Preresection hyperextension is associated with greater initial extension laxity and site‐specific reductions in bone resection without substantial joint‐line lowering in robot‐assisted cruciate‐retaining total knee arthroplasty

PURPOSE: Associations of preresection hyperextension with intraoperative laxity, bone resections, joint-line height and clinical outcomes were examined in cruciate-retaining (CR) robot-assisted total knee arthroplasty (TKA). METHODS: In this retrospective single-surgeon cohort, knees were classified by initial intraoperative extension angle as control (0° to +5°), mild hyperextension (-5° to <0°) or greater hyperextension (<-5°). Intraoperative balance, surgical adjustments, joint-line height, 6-week clinical extension and 1-year Knee injury and Osteoarthritis Outcome Score for Joint Replacement (KOOS-JR) outcomes were evaluated. Adjusted associations were assessed using regression models. RESULTS: Among 258 knees (149 control, 76 mild and 33 greater), greater initial extension laxity was observed in both hyperextension groups versus controls, with adjusted differences of 1.1° (95% CI, 0.4-1.9) and 2.4° (1.1-3.7), respectively. Postoperative hip-knee-ankle angle differences versus controls (n = 256) were 0.1° (95% CI, -0.9 to 1.0) and -0.6° (-1.8 to 0.7), respectively. Among 109 hyperextension knees, each 5° greater hyperextension was associated with smaller distal lateral femoral and proximal medial tibial resections (adjusted β, -1.0 mm [95% CI, -1.6 to -0.3] and -1.2 mm [-1.7 to -0.6]; false-discovery-rate-adjusted p = 0.016 and p < 0.001). Mean joint-line lowering was 0.3 mm (95% CI, 0.02-0.60), without a significant association with hyperextension magnitude. No significant differences were detected in final extension imbalance or 6-week extension. Paired KOOS-JR scores were available for 208 knees, without significant group differences in scores or improvement. Greater hyperextension was associated with higher adjusted odds of patient acceptable symptom state (PASS) failure versus combined control and mild-hyperextension groups (OR, 2.69; 95% CI, 1.02-7.08; p = 0.045). CONCLUSIONS: Greater preresection hyperextension was associated with greater initial extension laxity, smaller selected resections and higher adjusted odds of PASS failure. Minimal average joint-line change was observed. Using initial sagittal assessment to individualise resection planning is supported. LEVEL OF EVIDENCE: Level III, retrospective cohort study.