion angle (FA). The contribution of the PA to the FA at maximum squatting was evaluated as the pelvic posterior tilting ratio (PA/FA). In addition, when both components were positioned most closely during squatting, we estimated the minimum angle (MA) up to theoretical prosthetic impingement.

No prosthetic impingement occurred in any hips. The maximum hip flexion ROM was mean 92.7° (SD; 15.7°, range; 55.1°–119.1°) and was not always consisted with the maximum squatting. The maximum pelvic posterior tilting angle (PA) was mean 27.3° (SD; 11.0°, range; 5.5°–46.5°). The pelvis began to tilt posteriorly at 50°–70° of the hip flexion ROM. The maximum femoral flexion angle (FA) was mean 118.9° (SD; 10.4°, range; 86.4°–136.7°). At the maximum squatting, the ratio of the pelvic posterior tilting angle to the femoral flexion angle (pelvic posterior tilting ratio, PA/FA) was mean 22.9% (SD; 10.4%, range; 3.8%–45.7%). The minimum angle up to the theoretical prosthetic impingement was mean 22.7° (SD; 7.5°, range; 10.0°–37.9°). The maximum hip flexion of ROM in 36 mm head cases was larger than that in 32 mm or 28 mm head cases, while the minimum angle up to the prosthetic impingement in 36 mm head cases was also larger than that in 32 mm or 28 mm head cases.

Three-dimensional assessment of dynamic squatting motion after THA using the 2D/3D registration technique enabled us to elucidate hip ROM, and to assess the prosthetic impingement, the contribution of the pelvic posterior tilting, and the minimum angle up to theoretical prosthetic impingement during squatting.

I.H. Park H.S. Yoon S.H. Cheon S.H. Seo H.S. Cho

Recently, several preliminary reports have been issued on the application of computer assistance to bone tumour surgery. Surgical navigation systems can apply three-dimensional images such as CT and MR images to intraoperative visualization. Although CT is better at describing cortical bone status, MRI is considered the best method for defining the extent of marrow involvement for bone tumours and for planning surgical resection in bone tumour surgery. There have been a few reports on the application of MR imaging to navigation-assisted bone tumour surgery through CT–MR image fusion. However, the CT–MRI fusion technique requires additional costs and exposure of the patient to radiation from the preoperative CT, as well as additional time for image fusion. Above all, the image fusion process is a kind of registration (image to image registration) that inevitably leads to registration error. Herein we describe a new method for the direct application of MR images to navigation-assisted bone tumour surgery as an alternative to CT–MRI fusion.

Six patients with an orthopaedic malignancy were employed for this method during navigation-assisted tumour resection. Resorbable pin placement and rapid 3-dimensional spoiled gradient echo sequences made the direct application of MR images to computer-assisted bone tumour surgery without CT–MR image fusion possible. A paired-point registration technique was employed for patient-image registration in all patients. It took 20 min on average to set up the navigation (range 15 to 25 minutes). The mean registration error was 0.98 mm (range 0.4 to 1.7 mm). On histologic examination, distances from tumours to resection margins were in accord with preoperative plans. Mean duration of follow-up was 25.8 months (range 18 to 32 months). No patient had a local recurrence or distant metastasis at the last follow-up.

Direct patient-to-MRI registration is a very useful method for bone tumour surgery, permitting the application of MR images to intraoperative visualization without any additional costs or exposure of the patient to radiation from the preoperative CT scan.

R. Lamdan N. Simanovsky L. Joskowicz M. Liebergall A. Gefen E. Peleg

Supra-condylar humerus fractures (SCHF) are amongst the most common fractures requiring surgical stabilisation in the pediatric age group (1). Closed reduction and percutaneous fixation with Kirschner wires (KW) is currently the standard of care (2). The number of KW used and their configuration has been the subject of much research (3, 4). The failure modes leading to loss of fracture reduction are not clear and have not been quantified. The aim of this study is to compare the mechanical stability of the opt-used configurations for various loading modes and contact interactions at the KW/bone interface.

A Gartland type-III SCHF was introduced to a fourth generation composite saw bone (Sawbones®, Vashon, Washington, USA). The model was CT scanned with a slice spacing of 0.5mm and pixel size 0.3×0.3mm. The CT data set was imported into AmiraDev (AmiraDev 5.2 Visage Imaging, Inc). A uniaxial mechanical test was conducted in order to measure the KW pullout forces from the distal humerus.

A model of the fractured humerus was constructed with the following steps: 1) manual segmentation; 2) surface generation of each fragment, and; 3) automatic volumetric grid generation for each fragment. The fracture was then virtually reduced and KWs were placed at the desired configurations (Fig 1a-b). For each configuration, a separate model was generated. Material properties were assigned to the bone-model elements according to the manufacturer's data sheet; Young's modulus E = 16GPa and E = 150MPa for the cortical and cancellous bone respectively. The KW were assigned a Young's modulus of 200GPa. Each of the models created in Amira was imported to a finite element application (Abaqus 6.9, DS-Simula) for structural analysis. For each of KW configuration four different torque forces load types were simulated (Fig 1c left): 1) a clockwise and counterclockwise torque with a magnitude of 1.5 NM (Newton/Meters); 2) a translational force with a magnitude of 30 N (Newtons) in the direction of the humerus shaft, and; 3) a shear force with a magnitude of 30 N in the direction parallel to the fracture plane. The results were normalised such that the maximum displacement for the crossed pin configuration with a coefficient of friction equal to zero (μ = 0) was used as unity for each load configuration. Similarly, for each of KW configuration four different translational forces load types were simulated (Fig 1c right): 1) a clockwise and counter clock-wise torque with a magnitude of 1.5 NM (Newton/Meters); 2) a translational force with a magnitude of 30N in the direction of the humerus shaft, and; 3) a shear force with a magnitude of 30N in the direction parallel to the fracture plane. The results were normalised as described above.


Torque forces: the crossed configuration was found to be almost independent of the bone-implant friction and was symmetric in terms of direction of the applied torque. The diverging configuration exhibited larger dependency on the bone-implant interface. This is especially noticed as the coefficient of friction (COF) reduced to values below μ = 0.2. Translational forces: the diverging configuration exhibited high sensitivity to reduction of the COF μ = 0. Displacement of the fracture for μ = 0 was substantially larger for the diverging configuration relative to the crossed configuration: 13.5 times and 19 times for the transverse and pullout directions, respectively. As the COF increased to values above μ = 0.5, both fixation configurations performed in a similar manner.

Stabilisation of SCHF has been the subject of numerous studies. Relative stability of the different configurations and the risk for iatrogenic ulnar nerve injury has been in the center of the debate. Crossed KW configuration was shown in some clinical studies to be more stable than two lateral KW while others demonstrated no significant difference in stability. As ulnar nerve injury may occur in up to 15.4% of surgeries even if insertion of a medial KW is performed under direct vision, utilisation of two lateral KW configurations offers the advantage of reducing this risk significantly. The main finding of this study is that for a COF exceeding a threshold level (µ = 0.2) the crossed KW configuration did not offer any mechanical advantage over the diverging lateral KW configuration. However, for very low COF values (µ<0.2) the crossed configuration exhibited improved performance when compared with divergent lateral KW (figure 1d). The data demonstrates that the KW-bone bonding has a profound effect on the stability of the fixated bone construct. This is mostly evident when distraction forces are applied but also occurs, to a lesser degree, with rotational or translational forces. This may be a clinically important consideration in the rare SCHF in children with abnormal bones and possibly more commonly, when the KW-bone bonding was compromised after multiple attempts of passing the KW through the same entry point.

We have conducted a combined in-vitro mechanical test and finite element-based simulations of a fixated SCHF with different KW configurations, under various friction conditions. Under normal bone-implant interface bonding conditions, the two diverging lateral KW configuration offers adequate mechanical stability and may be the preferred choice of SCHF fixation.

S. Weidert L. Wang P.H. Thaller J. Landes A.G. Brand N. Navab E. Euler

The verification of the alignment of the lower limb is critical for reconstructive surgery as well as trauma surgery in order to prevent osteoarthritis. The mechanical axis is a straight line defined by the center of the femoral head and the center of the ankle joint, ideally passing the knee joint in its center.

Whereas the usual preoperative method to determine the mechanical axis of the lower limbs is still the long standing radiograph, common intra-operative methods are the use of an electrocautery cord or an X-ray grid consisting of wire lines underneath the patient. Both methods require the surgeon to bring the femoral head and the ankle joint exactly to overlay with a radiopaque line that passes through both points. The distance of the knee center from this line is defined as the mechanical axis deviation (MAD). In order to reduce the errors introduced by perspective projection effects, the joint centers must be placed in the center of the c-arm images, which definitely requires time, experience and additional radiation.

We propose a computer aided X-ray stitching method that puts individual X-ray images into a panoramic image frame combining the Camera Augmented Mobile C-arm (CamC) system, which features a video camera with its optical center virtually coinciding with the origin of the X-rays, with an optical tracking marker pattern underneath the operating table. The camera image of the marker pattern is used to perform pose estimation of the C-arm, allowing the calculation of the x-ray source motion between the positions in which the individual X-rays were taken. By estimating the homography, the different X-rays can be registered into a panoramic frame, enabling perfect alignment and metric measurements.

In order to reduce parallax effects that lead to axis and metric measurement errors, we applied a method requiring two constraints: The bone plane has to be roughly parallel to the planar marker pattern and the distance between the marker plane and the bone plane has to be estimated.

In order to evaluate the method, we used a life-size synthetic skeleton leg. After tightening a straight wire between the centers of the hip and ankle joint, the knee joint was bent into a MAD of 55 mm, which was confirmed by measuring the distance between the knee center and the wire with a ruler. The leg phantom was then placed on a radiolucent operating table, parallel to the pattern plane 130 mm underneath. The operating table was moved through the C-arm while acquiring the three desired X-ray images. which were registered into a panoramic image frame. The centers of the femoral head, the ankle, and the knee were manually determined on the generated panoramic image by a surgeon. The mechanical axis was automatically displayed and the MAD was visualised in the image and computed as 55.23 mm.

We presented a new solution to intra-operatively verify alignment of the lower extremity. When using the CamC system, only a marker pattern has to be used for tracking. No additional tracking devices and calibration procedures are needed. Furthermore, the presented method only requires three x-rays that cover the femoral head, the knee and the ankle and marking of the three spots. Due to the parallax correction, these spots do not have to be exactly in the center of the picture. For this reason, compared to using an X-ray grid or an electrocautery cord, our method allows the procedure to be much faster and reduces the number of x-ray images. However, for clinical evaluation, a patient study will be conducted in the future.

R.J. Murphy T.K. Subhawong A. Chhabra J.A. Carrino M. Armand M. Hungerford

Standard evaluation and diagnosis of pincer-type femoroacetabular impingment (FAI) relies on anteroposterior (AP) radiographs, clinical evaluation, and/or magnetic resonance imaging (MRI). However, the current evaluation techniques do not offer a method for accurately defining the amount of acetabular rim overcoverage in pincer-type FAI. Several studies have remarked on the particular problems with radiographic evaluation, including beam divergence, difficulty with defining the acetabular rim, and pelvic tilt. Some studies have proposed methods to mitigate these issues; however, radiographic analysis still relies on projected and distorted images, making it difficult to acquire an accurate quantitative estimate of the amount of crossover. We propose a technique that utilises computed tomography (CT) data to accurately quantify the amount of acetabular crossover while accounting for known diagnostic problems, specifically pelvic tilt.

This work describes a novel method that utilises CT data of a patient's afflicted hip joint region to assess the amount of acetabular overcoverage due to pincer deformity. The amount of overcoverage was assessed using a spline curve defined through the segmentation of the acetabular rim from CT data. To mitigate pelvic tilt, the user selected points to define both the pubic symphysis and the promontory in a lateral digitally reconstructed radiograph. The algorithm corrected the pelvic tilt by adjusting to a defined neutral position (in our case, a 60°), and the user adjusted for slight rotation differences ensuring there was a vertical line connecting the symphysis and the sacrococcygeal joint.

After successfully repositioning the pelvis, the algorithm computed the amount of acetabular overcoverage. The algorithm identified the superolateral point of the acetabulum and the most inferior points of the anterior and posterior rim. A line, the mid-acetabular axis, was constructed between the superolateral point and the midpoint of the most inferior points on the anterior and posterior rims; the mid-acetabular axis was extended anterior and posterior to create a plane. Crossover occurred when the anterior rim of the acetabulum intersected this plane. If an intersection occurred, the algorithm measured the length of the mid-acetabular axis, and the length and width of the section representing overcoverage. These points were then projected onto anteroposterior DRRs and again measured to generate a basis of comparison.

We tested our method on four cadaveric specimens to analyze the relationship between radiographic assessment and our technique. We simulated varying degrees of impingement in the cadavers by increasing the amount of pelvic tilt and defining that as the neutral position for a given trial. Moreover, we assessed interobserver variability in repositioning the pelvis as to the effect this would have on the final measurement of crossover length and width.

The software achieved consistent, quantitative measurements of the amount of acetabular overcoverage due to pincer deformity. When compared with conventional radiographic measurements for crossover, there was a significant different between the two modalities. Specifically, both the ratios of crossover length to acetabular length and crossover width to crossover length were less using the CT-based approach (p < 0.001). Moreover, there were no significant differences between observers using our approach.

The proposed technique can form the basis for a new way to diagnosis and measure acetabular overcoverage resulting in pincer impingement. This computational method can help clinicians to accurately correct for tilt and rotation, and subsequently provide consistent, quantitative measurement of acetabular overcoverage.

B.W. McCoy M.A. Yaffe S.D. Stulberg

Custom instrumentation in TKA utilises pre-operative imaging to generate a customised guide for cutting block placement. The surgeon is able to modify the plan using three-dimensional software. Although this technology is increasingly gaining acceptance, there is a paucity of clinical data supporting it.

One hundred and eleven patients underwent primary TKA using the Zimmer Patient-Specific Instrumentation (PSI) system, in 28 of the cases surgical navigation was used to validate the PSI-generated cuts. Alignment measurements included long-leg alignment and biplanar distal femoral and proximal tibial cuts. Further measurements evaluated femoral implant placement in the AP plane, femoral component rotation, measured bone resection and implant sizing accuracy.

The mean final limb alignment as recorded by computer-assisted surgical (CAS) tools was 0.3° of varus. Only two limbs were malaligned by greater than 3°. The femoral component had a mean alignment of 0.3° of valgus and 4.5° of flexion (PSI plan 3° flexion). The predicted femoral size was accurate in 89% of cases and the anterior femoral cut was congruent with the anterior cortex in 92% of cases. The PSI-directed femoral component rotation was consistent with the surgeon's perceived rotation in 95% of cases. The posterior condylar bone resection had a mean difference of < 1mm from the predicted resection.

The tibial component had a mean alignment of 0.5° of varus and 8.5° of posterior slope (PSI plan 7° posterior slope). The only statistically significant deviation in alignment was the increased tibial slope (p = 0.046). The tibial component size was accurately predicted in 66% of cases.

Custom instrumentation in total knee arthroplasty accurately achieved implant and limb alignment in our study. The plan was more reproducible on the femoral slide. The overestimation of tibial slope and tibial sizing incongruity were related to some of the reference points for the software. A potential benefit of this technology is improved mid-flexion stability by accurately determining femoral component size, placement, and rotation. Further studies will need to be conducted to determine the efficiency and cost-effectiveness of this technology.

E.K. Song J.K. Seon K.D. Kang C.H. Park J.H. Yim

The elevation of the joint line is considered a possible cause of mid-flexion instability in total knee arthroplasty (TKA). The authors evaluated the effects of joint line change on mid-flexion stability in cruciate retaining TKA.

Seventy-nine knees treated by cruciate retaining TKA using a modified balanced gap technique were included in this prospective study. After prosthesis insertion, valgus and varus stabilities were measured under valgus and varus stress using a navigation system at 0, 30, 60 and 90° of knee flexion. Changes of joint lines were measured preoperatively and postoperatively and compared. The knees were allocated to a “No change group (≤4mm, 62 patients)” or to an “Elevation group (>4mm, 17 patients)”. Medio-lateral stabilities (defined as the sums of valgus and varus stabilities measured intra-operatively) were compared in the two groups.

The mean joint line elevation was 4.6mm in the no change group and 1.7mm in the elevation group. Mean medio-lateral stability at 30° of knee flexion was 4.8±2.3 mm in the no change group and 6.3±2.7 mm in the elevation group, and these values were significantly different (p = 0.02). However, no significant differences in medio-lateral stability were observed at other flexion angles (p>0.05).

Knees with a < 5mm joint line elevation provide better mid-flexion stability after TKA. The results of this study suggest that a < 5mm elevation in joint line laxity is acceptable for cruciate retaining TKA.

B.J. Rasquinha J. Sayani A.W.L. Dickinson J.F. Rudan G.C.A. Wood R.E. Ellis

Developmental dysplasia of the hip is a condition in which the acetabulum provides insufficient coverage of the femoral head in the hip joint. This configuration gives poor biomechanical load distribution, with increased stress at the superior aspect of the joint surfaces, and can often lead to degenerative arthritis. Morphologically, the poor coverage may be due to an acetabulum that is too shallow or oriented in valgus.

The dysplastic deformity can be treated surgically with a group of similar procedures, often labeled periacetabular osteotomies or rotational acetabular osteotomies. Each involves separating the acetabulum from the pelvis and fixating the fragment back to the pelvis in an orientation with increased coverage of the femoral head. This redistributes the biomechanical loads relative to acetabulum.

Bone remodeling at the level of trabeculae is an accepted concept under research; however, it is unclear whether the hip undergoes gross morphology changes in response to changes in biomechanical loading. An understanding of the degree to which this remodeling occurs (if at all) may have an impact on surgical planning.

In this retrospective study, computed tomography (CT) scans of 13 patients (2 male, 11 female, 40 ± 9 years of age) undergoing unilateral periacetabular osteotomies were examined; scans were taken both pre-operatively and at least a year post-operatively with an in-plane resolution of 0.55 mm and a slice thickness of 1.25 mm. Scans were segmented to produce triangulated meshes for the proximal femurs and the pelvis. These scans were manually processed to isolate the articular portions of the femoral heads and acetabulums, respectively; the fovea, acetabular fossa, any osteophytes and any segmentation artifacts were excluded.

Post-operative meshes were registered to their pre-operative counterparts for both the femoral head and the acetabulum, for both the operative and non-operative hips, using the iterative closest point (ICP) algorithm to 20 iterations. To account for differences in defining the edges of the articular surfaces in the manual isolation, metrics were only calculated using points that were within 0.3 mm of a normal from the opposing mesh. With the resulting matched data, nearest neighbour distances were calculated to form the remodeling metrics. Select spurious datapoints were removed manually.

For the operative femoral heads, the registered post-operative points were 0.24±0.53 mm outside of the pre-operative points. The maximum deviation was on average 1.94 mm with worst-case of 2.99 mm; the minimum deviation was −0.62 mm with worst-case of −2.06 mm. Positive numbers indicate the post-operative points are ‘outside’ of the pre-operative points – that is, farther from the head centre. The non-operative femoral heads have similar deviation values, 0.21±0.46 mm outside, with maximum and minimum deviation averaging to 1.24 mm and −0.74 mm respectively, with worst cases of 2.99mm and −1.80mm.

For the operative acetabulums, the post-operative deviations were −0.08±0.43mm. The maximum and minimum deviations averaged to 0.62mm and −0.82mm, with worst cases of 2.14mm and −1.51mm across the set. Again, the non-operative acetabulums were very similar; post-operative deviations were −0.02±0.43mm, maximum and minimum deviations averaged to 1.24mm and −0.65mm, with worst cases of 1.97mm and −2.00mm.

These quantitative measurements were reflected in manual examination of the meshes; generally speaking, there were small deviations with no overarching patterns across the anatomy.

All metrics were very similar across the same anatomy (that is, femoral head or acetabulum) regardless of whether the hip operative or non-operative. Femurs tended to ‘grow’ slightly post-operatively, but by less than a half voxel in size. Given that the CT voxels are large compared to the measured deviations, it is possible the results may be sensitive to the manual segmentations used as source data.

Manual examination of the deviations indicated a few potential trends. Seven operative and eleven non-operative acetabulums had a small patch of positive deviation (1mm to 1.5mm) in the anterosuperior aspect. This can be seen in the plot as the yellow-red area near the top right of the leftmost rendering. Other high-deviation areas included the superior aspect of the acetabulum (both positive and negative) and the superior aspect of the femoral head (generally positive).

The edges of the mesh were often a source of high deviation. This is likely an artifact of over-inclusion the manual isolation of the articular surfaces, as joint surfaces become non-articular as they move away from the joint interface.

Overall, the superior and anterosuperior aspects of the acetabulum and the superior aspect of the femoral head showed some indication of systemic changes; further study may clarify whether these data represent consistent anatomical changes. However, as the magnitude of the deviations between pre- and post-operative scans are on or below the order of the CT voxel size, we conclude that (in the absence of other strongly compelling evidence) periacetabular osteotomies for adults should be planned without the expectation of gross remodeling of the articular surfaces.

C.X.B. Yan B. Goulet S.J.S. Chen D. Tampieri D.L. Collins

Image-guided spine surgery requires registration between the patient anatomy and the preoperative computed tomography (CT) image. We have previously developed an accurate and robust registration technique for this application by using intraoperative ultrasound to acquire patient anatomy and then registering the ultrasound images to the CT images by aligning the posterior vertebral surfaces extracted from both modalities. In this study, we validate our registration technique across 18 vertebrae on three porcine cadavers.

We applied the ultrasound-registration technique on the thoracic and lumbar vertebrae of the porcine cadavers using both single sweeps and double orthogonal sweeps. For each sweep pattern at each vertebra, we also randomly simulated 100 different initial misalignments and registered each misalignment. The resulting registration transformations are compared to gold standard registrations to assess the accuracy and the robustness of the technique.

Orthogonal-sweep acquisition was found to be the sweep-pattern that performed the best and yielded a registration accuracy of 1.65 mm across all vertebrae on all porcine cadavers. It was found that the target registration errors (TRE) stay relatively constant with increasing initial misalignment and that the majority (82.7%) of the registrations resulted in TREs below the clinically recommended 2 mm threshold. In addition, it was found that the registration accuracy varies by the sweep pattern and the vertebral level, but neighbouring vertebrae tend to result in statistically similar accuracy.

We found that our ultrasound-CT registration technique yields clinically acceptable accuracy and robustness on multiple vertebrae across multiple porcine cadavers.

E.K. Song J.K. Seon K.D. Kang C.H. Park J.H. Yim

This study was performed to measure intra-operative varus-valgus laxities from 0° to 90° of flexion during cruciate retaining total knee arthroplasty (TKA) using the modified balanced gap technique. Forty nine patients awaiting unilateral TKA for osteoarthritis were enrolled into this prospective study. Flexion and extension gaps were measured at full extension and at 90° of flexion using a tensioning device before femoral bone cutting. After implantation and closing the medial parapatellar arthrotomy, varus-valgus laxities at 0, 30, 60 and 90° of flexion were also measured using a navigation system.

Mean total varus-valgus laxities were significantly less at 0° of flexion (3.8±1.7°) than at the other selected flexion angles. Mean varus laxity was peaked at 3.1±2.2° at 60° of flexion and reached a nadir of 2.0±1.0° at 0° of flexion, which represented a significant difference. On increasing flexion from 0° to 60°, mean valgus laxity increased from 1.8±1.3° to 2.9±1.6°, which was significant, but no significant difference was found for other angles.

The use of the balanced gap technique for cruciate retaining TKA using a navigation system, which allows accurate soft tissue balancing via real time gap size feedback, could be helpful for achieving good in vivo laxities throughout range of motion without significant mid flexion laxity.

S.J. Spencer A.H. Deakin J.V. Clarke

Range of motion (ROM) is a well recognised outcome measure following total knee arthroplasty (TKA). Reduced knee flexion can lead to poor outcome after TKA and therefore identification at an early stage is important as it may provide a window for intervention with targeted physiotherapy, closer follow-up and in resistant cases possible manipulation or arthrolysis. ROM combines both flexion and extension and in contrast to flexion, fewer studies have recognised the importance of a lack of full extension or fixed flexion deformity (FFD) following TKA. A residual FFD can increase energy cost, decrease velocity during ambulation and result in pain with knee scores more likely to be diminished than if knee extension was normal. Recognition and early detection of FFD is therefore important. Methods of assessment include by visual estimation or goniometric measurement of knee flexion angle. While goniometers are inexpensive, easy to use and provide more accurate than visual estimates of angles, they have been shown to exhibit poor inter-observer reliability. Therefore they may not be sensitive enough to consistently identify FFD and therefore distinguish between grading systems based on absolute angular limits. The aim of this study was to investigate the accuracy of standard clinical ROM measurement techniques following TKA and determine their reliability for recognising FFD.

Ethical approval was obtained for this study. Thirty patients who were six weeks following TKA had their knee ROM measured. An infrared (IR) tracking system (±1°accuracy) that had been validated against an electro-goniometer was used to give a “true” measurement of the lower limb sagittal alignment with the knee fully extended and maximally flexed while the patient was supine. The patients were also assessed independently by experienced arthroplasty practitioners using a standardised goniometric measurement technique. For goniometric clinically-measured flexion (Clinflex) and extension (Clinext) linear models were generated using IR-measured flexion and extension (IRflex and IRext), BMI and gender as covariables. Data for extension were categorised in none, moderate and severe postoperative FFD as per Ritter et al. 2007 and agreement in classification between the two methods was assessed using the Kappa statistic.

