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.

Introduction

Differing descriptions of patellar motion relative to the femur have resulted from many in-vitro and in-vivo studies. The aim of this study was to examine the tracking behaviour of the patella. We hypothesized that patellar kinematics would correlate to the trochlear geometry.

Introduction

Tibial patho-morphology has been described as a factor that predisposes to medial compartment osteoarthritis of the knee in 2D analysis. The aim of this study was to investigate whether the morphology of normal and pre-OA medial compartments are really different in 3 dimensions.

Introduction

SPECT/CT might be a promising diagnostic modality in patients with painful total knee arthroplasty. It was the purpose of our study to introduce a novel standardised SPECT/CT algorithm for assessing patients with painful primary total knee arthroplasty and to evaluate its clinical applicability and inter- and intra-observer variation and reliability.

Introduction

The trochlear groove plays a major role in the mechanics and patho-mechanics of the patellofemoral joint. Our primary goal was to compare normal, osteoarthritic and dysplastic PFJs in terms of angles and distances.

Controversy still exists in the literature regarding efficacy and usefulness of CASN in knee arthroplasty. However, obsession with basic alignments and proper correction of mechanical axes fails to recognise the full future potential of CASN which seems to lie in enhanced dynamic assessment. Basic dynamics usually at least includes intraoperative assessment of limb alignments, flexion-extension gap balancing and simple testing through ranges of motion. However our upgraded CASN system (Brainlab) is also capable of enhanced assessment not only including the provision of data on initial to final alignments but also contact point observations. The system can also perform an enhanced ‘Range Of Motion’ (ROM) analysis including observation of epicondylar axis motion, valgus and varus, antero-posterior shifts as well as flexion and extension gaps. Tracking values for both tibiofemoral and patellofemoral motion have also been obtained after performing registration of the prosthetic trochlea.

Observations were then made using a set of standardised dynamic tests. Firstly, the lower leg was placed in neutral alignment and the knee put through a flexionextension 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.

We have been able to carry out these observations in a limited case series of 15 total knee arthroplasties and have found it possible to observe and quantify marked intra-operative variation in the stability characteristics of the implanted joints before corrections have been made and final assessments performed. Indeed contact point observation has found several cases of edge loading before corrections have been made. Also ROM analysis has demonstrated the ability of the system in other cases to observe and then make necessary adjustments of implant positions and ligament balance which alter the amounts of antero-posterior and lateral translations. In this way paradoxical antero-posterior and larger rotational movements have been minimised. Cases where conversion to posterior stabilisation has been necessary have been encountered. Also patellar tracking has been observed during such dynamic tests and appropriate adjustments made to components and soft tissue balancing.

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 adjustments to ligament balance, component sizes, types and positions. In this way CASN becomes a more useful tool.

A robust frame of reference is required to accurately characterize pathoanatomy in the proximal femur and quantify the femoral head-neck relationship. A three dimensional (3D) femoral neck axis (FNA) could serve such a purpose, but has not yet been established in the current literature.

The primary aim of this study was to develop and evaluate a reliable method of determining the 3D femoral neck axis. Secondly, we wanted to quantify the translational relationship between the femoral head and neck in normal and cam type hips.

Pelvic computed tomographic scans (CT) and radiographs were retrieved from our database of patients who had undergone navigated hip surgery or CT colonography. All patients had given informed consent for their medical files and imaging to be used for research purposes, as approved by the institutional review board.

Pre-operative scans were performed using the Siemens Sensation 64 slice scanner (Siemens Medical Solutions, Erlangen, Germany). The Imperial Protocol developed at the authors’ orthopaedic unit was applied, allowing acquisition of Digital Imaging and Communications in Medicine (DICOM) files of 0.75mm thickness.

Normal and cam type hips (n=30) were identified for analysis. ‘Normal’ hips (n=15) were defined in asymptomatic patients with no previous history of hip disease, and, no obvious abnormality on radiographs or CT. The ‘cam’ hip type (n=15) was defined by the presence of an anterior osseous bump at the head-neck junction, and an alpha angle greater than 50° on hip radiographs.

DICOMs were converted to 3D stereolith (STL) images using validated commercial image processing and analysis software (3-Matics, Materialise Group, Leuven, Belgium).

