Assessment of intraoperative soft tissue balance in functional alignment total knee arthroplasty: A comparative study with anatomic alignment.

Purpose: The purpose of this study is to evaluate the gap balance after implantation in patients who have undergone total knee arthroplasty (TKA) via functional alignment (FA) by comparing the virtual gap in preoperative simulation. To facilitate the evaluation of balance characteristics, a control group consisted of patients who underwent anatomical alignment (AA) with the articular surfaces tilted medially and set up in neutral alignment. Methods: We retrospectively analyzed data from 321 consecutive knees of patients with varus knee osteoarthritis who underwent primary cruciate-retaining TKA from September 2017 to July 2023. The FA group included 140 knees that were operated on via a robotic arm based on kinematic alignment, and the AA group included 93 knees that were operated on via a navigation system. The two groups were stratified according to the timing of surgery that corresponded to changes in the surgeon's surgical technique. After osteotomy, the implant gap was measured via a digital tensioner at 0°, 30°, 60°, 90° and 120° flexion angles with 30 pounds of tension stress applied, and the gaps were compared between the two groups. Results: The gap between the implants in the medial compartment was stable in FA, with no widening of more than 2 mm, including the mid-flexion position. In AA, the gap tended to open with flexion and was significantly larger than in FA at 60° (FA: 1.1 ± 1.5 mm, AA: 1.8 ± 1.9 mm), 90° (FA: 1.2 ± 1.5 mm, AA: 2.2 ± 2.0 mm) and 120° (FA: 1.4 ± 1.6 mm, AA: 2.8 ± 2.1 mm) ( Conclusion: Compared with AA, FA achieved balance at all knee flexion angles. FA can achieve a well-balanced knee with high reproducibility and may allow the surgeon to consistently obtain better balance. Level of Evidence: Level III prospective cohort study.