Weight-bearing line at the ankle joint level shifted laterally after total knee arthroplasty for varus knee osteoarthritis: Evaluation of the hip-to-calcaneus line.

BACKGROUN: Few studies have comprehensively examined how alignment beyond the ankle joint changes after Total knee arthroplasty (TKA). This retrospective study aimed to answer the questions: (1) do the radiological parameters beyond the ankle joint, including the weight-bearing line at the ankle joint level, change after TKA in varus knee osteoarthritis? (2) does the lower extremity alignment correlate with alignment beyond the ankle joint?.

HYPOTHESIS: Our hypothesis was that TKA altered the radiological parameters beyond the ankle, which correlated with the lower extremity alignment.

PATIENTS AND METHODS: This retrospective study included 67 patients who underwent mechanically aligned TKA for varus knee osteoarthritis. The hip-knee-ankle angle (HKA), tibial plafond inclination angle (TPIA), hindfoot alignment angle (HA), talar tilt (TT), and weight-bearing line at the ankle joint level (mechanical ankle joint axis point [MAJAP]) were measured using radiographs taken before and 6 months after TKA; each parameter was compared between the two time points. The correlations between HKA and other parameters were examined preoperatively and postoperatively.

RESULTS: The mean HKA, TPIA, HA, TT, and MAJAP changed significantly after TKA from 15.0°±6.1° to 2.6°±3.5°, from 10.8°±5.7° to 4.7°±3.9°, from 2.3°±5.5° to -2.5°±4.5°, from 0.2°±1.3° to -0.5°±1.4°, and from 29.3±23.3 to 54.3±20.6, respectively. Correlations were noted between the preoperative HKA and the preoperative TPIA (r = 0.58), HA (r = 0.36), and MAJAP (r = -0.59), and between the postoperative HKA and the postoperative TPIA (r = 0.54) and MAJAP (r = -0.38).

DISCUSSION: TKA for varus knee osteoarthritis altered radiological parameters beyond the ankle, with the weight-bearing line at the ankle joint level passing more laterally. The weight-bearing line at the ankle joint level was correlated with lower extremity alignment both preoperatively and postoperatively.

LEVEL OF EVIDENCE: IV; single-centre retrospective observational study.