In order to determine the 3D-FNA, a best fit sphere was applied to the femoral head with a root mean square error of less than 0.5mm. The border between sphere and femoral neck defined the head -neck junction. The bone surface was marked here (including the anterior bump in cam hips) and at the neck base, providing two anatomical rings that defined the superior and inferior limits of the femoral neck. The centre point of each ring was calculated. A line connecting these points defined the femoral neck axis, and was verified on a DICOM viewer in sagittal, axial and coronal planes. The offset between the femoral head centre and neck axis was measured.

The 3D image and axis were further analysed to examine the femoral head-neck relationship, using customized software developed at our institution and previously validated in previous research projects.

To standardize rotational alignment, the femoral neck was aligned vertically in two planes by creating an axis between the tip of the greater trochanter and the center of the lesser trochanter. The aligned proximal femur was viewed end on, and the version of the head relative to the neck determined by calculating the angle between the head centre and a vertical marker placed at the 12 o’clock position. Angles below 180° demonstrated anteversion, while those above 180° demonstrated retroversion.

A profound understanding of the pathoanatomy of the patellofemoral joint is considered to be fundamental for navigated knee arthroplasty. Previous studies used less sophisticated imaging modalities such as photography and plain radiographs or direct measurement tools like probes and micrometers to define the morphology of the trochlear groove, with differing results. This may be due to the complexity of the biomechanics and the geometry of this joint. Our primary goal was to compare normal, osteoarthritic and dysplastic PFJs in terms of angles and distances. To do this we first had to establish a reliable frame of reference.

Computed tomography scans of 40 normal knees (> 55 years old), 9 knees with patellofemoral osteoarthritis (group A) and 12 knees with trochlear dysplasia (group B) were analyzed using 3D software. The femurs were orientated using a robust frame of reference. A circle was fitted to the trochlear groove. The novel trochlear axis was defined as a line joining the centres of two spheres fitted to the trochlear surfaces, lateral and medial to the trochlear groove. The relationship between the femoral trochlea and the tibiofemoral joint was measured in term of angles and distances (offsets). T-test for paired samples was used (p< 0.05). The study was approved by the institutional review conforming to the state laws and regulations.

The normal trochlear groove closely matched a circle (RMS 0.3mm). It was positioned laterally in relation to the mechanical, anatomical, and trans-condylar axes of the femur. It was not co-planar with any of the three axes. After aligning to the new trochlear axis, the trochlear groove appeared more linear than when other axes were used. In comparison to the normal knees; the medial trochlear was smaller in group A (p=0.0003)- see figure 2. The lateral trochlear was smaller in group B (p=0.04). The trochlear groove was smaller in groups B (p=0.0003). Both trochlear centers in groups A+B were more centralized (p=0.00002–0.03). The medial trochlear center was more distal in group A (p=0.03) and the lateral trochlear center was more distal in group B (p=0.00009). The trochlear groove started more distal in group B (p=0.0007).

A better understanding of the 3-dimensional geometry can help better treat or even prevent the progression of disease to the stage of patellofemoral osteoarthritis. In osteoarthritic and dysplastic patellofemoral joints, the trochlea is both smaller and more distally located along the femur. These two factors may contribute to excessive loads that lead to early joint wear. These differences could have biomechanical implications and give us an insight into why joints fail. The data collected may also help in improving current designs and current navigational and surgical techniques used for the treatment of patellofemoral osteoarthritis.

When navigating patellofemoral/unicompartmental knee surgery, the surgeon makes assumptions based upon algorithms developed for total knee arthroplasty. In this study we set out to show how variable the normal knee is. Minor anatomical variations in the shape of our knee may make a big difference in terms of orientation and joint wear patterns. Tibial patho-morphology has been described as a factor that predisposes to medial compartment osteoarthritis of the knee (anteromedial-OA), yet this is limited to 2D analysis. We aimed to describe the 3D morphology of both the tibial and femoral components of the medial compartment of the knee. We hypothesized that morphological differences do exist between normal knees and those predisposed to osteoarthritis.

