Background: Anterior cruciate ligament (ACL) tears are common injuries occurring upwards of 250,000 times annually only in the US. These injuries create an annual billion dollar expense in the US alone. It is important to understand these mechanisms of injury 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. Objectives: To perform a parametric study of knee joint movement to highlight which movement combinations are most detrimental to the ligaments of the knee joint, and also to study the effect of ramping rate on varus and valgus failure angle. The study provides a spectrum of injury propensity based on internal and external femoral rotation and varus or valgus angle as well as showing the effect of ramping rate on the prescribed boundary conditions. Methods: Bones and ligaments were digitized from magnetic resonance images (MRI). Bone was modeled as rigid, and a transversely isotropic visco-elastic material was applied to the ligament structures. This study incorporates a novel approach for developing bundle specific prestrain within 3D 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 at different ramping rates to investigate the effect of rate dependency on the valgus and varus failure angle of the ACL. Results: Loading rate had negligible effect on the valgus or varus ACL failure angle. External femoral rotation scenarios were more susceptible to ligament failure than internal femoral rotation. The postero-lateral bundle showed more injury susceptibility compared to the anteromedial bundle. Varus simulations showed less angle needed for failure when compared to valgus simulations. Conclusions: 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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