Association of preoperative flexion contracture and limited knee flexion with short-term patient-reported outcomes after medial unicompartmental knee arthroplasty

BACKGROUND AND PURPOSE: We aimed to clarify whether preoperative flexion contracture (FC) and limited knee flexion in patients with anteromedial osteoarthritis (OA) were associated with postoperative outcomes in mobile-bearing medial unicompartmental knee arthroplasty (UKA). METHODS: We retrospectively evaluated all patients who underwent mobile-bearing medial UKA. Preoperative extension and flexion angles, and the Oxford Knee Score (OKS) and Forgotten Joint Score-12 (FJS) recorded preoperatively and at 3, 12, and 24 months, were assessed. Patients were categorized by preoperative extension (no FC, < 10°, ≥ 10°) and flexion angles (< 120°, ≥ 120°). Changes in OKS and FJS were compared using general linear models with adjustment for age, sex, body mass index, and preoperative scores. RESULTS: 1,199 patients were included. Preoperatively, 85% of patients had full extension, 12% had < 10° FC, and 3.1% had ≥ 10° FC. Regarding flexion, 40% had < 120°, while 60% had ≥ 120°. At 12 months, the < 10° FC group showed greater improvements than the ≥ 10° FC group in OKS (adjusted mean difference, 5.5 points; 95% confidence interval [CI] 0.9-10.2; P = 0.01) and FJS (17.0 points; CI 1.0-33.0; P = 0.03). The < 120° flexion group showed greater FJS improvement than the ≥ 120° flexion group (adjusted mean difference, 4.8 points; CI 0.1-9.6; P = 0.045). At 24 months, the CIs for all between-group differences included no difference. CONCLUSION: Mild preoperative FC and mild-to-moderately limited knee flexion were not associated with inferior outcomes. Patients with more pronounced FC showed slower recovery but still achieved the same increase in patient-reported outcome measures (PROMs) as those with mild FC.