Greater valgus severity is associated with larger lateral flexion‐gap changes between intraoperative assessments during robot‐assisted total knee arthroplasty: A retrospective cohort study
Yugo Morita, Jonggu Shin, Mohammed El‐Hassan, Ittai Shichman, Skylar Lewis, John M. Dundon, William J. Long, Peter K. SculcoAbstract
Purpose
To characterize differences between intraoperative flexion‐gap assessments and their association with deformity severity during primary robot‐assisted total knee arthroplasty for osteoarthritis.
Methods
This single‐centre retrospective cohort included 512 eligible knees identified from an institutional database between August 2022 and July 2025. One surgeon used different alignment strategies for varus and valgus knees. Robot‐recorded gaps were assessed at 90° using manual varus–valgus stress before bone resection and a quantitative tensioner after distal femoral and proximal tibial resections, extension balancing and selective releases when required. The primary outcome was compartment‐specific gap change: post‐resection measurements minus tibial resection depths and initial gaps. Multivariable models examined associations with deformity severity and clinical outcomes.
Results
Paired measurements were available for 505 knees (438 patients); 465 had final measurements. Tibial‐resection‐adjusted gaps exceeded initial gaps by a mean of 1.5 mm medially (95% confidence interval [CI] 1.2–1.7) and 0.8 mm laterally (95% CI 0.5–1.1; both p < 0.001). In 139 valgus knees, each additional degree of valgus was associated with 0.24‐mm greater lateral gap change (95% CI 0.14–0.34; Holm‐adjusted p < 0.001). Using an absolute mediolateral difference ≤2.5 mm to define balance, 21/42 (50.0%) initially balanced valgus knees were reclassified as imbalanced, 20 with larger lateral gaps; 16/21 (76.2%) were balanced at final assessment. Reclassification rates were 58.3% and 44.4% with 2‐ and 3‐mm thresholds. Medial, lateral and mean compartment‐specific gap changes were not associated with the 1‐year Knee injury and Osteoarthritis Outcome Score for Joint Replacement in 342 knees (all Holm‐adjusted p = 0.959).
Conclusion
Flexion‐gap measurements differed between intraoperative checkpoints, with greater valgus severity associated with larger lateral opening. Initial balance did not consistently persist at the later checkpoint, demonstrating that the information available for femoral planning may differ from that obtained during the initial manual‐stress assessment.
Level of Evidence
Level III, retrospective cohort study.