Barriers to the adoption of unicompartmental knee arthroplasty (UKA) by new consultants could be explained by its higher revision rate, to which mal-positioned components contribute. The aim of this study was to determine whether robotic technology enables inexperienced surgeons to perform accurate UKAs when compared to current conventional methods

After randomisation, sixteen trainees who had never performed UKAs performed three medial UKAs (Corin Uniglide), one per week, on dry-bone simulators by either robotic (Sculptor RGA) or conventional methods. They were instructed to match a universal 3D-CT based pre-operative plan that would result from a UKA based on the conventional jigs and operating guide. The knees were laser scanned and software used to compare the planned and actual implant positions. Feedback was given to trainees between attempts. Translational and rotational positioning errors were measured in all six degrees of freedom for both components

At all attempts robotic medial UKAs were more accurate in both translational and rotational alignments for both components reaching statistical significance (p<0.005) at all attempts for rotational errors. Considering outliers, the maximum rotational errors of the robot group was 9° and 7° for the tibial and femoral components respectively. For the conventional group this reached 18° and 16° for the tibial and femoral components respectively

Robotic technology allows inexperienced surgeons to perform medial UKAs on dry bone models with acceptable accuracy and precision on their first attempt. Conventional jigs do not. The adoption of robotic technology might provide new consultants with the confidence to offer UKAs to their patients by limiting the inaccuracies inherent in conventional equipment.

Osteoarticular loss in a major weightbearing joint is one of the many consequences of military conflict. While minor in terms of life and limb salvage, when rehabilitation is being planned, a small amount of joint damage can make a large impact on the level of long term disability. Reconstruction methods include allograft, massive replacement, arthrodesis and amputation.

We have been developing a suite of technologies that contribute to the reconstruction of such injuries including assessment of disability in a fully instrumented gait lab, modelling of the injury using low dose CT, analysis of the extent of loss and creation of stereolithograph files of the bones, planning of the surgical procedure including implants as needed, custom manufacture of osteotomy guides and prostheses if required and technology assisted surgery, including active constraint robots.

We report 3 cases of soldiers who have suffered osteoarticular loss to part of the knee, two from high velocity rounds and one from an IED. All 3 have received custom partial knee replacements preserving their cruciates, the other compartment and the patella-femoral joint.

No major technical issues have been encountered. The surgery is quick and recovery simple, with the prospect of normal painfree pedestrian life. Exchange of the bearing will be necessary. It is expected and planned for once a decade as a day case procedure. While the surgery is expensive, it is highly cost-effective, as it restores near normal gait. It does not appear to be particularly risky, as the volume of tissue being excised is small, and highly constrained. The same approach is now being developed for deployment in civilian life for post traumatic osteoarthritis.

Introduction

The introduction of the Stanmore Implants Savile Row mobile-bearing UKA procedure in July 2011 marked a world first – the use of a patient-specific knee implanted with robotic technology – the Sculptor Robotic Guidance Arm (RGA). This union gives a truly personalised solution by designing an implant for each patient based upon preoperative CT data and using Sculptor RGA to prepare the bone accurately so that the implant is correctly positioned as planned. The purpose of this study is to evaluate the accuracy of Sculptor RGA both in-vitro and in-vivo. We report on the accuracy of our first clinical procedures.

Background

High functional aspirations and an active ageing population equate to a growing number of patients awaiting hip arthroplasty demanding superior biomechanical function. The purpose of this study was to compare the biomechanics of top walking speed between two commonly used hip arthroplasty procedures to determine if a performance advantage existed.

Background

The combination of patient-specific “just-in-time” implant manufacture and robotic technology has not yet been reported. The robot enables accurate placement of anatomically-matched implants. It should be cost-effective, simplify the procedure, and reduce instrumentation. The aims of this study were to determine whether the procedure was safe, radiographically accurate, and comparable in time and cost to conventional arthroplasty.

Introduction

Are there really ‘conventional’ bearings, offering more security and less risk than the ‘alternative’ bearings that feature in the programme?

Alternative, when used as an adjective has 2 meanings:

offering or expressing a choice, as in several alternative plans

eg alternative newspaper, alternative rock music, alternative medicine

This paper reviews the elements that make up the bearing couples available today in the developed world, and tests each bearing against these meanings.

Metal on Metal hip resurfacing (MoM HR) can be an effective operation for the young arthritic hip population. However, errors in cup orientation have been associated with increased wear, circulating blood metal ions, and soft tissue abnormalities that can lead to premature failure of the bearing surface and subsequent revision surgery. While image free computer guidance has been shown to increase surgical accuracy in total hip arthroplasty, the role of image based technology in MoM HR is unclear. In this study, we compared the accuracy of cup orientation in MoM HR performed by either freehand technique or CT based navigation.

Seventy five patients (81 hips) underwent either freehand (n=42) or navigation (n=39) surgery, both requiring a three dimensional (3D) CT surgical plan. Surgery was conducted by hip specialists blind to the method of cup implantation until the operation. Deviation in inclination and version from the planned orientation, as well as, number of cups within a 10° safe zone and 5° optimal zone of the target position was calculated using post operative 3D CT analysis.

Error in inclination was significantly reduced with navigation compared to freehand technique (4° vs 6°, p=0.02). We could not detect a difference between the two groups for version error (5° vs 7°, p=0.06). There was a significantly greater number of hips within a 10° (87% vs 67%, p=0.04) and 5° (50% vs 20%, p=0.06) safe zone when navigated.

Image based navigation can substantially improve accuracy in cup orientation. The results of our freehand group appear better than historic controls, suggesting the use of a 3D plan may help to reduce technical error and improve the learning curve in this technically demanding procedure. We advocate the use of image based navigation in MoM hip resurfacing arthroplasty.

The current generation of knee replacements are based upon assumptions from kinematic studies that preceded their designs. These implants were further limited by practical restrictions imposed by affordability, materials and manufacturing, and finally by the methods available to surgeons to prepare the bone and implant them. The early designs of knee seldom distinguished left from right, as the early kinematic work had not appreciated the very different functions of the medial and lateral compartments. Trochlea shape and position within devices was also limited by the published work on the way the knee bends. Surgical insertion has been limited to landmark based registration, and adjustment of the kinematics by soft tissue releases. However accurately such operations were performed, they could not restore normal function, as the kinematics of the joint were quite different from the normal knee.

Recently, we have begun to appreciate three distinct axes of the knee joint: the flexion axis, the extension axis and the trochlea axis. These can be reliably found from 3d imaging, but cannot be immediately established by eye, or by conventional jigs, which must rely on unreliable landmarks acquired in surgery.

The current market leaders in knee joint sales do not reflect these three axes in their joint designs, so the instrumentation used to insert them cannot restore the kinematics of the normal knee. The emerging partial replacements can be designed to take the axes and their resulting kinematics into account. If they are then inserted using robotic assistance, or patient specific guides, they can restore joints to these axes reliably. Knee function following such conservative surgery reflects this improvement in kinematics with higher functional scores and faster top walking speeds than has ever been possible using conventional devices inserted using the conventional landmark based surgical techniques.

The radiographic analysis of over 5000 metal on metal (MoM) hips using Ein Bild Roentgen Analyse (EBRA) software have been recently published in an attempt to determine the influence of cup orientation on bearing function. The validation of this software relies one study, conducted in a phantom pelvis without a femoral head in situ. Three dimensional computed tomographic (3D-CT) has been shown to be more accurate for hip and knee arthroplasty than plain radiographs for measurements of component orientation and position. The accuracy of EBRA when compared to 3D-CT for MoM hips specifically is unclear.

