Mechanical axis correction after total knee arthroplasty alters ankle alignment but does not affect ankle function

PURPOSE: Total knee arthroplasty (TKA) restores knee function, but correction of coronal plane deformities may alter the biomechanics of adjacent joints. This study aimed to determine whether changes in knee alignment after TKA have clinical or radiological effects on the ankle. METHODS: A prospective observational study included 158 patients undergoing TKA, of whom 128 completed 1-year follow-up. Clinical outcomes were assessed preoperatively and at 3, 6 and 12 months using the Manchester-Oxford Foot Questionnaire (MOXFQ), Knee Society Score (KSS), Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), European Quality of Life (EuroQol) and Forgotten Joint Score-12 (FJS-12). Radiographic parameters, including the mechanical axis of the limb and the ankle joint orientation angle (AJOA), were measured preoperatively and at 6 months postoperatively. RESULTS: The mean mechanical axis improved from 5.8° varus to 1.3° varus (p < 0.001). The AJOA changed from -0.8° to 0.8° (p = 0.001). A moderate positive correlation was found between the change in AJOA and the degree of knee axis correction (rho = 0.4, p < 0.001). No significant correlation was found between mechanical axis correction and MOXFQ scores at 3, 6 or 12 months. All patient-reported outcome measures improved after surgery compared with preoperative values. CONCLUSIONS: Correction of the mechanical axis following TKA modifies ankle radiographic alignment but does not significantly affect ankle function at 1 year in patients with moderate deformities. Functional outcomes improved after surgery regardless of the degree of alignment change. LEVEL OF EVIDENCE: Level II.