Impact of soft-tissue management technique on functional alignment in total knee arthroplasty among Japanese population with severe varus deformity.

Purpose: Simultaneously optimizing alignment and soft-tissue balance in total knee arthroplasty (TKA) still remains challenging. Functional alignment (FA), particularly with navigation or robotic systems, enables real-time intraoperative assessment of implant positioning, joint gaps, and limb alignment. This study compared one-year outcomes of TKAs performed with FA principles by a single surgeon. Two soft-tissue management techniques were compared, a conventional method (manual FA) and an instrumented gap-balancing method (instrumented FA). Methods: A retrospective review was conducted on 50 consecutive TKA cases, with 25 manual FA-TKAs followed by 25 instrumented FA-TKAs. Both cohorts were performed using an image-free navigation system with a tibia-first workflow. In manual FA-TKAs, the femoral cuts were determined by assessing flexion and extension gaps using spacer blocks. In instrumented FA-TKAs, femoral cuts and component positioning were guided by gap balancing using a force-controlled tensioning device maintaining constant distraction force. The primary laxity goal was a stable medial compartment throughout the range of motion. Results: Baseline demographics (age, height, weight, body mass index) did not differ between groups. Femoral component rotational alignment was significantly different between groups (5.2° ± 2.6°, instrumented FA-TKA vs. 1.5° ± 1.0°, manual FA-TKA, p < 0.0001). The hip-knee-ankle angle was not significantly different between groups, preoperatively (169.8° ± 4.0° vs. 169.7° ± 3.5°) or postoperatively (176.1° ± 2.1° vs. 176.0° ± 3.1°). The average total KOOS and Forgotten Joint Score-12 scores were not significantly different between groups (79.5 vs. 72.1, and 65.8 vs. 52.3, respectively). The KOOS Sports sub-score was significantly higher in instrumented FA-TKAs than manual FA-TKAs (68.3 vs. 45.8, p = 0.029). Conclusion: Instrumented FA-TKA using tibia-first, force-controlled balancing achieved superior outcomes in demanding activities measured by KOOS Sports subscale compared to manual FA-TKA. Objective gap balancing data may enhance postoperative outcomes in TKA by more accurately addressing the individual soft-tissue balance.