For the linear models for Clinflex and Clinext neither BMI nor gender were significant variables. Therefore the final models were:

Clinflex = 0.54 + 0.66∗IRflex (r2adj = 0.521)

Clinext = 0.23 + 0.50∗IRext (r2adj = 0.247)

The model for Clinflex showed that the IR and clinical measurements coincided at approximately 90° so that for every 10° increase in flexion above 90° clinical measurement only increased by 7° but for every 10° decrease in flexion below 90° clinical measurement only decreased by 7°. The model for Clinext showed that the IR and clinical measurements coincided at approximately 0° so that for every 10° increase in FFD angle, clinical measurement only increased by 5° but if the knee went into hyperextension this would be underestimated by the clinical measure. In identifying FFD there was moderate agreement between the two measurements (κ = 0.44). Clinically nine patients were assessed as having FFD but the IR measurements showed 18 patients having FFD, of which nine patients were not identified clinically.

When assessing knee ROM following joint arthroplasty manual goniometric measurements provided a poor estimate of the range when compared to the “true” angle as measured with a validated IR measurement tool. When the knee was held in maximum flexion there was a tendency to both underestimate and overestimate the true angle. However when the knee was held in extension there was a tendency to underestimate which we believe is important as it would underreport both the frequency and magnitude of FFD. In our study, 18 patients had a moderate FFD as identified by the IR system, only half of which were identified by goniometer measurement alone. Studies of comparisons of both visual and manual goniometry measurements of the knee in maximum flexion with lateral radiographs have shown most errors involved an underestimate of true flexion. It has been concluded that it was safer to underestimate knee flexion angle as it would result in higher pick up rate of cases being performing less well. In contrast however, underestimation while in extension is less desirable as it fails to pick-up FFD which may have benefited from intervention had they been identified. It is known that residual FFD can increase energy cost and decrease velocity during ambulation with pain and functional knee scores more likely to be reduced. Recognition and early detection is therefore important. With the use of more accurate systems to identify and measure FFD, such as the one used for this study may in turn allow more timely treatment and therefore hopefully improved outcomes.

A. Ladenburger S. Nebelung C. Buschmann M. Strake J.A. Ohnsorge K. Radermacher M. de la Fuente

Fluoroscopic guidance is common in interventional pain procedures. In spine surgery, injections are used for differential diagnosis and determination of indication for surgical treatment as well. Fluoroscopy ensures correct needle placement and accurate delivery of the drug. Also, exact documentation of the intervention performed is possible. However, besides the patient, interventional pain physicians, surgeons and other medical staff are chronically exposed to low dose scatter radiation. The long-term adverse consequences of low dose radiation exposure to the medical staff are still unclear. Especially in university hospital settings, where education of trainees is performed, fluoroscopy time and total radiation exposure are significantly higher than in private practice settings. It remains a challenge for university hospitals to reduce the fluoroscopic time while maintaining the quality of education.

Multiple approaches have been made to reduce radiation exposure in fluoroscopy, including the wide spread use of pulsed fluoroscopy, or rarely used techniques like laser guided needle placement systems. The Zero-Dose-C-Arm-Navigation (ZDCAN) allows an optimal positioning of the c-arm without exposure to radiation. For training purposes, relevant anatomical structures can be highlighted for each interventional procedure, so injection needles can be best positioned next to the target area.

The Zero-Dose-C-Arm-Navigation (ZDCAN) module was developed to display a radiation free preview of the expected fluoroscopic image of the spine. Using an optical tracking system and a registered 3D-spine model, the expected x-ray image is displayed in real-time as a projection of the model. Additionally, selected anatomical structures including nerve roots, facet joints, vertebral discs and the epidural space, can be displayed. A seamless integration of the ZDCAN in a c-arm system already used in clinical practice for years could be achieved. For easy use, a tool was developed allowing the admission and use of regular single-use syringes and spinal needles. Accordingly, these can be used as pointers in the sterile area, a sterilization of the whole tool after every single injection is not required. We evaluated the efficiency and accuracy of this procedure compared to conventional fluoroscopically guided interventional procedures. In sawbones of the lumbar spine, facet joint injections (N = 50), perineural injections (N = 46) and epidural injections (N = 20) were performed.

Highlighting the target area in the radiation free preview model, an optimal positioning of the c-arm could be achieved even by unskilled medical staff. The desired anatomical structures could be identified easily in the x-rays taken, as they were displayed in the 3D model aside. As already seen evaluating a previous version of the ZDCAN module for the lower limb, the total number of x-ray images taken could be reduced significantly. Compared to the conventional group, the number of x-ray images required for facet joint injections could be reduced from 12.5 (±1.1) to 5.7 (±1.1), from 5.4 (±1.8) to 3.8 (±1.3) for perineural injections and from 4.1 (±0.9) to 2.1 (±0.3) for epidural injections. Total radiation time was reduced accordingly. Likewise, the mean time needed for the interventional procedure could be reduced from 168.3 s (±19.1) to 131.4 s (±16.8) for facet joint injections, was unchanged from 97.7 s (±26.0) to 104.7 s (±31.0) for perineural injections and from 60 s (±14.9) to 52 s (±7.1) for epidural injections.

The ZDCAN is a powerful tool advancing conventional fluoroscopy to another level. Using the radiation free preview model, the c-arm can easily be positioned to show the desired area. The accentuated display of the target structures in the preview model makes the introduction to fluoroscopy guided interventional procedures easier. This feature might reduce the learning curve to achieve better clinical results with lower radiation dose exposure. Thus, the ZDCAN can be a tool to improve education in university hospital settings for physicians as well as for medical staff while reducing radiation dose exposure in general use.

E.M. Vasarhelyi M. Kunz J.F. Rudan

The purpose of this study was to compare the accuracy and precision of acetabular component placement in cadavers using conventional techniques and CT-based individualised guides by both orthopaedic trainees and surgeons.

Seven cadaveric pelvises underwent a computerised tomography (CT) scan and a three-dimensional virtual model was created. Based on this model, cup orientation was planned for 40 degrees of inclination and 20 degrees of anteversion and an individualised guide was designed. A physical model of the individualised guide was created using a Rapid Prototyping machine (dimension SST, Stratasys, Inc., USA).

The pelvises were mounted in the lateral position and covered with a soft tissue envelope exposing only the acetabulum as would be visualised during a lateral approach to the hip. A total of 26 participants (16 orthopaedic surgery residents, 10 orthopaedic surgeons) were asked to use an acetabular cup impactor to place the cup in 40 degrees of inclination and 20 degrees of anteversion. This was first completed for all seven pelvises using conventional placement. Each participant was then instructed on how to use the individualised guide. They were provided with the guide and an individualised acetabular model to practice placement. Once they were comfortable with the system they were then asked to use the individualised guides in each of the seven pelvises.

An optoelectronic navigation system was used to evaluate the accuracy of the placement of the acetabular cup. An Optotrak Certus Motion Tracking System (Northern Digital Inc., Waterloo, Canada) was used. An optoelectronic marker was attached to the acetabulum and a combined pair-point and surface matching was performed. After the guide was placed in the acetabulum, a tracked axial pointing device was aligned inside the guidance cylinder and its three-dimensional orientation stored. The angle deviation between the achieved position and the planned cup orientation was calculated.

There were no statistically significant differences between trainees and surgeons in either conventional placement or use of the individualised guides. There were no statistically significant differences in anteversion between the groups. The individualised guide showed statistical improvement in the absolute deviation from planned inclination compared to conventional placement (4.2° vs. 9.1°, p< 0.001) as well as a reduction in standard deviation (3.3 vs. 5.9, p< 0.001).

The use of individualised guides can improve the accuracy and precision in the placement of acetabular component positioning. The current guide design controls well for inclination, which is a key factor in the function of a total hip arthroplasty. Based on this data, we will implement design changes to better address version of the component. Future work will likely include comparison to computer-assisted cup placement as well.

T.D. Goldberg W.T. Curry Q. Qing

In May 2010, MyKnee® patient-specific instrumentation was approved for use in this procedure in the USA. This technique uses a pre-operative CT scan of the lower extremity to plan the surgery. Images of the hip, knee, and ankle are reconstructed digitally to assess pre-operative deformity as well as size of the knee. Surgery is then planned with the goals of restoring a neutral mechanical axis of limb and providing correct sizing and placement of implants after the surgery. From this plan, patient-specific jigs are created to perform the surgery achieving the planned result without sacrificing speed of surgery or increasing complexity of the procedure. The present study seeks to evaluate both intraoperative and radiographic results of this procedure. IRB approval for retrospective research was obtained prior to evaluation of the data.

Thirty consecutive patients (14 males, 16 females) underwent TKA using the MyKnee technique by the senior author. Pre-operative long-standing radiographs were taken and compared to 6-week post-operative radiographs. Intraoperative data includes the femoral and tibial resection thickness: distal medial femoral, distal lateral femoral, posterior medial femoral, posterior lateral femoral, medial tibia, and lateral tibia. These were compared to the planned vs. actual resections. Tourniquet time was recorded as a measure of speed of surgery. These were compared to 30 consecutive patients using standard TKA technique by the same author. Intraoperative complications were also recorded.

For patients with varus pre-operative deformities (n = 21), the mechanical alignment was 7.8° (range 1.2° to 15.2°). Seven patients had pre-operative valgus deformities averaging 6.93° (range 1.3° to 14.5°). Two patients were neutral. Post-operative alignment for all patients (n = 30) was varus 1.92° (range 0° to 5.8°). Seventy-eight percent of patients were within 3° and 97% of patients were within 3.6°. In comparison, post-operative alignment for standard TKA patients measured varus 1.85°, which was not statistically significant. Seventy-nine percent of patients were within 3°; however the outliers were more dramatic ranging 3.5° to 9.2°.

Thirty femoral and 21 tibial resections were available for review using the MyKnee technique. The actual vs. planned resections for the distal medial femoral resection was 9.5 vs. 9.1mm respectively. Further actual vs. planned femoral resections include distal lateral femoral 8.4 vs. 6.3mm; posterior medial femoral 9.3 vs. 9.5mm; and posterior lateral femoral 8.6 vs. 7.0mm. The actual vs. planned tibial resections recorded include medial 6.07 vs. 6.29mm and lateral 9.36 vs. 8.19mm. Tourniquet time averaged 32.97 minutes (range 25 to 54) in the standard TKA group vs. 37.03 minutes (range 1 to 71) in the MyKnee group. This difference was not significant. However, the final 15 MyKnee patients had an average time of 33.46 minutes. No intraoperative complications occurred.

Many techniques exist for performance of TKA. Patient-specific cutting blocks allow the surgeon to pre-operatively determine resection depths, rotations, alignment, and sizing prior to the operative procedure itself. The present study shows that intraoperative resections and post-operative alignments can be accurately achieved with pre-operative CT planning and using patient-specific instrumentation.

For the typical varus knee deformity, cartilage will exist on the lateral side of the knee. This can cause measurement error when measuring the lateral compartments as the CT scan is based on bone only. This can be seen in 2.1mm and 1.6mm differences in the distal lateral femoral and posterior lateral femoral resections respectively. Thus, this difference can be explained by the false measurement of intact cartilage. More accurate results could be obtained if the cartilage was removed and bone measured. Valgus knees, being diseased in the lateral compartment, did not show such variance as expected in planned vs. actual resections. Intraoperative speed of surgery is important to all participants in TKA: surgeon, hospital, and patient. Obviously accuracy should not be sacrificed for speed so it is important for any new technology introduced to the market to accelerate surgery not compromise results. In the current study, the average times of MyKnee vs. standard TKA surgery were comparative and not significantly different using a two-sample T-test. The standard TKA average tourniquet time may appear faster than other reported literature; however the surgeon is on the end of learning curve with the system. The MyKnee average tourniquet time represents the initial procedures in the learning curve and can be considered slower than what they will eventually be as the author gains more experience with the technique. Efficiency was demonstrated with the decrease in tourniquet time for the last 15 patients.

Furthermore, the goals of surgery were maintained radiographically. Regardless of the deformity, the patient's post-operative mechanical axes averaged 1.85° for standard technique and 1.92° for the MyKnee group, not statistically significantly different. These results were obtained via long-standing x-rays, which are well known to be prone to error in alignment secondary to potential flexion and rotation of the extremity. The standardised protocol for acquisition of the X-ray, attempts to prevent these errors and X-rays are routinely re-done if the technician feels error has occurred. The technique also appears safe as no intra-operative complications occurred and were recognised within the first six weeks post-operative.

In conclusion, using patient-specific instrumentation (MyKnee) is safe, quick, and accurate in performance of TKA.

E.K. Song J.K. Seon K.D. Kang C.H. Park J.H. Yim

Recently, axial radiography has received attention for the assessment of distal femur rotational alignment, and satisfactory results have been as compared with the CT method. The purpose of this study was to assess rotational alignment of the femoral component in knee flexion by axial radiography and to compare flexion stabilities achieved by navigational and robotic total knee arthroplasty (TKA). In addition, the authors also evaluated the effects of flexion stability on functional outcomes in these two groups.

Sixty-four patients that underwent TKA for knee osteoarthritis with a minimum of follow-up of 1 year constituted the study cohort. Patients in the navigational group (N = 32) underwent TKA using the gap balancing technique and patients in the robotic group (N = 32) underwent TKA using the measured resection technique. To assess flexion stability using axial radiography a novel technique designed by the authors was used. Rotations of femoral components and mediolateral gaps in the neutral position on flexion radiographs was measured and compared. Valgus and varus stabilities under valgus-varus stress loading, and total flexion stabilities (defined as the sum of valgus and varus stability) were also compared, as were clinical outcomes at final follow up visits.

A significant difference was found between the navigation and robotic groups for mean external rotation of the femoral component (2.1° and 0.4°, respectively; p = 0.003). Mean mediolateral gap in neutral at 90° flexion position was 0.17° in the navigation group and 0.07° in the robotic group (p = 0.126), and mean total stability was 7.82° in the robotic group and 8.10° in the navigation group (p = 0.35). Clinically, no significant intergroup difference was found in terms of ranges of motion, HSS scores, KS scores, or WOMAC scores.

Both navigational and robotic techniques provide excellent clinical and flexion stability results. Furthermore, axial radiography was found to provide a useful, straightforward means of detecting rotational alignment, flexion gaps, and flexion stability.

E.K. Song J.K. Seon K.D. Kang C.H. Park J.H. Yim

The purpose of this study was to compare posterior tibial slope preoperatively and postoperatively in patients undergoing navigational opening-wedge High tibial osteotomy (HTO) and to compare posterior slope changes for 2 and 3-dimentional (D) navigation versions.

Between May 2009 and September 2010, 35 patients with unicompartmental osteoarthritis and varus deformity were treated by navigation-assisted open-wedge HTO. Patients were randomly divided into two groups according to the version of the Orthopilot (Aesculap) navigation system used; 2D group (18 patients, 2-D version) and 3D group (17 patients, 3-D version). Radiologic evaluations were conducted using pre- and postoperative leg axes. Posterior slope of proximal tibiae were measured using the proximal tibial anatomic axis method.

Postoperatively the mechanical axis was corrected adequately to a mean valgus of 2.81° in 2D group and of 3.15° in 3D group. Mean posterior slopes were well maintained, and measured 7.9° and 10.3° preoperatively and 8.99° and 9.14° postoperatively in 2D and 3D groups, respectively. No significant difference was found between the two navigation versions with respect to posterior tibial slope; mean tibial slope changes were 1.09° and −0.2° in 2D and 3D groups (p = 0.04).

Navigation-assisted opening-wedge HTO greatly improves the accuracy of the desired postoperative mechanical femorotibial axis and posterior tibial slope, and the use of 3D navigation results in significantly less change in posterior tibial slope. The authors recommend the use of the 3D navigation because they provide real time intraoperative information about coronal, sagittal, and transverse axis, which are important for the maintenance of a normal posterior tibial slope.

G.C. Claasen P. Martin F. Picard

Over the past fifteen years, computer-assisted surgery systems have been more commonly used, especially in joint arthroplasty. They allow a greater accuracy and precision in surgical procedures and thus should improve outcomes and long term results.

New instruments such as guided handheld tools have been recently developed to ultimately eliminate the need for drilling/cutting or milling guides.

To make sure that the handheld tool cuts and/or drills in the desired plane, it has to be servo-controlled. For this purpose, the tool joints are actuated by computer-controlled motors. A tracking system gives the tool position and orientation and a computer calculates the corrections for the motors to keep the tool in the desired plane.

For this servo-control, a very fast tracking system would be necessary. It should be fast enough to follow human motion. Current optical tracking systems used for computer-assisted surgery have a bandwidth of about 10–60 Hz [3]. For servo-control, a bandwidth of about 200–300 Hz would be required to be faster than human reaction; the latency of the system should also be small, about 2–3 ms. Optical tracking systems with a higher bandwidth exist but are too expensive for applications in surgery; besides the latency – due to the complex computer vision treatment involved – is too big.

We have developed a hybrid tracking system consisting of two cameras pointed at the operating field and a sensor unit which can be attached to a handheld tool.

The sensor unit is made up of an inertial measuring unit (IMU) and numerous optical markers. The data from the IMU (three gyroscopes and three accelerometers placed such that their measurement axes are perpendicular to each other) and the marker images from the cameras looking at the optical markers are fed to a data fusion algorithm. This algorithm calculates the position and the orientation of any handheld tool. It can do so at the higher of the two sensor sample rates which is the IMU sample rate in our case.

Our experimental setup consists of an ADIS 16355 IMU which runs at a sample rate of 250 Hz and a pair of stereo cameras which are sampled at 16.7 Hz. The data collected from these sensors are processed offline by the data fusion algorithm. To compare the results of our hybrid system to those of a purely optical tracking system, we use only the marker image data to recalculate the sensor unit's position by triangulation.

The experiment we conducted was a fast motion in a horizontal direction starting from a rest position. The sensor unit position was calculated by the hybrid system and by the optical tracking system using the experimental data. The fast motion started right after the optical sample at t1 and the hybrid system detects it at once. The optical tracking system, on the other hand, only sees the motion at the next optical sample time t2.

These results show that our hybrid system is able to follow a fast motion of the sensor unit whereas a purely optical tracking system is not.

The proposed hybrid tracking system calculates position and orientation of any handheld tool at a high frequency of 250 Hz and thus makes it possible to servo-control the tool to keep it in the desired plane.

Several similar systems fusing optical and inertial data have been described in the literature. They all use processed optical data, i.e. 3D marker positions. Our algorithm uses raw image data to considerably reduce computation time. This hybrid tracking system can be used with any handheld tool developed to substitute existing drilling, cutting or milling instruments used in orthopaedic surgery and particularly in arthroplasty.

The sensor unit can be easily implemented into an existing optical tracking system. For the surgeon, the only change is an additional small inertial sensor besides the optical markers already attached to the tool.

The authors would like to thank the AXA Research Fund for funding G.C. Claasen's work with a doctoral grant and Guillaume Picard for his contributions to the experimental setup.

W.T. Wilson A.H. Deakin F. Picard P.E. Riches J.V. Clarke

Clinical laxity tests are frequently used for assessing knee ligament injuries and for soft tissue balancing in total knee arthroplasty (TKA). Current routine methods are highly subjective with respect to examination technique, magnitude of clinician-applied load and assessment of joint displacement. Alignment measurements generated by computer-assisted technology have led to the development of quantitative TKA soft tissue balancing algorithms. However to make the algorithms applicable in practice requires the standardisation of several parameters: knee flexion angle should be maintained to minimise the potential positional variation in ligament restraining properties; hand positioning of the examining clinician should correspond to a measured lever arm, defined as the perpendicular distance of the applied force from the rotational knee centre; accurate measurement of force applied is required to calculate the moment applied to the knee joint; resultant displacement of the knee should be quantified.

The primary aim of this study was to determine whether different clinicians could reliably assess coronal knee laxity with a standardised protocol that controlled these variables. Furthermore, a secondary question was to examine if the experience of the clinician makes a difference. We hypothesised that standardisation would result in a narrow range of laxity measurements obtained by different clinicians.

Six consultant orthopaedic surgeons, six orthopaedic trainees and six physiotherapists were instructed to assess the coronal laxity of the right knee of a healthy volunteer. Points were marked over the femoral epicondyles and the malleoli to indicate hand positioning and give a constant moment arm. The non-invasive adaptation of a commercially available image-free navigation system enabled real-time measurement of coronal and sagittal mechanical femorotibial (MFT) angles. This has been previously validated to an accuracy of ±1°. Collateral knee laxity was defined as the amount of angular displacement during a stress manoeuvre. Participants were instructed to maintain the knee joint in 2° of flexion whilst performing a varus-valgus stress test using what they perceived as an acceptable load. They were blinded to the coronal MFT angle measurements. A hand-held force application device (FAD) was then employed to allow the clinicians to apply a moment of 18Nm. This level was based on previous work to determine a suitable subject tolerance limit. They were instructed to repeat the test using the device in the palm of their right hand and to apply the force until the visual display and an auditory alarm indicated that the target had been reached. The FAD was then removed and participants were asked to repeat the clinical varus-valgus stress test, but to try and apply the same amount of force as they had been doing with the device.

Maximum MFT angular deviation was automatically recorded for each stress test and the maximum moment applied was recorded for each of the tests using the FAD. Means and standard deviations (SD) were used to compare different clinicians under the same conditions. Paired t-tests were used to measure the change in practice of groups of clinicians before, during and after use of the FAD for both varus and valgus stress tests.

All three groups of clinicians initially produced measurements of valgus laxity with consistent mean values (1.5° for physiotherapists, 1.8° for consultants and 1.6° for trainees) and standard deviations (<1°). For varus, mean values were consistent (5.9° for physiotherapists, 5.0° for consultants and 5.4° for trainees) but standard deviations were larger (0.9° to 1.6°). When using the FAD, the standard deviations remained low for all groups for both varus and valgus laxity. Introducing the FAD overall produced a significantly greater angulation in valgus (2.4° compared to 1.6°, p<0.001) but not varus (p = 0.67) when compared to the initial examination. In attempting to reach the target moment of 18Nm, the mean ‘overshoot’ was 0.9Nm for both varus and valgus tests. Standard deviations for varus laxity were lower for all groups following use of the FAD. The consultants' performance remained consistent and valgus assessment remained consistent for all groups. The only statistically significant change in practice for a group before and after use of the FAD was for the trainees testing valgus, who may have been trained to push harder (p = 0.01). Standardising the applied moment indicated that usually a lower force is applied during valgus stress testing than varus. This was re-enforced by clinicians, one third of whom commented that they felt they had to push harder for valgus than varus, despite the FAD target being the same.

We have successfully standardised the manual technique of coronal knee laxity assessment by controlling the subjective variables. The results support the hypothesis of producing a narrow range of laxity measurements but with valgus laxity appearing more consistent than varus. The incorporation of a FAD into assessment of coronal knee laxity did not affect the clinicians' ability to produce reliable and repeatable measurements, despite removing the manual perception of laxity. The FAD also provided additional information about the actual moment applied. This information may have a role in improving the balancing techniques of TKA and the management of collateral ligament injuries with regard initial diagnosis and grading as well as rehabilitation.

Finally, the results suggest that following use of the FAD, more experienced clinicians returned to applying their usual manual force, while trainees appeared to use this augmented feedback to adapt their technique. Therefore this technique could be a way to harness the experience of senior clinicians and use it to enhance the perceptive skills of more junior trainees who do not have the benefit of this knowledge.

S.M.A. Arachchi A. Augustine A.H. Deakin F. Picard P.J. Rowe

Computer assisted surgery is becoming more frequently used in the medical world. Navigation of surgical instruments and implants plays an important role in this surgery. OrthoPilot™ Hip Suite (BBraun Aesculap) is one such system used for hip navigation in orthopaedic surgery. However the accuracy of this system remains to be determined independently of the manufacturer. The manufacturer supplies a technical specification for the accuracy of the system (± 2 mm and ± 2°) and previous research has been undertaken to compare its clinical accuracy against conventional hip replacements by x-ray. This clinical validation is important but contains many sources of error or deviation from an ideal outcome in terms of the surgeons' use of the system, inaccurate palpation of landmarks, variation in actual cup position from that given by the navigation system and measurement of the final cup position. It is therefore not possible to validate the claims of the manufacturer from this data. There is no literature evaluating the technical accuracy of the software i.e. the accuracy of the system given known inputs. This study had two main aims 1) validating the accuracy of the OrthoPilot data while navigating the surgical instruments and 2) validating the accuracy of navigation algorithm inside the OrthoPilot system which determines cup implant placement. The OrthoPilot validation was performed and compared against the gold standard of a VICON movement analysis system.

The system used was OrthoPilot™ with a Spectra camera from Northern Digital Inc. (Ontario, Canada). Software investigated was the Hip Suite THA cup only navigation software Version 3.1. The validation was performed and compared against the VICON Nexus version 1.4.116 with Bodybuilder software version 3.55. An aluminium pelvis phantom was used for measurement allowing accurate and repeatable inputs. The OrthoPilot system has three types of instruments sets; passive, active and hybrid. This study was carried out with the passive instruments set. Data were captured simultaneously from both the OrthoPilot and VICON systems for the supine position of the phantom. Distances between the anatomical land marks on the phantom were compared to test the data capturing accuracy of the OrthoPilot system. Anatomical land marks of right anterior superior iliac supine (RASIS), left anterior superior iliac supine (LASIS) and Pubic Symphasis (PS) were palpated to define the Anterior Pelvic Plane (APP). Distances between the anatomical landmarks of RASIS to LASIS, RASIS to PS and LASIS to PS were considered for comparison. Width and height of the pelvis was varied to examine different APPs. The width and height used were 170 mm and 53 mm, 230 mm and 88 mm, and 290 mm and 123 mm respectively. One hundred APP data sets were captured at each instance.

