Anterior cruciate ligament (ACL) tears are common injuries occurring upwards of 250,000 times annually only in the U.S. These injuries create an annual billion dollar expense. It is important to understand these injury mechanisms as these injuries continue to be one of the largest problems in orthopedic sports medicine. Improved understanding of the injury mechanisms may improve prevention, rehabilitation and surgical procedures.

This study used a 3-Dimensional finite element (FE) knee joint model to investigate the combinations of movement, which cause ACL injury.

Digital bone structures were created from magnetic resonance images (MRI). Ligament bundles were modeled based on the origins and insertion sites determined from MRI. Bone was modeled as rigid, and a transversely isotropic Mooney-Rivlin material was applied to the ligament structures. This study incorporates a novel approach for developing bundle specific prestrain within 3-D ligament structures. The bundles were stretched from their zero load lengths to their reference lengths, producing a strain field mimicking the in vivo strain conditions of the ACL at full knee extension. A failure locus was created by performing multiple FE simulations of knee joint motion combinations until ACL failure.

The relationship between knee joint orientation and ligament rupture was plotted providing a spectrum for the propensity of ACL injury based on knee joint orientations, known as a failure locus. The locus shows which combinations of internal/external femoral rotation and varus/valgus angle cause ACL failure. The results show the posterolateral bundle more susceptible to rupture than the anteromedial bundle in 17 of the 22 simulations. The results also show 45% less varus angle needed for ligament failure relative to valgus angle. The results highlight femoral external rotation as an important factor for ACL injury as it decreases the failure angle by an average of 23% compared to femoral internal rotation.

These results have various clinical applications. In sports where ACL injuries are prevalent, training programs can be adapted to address the avoidance of harmful knee orientations.


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