Beyond the coronal plane in robotic total knee arthroplasty—Part 2: Combined flexion does not affect outcomes

Abstract

PURPOSE: Personalized alignment in total knee arthroplasty (TKA) is becoming increasingly widespread, driven in part by advancements in robotic-assisted surgery. However, true personalization must extend beyond the coronal plane to include sagittal and axial planes. This study investigates the impact of combined flexion (CF) of the femoral and tibial components in robotic-assisted TKA within functional alignment (FA), also analyzing its variation (ΔCF) from native anatomy and its correlation with functional outcomes and complications. METHODS: A retrospective analysis was conducted on 310 patients who underwent primary TKA using an image-based robotic system between March 2021 and January 2023. CF was calculated based on tibial slope (TS) and distal femoral flexion (DFF). Patients were stratified into groups based on CF (≤7.5° vs. >7.5°) and CF variation (ΔCF < -3, -3 to 3 and >3). Clinical scores, radiographic parameters and complication rates were analyzed. RESULTS: Patients with CF ≤ 7.5° had lower preoperative maximum flexion values (p = 0.005). No significant differences in complication rates or clinical outcomes were observed between groups. ΔCF were associated with post-operative coronal alignment changes (mechanical hip-knee-ankle angle and medial proximal tibial angle) but did not impact patient-reported outcomes. Additionally, subgroup analysis revealed that ΔCF were not linked to differences in implant survival, revision rates or mechanical failure. CONCLUSIONS: Although CF influences knee biomechanics, its direct impact on clinical outcomes remains unclear. It is evident that a personalized approach to sagittal alignment can be an integral component of functional knee positioning. LEVEL OF EVIDENCE: Level III.