The effect of cartilage deformation on the laxity of the knee joint

In this paper, deformation of the articular cartilage layers is incorporate d into an existing two-dimensional quasi-static model of the knee joint. Th e new model relates the applied force and the joint displacement, as measur ed in the Lachmann drawer test, and allows the effect of cartilage deformat ion on the knee joint laxity to be determined. The new model augments the p revious knee model by calculating the tibio-femoral contact force subject t o an approximate 'thin-layer' constitutive equation, and a method is descri bed for finding the configuration of the knee under a specified load, in te rms of a displacement from a zero-load reference configuration. The results show that inclusion of deformable cartilage layers can cause a reduction o f between 10 and 35 per cent in the force required to produce a given tibia l displacement, over the range of flexion angles considered. The presence o f cartilage deformation was found to be an important modifier of the loadin g response but is secondary to the effect of ligamentous extension. The fle xion angle dependence of passive joint laxity is much more strongly influen ced by fibre recruitment in the ligaments than by cartilage deformation.