A total of 20 normal (group A) and 20 pre-OA knees (group B) were included. Group A consisted of contra lateral knees of young patients (< 55 years) awaiting hip surgery and group B of asymptomatic contra lateral knees of patients awaiting unicompartmental knee arthroplasty (UKA). Using 3D reconstructions from CT scans, we analyzed the tibiofemoral joint, which consists of the femoral condyles and the tibial plateau. The femur was aligned to the transcondylar and anatomical axes. The medial femoral extension facet (MFEF) was modeled as a segment of a sphere. The offsets between the MFEF centre and the medial femoral flexion facet centre were measured. The MFEF radius and the MFEF 2D arc angle in the sagittal plane were also measured. The tibias were aligned for flexion-extension and varus-valgus to a flat portion of the flexion facet (flexion facet plane), which lie’s roughly perpendicular to the tibial mechanical axis. To control for axial rotation, the anatomical tibial axis was used. A model of analysis was developed by rotating several increments towards and away from the midline to obtain several sagittal section images. For each sagittal section the medial tibial extension facet (MTEF) slope angle, its length, and the medial tibial submeniscal plane (MTSP) angle and length were analyzed. The relative length proportions of the MTEF, medial tibial flexion facet and MTSP were also measured.

The MFEF was larger and more offset in pre-OA knees. Pre-OA knees also had a significantly larger MFEF arc angle than normals (p< 0.05). The MTEF appeared similar between normal and pre-OA knees. The submeniscal plane was highly variable between subjects but on average horizontally inclined (median 0o, range −15–14o) and formed a crescent shape anteriorly. There was no significant difference in tibial measured parameters between normal and pre-OA tibias (p> 0.05). The method showed good reproducibility using intraclass correlation coefficient (ICC value> 0.9) and Bland-Altman plot analysis.

This study gives the CAOS surgeon some interesting insights into the anatomical variation of the normal knee. We have found evidence of a predisposing patho-morphology to medial-OA in the femoral condyle, but not the tibia. There is evidence of an enlarged flatter extension facet on the medial femoral condyle in the pre-OA knees, with no significant difference in the geometry of the medial tibial plateau, which is now reliably defined based upon a flexion plateau frame of reference.

Reconstructive knee arthroplasty in patients with limb deformity can be a daunting and complex task. These patients are often younger and so post traumatic osteoarthritis poses a real challenge. In view of their relative youth, bone preservation would be favourable; however accurate implantation of components is essential. Formulation of a well calculated plan and accurate execution is essential for successful surgery.

We report on a novel method which combines 3D CT joint analysis and computer navigation to define the deformity present pre-operatively and determine whether the proposed reconstruction is feasible. If the reconstructive surgery is feasible, an accurate calculation the correction required is performed. The planned surgery is executed using computer aided navigation surgery.

Eight patients have benefited from the technique. Four patients presented with isolated medial compartment osteoarthritis and intact anterior cruciate ligament. These patients underwent 3D CT joint analysis and computer assisted navigation surgery to accurately implant unicondylar knee replacements.

Four Patients presented with two or three compartment disease. These patients underwent similar 3D CT analysis and navigated Total Knee Replacement.

The series demonstrates the merits of 3D CT joint analysis to accurately define deformity and therefore determine pre-operatively feasibility of corrective surgery proposed. The technique is then complimented by computer assisted navigation surgery to ensure the proposed surgical plan is accurately executed.

Cam femoroacetabular impingement (FAI) is currently treated by resecting the femoral cam lesion. Some surgeons advocate additional anterosuperior acetabular rim resection. However, the exact acetabular contribution to cam-FAI has yet to be described. Using 3D-CT analysis, we set out to quantify the acetabular rim shape and orientation in this condition, and to determine the roles of these factors in cam-FAI.

The acetabula of twenty consecutive cam hips (defined by α-angle of Notzli greater than 55° on plain radiographs) undergoing image based navigated surgery. These were compared with twenty normal hips (defined as disease free sockets with a normal femoral head-neck junction) obtained from a CT colonoscopy database.

Using 3D reconstruction software, the pelvis was aligned to the anterior pelvic plane (APP). Starting at the most anterior rim point, successive markers were placed along the rim. A best-fit acetabular rim plane (ARP) was derived, and the subtended angle (SA) between each rim marker and a normal vector from the acetabular centre was calculated. Values above 90° indicated a peak, with less than 90° representing a trough. Inclination and version were measured from the APP.

