BACKGROUND : Flexion contracture is a serious complication after total knee arthroplasty (TKA). Polyethylene insert (PE) thicker than the extension gap during surgery causes flexion contracture. The purpose of this study was to identify the changes over time in postoperative extension angle and the correlation between extension gap and PE thickness. METHODS : Overall, 127 knees were analyzed to evaluate the changes in extension angle over time. "Gap difference" was defined as the distance obtained by subtracting the extension gap from the PE thickness. Patients were divided into four groups according to gap difference (0-3 or greater mm). We evaluated the correlation of gap difference and changes in postoperative extension angle over time. RESULTS : The extension angles of all knees were -4.2 ± 3.9 (deg. mean ± s.d.) at surgery, -5.9 ± 5.7 at 3 months, -3.4 ± 5.4 at 6 months, and -1.3 ± 5.4 at 2 years. Gap difference was negatively correlated with the extension angle at surgery and at 2 years (r = 0.40, r = 0.36). The extension angles at 2 years in each group (groups 0, 1, 2, 3 or greater) were 2.0°±4.7°, -0.8°±5.3°, -3.4°±5.3°, -4.0°±4.1° (p < 0.05), respectively. CONCLUSIONS : The extension angle became worse at 3 months compared to that at surgery (p = 0.01) and then improved until 2 years. The correlation between gap difference and extension angle was maintained at 2 years, and thicker PE caused flexion contracture even after 2 years. Hence, PE selection considering the extension gap is important.
