Model studies of the stiffness characteristics of the human knee joint.

A three-dimensional mathematical model of the ligamentous knee joint has been developed and applied to studies of the mechanical response of the knee joint. The model includes a representation of the bony portion of the knee including the proximal tibia, distal femur, soft tissue structure and the contacting surfaces of the medial and lateral condyles. The bony portions of the model are represented by rigid bodies while the soft tissue structures are represented by spring and beam type elements. The model employs the direct stiffness approach from structural mechanics and uses an incremental linearization procedure for the geometric and material non-linearities. Studies with the model indicate that the geometric type non-linearities contribute to the overall non-linear response of the knee joint. It was also found that the load-displacement response of the knee is highly dependent on constraints to coupled degrees of freedom. This finding may be an important consideration when interpreting the results of standard laxity tests at the knee, which by their nature may impose constraints to motion.