Our results showed that the rim profile of both cam-type and normal acetabular is an asymmetric succession of three peaks and three troughs. However, the cam-type acetabulum is significantly shallower overall than normal (Mean SA: 84±5° versus 87±4°, p< 0.0001). In particular, at anatomical points in the impingement zone between 12 and 3 o’clock, the subtended angle of cam hips were never higher than normal, and, in fact, at certain points were lower (iliac eminence: 90±5° vs. 93±4° p=0.0094, iliopubic trough: 79±5° vs. 83±4° p=0.0169, pubic eminence 83±7° vs. 84±4° p=0.4445). The orientation of cam and normal hips were almost identical (Inclination: 53±4°vs. 51±3° p=0.2609 and Anteversion: 23±7° vs. 24±6° p=0.3917).

We concluded that cam-type acetabula are significantly shallower than normal. The subtended angles at all points around the hip were lower, and in particular, in the impingement zone between 12 and 3 o’clock not one cam had a subtended angle over 90°. We have therefore been unable to support the hypothesis of mixed-type FAI in cam-type hips.

Bony rim resection in cam hips therefore runs the risk of rendering the acetabulum more morphologically abnormal and even functionally dysplastic. We do not recommend acetabular rim resection in patients with pure cam-type impingement, and await the longer-term results of this practice with apprehension.

Differing descriptions of patellar motion relative to the femur have resulted from many in-vitro and in-vivo studies. The aim of this study was to examine the tracking behaviour of the patella. We hypothesized that patellar kinematics would correlate to the trochlear geometry and that differing previous descriptions could be reconciled by accounting for differing alignments of measurement axes.

Seven normal fresh-frozen knees were CT scanned and their kinematics with quadriceps loading was measured by an optical tracker system and calculated in relation to the previously-established femoral axes. CT scans were used to reliably define frames of reference for the femur, tibia and the patella. A novel trochlear axis was defined, between the centres of best-fit medial and lateral trochlear articular surfaces spheres.

The path of the centre of the patella was circular and uniplanar (RMS error 0.3mm) above 16°±3° knee flexion. The distal end of the median ridge of the patella entered the groove at 6° knee flexion, and the midpoint at 22°. This circle was aligned 6.4° ± 1.6° (mean± SD) from the femoral anatomical axis, 91.2°±3.4° from the epicondylar axis, and 88.3°±3° from the trochlear axis, in the coronal plane. In the transverse plane it was 91.2°±3.4° and 88.3°±3° from the epicondylar and trochlear axes. Manipulation of the data to different axis alignments showed that differing previously-published data could be reconciled. When the anatomic axis of the femur was used to align the coordinates, there was an initial medial and then a lateral translation. Comparing this with the uniplanar and circular path of the center of the patella, it shows that the orientation of the femoral coordinate system affects the description of the patellar medial-lateral translation.

This study has shown the effect of using different coordinate systems on reporting the patellar translation. Choosing a femoral reference that is more in line with the plane of the circular path of motion and the trochlear groove in the coronal plane diminishes the reported subsequent lateral translation of the patella. Once the frame of reference had been aligned to the trochlear axis, there was minimum medial-lateral translation of the patella.

A 10° deviation from the ideal cup orientation in Metal on Metal (MoM) bearing couples leads to increased wear and the subsequent risk of early revision surgery. We assessed the accuracy of orthopaedic trainees and consultants in achieving optimal acetabular cup orientation.

49 trainees and 18 consultants were asked to orientate an acetabular component to 40° inclination and 20° anteversion in 3 consecutive pelvic models:

osteoarthritic (OA),

The trainee group experience in performing hip arthroplasty procedures ranged from novice to expert (> 100 procedures performed). Performance was measured using an image based navigation system.

Average angular error in all tasks was less than 10°, but the range in anteversion or inclination was up to 65°. Eighteen percent of trainees were +/− 10° of the target orientation in Station A, 29% in B and 2% in C. Forty four percent of consultants achieved the safe zone in A, 16% in B and 0% in C. There was no significant difference in accuracy between the two groups in any of the tasks (p> 0.01). There was no correlation between experience and angular accuracy.

We have been unable to demonstrate trainees have the ability to achieve the optimal cup orientation in a clinically relevant safe zone. A similar range of error is found in experienced surgeons. Focused training or intra-operative computer assistance may provide the solution to improving accuracy in this core orthopaedic skill.

Pincer femoroacetabular impingement (FAI) is cited as being the result of a socket that is either too deep or retroverted, or both. Using 3D-CT analysis, we set out to quantify the acetabular rim shape and orientation to determine the roles of these two factors in FAI.