We measured the cup orientation of 96 large diameter MoM hips using EBRA analysis of plain radiographs and compared this to 3D-CT. All measurements were made using the radiographic definition of cup orientation. The mean difference in version between the two imaging modalities was 8°; with wide limits of agreement of −21.2° and +5.6°. Three retroverted cups were not detected by EBRA. The mean difference in inclination values was 0.3°, but there was up to 9°difference between imaging modalities. When measured by 3D CT, 64% of hips were within a 10° safe zone around 45° inclination and 20° version, compared to only 24% when measured by EBRA (Fishers Exact test, p< 0.0001).

The measurement of cup orientation of MoM hips using EBRA software is insufficiently accurate, particularly for the assessment of cup version. The cup rim is obscured by the large diameter femoral head on plain radiographs. Research studies using EBRA analysis for version have limited value if accuracy of more than 20 degrees is required to draw conclusions. This software may not be suitable to measure the performance of a device or surgeon. The limitations of EBRA can be overcome, if 3D-CT with an extended Hounsfield scale for data capture is used.

Laboratory experiments and computational models were used to predict bone-implant micromotion and bone strains induced by the cemented and cementless Biomet Oxford medial Unicompartmental Knee Replacement (UKR) tibial implants.

Introduction

Metal Artefact Reduction Sequence (MARS) MRI is being increasingly used to detect soft tissue inflammatory reactions surrounding metal-on-metal hip replacements. The UK MHRA safety alert announced in April 2010 recommended cross-sectional imaging such as MRI for all patients with painful MOM hips. The terms used to describe the findings include bursae, cystic lesions and solid masses. A recently used term, pseudotumour, incorporates all of these lesions. We aimed to correlate the pattern of abnormalities on MRI with clinical symptoms.

Background

In large diameter hip arthroplasty, the femoral head size and shape have to be optimised to avoid neck on socket impingement if the head is too small, or psoas tendonopathy if the head is too large, overhanging the normal head neck junction in the sagittal plane. Currently there is no published guideline to help the surgeon select an optimal size femoral head. Instead, the novice surgeon may inadvertently oversize the femoral component through fear of notching the femoral neck—causing psoas impingement, especially in female patients. We sought to provide anatomically based advice for surgeons to optimise both the position of the femoral head and the head neck ratio.

Introduction

In 2009, surgeon error is a major factor contributing to premature failure in conventional arthroplasty. Technology has revolutionised quality control in all manufacturing industries, yet it has made little or no impact on practice in arthroplasty. Currently, no agreed standards exist—in either the UK or Australia— that allows us to state whether or not the operation was performed correctly.

In hip arthroplasty, acetabular orientation may be considered a non-controversial metric for assessing surgical precision in hip arthroplasty. We considered that a trained surgeon should be able to orientate the acetabular component within the safe zone 19 times out of 20.

Introduction: The London Implant Retrieval Centre (LIRC) was founded to investigate the high incidence of unexplained failures of Metal-on-Metal (MoM) hips. A multidisciplinary team analyse the failed hips, investigations include CT and MRI scans, blood and synovial fluid tests, wear measurements, X-rays and clinical data from the explanting surgeons.

Wear measurements of 100 explanted hips have been carried out on a Taylor Hobson 365 Roundness Machine using the LIRC Wear Protocol. It was found that 50% of explanted cups were wearing less than 5 μm/year and 60% of components were wearing less than 10 μm/year. Wear tests on hip joint simulators predict wear rates between 2 and 8 μm/year. However, 6% of cups are wearing faster than 100 μm/year, with 16% of cups have wear patches deeper than 100 μm and that 4% have a wear patch deeper than 300 μm.

Discussion: This paper considers the common characteristics of components in this very high wearing category. Engineering parameters such as head/cup clearance, surface finish, form errors and head cup contact conditions are investigated. This is correlated with clinical data and other results from the LIRC.

Cup position is an important factor, all of the high wearing components are outside the Lewinick’s Box, however it is shown that mal position is does not always lead to extreme wear. Further analysis is taking place to calculate the size of the contact patch between head and cup (based on patient data and biomechanics) and the proximity of the contact patch to the edge of the cup.

Conclusion: The study of explanted components shows that 6% exhibit extreme wear, and although several “risk” factors can be identified, it is not clear why only a proportion of these components show extremely high wear rates. This is the subject of current investigation.

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.

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.

Introduction: Preoperative planning plays an important role in any surgical procedure and total hip replacement (THR) is no exception. Templating of radiographs allows preoperative assessment of the correct size of implant to be used, lowers the risk of periprosthetic fracture, helps restore femoral offset and leg length, facilitates the optimisation of alignment and ensures the correct implants sizes are available.

With the wide scale use of Picture Archiving and Communication Systems (PACS) in the National Health Service (NHS), the potential exists for faster and more accurate templating of THRs.

Aim: In performing this study, we assessed whether there is adequate provision of the current NHS PACS to allow optimal digital templating for THRs. We also made comparison between the availability and overall ease of conventional versus digital templating.

Methods: Data was collected using a telephone questionnaire requesting information from the on-call orthopaedic Specialist Registrar (SpR) in 28 Greater London and surrounding area NHS Hospitals. Data on the availability of PACS and the ability to template using hard-copy or digital templating was collected and analysed.

Results: PACS were used in all 28 (100%) hospitals that were contacted. None performed conventional templating regularly and only 8 (28.6%) admitted to occasional templating. The predominant reason for this was difficulty in obtaining hard copies of x-rays in 12 (42.9%) hospitals, as well as lack of availability acetate templates, with 13 (46.4%) claiming that this was the case.

Digital templating software was available in 14 (50%) hospitals. Despite this, none of them performed digital templating regularly. In the 50% that did have digital templating, this was not routinely done for the following reasons:

only 3 (10.7%) allowed easy access to the software to the SpRs

Discussion: Digital PACS systems have made great improvement in the access of radiographs in the NHS. With regards to orthopaedic practice, however, we have shown that the benefit of digital templating is being overlooked within the NHS.

Introduction: The successful outcome from metal-on-metal hip resurfacing is partly dependent on the restoration of the natural biomechanics of the hip joint. Valid measurement of the geometry of the reconstructed hip is challenging using plain radiographs. CT is more accurate and precise yet rarely used to assess hip geometry. Our aims were 1) to quantify the agreement between radiographic and CT measurement of horizontal femoral offset (HFO); 2) to determine the relationship between HFO and patient gender and size; and 3) To compare HFO of the reconstructed hip to the contralateral hip.

Method: We used plain radiograph and CT data from 42 patients (23 male and 19 female) from a consecutive series with unilateral metal-on-metal hip resurfacings. We measured HFO of both hips (component and contralateral) using plain radiographs (with PACS) and CT (with Robin 3D software). Pelvic width and radial head sizes were measured on CT. Measurements were made in triplicate by 2 observers.

We graded the contralateral hip for severity of joint space narrowing on plain radiographs.

Results: There was considerable disagreement between CT and plain radiographs for HFO. HFO was statistically different between genders (p=0.0004). HFO correlated with femoral head radius (0.57, p=0.0002), but not patient size (for height (0.29, p=0.13), or pelvic width (0.25, p=0.11). There was a wide range of HFO of the contralateral hips that was comparable to the reconstructed hip.

