[Biomechanics of the normal and prosthetic knee]

The knee joint is characterized by particular kinetics due to asymmetry of the femoral condyles and to angulation of the extensor system. During flexion, this results in a rolling/sliding movement in which the upper surface of the tibia recoils under the femoral condyles, combined with an automatic movement of medial rotation from extension towards flexion of the tibia beneath the femur. The articular contact areas are very small, ranging from 10 to 20 cm2 for the femorotibial articulation and barely reaching 4 cm2 for the femoropatellar articulation. These areas are subjected to extremely strong pressures amounting to 7 times the body weight in the femorotibial articulation during walking, and up to 1500 N (20 times the body weight) during simple flexion. However, the most fundamental element of biomechanics is the instability of the knee joint which is dependent on its menisco-ligamentous system. In addition, the menisci increase the femorotibial contact area, thereby relieving some of the pressure. The knee prosthesis concept is based on the stability of the prosthetic knee. In patients with healthy ligaments, non-strained prosthesis can be used, in which case part of the forces proceeds directly from one bone to the other via the ligaments, and there are only minimal stresses between the two prosthetic pieces and at the prosthesis/bone interface. But when the ligamentous system is defective or incomplete, or when the axial alignment is so upset that ligaments must be severed, one must use prostheses that are partly or totally constrained.(ABSTRACT TRUNCATED AT 250 WORDS)