Femoro-Tibial Size Mismatch Does Not Increase Polyethylene Wear in Fixed-Bearing Total Knee Arthroplasty: A Retrieval Analysis.

Abstract

This study aimed to determine whether femoro-tibial component size mismatch in a fixed-bearing total knee arthroplasty (TKA) design is associated with increased polyethylene (PE) wear. We hypothesized that mismatched knees, particularly a femur greater than a tibia, would demonstrate higher PE wear. A retrospective retrieval analysis was performed on a fixed-bearing design revised between 2008 and 2024. Implants were categorized as matched (femur = tibia), femur (F) greater than tibia (T), or femur less than tibia. A total of 105 implants were included. The PE inserts were graded using the modified Hood scoring system. Mixed-effects models evaluated associations between mismatch and wear severity across regions and surfaces, and Cox regressions analyzed time in situ. Total PE wear did not differ between groups (mean matched 47.6 ± 23.6, F greater than T 46.1 ± 22.6, F less than T 51.3 ± 19.4; P = 0.81; Cohen's d < 0.20). In multilevel modeling, mismatch was not associated with overall wear (Wald Chi-square 0.42; P = 0.81). A significant surface × mismatch interaction was identified (P < 0.001): F greater than T knees exhibited greater backside wear compared with matched (P = 0.021; d approximately 0.37), but there was no increase in bearing-surface wear (P = 0.24). Time in situ (P < 0.001) and older age (P = 0.014) were the only independent predictors of wear. Burnishing, scratching, and pitting were the dominant wear mechanisms. Mismatch did not affect implant survival, with no difference in time in situ (Cox P = 0.456). Femoro-tibial size mismatch did not result in greater total or bearing-surface PE wear. The modest increase in backside wear observed in F greater than T did not translate into reduced survivorship. These findings suggest that the higher revision risk reported in mismatched TKAs may involve mechanisms beyond PE wear alone.