The accuracy of the hip navigation algorithm was tested by applying similar algorithm to calculate the native anteversion and inclination angles of the acetabulum using the VICON system. Data were captured simultaneously from both OrthoPilot and VICON systems. Radiographic anteversion and inclination angles were obtained with phantom model, which had 14° of anteversion angle and 45° of inclination angle. APP of 230 mm in width and 88 mm in height was used to obtain anterior pelvic plane data. Position vectors for each anatomical land mark from the OrthoPilot system were extracted from relevant transformation matrices, while position vectors from the VICON system were extracted from static trial modelling.

The distance data from both systems were compared with calibrated distance data from the phantom model. Mean values of the distances between anatomical landmarks were found to be similar for both OrthoPilot and VICON systems. In addition, these distances were comparable with the pelvic phantom model data, within 1 mm for all measured distances for the VICON and 2 mm for the OrthoPilot. Furthermore, the standard deviations were less than 1% of the measured value. Comparison was also made for the anteversion and inclination angles of the acetabulum of the pelvic model with OrthoPilot and VICON data. Both systems produced similar results for the mean angle values, within 0.5° of the known angles for the VICON and 1° for the OrthoPilot and with standard deviations of the measured values of less than 1%.

All the data were captured simultaneously from both OrthoPilot and VICON systems under the same laboratory conditions. According to the above results it is clear that the distance readings obtained from the OrthoPilot are comparable to the results obtained from the gold standard VICON system and the calibrated distance readings of the phantom. In addition, acetabular angle results obtained from OrthoPilot are almost equivalent to results obtained from VICON and the calibrated phantom angles. Finally it is can be concluded that, both the data palpation with OrthoPilot system and acetabular angle calculation algorithm of the OrthoPilot system are accurate enough for the real world clinical tasks they are expected to perform.

J. Eschweiler L. Fieten F. Schmidt K. Kabir S. Gravius M. de la Fuente K. Radermacher

Consideration of biomechanical aspects during computer assisted orthopaedic surgery (CAOS) is recommendable in order to obtain satisfactory long-term results in total hip arthroplasty (THA). In addition to the absolute value of the hip joint resultant force R the pre- and post-operative orientation of R is an important aspect in the context of the development of a planning module for computer-assisted THA and furthermore for planning of acetabular orientation in periacetabular osteotomy interventions. It is possible to estimate the orientation of hip joint resultant force R for individual patients based on geometrical and anthropometrical parameters. The aim of this study was to examine how far the choice of the mathematical model influences the computational results for the orientation of R in the frontal plane. A further aspect was the comparison of the results with in-vivo data published in the open access OrthoLoad database (www.orthoload.com).

Our comparative study included the 2D-models suggested by Pauwels, Blumentritt and Debrunner as well as the 3D-model suggested by Iglič and three patient datasets from the Orthoload database. As computation of R according to each model relies on standardized X-ray imaging, three anterior-posterior (a.p.) digitally reconstructed radiographs (DRRs) were generated from CT data (x21_x21, x8_x8, x12_x12). The orientation of R was expressed in terms of the angle δ for these three patient individual datasets. The angle δ is defined as the angle between the longitudinal axis and R. The computation results were also compared with in vivo telemetric measurement data from the OrthoLoad database. The following data were used to evaluate R in the frontal plane: the highest load peak of the single leg stance (static conditions) of three patients (EBL, HSR, KWR) respectively in the same manner for planar gait (dynamic conditions) of one patient (KWR). The mean value of the orientation of R under static conditions in single leg stance was calculated in order to get a reference value. For the orientation of R under dynamic conditions δ was calculated by using only the highest peak of three cycles (heel strike to toe off) determined in one single patient (among the three patients involved in the measurements under static conditions) of the database.

The following values of δ were obtained:

Pauwels: 18.26°/20.34°/17.31° (x21_x21/x8_x8/x12_x12) Debrunner: 12.37°/14.30°/12.59° Blumentritt: 5.18°/6.52°/6.14° Iglič: 9.24°/9.01°/9.20°

OrthoLoad database (in-vivo): 28.41°/17.08°/13.32°-static (EBL/HSR/KWR) 16.44°-dynamic (KWR)

The differences in the computational results appear to depend more on the hip model than on the variability of patient-specific geometrical and anthropometrical parameters. The results obtained with in-vivo measurement data are best approximated by using Pauwels' model. The mean values of Pauwels (18.64°), Debrunner (13.09°) and Iglič (9.15°) are a little bit more vertically orientated than the mean value of the static in-vivo results (19.60°). Only Pauwels' model result has a larger angle δ than the in-vivo dynamic result (KWR = 16.44°). By comparing the in-vivo values obtained under dynamic conditions, i.e. gait, (16.44°) with the static in-vivo values of the same patient (13.32°), it could be recognized that the static values are a little bit more vertically orientated than the dynamic result. But both are in the same range as the mathematical models.

The computational biomechanical hip models try to approximate the physiological conditions of the hip joint and the OrthoLoad database represents the physiological reconstructed (artificial) hip joint. Therefore, we think our validation approach is useful for a comparison of the biomechanical computation models.

In contrast, Blumentritt's model outcomes have the largest deviation from the other models as well as from the in-vivo data (static and dynamic conditions). Blumentritt used the weight bearing surface as a reference. He defined it being perpendicular to the longitudinal axis [3]. He postulated that a valid and optimal orientation of R is approximately perpendicular on the weight bearing surface respectively parallel to the longitudinal axis. This approach for validation is questionable because the results show that in the three included and analysed DDR's the orientation is in the mean value 5.95° to the longitudinal axis. It can be concluded that Blumentritt's model assumptions have to be carefully reviewed due to the deviations from in-vivo measurement data.

Among the limitations of our study is the fact that the OrthoLoad database offers only a small number of patient datasets. There is only one dataset for the direct comparison of static (single leg stance) and dynamic (free planar gait) in-vivo measurement data of the same patient included. Furthermore, the individual anatomic geometry data of the patients included in the database are not revealed. Additionally, a source of errors could be an inaccuracy during the data acquisition from the DRR.

Further research seems to be recommendable in the context of implementing a biomechanical hip model in a planning module for computer-assisted THA or periacetabular osteotomy interventions, respectively. Sensitivity analyses and parameter studies for different mathematical models using a multi-body-simulation system are objectives of our ongoing work.

M. Haimerl L. Poitzsch S. Gneiting M. Schubert E. Sendtner M. Wörner R. Springorum T. Renkawitz

Incorrect restoration of leg length (LL) and offset is a major source of patient dissatisfaction and dysfunction after total hip arthroplasties (THAs). Evaluations on anterior-posterior x-ray images are state-of-the-art to assess the accuracy of such techniques. However, x-ray based measurements of LL and offset are challenging and limited in terms of accuracy. Within this study, we evaluated the accuracy of such measurements by analysing a series of clinical data. We evaluated the results on the non-treated side, since we know that there should be no significant difference between pre- and postoperative measurements on this side.

A series of 44 consecutive patients was analysed regarding changes in the difference between pre- and post-operative LL and offset measurements. Anterior-posterior x-rays were taken pre- (pre-OP) and post-operatively (post-OP) with a calibration by a scaling ruler (pre-OP) or implant size (post-OP). The LL and offset measurements were performed with a digital planning software based on the teardrop and transischial line. The distance between the teardrop/transischial line and the trochanter minor was measured to assess LL differences. Femoral offset (FO) was calculated as the orthogonal distance between the centre of the femoral head and the proximal shaft axis. Global offset (GO) was calculated as the distance between the inferior aspect of the teardrop figure and the shaft axis along the teardrop line. Descriptive statistics (mean value ± standard deviation) were calculated for the different types of measurements. Statistically significant differences were checked according to a student's t-test (α = 0.05).

The differences between the pre-and post-operative situation was 0.8±3.2 mm for LL, 0.2±3.5 mm for GO, and −0.5±2.5 mm for FO when referencing to the teardrop line and 0.9±4.0 mm (LL) and −0.3±2.7 mm (FO) for the transischial line. The error distributions did not show statistically significant differences when referencing to the teardrop or transischial line. But high differences (0.1±6.6 mm) were found when comparing the LL values (teardrop vs. transischial) case-by-case.

Within this study we demonstrated that x-ray based offset and LL measurements show reasonable inaccuracies. X-ray based evaluations of navigation-based techniques to assist LL and offset restoration cannot produce significantly better results than these analysed limits. That is, even if the navigation technique would be perfectly accurate, the evaluation would not achieve better accuracies than approximately ±3.5 mm for LL, ±3.5 mm for GO, and ±2.5 mm for FO.

A.V. Kaminskiy E.V. Gorbunov


Two aspects are very important for knee joint replacement – restoration of biomechanical limb axis and achieving ligaments balance. Computer navigation allows us to do all this.

Material and methods

We analysed 94 knee joint replacement surgeries using computer navigation by “STRYKER”.

J.V. Clarke A.H. Deakin F. Picard P.E. Riches

Knee alignment is a fundamental measurement in the assessment, monitoring and surgical management of patients with osteoarthritis [OA]. In spite of extensive research into the consequences of malalignment, our understanding of static tibiofemoral alignment remains poor with discrepancies in the reported weight-bearing characteristics of the knee joint and there is a lack of data regarding the potential variation between supine and standing (functional) conditions. In total knee arthroplasty [TKA] the lower limb alignment is usually measured in a supine condition and decisions on prosthesis placement made on this. An improved understanding of the relationship between supine and weight-bearing conditions may lead to a reassessment of current surgical goals.

The purpose of this study was to explore the relationship between supine and standing lower limb alignment in asymptomatic, osteoarthritic and prosthetic knees. Our hypothesis was that the change in alignment of these three groups would be different.

A non-invasive infrared position capture system (accuracy ±1° in both coronal and sagittal plane) was used to assess the knee alignment for 30 asymptomatic controls and 31 patients with OA, both before and after TKA. Coronal and sagittal mechanical femorotibial (MFT) angles in extension (negative values indicating varus in the coronal plane and hyperextension in the sagittal plane) were measured with each subject supine and in bi-pedal stance. For the supine test, the lower limb was supported at the heel and the subject told to relax. For the standing position subjects were asked to assume their normal stance. The change in alignment between these two conditions was analysed using a paired t-test for both coronal and sagittal planes. To quantify the change in 3D, vector plots of ankle centre displacement relative to the knee centre from the supine to standing condition were produced.

Alignment in both planes changed significantly from supine to standing for all three groups. For the coronal plane the supine and standing measurements (in degrees, mean(SD)) were 0.1(2.5) and −1.1(3.7) in the asymptomatic group, −2.5(5.7) and −3.6(6) in the OA group and −0.7(1.4) and −2.5(2) in the TKA group. For the sagittal plane the numbers were −1.7(3.3) and −5.5(4.9); 7.7(7.1) and 1.8(7.7); 6.8(5.1) and 1.4((7.6) respectively. This change was most frequently towards relative varus and extension. Vector plots showed that the trend of relative varus and extension in stance was similar in overall magnitude and direction between the three groups.

Knee alignment can change from supine to standing for asymptomatic and osteoarthritic knees, most frequently towards relative varus and hyperextension. The similarities between each group did not support our hypothesis. The consistent kinematic pattern for different knee types suggests that soft tissue restraints rather than underlying joint deformity may be more influential in dynamic control of alignment from lying to standing. In spite of some evidence suggesting a difference between supine and standing knee alignment a mechanical femorotibial (MFT) angle of 0° is a common intra-operative target as well as the desired post-operative weight-bearing alignment. These results indicated that arthroplasties positioned in varus intra-operatively could potentially become ‘outliers’ (>3° varus) when measured weight-bearing. Mild flexion contractures may correct when standing, reducing the need for intra-operative posterior release. These potential changes should be considered when positioning TKA components on supine limbs as post-operative functional alignment may be different.

A. Augustine A.H. Deakin P. Rowe F. Picard

There is increasing interest in the use of image free computer assisted surgery (CAS) in total hip arthroplasty (THA). Many of these systems require the registration of the Anterior Pelvic Plane (APP) via the bony landmarks of the anterior superior iliac spines (ASIS) and pubic tubercles (PT) in order to accurately orient the acetabular cup in terms of anteversion and inclination. Given system accuracies are within 1mm and 1° and clinical validation studies have given accuracy by cup position. However, clinical outcomes contain not only system inaccuracies but also variations due to clinical practice. To understand the effects of variation in landmark acquisition on the identification of the acetabular cup orientation, independent bench testing is required. This requires a phantom model that can represent the range of pelvises, male and female, encountered during THA and introduce deliberate known errors to the acquisition to see the effect on anteversion and inclination angles. However, there is a paucity of information in the literature with regards to these specific pelvic dimensions (pelvic width and height). Therefore the aims of this work were to generate the normal expected range of sizes of the APP for both males and females and to use these to manufacture a phantom model that could be used to assess CT free navigation systems.

In the first part of the study 35 human cadavers and 100 pelvic computed tomography (CT) scans were examined.

All cadavers had no gross pelvic abnormalities or previous surgeries. Measurements were carried out with cadavers placed in a supine position. The first author made three sets of measurements using a millimeter ruler. Solid steel pins were used to identify the palpated ASISs and PTs. String was tied between the two ASIS pins and the pelvic width measured. The midpoint of the pubic tubercles was taken to be the midpoint of the pubic symphysis. Pelvic height was measured from the midpoint of the ASIS distance (marked on the string) to the midpoint of the PTs. One hundred pelvic CT scans with no bony abnormalities, previous surgery or metal prosthesis (due to artefacts) were obtained retrospectively from the hospital radiological online system (PACS, Kodak). Mimics software (Mimics12 Materialise, Leuven, Belgium) was used to automatically reconstruct three-dimensional (3D) models using the ‘Bone’ thresholding function. This eliminated any soft tissue from the 3D models. The most anterior ASIS and PT points were then identified on the 3D model surface and measurements of distances made. As the software did not allow identification of points not on the model surface it was not possible to directly obtain the midpoint of the ASIS distance. Therefore to obtain the pelvic height measurements the distance between each ASIS and the ipsilateral and contralateral PTs was also measured. The pelvic height was then calculated using trigonometric functions. The ratio of width to height was calculated (ratio > 1 indicating pelvis width greater than pelvis height). Student's t test was used analyse any differences between male and female pelvic measurements with a p<0.05 being statistically significant.

Using the results from above an aluminium pelvic phantom model was designed and manufactured. It was machined from a billet of marine grade aluminium alloy using a vertical computer numerical controlled (CNC) milling machine. The top surface represented the APP and sides (which represented the acetabuli) were angled to give anteversion and inclination angles of 20° and 45° respectively. Co-ordinates for ASIS and PT points were given based on the 99% prediction intervals from the pelvic data and additional points were milled to give up to a 20 mm error mediolaterally and also in height. Each co-ordinate point was drilled with a 2.0mm diameter ball-nose cutter to a depth of 1.0mm, these holes designed to accommodate the ball-nosed pointer tip to ensure it remained at the same position in space at all orientations of the pointer. Further to this, known errors in height were introduced using accurately manufactured blocks with similar points milled on the surface to fit a ball-nosed pointer. These blocks could be secured to the top surface of the model using screws. A Perspex base unit with tracker attachments was made to hold the phantom and provide the reference frame. A further support that enables the phantom to also be used in the “lateral” position was manufactured.

For the assessment of pelvic size there were 66 females and 69 males, mean age 62.3 years (range from 20 to 99 years). The mean width was 238 mm (SD 20 mm) and mean height was 93 mm (SD 11 mm) with a mean ratio of 2.6 (SD 0.3). There were no statistically significant differences in mean between males and females (p>0.4 in all cases). From this data set the range of APP sizes required to cover 99% of population (width 186 to 290 mm and height 66 to 120 mm) and therefore the measurements for the model were generated. The manufactured model can be used to give the range of pelvis sizes from 170mm to 290mm in width and 60mm to 120mm in height and also to add up to 20 mm of error in palpation of each of the ASISs and PT.

This study generated APP sizes to cover 99% of the general population over a wide age range. It illustrated that a single pelvic model would fit both sexes. The model allows the determination of the effects of changes of the pelvic dimensions may have on the acetabular orientation measured on an image free CAS system including the assessment of point acquisition and deliberate errors. The model has been successfully used in preliminary testing and can be used to assess any CT free system.

E. Schkommodau F. Coigny C. Findeisen M. Hirschmann C. Ballweg P. Jürgens R. Thoranaghatte S. Hemm B. Knobel


Currently existing optical navigation systems have ergonomic disadvantages such as size, the “line of sight” problem and extended registration procedures. The operation room becomes crowded by additional installations and competitive supporting devices around the patient. These points reduce and limit the acceptance of navigation systems for further applications. But especially for surgical quality management, navigation systems have a high potential as objective measurement systems.


A miniaturised measuring and navigation system, which is directly fixed at the surgical tool, could overcome these limitations and fulfil the requirements demanded by current and future operation rooms. Minimising the distance between situ and camera promises an increased accuracy, a reduced “line of sight problem,” intuitive handling and one coordinate transformation (Tool2DRB) less. However, such a setting reduces the navigation working space available, needs a sterile system, a new marker design and special requirements for the cameras. The developed prototypes were tested in vitro using Synbones™ and ex vivo at anatomical specimen. Following surgical pilot applications were defined and considered during the studies: maxillofacial restoration osteotomy, hip replacement and unicondylar knee replacements (UKR). Special emphasis was placed on measured and recorded accuracy and miniaturised hardware.

G. Zheng J. von Recum L.-P. Nolte P.A. Grützner J. Franke

The goal of this study was to validate accuracy and reproducibility of a new 2D/3D reconstruction-based program called “HipRecon” for determining cup orientation after THA. “HipRecon” uses a statistical shape model based 2D/3D deformable registration technique that can reconstruct a patient-specific 3D model from a single standard AP pelvic X-ray radiograph. Required inputs include a digital radiograph, the pixel size, and the film-to-source distance. No specific calibration of the X-ray, or a CAD (computer-assisted design) model of the implant, or a CT-scan of the patient is required. Cup orientation is then calculated with respect to the anterior pelvic plane that is derived from the reconstructed 3D-model.

The validation study was conducted on datasets of 29 patients (31 hips). Among them, there were 15 males and 14 females. Each dataset has one post-operative X-ray radiograph and one post-operative CT-scan. The post-operative CT scan for each patient was used to establish the ground truth for the cup orientation. Radiographs with deep centering (7 radiographs), or of pelvises with fractures (2 radiographs), or with both (1 radiograph), or of non-hemispherely shaped cup (1 radiograph) were assessed separately from the radiographs without above mentioned phenomena (18 radiographs) to estimate a potential influence on the 2D/3D reconstruction accuracy. To make the description easier, we denote those radiographs with above mentioned phenomena as non-normal cases and those without as normal cases. The cup anteversions and inclinations that were calculated by “HipRecon” were compared to the associated ground truth. To validate the reproducibility and the reliability, one observer conducted twice measurements for each dataset using “HipRecon”.

The mean accuracy for the normal cases was 0.4° ± 1.8° (−2.6° to 3.3°) for inclination and 0.6° ± 1.5° (−2.0° to 3.9°) for anteversion, and the mean accuracy for the non-normal cases was 2.3° ± 2.4° (−2.1° to 6.3°) for inclination and 0.1° ± 2.8° (−4.6° to 5.1°) for anteversion. Comparing the measurement from the normal radiographs to those from the non-normal radiographs using the Mann-Whitney U-test, we found a significant difference in measuring cup inclination (p = 0.01) but not in measuring cup anteversion (p = 0.3). Bland-Altman analysis of those measurements from the normal cases indicated that no systematical error was detected for “HipRecon,” as the mean of the measurement pairs were spread evenly and randomly for both inclination and anteversion. “HipRecon” showed a very good reproducibility for both parameters with an intraclass correlation coefficient (ICC) for inclination of 0.98 (95% Confidence Limits (CL): 0.96–0.99) and for anteversion of 0.96 (95% CL: 0.91–0.98).

Accurate assessment of the acetabular cup orientation is important for evaluation of outcome after THA, but the inability to measure acetabular cup orientation accurately limits one's ability to determine optimal cup orientations, to assess new treatment methods of improving acetabular cup orientation in surgery, and to correlate the acetabular cup orientation to osteolysis, wear, and instability. In this study, we showed that “HipRecon” was an accurate, consistent, and reproducible technique to measure cup orientation from post-operative X-ray radiographs. Furthermore, our experimental results indicated that the best results were achieved with the radiographs of non-fractured pelvises that included the anterior superior iliac spines and the cranial part of the non-fractured pelvis. Thus, it is recommended that these landmarks should be included in the radiograph whenever the 2D/3D reconstruction-based method will be used

S.T. Goudie K. Deep

The success of total hip replacement (THR) is closely linked to the positioning of the acetabular component. Malalignment increases rates of dislocation, impingement, acetabular migration, pelvic osteolysis, leg length discrepancy and polyethylene wear. Many surgeons orientate the cup in the same anteversion and inclination as the inherent anatomy of the acetabulum. The transverse acetabular ligament and acetabular rim can be used as a reference points for orientating the cup this way. Low rates of dislocation have been reported using this technique. Detailed understanding of the anatomy and orientation of the acetabulum in arthritic hips is therefore very important. The aim of this study was to describe the anteversion and inclination of the inherent acetabulum in arthritic hips and to identify the number that fall out with the ‘safe zone’ of acetabular position described by Lewinnek et al. (anteversion 15°±10°; inclination 40°±10°).

A series of 65 hips, all with symptomatic osteoarthritis undergoing THR were investigated. Patients with developmental dysplastia of hip (DDH) were excluded. All patients had a navigated THR as part of their normal clinical treatment. A posterior approach to the hip was used. A commercially available non image based computer navigation system (Orthopilot BBraun Aesculap, Tuttlingen, Germany) was used. Rigid bodies (using active trackers) were attached to pelvis and femur. Anterior pelvic plane was registered using the two anterior superior iliac spines and pubic symphysis. The femoral head dislocated and removed and the labrum and soft tissue were excised to clear floor and rim of the acetabulum. Inner size of the empty acetabulum was sized with cup trials and appropriately size trial fixed with a computer tracker was then aligned in the orientation of the natural acetabulum as defined by the acetabular rim ignoring any osteophytes. The inclination and anteversion were calculated by the software. Surgery then proceeded with guidance of the computer navigation system. The computer software defines the anatomical values of orientation, to allow comparison with radiographs these were converted to radiological values as described by Murray et al. The acetabular inclination in all hips was also measured on pre-operative anteroposterior pelvic radiographs. This was done using digital radiographs analysed with the PACS system (Kodak, Carestream PACS Client, version 10.0). Acetabular inclination was measured using as the angle between a line passing through the superior and inferior rim of the acetabulum and a line parallel to the pelvis as identified by the tear drops, using the method described by Atkinson et al.

All patients were Caucasian and had primary osteoarthritis. There were 29 males and 36 females. The average age was 68 years (SD 8). Mean anteversion was 9.3° (SD 10.3°). Anteversion for males was significantly lower than females with a mean difference of −5.5° (95%CI −10.5°,−0.5°) p = 0.033 but there was no significant difference in the number falling outside the “safe zone”. Mean inclination was 50.4° (SD 7.4°). There was no significant difference between males and females with respect to inclination angle or the number that fell outside the “safe zone”. Overall 69% of patients had a combined inclination and anteversion of the native acetabulum that fell outside the “safe zone” of Lewinnek.

Mean acetabular inclination falls out with the ‘safe zone’. This trend has been seen in a recent study of arthritic hips using CT scans which found that the average angle of inclination in both males and females was greater than the upper limit of the safe zone. This study using CT also demonstrated a statistically significant 5.5° difference between males and females in terms of anteversion. This is the same as the figure we have found in our work. Inherent acetabular orientation in arthritic hips falls out with the safe zone defined by Lewinnek in 69% of cases. When using the natural acetabular orientation as a guide for positioning implants it should therefore not be assumed this will fall with in the safe zone although the validity of safe zones itself is questionable. Variation between patients must be taken into account and the difference between males and females, particularly in terms of anteversion, should also be considered.

K. Deep C. Menna F. Picard

The aim of the study was to investigate rotational behaviour of the arthritic knee before (preimplant) and after (postimplant) total knee replacement (TKR) using (image-free navigation system as a measurement tool which recorded the axial plane alignment between femur and tibia, in addition to the coronal and sagittal alignment as the knee is flexed through the range of motion. The data on the rotation of the arthritic knee was collected after the knee exposure and registration of the lower limb (preimplant data). The position of rotation between the femur and tibia was recorded in 30° flexion, 45°, 60°, 90° and maximum degrees of flexion of the knee. The data was divided into subsets of varus and valgus knees and these were analysed pre and postimplant for their rotational position using SPSS for statistics.

The system was used in 117 knees of which 91 had full data set available (43 male 48 female). These included 71 varus knees, 16 valgus knees and 4 neutral knees to start in extension. Preimplant data analysis revealed there is tendency for the arthritic knees to first go in internal rotation in the initial part of flexion to 30 degrees and then the rotation is reversed back. This happens irrespective of the initial starting rotational relationship between femur and tibia in full extension. This happens in both varus as well as valgus arthritic knees. This trend of internal rotation in this initial part of flexion is followed in TKR as well implanted with fixed bearing CR knees irrespective of the preoperative deformity. Also noteworthy was the difference in rotation at 30°, 60° and 90 degrees of flexion between preimplant and postimplant knees (irrespective of varus and valgus groups).

