Development of the femoral bicondylar angle in hominid bipedalism

The bicondylar angle is the angle between the diaphysis of the femur and a line perpendicular to the infracondylar plane. The presence of a femoral bicondylar angle in Australopithecus afarensis indicates that these 3.5-million-year-old hominids were bipedal. Many studies have linked the formation of the femoral bicondylar angle with bipedality, but the mechanism for the formation of the angle is poorly understood. Mechanical factors, such as stresses and strains, influence the growth process. In particular, previous studies have demonstrated that hydrostatic compressive stress inhibits growth and ossification, and octahedral shear stress promotes growth and ossification. In this study we implemented these mechanobiological principles in a three-dimensional finite-element model of the distal femur. We applied loading conditions to the model to simulate loading during the single-leg stance phase of bipedal gait. The stresses in the physis of the distal femur that result from bipedal loading conditions promote growth and ossification more on the medial side than on the lateral side of the femur, forming the bicondylar angle. This model explains the presence of the bicondylar angle in hominid bipedalism and also the ontogenetic development of the bicondylar angle in growing children. The mechanobiological relationship between endochondral ossification and mechanical loading provides valuable insight into bone development and morphology.