Conclusion: To our knowledge this is the first study to show the importance of measuring HFO using CT. HFO was found to be correlated to gender and femoral head radius, but not with any other parameters of patient size. The wide range of offset was considerably greater than is available from current total hip replacement designs. Hip resurfacing may overcome this.

Introduction: Unicompartmental knee Arthroplasty (UKA) is a commonly used and accepted treatment for Osteoarthritis (OA) in the medial compartment. How-ever, despite some good results¹ there is still a reluctance to use this procedure in the lateral compartment for the same indications, as the procedure is considered technically difficult, and not as successful². This study reports the clinical outcome of lateral UKAs in comparison with medial UKAs, TKAs and a normal population group using a knee score designed to highlight the shortcomings of TKA³.

Methods: 20 consecutive patients over 2 years following lateral UKA were functionally assessed. They were compared with 3 groups of 20 age and sex matched patients: those who had undergone medial UKA or TKA in the same time period, or normal controls from an upper limb clinic. Clinical function was assessed at least 2 years postoperatively, using the ‘total knee questionaire’³. This consists of 55 scaled multiple choice questions. The score is derived from the product of three scales: the importance of a specific activity, the frequency with which it is undertaken, and the ease with a patient can perform it.

Results: 90% of the patients reported that they were either satisfied or very satisfied with their lateral UKA, with 95% of the patients in the medial UKA group and 75% in the TKA group reaching this level of satisfaction. The average Composite Score for the lateral UKA group was significantly better compared with the TKA group (p < 0, 05). (Kneeling – (5,72/4,45), Gardening – (7,32/5,18), Pivoting – (7,83/6,78) and Walking with heavy bags (8,2/5,97)). The Total Composite Score was significantly better (p< 0, 05) in Patients after lateral UKA (7,14) compared to patients who underwent TKA (5,99). No statistically significant differences in the Total Composite Score was found between both the lateral & medial UKA patients taken as a single group compared with the control group.

Conclusion: Lateral Unicompartmental Knee Arthroplasty achieves superior knee function in comparison to Total Knee Arthroplasty, so is worth considering as an option in for early OA of the lateral compartment.

Introduction: Patellofemoral complications are one of the major causes for revision surgery. In the prosthetic knee, the main determinant within the patellofemoral mechanism is said to be the design of the groove (Kulkarni et al., 2000). Other studies characterising the native trochlear groove used indirect methods such as photography, plain radiographs and measurements using probes and micrometer. The aim of this study was to define the 3-dimensional geometry of the femoral trochlear groove. We used CT scans to describe the geometry of the trochlear groove and its relationship to the tibiofemoral joint in terms of angles and distances.

Materials and Methods: CT scans of 45 normal femurs were analysed using custom designed imaging software. This enabled us to convert the scans to 3D and measure distances and angles. The flexion axis of the tibiofemoral joint was found to be a line connecting the centres of the spheres fitted to posterior femoral condyles. These two centres and the femoral head centre form a frame of reference for reproducible femoral alignment. The trochlear geometry was defined by fitting circles to cross sectional images and spheres to 3D surfaces. Axes were constructed through these centres. The deepest points on the trochlear groove were identified using quad images and Hounsfield units. After aligning the femur using different axes, the location of the groove was examined in relation to the mid plane between the centres of flexion of the condyles.

Results: The deepest points on the trochlear groove can be fitted to a circle with a radius of 23mm (S.D. 4mm) and an R.M.S error of 0.3mm. The groove is positioned laterally (especially in its mid portion) in relation to the femoral mechanical and anatomical axes. It was also lateral to the perpendicular bisect of the transcondylar axes. After aligning the anatomical axis in screen the trochlear groove can be described on average to be linear with less than 2 mm medial/lateral translation.

In the sagital view, the centre of the circle is offset by 21mm (S.D.3mm) at an angle of 67° (S.D. 7°) from a line connecting the midpoint between the centres of the femoral condyles and the femoral head centre.

On either end of this line, the articular surface of the trochlea can be fitted to spheres of radius 30mm (S.D. 6mm) laterally and 27mm (S.D. 5mm) medially, with an rms of 0.4mm.

Discussion: The location and configuration of the inter-condylar groove of the distal femur is clinically significant in the mechanics and pathomechanics of the patellofemoral articulation. This investigation has allowed us to characterise the trochlear groove.

This can be of use in planning and performing joint reconstruction and have implications for the design of patello-femoral replacements and the rules governing their position.

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.

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.

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.

Radiological measurements are an essential component of the assessment of outcome following knee arthroplasty. However, plain radiographic techniques can be associated with significant projectional errors because they are a two-dimensional (2D) representation of a three-dimensional (3D) structure. Angles that are considered within the target zone on one film may be outside that zone on other films. Moreover, these parameters can be subject to significant inter-observer differences when measured. The aim of our study therefore was to quantify the variability between observers evaluating plain radiographs following Unicompartmental knee arthroplasty.

Twenty-three observers, made up of Orthopaedic Consultants and trainees, were asked to measure the coronal and sagittal alignment of the tibial and femoral components from the post-operative long-leg plain radiograph of a Unicompartmental knee arthroplasty. A post-operative CT scan using the low dose Imperial knee protocol was obtained as well and analysed with 3D reconstruction software to measure the true values of these parameters. The accuracy and spread of the pain radiographic measurements were then compared with the values obtained on the CT.

On the femoral side, the mean angle in coronal alignment was 1.5° varus (Range 3.8, SD 1, min 0.1, max 3.9), whereas the mean angle in sagittal alignment was 8.6° of flexion (Range 7.5, SD 1.5, Min 3.7, Max 11.2). The true values measured with CT were 2.4° and 11.0° respectively. As for the tibial component, the mean coronal alignment angle was 89.7° (Range 11.6, SD 3.3, Min 83.8, Max 95.4), and the mean posterior slope was 2.4° (Range 8.7, SD 1.6, Min -2, Max 6.7). The CT values for these were 87.6° and 2.7° respectively.

We conclude that the plain radiographic measurements had a large scatter evidenced by the wide ranges in the values obtained by the different observers. If only the means are compared, the plain radiographic values were comparable with the true values obtained with CT (that is; accuracy was good) with differences ranging from 0.3° to 2.4°. The lack of precision can be avoided with the use of CT, particularly with the advent of low-dose scanning protocols.

32 students of surgical technology were instructed in hip resurfacing, and shown detailed plans of the desired operative outcome for the 3 cam type hips. They then used conventional instruments, image-free navigation (brainlab) and image based navigation(Acrobot).

Only image based navigation performed well enough at navigating these difficult cam type hips with novice surgeons. Conventional instruments were not sufficient, with a tendency for the novice to put the hip in varus and translated low on the femoral neck. Image free navigation was more accurate than conventional instruments, avoiding the serious complication of notching but the range of error was 18mm and 10¡.

Image based navigation appears to be fit for purpose in delivering both the accuracy and the precision needed by the novice surgeon in the skills laboratory who needs timely feedback so his clinical experience may start substantially further along the learning curve of this or any other technically demanding operation.

INTRODUCTION: Periprosthetic bone remodeling after uncemented hip replacement has always been a matter of research and debate. DEXA analysis of BMD was studied by previous groups but not the cross sectional cortical volume. We report a validated CT based algorithm for accurate measurement of cortical volume in these group of patients.