When calculated at different points of flexion, there was statistically significant difference in the change of rotation at each point of flexion except 45 degree of flexion. The pre-operative values of change in rotation (internal being positive) at each step from the extended position being 5.4° (SD 4.5°) at 30 ° flexion, 4.7°(5.2°) at 45°, 3.6°(6.1°) at 60°, 3.5°(7.2°) at 90° and 4.2°(8.3°) at maximum flexion. Corresponding post-operative rotations were 2.2°(4.8°), 4.1°(6.4°), 6.6°(7.3°), 9.9°(8.8°) and 7.7°(8.9°). There was also an increase in the total range of rotation that the knee goes through after it has been implanted with prosthesis although it may not happen in every knee. This is statistically significant (p value <0.001) and seems more so in valgus group. The rotational movements and interrelationship of the femur and tibia is a complex issue, especially in the arthritic knees. Preimplant arthritic knee behaved generally similarly to normal knees according to the literature. Normal gait pattern demonstrates that the tibia moved through a 4° to 8° arc of internal rotation relative to the femur. The overall range (10.2° =/−4.2°) of knee rotation in this study greater than 8° might be explained by preimplant data acquired after the knee was approached and therefore releasing knee soft tissue envelop. This study confirmed that during the first 30° both varus and valgus knees moved internally. In our study there is increased range of total rotation postimplant (14° =/−6.8°) which may be explained by the fact that the anterior cruciate ligament is lost in all the TKRs and the posterior cruciate ligament may be dysfunctional as well. Thus the constraints on the knee rotation are decreased postimplant leading to increased rotation. We found some difference between varus and valgus post implant knees in that internal rotation seen in initial 30 degrees of flexion is much more pronounced in valgus knees as compared to varus knees (p value <0.001). This study confirmed knee internal rotation in initial stages of flexion, preimplant in arthritic knees during a passive knee flexion assessment. Varus and valgus knee seemed to behave similarly. This mimics the normal knee rotation. Postimplant knees in TKR behave differently.

S. Schumann L.P. Nolte G. Zheng

The integration of statistical shape models (SSMs) for generating a patient-specific model from sparse data is widely spread. The SSM needs to be initially registered to the coordinate-system in which the data is acquired and then be instantiated based on the point data using some regressing techniques such as principal component analysis (PCR). Besides PCR, partial least squares regression (PLSR) could also be used to predict a patient-specific model. PLSR combines properties of PCR and multiple linear regression and could be used for shape prediction based on morphological parameters.

Both methods were compared on the basis of two SSMs, each of them constructed from 30 surface models of the proximal femur and the pelvis, respectively. Thirty leave-one-out trials were performed, in which one surface was consecutively left out and further used as ground truth surface model. Landmark data were randomly derived from the surface models and used together with the remaining 29 surface models to predict the left-out surface model based on PCR and PLSR, respectively. The prediction accuracy was analysed by comparing the ground truth model with the corresponding predicted model and expressed in terms of mean surface distance error.

According to their obtained minimum error, PCR (1.62 mm) and PLSR (1. 63 mm) gave similar results for a set of 50 randomly chosen landmarks. However PLSR seems to be more susceptible to a wrong selection of number of latent vectors, as it has a more variation in the error.

Although both regression methods gave similar results, decision needs to be done, how to select the optimal number of regressors, which is a delicate task. In order to predict a surface model based on morphological parameters using PLSR, the choice of the parameters and their optimal number needs to be carefully selected.

I. Hacihaliloglu R. Abugharbieh A.J. Hodgson P. Gug

Due to its ease of use, portability, low cost, real-time response and absence of ionising radiation, ultrasound (US) imaging could potentially be an important tool for non-invasive diagnostic imaging in orthopaedics. Unfortunately, nonlinear characteristics of ultrasound, low signal-to-noise ratio and speckle make it difficult to accurately and reliably determine the location and shape of the bone surface. Recently, local phase-based image processing methods, named phase symmetry (PS), have been shown to perform very well at locating bone surfaces in ultrasound images, with reported accuracies of better than 0.4mm. The local phase features are extracted by filtering the B-mode US image in the frequency domain with a Log-Gabor filter. Although successful results were achieved, accurate localization is highly affected by the choice of filter parameters. Recently, our group proposed a method of automatically selecting the scale, bandwidth and orientation parameters of Log-Gabor filters. Previously, we showed our first clinical results using local phase information to identify distal radius fractures from B-mode US images using automatically selected filter parameters.

The objective of the current study was to determine if the proposed automatic parameter selection method could produce accurate pelvic bone surface shapes in a live clinical setting.

CT scans were obtained as part of normal clinical care from ten patients admitted to Vancouver General Hospital for pelvic fractures. A ‘gold standard’ bone surface was computed from the CT scan. After obtaining informed consent, we performed an additional US scan using a commercially-available real-time scanner (Voluson 730, GE Healthcare, Waukesha, WI) with a 3D US transducer. The PS bone surfaces were extracted from the US scans using the empirical Log-Gabor filter parameters and optimised Log-Gabor filter parameters. The bone surfaces on CT were extracted using a standard thresholding approach that minimises the intra-class variance. The US images were then registered to the CT images using a feature-based rigid registration algorithm with manual landmarking. The quality of the resulting surface matching was evaluated by computing the root mean square distance between the two surface representations.

The average fiducial registration error was 0.31mm (SD 0.25mm). The average surface fitting error (SFE) was 0.72mm (SD 1.24 mm) for PS surfaces extracted using empirical filter parameters and 0.41mm (SD 0.44 mm) using the optimized filter parameters.

In this study, we have demonstrated that our automatic filter parameter selection process can be applied successfully to a bone surface extraction task on 3D US images acquired under clinically realistic conditions. The accuracy of the resulting bone surface is excellent, with an average discrepancy relative to a CT standard of well under a millimeter. This level of accuracy is likely to be sufficiently good for a number of important surgical tasks, including CT to US registration.

M. Haselbacher K. Sekyra E. Mayr M. Thaler M. Nogler

In the last years custom-fit cutting guides using magnetic resonance imaging (MRI) were introduced by orthopedic surgeons for total knee arthroplasty (TKA). One of the advantages of these shape-fitting jigs is the possibility to transfer the preoperative planning of the TKA directly to the individual patient's bone. However, one has to be aware, that the jigs are designed for single-use and have to be custom made by an external manufacturer. This increases the cost of implantation and unlinks the surgeon from this process. In addition a potentially necessary adjustment of the preoperatively planned implant size and position in a surgical situation is not possible.

The purpose of our development was to combine the advantages of custom-fit cutting guides as a 3-D-computer-assisted planning tool with the option to adjust and improve the preoperative planning and the jig in the actual surgical situation. In addition no outside jig manufacturing would occur in this concept. This leaves the surgeon in control of the entire process.

The purpose of this study was to examine the reliability of this screw-based shape – fitting system. In order to do this we assessed the inter- and intra-observer reliability of the recurrent placement of the plate on a set of bone samples with preset screws.

We developed a plate with the dimension of 66 × 76 × 10 mm, containing 443 threaded holes. A connector for further instrumentation is mounted on the proximal part of the plate,. As the plate and the screws are made of aluminum and steel, sterilization is possible.

After computer tomography (CT) scans were taken from three human femoral bones, eight to nine variably positioned screws (50.45 mm length, 2.75 mm diameter), reversibly fixed by locknuts, formed an imprint of a bone's surface. For calculating precise screw positions, a computer-based planning software was developed resulting in a three-dimensional reconstruction of the bony surfaces. The plate was integrated in the 3-D reconstruction software. With a defined distance to the distal part of the femurs, allowed the proper length and position of the screws to be calculated. These calculations were transferred to the screws on the real plate.

In the next step the plate was positioned on the bony surface and after reaching the planned position the plate's connector was rigidly fixed to the bone. The plate was removed to give place to link saw jigs to the connector.

Planning and setting of the plate and the screws were conducted on three femoral bones.

Examinations were performed by five investigators with ten repetitions on each bone with three distinct plates. Intra- and inter-observer variability was assessed by measuring the variation in plate position between the trials.

The jigs were placed in a mean frontal tilting (medial to lateral) of 0.83°. The mean axial tilting (proximal to distal) was 1.66° and the mean shift on the axis from proximal to distal 8.48 mm. The shift and the tilting were significantly bone dependent but not user dependent. Compared with previous studies the deviation from the mechanical axis were comparable with conventional TKA (2.6° and 0.4°), computer assisted TKA (1.4° and 1.9°) and Custom-fit TKA (1.2°).

We developed a preoperative planning system for TKA that allows a transfer of the planning and the calculated imprint of the bones surface on a grid-plate during surgery by the surgeons themselves. Neither external manufacturers to create a fixed device nor a navigation system is necessary. Results showed the functioning of the screw – based shape fitting technique within the accuracy mentioned above. These findings are encouraging to do further research to examine the ideal number of screws to offer a perfect fitting.

C. Belvedere A. Ensini J.L. Moctezuma De La Barrera A. Feliciangeli A. Leardini F. Catani

During total knee replacement (TKR), surgical navigation systems (SNS) allow accurate prosthesis component implantation by tracking the tibio-femoral joint (TFJ) kinematics in the original articulation at the beginning of the operation, after relevant trial components implantation, and, ultimately, after final component implantation and cementation. It is known that TKR also alters normal patello-femoral joint (PFJ) kinematics resulting frequently in PFJ disorders and TKR failure. More importantly, patellar tracking in case of resurfacing is further affected by patellar bone preparation and relevant component positioning. The traditional technique used to perform patellar resurfacing, even in navigated TKR, is based only on visual inspection of the patellar articular aspect for clamping patellar cutting jig and on a simple calliper to check for patellar thickness before and after bone cut, and, thus, without any computer assistance. Even though the inclusion in in-vivo navigated TKR of a procedure for supporting also patellar resurfacing based on patient-specific bone morphology seems fundamental, this have been completely disregarded till now, whose efficacy being assessed only in-vitro. This procedure has been developed, together with relevant software and surgical instrumentation, as an extension of current SNS, i.e. TKR is navigated, at the same time measuring the effects of every surgical action on PFJ kinematics. The aim of this study was to report on the first in-vivo experiences during TKR with patellar resurfacing.

Four patients affected by primary gonarthrosis were implanted with a fixed bearing posterior-stabilised prosthesis (NRG, Stryker®-Orthopaedics, Mahwah, NJ-USA) with patellar resurfacing. All TKR were performed by means of two SNS (Stryker®-Leibinger, Freiburg, Germany) with the standard femoral/tibial trackers, the pointer, and a specially-designed patellar tracker. The novel procedure for patellar tracking was approved by the local ethical committee; the patients gave informed consent prior the surgery. This procedure implies the use of a second system, i.e. the patellar SNS (PSNS), with dedicated software for supporting patellar resurfacing and relative data processing/storing, in addition to the traditional knee SNS (KSNS). TFJ anatomical survey and kinematics data are shared between the two. Before surgery, both systems were initialised and the patellar tracker was assembled with a sterile procedure by shaping a metal grid mounted with three markers to be tracked by PSNS only. The additional patellar-resection-plane and patellar-cut-verification probes were instrumented with a standard tracker and a relevant reference frame was defined on these by digitisation with PSNS. Afterwards, the procedures for standard navigation were performed to calculate preoperative joint deformities and TFJ kinematics. The anatomical survey was performed also with PSNS, with relevant patellar anatomical reference frame definition and PFJ kinematics assessment according to a recent proposal. Standard procedures for femoral and tibial component implantation, and TFJ kinematics assessment were then performed by using relevant trial components. Afterwards, the procedure for patellar resection begun. Once the surgeon had arranged and fixed the patellar cutting jig at the desired position, the patellar-resection-plane probe was inserted into the slot for the saw blade. With this in place, the PSNS captured tracker data to calculate the planned level of patellar bone cut and the patellar cut orientation. Then the cut was executed, and the accuracy of this actual bone cut was assessed by means of the patellar-cut-verification probe. The trial patellar component was positioned, and, with all three trial components in place, TFJ and PFJ kinematics were assessed. Possible adjustments in component positioning could still be performed, until both kinematics were satisfactory. Finally, final components were implanted and cemented, and final TFJ and PFJ kinematics were acquired. A sterile calliper and pre- and post-implantation lower limb X-rays were used to check for the patellar thickness and final lower limb alignment. The novel surgical technique was performed successfully in all four cases without complication, resulting in 30 min longer TKR. The final lower limb alignment was within 0.5°, the resurfaced patella was 0.4±1.3 mm thinner than in the native, the patellar cut was 1.5°±3.0° laterally tilted. PFJ kinematics was taken within the reference normality. The patella implantation parameters were confirmed also by X-ray inspection; discrepancies in thickness up to 5 mm were observed between SNS- and calliper-based measurements.

At the present experimental phase, a second separate PSNS was utilised not to affect the standard navigated TKR. The results reported support relevance, feasibility and efficacy of patellar tracking and PFJ kinematics assessment in in-vivo navigated TKR. The encouraging in-vivo results may lay ground for the design of a future clinical patella navigation system the surgeon could use to perform a more comprehensive assessment of the original whole knee anatomy and kinematics, i.e. including also PFJ. Patellar bone preparation would be supported for suitable patellar component positioning in case of resurfacing but, conceptually, also in not resurfacing if patellar anatomy and tracking assessment by SNS reveals no abnormality. After suitable adjustment and further tests, in the future if this procedure will be routinely applied during navigated TKR, abnormalities at both TFJ and PFJ can be corrected intra-operatively by more cautious bone cut preparation on the femur, tibia and also patella, in case of resurfacing, and by correct prosthetic component positioning.

H. Enomoto T. Nakamura H. Shimosawa A. Waseda Y. Niki Y. Toyama Y. Suda

Although optimal alignment is essential for improved function and implant longevity after TKA, we have less bony landmarks of tibia relative to femur. Trans-malleolar axis (TMA) is a reference line of distal tibia in the axial plane, which externally rotated relative to a ML axis of proximal tibia. We originally defined another reference axis associated with the orientation of tibial plafond, and then measured tibial torsion in the 3D-coordinate system.

Three-dimensional CAD models of 20 tibiae were reconstructed based on pre-operative CT data from OA patients (16 females and 4 males, 73.8 ± 6.9 years old). TMA was a line connecting each apex of medial and lateral malleolus. The plafond axis (PLA) that we originally defined in this study was a line connecting each midpoint of medial and lateral margin of talocrural facet. In terms of interobserver correlation coefficiency and mean errors of the designated points to define those axes, TMA was found out to be 0.982, 3.14 ± 0.47 mm (medial), and 0.988, 4.88 ± 0.59 mm (lateral). Those of PLA were 0.997, 1.97 ± 0.53 mm (medial), and 0.995, 2.02 ± 0.44 mm (lateral). The tibial torsion was 16.3 ± 6.3°with reference to TMA, and 10.2 ± 8.4°to PLA.

Based on these results, as for the rotational reference axis in the axial plain of distal tibia, we consider the plafond axis to be another reliable and reproducible axis, which is expected to be applicable in preoperative planning in TKA to reduce outliers of coronal alignment.

E. Beretta M. Valenti E. De Momi G. Ferrigno

The location of the hip joint center (HJC) allows correct prosthesis aligning and positioning in Computer-Assisted Orthopaedic Surgery (CAOS) applications. For the kinematic HJC localisation, the femur is moved around the pelvis with ad hoc motion trials (“pivoting”). The “Pivoting algorithm” [Siston et al., J Biomech 39 (2006) 125–130] is the functional state-of-the-art method for the hip center localisation. A source of systematic error in HJC localisation algorithms is represented by the pelvis motion during the pivoting. In computer assisted total knee arthroplasty applications, the pelvis pose is not acquired during passive movements. In motion capture applications, Kalman Filter (KF) methodology was used to estimate the pose of hidden segment for rigid body pose estimation.

The purpose of this study was to validate the accuracy and robustness of a Kalman Filter algorithm, applied to a state space formulation based on two links model of the hip joint, to track the HJC position during passive movements of the articulation in CAOS procedure.

The state space model describes femur and pelvis kinematics under the hypothesis of non-laxity of the articulation (ideal spherical joint). The first link models the femoral bone, while the second link models the pelvis. The femur is tracked with a Dynamic Reference Frame (DRF) attached to the distal end, composed by four active markers, while the pelvis is tracked attaching a marker to it. The kinematic relations between the state vector and the observations are non linear function. The state space has been implemented with II order linear dynamics. The position of HJC in the Femur Reference Frame is modeled with non-dynamic state variables.

In order to validate the proposed algorithm, a physical model of the hip joint (femur and pelvis) was realised using SawBones models. An active optical localisation system (Certus, NDI, Ontario, Canada) was used in order to track the coordinates of two DRF rigidly connected on each segment and the coordinates of a marker attached to the pelvis segment (on the Anterior Superior Iliac Spine ASIS). The pelvis phantom is locked on a Mass-Spring-Damper platform with 2 DoFs, which mimics soft tissues behaviour. During the pivoting motion, the poses of the femur DRF and the positions of the ASIS marker of the pelvis DRF were collected. The acquired data were the observable outputs to the KF algorithm, which computes an estimation of the state parameters. The accuracy is evaluated as the Euclidean distance between respectively the estimated and Gold Standard HJC positions in FRF. The KF method performances were compared with the “Pivoting” algorithm. The localisation errors computed for both the methodologies were evaluated with respect to the HJC translation, to the Range Of pivoting Motion (ROM) and to the velocity of femur DRF trajectory (Pearson correlation analysis).

The positive correlation coefficients between HJC translation and the localization errors result statistically significant (p<0.01) for both “Pivoting” (correlation index equal to 0.838) and KF (correlation index equal to 0.415) algorithms; while a negative (correlation index equal to −0.355) and positive (correlation index equal to 0.263) correlation respectively for ROM and Velocity is computed as statistically significant (p<0.05) only for KF algorithm errors. Statistically significant difference (Kruskal-Wallis, p<0.01) between “Pivoting” [median 26.71 mm and inter-quartile range (24.04, 32.18)mm] and KF [median 11.71mm and inter-quartile range (7.74, 18.82)mm] algorithms was assessed for HJC translation greater than 7 mm.

The new method KF proved to be applicable in current CAOS systems. The substantial improvement of KF method is the possibility of reducing the systematical error, caused by pelvis motion during passive movement of the femur, to compute HJC position. On the other hand, tracking the HJC trajectory in real time is a nontrivial task and requires a very accurate filter parameters tuning. Further tests must be made to estimate the in-vivo range of HJC translation during passive pivoting movements and evaluate the performances of KF method with respect to others state-of-the-art methods.

W. Xie J. Franke P.A. Gruetzner L.P. Nolte G. Zheng

The existing image-free Total Hip Arthroplasty (THA) navigation systems conventionally utilise the patient-specific Anterior Pelvic Plane (APP) as the reference to calculate orientations of the implanted cup, e.g. anteversion and inclination angles. The definition of APP relies on the intra-operative digitisation of three anatomical landmarks, the bilateral Anterior Superior Iliac Spine (ASIS) and the pubicum.

Due to the presence of the thick soft tissue around the patient's pubic region, however, the landmark on pubic area is hard to be digitised accurately. A novel reference plane called Intra-operative Reference Plane (IRP) was proposed by G. Zheng et al to address this issue. To determine the IRP, bilateral ASIS and the cup center of the operating side instead of the pubicum are digitised intra-operatively. It avoids the error-prone digitisation of pubicum, and the angle between the patient-specific APP and the suggested IRP can be computed pre-operatively by a single X-ray radiograph-based 2D/3D reconstruction approach developed by G. Zheng et al. Based on this angle, the orientation of the APP can be intra-operatively estimated from that of the IRP such that all measurements with respect to IRP can be transformed to measurements with respect to APP.

In order to implement and validate this new reference plane for image-free navigation of acetabular cup placement, we developed an IRP-based image-free THA navigation system. All cup placement instruments were mounted with passive markers whose positions could be traced by a NDI Polaris® infrared camera (Northern Digital Inc, Ontario, Canada). The cup center was obtained by first pivoting a tracked impactor with appropriate size of the mounted trial cup and then calculating the pivoting center through a least-squares fitting. The bilateral ASIS landmarks were acquired through the percutaneous pointer-based digitisation.

We tested this new IRP-based image-free THA navigation system in our laboratory by conducting twelve studies on two dry cadaver pelvises and two plastic pelvises. The ground truth for each study was established using the conventional APP-based method, i.e., in addition to those landmarks required by our IRP-based method, we also digitised the pubicum on respective pelvic bones and calculated cup orientations on the basis of the digitised APP.

The mean and standard deviation of differences between the proposed IRP-based anteversion measurement and the ground truth are 1.0 degree and 0.7 degree, while the maximal and minimal differences are 2.1 degree and 0.3 degree respectively.

The mean and standard deviation of differences between the proposed IRP-based inclination measurement and the ground truth are respective 0.2 degree and 0.2 degree. Moreover, the maximum of differences is 0.5 degree and the minimum is 0.0 degree.

Our laboratory experimental results demonstrate that the new IRP-based image-free navigation system is accurate enough for acetabular cup placement. In comparison to existing image-free navigation systems that use APP as the reference plane, the newly developed system employs IRP as the reference plane, which has the advantage to eliminate the digitisation of landmarks around the pubic region. The successful validation with the laboratorial study has led us to the next step of clinical trials. We expect to report preliminary clinical cases in the near future.

C. Myden C. Anglin G. Kopp C.R. Hutchison

Orthopaedic surgery residents typically learn total knee arthroplasty (TKA) through an apprenticeship-type model, which is a necessarily slow process. If residents could learn the required technical and cognitive skills more quickly, they could make better use of reduced hours in the operating room, surgeons could teach at a higher level, patients could have shorter operating times with better outcomes, and the healthcare system would have reduced costs and better-trained surgeons.

Surgical skills courses, using artificial bones, have been shown to improve technical and cognitive skills significantly within a couple of days. Computer-assisted surgery (CAS) provides real-time feedback and component position planning, leading to improved alignment and a shorter learning curve. Combining these two approaches challenges the participants to consider the same task in different contexts, promoting cognitive flexibility.

We designed a hands-on educational intervention for junior residents incorporating a conventional tibiofemoral TKA station, two different tibiofemoral CAS stations and a conventional and CAS patellar resection station. The same implant system was used in all cases. Both qualitative and quantitative analyses were performed. Qualitatively, structured interviews before and after the course were analysed for recurring themes. Quantitatively, subjects were evaluated on their technical skills in a timed conventional TKA test before and after the course, and on their knowledge and error-detection skills after the course. Their performance was compared to senior residents who performed only the testing.

Four themes emerged: increased confidence, improved awareness, deepening knowledge and changed perspectives. The residents' attitudes to CAS changed from negative before the course to neutral or positive after the course. They expected it to be difficult to use and found that it was easy. They originally distrusted the system, but came to think they would use it for their most difficult cases. The junior resident group improved their task completion rate from 23% to 75% of tasks (p<0.01), compared to 45% of tasks completed by the senior resident group.

As a result of the course, the residents will be more aware what to focus on in the operating room. High impact educational interventions, promoting cognitive flexibility and including real-time feedback from computer-assisted surgery simulations, would benefit trainees, surgeons, the healthcare system and patients.

H. Bäthis S. Shafizadeh M. Banerjee T. Tjardes B. Bracke T. Neubauer B. Bouillon

In order to enhance the acceptance of computer assisted surgery in joint replacement, a development-cooperation with BrainLAB, Germany was set up to develop a user-friendly handheld navigation device. A sterile draped Apple® IPod-Touch which is placed into a hardcover cradle, is used as navigation monitor and touchscreen control. Different instruments, such as navigation-pointer are attached to the cradle. In addition the workflows for TKR and THR procedures have been optimised. Therefore the main focus for TKR is navigation of femoral and tibial resection as well as leg alignment control. For the THR the system enables an intraoperative control of leg-length and femoral-offset measurement in comparison with the preoperative situation. Each step of the procedure is supported by video animations of the specific navigation workflow.

Between September and December 2010 the first clinical study on the usability in TKR and THR was performed for 20 cases using a prototype system. The study was approved by the local ethic committee and the “German Federal Institute for Drugs and Medical Devices (BfArM)”. Special interest was taken to the aspects of usability and the necessary time periods for specific steps of the procedure. Usability was measured for specific time periods of the procedure assessment of the usability of the surgical team. In addition postoperative x-rays were evaluated for implant position, leg alignment for TKR and hip joint geometry for THR cases.

Throughout the study for each assigned patient the procedure could be performed as planned. Several design inputs were identified for further improvement of the final system. Therefore time measurements of the first five cases were excluded.

For the TKR cases the registration process of the last 5 cases was less than 3 minutes. The interval for the tibial resection was between 3 and 7 minutes (aligning tibial cutting block – end of tibial verification). The interval for the distal femur resection was between 7 and 11 minutes (aligning femoral cutting block – end of femoral verification). All 10 Patients showed a final leg alignment on the postoperative standing x-ray within the save-zone of +/− 3° from neutral alignment. For the THR cases the preoperative registration period including the femoral head resection and acetabular registration was between 7 and 12 Minutes. Each final measurement of the hip geometry was done in less than 2 minutes. The evaluation of the pelvic ap-x-ray pre- and postoperative showed equivalent measurements of the new hip geometry compared with the intraoperative measured values. No specific complications occurred throughout the study.

In conclusion the BrainLAB–DASH-System has shown a high grade of usability and very short learning curve within this first clinical study. The use of a standard Apple® IPod-touch as a user interface seems to enhance the acceptance of the navigation technique. Equivalent precision compared to standard navigation systems have been demonstrated.

