Femoral rotation and medial flexion gap tightness in functional alignment robotic total knee arthroplasty for varus knees.

Abstract

Functional-alignment (FA) robotic total knee arthroplasty (TKA) combines component positioning and soft-tissue management. This study described the association between medial flexion-gap tightness and surgeon-selected femoral rotation, together with release grades and intraoperative gap asymmetry in varus knees. This observational single-surgeon study included the 65 varus knees in an 89-knee database. Medial tightness was defined as lateral minus medial preoperative flexion gap. Correlation and ordinary least-squares regression used 63 complete observations. Recorded preoperative, planned and final trial gaps were analysed with endpoint-specific denominators. An exploratory model considered tibial coronal positioning and posterior slope. Selected rotation was associated with medial tightness (Spearman ρ = 0.571; p < 0.001). The univariable fit was rotation = 1.93° + 0.343°/mm × tightness (R² = 0.257; slope 95% CI, 0.194-0.492). The adjusted slope was 0.296°/mm (95% CI, 0.131-0.460; n = 60). Release was mild in 64/65 and moderate in 1/65 knees. Mean absolute final asymmetry was 0.38 mm in extension and 0.29 mm in flexion. Asymmetry <1 mm occurred in 50/60 (83.3%) and 55/59 (93.2%), respectively; 53/58 (91.4%) met ≤1 mm in both positions. Greater medial flexion tightness was associated with greater selected femoral rotation. Predominantly mild release and small final mean asymmetry accompanied the combined workflow. The equation describes one surgeon's decisions and is not a validated surgical rule. These observations do not establish an independent release-sparing effect, clinical superiority of submillimetre symmetry, patellofemoral safety or long-term benefit.