METHODS: Twenty two patients (34 hips) who have undergone Uncemented Furlong total hip replacement agreed to undergo CT scan of their hips for our study. The mean age was 74.6 yrs. The mean follow up was 5.4 yrs. 12 patients had bilateral replacement.

Using software adapted for the specific purpose, femoral cortical volume was measured at three different levels at a fixed distance from the lower border of the lesser trochanter on both sides: 6mm distal to the tip of the prosthesis (z), At the top of the cylindrical portion(x) Midway between x and z (y).

Accuracy and precision of the of the method was also assessed.

RESULTS: The mean cortical volume in the proximal cylindrical portion (x), midpoint(y) and the portion of bone distal to the prosthesis (z) were 458 mm3, 466 mm3, 504 mm3 respectively. The corresponding cortical volumes in the contralateral native femur in unilateral hip replacements were 530 mm3(x), 511 mm3(y), 522 mm3 (z) giving a ratios of 0.86(x), 0.91(y) and 0.97(z). The mean cortical volumes on the left side of bilateral hips were 490 mm3(x), 499 mm3(y) and 528 mm3 (z). The mean cortical volumes on the right side were 456 mm3(x), 463 mm3 (y) and 516 mm3 (z).

No significant trend was noted with change of volume of bone with time.

In the three cases who had cemented hips on their other side, the cemented hips exhibited substantially more stress shielding than their cementless controls (ratios of 0.82, 0.74 and 0.85).

A high correlation between the test and standard measurements was noted. The interobserver agreement between two observers was also good.

DISCUSSION & CONCLUSION: In a fully coated uncemented femoral component, with documented long term results, it is to be expected that load will be shed steadily along the length of the prosthesis. In this study we have confirmed this supposition, with volumetric data, by showing that an almost normal bone just below the tip of the stem (97% volume) reduces to a bone volume of 91% by the middle of the stem and then 86% by the shoulder of the prosthesis. This decrease in the volume of cortical bone effectively normal at the tip of the prosthesis while not optimal appears to stabilize early with no trend of continued reduction over a decade. The effect of cementation on stress shielding was only examined incidentally in this study but appears to contribute to more marked bone loss.

Introduction: The normal relationships of the patellofemoral joint provide a basis for the evaluation of patients with patellofemoral abnormalities. Previous studies have often described the patellofemoral joint using X-rays which are encumbered with projectional inaccuracies. We have used CT to describe the geometry of this joint and its relationship to the tibiofemoral joint in terms of angles and distances.

Materials and method: 33 patients had a CT scan prior to medial unicompartmental knee replacement. These patients have minimum patellofemoral joint disease. Special software was used to convert the scans to 3D and measure the distances and angles. The flexion axis of the tibiofemoral joint was found as the line connecting the centres of the spheres fitted to posterior femoral condyles. These two centres and femoral head centre form a frame of reference for reproducible femoral alignment. The trochlear geometry was defined by fitting circles and spheres to slices and surfaces, then constructing an axis through their centres. The geometry of the patella was established by fitting two planes to the proximal and anterior extra-articular surfaces of the patella. The relationships between these planes and the rest of the patella were explored.

Results: The deepest points on the trochlear groove can be fitted to a circle with radius of 23mm (stdev 4mm) and an rms of 0.3mm. This centre is offset by 21mm (stdev 3mm) at an angle of 68° (stdev 8°) from the line connecting the midpoint between the centres of the femoral condyles and a point in the piriform fossa.

On either end of this line, the articular surface of the trochlea can be fitted to spheres of radius 30mm (stdev 6mm) laterally and 27mm (stdev 5mm) and an rms of 0.4mm medially. The centres of the circle and the two spheres fall on a line with an rms of 1.1mm.

The anterior and proximal patellar planes could be described as flat surfaces (rms of 0.4 and 0.3mm). The median ridge could be described as a straight line (rms of 0.2mm). The angle between planes was 112° (stdev 5°); the average angle between the proximal plane and the line on the medial ridge was 62° (stdev4°).

The functional centre of the patella was defined as a point in the centre of 2 planes orthogonal to the sagital plane from the midpoint between the most proximal and most distal points on the median ridge. The length, width and thickness of the patellae were measured at 22mm +/−4mm, 47mm +/− 3mm and 24 mm+/− 2 mm.

Discussion: This investigation has allowed us to characterise the patello-femoral joint geometry. The knowledge of the shapes of the surfaces of this joint and their relationships may help identify and explain the aetiology of patello-femoral dyplasia and other pathologies. It may also be of use in planning and performing joint reconstruction and may have implications for the design of patello-femoral replacements and the rules governing their position.

Patellofemoral symptoms are a prominent cause of dissatisfaction following knee arthroplasty. This may relate to difficulty in knowing where to resect the bone and in placing prosthetic components to reproduce the anatomy accurately. This study developed geometrical data to facilitate these procedures during TKR.

Thirty CT scans of patients above the age of 55 without patellofemoral disease were performed. Three dimensional images were reconstructed using computer software that enabled manipulation of these images and measurements to be taken. These models allowed the shape of the patella to be modelled, its size and the track it takes in the normal trochlea.

The anterior and proximal patellar planes could be described as flat surfaces with an rms of 0.4 and 0.3mm. The angle between these planes was 112° (stdev 5°). The median ridge of the articular surface was a straight line with an rms of 0.2mm and the average angle between the anterior plane and this line was 12° (stdev4°). The angle between the anterior plane and a line fitted to the posterior aspect of the apex of the patella was 56° (stdev 2°). Having oriented the patella with the proximal plane vertical, the distal pole of the patella was within 2mm of the same sagittal plane as the median ridge of the articular surface in all cases. The functional centre of the patella was defined as a point in the centre of 2 planes orthogonal to the sagittal plane at the midpoint between the most proximal and most distal points on the median ridge. In the transverse section this centre was always on the line separating the superficial and deep surfaces of the patella. Also the length, width and thickness of the patellae were measured at 22mm +/−4mm, 47mm +/− 3mm and 24 mm+/− 2 mm. The average ratio of the lateral facet to medial facet width was 1.3 (range 0.8–1.6). The average ratio of the patellar width to thickness was 2.0 (S.D. 0.106, 95%CI 1.96 to 2.03) with a strong correlation(r= 0.89).

From this work we have concluded that the anterior and proximal planes of the patella, which will not be affected by the disease, can be defined and used as a frame of reference for the patella, which will be helpful for navigating the patella and restoring its anatomical form in the presence of erosive changes.

The patella has a constant shape, so that its articular surface can be defined in relatively simple terms, and can be referenced off its non articular surface.

Wear and loosening are the major causes for long tem failure in Total Hip Replacement (THR). Accurate three dimensional wear analysis of radiographs has its own limitations. We report the results of our clinical study of three dimensional volumetric wear measurements using our custom low radiation risk CT based algorithm and special software

Twenty four patients (32 hips) agreed to take part in our study. The male: female ratio was 1:4. The mean age was 75 years and the mean follow up was 5.4 years. All patients had 28 mm diameter ceramic heads. Of the 32 hips, 17 hips had polyethylene inserts and 15 hips had ceramic inserts. The maximum follow up for the polyethylene and ceramic groups were 12 years and 5.5 years respectively. All the patients were scanned using Somatom Sensation 4 scanner. Using custom software, 3D reconstruction of the components was done and landmark acquisition done on the femoral head, acetabular metal component and the insert. From these landmarks, a dedicated program was used to calculate the centre of the femoral head in relation to the centre of the acetabular component in all three axes and an indirect measurement of wear obtained. Using the axes measurements graphical 3D models of migration of the femoral head component into the acetabular liner were created and volume of wear measured using special software. Accuracy of the method was assessed by measuring the radius of the femoral head since all patients had 28mm diameter heads implanted in them. Assessment of precision of method was done by calculating the level of agreement between two independent observers.

