Preoperative coronal phenotype is associated with the initial balance state but only weakly with final balance in robot-assisted TKA.

PURPOSE: To determine how preoperative coronal phenotype and structural remodelling relate to initial and final intraoperative balance in robot-assisted total knee arthroplasty.

METHODS: Initial analyses included 429 knees; final/change analyses included 389. Preoperative phenotype was characterised by hip-knee-ankle angle (HKA), arithmetic HKA (aHKA), functional knee phenotype (FKP), and categorical aHKA-joint-line obliquity (JLO) components. Structural factors included medial and lateral osteophyte burden and coronal tibiofemoral subluxation. Medial and lateral gaps and coronal laxity ranges were recorded in extension and flexion before and after balancing. Signed mediolateral gap difference was defined as lateral minus medial, and change as final minus initial. Initial extension signed gap difference was the primary outcome; corresponding initial flexion, final, and change outcomes were secondary. Compartment-specific gaps, laxity ranges, and varus-valgus analyses were exploratory. Multivariable linear regression adjusted for age, sex, and body mass index, with patient-level cluster-robust standard errors and Benjamini-Hochberg false discovery rate correction.

RESULTS: The FKP model incorporating all structural factors yielded adjusted R2 values of 0.201 for the primary outcome and 0.125 for initial flexion, leaving approximately 80% of primary-outcome variability unexplained. In the overall cohort, the maximum adjusted R2 across final outcomes was 0.064. For extension and flexion changes, FKP alone and HKA alone yielded adjusted R2 values of 0.234 and 0.122, respectively. In exploratory comparisons, valgus knees had greater initial extension and flexion signed gap differences than varus knees (adjusted mean differences, 3.10 mm [95% confidence interval, 2.39-3.81] and 2.98 mm [2.23-3.74], respectively; both false discovery rate-adjusted p values were < 0.001).

CONCLUSIONS: Preoperative coronal phenotype and structural remodelling explained only modest variability in the initial intraoperative balance state and changes between assessments, and little variability in final balance, particularly in flexion. These findings support direct intraoperative assessment.

LEVEL OF EVIDENCE: Level III, retrospective cohort study.