Prediction of ACL Force Produced by Tibiofemoral Compression During Controlled Knee Flexion: A New Robotic Testing Methodology.

Tibiofemoral compression force (TCF) is an important component of anterior cruciate ligament (ACL) injuries. A new robotic testing methodology was utilized to predict ACL forces generated by TCF without loading the ligament. We hypothesized that ACL force, directly recorded by a miniature load cell during an unconstrained test, could be predicted by measurements of anterior tibial restraining force (ARF) recorded during a constrained test. The knee was first flexed under load control with 25N TCF (tibia unconstrained) to record a baseline kinematic pathway. Tests were repeated with increasing levels of TCF, while recording ACL force and knee kinematics. Then tests with increasing TCF were performed under displacement control to reproduce the baseline kinematic pathway (tibia constrained), while recording ARF. This allowed testing to 1500N TCF since the ACL was not loaded. TCF generated ACL force for all knees (n=10) at 50° flexion, and for 8 knees at 30° flexion. ACL force and ARF had strong linear correlations with TCF at both flexion angles (R2 from 0.85 to 0.99), and ACL force was strongly correlated with ARF at both flexion angles (R2 from 0.76 to 0.99). Under 500N TCF the mean error between ACL force prediction from ARF regression and measured ACL force was 4.8 ± 7.3 N at 30° and 8.8 ± 27.5 N at 50° flexion. Our hypothesis was confirmed for TCF levels up to 500N, and ARF had a strong linear correlation with TCF up to 1500N TCF.