Twenty pincer acetabulae were selected from patients undergoing image based navigated surgery, where the lateral centre edge angle was greater than 40° on plain radiographs. The normal group of disease free sockets were obtained from a CT colonography database.

Using 3D reconstruction of their CT scans, a novel method of mapping the acetabular rim profile was created. The pelvis was aligned to the anterior pelvic plane. Starting at the most anterior rim point, successive markers were placed along the rim. A best fit plane (ARP) through the acetabulum was derived, and the subtended angle (SA) between each rim marker and a normal vector from the acetabular centre was calculated. Values above 90° indicated a peak, with less than 90° representing a trough. Inclination and version were measured from a horizontal plane and the ARP, in the coronal and axial view respectively.

The results showed that asymmetric acetabular rim profiles in normal and pincer hips were very similar. However, pincer hips are significantly deeper overall (Mean SA 96±5° vs. 87±4° p< 0.00001) and at each anatomical point of the three eminences (pubic [SA: Normal 84±4° vs. Pincer 94±7° p< 0.00001], iliac [SA: 93±4° vs. 100±6° p=0.00021] and ischial [SA: 92±3° vs. 102±8° p=0.00005]) and two troughs (ilio-pubic [SA: Normal 83±4° vs. Pincer 94±8° p=0.00001] and ilio-ischial [SA: 92±3° vs. 102±8° p=0.00002]).

The orientation of normal and pincer were almost identical (Inclination: 51±3° vs. 51±6° p=0.54 and Version: 24±6° vs. 25°±7° p=0.67).

We conclude that the rim shape of pincer hips follows the same contour as normal hips. In agreement with current radiographic diagnosis, pincer-type hips are characterised by a deeper acetabulum. This ‘overcoverage’ of the femoral head confirms the biomechanical model of pincer-type impingement.

Both inclination and version in these two groups were almost identical, with no truly retroverted acetabulum seen. Pincer impingement resulting from ‘acetabular retroversion’ is a concept currently based upon radiographic signs that we have been unable to confirm in this small 3D study using the subtended angle as the key descriptor of acetabular morphology.

Acetabular centre positioning in the pelvis has a profound effect on hip joint function. The force–and moment-generating capacities of the hip muscles are highly sensitive to the location of the hip centre. We describe a novel 3D CT-based system that provides a scaled frame of reference (FOR) defining the hip centre coordinates in relation to easily identifiable pelvic anatomic landmarks. This FOR is more specific than the anterior pelvic plane (APP) alone, giving depth, height and width to the pelvis for both men and women under-going hip surgery.

CT scans of 22 normal hips were analysed. There were 14 female and 8 male hips. The APP was used as the basis of the coordinate system with the origin set at the right anterior superior iliac spine. After aligning the pelvis with the APP, the pelvic horizontal dimension (Dx) was defined as the distance between the most lateral points on the iliac crests, and its vertical dimension (Dy) was the distance between the highest point on the iliac wing and the lowest point on ischial tuberosity. The pelvic depth (Dz) was defined as the horizontal distance between the posterior superior iliac spine and the ipsilateral ASIS. The ratios of the hip centre’s x, y, and z coordinates to their corresponding pelvic dimensions (Cx/Dx, Cy/Dy, Cz,Dz) were calculated. The results were analysed for men and women.

For a given individual the hip centre coordinates can be derived from pelvic landmarks. We have found that the mean Cx/Dx measured 0.09 ± 0.02 (0.10 for males, 0.08 for females), Cy/Dy was 0.33 ± 0.02 (0.30 for males, 0.35 for females), and Cz/Dz was 0.37 ± 0.02 (0.39 for males and 0.36 for females). There was a statistically significant gender difference in Cy/Dy (p=0.0001) and Cz/Dz (p=0.03), but not in Cx/Dx (p=0.17). Anteversion for the male hips averaged 19° ± 3°, and for the female hips it was 26° ± 5°. Inclination measured 56° ± 1° for the males and 55° ± 4° for the females. Reliability testing showed a mean intra-class correlation coefficient of 0.95. Bland-Altman plots showed a good inter-observer agreement.

