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

Abstract 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 ( β = −0.055; p = 0.002). Corresponding associations were found for |ΔJLO| ( β = −0.046; p = 0.018 and β = −0.070; p = 0.004). Neither signed nor absolute aHKA change was associated with final extension or flexion imbalance. In varus knees, tibial‐varus reduction was associated with greater JLO and aHKA changes, coronal femoral adjustment and smaller final flexion imbalance. Neither |ΔaHKA| nor |ΔJLO| was associated with improvement in the Knee injury and Osteoarthritis Outcome Score for Joint Replacement. 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.