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

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.

There were six nonarthritic cadaver knees that underwent robotic-assisted TKA using restricted kinematic alignment 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.

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, zero and six, 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.

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.