This method relies on a small number of anatomical points that are easily identifiable. The fairly constant relationship between the centre coordinates and pelvic dimensions allows derivation of the hip centre position from those dimensions. Even in this small group, it is apparent that there is a difference between the sexes in all three dimensions. Without the need for detailed imaging, the pelvic points allow the surgeon to scale the patient’s pelvis and thereby know within a few millimetres the ‘normal’ position of the acetabulum for both men and women. This knowledge may be of benefit when planning or undertaking reconstructive hip surgery especially in patients with hip dysplasia or bilateral hip disease where there is no reference available for planning the surgery.

Objective: The aim of the study was to test the hypothesis that insertion of a total knee replacement (TKR) may effect range of motion as a consequence of excessive stretching of the retinaculae.

Methods: 8 fresh frozen cadaver knees were placed on a customised testing rig. The femur was rigidly fixed allowing the tibia to move freely through an arc of flexion. The quadriceps were loaded to 175N in their physiologic lines of action using a cable, pulley and weight system. The iliotibial tract was loaded with 30N. Tibiofemoral flexion and extension was measured using an optical tracking system. Monofilament sutures were passed along the fibres of the medial patellofemoral ligament (MPFL) and the deep transverse band in the lateral retinaculum with the anterior ends attached to the patella. The posterior suture ends were attached to ‘Linear Variable Displacement Transducers’. Thus small changes in ligament length were recorded by the transducers. Ligament length changes were recorded every 10° from 90° to 0° during an extension cycle. A transpatellar approach was used when performing the TKR to preserve the medial and lateral retinaculae. Testing was conducted on an intact knee and following insertion of a cruciate retaining TKR (Genesis II). Statistical analysis was performed using a two way ANOVA test.

Results: The MPFL had a mean behaviour close to isometric, while the lateral retinaculum slackened by a mean of 6mm as the knee extended from 60 degrees (Fig 1). After knee replacement there was no statistically significant difference seen in ligament length change patterns in the MPFL, however the lateral retinaculum showed significant slackening from 10 to 0°.

Conclusion: The data does not support the hypothesis that insertion of a TKR causes abnormal stretching of the retinaculuae. This result relates specifically to the TKR design tested.

Although acetabular centre positioning has a profound effect on hip joint function, there are very few studies describing accurate methods of defining the acetabular centre position in 3D space. Clinical and plain radiographic methods are inaccurate and unreliable. We hypothesize that a 3D CT-based system would provide a gender-specific scaled frame of reference defining the hip centre coordinates in relation to easily identifiable pelvic anatomic landmarks.

CT scans of thirty-seven normal hips (19 female and 18 male) were analysed. The ratios of the hip centre coordinates to their corresponding pelvic dimensions represented its horizontal (x), vertical (y), and posterior (z) scaled offsets (HSO, VSO, and PSO).

The mean HSO for females was 0.08 ± 0.018, mean VSO was 0.35 ± 0.018, and mean PSO was 0.36 ± 0.017. For males HSO averaged 0.10 ± 0.014, VSO was 0.32 ± 0.015, and PSO was 0.38 ± 0.013. There was a statistically significant gender difference in all three scaled offsets (p=0.04, 0.002, and 0.03 for HSO, VSO, and PSO respectively). Inter-observer agreement tests showed a mean intra-class correlation coefficient of 0.95.

We conclude that this frame of reference is gender-specific giving a unique scale to the patient and allowing reliable derivation of the position of the hip centre from the pelvic dimensions alone. The gender differences should be borne in mind when positioning the centre of a reconstructed hip joint. Using this method, malpositioning, particularly in the antero-posterior (or z) axis, can be identified and addressed in a malfunctioning hip replacement. Pathological states, such as dysplasia and protrusio, can also be accurately described and surgery addressing them can be precisely planned.

Objective: The aim of this study was to test the hypothesis that malrotation of the femoral component following total knee replacement (TKR) may lead to patellofemoral complications as a consequence of excessive stretching of the retinaculae.