In the polyethylene group, there was no significant (< 1mm) wear in x and y axis with time. However there was significant evidence of wear in relation to time in the z axis (max wear = −2.5 mm). In the ceramic group with relatively shorter follow up, there was no evidence of significant wear in all three axes. The mean volume measured in the polyethylene group was 685 mm3 (max = 1629 mm3, min = 132mm3 ). The mean volume measured in the ceramic group was 350mm3 (max = 1045 mm3, min = 139mm3 ). The mean radius of the femoral head measured in both groups was 14.02mm (range =13.8 to 14.4 mm). Accuracy was limited by artifacts particularly in bilateral hip arthroplasties and further in the ceramic group because of the restricted access to the ceramic head for placement of markers. Measurements obtained by two independent observers showed a strong correlation (0.99, p value = 0.001) for the polyethylene group. In the ceramic group the correlation (0.69, p value=0.0126) was not as strong as the polyethylene group.

This study has produced a method for three dimensional estimation of wear that can be obtained from low dose CT scans with better accuracy and repeatability (< 0.5 mm) even than to ex vivo studies particularly in polyethylene bearings(wear rate 0.14mm/yr). Noise reduction with appropriate artefact reduction software may further improve the accuracy of this simple and repeatable method.

Periprosthetic bone remodeling after uncemented hip replacement has always been a matter of research and debate. DEXA analysis of bone density was studied by previous groups but not the cross sectional cortical volume. We report a validated CT based algorithm for accurate measurement of cortical volume in these group of patients.

Twenty two patients who have undergone Uncemented Furlong total hip replacement agreed to undergo CT scan of their hips for our study. The mean age was 74.6 yrs. The mean follow up was 5.4 yrs. Using software adapted for the specific purpose, femoral cortical volume was measured at three different levels at a fixed distance from the lower border of the lesser trochanter on both sides:

6mm distal to the tip of the prosthesis (z),

Accuracy of the method was assessed by measuring the volume of artificial cavities created on a polyurethane pelvis. Assessment of precision of method was done by calculating the level of agreement between two observers.

The mean cortical volume in the proximal cylindrical portion (x), midpoint(y) and the portion of bone distal to the prosthesis (z) were 458 mm3, 466 mm3, 504 mm3 respectively. The corresponding cortical volumes in the contralateral native femur in unilateral hip replacements were 530 mm3(x), 511 mm3(y), 522 mm3 (z) giving a ratios of 0.86(x), 0.91(y) and 0.97(z). The mean cortical volumes on the left side of bilateral hips were 490 mm3(x), 499 mm3(y) and 528 mm3 (z). The mean cortical volumes on the right side were 456 mm3(x), 463 mm3 (y) and 516 mm3 (z). No significant trend was noted with change of volume of bone with time. In the three cases who had cemented hips on their other side, the cemented hips exhibited substantially more stress shielding than their cementless controls (ratios of 0.82, 0.74 and 0.85). A high correlation between the test and standard measurements was noted. The interobserver agreement between two observers was also good.

In a fully coated uncemented femoral component, with documented long term results, it is to be expected that load will be shed steadily along the length of the prosthesis. In this study we have confirmed this supposition, with volumetric data, by showing that an almost normal bone just below the tip of the stem (97% volume) reduces to a bone volume of 91% by the middle of the stem and then 86% by the shoulder of the prosthesis. This decrease in the volume of cortical bone effectively normal at the tip of the prosthesis while not optimal appears to stabilize early with no trend of continued reduction over a decade. The effect of cementation on stress shielding was only examined incidentally in this study but appears to contribute to more marked bone loss.

Acetabular and pelvic fractures are amongst the most challenging to treat, still requiring major open surgery. The operations to reduce and fix them entail lengthy operative time, significant blood loss and use of ionising radiation.

We report on the initial stages of developement of a minimally invasive method for navigated reduction and percutaneous fixation of acetabular fractures (NRFA). A commercial navigation platform (Acrobot Ltd.) will be adapted for use with this technique. CT based planning will be used to identify the correct realignment of the the bone fragments, which will then be reduced percutaneously with the aid of two tracked arms attached to the navigation system. Schanz pins, which are inserted in pre-operatively planned sites in each fragment using safe trajectories, are handled as joysticks to manipulate the fracture under computer assistance. Registration of the fragments after insertion of the joysticks will be carried out by means of fluoroscopic images of the AP and Judet views of the fractured acetabulum. Once reduction is achieved by following on-screen instructions, the joysticks are held in place by a custom clamping system connected to one of the arms, while the other is used for percutaneous insertion of column screws.

This technique is potentially suitable for a number of acetabular fractures which include transverse, anterior column, posterior column, T-fractures and some associated both columns fractures. These constitute over 50% of Letournel’s and 60% of Matta’s original series of acetabular fractures. Furthermore, this percutaneous technique could reduce bleeding, wound complications, hospital stay and cost of treatment. Intra operative ionising radiation would be greatly reduced for both patients and the surgeons.

Adequate training with the use of this software may provide a greater number of surgeons the capability to surgically treat these complex fractures.

A Prospective, randomised controlled trial demonstrates superior outcomes using an active constraint robot compared with conventional surgical technique in unicompartmental knee arthroplasty (UKA). Computer assistance should extinguish outliers in arthroplasty, with robotic systems being able to execute the preoperative plan with millimetre precision.

We used the Acrobot system to deliver tailor made surgery for each individual patient. A total of 27 patients (28 knees) awaiting unicompartmental knee arthroplasty were randomly assigned to have the operation performed either with the assistance of the Acrobot or conventionally. CT scans were obtained with coarse slices through hips and ankles and fine slices through the knee joint. Preoperative 3D plans were made and transferred to the Acrobot system in theatre, or printed out as a conventional surgical aid. Accurate co-registration was confirmed, prior to the surfaces of the femur and tibia being milled. The outcome parameters included measurements of the American Knee Society (AKS) score and Western Ontario and McMaster Universities Osteoarthritis (WOMAC) index. These measurements were performed pre-operatively and at six, 18 weeks, and 18 months post-operatively. After 18 months two UKA out of the conventional trial (n =15) had been revised into a total knee replacement (TKA), whereas there were no revisions in the Acrobot trial group (n = 13).

Using an active constrained robot to assist the surgeon was significantly more accurate than the conventional surgical technique. This study has shown a direct correlation between accuracy and improvement in knee scores at 6, 18 weeks and 18 months after surgery. At 18 months there continues to be a significant improvement in the knee scores with again a marked correlation between radiological accuracy and clinical outcome with higher accuracy leading to better function based on the WOMAC and American Knee Society Score.

Periacetabular osteolysis is now considered one of the major long term complications following uncemented total hip replacement. Radiographs are inaccurate and lack sensitivity in detecting lesions even with multiple views. Very few clinical studies have shown the use of CTscan for measuring these lesions. We report our clinical experience with CT based algorithm for measuring it.