R. Blanc G. Székely

Bone shape estimation from partial observations, such as fluoroscopy or ultrasound, has been subject of significant interest over the past decade and can be regarded as the driving force behind several advances in statistical modelling of shape. While statistical models were initially used mostly as regularisers constraining shape matching algorithms, they are now increasingly employed due to their predictive ability, when only limited observations are available. With the current efforts toward minimal invasiveness, radiation exposure reduction, and optimization of the cost-effectiveness of procedures, two major challenges emerge on the field of statistical modelling. The first one is to develop methods that enable the use of as much information as possible that can be relevant for a specific shape prediction task, within the aforementioned limits. The second challenge concerns the accuracy of the resulting predictions, which needs to be quantified in order to evaluate the associated risks, and to optimise the data acquisition procedures.

In terms of shape prediction, most studies so far have concentrated on individualizing statistical atlases based on imaging data. However, relevant information about skeletal morphology can also be obtained from simple anthropometric and morphometric measurements such as gender, age, body-mass index, and bone specific measurements. We develop a multivariate regression framework that enables to take into account such combinations of predictors simultaneously with sparse observations of the bone surface for improved prediction of the complete bone shape. In particular, we describe in a quantitative and localised fashion the individual contributions but also the complementarities of the heterogeneous sources of information with respect to bone morphology assessment. To do so, we compare the prediction errors obtained with different combinations of predictors, relying on cross-validation experiments. In addition to providing valuable and complementary predictive information, non-imaging measurements can be exploited to automatically initialise surface registration algorithms which increase their robustness for the determination of patient specific morphologies.

A statistical model, by essence, is a mathematical model resulting from a learning phase using a set of training data. Statistical model based prediction is affected by three main sources of errors. The pre-processing of the training data, in particular the establishment of anatomical correspondences between the different samples, and the limited number of training elements constitute a first source of uncertainties. Second, the predictors can be affected by measurement noise, which will then propagate through the prediction process. Finally, and this is particularly important in the context of sparse observation data, the limited correlations between the predictors and the shape to predict imply theoretical limits for the achievable accuracy of such approaches. We have developed a framework enabling to account for these various sources of uncertainty, and propagating them through the prediction pipeline to generate confidence regions around the predicted shape. It relies extensively on cross-validation experiments in order to quantify the limitations of the statistical model with respect to the representation of new shapes (generalization ability) and to their prediction from partial data. Furthermore, we demonstrate the reliability of the obtained regions, following the procedure initially proposed in.

We evaluate our approaches on a database of 140 femur bones, age range: 23–83, mean 62.57, stdv 15; 46% males and 54% females, with known age, height and weight. Morphometric measurements such as bone length, inter-condyle distance or anteversion angle are considered, either as predictors, together with sparse point clouds around the femoral head and greater trochanter, or as a pose-independent quality-of-fit metric. Cross-validation experiments indicate that a higher accuracy can be reached when complementing surface-based predictors with relevant anthropometric and morphometric information, and that reliable confidence regions can be estimated.

E.J. Smith J. Anstey M. Kunz B. Rasquinha J. Rudan P.J. St. John G. Wood R.E. Ellis

Femoroacetabular impingement is a condition in which the femoral head/neck region abnormally contacts the acetabulum, limiting the range of motion of the hip and often associated with pain, damage, and loss of function. The pathophysiology of osteoarthritic changes stemming from impingement syndromes has been linked to the shape of the hip; however, little is known about the influence of the soft tissues to this process.

In this pilot study, we used computer-assisted navigation technology to track motion on a cadaver that had mild bilateral cam-impingement lesions, and then performed a virtual simulation to locate sites of impingement. We hypothesised that soft tissues contribute to the degree and location of impingement, so we compared impingements across three different dissection states: (i) all soft tissues intact; (ii) post-capsulectomy; with only the labrum and ligamentum teres remaining; and (iii) disarticulated, with labrum and ligamentum teres removed.

With ethical approval, we used one fresh frozen cadaver pelvis that was sectioned above the fifth lumbar vertebra and at the knee. The femurs and pelvis were implanted with fiducial screws as an accurate means for surface-based image registration. With all soft tissues intact, tissues were imaged using computed tomography with a slice thickness of 0.625 mm. The CT scans were imported into Mimics (v13.0, Materialise, Belgium) and carefully segmented, with particular detail to the articular regions and fiducials, to create 3D digital models of the pelvis and femurs.

On each side, optical local coordinate reference (LCR) bodies were attached at the proximal femur and iliac crest to permit spatial tracking with an Optotrak Certus camera (Northern Digital Inc., Waterloo, Canada). The 3D digital models were imported into the VSS navigation system (iGO Technologies, Kingston, Canada) and scrupulously registered to the anatomy using the fiducial screws and a calibrated probe. The pose of the femur and pelvis were recorded throughout a series of twelve movements involving various combinations of flexion-extension, abduction-adduction, internal-external rotation and circumduction, as well as functional movements typical of a clinical hip screening. Soft tissues were selectively removed and the movements were repeated post-capsulectomy and completely disarticulated.

The recorded pose data were applied to the 3D digital models to perform a computational simulation of the movements during the trials. The pose data were expressed in coordinates of the anterior pelvic plane to compute angles of motion in the principal directions (flexion, abduction, rotation). The motion data were further filtered so that only comparable ranges of motion were present for data analysis. Algorithms were developed to determine bone-on-bone impingement locations by finding contact points between the models.

Impingement locations were plotted on the digital models of the femur and pelvis in order to establish zones of impingement. The surface area of each impingement zone was computed by using a Crust-based algorithm that triangulated impingement points encompassing a region, and then summed the surface area of each triangle to estimate the total impingement surface area.

Upon visual inspection, it was immediately apparent that impingements tended to occur in well-defined regions. On the femur, these were found along aspects of the head-neck junction, especially on or near osteophytes. On the pelvis, impingement regions were found along the acetabular rim and extending into the lunate region.

With soft tissues intact, both femurs and pelvis had prominent anterior and posterior impingement zones. In contrast, post-capsulectomy impingement zones were predominately confined to the anterior region. It should be noted, however, that the total impingement area decreased post-capsulectomy, representing only about 25% of the total area of impingements when all soft tissues were intact. This was also true in the disarticulated state.

Both femurs had mild posterior cam lesions, the right worse than the left. Impingements were seen at these sites with soft tissues intact, but diminished almost entirely post-capsulectomy. The anterior lesions were located contra coup to these cam lesions.

With soft tissues intact, impingements tended to occur in external rotation and abduction. With soft tissues removed there was a pronounced shift towards impingements occurring in internal rotation. Impingements were also noted in large flexion angles and large abduction-adduction angles in the absence of soft tissues.

Although it is widely accepted that the hip is spherical in shape and has ball-and-socket kinematics, recent work suggests that the osteoarthritic hip is aspherical and that translational motion is present. On a very limited series, this work is supportive of the latter observation: if hip motion is purely spherical, a decrease in impingements post-capsulectomy is exceedingly hard to describe. However, if soft tissues cause translatory motion, then their absence logically should lead to a change in the impingement pattern (which we found).

This preliminary study provides a methodology for studying the effects of soft tissue on impingements. We conclude that soft tissues do indeed play an important role in impingement and may even contribute to the development of impingement lesions. Limitations include a small sample size, so further studies are required prior to conclusively establishing impingement patterns in passive kinematics of cadaver hips.

M.S. Goddard J.E. Lang G.G. Poehling M.A. Conditt R.H. Jinnah

Unicompartmental knee arthroplasty (UKA) was first described over 30 years ago and allows replacement of a single compartment in patients who have isolated osteoarthritis. However, UKA is more technically challenging than total knee arthroplasty due to limited exposure as a minimally invasive procedure. In addition to component alignment and fixation, ligament balancing plays an important role in implant survival. Some failures of early UKA systems were attributed to a failure to adequately balance the knee. The development of robots to aid in performing the procedure has lead to renewed interest in this surgical technique. The use of a robot-assisted system allows the orthopaedic surgeon to verify that balancing sought pre-operatively correlates with that obtained at surgery. Some studies have shown good post-operative mechanical alignment utilizing this method. The aim of this study was to examine the variation in pre-operative templated ligament balance and that obtained during the operation.

Data were prospectively collected on 51 patients (52 knees) undergoing robot-assisted unicompartmental knee arthroplasty by a single surgeon. For pre-operative planning, dynamic ligament balancing was obtained of the operative knee under valgus stress, prior to any bony cuts. Final intra-operative images with the prosthesis in place were taken without valgus stress. Positive values denoted loose ligamentous balancing while negative values indicated ligament tightness.

A small variation of less than 1 mm was measured between the pre-operative plan and the final image with the implant in place. At 0 degrees the mean change was −0.26 mm (range, −4.40 to 2.20 mm), at 30 degrees −0.53 mm (range, −5.30 to 1.80 mm), at 60 degrees −0.04 mm (range, −3.10 to 2.30 mm) and at 90 degrees 0.16 mm (range, −2.70 to 2.00 mm). These results show that planned dynamic ligament balancing is accurate to within 0.52 mm.

The technological advancements with robotic feedback in orthopaedic surgery can aid in the success of unicompartmental knee replacement surgery. Ensuring that pre-operative templated changes match those performed during surgery is an important predictor of outcome. With proper planning prior to surgery, the use of a robot in UKA can improve ligament balancing. This can be done at various angles, ensuring excellent ligament balancing throughout the entire range of motion. Correct component alignment reduces the risk of prosthetic failure and may increase the length of implant survival. Further fine-tuning of the accuracy of feedback between the robot and the anatomical points will improve the accuracy of UKA.

M.S. Goddard J.E. Lang J.S. Bircher B. Lu G.G. Poehling R.H. Jinnah

Osteoarthritis of the knee is a debilitating condition affecting millions of persons, often requiring arthroplasty to relieve pain and improve mobility. For those patients with disease in only one compartment of the knee, unicompartmental knee arthroplasty (UKA) can be a viable surgical alternative. To date, there has not been a large series reported in the literature of UKAs performed with robotic assistance. The aim of this study was to examine the clinical outcomes of patients who underwent this procedure.

Five hundred and ten procedures in patients with a mean age of 63.7 years (range, 28 to 88 years) who underwent unicompartmental knee arthroplasty using a robotic-assisted system between July, 2008 and June, 2010 were identified. Clinical outcomes were evaluated using the Oxford Knee Score and patients without recent follow-up were contacted by telephone. The revision rate and time to revision were also examined.

The average length of stay for patients who underwent robot-assisted UKA was 1.4 days (range, 1 to 7 days). There was minimal blood loss with most procedures. At latest clinical follow-up, most patients were doing well after UKA with a mean Oxford Knee Score of 36.1 + 9.92. The revision rate was 2.5% with 13 patients being either converted from an inlay to onlay prosthesis or conversion to total knee arthroplasty. The most common indication for revision was tibial component loosening, followed by progression of arthritis. Mean time to revision was 9.55 + 5.48 months (range, 1 to 19 months).

Unicompartmental arthroplasty with a robotic system provides good pain relief and functional outcome at short-term follow-up. Ensuring correct component alignment and ligament balancing increases the probability of a favorable outcome following surgery. Proper patient selection for appropriate UKA candidates remains an important factor for successful outcomes.

E.K. Song J.K. Seon K.D. Kang C.H. Park J.H. Yim

This prospective study was undertaken to compare the clinical and radiological results and the in vivo stabilities of anteromedial (AM) and posterolateral (PL) bundle augmentation during anterior cruciate ligament (ACL) reconstruction.

Forty-two ACL partial tears that underwent isolated bundle augmentation (22 AM and 20 PL bundles) were evaluated with a minimum follow-up of 1 year. For in vivo intraoperative stability testing, anteroposterior and external/internal rotation stabilities were measured at 0, 30, 60, and 90° of flexion using a navigation system. Ranges of motion, Lachman and pivot shift test results, Tegner activity scores, and Lysholm knee scores of the AM and PL bundle groups were compared. In addition, Telos arthrometer determined stabilities were compared.

In-vivo intraoperative stability testing showed that mean preoperative anterior translation at 30° of flexion was greater in the AM group (8.7 vs. 6.5, p = 0.04), whereas mean rotational amount was larger in the PL group (by 2.9 at 0° and 3.6 at 30° of flexion). After ACL reconstruction, no significant differences were found between the two groups in terms of anterior and rotational stabilities at any flexion angle. Furthermore, clinical outcomes in the two groups were not significantly different. Lachman and pivot shift test results and instrumented laxity findings were similar for the two groups at final follow up.

In this study, the authors carefully preserved the remnant injured ACL, and achieved excellent anterior stability recoveries and good rotatory stabilities. No significant intergroup difference was found in terms of intraoperative stability or clinical parameters after ACL reconstruction.

E.K. Song J.K. Seon K.D. Kang C.H. Park J.H. Yim

The purpose of this study were to evaluate early intra-operative experiences of a custom-fit total knee arthroplasty (TKA) system and to determine the precision of long leg alignment and component placement achieved using this system.

Seventeen patients underwent sagittal MRI of an arthritic knee to determine component placement for TKA from October 2010 and March 2011. Cutting guides were machined to control all intra-operative cuts, and cutting guide placements were recorded by navigation system. Radiographic parameters regarding mechanical axis changes, and inclinations of the femoral and tibial components were measured. Outcome was defined as “excellent” when values of each parameters were within ± 2°, as “acceptable” when within ± 3°, and as “outliers” when >± 3° of optimum.

The cutting guide placement was within ±2° of the target angle for inclinations of femoral and tibial components. The cutting heights were within 2mm for distal femoral and proximal tibia. Mechanical axis changed from a mean of 8.57° varus to 0.49° valgus, and mean coronal inclinations of femoral and tibial components were 89.52° and 90.12°, respectively, at last follow up visits. There were no outliers and all of them were classified as excellent. Mean sagittal inclinations of the femoral and tibial components were 1.06° and 84.56°, respectively. There were no intra-operative or acute post-operative complications.

The custom-fit TKA system system provides an effective, safe means of achieving an accurate mechanical axis and of reducing prosthetic alignment outliers. However, further long term follow-up is needed.

J.E. Schroeder L. Fliri M. Liebergall G. Richards M. Windolf

The common practice for insertion of distal locking screws of intramedullary (IM) nails is a freehand technique under fluoroscopic control. The process is technically demanding, time-consuming and afflicted to considerable radiation exposure to patient and surgical personnel. A new technique is introduced which guides the surgeon by landmarks on the X-ray projection.

18 fresh frozen human below-knee specimens (incl. soft tissue) were used. Each specimen was instrumented with an Expert Tibial Nail (Synthes GmbH, Switzerland) and was mounted on an OR-table. Two distal interlocking techniques were performed in random order using a Siemens ARCADIS C-arm system (Siemens AG, Munich, Germany). The newly developed guided technique, guides the surgeon by visible landmarks projected onto the fluoroscopy image. A computer program plans the drilling trajectory by 2D-3D conversion and provides said guiding landmarks for drilling in real-time. No additional tracking or navigation equipment is needed.

All four distal screws (2 mediolateral, 2 anteroposterior) were placed in each procedure. Operating time, number of taken X-rays and radiation time were recorded per procedure and for each single screw.

8 procedures were performed with the freehand technique and 10 with the guided technique. A 58% reduction in number of fluoroscopy shots per screw was found for the guided technique (7.4±3.4 vs. 17.6±10.3; p < 0.001). Total radiation time was 55% lower for the guided technique (17.1 ± 3.7s vs. 37.9 ± 9.1s) (p = 0.001). Operating time was shorter by 22% in the guided technique (3.2±1.2 min vs. 4.1±2.1 min p = 0.018).

In an experimental setting, the newly developed guided freehand technique has proven to markedly reduce radiation exposure when compared to the conventional freehand technique. The method enhances established clinical workflows and does not require cost intensive add-on devices or extensive training.

A newly developed simple navigated technique has proven to markedly reduce radiation exposure and time for distal locking of intramedullary nails.

D. Saragaglia M. Grimaldi B. Rubens-Duval S. Plaweski

Navigation of Uni knee arthroplasty (UKA) is not common. Usually the software includes navigation of the tibial as well as the femoral implant. In order to simplify the surgical procedure we thought that navigation of the tibial plateau alone could be a good option. Since 2005 we have been using a mobile bearing UKA of which the ancillary is based on dependent bone cuts. The tibial cut is made first and the femoral cut is automatically performed using cutting blocks inserted between the tibial cut and the distal end of the femur. Although we are satisfied with this procedure, it is not rare we have some difficulties getting the right under correction needed to get a good long-term result. The aim of this paper was to present our computer-assisted UKA technique and our preliminary radiological results in genu varum (17 cases) as well as genu valgum (6 cases) deformities.

The series was composed of 23 patients, 10 females and 13 males, aged from 63 to 88 years old (mean age: 75 +/− 8). The mean preoperative HKA (Hip-Knee-Ankle) angle was: 172.35° +/− 2.31° (167° to 176°) for the genu vara and 186.33° +/− 2.87° (182° to 189°) for the genu valga.

The goal of the navigation was to get an HKA angle of 177° +/− 2° for genu varum deformity and 183° +/− 2° for genu valgum.

We used the SURGETICS® device (PRAXIM, GRENOBLE, FRANCE) in the first six cases and the ORTHOPILOT® device (B-BRAUN-AESCULAP, TUTTLINGEN, GERMANY) in the other cases. The principles are the same for both devices. The 1rst step consists in inserting percutaneously the rigid-bodies on the distal end of the femur and on the proximal end of the tibia. Then, we locate the center of the hip by a movement of circumduction, the center of the ankle by palpating the malleoli and the center of the knee by palpating intra articular anatomic landmarks to get the HKA angle in real time. This step is probably the most important because it allows checking the reducibility of the deformity in order to avoid an over correction when inserting a mobile bearing prosthesis. The 3rd step consists in navigation of the tibial cut such as the height of the resection, the tibial slope (3 to 5° posterior tibial slope) and the varus of the implant (2 to 3°). Once the tibial cut was done, we must use the conventional ancillary to perform the femoral bone cuts (distal and chamfer). The last step consists in inserting the trial implants and checking the HKA angle and the laxity of the medial or lateral side.

We used postoperative long leg X-Rays to evaluate the accuracy of navigation and plain radiographs to evaluate the right position of the implant.

As far as genu varum deformity was concerned, the mean postoperative HKA angle was 177.23° +/− 1.64° (173°–179°). The preoperative goal was reached in 94% of the cases. Moreover, this angle could be superimposed on the peroperative computer-assisted angle, which was 177° +/− 1.43° (p>0.05). For genu valgum, the mean postoperative HKA angle was 181° +/− 1.41° (179°–183°). The preoperative goal was reached in 66% of the cases but the series is too short to give any conclusion.

The navigation of tibial plateau alone can be used with accuracy, provided one has the right ancillary to use dependent bone cuts. The procedure is quick and needs only one tibial cutting guide equipped with a rigid-body. Our results, especially in genu varum deformity, are quite remarkable. Regarding genu valgum, the results seem to be less accurate, but the software was designed for medial UKA and the series is short, so, it is too soon to extrapolate any conclusion. The main interest in this navigation is to avoid too much under correction and even better to avoid over correction when the deformity is over reducible. Indeed, when one uses a mobile bearing plateau, the risk is to have a dislocation of the meniscus. So, when tightening the collateral ligaments, checking the lower limb axis may persuade not to use a mobile bearing plateau but rather a fixed plateau.

S. Yanagimoto M. Tezuka M. Kameyama K. Inoue S. Nakayama T. Komiyama E. Okada K. Takeda Y. Fujita A. Funayama

We have used CT-based total hip navigation system from 2003, to set the acetabular socket in optimal position. At first, we had used CT-based land-mark matching system. It needs matching procedure during surgery, touching paired points in surgical exposure. From 2006, we started to use new navigation system, called CT-based fluoroscopy-matching system, which was developed by BrainLAB Company (Vector-vision 2.7.1., 3.5.1.). For this new system, pre-surgical image matching procedure is need. Fluoroscopic images with 2 different directions must be taken in operation room. Then fluoroscopic images and CT reconstructive images were matched in computer with special program. Matching procedure was done before surgical incision. We compared the advantage of these two systems about technical problem, radiation exposure, time need for procedure, and accuracy. And then we discussed how to use these two different systems for THA patients.

Accuracy was compared for 241 THA patients using these navigation systems. 152 cases were with CT-based land-mark matching system and 89 patients with CT-based fluoroscopy matching system. Final verification angle of acetabular socket setting in navigation during surgery was recorded for each case. The operative angle, which is referred from Murray, is used to show the socket setting angle (inclination and anteversion) in these navigation systems. Post-operative CT scan was taken to evaluate the actual socket setting angle. The values between verification angle during surgery and post-operative CT measured angle were calculated and compared statistically.

Results were followed. New CT-based fluoroscopy matching method (F method): Average setting angle (operative angle) of socket in these 89 cases were 42.9 +/− 5.1 degree in inclination angle, and 28.5 +/− 7.9 degree in anteversion angle. The absolute difference in 89 cases between final verification angle and post-operative CT measurement angle was 2.9 degree (on average) +/− 2.5 degree in inclination angle, and 2.8 degree (on average) +/− 2.6 degree in anteversion angle. Conventional CT-based land-mark matching method (L method): The absolute difference in 152 cases between final verification angle and post-operative CT measurement angle was 4.2 degree (on average) +/− 3.2 degree in inclination angle, and 4.4 degree (on average), +/− 3.7 degree in anteversion angle. Absolute differences of setting angle in fluoroscopy matching groups showed statistically low compared with land-mark matching groups (P<0.01).

Technical problems: L method is difficult to learn actual procedure. F method is easy to learn procedure. Image matching was done automatically by computer program. Radiation exposure during surgery: L method needs no additional radiation. F method needs radiation to get 2 fluoroscopic images. Total amount time need for navigation: L method needs extra 10 minutes during surgery in case of skilled-doctor. F method needs extra 20 minutes before starting surgery in case of all kind doctors. The accuracy of acetabular socket setting: Absolute errors in socket setting with theses two systems were within 5 degree together on average. These results showed the usefulness of both systems. Compared the accuracy between these 2 systems, F methods showed high accuracy. The accuracy of F methods is always high. It has no influence with deformity around hip joint, because fluoroscopic image matching was done with lower part of pubic bone, especially around symphysis pubis.

For ordinary THA cases with skilled-doctor, CT-based land-mark matching system is useful, because this system is very convenient and needs only extra 10 minutes during surgery. For severe deformed cases with all kind doctors, CT-based fluoroscopy matching system is useful, because this system showed high accuracy even for severe deformed cases. Before surgical incision, fluoroscopic matching procedure has finished. This system needs no extra time after surgery starts.

J.Y. Lazennec M.A. Rousseau A. Rangel V. Gozalbes S Chabane A. Brusson C Picard Y. Catonne


Recent literature points out the potential interest of standing and sitting X-rays for the evaluation of THA patients. The accuracy of the anterior pelvic plane measures is questionable due to the variations in the quality of lateral standing and sitting X-rays. The EOS® (EOS imaging, Paris, France) is an innovative slot-scanning radiograph system allowing the acquisition of radiograph images while the patient is in weightbearing position with less irradiation than standard imagers. This study reports the “functionnal” positions of a 150 THA cohort, including the lateral orientation of the cups.


The following parameters were measured: sacral slope (SS), pelvic tilt (PT), pelvic incidence (PI) and anterior pelvic plane (APP) sagittal inclination (ASI), frontal inclination (AFI) and planar anteversion (ANT). Irradiation doses were calculated in standing and sitting acquisitions. Variations of sagittal orientation of the cup were measured on lateral standing and sitting images. Descriptive and multivariate analysis were performed for the different parameters studied.

L. Paul P L. Docquier O. Cartiaux C. Delloye X. Banse

Primary malignant bone tumor often requires a surgical treatment to remove the tumor and sometimes restore the anatomy using a frozen allograft. During the removal, there is a need for a highest possible accuracy to obtain a wide safe margin from the bone tumour. In case of reconstruction using a bone allograft, an intimate and precise contact at each host-graft junction must be obtained (Enneking 2001). The conventional freehand technique does not guarantee a wide safe margin nor a satisfying reconstruction (Cartiaux 2008). The emergence of navigation systems has procured a significant improvement in accuracy (Cartiaux 2010). However, their use implies some constraints that overcome their benefits, specifically for long bones. Patient-specific cutting guides become now available for a clinical use and drastically simplify the intra-operative set-up. We present the use of pre-operative assistances to produce patient-specific cutting guides for tumor resection and allograft adjustment. We also report their use in the operative room.

We have developed technical tools to assist the surgeon during both pre-operative planning and surgery. First, the tumor extension is delineated on MRI images. These MRI images are then merged with Computed Tomography scans of the patient. The tumor and the CTscan are loaded in custom software that enables the surgeon to define target (desired) cutting planes around the tumor (Paul 2009) including a user-defined safe margin. Finally, cutting guides are designed on the virtual model of the patient as a mould of the bone surface surrounding the tumor, materialising the desired cutting planes. When required, a massive bone allograft is selected by comparing shapes of the considered patient's bone and available allografts. The resection planes are transferred onto the selected allograft and a second guide is designed for the allograft cutting. The virtually-designed cutting guides are then manufactured by a rapid prototyping machine using biocompatible material. This procedure has been used to excise a local recurrence of a tibial sarcoma and reconstruct the anatomy using a frozen tibial allograft.

The pre-operative planning using virtual models of the patient's bone, tumor and the available allografts enabled the surgeon to localise the tumor, define the desired cutting planes and select the optimal allograft. Patient- and allograft-specific guides have been designed and manufactured. A stable and accurate positioning of guide onto the patient's tibia was made easier thanks to the plate formerly put in place during the previous surgery. An accurate positioning of the allograft cutting guide has been obtained thanks to its design. The obtained reconstruction was optimal with a adjusted allograft that was perfectly fitting the bone defect. The leg alignment was also optimally restored.

