Abstract
PurposeThe Oxford unicompartmental knee replacement (UKR) has a fully congruent mobile bearing to minimise wear. However, with younger higher demand patients, wear remains a concern. The aim of this study was to quantify the wear rate of Phase 3 Oxford UKR bearings over the course of 5 years and to identify the factors that influence it.Methods40 medial Oxford UKRs recruited for a randomised study of cemented and cementless fixation were studied with Radiostereometric analysis (RSA) at 1 week, 3 months, 6 months, 1 year, 2 years, and 5 years post-operatively and bearing thickness was calculated. Penetration, defined as the change in thickness compared to the 1-week measurement, was determined. Creep (early penetration) and wear (late penetration at a constant rate) were calculated. The influence of demographic factors, Oxford Knee Score (OKS), Tegner score, fixation and bearing overhang (determined by RSA) on wear was analysed.ResultsAfter 6 months the penetration rate was constant, indicating that wear alone was occurring. The wear rate was 0.07 mm/year (SD 0.03). The creep was 0.06 mm with about 95% occurring during the first 3 months. There was no significant relationship between fixation (cemented/cementless), age, component size, OKS and Tegner score with wear rate. Increasing BMI was associated with decreasing wear (p = 0.042). 37/40 bearings overhung the tibia to some extent and 23/40 overhung the tibia medially. An increase in the area of overhang (p = 0.036), amount of medial overhang (p = 0.028) and distance between the bearing and tibial wall (p = 0.019) were associated with increased wear. Bearings that did not overhang (0.06 mm/year) had less wear (p = 0.025) than those that did (0.08 mm/year). There was no relationship (p = 0.6) between the femoral contact area and wear.ConclusionDuring the first three to six months after implantation, the bearing becomes 0.06 mm thinner due to creep. The combined wear rate of the upper and lower surfaces of the bearing is constant (0.07 mm/year). The wear is lower if the bearing does not overhang the tibia so surgeons should aim for the bearing to be close to the tibial wall. The orientation of the femoral component does not influence wear.Level of evidenceRetrospective Study, Level III.
Highlights
Polyethylene wear is a significant concern following unicompartmental knee replacement (UKR) as it is important to use thin bearings to preserve bone
There was no significant relationship between Oxford Knee Score (OKS) and linear wear (p = 0.351)
There was no significant relationship between the size of either the tibial component or femoral component and the wear rate
Summary
Polyethylene wear is a significant concern following unicompartmental knee replacement (UKR) as it is important to use thin bearings to preserve bone. Linear wear can potentially cause catastrophic failure with bearing wear through or fracture. Marked linear wear may result in deterioration of kinematics and abnormal implant loading with loosening and patellofemoral joint problems. Wear particleinduced osteolysis secondary to volumetric wear can be an issue this usually only happens if there is marked linear wear [1, 19]. It is most important to minimise linear wear [3]. Unicompartmental knee replacements are either fixed bearing or mobile bearing. The mobile bearing of the Oxford UKR is fully congruent with a spherical femoral component above and a flat tibial component below.
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