Twenty two patients (32 hips) who have undergone Uncemented Furlong total hip replacement agreed to undergo CT scan of their hips for our study. The mean follow up was 5.4 yrs. Of the 34 hips,17 were polyethylene bearings and 15 were ceramic bearings. Nine patients had bilateral replacement in this group. Using custom reconstruction software, 3D models were created and volume measurements made after identifying the lesions in the slices and painting them using appropriate tools available in the software.

Accuracy of the method was assessed by measuring the volume of artificial cavities created on polyurethane pelvis with and without the components. In our control experiments, a high correlation between the test and standard measurements was noted in the cavities above the component, while medial to the acetabular component in bilateral cases it was difficult to be accurate, with cavities less than 10mm in diameter being hard to detect reliably.

In our clinical group of 32 hips, degenerative cysts were noted in 13, secondary rheumatoid cysts in 2 and wear cysts were noted in 2, the largest having a maximum dimension of 10mm. All the degenerative cysts were in the peripheral zone and both the wear cysts were seen in the central zone communicating with the screw holes. These cysts were identified by the characteristic absence of sclerosis surrounding the cyst and obvious communication with screw holes. Both the wear cysts were found with polyethylene bearings at a minimum of 5yrs follow up.

The mean volume of the degenerative cysts was 799 mm3 (71–3500) and the mean volume of the wear cysts was 567 mm3 (550–585)

The low dose CT method we describe and the results we report show that cavities can be measured reliably, above or below the acetabular component. On the medial side, in bilateral cases in particular, although location is possible, volumetric analysis of anything less than 10mm in diameter is not.

Regarding surveillance strategy for wear cysts, we have established that in this series the incidence is 14%, with one at 5 yrs and another noted at 12 yrs, with 10mm in maximum dimension. The absence of any wear cysts at all in the ceramic group, albeit after a shorter follow up of only 5 years is encouraging.

Based on these figures, with these implants, we would recommend that there is no need to undertake surveillance more frequently than every 10 years.

We have used CT to describe the geometry of the patel-lofemoral joint and its relationship to the tibiofemoral joint.

33 CT scans of patients without patellofemoral disease were performed. 3D images were reconstructed and measured using computer software. The flexion axis of the tibiofemoral joint was found as the line connecting the centres of the spheres fitted to posterior femoral condyles.

The deepest points on the trochlear groove can be fitted to a circle with radius of 23mm (stdev 4mm) and an rms of 0.3mm. This centre is offset by 21mm (stdev 3mm) at an angle of 68° (stdev 8°) from the line connecting the midpoint between the centres of the femoral condyles and a point in the piriform fossa.

On either side of this line, the articular surface of the trochlea can be fitted to spheres of radius 30mm (stdev 6mm) laterally and 27mm (stdev 5mm) and an rms of 0.4mm medially. The centres of the circle and the two spheres fall on a line with an rms of 1.1mm.

The anterior and proximal patellar planes could be described as flat surfaces (rms of 0.4 and 0.3mm). The median ridge could be described as a straight line (rms of 0.2mm). The angle between planes was 112° (stdev 5°); the average angle between the proximal plane and the line on the medial ridge was 62° (stdev4°).

The length, width and thickness of the patellae were measured at 34.2mm +/−4mm, 44.8mm +/− 4.8mm and 22.4 mm+/− 2.3 mm (table).

This investigation has allowed us to characterise the patello-femoral joint geometry which may help identify and explain the aetiology of patello-femoral pathologies. It may have implications for the design of patello-femoral replacements and the rules governing their position.

Whilst computer assistance enables more accurate arthroplasty to be performed, demonstrating this is difficult. The superior results of CAOS systems have not been widely appreciated because accurate determination of the position of the implants is impossible with conventional radiographs for they give very little information outside their plane of view.

We report on the use of low dose (approximately a quarter of a conventional pelvic scan), low cost CT to robustly measure and demonstrate the efficacy of computer assisted hip resurfacing. In this study we demonstrate 3 methods of using 3D CT to measure the difference between the planned and achieved positions in both conventional and navigated hip resurfacing.

The initial part of this study was performed by imaging a standard radiological, tissue equivalent phantom pelvis. The 3D surface models extracted from the CT scan were co-registered with a further scan of the same phantom. Subsequently both the femoral and acetabular components were scanned encased in a large block of ice to simulate the equivalent Hounsfield value of human tissue. The CT images of the metal components were then co-registered with their digital images provided by the implant manufactures. The accuracy of the co-registration algorithm developed here was shown to be within 0.5mm.

This technique was subsequently used to evaluate the accuracy of component placement in our patients who were all pre-operatively CT scanned. Their surgery was digitally planned by first defining the anterior pelvic plane (APP), which is then used as the frame of reference to accurately position and size the wire frame models of the implant. This plan greatly aids the surgeon in both groups and in the computer assisted arm the Acrobot Wayfinder uses this pre-operative plan to guide the surgeon.

Following surgery all patients, in both groups were further CT scanned to evaluate the achieved accuracy. This post-operative CT scan is co-registered to the pre-operative CT based plan. The difference between the planned and achieved implant positions is accurately computed in all three planes, giving 3 angular and 3 translational numerical values for each component.

Further analysis of the CT generated results is used to measure the implant intersection volume between the pre-operatively planned and achieved positions. This gives a single numerical value of placement error for each component. These 3D CT datasets have also been used to quantify the volume of bone resected in both groups of patients comparing the simulated resection of the planned position of the implant to that measured on the post-operative CT.

This study uses 3D CT as a surrogate outcome measure to demonstrate the efficacy of CAOS systems.

Complications following hip resurfacing occur primarily because of the surgeon’s inability to achieve optimal implant positioning, and the significant learning curve associated with it. Our study sought to look at the impact of navigation technologies on this learning curve.

Twenty medical students doing their BSc project took part in the study. Four types of synthetic femurs were used for the study viz., Normal anatomy (11students), Osteoarthritis (5), Coxa Vara(2) and Coxa Valga(2). Each student was allowed to insert the guide wire according to their judgement in the femoral head using 3 systems: Conventional instrumentation, 3D plan based on a CT scan of the particular bone, helped by a conventional jig and Navigation system.

This achieved angle was then compared with the angle originally planned for each bone in all three groups using digitizing arm.

The range of error using the conventional method to insert a guide wire was 23deg (range −9 to 14, SD= 6.3), using the CT plan method, it was 22 deg (range −9 to 13, SD=6.6). Using the Navigation method it was 7 deg (range −5 to 2, SD=2.). Students who progressed from conventional through planning to navigation (group 1) were no more accurate than students who went straight to navigation without ever having used conventional instrumentation (group 3). Students produced similar accuracy even in their maiden attempt, on difficult anatomy when provided with navigation technology.

This study has shown that motivated and enthusiastic students can achieve an expert level of accuracy very rapidly when provided with the appropriate level of technology. he development of surgeons who are able to deliver excellent outcomes depends more on technology than training.

Introduction: A subjective observation suggests that a significant percentage of patients offered a TKR could benefit from a relatively more conservative, less invasive unicompartmental knee arthroplasty. We set out to challenge this hypothesis.