Methods: 8 fresh frozen cadaver knees were placed on a customised testing rig. The femur was rigidly fixed allowing the tibia to move freely through an arc of flexion. The quadriceps and iliotibial tract were loaded to 205N in their physiologic lines of action using a cable, pulley and weight system. Tibiofemoral flexion and extension was measured using an optical tracking system. Monofilament sutures were passed along the fibres of the medial patellofemoral ligament (MPFL) and the deep transverse band in the lateral retinaculum with the anterior ends attached to the patella. The posterior suture ends were attached to ‘Linear Variable Displacement Transducers’. Thus small changes in ligament length were recorded by the transducers. Ligament length changes were recorded every 10° from 90° to 0° during an extension cycle. A transpatellar approach was used when performing the TKR to preserve the medial and lateral retinaculae. Testing was conducted following insertion of a cruciate retaining TKR (Genesis II). The femoral component was rotated using a custom built intramedullary device. Ligament length changes were measured at neutral rotation, 5° internal and 5° external rotation. Statistical analysis was performed using a two way ANOVA test.

Results: Internal rotation resulted in the MPFL slackening a mean of 1.7mm from 70-0° extension (p< 0.001). External rotation resulted in the MPFL tightening a mean of 1.5mm over the same range (p< 0.01). The lateral retinaculum showed less significant differences.

Conclusion: External rotation resulted in smaller length changes than internal rotation. Patellar tilting as a result of internal rotation may be caused by MPFL slackening and not lateral retinacular tension, contrary to popular understanding.

Objective: This study tested the hypothesis that complications resulting from overstuffing the patellofemoral joint after total knee replacement (TKR) may be a consequence of excessive stretching of the retinaculae.

Methods: 8 fresh frozen cadaver knees were placed on a customised testing rig. The femur was rigidly fixed and the tibia moved freely through an arc of flexion. The quadriceps and iliotibial tract were physiologically loaded to 205N using a cable, pulley and weight system. Tibiofemoral flexion/extension was measured using an optical tracking system. Monofilament sutures were passed along the fibres of the medial patellofemoral ligament (MPFL) and the deep transverse band in the lateral retinaculum with the anterior ends attached to the patella. The posterior suture ends were attached to ‘Linear Variable Displacement Transducers’. Thus, small changes in ligament length were recorded by the transducers. Length changes were recorded every 10° from 90°- 0° during an extension cycle. A transpatellar approach was used when performing the TKR to preserve the medial and lateral retinaculae. Testing was conducted following insertion of a cruciate retaining TKR (Genesis II). The patella was resurfaced and various patellar thicknesses were achieved by placing 2mm thick nylon washers behind the ‘onlay’ button. The thicknesses measured were 2mm understuff, pre-cut thickness, 2 and 4mm overstuff. Statistical analysis was performed using a two way ANOVA test.

Results: Patellar understuff resulted in the MPFL slackening an average of 1.6mm from 60 to 0° (p< 0.05). Overstuffing the patella 2mm resulted in no significant length changes whereas 4mm overstuff resulted in a mean increase in MPFL length of 2.3mm throughout extension (p< 0.001). No significant length changes seen in the lateral retinaculum

Conclusion: Overstuffing the PFJ stretches the MPFL, because it attaches directly between two bones. The lateral retinaculum attaches to the relatively mobile ITT, so overstuffing does not stretch it.

The rotational alignment of the tibia is an as yet unresolved issue for arthroplasty surgeons. Functional variation may be due to minor malrotation of the tibial component. The aim was to find a reliable method for positioning the tibial component in arthroplasty.

CT scans of 21 knees were reconstructed in three dimensions and oriented vertically. A plane was taken 20 mm below the tibial spines. The centre of each tibial condyle was calculated from points taken round that condylar cortex. A tibial tubercle centre was also generated as the centre of the circle that best fit points on the surface of the tubercle in the plane of its most prominent point.

The derived points were identified by three observers with errors of 0.6 – 1mm. The medial and lateral tibial centres were constant features (radius 24mm ± 3mm, and 22mm ± 3mm respectively). An ‘anatomic’ axis was created perpendicular to a line joining these two points. The tubercle centre was found 20mm ± 7mm lateral to the medial tibial centre. Compared to this axis, an axis perpendicular to the posterior condylar axis was internally rotated by 6° ± 3°. An axis based on the tibial tubercle and the tibial spines was also internally rotated by 6° ± 10°.

We conclude that alignment of the knee when based on this ‘anatomic’ axis is more reliable than either of the posterior surfaces. It is also more reliable than any axis involving the tubercle, which is the least reliable feature in the region. The ‘anatomic’ axis can be used in navigated knee arthroplasty for referencing the rotational alignment of the tibial component.