Computer assistances for tumor surgery are progressively appearing. We have presented at CAOS 2010 an optical navigation system for tumor resection in the pelvis that was promising. However, such a tool is not well adapted for long bones. We have used patient-specific guides on a clinical case to assess the feasibility of the technique and check its accuracy in the resection and reconstruction. The surgeon has benefited from the 3D planning to define his strategy. He had the opportunity to select the optimal transplant for his patient and plan the same cuttings for the allograft and the patient. During the surgery, guide positioning was straightforward and accurate. The bone cuttings were very easy to perform. The use of custom guides decreases the operating time when compared to the conventional procedure since there is no need for measurements between cutting trajectories and anatomical landmarks. Furthermore, the same cutting planes were performed around the tumor and onto the allograft to obtain a transplant that optimally fills the defect. We recommend the use of such an intra-operative assistance for tumor surgery.

C. Belvedere A. Ensini D.P. Notarangelo S. Tamarri A. Feliciangeli A. Leardini

During total knee replacement (TKR), knee surgical navigation systems (KSNS) report in real time relative motion data between the tibia and the femur from the patient under anaesthesia, in order to identify best possible locations for the corresponding prosthesis components. These systems are meant to support the surgeon for achieving the best possible replication of natural knee motion, compatible with the prosthesis design and the joint status, in the hope that this kinematics under passive condition will be then the same during the daily living activities of the patient. Particularly, by means of KSNS, knee kinematics is tracked in the original arthritic joint at the beginning of the operation, intra-operatively after adjustments of bone cuts and trial components implantation, and after final components implantation and cementation. Rarely the extent to which the kinematics in the latter condition is then replicated during activity is analysed. As for the assessment of the active motion performance, the most accurate technique for the in-vivo measurements of replaced joint kinematics is three-dimensional video-fluoroscopy. This allows joint motion tracking under typical movements and loads of daily living. The general aim of this study is assessing the capability of the current KSNS to predict replaced joint motion after TKR. Particularly, the specific objective is to compare, for a number of patients implanted with two different TKR prosthesis component designs, knee kinematics obtained intra-operatively after final component implantation measured by means of KSNS with that assessed post-operatively at the follow-up by means of three-dimensional video-fluoroscopy.

Thirty-one patients affected by primary gonarthrosis were implanted with a fixed bearing posterior-stabilized TKR design, either the Journey® (JOU; Smith&Nephew, London, UK) or the NRG® (Stryker®-Orthopaedics, Mahwah, NJ-USA). All implantations were performed by means of a KSNS (Stryker®-Leibinger, Freiburg, Germany), utilised to track and store joint kinematics intra-operatively immediately after final component implantation (INTRA-OP). Six months after TKR, the patients were followed for clinical assessment and three-dimensional video fluoroscopy (POST-OP). Fifteen of these patients, 8 with the JOU and 7 with the NRG, gave informed consent and these were analyzed. At surgery (INTRA-OP), a spatial tracker of the navigation system was attached through two bi-cortical 3 mm thick Kirschner wires to the distal femur and another to the proximal tibia. The conventional navigation procedure recommended in the system manual was performed to calculate the preoperative deformity including the preoperative lower limb alignment, to perform the femoral and tibial bone cuts, and to measure the final lower limb alignment. All these assessment were calculated with respect to the initial anatomical survey, the latter being based on calibrations of anatomical landmarks by an instrumented pointer. Patients were then analysed (POST-OP) by three-dimensional video-fluoroscopy (digital remote-controlled diagnostic Alpha90SX16; CAT Medical System, Rome-Italy) at 10 frames per second during chair rising-sitting, stair climbing, and step up-down. A technique based on CAD-model shape matching was utilised for obtaining three-dimensional pose of the prosthesis components. Between the two techniques, the kinematics variables analysed for the comparison were the three components of the joint rotation (being the relative motion between the tibial and femoral components represented using a standard joint convention, the translation of the line through the medial and lateral contact points (being these points assumed to be where the minimum distance between the femoral condyles and the tibial baseplate is observed) on the tibial baseplate and the corresponding pivot point, and the location of the instantaneous helical axes with the corresponding mean helical axis and pivot point.

In all patients and in both conditions, physiological ranges of flexion (from −5° to 120°), and ab-adduction (±5°) were observed. Internal-external rotation patterns are different between the two prostheses, with a more central pivoting in NRG and medial pivoting in JOU, as expected by the design. Restoration of knee joint normal kinematics was demonstrated also by the coupling of the internal rotation with flexion, as well as by the roll-back and screw-home mechanisms, observed somehow both in INTRA- and POST-OP measurements. Location of the mean helical axis and pivot point, both from the contact lines and helical axes, were very consistent over time, i.e. after six months from intervention and in fully different conditions. Only one JOU and one NRG patient had the pivot point location POST-OP different from that INTRA-OP, despite cases of paradoxical translation.

In all TKR knees analysed, a good restoration of normal joint motion was observed, both during operation and at the follow-up. This supports the general efficacy of the surgery and of both prosthesis designs. Particularly, the results here reported show a good consistency of the measurements over time, no matter these were taken in very different joint conditions and by means of very different techniques. Intra-operative kinematics therefore does matter, and must be taken into careful consideration for the implantation of the prosthesis components. Joint kinematics should be tracked accurately during TKR surgery, and for this purpose KSNS seem to offer a very good support. These systems not only supports in real time the best possible alignment of the prosthesis components, but also make a reliable prediction of the motion performance of the replaced joint. Additional analyses will be necessary to support this with a statistical power, and to identify the most predicting parameters among the many kinematics variables here analysed preliminarily.

J.G. Gerbers P.C. Jutte

Adamantinoma are rare, low grade malignant, bone tumors, making up only 0.1–0.48 percent of primary malignant bone tumors. They occur predominantly in the long bones, especially the tibia. Histogenetically it is thought that it originates from embryological displacement of basal epithelium of the skin, although other hypotheses have been proposed. Clinically most patients present with swelling and possible bending of the tibia, painful or painless. It's often noticed in an earlier stadium, but symptoms are non-specific and have a slow progressive character. Median patient age is 25 to 35 years, with a range from two to 86 years. It is slightly more common in men than woman, with a ratio of 5:4. Occurrence in children is even rarer. A study by Van Rijn et al. finds only 119 references, and presents six more cases. Treatment is the same. An MRI-scan should be performed to check for metastasis, loco regional staging and for operative planning. Operative excision and reconstruction is necessary to prevent metastasis and maintain load bearing capacity.

Generally these resections and reconstructions are done without objective measurements. The surgeon uses a rule of thumb, like a sculptor, or ruler approach to recreate the excised bone, either with allo- or autograft materials. An optimal fit, i.e. a minimal space between tibia and graft, is not always achieved, possibly resulting in pathological fractures.

This risk of pathological fractures lengthens recovery time. The fractures elongate hospitalization time and recovery time and are a heavy burden to patients. Computer assisted surgery (CAS) systems, used for example in prosthesis placement, offer objective measurements in 3d space of hard structures with high accuracy. These can be used to produce an accurate copy of the resected bone. If the reconstruction accurately fits the bone defect that's left after the resection, it's likely that the occurrence of pathological fractures decreases.

An adamantinoma in the tibia of a 12 year old boy was treated. Surgery consisted of hemicortical resection and inlay allograft reconstruction. The software used was the Orthomap navigation software (Stryker). A donor bone was supplied with help from the bone bank. The technical approach to the reconstruction was the planning of resection planes around the tumor. As the CT scale for both the patient and allograft bone is the same, the resection planes in the patient navigation setup could be copied to the allograft creation setup. Normal CAS setup was performed after first incision, with a tracker attached to the tibia. It was planned that a navigated bone saw would be used for the cutting. The tracker was attached to the saw with a new attachment, and calibrated in the universal calibration tool. During the surgery the oscillating saw proved to be impossible to navigate. The instrument calibration module was not able to accurately registered the saw, this despite accurate registrations in pre-operative testing. The CAS system was used however for accurately determining the saw planes. The planes were traced with the pointer tool. Then a non-navigated saw was used to perform both trapezoid shaped resections. A similar CAS setup was performed on the donor bone.

The reconstruction was a good fit. The skin was closed in layers. Post-operative x-ray control was performed. Operation time was just over two hours. Currently the follow-up time is five months. There have been no complications and the control x-rays show good allograft ingrowth.

While the original operation plan couldn't be performed the principle of computer assisted reconstruction has its merits. This was a proof of concept. The navigation was accurate to less than 1 mm, and the trapezoid resection shape guarantees a good fit. However the method of resection of the drawn planes by non-navigated bone saw was not accurate enough, because of the saw oscillations. There was improvement in operation time. With more accurate means of resection, as for example a computer controlled laser or water-jet, this type of reconstruction could have other very interesting applications.

V. D'Angeli A. Visentini C. Belvedere A. Leardini M. Romagnoli S. Giannini

Restoration of natural range and pattern of motion is the primary goal of joint replacement. In total ankle replacement, proper implant positioning is a major requirement to achieve good clinical results and to prevent instability, aseptic loosening, meniscal bearing premature wear and dislocation at the replaced ankle. The current operative techniques support limitedly the surgeon in achieving a best possible prosthetic component alignment and in assessing proper restoration of ligament natural tensioning, which could be well aided by computer-assisted surgical systems. Therefore the outcome of this replacement is, at present, mainly associated to surgeon's experience and visual inspection. In some of the current ankle prosthetic designs, tibial component positioning along the anterior/posterior (A/P) and medio/lateral axes is critical, particularly in those designs not with a flat articulation between the tibial and the meniscal or talar components. The general aim of this study was assessing in-vitro the effects of the A/P malpositioning of the tibial component on three-dimensional kinematics of the replaced joint and on tensioning of the calcaneofibular (CaFiL) and tibiocalcaneal (TiCaL) ligaments, during passive flexion. Particularly, the specific objective is to compare the intact ankle kinematics with that measured after prosthesis component implantation over a series of different positions of the tibial component.

Four fresh-frozen specimens from amputation were analysed before and after implantation of an original convex-tibia fully-congruent three-component design of ankle replacement (Box Ankle, Finsbury Orthopaedics, UK). Each specimen included the intact tibia, fibula and ankle joint complex, completed with entire joint capsule, ligaments, muscular structures and skin. The subtalar joint was fixed with a pin protruding from the calcaneus for isolating tibiotalar joint motion. A rig was used to move the ankle joint complex along its full range of flexion while applying minimum load, i.e. passive motion. In these conditions, motion at the ankle was constrained only by the articular surfaces and the ligaments. A stereofotogrammetric system for surgical navigation (Stryker-Leibinger, Freiburg, Germany) was used to track the movement of the talus/calcaneus and tibial segments, by using trackers instrumented with five active markers. Anatomical based kinematics was obtained after digitization by an instrumented pointer of a number of anatomical landmarks and by a standard joint convention. The central point of the attachment areas of CaFiL e TiCaL was also digitised. Passive motion and ankle joint neutral position were acquired, and the standard operative technique was performed to prepare the bones for prosthesis component implantation. The final component for the talus was implanted, the tibial component was initially positioned well in front of the nominal right (NR) position, the meniscal bearing was instrumented with an additional special tracker, and passive motion was collected again in passive flexion. Data collection was repeated for progressively more posterior locations for the tibial component, for a total of six different locations along the tibial A/P axis: three anterior (PA), the NR, and two more posterior (PP), approximately 3 to 5 mm far apart each. The following three-dimensional kinematics variables were analyzed: the three anatomical components of the ankle joint (talus-to-tibial) rotation (dorsi/plantar flexion, prono/supination and internal/external rotation respectively in the sagittal, frontal and transverse planes), the meniscal bearing pose with respect to the talar and tibial components, the ‘ligament effective length fraction’ as the ratio between the instantaneous distance between the ligament attachment points and the corresponding maximum distance, and the instantaneous and mean helical axes in the tibial anatomical reference frame.

In all specimens and in all conditions, physiological ranges of flexion, prono/supination and internal/external rotation were observed at the ankle joint. A good restoration of motion was observed at the replaced joint, demonstrated also by the coupling between axial rotation and flexion and the physiological location of the mean helical axis, in all specimens and in most of the component positions. Larger plantar- and smaller dorsi-flexion were observed when the tibial component was positioned more anteriorly than NR, and the opposite occurred for more posterior positions. In regards to the meniscal bearing, rotations were small and followed approximately the same patterns of the ankle rotations, accounted for the full conformity of the articulating surfaces. Translations in A/P were larger than in other directions, the bearing moving backward in plantarflexion and forward in dorsiflexion with respect to both components. It was observed that the closer to NR the position of the tibial component is, the larger this A/P motion is, accounted mainly to the associated larger range of flexion. The change of CaFiL and TiCaL effective length fraction over the flexion arc was found smaller than 0.1 in three specimens, smaller than 0.2 in the fourth, larger both in more anterior and more posterior locations of the tibial component. The simulated malpositioning did not affect much position and orientation of the mean helical axis in both the transversal and frontal planes.

The experimental protocol and measurements were appropriate to achieve the proposed goals. All kinematics variables support the conclusion that the ankle replaced with this original prosthesis behaves as predicted by the relevant computer models, i.e. physiological joint motion and ligament tension is experienced resulting in a considerable A/P motion of the meniscal bearing. These observations are particularly true in the NR postion for the prosthesis, but are somehow correct also in most of the tibial malpositions analysed, in particular those on the back.

E.K. Song J.K. Seon K.D. Kang C.H. Park J.H. Yim

The preoperative prediction of gap balance after robotic total knee arthroplasty (TKA) is difficult. The purpose of this study was to evaluate the effectiveness of a new method of achieving balanced flexion-extension gaps during robotic TKA.

Fifty one osteoarthritic patients undergoing cruciate retaining TKA using robotic system were included in this prospective study. Preoperative planning was based on the amount of lateral laxity in extension and flexion using varus stress radiograph. After complete milling by the robot and soft tissue balancing, intra-operative extension and flexion gaps were measured using a tensioning device. Knees were subdivided into three groups based on lateral laxities in 0° and 90° of flexion, as follows; the tight extension group (≥ 2mm smaller in extension than flexion laxity), the tight flexion group (≥ 2mm smaller in flexion than extension laxity), and the balanced group (< 2mm difference between laxities). In addition, intra-operative gap balance results were classified as acceptable (0–3mm larger in flexion than in extension), tight (larger in extension than in flexion) or loose (> 3mm larger in flexion than in extension) based on differences between extension and flexion gaps.

During preoperative planning, 34 cases were allocated to the balanced group, 16 to the tight extension group and 1 case was allocated to the tight flexion group. Intra-operative gap balance was acceptable in 46 cases, 4 cases had a tight result, and one case had a loose flexion gap.

We concluded that preoperative planning based on the amount of lateral laxity determined using varus stress radiographs may be useful for predicting intraoperative gap balance and help to achieve precise gap balance during robotic TKA.

J.Y. Jenny R.K. Miehlke D. Saragaglia

Polyethylene wear is one of the reasons for failure of total knee replacement (TKR). There are several reasons for wear, and the femoro-tibial contact area is an important factor. Mobile bearing, highly congruent prostheses might be more resistant to polyethylene wear than fixed bearing, incongruent prostheses. We evaluated the five- to eight-year experience of three university departments by using an original system with following highlights: implantation with a navigation system, extended congruency up to 90° of flexion, floating polyethylene component with non-limited movements of rotation, antero-posterior translation and medio-lateral translation.

347 patients have been operated on in the three participating departments with this new prosthesis system between 2001 and 2004, and have been prospectively followed with clinical and radiologic examination with a minimal follow-up time of five years. There were 246 women and 101 men, with a mean age of 67 years.

Clinical and functional results have been analyzed according to the Knee Society scoring system. Accuracy of implantation has been assessed on post-operative long leg antero-posterior and lateral X-rays. Survival rate up to eight years has been calculated according to Kaplan and Meier, with mechanical revision or any revision as end-points.

Complete patient history was obtained by 319 cases (92%). The mean clinical score was 93 points. The mean pain score was 47 points. The mean flexion angle was 118°. The mean functional score was 87 points. An optimal correction of the coronal femoro-tibial axis was obtained in 94% of the cases. Survival rate after eight years was 98.8% for mechanical revisions and 95.5% for all revisions.

We confirmed the influence of the navigation system on the accuracy of implantation. The clinical and functional results after five to eight years are in line with the better results of the current literature after conventional implantation of non-congruent prostheses. The survival rate is comparable to the current standards. The influence of the design on polyethylene wear will need a longer follow-up.

F. Schmidt M. Asseln J. Eschweiler P. Belei K. Radermacher

The alignment of prostheses components has a major impact on the longevity of total knee protheses as it significantly influences the biomechanics and thus also the load distribution in the knee joint.

Knee joint loads depend on three factors: (1) geometrical conditions such as bone geometry and implant position/orientation, (2) passive structures such as ligaments and tendons as well as passive mechanical properties of muscles, and (3) active structures that are muscles. The complex correlation between implant position and clinical outcome of TKA and later in vivo joint loading after TKA has been investigated since 1977. These investigations predominantly focused on component alignment relative to the mechanical leg axis (Mikulicz-line) and more recently on rotational alignment perpendicular to the mechanical axis. In general four different approaches can be used to study the relationship between implant position and knee joint loads: In anatomical studies (1), the influence of the geometrical conditions and passive structures can be analyzed under the constraint that the properties of vital tissue are only approximated. This could be overcome with an intraoperative load measurement approach (2). Though, this set up does not consider the influence of active structures. Although post-operative in vivo load measurements (3) provide information about the actual loading condition including the influence of active structures, this method is not applicable to investigate the influence of different implant positions. Using mathematical approaches (4) including finite element analysis and multi-body-modeling, prostheses positions can be varied freely. However, there exists no systematical analysis of the influence of prosthesis alignment on knee loading conditions not only in axial alignment along and rotational alignment perpendicular to the mechanical axis but in all six degrees of freedom (DOF) with a validated mathematical model. Our goal was therefore to investigate the correlation between implant position and joint load in all six DOF using an adaptable biomechanical multi-body model.

A model for the simulation of static single leg stance was implemented as an approximation of the phase with the highest load during walking cycle. This model is based on the AnyBody simulation software (AnyBody Technology A/S, Denmark). As an initial approach, with regard to the simulation of purely static loading the knee joint was implemented as hinge joint. The patella was realised as a deflection point, a so called “ViaNode,” for the quadriceps femoris muscle. All muscles were implemented based on Hill's muscle model. The knee model was indirectly validated by comparison of the simulation results for single and also double leg stance with in-vivo measurements from the Orthoload database (www.orthoload.de). For the investigation of the correlation between implant position and knee load, major boundary conditions were chosen as follows:

Flexion angle was set to 20° corresponding to the position with the highest muscle activity during gait cycle.

Muscle lengths and thereby also muscle loads were adapted to the geometrical changes after each simulation step representing the situation after post-operative rehabilitation. As input parameters, the tibial and femoral components' positions were independently translated in a range of ±20mm in 10 equally distant steps for all three spatial directions. For the rotational alignment in adduction/abduction as well as flexion/extension the tibial and femoral components' positions were varied in the range of ±15° and for internal/external rotation within the range of ±20°, also in 10 equally angled steps. Changes in knee joint forces and torques as well as in patellar forces were recorded and compared to results of previous studies.

Comparing the simulation results of single and double leg stance with the in-vivo measurements from the Orthoload database, changes in knee joint forces showed similar trends and the slope of changes in torques transmitted by the joint was equal. Against the background of unknown geometrical conditions in the Orthoload measurements and the simplification (hinge joint) of the initial multi-body-model compared to real knee joints, the developed model provides a reasonable basis for further investigations already – and will be refined in future works.

As influencing parameters are very complex, a non-ambiguous interpretation of force/torque changes in the knee joint as a function of changes in component positions was in many cases hardly possible. Changes in patella force on the other hand could be traced back to geometrical and force changes in the quadriceps femoris muscle. Positional changes mostly were in good agreement with our hypotheses based on literature data when knee load and patellar forces respectively were primarily influenced by active structures, e.g. with regard to the danger of patella luxation in case of increased internal rotation of the tibial component. Whereas simulations also showed results contradicting our expectations for positional changes mainly affecting passive structures, e.g. cranial/caudal translation of the femoral component. This shows the major drawback of the implemented model: Intra-articular passive structures such as cruciate and collateral ligaments were not represented. Additionally kinematic influences on knee and patella loading were not taken into account as the simulations were made under static conditions. Implementation of relative movements of femoral, tibial and patella components and simulation under dynamic conditions might overcome this limitation. Furthermore, the boundary condition of complete muscle adaptations might be critical, as joint loads might be significantly higher shortly after operation. This could lead to a much longer and possibly ineffective rehabilitation process.

J.Y. Jenny L. Wasser

We wanted to assess the possible correlation between the intra-operative kinematics of the knee and the clinical results after total knee replacement (TKR).

187 cases of TKR implanted with help of a navigation system for end-stage osteoarthritis have been prospectively analyzed. There were 127 women and 60 men, with a mean age of 71 years. Indication for TKR was osteoarthritis in 161 cases and inflammatory arthritis in 26 cases.

A floating platform, PCL preserving, cemented TKR was implanted in all cases. A non-image based navigation system was used in all cases to help for accuracy of bone resections and ligamentous balancing. The standard navigation system was modified to allow recording the three-dimensional tibio-femoral movement during passive knee flexion during the surgical procedure. Two sets of records have been performed: before any intra-articular procedure and after final implantation. Only antero-posterior femoral translation (in mm) and internal-external femoral rotation (in degrees) have been recorded. Kinematic data have been analyzed in a quantitative manner (total amount of displacement) and in a qualitative manner (restoration of the physiological posterior femoral translation and femoral external rotation during knee flexion). Clinical and functional results have been analysed according to the Knee Society scoring system with a minimal follow-up of one year. Statistical links between kinematic data and Knee Society scores have been analysed with an ANOVA test and a Spearman correlation test at a 0.05 level of significance.

101 knees had a posterior femoral translation during flexion before and after TKR. 18 knees had a paradoxical anterior femoral translation during flexion before and after TKR. 51 knees had the pre-TKR paradoxical anterior femoral translation corrected to posterior femoral translation after TKR. 14 knees had the pre-TKR posterior femoral translation modified to a paradoxical anterior femoral translation after TKR. 91 knees had a femoral external rotation during flexion before and after TKR. 34 knees had a paradoxical femoral internal rotation during flexion before and after TKR. 50 knees had the pre-TKR paradoxical femoral internal rotation corrected to a femoral external rotation after TKR. Nine knees had the pre-TKR femoral external rotation modified to a paradoxical femoral internal rotation after TKR. There was a moderate statistical link between the reconstruction of a physiological kinematics after TKR and the Knee Society scores, with higher scores in the group of physiological kinematics after reconstruction. There was no correlation between the quantitative data and the Knee Society scores.

To record the knee kinematics during TKR is feasible. This information might help the surgeon choosing the optimal reconstruction compromise. However, it is not well defined how to influence final kinematics during knee replacement. The exact influence of the quality of the kinematic reconstruction measured during surgery on the clinical and functional results has to be investigated more extensively.

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J. Chaoui G. Moineau E. Stindel C. Hamitouche P. Boileau

For any image guided surgery, independently of the technique which is used (navigation, templates, robotics), it is necessary to get a 3D bone surface model from CT or MR images. Such model is used for planning, registration and visualization. We report that graphical representation of patient bony structure and the surgical tools, inter-connectively with the tracking device and patient-to-image registration, are crucial components in such system. For Total Shoulder Arthroplasty (TSA), there are many challenges. The most of cases that we are working with are pathological cases such as rheumatoid arthritis, osteoarthritis disease. The CT images of these cases often show a fusion area between the glenoid cavity and the humeral head. They also show severe deformations of the humeral head surface that result in a loss of contours. These fusion area and image quality problems are also amplified by well-known CT-scan artefacts like beam-hardening or partial volume effects. The state of the art shows that several segmentation techniques, applied to CT-Scans of the shoulder, have already been disclosed. Unfortunately, their performances, when used on pathological data, are quite poor.

In severe cases, bone-on-bone arthritis may lead to erosion-wearing away of the bone. Shoulder replacement surgery, also called shoulder arthroplasty, is a successful, pain-relieving option for many people. During the procedure, the humeral head and the glenoid bone are replaced with metal and plastic components to alleviate pain and improve function. This surgical procedure is very difficult and limited to expert centres. The two main problems are the minimal surgical incision and limited access to the operated structures. The success of such procedure is related to optimal prosthesis positioning. For TSA, separating the humeral head in the 3D scanner images would allow enhancing the vision field for the surgeon on the glenoid surface. So far, none of the existing systems or software packages makes it possible to obtain such 3D surface model automatically from CT images and this is probably one of the reasons for very limited success of Computer Assisted Orthopaedic Surgery (CAOS) applications for shoulder surgery. This kind of application often has been limited due to CT-image segmentation for severe pathologic cases and patient to image registration.

The aim of this paper is to present a new image guided planning software based on CT scan of the patient and using bony structure recognition, morphological and anatomical analysis for the operated region. Volumetric preoperative CT datasets have been used to derive a surface model shape of the shoulder. The proposed planning software could be used with a conventional localisation system, which locates in 3D and in real time position and orientation for surgical tools using passive markers associated to rigid bodies that will be fixed on the patient bone and on the surgical instruments.