Materials & Methods: 1147 TKRs were performed between 2002 and 2005 at Ravenscourt Park Hospital. 50 consecutive knee x-rays of patients who underwent a TKR were reviewed by three independent observers. Medial and lateral articular cartilage height, varus angulation, and femero-tibial anteroposterior and mediolateral translation were measured on antero-posterior and lateral weight bearing radiographs. Skyline views were analysed for patellofemoral disease. The most appropriate procedure according to local radiological criteria was recorded for all three observers. Unicompartmental arthroplasty was considered when the following criteria was met 1) anteromedial disease with preservation of posterior slope, 2) preservation of the tibial spines, 3) no anteroposte-rior or mediolateral translation, 4) normal tibiofemoral alignment and 5) preservation of patellofemoral joint. Osteophytes were disregarded. Tricompartmental disease merited a TKR while isolated patellofemoral (PFJ) disease considered for PFJ replacement. Patients were not formally examined. Preoperative Knee Society Scores (KSS) and WOMAC scores were noted.

Results: The three observers indicated that 26 (52%), 21 (42%) and 22 (44%) patients respectively could potentially benefit from a unicompartmental arthroplasty given the right clinical setting. Consensus was reached for unicompartmental replacement in 16 (31.2%) and for TKR in 18 (36%) of cases. There was no correlation between the operation performed and operation proposed (42% ± 8) suggesting that the surgeon’s preference is a dominating factor. Interestingly within the proposed unicompartmental group Knee Society Scores were higher (100 ± 22 vs 71 ± 26) giving an indication to the disease severity.

Conclusion: The clinical benefit and economic value of opting for a unicondylar knee arthroplasty when indicated is considerable. None the less it was only considered by a minority of surgeons who undertake knee arthroplasty.

1282 Primary total hip replacements were performed over the past 3 years at the Ravenscourt Park Hospital. Standardised post operative x-rays of the pelvis were archived on the Hammersmith Trust Picture Archiving and Communication System. 100 X-rays were randomly selected and reviewed by 3 independent observers (SHO, SPR, and a Consultant), and they were blinded as regards the Surgeon and their colleagues’ assessments. Surgeons who performed their procedure were excluded.

Digital radiographic analysis was performed using the OrthoView system (Meridian Technique Limited, Southamptom, UK). The acetabular component was studied with respect to cup version, the angle of inclination, the quality of cement technique, and the site of cup placement. The stems were studied for cementing technique and quality, stem alignment and limb length discrepancy. A hit was declared when excellence was achieved, whilst all others were declared as a miss. Inter observer rate in declaring a hit or miss was calculated (kappa). 58% of the radiographs studied were declared a hit, and 42% a miss.

All radiological reports were reviewed, and it was noted that no mention was made as regards the cup angles and the cementing quality. Each assessment took 3 minutes

(1.5). The aim of this post operative radiological assessment is to introduce a tool that could be used for appraisal of Surgeons, the surgical technique and for quality control.

The authors conclude that it is an easily reproducible technique, and can be performed by independent observers. These assessments will generate valuable data for research/auditing purposes, and act as an educational tool for trainees. They cautiously recommend this hit or miss approach, believing that it is a cost effective and efficient tool towards achieving better patient quality care and enhancing hip arthroplasty training skills.

Last year at CAOS UK we reported on the development of the Acrobot® Navigation System for accurate computer-assisted hip resurfacing surgery. This paper describes the findings of using the system in the clinical setting and includes the improvements that have been made to expedite the procedure. The aim of our system is to allow accurate planning of the surgery and precise placement of the prosthesis in accordance with the plan, with a zero intra-operative time penalty in comparison to the standard non-navigated technique.

The system uses a pre-operative CT-based plan to allow the surgeon to have full 3D knowledge of the patient’s anatomy and complete control over the sizes and positions of the components prior to surgery.

At present the navigation system is undergoing final clinical evaluation prior to a clinical study designed to demonstrate the accuracy of outcome compared with the conventional technique. Whilst full results are not yet available, this paper describes the techniques that are being used to evaluate accuracy by comparing pre-operative CT-based plans with post-operative CT scans, and gives initial results.

This approach provides a true measure of procedure outcome by measuring what was achieved against what was planned in 3D. The measure includes all the sources of error present within the procedure protocol, therefore these results represent the first time that the outcome of a navigated orthopaedic procedure has been measured accurately.

We used computer tomography (CT) to measure the outcome of knee-arthroplasty in our prospective double-blind randomised controlled study of our active constraint robotic system ACROBOT.

All patients in our trial had pre-operative CT scan and proprietary software used to plan the size, position and orientation of the implants. Post operatively a further CT scan was performed and measurement studies performed using 3 different methods of manipulating the CT dicom data.

Method 1, a quick and simple method of implant assessment that measures the varus-valgus orientation of the implants relative to the axes of the long bones

Two landmarks each are used to define the individual mechanical axis for both the femur and tibia, for consistency these landmarks are the very ones used in the planning stage on the pre-operative CT.

Landmarks are then placed on the implants in order to measure their tilt relative to the mechanical axes. An appropriate Hounsfield threshold (2800) was used to image the metal components. The angle between the individual mechanical axis and the prosthetic component was calculated.

Method 2, detailed and accurate comparisons between the planned and achieved component positions in 3D are made. Co-registration of the precisely planned CT based models with surface models from the post-op scan gives real measurements of implant position enabling the measurement of the accuracy of component in an all six degrees of freedom giving both translation and rotation errors in all three planes.

The process of alignment was achieved by surface-to-surface registration. An implementation of the iterative closest point algorithm was used to register matching surfaces on the objects to be registered. A polygon mesh of the implant, provided by the manufacturer, defined the surface shape of each size of implant. This was used both to define the planned position and to register to the post-operative scan. Method 3, in this study we quantified post-operative error in knee arthroplasty using one value for each component whilst retaining 3D perspective.

The position of the prosthetic components in the post-op scan is calculated and individual transformation matrix computed which is matched to the transformation matrices for the planned components.

The pre-operative CT based component positions were co-registered to the post-operative CT scan and values for the intersection (volumetric) between the digitised images (both planned and achieved) were calculated. Both the co-registered femoral and tibial component’s intersection was quantified with software packages supporting Boolean volume analysis

Method 1, the sum of the two, independently measured, angles allows an estimate of the post-operative alignment of the load bearing axes in the two bones.

Method 2, 3D CT allows precise measurements of the achieved position for each component in all three planes. Six values, three angular and three translational, define the achieved component position relative to the planned position.

Method 3, the greater the percentage intersection between the planned and achieved images, the greater the accuracy of the surgery. Owing to the shape of the components (large articular surface) large intersections demonstrate more accurate reconstruction of the joint line.

In the recent past the lack of a sufficiently accurate tool to plan and measure the accuracy of component placement has resulted in an inability to detect and study radiological and functional outliers and hence the hypnotised relationship between prosthetic joint placement and outcome has been difficult to prove.

CT offers us the ability to accurately describe the actual position and deviation from plan of component placement in knee arthroplasty. Whilst X-ray has the intrinsic problems of perspective distortion magnification errors and orientation uncertainties CT can be used to define ‘true’ planes for two dimensional (2D) measurements and permits the comparison in three dimensions (3D) between the planned and achieved component positions.

Hip resurfacing has advantages over hip replacement for younger, more active patients. However, it requires that surgeons learn new techniques for correctly cutting bone and positioning the components. Pre-operative planning systems exist for conventional hip replacement. Planning software for hip resurfacing is described, with the resulting plans available as a visual aid during surgery, or transferred to the Acrobot^® Navigation system for intra-operative guidance.

CT data is acquired from the top of the pelvis to immediately above the acetabulae in 4 mm slices, and from there down to just below the lesser trochanter in one mm slices. This keeps radiation doses low while providing high image quality in the important regions for planning. This is segmented semi-automatically, and bone surface models are generated.

