An Asymmetric Medio-Lateral Flexion Laxity Target Yields Excellent Results in Robotic-Assisted Total Knee Arthroplasty With Functional Knee Positioning

BACKGROUND: Functional knee positioning (FKP) in robotic-assisted (RA) total knee arthroplasty (TKA) aims to optimize soft-tissue balance through patient-specific implant fine-tuning. However, it remains unclear (1) how intraoperative ligament laxity affects joint awareness and (2) whether the discrepancy between intraoperative planned and achieved laxity is affected by preoperative coronal limb alignment or fixed flexion deformity (FFD). METHODS: This retrospective study analyzed 130 patients undergoing image-based RA TKA with FKP using a cementless cruciate-retaining implant. Intraoperative adjustments were made to balance the flexion-extension laxity, targeting one to two mm larger lateral flexion laxity. Medial and lateral gaps before and after implant positioning were recorded at near extension and 90° flexion. Patient-reported outcome measures (PROMs), including the Forgotten Joint Score (FJS-12) and a 5-point Likert scale for satisfaction, were assessed at a minimum of one year postoperatively to investigate correlations between PROMs and final intraoperative laxities. The impact of preoperative coronal limb alignment, measured on long-leg radiographs, and FFD on final laxities was also evaluated. RESULTS: Final laxities were: median medial extension laxity 1.5 mm (1.0 to 2.0), lateral extension laxity 1.5 mm (1.1 to 2.0), medial flexion laxity 1.0 mm (0.5 to 1.5), and lateral flexion laxity 2.0 mm (1.5 to 2.5). At one year, the FJS-12 was 87.4 ± 14.5. A weak inverse correlation was observed between medial flexion laxity and FJS-12 (r = -0.18, P = 0.047); no other laxities were associated with PROMs. Sensitivity analyses showed no relevant laxity cutoffs influencing PROMs. Valgus knees exhibited larger changes between planned and final medial extension laxity compared with varus knees (P < 0.002). Unlike cases without FFD, those with preoperative FFD >10° showed no discrepancy between planned and final extension laxities (P < 0.003). CONCLUSIONS: Image-based RA TKA using FKP achieves good-to-excellent clinical outcomes at one year, with a balanced symmetric extension laxity. Coronal limb alignment and FFD significantly influence the deviation between planned and final extension laxity and should be considered during surgical planning.