BIOMECHANICAL REASONS FOR THE DIVERGENT MORPHOLOGY OF THE KNEE-JOINT AND THE DISTAL EPIPHYSEAL SUTURE IN HOMINOIDS

The obliquity of the femoral diaphysis accounts for the valgus positio n of the human knee joint and reduces bending moments in the frontal p lane, A high angle of obliquity is considered a hallmark of hominid bi pedality, but its functional importance has rarely been identified cor rectly. A biostatic investigation of the knee joint in various realist ic positions unveils resultant joint forces which do not deviate great ly from the long axis of the femoral shaft. This is due to the length of the femur and to the shortness of the human foot, The flat epiphyse al suture is more or less perpendicular to these joint forces, and the equal size of the femoral condyles reflects the even distribution of forces between them. In great apes the resultant forces acting in the knee joint vary considerably in dependence on the degree of flexion an d rotation of the knee joint. The resultant joint force may be in line with the femur shaft or diverge. The epiphyseal surfaces offer facets to all joint forces found in the course of the study. Due to the pron ounced varus position of the knee joint, the joint itself and the adja cent part of the femur are under medially concave bending moments, whi ch lead to higher compressive forces at the medial than at the lateral condyle. The enlarged medial condyle allows the distribution of media lly displaced joint forces over a relatively large area, and the ellip tic cross-section yields high bending resistance in the frontal plane. A human-like angle of obliquity is present in the early australopithe cines, the values being mostly within the range of variation of childr en. The valgus position of the australopithecine knee joint is conside red to be a functional, and epigenetic consequence of habitual bipedal ity. It is particularly pronounced because of the short length of the femur and the great bitrochanteric width.