Frames of reference are generated for the pelvis and femur, and the acetabular and femoral head positions are computed relative to these.

Prosthesis components are initially positioned and sized to match the computed anatomy. They can then be adjusted as required by the surgeon. While adjusting their positions, he is able to visualize their fit onto the bone to ensure good placement without problems such as femoral neck notching.

Twenty one hip resurfacings have been planned including two navigated cases. In addition, visualization of hip geometry for osteotomy and impingement debridement has been performed on 14 cases, giving the surgeon a good understanding of hip geometry prior to surgery. Initial evidence indicates surgeons find the planner useful, particularly when the anatomy is not straightforward.

Accurately planning the intervention and precisely measuring outcome in computer assisted orthopaedic surgery (CAOS) is essential for it permits robust analysis of the efficacy of these systems.

We demonstrate the use of low dose computer tomography (CT) radiation for both the planning and outcome measurement of robotic and conventionally performed knee arthroplasty.

Studies were initially performed on a human phantom pelvis and lower limb. The mAs (milliampere seconds) were varied from 120 to 75 at the pelvis and from 100 to 45 for both the knee and ankle whilst keeping the kV (kilovolt) between 120 and 140. Image quality was evaluated at the different doses.

The volumes scanned were defined on the scout film; they included the whole femoral head (0.5cm above and below the head), 20cm at the knee (10cm on either side of the joint line) and 5cm at the ankle (the distal tibia and the talus). Effective dose (mSv) was calculated using two commercially available software packages. This protocol was subsequently used to image patients in our prospective double-blind randomised controlled study of our active constraint robotic system ACRO-BOT.

With the reduction in the mA and scanned volume the effective dose was reduced to 0.761 mSv in females and 0.497 mSv in males whilst maintaining a sufficient image resolution for our purposes. We found that a mAs of 80 for the hip joint, 100 for the knee and 45 for the ankle was sufficient for imaging in both pre-op planning and pos-operative assessment in knee arthroplasty. This contributed on an average effective dose to the hip of 0.61 mSv, the knee 0.120 mSv and to the ankle 0.0046 mSv.

The results of our study show that we have considerably reduced the effective dose (0.8 mSv) to one third of the Perth Protocol (2.5mSv) by reducing the areas of the body scanned and adjusting the mA for the various parts of the body whist maintaining the x, y and z axis throughout the scan. The areas between the knee, hip and ankle that were not exposed to radiation are not strictly necessary for the planning of knee arthroplasty, but it is essential that the leg does not move during the scanning process. In order to prevent this leg was placed in a radiolucent splint. For post op three dimensional (3D) assessments only the knee component of the protocol is necessary.

This paper presents initial results of the Acrobot® Navigation System for Minimally Invasive (MI) Hip Resurfacing (HR) which addresses the problems of conventional HR. The system allows true MI HR – mini-mising the incision and tissue retraction required, and conservation of bone in contrast to other MI total hip procedures.

Pre-operative CT-based software allows the surgeon to plan the operation accurately. Use of CT gives the greatest accuracy, and is the only method which can give an accurate assessment of procedure outcome (planned versus achieved implant position). Intra-operatively, the bones are registered by touching points using a probe connected to a digitising arm. Next a series of tools is connected so that bone preparation and implant insertion is performed using on-screen guidance.

The accuracy of the registration probe is within 0.6mm, inside the acceptable margin for optical tracker systems. We have validated this acceptability using registration simulations leading to a protocol which restricts registration errors to within 1.5mm and three degree. These error margins are within those in the literature for acetabular component placement using optical tracker based systems (five degree inclination, six degree anteversion). No comparable data could be found regarding the accuracy of femoral component placement during computer-assisted HR.

The system is currently undergoing clinical tests at one alpha site, with three further beta sites planned for early 2006. The methods described by Henckel et al (CAOS International Proceedings 1994, pp. 281–282) are being used to evaluate the performance of the system, comparing pre-operative to post-operative CTs to obtain a true, accurate measure of performance.

The primary objective of this study was to evaluate the performance of the Acrobot^® Sculptor system in achieving a surgical plan for implantation of unicompartmental knee prostheses, compared with conventional surgery. The Acrobot^® Sculptor is a novel hands-on medical device, consisting of a high speed cutter mounted on a robotic device which the surgeon holds and directs.

A prospective, randomised, double-blind (patient and evaluator), controlled versus conventional surgery study was undertaken and has been fully reported in Journal of Bone and Joint Surgery (British), 88-B.

All (13 out of 13) of the Acrobot^® cases were implanted with tibio-femoral alignment in the coronal plane within ±2° of the planned position, while only 40% (six out of 15) of the conventionally performed cases achieved this level of accuracy.

There was also a significant enhancement in the extent of post-operative improvement, as measured by American Knee Society (AKS) Scores at six weeks, in the cases implanted with the Acrobot^®. The difference between type of surgery is statistically significant (p=0.004, Mann-Whitney U test). Operating time (skin to skin) is higher in Acrobot treated subjects, but the difference between the two types of surgery fails to reach significance.

The Acrobot^® System was found to significantly improve both accuracy and short term outcome in this investigation. By permitting the creation of bone surfaces that can be machined by means other than an oscillating saw, the Acrobot^® System paves the way for novel implant designs to be developed, facilitating bone conserving arthroplasty in the knee, hip and spine with a new generation of even less invasive but more reliable procedures.

Introduction: Malignant tumours of the fibula are rare and can be difficult to treat. We discuss the management and outcome of 52 patients who presented with malignant tumours of the fibula over a 15-year period between 1983 and 1998.

Methods: Data was collected prospectively and reviewed from the Bone Tumour database, medical records and by clinical review. Consecutive patients were studied and survival was calculated using the Kaplan-Meier curve.

Patients: The tumour type was Osteosarcoma (23 patients), Ewing’s sarcoma (16), Chondrosarcoma (11 – of which 10 low grade) and Malignant Fibrous Histiocytoma (2). We concentrate on the two most common frankly malignant groups: Osteosarcoma and Ewing’s. The male:female ratio of patients with Osteosarcoma was 11:12 and with Ewing’s Sarcoma was 11:5. Mean age for Osteosarcoma was 21.5 years and for Ewing’s Sarcoma was 14.2. The most common site of tumour was in the proximal fibula in both Osteosarcoma (19 / 23) and Ewing’s Sarcoma (10 / 16). The stage of disease at presentation was IIa or IIb in the majority of patients, with seven patients presenting with metastases.

Management: The current investigative procedures are Radiographs, Magnetic Resonance Imaging, Radioisotope Bone Scans, Computerised Tomography of the chest and needle biopsy whereas in the past CT of the lesion and open biopsy were common. Chemotherapy was administered as per protocol at the time of diagnosis and radiotherapy was given in selected cases. Surgery was performed on all but 3 patients, who were unfit and died. This consisted of local en bloc resection in 86.3% and above knee amputation in 6.8%.

Outcome: Whereas all the diaphyseal and distal lesions were completely excised, 9 out of 26 proximal lesions had a marginal excision, 4 of which had open biopsies. The common peroneal nerve was sacrificed in 50% of cases and this had no link to survival. The overall 5-year survival was 33% for Osteosarcoma and 40% for Ewing’s Sarcoma, with proximal lesions doing much worse than diaphyseal and distal lesions. Patients who had marginal excisions all died within 2.5 years.