Medial unicompartmental replacement (UKR) has been shown to have superior functional results to total knee replacement (TKR) in appropriately selected patients, and this has been associated with a resurgence of interest in the procedure. This may relate to evidence showing that the kinematic profile of UKR is similar to the normal knee, in comparison to TKR, which has abnormal kinematics. Concerns remain over the survivorship of UKR and work has suggested the anterior cruciate ligament (ACL) may become dysfunctional over time. Cruciate mechanism dysfunction would produce poor kinematics and instability providing a potential mechanism of failure for the UKR.

Aim: To test the hypothesis that the sagittal plane kinematics (and cruciate mechanism) of a fixed bearing medial UKR deteriorate over time (short to long term).

A cross sectional study was designed in which 24 patients who had undergone successful UKR were recruited and divided into early (2–5 years) and late (> 9 years) groups according to time since surgery. Patients performed flexion/extension against gravity, and a step up. Video fluoroscopy of these activities was used to obtain the Patellar Tendon Angle (PTA), the angle between the long axis of the tibia and the patella tendon, as a function of knee flexion. This is a previously validated method of assessing sagittal plane kinematics of a knee joint.

This work suggests the sagittal plane kinematics of a fixed bearing UKR is maintained in the long term. There is no evidence that the cruciate mechanism has failed at ten years. However, increased tibial bearing conformity from ‘dishing’, and adequate muscle control, cannot be ruled out as possible mechanisms for the satisfactory kinematics observed in the long term for this UKA.

Functional outcome after patellofemoral joint replacement (PFA) for osteoarthritis remains inconsistent. It is believed that functional outcome for joint replacement is dependent upon postoperative joint kinematics. Minimal disruption of the native joint, as in PFA, should produce more normal kinematics and improved outcome. No previous studies have examined joint kinematics after isolated PFA.

Aim: To investigate the sagittal plane kinematics of patellofemoral replacement and compare with the normal knee.

Twelve patients who had undergone successful PFA at least two years previously were recruited. Patients performed flexion/extension against gravity, and a step up. Video fluoroscopy of these activities was used to obtain the Patellar Tendon Angle (PTA), the angle between the long axis of the tibia and the patella tendon, as a function of knee flexion. This is a previously validated method of assessing sagittal plane kinematics of a knee joint. The kinematic profile of the PFA joints was compared to the profiles for fourteen normal knees.

Overall, the kinematic plot obtained for PFA reflected similar trends to that for normal knees; but the PTA was slightly but significantly increased throughout the entire range of flexion (two degrees). This is equivalent to an average displacement of the lower pole of the patella of 1.5mm.

Sagittal plane knee kinematics after PFA are much more normal than after TKR and this should give improved functional outcome. The observed increase in PTA through range may result from increased patella thickness or a shallow trochlear groove and may influence patellofemoral contact forces.

Patellofemoral pain is a significant problem for patients with Total Knee Replacements (TKRs). It is hypothesized that pain is related to high patellofemoral forces (PFF). The aim of this study is to validate a model to estimate PFF after TKR, using a combination of non-invasive measurement and theoretical modeling.

Experiments were performed on four cadaver knee specimens to compare the PFF and the quadriceps force (QF) estimated by a model, with those measured using force transducers. Each knee was tested in its initial state and after implantation of three Scorpio designs: Cruciate Retaining (CR), Posterior Stabilised (PS), and the Posterior Stabilised Mobile Bearing (PS+). Each knee was extended/flexed under a simulated quadriceps load with 3 kg hung from the distal tibia. Relative movement of the bones was measured using a Vicon 612 motion analysis system. A 6DOF force transducer was used to measure PFFs and a uni-axial transducer was used to measure QFs. A fluoroscope simultaneously captured images of the leg extension activity. Parameters measured from the images were used as inputs to the model.

