The design of guide surfaces for fixed-bearing and mobile-bearing knee replacements

In the natural knee, the femoral–tibial contacts move posteriorly as the knee is flexed, guided primarily by the cruciate ligaments. This kinematic behaviour is important regarding muscle lever arms and in achieving a high flexion range. Most contemporary total knee designs use either posterior cruciate preservation or a cam system to produce posterior displacement with flexion, but there is no specific provision for anterior displacement. In this study, a method for the design of cams is described where the cams would guide the motion in both posterior and anterior directions, without requiring cruciate ligaments. The cams consist of a femoral Guide Surface interacting with a tibial Guide Surface while the main lateral and medial bearing surfaces carry the forces across the knee. It is shown that Guide Surfaces can be designed which provide the required motion, but with some laxity at different flexion ranges. It is then demonstrated that the Guide Surfaces can be applied to a range of possible knee designs including mobile-bearing types, rotating-platform types, and fixed-bearing types. The relative advantages of the different possibilities are discussed.