Measuring polyethylene wear in total knee arthroplasty by RSA: Differences between weight‐bearing and non‐weight‐bearing positioning

Measuring the minimum‐joint‐space‐width (mJSW) in total knee arthroplasty (TKA) in Roentgen stereophotogrammetric analysis (RSA) provides valuable information on polyethylene wear, a leading cause for TKA failure. Most existing studies use non‐weight‐bearing (NWB) patient positioning. The latter may compromise mJSW measurements due to knee laxity with subsequent non‐contact between the TKA components. We investigated the difference in mJSW between weight‐bearing (WB) and NWB images and the association with mediolateral (ML) knee stability. At one‐year follow‐up, 23 TKAs were included from an ongoing RSA study, and ML stability was evaluated. For each examination, the mJSW and femoral‐tibial contact locations were measured. A linear regression model was used to analyze the association between the mJSW difference (NWB–WB) with the ML stability and contact locations. The mean mJSW difference was 0.28 mm medially and 0.20 mm laterally. Four TKAs had medium (5–9°) and 19 TKAs had high (<5°) ML stability. A higher mJSW difference was found for TKAs with medium stability (0.36 mm, P = 0.01). In conclusion, mJSW measurements in existing (NWB) RSA studies are influenced by knee laxity, but may still provide information on wear progression based on TKA with high ML stability. A direct comparison of mJSW measurements from WB and NWB data is not possible.