Introduction. Unicompartmental knee arthroplasty (UKA) in patients with isolated medial osteoarthritis of the knee is nowadays a standard procedure with good results, especially with the minimally-invasive approach. However, the survival rate of the unicompartmental knee prostheses is inferior to that of total knee prostheses. Therefore, further studying of UKA is still necessary. In most mobile bearing designs the femoral component has a spherical surface and therefore its positioning is not crucial. The role of the tibial slope in UKA has not been investigated so far. The manufacturers recommend tibial slopes with values between 10° positive slope and 5° negative slope. Most surgeons try to reconstruct the anatomical slope with a high failure by measuring the slope on x-rays. The aim of this study was to investigate the influence of the tibial slope on the wear rate of a medial UKA. Materials and methods. In vitro wear simulation of medial mobile bearing unicompartmental knee prosthesis with a spherical femoral surface (Univation ®) was performed with a customized four-station servo-hydraulic knee wear simulator (EndoLab GmbH, Thansau, Germany) reproducing exactly the walking cycle as specified in ISO 14243–1:2002(E). The tibial tray was inserted with 2 different medial tibial slopes: 0°, 8° (n=3 for each group). The lateral tibial slope of the space-holder was not changed (0° for every group). We performed a total of 5 million cycles for every different slope, the gravimetric wear rate was determined gravimetrically using an analytical balance every 500 000 cycles according to the ISO 14243–2. Results. The wear rate in the 0° slope group was 3.46±0.59 mg/million cycles, and in the 8° slope group it was 0.99±0.42 mg/million cycles. The difference between the 0° tibial slope group and the 8° tibial slope group was highly significant (p<0.01, alternate t-test). Discussion. An increase of the tibial slope leads to a reduced wear rate in a mobile bearing UKA. Therefore, a higher tibial slope should be recommended for mobile bearing UKA. However, the influence on the ligaments has to be considered as a higher tibial slope leads to an increased strain on the anterior cruciate ligament. This influences needs to be investigated in further studies before a definite optimal range for the tibial slope can be recommended

Background

Unicompartmental knee arthroplasty provides a good alternative to total knee arthroplasty in patients with isolated medial compartment osteoarthritis. There has been variable reporting in the literature as to whether age and sex affect the survival of unicompartmental knee arthroplasty. The aim of this study was to determine whether age and sex were predictors of failure for the Oxford unicompartmental knee arthroplasty.

Unicompartmental knee replacements (UKRs) have inconsistent and variable survivorships reported in the literature. It has been suggested that many are revised for ongoing pain with no other mode of failure identified. Using a medial UKR with an all-polyethylene non-congruent tibial component from 2004–7, we noted a revision rate of 9/98 cases (9.2%) at a mean of 39 months. Subchondral sclerosis was noted under the tibial component in 3/9 revisions with well fixed implants, and the aim of this study was to investigate this as a mode of failure. 89 UKRs in 77 patients were investigated radiographically (at mean 50 months) and with SF-12 and Oxford Knee scores at mean follow up 55 months. Subjectively 23/89 cases (25%) had sclerosis under the tibial component. We describe a method of quantifying this sclerosis as a greyscale ratio (GSR), which was significantly correlated with presence/absence of sclerosis (p<0.001). Significant predictors of elevated GSR (increasing sclerosis) were female sex (p<0.001) and elevated BMI (P=0.010) on both univariate and multivariate analysis. In turn, elevated GSR was significantly associated with poorer improvement in OKS (p<0.05) at the time of final follow up. We hypothesise that this sclerosis results from repetitive microfracture and adaptive remodelling in the proximal tibia due to increased strain. Finite element analysis is required to investigate this further, but we suggest caution should be employed when considering all polyethylene UKR implants in older women and in those with BMI >35.

The Oxford medial unicompartmental knee replacement has been shown to provide good long-tern results in numerous studies with survivorship at 10 years ranging from 82% to 100%. This prospective study describes the survival of 265 Oxford unicompartmental knee replacements implanted in one centre from 1995-2009. 8 were lost to follow up. 40 of the 265 knees were revised. For operation performed from1995-1999 the risk of revision at 5 years was 10%, operations from 2000-2004 the risk of revision was 15% and from 2005-2009 the risk of revision at 5 years was 36%. This study demonstrates that since 2005 there has been a significant increase in early failure of the Oxford unicompartmental knee at this institution and discusses the possible reasons for this

To our knowledge in medial unicompartmental knee arthroplasty (UKA) no study has specifically assessed the difference in outcome between matched gender groups. Previous unmatched gender studies have indicated more favourable results for women. Method. 2 groups of 40 of either sex was determined sufficient power for significant difference. These consecutively were matched with both the pre-operative clinical and radiological findings. Minimum follow up of 2 years, mean follow-up 5.9 years. Mean age at operation was 71 years. Results. In both groups, the mean IKS knee and function scores improved significantly (p< 0.001) post operatively. There were no significant differences were between the 2 groups. In both groups mean preoperative flexion was 130 degrees and remained unchanged at final follow-up. No significant differences in preoperative and postoperative axial alignment and in the number of radiolucent lines, between groups. With component size used there was a significant difference (p < 0.001) between the 2 groups. However the size of the femoral or tibial implant used was significantly related (p< 0.001) to patient height for both sexes. Radiolucent lines were more frequent on the tibial component, but were considered stable with none progressing. No revisions for component failure. 1 patient in each group developed lateral compartment degenerative change. Male group; one conversion to TKA for undiagnosed pain, three patients underwent reoperation without changing the implant. Female group; no implants were revised, and two patients required a reoperation. Kaplan-Meier 5-year survival rate of 93.46% (84.8; 100) for men and100% for women. The survival rate difference is not significant (p=0.28). Conclusion. These results suggest that there is no difference in outcome between sexes and that gender should not enter into the selection criteria for UKA. It also suggests that height and not gender is important to predict the appropriate UKA components