Preserving the native knee joint line obliquity in total knee arthroplasty benefits ankle movement during the five times sit‐to‐stand test

Abstract Purpose Total knee arthroplasty (TKA) alignment strategies affect lower limb biomechanics during functional activities. This study assessed sit‐to‐stand performance between individuals who underwent TKA with inverse kinematic alignment (iKA), adjusted mechanical alignment (aMA). Methods The iKA and aMA groups were compared to a healthy control group in their performance of the five times sit‐to‐stand (5xSTS) task. Phase‐specific timing and joint angles were analysed using statistical parametric mapping (SPM) and conventional metrics. Results This study included a 2‐year follow‐up after TKA, with a minimum follow‐up of 21.50 ± 5.20 months. Each group had 15 participants. The iKA group (72.52 ± 5.50 years) was older than the aMA group (66.10 ± 8.22 years; p = 0.036). The aMA group demonstrated a significantly shorter sit‐down phase compared to healthy controls (9.48 ± 0.72% vs. 10.80 ± 1.33%; p = 0.003). Additionally, the aMA group (15.92 ± 7.47°) exhibited approximately 7° less ankle range of motion (ROM) during the stand‐up phase compared to both the iKA group (22.83 ± 6.90°; p = 0.027) and healthy controls (23.61 ± 6.76°; p = 0.013). During the sit‐down phase, the aMA group displayed a significantly reduced average ankle ROM (14.05 ± 7.89°) compared to healthy controls (19.73 ± 5.34°; p = 0.036). The SPM analysis revealed no significant differences in ankle or knee flexion angles across the groups during the entire 5xSTS task. Conclusion This study demonstrates that the ankle ROM is influenced following TKA during the 5xSTS task, suggesting that alignment strategies may have biomechanical effects extending beyond the knee joint. A deeper understanding of these effects could inform rehabilitation strategies and optimise surgical outcomes following TKA. Level of Evidence Level III.