Joint line obliquity change was associated with femoral and tibial resection patterns and smaller final flexion imbalance after robot-assisted total knee arthroplasty.

Purpose: Associations between preoperative-to-post-operative changes in arithmetic hip-knee-ankle angle (aHKA) and joint line obliquity (JLO) and final balance after robot-assisted total knee arthroplasty (TKA) were evaluated. Relationships with resection redistribution and tibial-varus reduction were also examined. Methods: A retrospective single-surgeon series of 402 primary robot-assisted TKAs was analysed. Preoperative and post-operative aHKA, JLO and coronal plane alignment of the knee (CPAK) type were determined from full-length radiographs. Final extension and flexion imbalance and laxity ranges after trial implantation were the primary outcomes. Signed and absolute changes were assessed in multivariable models; resection redistribution and a varus-only tibial-boundary subgroup were examined in supportive analyses. Results: A change in CPAK type was observed in 300 knees (74.6%), including a JLO-zone change in 221 (55.0%). Greater ΔJLO was associated with smaller final flexion imbalance (β = -0.060; p < 0.001) and a narrower final extension laxity range ( Conclusions: Greater JLO change was associated with smaller final flexion imbalance and a narrower extension laxity range rather than impaired balance. In varus knees, JLO change accompanied tibial-varus reduction and coordinated femoral adjustment. JLO change may therefore reflect specific aspects of the final balancing state rather than an unintended deviation from the native phenotype. Level of Evidence: Level III, retrospective cohort study.