The measured and estimated PFF and QF matched closely between 20o and 80o of knee flexion for the TKRs. At higher flexion angles, the model overestimated the PFF by a maximum of 23N (7.6% max) for the PFF and by 31N for the QF (10.3% max). The estimated and measured Patellar Flexion Angles (PFA) were within 3.5o throughout the flexion range.

The model accurately predicts sagittal plane patellar kinematics and kinetics, using only fluoroscopy and externally measured forces as inputs. However, the model has a limitation in assuming that the extending moment is only due to the quadriceps.

Award for the best student biomaterials paper (US$ 2,000); a proper certificate

Introduction: Total knee replacement (TKR) is a common treatment for end stage osteoarthritis of knee. The best knee replacement is one in which the kinematics of the normal knee are reproduced. Amongst several factors affecting kinematics, variation in surface geometry and the retention/ sacrifice of the PCL are considered especially important. It is not known which of these two factors is most influential for establishing optimum joint kinematics after TKR.

Method: Four groups of patients who had undergone TKR at least one year previously were recruited. Two groups of patients had undergone replacement with a single axis design (Scorpio, Stryker Howmedica) in both PCL retaining (Scorpio CR, n=15) and PCL sacrificing (Scorpio CS, n=15) variants. The other two groups had undergone replacement with the traditional polyradial design prosthesis (Sigma, Depuy, Johnson & Johnson), again with both PCL retaining (Sigma CR, n=14) and PCL sacrificing (Sigma CS, n=13) variants. An in-vivo fluoroscopic analysis was carried out on all patients. Patients were asked to perform closed chain step up and open chain extension and flexion against gravity. The kinematic profile of each knee was obtained by measuring patella tendon angle (PTA) at specific angles of knee flexion (KFA) using an established fluoroscopic method. The data was also compared with the kinematic profile of normal knees. American Knee Society, Oxford and Patella Scores were recorded for all patients.

Results: All groups were comparable in terms of age and gender. In addition, no significant difference was found between groups in clinical outcome. PTA results for a step-up exercise are shown in the figure. A one way ANOVA between groups revealed that knee kinematics after total knee replacement is different to that for normal knees. No differences were found between groups when the data was analysed using CR/CS as the independent variable. The only differences between groups were found when surface geometry was used as the independent variable. It was shown that the kinematic profile of the single axis Scorpio design (in both CR & CS ) was closer to normal, especially near extension, than the traditional polyradial design (Sigma CR & CS).

Conclusions: Kinematics after a total knee replacement differ from that for a normal knee. Differences in surface design between knee replacements appear to have greater influence on kinematics than the presence or absence of the PCL.

Introduction and Aims: Single compartmental replacement procedures are increasingly preferred over total knee replacement (TKR) for single compartment osteoarthritis of the knee joint. Theoretically, reduced disruption of the native joint should produce more normal kinematics. This study aimed to describe and compare the sagittal plane kinematics of four different, commonly used devices.

Method: Four groups of patients who had undergone successful single compartment replacement at least two years previously were recruited. Fifteen following Oxford medial UKA, 12 following medial St Georg Sled UKA, five following Oxford lateral UKA, and 12 following Avon PFJ replacement. Patients performed flexion/extension against gravity, and a step-up during video fluoroscopy. The Patellar Tendon Angle (PTA), the angle between the long axis of the tibia and the patella tendon, was obtained as a function of knee flexion. This relationship provides indication of sagittal movement between femur and tibia through range and has been validated as a reliable measure of joint kinematics.

Results: The kinematic profile for each group was compared to that of the profile for 12 normal and 30 TKR (AGC) knees. All three tibiofemoral devices produced knee kinematics similar to the normal knee. The PTA was found to have a linear relationship to flexion angle, decreasing with increasing knee flexion angle. No such linear relationship exists for the TKR joint, which display abnormal kinematics. The PF device also reflected similar trends to that for normal knees except that the PTA was moderately increased throughout the entire range of flexion (three degrees).

