Restoration of the Native Joint Line Obliquity in Total Knee Arthroplasty Benefits Hip and Ankle Function

Preserving the native joint line obliquity (JLO) during total knee arthroplasty (TKA) may enhance postoperative function. This study compared inverse kinematic alignment (iKA) TKA, which preserves native JLO, with adjusted mechanical alignment (aMA) TKA, which standardizes JLO to 90°, and a healthy control group across eight functional activities. Both TKA groups exhibited hip-knee coupling patterns similar to healthy controls during gait, low step-down, high step-down, low step-up, and sit-down activities. However, the aMA group required significantly greater compensatory hip flexion during high step-up ( p  = 0.005, 0.016; Bonferroni-adjusted α = 0.017), despite demonstrating a lower maximum hip flexion (102.6° ± 23.10º) compared to the healthy control group (113.33° ± 11.08º) ( p  = 0.014). In contrast, the iKA group maintained hip-knee coupling patterns comparable to the healthy control group during more demanding activities, such as lunges and squats. Additionally, the aMA group exhibited reduced ankle plantarflexion compared to the healthy control group during low step-down ( p  = 0.008) and gait ( p  = 0.002), while the iKA group was more closely with the healthy control group. Preserving native JLO supports natural hip and ankle motion, particularly under extreme joint angles, emphasizing the need to address TKA-induced biomechanical adaptations to improve functional outcomes.