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.

To report a 15-year survival analysis of the Oxford Medial Unicompartmental Knee Arthroplasty (Oxford UKA) in an independent series.

We report the results of a series of 420 Oxford UKAs performed between 1983 and 2000. Indications for surgery were primary antero-medial osteoarthritis of the knee with an intact ACL, correctable varus deformity of < 15° and < 15° fixed flexion deformity. The state of the patello-femoral joint was not used as a selection criterion. Patients were contacted by a postal questionnaire or by telephone. The outcome of all 420 knees was established, with none lost to follow-up. Seventy-six knees were in patients who had died and the state of each arthroplasty was determined from hospital and GP records.

Seventeen patients (4%) had required revision. Indications for revision were lateral compartment arthrosis (7), component loosening (4), bearing dislocation (4) and infection (2). There were no failures for polyethylene wear. Cumulative survival at 15 years was 94.3% [95% CI 3.8%]. The worst case scenario was 94.3% as none were lost to follow-up.

The results from an independent series are important, as they avoid bias. The 15-year results of this independent series are better than any other reported series of unicompartmental device at 15 years and as good as the published independent 15 year survival results for total knee arthroplasty. The data illustrates that excellent long-term survival can be achieved with the Oxford UKA, allowing patients to benefit from the advantages that unicompartmental arthroplasty offers. We believe that provided patients are selected appropriately, this device provides the treatment of choice for anteromedial osteoarthritis of the knee.

Introduction: The indications for unicompartmental knee arthroplasty (UKA) can be contentious. Concerns exist about implanting a UKA in younger individuals and it has been reported that the procedure is contra-indicated in patients under the age of 60. The suggestion is that younger patients may compromise their outcome after surgery by their increased activity levels. However, the number of publications with data on UKA in young patients is minimal and the age/activity related criteria for UKA remains unknown.

Aim: The aim is twofold: 1) to confirm that younger UKA patients have higher activity levels than older patients, and 2) to test the hypothesis that younger, more active patients have inferior outcome when compared to older less active patients.

Materials and methods: Fifty consecutive patients undergoing UKA who were under 60 years old at the time of surgery (Group Y) and 50 patients over 60 years (Group O) were recruited. Indications were anteromedial arthritis with full thickness lateral compartment cartilage, a functioning ACL and a correctible varus deformity. All patients underwent Oxford UKA using the minimally invasive technique. This device employs an unconstrained fully congruous meniscal bearing. Outcome was evaluated using the American Knee Society Score (AKSS). Activity level was documented using the established Tegner Activity Score.

Results: The average age of patients for group Y and O was 55 years and 68 years respectively. Minimum follow up for both groups was 2 years. Pre-operative AKSS scores were comparable for each group (Group Y = 38.8, Group O = 35.8) and patients in both groups significantly improved after surgery. A significant difference in Tegner score was found between groups after surgery (Group Y = 3.9, Group O = 2.6). It was found that 40% of younger patients regularly participate in high demand activities like skiing, tennis, hard manual labour and swimming. No statistically significant difference in any other post operative knee scores or complication rate were found despite adequate study power.

Conclusions: Younger patients have increased activity levels after UKA when compared to an older age group. There is no evidence that the outcome of UKA in younger more active patients will be inferior to those who are older and more sedentary.

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

Method: 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.

Results: 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.

Conclusion: 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.

*Oxford Hip and Knee Group: P McLardy-Smith, C Dodd, D Murray & R Gundle