20 series of patients aged from 42 years to 91 years (mean age of 71 years) were analysed. The first step of this planning software is fully automatic segmentation method based on 3D shape recognition algorithms applied to each object detected in the volume. The second step is a specific processing that only treats the region between the humerus and the glenoid surface in order to separate possible contact areas. The third step is a full morphological analysis of anatomical structure of the bone. The glenoid surface and the glenoid vault are detected and a 3D version and inclination angle of the glenoid surface are computed. These parameters are very important to define an optimal path for drilling and reaming glenoid surface. The surgeon can easily modify the position of the implant in 3D aided by 3D and 2D view of the patient anatomy. The glenoid version/inclination angle and the glenoid vault are computed for each postion in real time to help the surgeon to evaluate the implant position and orientation.

In summary, preoperative planning, 3D CT modelling and intraoperative tracking produced improved accuracy of glenoid implantation. The current paper has presented new planning software in the world of image guided surgery focused on shoulder arthroplasty. Within our approach, we propose, to use pattern recognition instead of manual picking of landmarks to avoid user intervention, in addition to potentially reducing the procedure time. A very important role is played by 3D data sets to visualise specific anatomical structures of the patient. The automatic segmentation of arthritic joints with bone recognition is intended to form a solid basis for the registration. The results of this methodology were tested on arthritic patients to prove that it is not just easy and fast to perform but also very accurate so it realises all conditions for the clinical use in OR.

J.K. Bow M. Kunz J.F. Rudan G.C.A. Wood R.E. Ellis

Hip Resurfacing Arthroplasty (HRA) is a surgical technique that has become more popular in recent years for the treatment of hip osteoarthritis in young patients. For these patients, an HRA offers the advantages of preserving the physiologic anatomy of a patient's femoral head size and neck offset, which has been theoretically suggested to improve range of motion and muscle function, as well as preserving bone stock for future revision surgeries. Although the improvements in quality of life outcomes in patients undergoing total hip arthroplasty (THA) are well-documented, there is a lack of literature documenting the improvements in quality of life in patients undergoing HRA.


One hundred and four consecutive patients presenting for elective HRA at our institution were recruited between 2004 and 2008 for participation in this study, which was approved by the Ethics Review Board at our institution. The mean age was 51±6y, male:female ratio 79:24 and mean BMI of 29.7±4.4 Preoperative computed tomography (CT) scans were used to preoperatively plan each procedure, and intraoperative procedures were performed using individualized templates [Kunz M, Rudan JF, Xenoyannis GL, Ellis RE. Computer assisted hip resurfacing using individualized drill templates. J Arthroplasty 2010;25(4):600–6]. Surgery time was 90±28 min including time for intraoperative verification of templating accuracy. Mobilization with physiotherapy began within 24 hrs of surgery and continued until the patient was discharged, usually within 2–3 days of surgery. Each patient completed the modified Harris Hip Score (HHS), the UCLA activity rating, the SF-36 mental and physical health score and the Western Ontario and McMaster University Osteoarthritis Index (WOMAC) questionnaires at their preoperative appointment, then at 6 months, 1 year and 2 years postoperative. In addition, radiographs were taken at these appointments to confirm component position, and to check for signs of loosening or heterotopic ossification. Chi-square and t-tests were used for within and between group comparisons on selected variables and across times.


Only four patients required revision to THA, with one case of avascular necrosis of the femoral head, one femoral neck fracture and two infections.

The mean of the preoperative modified Harris Hip Scores was 51±19.7 with a significant improvement in the mean score at 6 months, 1 and 2 years postoperative (p<0.01). The preoperative UCLA activity index averaged 4 (range 2–9), improving to a mean of 6 at 6 months (p<0.001) then at 1 to 2 years to 7 (p<0.001). Mental state and further assessment of physical function were performed using the SF-36 scores, with the physical score initially 27.5 and improving to 45.2 after 2 years (p<0.01). The mental component score (MCS) means were almost unchanged, from 50.3 preoperatively to 51.5 after 2 years (p<0.21). Further data processing showed that patients who began with a below-average mental score also had significantly worse WOMAC scores for pain, stiffness and function; these patient showed a significantly higher MCS at 2 years (p<0.05). Those whose MCS were above average preoperatively showed little difference after 2 years.

J. Windley D. Nathwani

Unicompartmental knee replacement (UKR) is technically challenging, but has the advantage over total knee replacement (TKR) of conserving bone and ligaments, preserving knee range of movement and stability. Computer navigation allows for accurate placement of the components, important for preventing failures secondary to mal-alignment. Evidence suggests an increase in failure rates beyond 3 degrees of coronal mal-alignment.

Our previous work has shown superior functional scores in those patients having undergone UKR, when compared with those having had TKR. However, to a certain extent, this is likely to be due to differences in the two cohorts. Those selected for UKRs are likely to be younger, with less advanced and less widespread degenerative disease. It is almost inevitable, therefore, that functional outcomes will be superior. We aimed to compare the functional and radiological outcomes of UKR vs TKR in a more matched population.

Ninety-two patients having had one hundred consecutive computer navigated UKRs were reviewed both clinically and radiographically. The Smith & Nephew Accuris fixed-bearing modular prosthesis was used in all cases, with the ‘Brainlab’ navigation system. For our comparative group we identified patients who had actually undergone navigated TKR several years ago, but who, in retrospect, would have now been offered a UKR in line with our current practice. These patients were identified following review of pre-operative radiographs and operation notes, confirming degenerative disease confined mainly to one compartment of the knee, in the absence of any concern as to the integrity of the anterior cruciate ligament. This sub-group of patients were also assessed clinically and radiographically. Mean follow-up for the UKR group was 25 months, (range 8–45.) For our TKR sub-group, nineteen patients were identified. Average length of stay for the UKR group was 3.7 days, (range 2 to 7,) and for the TKR group this was 5.2 days, (range 3 to 10.)

Functional scores (Oxford Knee Score) were good to excellent for the majority of patients in both groups, although they were significantly better in the UKR group. Mean Oxford Knee Score in the UKR group was 7.5, (0–48, with 0 being best.) Mean score in the TKR sub-group was 12.1. (p = 0.02)

Reliably comparing TKR with UKR is difficult, due to the fundamental differences in the two groups. We have endeavoured to match these two cohorts as best possible, in order to compare the outcomes of both. Our use of computer navigation in both groups allows for accurate prosthesis placement. When measuring component position, there were no ‘outliers,’ outside of the widely accepted three degrees of deviation.

We propose that, with the correct patient selection, UKR gives a better functional outcome than TKR. Longer-term follow-up of our UKR group is required to monitor the onset of progressive arthrosis in other joint compartments, although our early results are very encouraging. Furthermore, we advocate the use of computer navigation to firstly allow for more accurate component positioning, and secondly to make challenging UKR surgery less technically demanding.

J. Windley S. Ball D. Nathwani

Computer navigation has the potential to revolutionise orthopaedic surgery, although according to the latest 7th Annual NJR Report, only 2% of the 5 800 unicompartmental knee replacements (UKRs) performed in 2009 were carried out using ‘image guidance.’ The report also states an average 3-year revision rate for UKRs of 6.5%. Previous NJR data has shown that this figure rises up to 12% for certain types of prosthesis. We suspect that a significant proportion of these revisions are due to failure secondary to component malpositioning. We therefore propose that the use of computer navigation enables a more accurate prosthesis placement, leading to a reduction in the revision rate for early failure secondary to component malpositioning. Our early results of one hundred consecutive computer navigated UKRs are presented and discussed.

Ninety-two patients having had one hundred consecutive computer navigated UKRs were reviewed both clinically and radiographically. The Smith & Nephew Accuris fixed-bearing modular prosthesis was used in all cases, with the ‘Brainlab’ navigation system. Pre-operative aim was neutral tibial cut with three degrees posterior slope. Post-operative component alignment was measured with PACs web measuring tools. Patients were scored clinically using the Oxford Knee Score.

Our patient cohort includes 54 male knees and 46 female knees. Average age is 66.6yrs. Average length of stay was 3.7 days, (range 2–7.) With respect to the tibial component, average alignment was 0.7° varus, and 2.32° posterior slope. All components were within the acceptable 3 degrees deviation. Functional scores are very satisfactory, with an overall patient satisfaction rate of 97%.

To date, only one UKR has required revision. This was due to ongoing medial pain due to medial overhang, not related to computer navigation. There was one superficial infection, with full resolution following a superficial surgical washout, debridement and antibiotics. Unlike complications reported in the NJR, we report no peri-prosthetic fractures or patella tendon injuries.

Our results demonstrate accurate prosthesis placement with the use of computer navigation. Furthermore, clinical scores are highly satisfactory. Our current revision rate is 1% at a mean of 27 months post-op. Although longer-term follow-up of our group is required, our results compare very favourably to statistics published in the NJR, (average 3-year revision rate 6.5%.) The only major differences appear to be the type of prosthesis used and the use of computer navigation. It is our proposal that computer navigation reduces the number of revisions required due to component malpositioning and subsequent failure. Furthermore, we believe that this challenging surgery is made easier with the use of computer navigation. We expect our longer-term results to show significant benefits of computer navigation over conventional techniques.

E.M. Suero J.C. Rozell M.L. Inra M.B. Cross A.S. Ranawat A.D. Pearle

Unicompartmental knee replacement (UKR) has good outcomes for the treatment of compartmental osteoarthritis of the knee. Mechanical alignment overcorrection is associated with early failure of the femoral and tibial components. Preoperative mechanical alignment is the most important predictor of postoperative alignment. However, most studies do not take into consideration the magnitude of preoperative deformity when reporting on mechanical alignment outcomes after UKR.

We aimed to determine the magnitude of postoperative mechanical alignment achieved based on the magnitude of preoperative alignment; and to compare the number of cases of overcorrection into valgus to historical data.

This was a radiographic review of patients who underwent robotic medial UKR by a single surgeon between 2007 and 2011. Two examiners measured pre- and postoperative mechanical alignment for all patients on long-leg radiographs. Patients were classified into three groups of preoperative mechanical alignment: mild varus (0–5®); moderate varus (5–10®); and severe varus (>10®). Patients with valgus alignment (<0®) were excluded. Linear regression was used to estimate the magnitude of postoperative alignment for each group, adjusting for age, BMI, gender, side, implant type, and polyethylene thickness.

89 patients were included. Mean preoperative alignment was 7.3® varus (95% CI = 6.6®–8®; range, 0.1–15® varus). Mean postoperative alignment was 2.8® varus (95% CI = 1.9®–3.8®; range, 1.4® valgus–9.7® varus). There was a significant difference in postoperative mechanical alignment between the three groups (Table 1) (P<0.05). Four overcorrections (4.5%) were detected, all under 1.5® valgus. This percentage of overcorrection was significantly better than previous conventional UKR reports (mean = 12.6%; P = 0.04).

The magnitude of postoperative alignment in medial UKR depends on the severity of the preoperative deformity. Reports on radiographic outcomes of UKR should be stratified by the magnitude of preoperative alignment. The risk of overcorrection is reduced when using robotic assistance compared to using the conventional manual technique.

V.R. Hofbauer T. Bittrich J. Glasbrenner M. Schulze M. Burger T. Zantop D. Rosenbaum A. Ruebberdt M.J. Raschke

The medial patellofemoral ligament (MPFL) has been recognised as the most important medial structure preventing lateral dislocation or subluxation of the patella (LeGrand 2007). After MPFL rupture the patella deviates from the optimal path resulting in an altered retropatellar pressure distribution. This may lead to an early degeneration with loss of function and need for endoprosthetic joint replacement. The goal of this study was to develop a dynamic knee-simulator to test the influence of ligament instabilities to patella-tracking under simulation of physiological quadriceps muscle loading.

On 10 fresh-frozen cadaveric knees the quadriceps muscle was divided into five parts along their anatomic fibre orientation analogous to Farahmand 1998. The muscular loading was achieved by applying weights to each of the fife components in proportion to the cross sectional muscle area. A total of 175 N was connected to the muscles using modified industrial cable connecting systems [Lancier Calbe, Drensteinfurt/Germany].

A novel light digital patellar reference base (DRB) was developed and attached to the patella with four bone screws. On addition a femoral and tibial digital reference base was constructed and secured to these two bones.

Position data of the patella, the femur and tibia was tracked by a conventional tracking system [Optotrak, NDI Europe]. The relative movement between femur and tibia (“flexion path”) and patella and femur (“patella tracking”) was recorded. For retropatellar pressure measurement a custom made sensor was introduced between the patella and femur [Pliance, Novel/Germany]. The sensor consists of 85 single pressure measuring cells. The robot-control-unit is liked to a force-torque sensor (hybrid method). The force free knee-flexion-path from 0° to 90° was calculated during three “passive path” measurements using this hybrid robotic method. The actual measurements followed with identical parameters.

The 3D-patella position was recorded (six degrees of freedom) along with the corresponding retropatellar pressure distribution according to knee-flexion and medial forces (intact vs. cut MPFL). Measurements were performed for the intact knee (“native”), with muscular quadriceps loading, after opening the joint capsule and with introduced pressure sensor to differentiate each of their influences.

The load free knee-flexion-path (“passive path”) could be calculated for all of the ten knees and was utilised as the basis for all dynamic measurements. There was no alteration of the “flexion-path”. Thus the measurements were only influenced by the variables “capsular joint opening,” “muscular quadriceps loading” and “MPFL-tension”.

The custom made connections between the five quadriceps components and weights proved to be a secure way to prevent rupture due to the applied forces of up to 70 N during the average measuring time of 7.5 h/knee. Only on one knee the Vastus lateralis obliquus muscle ruptured proximally. All reference bases were 100% visible despite the knee flexion form 0°–90°. No loosening of the reference base screws occurred.

Overall the combination of a robotic-assisted, force free dynamic knee-flexion under quadriceps simulation and 3D-patella-tracking seems to be a promising method to evaluate the biomechanical influences of ligaments on the human knee.

V. Dubois-Ferriere P. Hoffmeyer M. Assal

In foot and ankle surgery incorrect placement of implants, or inaccuracy in fracture reduction may remain undiscovered with the use of conventional C-arm fluoroscopy. These imperfections are often only recognized on postoperative computer tomography scans. The apparition of three dimensional (3D) mobile Imaging system has allowed to provide an intraoperative control of fracture reduction and implant placement. Three dimensional computer assisted surgery (CAS) has proven to improve accuracy in spine and pelvic surgery. We hypothesized that 3D-based CAS could improve accuracy in foot and ankle surgery.

The purpose of our study was to evaluate the feasibility and utility of a multi-dimensional surgical imaging platform with intra-operative three dimensional imaging and/or CAS in a broad array of foot and ankle traumatic and orthopaedic surgery.

Cohort study of patients where the 3D mobile imaging system was used for intraoperative 3D imaging or 3D-based CAS in foot and ankle surgery.

The imaging system used was the O-arm Surgical Imaging System and the navigation system was the Medtronic's StealthStation.

Surgical procedures were performed according to standard protocols.

In case of fractures, image acquisition was performed after reduction of the fracture. In cases of 3D-based CAS, image acquisition was performed at the surgical step before implants placement. At the end of the operations, an intraoperative 3D scan was made.

We used the O-arm Surgical Imaging system in 11 patients: intraoperative 3D scans were performed in 3 cases of percutaneus fixation of distal tibio-fibular syndesmotic disruptions; in 2 of the cases, revision of reduction and/or implant placement were needed after the intraoperative 3D scan.

Three dimensional CAS was used in 10 cases: 2 open reduction and internal fixation (ORIF) of the calcaneum, 1 subtalar fusion, 2 ankle arthrodesis, 1 retrograde drilling of an osteochondral lesion of the talus, 1 Charcot diabetic reconstruction foot and 1 intramedullary screw fixation of a fifth metatarsal fracture. The guidance was used essentially for screw placement, except in the retrograde drilling of an osteochondral lesion where the guidance was used to navigate the drill tool. Intraoperative 3D imaging showed a good accuracy in implant placement with no need to revision of implants.

We report a preliminary case series with use of the O-arm Surgical Imaging System in the field of foot and ankle surgery. This system has been used either as intraoperative 3D imaging control or for 3D-based CAS. In our series, the 3D computer assisted navigation has been very useful in the placement of implants and has shown that guidance of implants is feasible in foot and ankle surgery. Intraoperative 3D imaging could confirm the accuracy of the system as no revisions were needed. Using the O-arm as intraoperative 3D imaging was also beneficial because it allowed todemonstrate intraoperative malreduction or malposition of implants (which were repositioned immediately). Intraoperative 3D imaging system showed very promising preliminary results in foot and ankle surgery. There is no doubt that intraoperative use of 3D imaging will become a standard of care. The exact indications need however to be defined with further studies.

S. Hammoud E.M. Suero T.G. Maak J.C. Rozell M.L. Inra K. Jones M.B. Cross A.D. Pearle

Controversies about the management of injuries to the soft tissue structures of the posteromedial corner of the knee and the contribution of such peripheral structures on rotational stability of the knee are of increasing interest and currently remain inadequately characterised. The posterior oblique ligament (POL) is a fibrous extension off the distal aspect of the semimembranosus that blends with and reinforces the posteromedial aspect of the joint capsule. The POL is reported to be a primary restraint to internal rotation and a secondary restraint to valgus translation and external rotation. Although its role as a static stabiliser to the medial knee has been previously described, the effect of the posterior oblique ligament (POL) injuries on tibiofemoral stability during Lachman and pivot shift examination in the setting of ACL injury is unknown.

The objective of this study was to quantify the magnitude of tibiofemoral translation during the Lachman and pivot shift tests after serial sectioning of the ACL and POL.

Eight knees were used for this study. Ligamentous constraints were sequentially sectioned in the following order: ACL first, followed by the POL. Navigated mechanised pivot shift and Lachman examinations were performed before and after each structure was sectioned, and tibiofemoral translation was recorded.

Lachman test: There was a mean 6.0 mm of lateral compartment translation in the intact knee (SD = 3.3 mm). After sectioning the ACL, translation increased to 13.8 mm (SD = 4.6; P<0.05). There was a nonsignificant 0.7 mm increase in translation after sectioning the POL (mean = 14.5 mm; SD = 3.9 P>0.05).

Mechanised pivot shift: Mean lateral compartment translation in the intact knee was −1.2 mm (SD = 3.2 mm). Sectioning the ACL caused an increase in anterior tibial translation (mean = 6.7 mm; SD = 3.0 mm; P<0.05). No significant change in translation was seen after sectioning the POL (mean = 7.0 mm, SD = 4.0 mm; P>0.05).

Sectioning the POL did not significantly alter tibiofemoral translation in the ACL deficient knee during the Lachman and pivot shift tests. This study brings into question whether injuries to the POL require reconstruction in conjunction with ACL reconstruction. More studies are needed to further characterise the role of the injured POL in knee stability and its clinical relevance in the ACL deficient and reconstructed knee.

S.L. Sherman E.M. Suero D. Delos J.C. Rozell K. Jones M.F. Sherman A.D. Pearle

Over the last two decades, anatomic anterior cruciate ligament (ACL) reconstructions have gained popularity, while the use of extraarticular reconstructions has decreased. However, the biomechanical rationale behind the lateral extraarticular sling has not been adequately studied. By understanding its effect on knee stability, it may be possible to identify specific situations in which lateral extraarticular tenodesis may be advantageous. The primary objective of this study was to quantify the ability of a lateral extraarticular sling to restore native kinematics to the ACL deficient knee, with and without combined intraarticular anatomic ACL reconstruction. Additionally, we aimed to characterise the isometry of four possible femoral tunnel positions for the lateral extraarticular sling.

Eight fresh frozen hip-to-toe cadavers were used in this study. Navigated Lachman and mechanised pivot shift examinations were performed on ACL itact and deficient knees. Three reconstruction strategies were evaluated: Single bundle anatomic intraarticular ACL reconstruction, Lateral extraarticular sling, Combined intraarticular ACL reconstruction and lateral extraarticular sling. After all stability tests were completed, we quantified the isometry of four possible femoral tunnel positions for the lateral extraarticular sling using the Surgetics navigation system. A single tibial tunnel position was identified and digitised over Gerdy's tubercle. Four possible graft positions were identified on the lateral femoral condyle: the top of the lateral collateral ligament (LCL); the top of the septum; the ideal tunnel position, as defined by the navigation system's own algorithm; and the actual tunnel position used during testing, described in the literature as the intersection of the linear projections of the LCL and the septum over the lateral femoral condyle. For each of the four tunnel positions, the knee was cycled from 0 to 90® of flexion and fiber length was recorded at 30® intervals, therefore quantiying the magnitude of anisometry for each tunnel position.

Stability testing: Sectioning of the ACL resulted in an increase in Lachman (15mm, p = 0.01) and mechanised pivot shift examination (6.75mm, p = 0.04) in all specimens compared with the intact knee. Anatomic intraarticular ACL reconstruction restored the Lachman (6.7mm, p = 3.76) and pivot shift (−3.5mm, p = 0.85) to the intact state. With lateral extraarticular sling alone, there was a trend towards increased anterior translation with the Lachman test (9.2mm, p = 0.50). This reconstruction restored the pivot shift to the intact state. (1.25mm, p = 0.73). Combined intraarticular and extraarticular reconstruction restored the Lachman (6.2mm, p = 2.11) and pivot shift (−3.75mm, p = 0.41) to the intact state. There was no significant difference between intraarticular alone and combined intraarticular and extraarticular reconstruction. (p = 1.88)

Isometry: The ideal tunnel position calculated by the navigation system was identified over the lateral femoral condyle, beneath the mid-portion of the LCL. The anisometry for the ideal tunnel position was significantly lower (5.9mm; SD = 1.8mm; P<0.05) than the anisometry of the actual graft position (14.9mm; SD = 4mm), the top of the LCL (13.9mm; SD = 4.3mm) and the top of the septum (12mm; SD = 2.4mm).

In the isolated acute ACL deficient knee, the addition of a lateral extraarticular sling to anatomic intraarticular ACL reconstruction provides little biomechanical advantage and is not routinely recommended. Isolated lateral extraarticular sling does control the pivot shift, and may be an option in the revision setting or in the lower demand patient with functional instability. Additionally, the location of the femoral tunnel traditionally used results in a significantly more anisometric graft than the navigation's system mathematical ideal location. However, the location of this ideal tunnel placement lies beneath mid-portion of the fibers of the LCL, which would not be clinically feasible.

T. Hiranaka Y Kawakami Y. Hida H. Uemoto M. Doita M Tsuji

Thirty-three knees in thirty-three patients who underwent ACLR using four-strand semitendinousus and gracilis tendon in our hospital were included in this study. In 17 knees, we use a fluoroscopic-based navigation system (Vector Vision ACL, BrainLab. Inc.) for positioning of the tunnels (Group 1). In the remaining 16 knees, positioning of the femoral and tibial tunnels was done without navigation (Group 2).

In navigation operation, anteroposterior and lateral images of the knee were taken with a fluoroscope and captured into the computer. The optimal target points for bone tunnels were semi-automatically calculated and displayed on the screen. Femoral placement was determined based on the quadrant method. The target for tibial tunnel was set at 43% of tibial plateau AP length. Intraoperatively, positions of the drill guides were decided referring to both navigation image and arthroscopic image. We evaluated Lysholm score, International Knee Documentation Committee (IKDC) subjective score, Lachman test and pivot shift test at 1 year after operation and calculated bone tunnel position on the postoperative lateral x-ray films and expressed them as relative values against total AP length of the Blumensaat's line and of the tibia plateau.

Lysholm score, IKDC subjective score, Lachman test and pivot shift test were not significantly differed between the groups. The femoral tunnels were 74.2±3.3% in Group 1 and 71.7±6.0% in Group 2 along and the tibial tunnels were 42.1±1.4% in group 1 and 43.0±4.6% in group 2 along the tibia plateau. Although femur and tibial tunnel positions were not significantly differed between the groups, variation of bone tunnel position was significantly smaller in Group 1, indicating a good reproducibility. One pin tract infection occurred in Group 1. This case successfully treated with debridment and antibiotics containing cement filling.

Fluoroscopic navigation system is quite helpful for precise and reproducible creation of both femur and tibial tunnel. The results encourage us to use this system for double-bundle anatomical ACLR. However, a special care must be taken to avoid complication caused by tracker pin placement.

L. Fieten J. Eschweiler K. Kabir S. Gravius T. Randau K. Radermacher

Biomechanical considerations are relevant to cup positioning in total hip replacement (THR) to optimise the patient-specific post-operative outcome. One goal is to place the hip centre of rotation (COR) such that parameters characterising the biomechanics of the hip joint lie within physiological ranges. Different biomechanical models have been developed and are based on exact knowledge about muscle insertion points whose positions can be estimated on the basis of bony landmarks. Therefore, accurate landmark localisation is necessary to obtain reliable and comparable parameter values.

As most biomechanical considerations are limited to the frontal plane, landmark localisation relying on standardised pre-operative radiographs has been established in clinical practice. One potential drawback of this approach is that user-interactive landmark localisation in radiographs might be more error-prone and subjective than localisation in 3D images. Therefore, we investigated the possibility of increasing the reproducibility of interactive landmark localisation by providing 3D localisation techniques. As the so-called BLB score based on Blumentritt's biomechanical hip model has already been introduced into clinical practice as a criterion for cup position planning, we examined the anatomical landmarks involved in BLB score evaluation. We developed a CT-based simulation tool allowing for the generation of 3D bone surface models and standardised digitally reconstructed radiographs (DRRs). Correspondences between points in the 2D DRR and rays in the 3D bone surface model are automatically established and optionally visualised by the tool.

Two modes of landmark localisation were examined: In the 2D-mode, only AP DRRs were displayed, and the users had to localise the landmarks by clicking within the DRR image. In the 3D-mode, additionally the arbitrarily rotatable bone surface models together with the aforementioned 2D/3D correspondences were visualised. The user could then choose between landmark localisation by clicking either within the DRR image or within the 3D view. In either case, the 2D landmark positions within the DRR were recorded.

