Kinematics of the Knee

Kinematics describes the angular and linear movements during functions, as well as the internal motions of the femur, tibia, and patella, relative to each other. There are 6 degrees of freedom to be considered; 3 rotations; and 3 displacements. These are described relative to axis systems defined in each of the moving bodies. The axis systems are defined so that the motions are related to clinical parameters, the most important being flexion–extension, internal–external, varus–valgus, and anterior–posterior displacement. Fluoroscopic techniques have determined the anterior–posterior displacements of the femoral condyles on the tibial plateau during various activities, a valuable measure of joint kinematics. An important aspect of kinematics is laxity, which defines the boundaries of motions for applied forces. The laxity boundaries themselves are partly determined by the cruciate and collateral ligaments, but also by the shapes of the femoral and tibial condyles and the menisci. Laxity is substantially reduced when axial forces are applied. In a total knee where cruciates are retained, to achieve normal kinematics, the challenges include design of components and surgical technique, where ligament lengths are maintained to an accuracy of around 2 mm. For components which substitute for the cruciates, the challenge is in the design reproducing the nuances of motion normally provided by the cruciates as well as the bearing surfaces.