Comparison between anatomical and approximate surfaces in a 3D kinetostatic model of the knee for the study of the unloaded and loaded joint motion

The aim of this study is to define a new kinetostatic model of the knee, in order to analyze the joint behavior both in unloaded and loaded conditions. This model was obtained as an extension of an existing kinematic model. Anatomical surfaces and a comprehensive representation of the ligamentous structures were modeled. A second model was also defined by replacing the anatomical surfaces with spherical ones to evaluate how accuracy of articular surface representation affects tibiofemoral motion. Model geometry was defined according to a specimen, ligament zero-load lengths were derived from unloaded kinematics, whereas ligament stiffness parameters were taken from the literature. All these quantities were not optimized in order to verify the predictive capabilities of the model and to understand to which extent the model could be exempted from optimization. The two models were validated by comparing the predicted motion with experimental measurements from the literature under several loading conditions. Specifically, drawer tests, torsion tests and ab/adduction tests were simulated at different flexion angles. Anatomical articular surfaces provided more similar results to the reference kinematics, but the accuracy of the two models was comparable. Predictions of the model with spherical surfaces were less accurate at higher loads. Ligament and contact forces were also analyzed and were in good agreement with previous studies. Both models effectively replicated the behavior of the knee in unloaded and loaded conditions.