Conclusion: In contrast to TKR, all single compartmental knee replacements provided kinematics similar to the normal joint. The kinematic pattern of the PFJ replacement may be of most interest as the observed increase in PTA through range could influence patello-femoral contact forces

Aim: To compare the kinematic profile of two types of TKRs – a single-axis design Vs a polyradial design, with that of the normal knee.

Methodology: An in-vivo fluoroscopic analysis was carried out as part of a four-armed prospective randomised trial comparing the clinical outcome of two commonly used types of TKRs each with posterior cruciate retaining -CR and sacrificing –CS models. The kinematic profile was obtained by measuring patella tendon angle at specific angles of knee flexion using an established fluoroscopic method whilst the patients performed close and open chain exercises. The data was compared with the kinematic profile of the normal knee.

Results: Fifty-five patients who had undergone TKR at least one year prior, were invited to take part in this ethically approved study. They were matched for age and gender and had a similar clinical outcome.

The kinematic profile of single axis design TKR was closer to normal especially near extension. During mid-flexion, abnormal anterior femoral translation was noticed with the polyradial design. No significant difference was noted between CR and CS designs.

Conclusions: Kinematics after a TKR differed from that of a normal knee. Reproducible differences were found between the two designs, which may predict mode of failure and longevity.

Aim: To study the sagittal plane kinematics of the Avon patello-femoral replacement (Stryker-Howmedica), PTA.

Introduction: Replacement of the patello-femoral joint for end stage osteoarthritis has previously been associated with inconsistent results. Retention of the cruciate ligaments is likely to be important in maintaining normal kinematics and hence improved functional outcome.

Methodology: Twelve patients who had undergone Avon PFR least two years previously were recruited following ethical approval. American Knee Society, Bristol and Oxford knee scores were obtained. Patients performed open chain flexion and extension against gravity, in addition to closed chain step up. Video fluoroscopy of these activities was used to obtain the Patellar Tendon Angle (PTA), which is the angle between the long axis of the tibia and the patella tendon, at specific angles of knee flexion. This is a previously validated method of assessing the kinematic profile of a knee joint. These measurements were used to determine the kinematic profile of each knee and they were then compared to a group of twelve normal knees.

Results: A one way ANOVA revealed no significant differences between the kinematic profile following Avon PFR and that of the normal knee. All patients had good or excellent knee scores.

Conclusion: The kinematic profile after Avon PFR is similar to that of the normal knee. In contrast all TKRs we have studied have abnormal kinematics, which are associated with abnormal patello-femoral joint loading. This suggests that isolated PFR should have a functional advantage over TKR.

Polyethylene wear can be an important cause of knee replacement failure.

Six TKRs in young, active patients with excellent Oxford Knee Scores and Knee Society Scores, mean 76 months post knee replacement and 5 control patients, 2 weeks post TKR, were selected. Each patient had weight bearing stereo radiographs of at 0, 15, 30, 45 and 60 degrees of flexion while standing in a calibration grid. These x-rays were analysed using our Radio Stereometric Analysis (RSA) system. The three-dimensional shape of the TKR (manufacturer’s computer aided design model) was matched to the TKR silhouette on the calibrated stereo radiographs for each angle of flexion. The relative positions of the femoral and tibial components in space were then determined and the linear and volumetric penetration was calculated using Matlab software.

The accuracy of the system was found to be 0.3mm (CAD model tolerance 0.25mm). The mean linear wear in the control patients was 0.02mm (range −0.19 to +0.23mm). Average linear penetration in the study group was found to be 0.6 mm at 6 years, giving an overall linear wear rate of 0.1mm/year. Average penetration volume at 76 months was 399mm3. The average volumetric wear rate was 63mm3/year.

It is possible to measure volumetric wear in vivo using RSA. Volumetric wear rate was found to be 63mm3 per year. Studies on retrieved normally functioning hip replacements have shown volumetric wear rates of 35mm3 per year. However, clinical outcomes of knee replacements are comparable to those of hip replacements, suggesting that the knee has a more effective mechanism for dealing with polyethylene wear particles.