Abstract

The entire flexion of the human knee joint was experimentally studied by means of spatial solid kinematic theory. This study used postmortem joints, initially intact, then with the menisci removed, and finally without the antero-external cruciate ligament. The instantaneous velocities were defined in kinematic theory; then they were transformed into incremental displacements of the tibia which was considered as a moving solid relative to the femur, which was considered as the reference solid. The rotational movement of tibia was computed as being the main degree of freedom; then it was divided into three physiological components: flexion-extension, axial rotation, and varus-valgus rotation. The computation of the instant axis of the motion allowed definition of an area that was characterized as a central zone of the joint motion. This 3-dimensional approach was in opposition to the classical but incorrect definitions of the instant center of rotation, which can be described only for movement within a plane. Eight anatomical knee joints were tested and their mean results were used to elucidate conceptual problems relative to the femoro-tibial knee-joint prosthesis design.