Lateral Femoral Joint-Line Distalization Increases Lateral Collateral Ligament Strain During Total Knee Arthroplasty.

BACKGROUND: Optimal coronal alignment strategy during total knee arthroplasty (TKA) remains controversial. Joint-line alteration has been associated with abnormal knee kinematics, ligament imbalance, and patient dissatisfaction; however, the isolated biomechanical consequences of lateral femoral joint-line distalization on lateral collateral ligament (LCL) strain remain poorly understood. METHODS: There were six non-arthritic cadaver knees that underwent robotic-assisted TKA using restricted kinematic alignment (rKA) to approximate native joint-line orientation. A motion capture technique utilizing reflective markers applied to the LCL quantified strain across knee flexion angles (0, 30, 60, and 90°). Progressive distalization of the lateral femoral condyle was simulated using 2-mm shims placed between the implant and the lateral distal femur. The median peak LCL strain across flexion arcs and ligament regions was recorded and compared using paired statistical testing. RESULTS: Native LCL strain demonstrated physiologic relaxation with increasing knee flexion. Following rKA TKA, the baseline median peak LCL strain measured 1.6%. Progressive lateral femoral joint-line distalization resulted in increases in strain to 1.9, 6.0, and 8.1% with 2-, 4-, and 6-mm distalization, respectively. Statistically significant increases in strain were observed between zero and four mm, zero and six mm, and four and six mm distalization (P < 0.05). A stepwise strain response was observed, demonstrating a potential biomechanical threshold between two and four mm distalization. CONCLUSIONS: Lateral distal femoral joint-line distalization independently increases LCL strain following TKA. Distalization exceeding approximately four mm results in strain magnitudes associated with a microstructural injury threshold based on in vitro studies of animal ligaments. These findings suggest that joint-line alteration may contribute to lateral soft-tissue imbalance and postoperative dissatisfaction.