Exposing implausible knee loading: 3D load-balancing reveals limitations in traditional two-step musculoskeletal modeling

Accurate estimation of medial and lateral knee compartment loading is crucial for understanding joint mechanics. The traditional two-step approach, which first estimates musculotendon forces and then derives joint reaction forces and moments, is often followed by a simplified 2D load-balancing method to compute tibiofemoral contact forces in the frontal plane. This study investigated the effect of full 3D knee equilibrium on tibiofemoral contact and ligament force estimation. Gait data from 48 healthy participants were analyzed using a lower-limb musculoskeletal model. The tibiofemoral joint was modeled as a hinge with a moving axis. Joint reaction forces and moments were obtained by subtracting musculotendon forces estimated via static optimization from the intersegmental loads derived through inverse dynamics. Medial and lateral contact forces were then computed either with the 2D or the proposed 3D load-balancing method, which uses all joint reaction load components to estimate ligament and contact forces. The 3D method yielded significantly higher medial and lateral contact forces throughout stance, peaking at 2.50 BW and 1.82 BW, respectively. Ligament forces peaked at 1.05 BW for the ACL, 1.12 BW for the PCL, and 0.44 BW for the LCL, with the MCL largely unloaded. Non-negligible least-squares residuals were observed, indicating limitations in force balance. The 3D load-balancing method revealed physiologically implausible loading patterns arising from joint reaction loads, particularly elevated internal/external rotation moments, computed after independently estimating musculotendon forces in the classical two-step approach. The 2D load-balancing approach is blind to these outcomes. Future work should favor one-step approaches that simultaneously resolve musculotendon, ligament, and contact forces.