The effects of geometry and fiber bundle orientation on the finite element modeling of the anterior cruciate ligament

The anterior cruciate ligament (ACL) has irregular geometry and spirally oriented fiber bundle organization, which are closely related to its physiological function. In previous finite element (FE) models, however, these two features are neglected due to the difficulty of obtaining its complex geometry and spiral fiber bundle orientation. Based on a previously developed and validated FE model, this study performed parametric studies to evaluate the effects of geometry and fiber bundle orientation on the FE modeling of the ACL. To evaluate the effect of the geometry, two models were compared: 1) with realistic ACL geometry obtained by using digitizer; 2) with ACL geometry reconstructed by directly connecting the femur and tibia insertion sites as commonly used in previous studies. To evaluate the effect of fiber bundle orientation, another two models were compared: 1) with realistic fiber bundle orientation obtained by using digitizer (α=38°); 2) with unrealistic fiber bundle orientation (α=0°). The same kinematics obtained by a Robotic/Universal Force-moment Sensor (UFS) system was input into the models as boundary conditions. The resultant forces calculated by the models were compared to the experimental data. The model with unrealistic geometry had a 40% higher ACL resultant force compared to the experimental data, while the model with the realistic ACL geometry well predicted the ACL resultant force, with an error less than 10%. When evaluating the effect of fiber bundle orientation, the model with unrealistic fiber bundle orientation predicted similar ACL resultant forces and stress distribution as the model with realistic fiber bundle orientation. The results revealed that ACL geometry has a significant effect on the FE model while fiber orientation does not.