Long term in vivo wear of different bearing types used for the Oxford Unicompartmental Knee Replacement

Objectives: To determine the polyethylene wear rate of Phase 3 Oxford Knee bearings and to investigate the effects of resin type and manufacturing process.
Methods: 63 patients with at least ten years follow up with three bearing types; 1900 resin machined, 1050 resin machined and 1050 resin moulded were recruited. Patients underwent full weightbearing model based radiostereometric analysis to determine the bearing thickness. The linear wear rate was estimated from the change in thickness divided by the duration of implantation.
Results: The wear rate for 1900 resin machined (n=19), 1050 machined (n=21) and 1050 moulded bearings (n=23) were 60µm/yr (SD42), 76µm/yr (SD32) and 57µm/yr (SD30). There was no significant difference between 1900 machined and 1050 machined (p=0.20), but 1050 moulded had significantly (p=0.05) less wear than the 1050 machined. Increasing femoral (p<0.001) and tibial (p<0.001) component size were associated with increasing wear.
Conclusion: Wear rate is similar with 1050 and 1900 resin, but lower with moulded than machined bearings. The currently used Phase 3 bearings wear rate is low (1050 moulded, 57µm/yr), but higher than the previously reported Phase 2 bearings (1900 moulded, 20µm/yr). This is unlikely to be due to the change in polyethylene but may relate to the minimally invasive approach, used with the Phase 3. This approach, as well as improving function, may increase the risk of surgical errors, such as impingement or bearing overhang, which can increase wear. Surgeons should aim to use 4mm rather than 3mm thick bearings in young patients, unless they are small and need conservative bone resections.