Predictive Value of Knee Flexion at Hospital Discharge for 6-Month Outcomes Following Total Knee Arthroplasty: A Prospective Cohort Study

Abstract

Purpose: To evaluate whether discharge knee flexion <90° after total knee arthroplasty (TKA) predicts 6-month functional outcomes and how postoperative ROM association with final outcomes evolves. Methods: In this single-center prospective cohort, 216 of 300 patients (72.0%; mean age 68.9 years, 87.0% women) completed 6-month follow-up after primary TKA and were classified by discharge ROM as <90° (n=32) or ≥90° (n=184). ROM was measured preoperatively, at discharge, 2 weeks, 6 weeks, 3 months, and 6 months. The primary outcome was 6-month knee flexion ROM. Secondary analyses examined the association between ROM at different postoperative time points and final ROM outcomes. Functional outcomes included the Knee Society Score (KSS) and Oxford Knee Score (OKS). Results: Six-month ROM (116.4±9.7° vs 119.0±7.0°, p=0.152), KSS (94.9±5.7 vs 96.2±7.4, p=0.280), and OKS (38.3±4.8 vs 38.3±5.7, p=0.996) were similar between the <90° and ≥90° groups, and all patients reached ≥90° flexion. Discharge ROM weakly predicted 6-month ROM (R²=0.026), whereas 3-month ROM was strongly associated with 6-month ROM (R²=0.591, β=0.643, 95% CI 0.564–0.722). In multivariable regression adjusted for age, sex, and BMI, preoperative ROM independently predicted 6-month ROM (p<0.001), discharge ROM group did not (p=0.149). Conclusions: Discharge ROM <90° did not predict poor 6-month outcomes. Among the time points assessed, 3-month ROM was the strongest predictor of 6-month ROM and may be a useful time point for prognostication.