Computational study of kinematics of the anterior cruciate ligament double-bundle structure during passive knee...

The anterior cruciate ligament (ACL) comprises an anteromedial bundle (AMB) and posterolateral bundle (PLB). Cadaver studies showed that this double-bundle structure exhibits reciprocal function during passive knee flexion-extension, with the PLB taut in knee extension and the AMB taut in knee flexion. In vivo measurements indicated that straight-line lengths of both bundles decrease with increasing knee-flexion angle (KFA). To interpret these seemingly conflicting facts, we developed a computational ACL model simulating the kinematics of the double-bundle structure during passive knee flexion-extension. Tibial and femoral shapes were reconstructed from computed-tomography images of a cadaver knee and used to construct an idealized model of an ACL including its bundles at the tibiofemoral joint. The ACL deformations at various KFAs were computed by finite element analysis. Results showed that the PLB was stretched in knee extension (KFA = 0^∘) and slackened with increasing KFA. The AMB was stretched in knee extension (KFA = 0^∘) and remained stretched on the medial side when the knee flexed (KFA = 90^∘), but its straight-line length decreased with increasing KFA. These findings are consistent with cadaver and in vivo experimental results and highlight the usefulness of a computational approach for understanding ACL functional anatomy.