The participants were given both an example AP pelvis radiograph with highlighted anatomical landmarks and the following landmark descriptions from the user's manual (v2.06) of the mediCAD software (Hectec GmbH, Landshut, Germany): P4: ca. 3cm distal lesser trochanter minor (in the imagined direction of pull of the rectus femoris muscle towards the medial upper edge of the patella); P5:lateral, most proximal edge of the trochanter major; P6: most cranial edge of the sclerotic area; P7:spina iliaca anterior inferior; P8/P9:most lateral/cranial point of the wing of the ilium.

(P1 and P2 are only needed to define the position of the mid-sagittal plane, and P3 is the pre-operative COR. Due to correct radiograph standardisation, we assumed this plane and P3 to be known prior to landmark localisation.)

Thirteen surgeons repeated the experiments on four hips (CT datasets of two male patients).

The following results were obtained (SD of relevant coordinates obtained with 2D localisation vs. SD of those obtained with 3D localisation) in the first patient (left hip: 1L; right hip: 1R) and the second patient (left hip: 2L; right hip: 2R):P4: 6.3 vs. 9.0 (1L); 6.7 vs. 5.6 (1R); 9.0 vs. 11.1 (2L); 7.1 vs. 8.6 (2R); P5: 4.4 vs. 2.8 (1L); 3.1 vs. 3.1 (1R); 4.3 vs. 2.4 (2L); 4.7 vs. 4.1 (2R); P6: 4.8 vs. 3.8 (1L); 2.9 vs. 2.8 (1R); 3.7 vs. 5.2 (2L); 6.9 vs. 3.5 (2R); P7: 12.2 vs. 6.1 (1L); 12.1 vs. 3.7 (1R); 7.6 vs. 4.6 (2L); 6.2 vs. 4.5 (2R); P8: 1.2 vs. 2.8 (1L); 2.0 vs. 2.6 (1R); 1.5 vs. 2.1 (2L); 2.0 vs. 1.6 (2R);P8: 4.1 vs. 2.1 (1L); 7.3 vs. 3.9 (1R); 1.6 vs. 2.6 (2L); 4.1 vs. 3.2 (2R).

The greatest differences in reproducibility were observed in P7, which was barely distinguishable in the radiographs and, hence, showed very low reproducibility only for the 2D-mode. P4 showed low reproducibility in both modes due to its vague description and the relatively small portions of the femurs contained in the CT-scanned volume. In P9 the low reproducibility obtained with the 2D-mode might be partly explained by truncation artefacts present in the DRRs.

Although our study needs to be extended to more datasets, we conclude that the availability of 3D data allows for higher landmark localisation reproducibility when compared with the conventional X-ray-based approach, which has additional drawbacks: Standardisation of X-ray imaging, which is necessary to retain comparability of biomechanical parameter values determined in different patients, is hard to achieve; specifications e.g. concerning the central beam may be met only after acquiring several radiographs. Moreover, once a 2D target cup position is defined based on the 2D biomechanical analyses, the transfer of this position into the 3D surgical site is difficult without additional 3D imaging.

Hence, the use of 3D imaging and 3D landmark localisation techniques seems more promising for cup positioning based on biomechanical models, which, however, need validation.

S. Hammoud E.M. Suero T.G. Maak J.C. Rozell M.L. Inra K. Jones M.B. Cross A.D. Pearle

Controversies about the management of injuries to the soft tissue structures of the posteromedial corner of the knee and the contribution of such peripheral structures on rotational stability of the knee are of increasing interest and currently remain inadequately characterised. The posterior oblique ligament (POL) is a fibrous extension off the distal aspect of the semimembranosus that blends with and reinforces the posteromedial aspect of the joint capsule. The POL is reported to be a primary restraint to internal rotation and a secondary restraint to valgus translation and external rotation. Although its role as a static stabiliser to the medial knee has been previously described, the effect of the posterior oblique ligament (POL) injuries on tibiofemoral stability during Lachman and pivot shift examination in the setting of ACL injury is unknown.

The objective of this study was to quantify the magnitude of tibiofemoral translation during the Lachman and pivot shift tests after serial sectioning of the ACL and POL.

Eight knees were used for this study. Ligamentous constraints were sequentially sectioned in the following order: ACL first, followed by the POL. Navigated mechanised pivot shift and Lachman examinations were performed before and after each structure was sectioned, and tibiofemoral translation was recorded.

Lachman test: There was a mean 6.0 mm of lateral compartment translation in the intact knee (SD = 3.3 mm). After sectioning the ACL, translation increased to 13.8 mm (SD = 4.6; P<0.05). There was a nonsignificant 0.7 mm increase in translation after sectioning the POL (mean = 14.5 mm; SD = 3.9 P>0.05).

Mechanised pivot shift: Mean lateral compartment translation in the intact knee was −1.2 mm (SD = 3.2 mm). Sectioning the ACL caused an increase in anterior tibial translation (mean = 6.7 mm; SD = 3.0 mm; P<0.05). No significant change in translation was seen after sectioning the POL (mean = 7.0 mm, SD = 4.0 mm; P>0.05).

Sectioning the POL did not significantly alter tibiofemoral translation in the ACL deficient knee during the Lachman and pivot shift tests. This study brings into question whether injuries to the POL require reconstruction in conjunction with ACL reconstruction. More studies are needed to further characterise the role of the injured POL in knee stability and its clinical relevance in the ACL deficient and reconstructed knee.

F.A. Petrigliano E.M. Suero C.G. Lane J.E. Voos M. Citak A.A. Allen T.J. Wickiewicz A.D. Pearle

Injuries to the posterior cruciate ligament (PCL) and the posterolateral corner (PLC) of the knee remain a challenging orthopaedic problem. Studies evaluating PCL and PLC reconstruction have failed to demonstrate a strong correlation between the degree of knee laxity as measured by uniplanar testing and subjective outcome or patient satisfaction. The effect that changing the magnitude of posterior tibial slope has on multiplanar, rotational stability of the PCL-deficient knee has yet to be determined. We aimed to evaluate the effect that changes in posterior tibial slope would have on static and dynamic stability of the PCL-PLC deficient knee.

Ten knees were used for this study. Navigated posterior drawer and standardised reverse mechanised pivot shift maneuvers were performed in the intact knee and after sectioning the PCL, the lateral collateral ligament (LCL), the popliteofibular ligament (PFL) and the popliteus muscle tendon (POP). Navigated high tibial osteotomy (HTO) was performed to obtain the desired change in tibial plateau slope (+5® or −5® from native slope). We then repeated the posterior drawer and the reverse mechanised pivot shift test for each of the two altered slope conditions.

Mean posterior tibial translation during the posterior drawer in the intact knee was 1.4 mm (SD = 0.48 mm). In the PCL-PLC deficient knee, posterior tibial translation increased to 18 mm (SD = 5.7 mm) (P < 0.001). Increasing the amount of posterior tibial slope by 5® reduced posterior tibial translation to 12 mm (SD = 4.7 mm) (P < 0.01). Decreasing the amount of posterior slope by 5® compared to the native knee, increased posterior tibial translation to 21 mm (SD = 6.8 mm) (P < 0.01). There was a significant negative correlation between the magnitude of tibial plateau slope and the magnitude of the reverse pivot shift (R2 = 0.71; P < 0.0001).

Mean posterior tibial translation during the reverse mechanised pivot shift test in the intact knee was 7.8 mm (SD = 2.8 mm). In the PCL-PLC deficient knee, posterior tibial translation increased to 26 mm (SD = 5.6 mm) (P < 0.001). Increasing the amount of posterior tibial slope by 5® reduced posterior tibial translation to 21 mm (SD = 6.7 mm) (P < 0.01). Decreasing the amount of posterior slope by 5® compared to the native knee, increased posterior tibial translation to 34 mm (SD = 8.2 mm) (P < 0.01). There was a significant negative correlation between the magnitude of tibial plateau slope and the magnitude of the reverse pivot shift (R2 = 0.72; P < 0.0001).

Decreasing the magnitude of posterior slope of the tibial plateau resulted in an increase in the magnitude of posterior tibial translation during the posterior drawer and the reverse mechanised pivot shift test in the PCL-PLC deficient knee. Conversely, increasing the slope of the tibial plateau reduced the amount of posterior tibial translation during the posterior drawer and the reverse mechanised pivot shift test. However, the effect of the increase in slope was not sufficient to reduce posterior tibial translation to levels similar to those of the intact knee.

J.A. Koenig E.M. Suero C. Plaskos

Robotic-guided arthroplasty procedures are becoming increasingly common. We introduced a new computer-navigated TKA system with a robotic cutting-guide into a community-based hospital and characterized the accuracy and efficiency of the technique.

We retrospectively reviewed our first 100 cases following IRB approval. Tourniquet time, intraoperative bone-cut accuracy and final limb alignment as measured by the computer were collected and divided into consecutive quartiles: Groups I, II, III, and IV; 25 cases per group. All resections were planned neutral to the mechanical axis. Postoperative component alignment and overall mechanical axis limb alignment were also measured on standing long-leg radiographs by two independent observers at minimum six weeks follow-up. Radiographic alignment was available for 62 cases.

Intraoperative Computer Data: Bone-cut accuracy was a mean 0.1° valgus, SD±0.8° for both the femur and tibia (range, femur: 2.0° valgus to 1.5° varus; range, tibia: 3.5° valgus to 1.5° varus). Final limb alignment was within 3° for 98% (97/99) of cases (range: 2.0° valgus to 3.5° varus). Radiographic Alignment: Pre-operative mechanical alignment ranged from −14.5° valgus to 21.5° varus. Radiographic femoral and tibial component alignment was within 3° of neutral in 98.4% of cases (61/62). Final limb alignment was within 3° for 87.1% (54/62) of cases (range: 4.5° varus to 4.5° valgus). Learning curve: Mean tourniquet time was 60minutes ±9.9SD (range 46–79) for Group I and 49.5minutes for Groups II, III, and IV (range 35–68), p = 0.0001. Mean tourniquet time for the first ten and second ten procedures was 65±10.6minutes and 55±8.3minutes, respectively, p = 0.034. There were no differences in accuracy among the four groups (p>0.05).

Imageless computer-navigated TKA with a robotic cutting guide allowed one surgeon to make bone resections within 3° of neutral in 98% of cases. Radiographic limb alignment was less precise, which is consistent with the known limitations inherent to this measurement technique. Surgeons can expect this procedure to take 15 additional minutes during the first ten cases and five additional minutes during the second ten cases on average, without compromising accuracy.

E.M. Suero C. Claps M. Citak A.D. Pearle C. Plaskos

Accurate and reliable registration of the ankle center is a necessary requirement in computer-assisted TKR. There is debate among surgeons over which registration procedure more accurately reflects the true center of the ankle joint. The aim of this study was to compare two different ankle registration landmarks on radiographs and determine how much they differed from the anatomic center of the talus in the frontal plane. Specifically, we asked what the average deviation in tibial mechanical axis registration would be when registering the ankle center using: A) the extreme medial and lateral points; and B) the most distal points, of the respective malleoli. A second question was whether or not BMI had any significant effect on mechanical axis registration error.

We reviewed the preoperative hip-to-ankle radiographs of 40 patients who underwent navigated TKR at our institution. The patient cohort was composed of 32 females and 7 males, with a mean age of 69 years (range, 45–84 years) and a mean BMI of 29.9 (range, 14.7–43.3). All radiographs were stored in and reviewed using PACS.

No clinically significant divergence from the anatomic center of the ankle was seen when using the Extremes Midpoint technique (mean divergence = 0.2® lateral; SD = 0.5®; 95% CI = −0.3®, −0.1®) or the Distal Midpoint technique (mean divergence = 0.2® lateral; SD = 0.6®; 95% CI = −0.39®, 0®). The mean difference between both techniques was 0.02® (SD = 0.3®; 95% CI = −0.1®, 0.1®; P = 0.68). BMI had no significant effect on the divergence from the true ankle center for either the Extremes Midpoint (R2 = 0.002; P = 0.78) or the Distal Midpoint techniques (R2 = 0.004; P = 0.90).(Figure 2)

The center of the ankle, as determined by using the Extremes Midpoint technique, lied 1.1 mm (SD = 2.6 mm; 95% CI = −1.9 mm to −0.3 mm) from the anatomic axis of the tibia. When determined using the Distal Midpoint technique, the center of the ankle lied 1.7 mm (SD = 2.3 mm; 95% CI = −2.5 mm to −0.98 mm) from the anatomic axis. Although statistically significant (P = 0.028), this difference was not clinically relevant (<3 mm). BMI had no significant effect on these differences (R2 = 0.07; P = 0.11; R2 = 0.02, P = 0.38).(Figure 3)

There is no significant difference between ankle registration using the Extremes Midpoint or the Distal Midpoint techniques and the anatomic center of the ankle. Patients' BMI does not seem to affect the registration of the ankle center with either technique.

A.M. Anaya L. Vigneron M. Diab S. Burch

The Bernese periacetabular osteotomy (PAO) described by Ganz, et al. is a commonly used surgical intervention in hip dysplasia. PAO is being performed more frequently and is a viable alternative to hip arthroplasty for younger and more physically active patients. The procedure is challenging because pelvic anatomy is prohibitive to visibility and open access and requires four X-ray guided blind cuts around the acetabulum to free it from the hemi-pelvis. The crucial step is the re-orientation of the freed acetabulum to correct the inadequate coverage of the femoral head by idealy rotating the freed acetabular fragment.

Diagnosis and the decision for surgical intervention is currently based upon patient symptoms, use of two-dimensional (2D) radiographic measurements, and the intrinsic experience of the surgeon. With the advent of new technologies allowing three-dimensional reconstructions of hip anatomy, previous two-dimensional X-ray definitions have created much debate in standardizing numerical representations of hip dysplasia. Recent work done by groups such as Arminger et al. have combined and expanded two-dimensional measurements such as Center-Edge (CE) angle of Wiberg, Vertical-Center-Anterior margin (VCA) angle, Acetabular Anteversion (AcetAV) and applied them to three-dimensional CT rendering of hip anatomy. Further, variability in pelvic tilt is a confounding factor and has further impeded measurement translatability.

Computer assisted surgery (CAS) and navigation also called image-guided surgery (IGS) has been used in clinical cases of PAO with mixed results. The first appearing study of CAS/IGS in PAO was conducted by Langlotz, et. al 1997 and reported no clinical benefit to using CAS/IGS. However, they did conclude that the use of CAS/IGS is undoubtedly useful for surgeons starting this technically demanding procedure. This is supported by a more recent study done by Hsieh, et. al 2006 who conducted a two year randomised study of CAS/IGS in PAO and concluded its feasibility to facilitate PAO, but there was not an additional benefit when conventional PAO is done by an experienced surgeon. A study done by Peters, et. Al 2006 studying the learning curve necessary to become proficient at PAO found that “The occurrence of complications demonstrates a substantial learning curve” and thus makes a compelling argument for the use of CAS/IGS.

A major obstacle to navigation and CAS/IGS revolves around consistency, intra-operative time and ease of use. Custom made guides and implants may help circumvent these limitations. The use of CAS/CAM in developing custom made guides has been proven very successful in areas of oral maxillofacial surgery, hip arthroplasty, and knee replacement surgeries. Additionally, a significant study in the development of rapid prototyping guides in the treatment of dysplastic hip joints was done by Radermacher et. al 1998. They describe a process of using CAS/CAM within the operational theatre using a desktop planning station and a manufacturing unit to develop what they termed as “templates” to carry out a triple osteotomy.

Our group is evaluating and developing strategies in PAO using CAS/IGS and more recently using CAS and computer aided modeling (CAM) to develop custom made guides for acetabular positioning. Our first study (Burch et al.) focused on CAS/IGS in PAO using cadavers and yielded small mean cut (1.97± 0.73mm) and CE angle (4.9± 6.0) errors. Our recent study used full sized high-resolution foam pelvis models (Sawbones®, Vashon, Washington) and used CAS/IGS to carry out the pelvic cuts and CAS/CAM to develop a acetabular positioning guide (APG) by rapid prototyping. The CAS/IGS pelvic cuts results were good (mean error of 3.18 mm ± 1.35) and support our and other studies done using CAS/IGS in PAO. The APG yielded high accuracy and was analysed using four angles with an overall mean angular error of 1.81 (0.550)and individual angulation was as follows: CE 0.83° ± 0.53, S-AC 0.28° ± 0.19, AcetAV 0.41° ± 0.37, and VCA 0.68° ± 0.27. To our knowledge this is the first developed APG for PAO.

The APG we developed was to demonstrate the concept of using a positioning guide to obtain accurate rotation of the acetabular fragment. For a clinical application a refined and sleeker design would be required. Further, because working space within the pelvis is extraordinary constrained, once fitted the APG would need to remain and serve as an implantable cage capable of holding bone graft. A potential material is polyetheretherketone (PEEK). Customised PEEK implants and cages have been established in the literature and is a potential option for PAO. The benefits of an implant not only serve to constrain the acetabular fragment in the ideal position based upon the pre-operative plan, but may also provide the structural support for rotations not other wise possible.

Though CAS/IGS is a proven viable option, we envision a potentially simpler method for PAO, the use of a cut guide and an acetabular positioning implant. Using customized guides and implants could potentially circumvent the need for specialised intra-operative equipment and the associated learning curves, by providing guides that incorporate the pre-operational plan within the guide, constraining the surgeon to the desired outcome.

P.L. Yen S.S. Hung Y.J. Chu

To close the surgeon in the control loop is able to take advantages of the fertile sensing information and intelligence from human being so that the complex and unpredictable surgical procedure can be better handled properly. However, there is also weakness to be strengthened. For example, the motion control of the human being is not as accurate as required. Assisted equipment for such purpose may be helpful to the procedure. Exerting large forces during the cutting process may exhaust the operator and cause fatigue. The operator may need power assistance during the surgery procedure. The response speed of human being's may not be adequate to take immediate action in certain critical situations. The constraints from the limbs and human's attention usually cause a significant delay to react the critical situations. This may lead to serious damage by failing to response immediately. For example, in knee replacement procedure, the shape of the knee joint has to be prepared by performing bone resection procedure. The surgeon cuts the bone at certain position and orientation with the help of the cutting jig from the surgical planning. The cutter cut the bone by inserting vibration saw through the slot of the cutting block. The surgeon has to stop the cutter right after the bone has been cut through by the saw so that no surrounding soft tissue, blood vessel, and even important nerves, be damaged. Currently the tactile sensing from the hand is heavily relied on by the surgeon. He has also to be experienced and dexterous. If he fails to draw back the cutter after the bone being cut through, damages to the patient may occur. Therefore there is need to develop a cutting tool, which is intelligent, knows where it is, and is able to judge what the cutting situation is, and further assists the operator to stop the cutter in case that the operator fails to do so, or too slow to response. The benefit to the patient as well as the surgeon will be significant. Therefore the purpose of the paper is to develop an algorithm to implement such functionality of auto-detection of bone cutting through for a bone cutting tool when the cutter has cut through the bone. By the developed method, the intelligent cutter can effectively detect the cutting through condition and then adopt an immediate reaction to prevent the cutter from going further and to avoid unexpected damage.

An auto-detection scheme has also been developed for assisting the operator in judging whether the cutter has reached the boundary or not. The auto-detection scheme is based on analyzing the cutting force pattern in conjunction with a bilateral force controller for the hand's on robot. The bilateral force controller consists of two force controllers. The force controller on the master calculates the feedrate of the cutter according to the push force by the operator. Meanwhile, the operator can also feel the cutting force by the force controller on the slave site, which revises the feedrate of the cutter by the measured cutting force. The operator can kinesthetically feel the cutting force from the variation of the feedrate. When the cutter breaks through the boundary of the bone, the cutting force will drop suddenly to almost zero. When this special force pattern occurs, an acknowledge signal of bone been broken through will be activated. The subsequent action will be followed to stop the cutter going further. We implement this concept by defining a “cutting admittance” indicating the resistance encountered by the cutting during bone resection at different cutting feedrate. During the bone resection, the cutting admittance varies from cutting through hard or soft portions of bone. As reaching the cutting boundary, the cutting admittance will suddenly increase. A threshold value will experimentally be determined to indicate cutting through condition. Together with pushing force by the human operator, the criterion for cutting-through detection is defined. Once cutting through is detected, the admittance for cutter movement will be set zero. The cutter stops going further and the operator feel like hitting a virtual wall in front.

In this paper we proposed a human robot cooperative operation method by which the robotic system can intelligently detect where the cutter has cut through the bone. Characterisation of bone cutting procedure was performed. This auto detection scheme was developed by analyzing the information of the motion and cutting force information during the bone cutting process, no medical images are required. The auto-detection bone cut through was able to transfer the experiences of human being to quantitative modeling. The developed model has been tested for its applicability and robustness by saw bones and pig's knee joints. Results have shown the virtual wall generated by the real-time bone detection scheme and active constraint control is very accurate and capable of providing a safety enhance module for computer assisted orthopaedic surgery, in particular, in total knee replacement. By this method the robot system can accomplish a safe and accurate bone cutting with a complement of an imageless navigation system and results in a low cost, but safe and effective surgical robot system.

T. Vrtovec M.M.A. Janssen F. Pernuš R.M. Castelein M.A. Viergever

Pelvic incidence is as a key factor for sagittal balance regulation that describes the anatomical configuration of the pelvis. The sagittal alignment of the pelvis is usually evaluated in two-dimensional (2D) sagittal radiographs in standing position by pelvic parameters of sacral slope, pelvic tilt and pelvic incidence (PI). However, the angle of PI remains constant for an arbitrary subject position and orientation, and can be therefore compared among subjects in standing, sitting or supine position. Such properties also enable the measurement of PI in three-dimensional (3D) images, commonly acquired in supine position. The purpose of this study is to analyse the sagittal alignment of the pelvis in terms of PI in 3D computed tomography (CT) images.

A computerised method based on image processing techniques was developed to determine the anatomical references, required to measure PI, i.e. the centre of the left femoral head, the centre of the right femoral, the centre of the sacral endplate, and the inclination of the sacral endplate. First, three initialisation points were manually selected in 3D at the approximate location of the left femoral head, right femoral head and L5 vertebral body. The computerised method then determined the exact centres of the femoral heads in 3D from the spheres that best fit to the 3D edges of the femoral heads. The exact centre of the sacral endplate in 3D was determined by locating the sacral endplate below the L5 vertebral body and finding the midpoint of the lines between the anterior and posterior edge, and between the left and right edge of the endplate. The exact inclination of the sacral endplate in 3D was determined from the plane that best fit to the endplate. Multiplanar 3D image reformation was applied to obtain the superposition of the femoral heads in the sagittal view, so that the hip axis was observed as a straight not inclined line and all anatomical structures were completely in line with the hip axis. Finally, PI was automatically measured as the angle between the line orthogonal to the inclination of the sacral endplate and the line connecting the centre of the sacral endplate with the hip axis.

The method was applied to axially reconstructed CT scans of 426 subjects (age 0–89 years, pixel size 0.4–1.0 mm, slice thickness 3.0–4.0 mm). Thirteen subjects were excluded due to lumbar spine trauma and presence of the sixth lumbar segment. For the remaining subjects, the computerised measurements were visually assessed for errors, which occurred due to low CT image quality, low image intensity of bone structures, or other factors affecting the determination of the anatomical references. The erroneous or ambiguous results were detected for 43 subjects, which were excluded from further analysis. For the final cohort of 370 subjects, statistical analysis was performed for the obtained PI. The resulting mean PI ± standard deviation was equal to 46.6 ± 9.2 degrees for males (N = 189, age 39.7 ± 20.3 years), 47.6 ± 10.7 degrees for females (N = 181, age 43.4 ± 19.9 years), and 47.1 ± 10.0 degrees for both genders (N = 370, age 41.5 ± 20.1 years). Correlation analysis yielded relatively low but statistically significant correlation between PI and age, with the correlation coefficient r = 0.20 (p < 0.005) for males, r = 0.32 (p < 0.0001) for females, and r = 0.27 (p < 0.0001) for both genders. No statistically significant differences (p = 0.357) were found between PI for male and female subjects.

This is the first study that evaluates the sagittal alignment of the pelvis in terms of PI completely in 3D. Studies that measured PI manually from 2D sagittal radiographs reported normative PI in adult population of 52 ± 10 degrees, 53 ± 8 degrees and 51 ± 9 degrees for 25 normal subjects aged 21–40, 41–60, and over 60 years, respectively [3], and 52 ± 5 degrees for a cohort of 160 normal subjects [4]. The PI of 47 ± 10 degrees obtained in our study is lower than the reported normative values, which indicates that radiographic measurements may overestimate the actual PI. Radiographic measurements are biased by the projective nature of X-ray image acquisition, as it is usually impossible to obtain the superposition of the two femoral heads. The midpoint of the line connecting the centres of femoral heads in 2D is therefore considered to be the reference point on the hip axis, moreover, the inclination of the sacral endplate in the sagittal plane is biased by its architecture and inclination in the coronal plane. On the other hand, the measurements in the present study were obtained by applying a computerized method to CT images that determined the exact anatomical references in 3D. Perfect sagittal views were generated by multiplanar reformation, which aligned the centres of the femoral heads in 3D. The measurement of PI was therefore not biased by acquisition projection or structure orientation, as all anatomical structures were completely in line with the hip axis. Moreover, the range of the PI obtained in every study (standard deviation of around 10 degrees) indicates that the span of PI is relatively large. It can be therefore concluded that an increased or decreased PI may not necessary relate to a spino-pelvic pathology.