Functional Alignment Achieved a More Balanced Knee After Robotic Arm-Assisted Total Knee Arthroplasty than Modified Kinematic Alignment

Background: The aim of this study was to evaluate the balance in extension and flexion achievable after total knee arthroplasty (TKA) using a modified kinematic alignment (KA) plan and the subsequent balance achievable after adjusting the component based on the functional alignment (FA) principle. Methods: This retrospective cohort study included 100 consecutive patients who underwent primary TKA for knee osteoarthritis through an image-based robotic system in a single center between October 2021 and February 2022. Whether modified KA or FA could achieve a balanced knee was evaluated by assessing the ligament balance in the medial and lateral compartments using a robotic system at extension and 90 degrees flexion. Balance was defined as a difference of <= 2 mm between the compartments. Component positioning was adjusted within limits based on the functional positioning principles to achieve balance. Implant positioning and balance in extension and 90 degrees flexion were compared between the modified KA plan (n = 100) and after FA adjustments (n = 100). Results: FA achieved significantly better balance in extension (FA, 99.0% vs. modified KA, 86.0%; p = 0.001) and flexion (98.0% vs. 43.0%; p < 0.001) than the modified KA plan. The mean difference in gap balance in extension (FA, 0.1 mm vs. modified KA, 0.6 mm; p = 0.001) and flexion (0.1 mm vs. 2.3 mm; p < 0.001) was also significant between the two techniques. The femoral component was positioned more externally rotated relative to the transepicondylar axis (FA, 2.5 degrees vs. modified KA, 0.0 degrees; p < 0.001) to obtain balanced targets. There were significant improvements in the patient-reported outcome measures between preoperative and postoperative assessments two years after TKA (all p < 0.05). Conclusions: FA consistently achieved superior balance in both extension and flexion following TKA compared with modified KA without altering the soft tissue envelope, leading to significant improvements in clinical outcomes at the two-year follow-up.