Association of joint line changes with patient-reported outcomes after cruciate-retaining robotic arm-assisted total knee arthroplasty with functional alignment.

PURPOSE: Robotic arm-assisted total knee arthroplasty (R-TKA) enables precise bone resection and real-time intraoperative gap adjustment, facilitating functional alignment (FA). However, performing FA with standardized implants may cause unintended changes in the joint line (JL), and their clinical relevance remains unclear. This study aimed to evaluate the association between compartment-specific JL changes and postoperative patient-reported outcomes after R-TKA with FA. METHODS: This retrospective study included 147 knees that underwent primary cruciate retaining (CR) R-TKA with FA. JL height was defined as the perpendicular distance from the transepicondylar axis to the most distal aspect of the femoral condyles on preoperative and postoperative computed tomography images. Patients were categorized into elevation, neutral, or depression groups based on a 2-mm deviation from the native JL in the medial and lateral compartments. Patient-reported outcomes were measured 1 year after surgery. RESULTS: Mean JL depression occurred in both compartments, with a greater change in the lateral compartment (3.1 ± 2.5 vs. 1.8 ± 2.5 mm). Among the groups in the medial compartment, the elevation group had the lowest Forgotten Joint Score-12 scores (mean difference = 28.5 points, Cohen's d = 1.09). In contrast, lateral JL changes were not significantly associated with 1-year patient-reported outcomes. Final gap asymmetry values were comparable among medial JL subgroups. CONCLUSION: Although JL depression was relatively well tolerated, medial JL elevation was associated with worse 1-year patient-reported outcomes after CR R-TKA with FA. These findings suggest that unnecessary medial JL elevation may warrant caution, whereas modest JL depression may be acceptable when needed to achieve balanced gaps.