SHEFELBINE, SJ; CARTER, DR: Effects of mechanical loads on morphological changes in the femur during development

Many morphologic changes occur during growth and development of the femur. In particular, the bicondylar angle (angle between the diaphysis and the intracondylar plane) increases about 8 degrees. The neck-shaft angle and anteversion angle of the proximal femur both decrease approximately 10 degrees. Coincident with these morphological changes are drastic changes in the mechanical loading on the femur after a child starts walking. During long bone growth, local mechanical factors at the growth front regulate the growth rate, thereby influencing the morphologic characteristics of the developing bone. Previous studies have indicated that the endochondral growth and ossification process during long bone development is promoted by intermittent octahedral shear stress and inhibited by intermittent hydrostatic compression. We applied these mechanobiological principles to a three-dimensional finite element model of the femur to predict how forces on the joint surface alter growth front morphology and ultimate bone shape of the proximal and distal femur. Results accurately predicted growth front morphologies in the proximal and distal femur. Asymmetric stresses in the distal femoral growth plate result in asymmetric growth with the medial side of the femur growing faster than the lateral side. In the proximal femur the direction of the resultant hip force influenced both the neck-shaft and anteversion angle. These concepts have broad implications for understanding normal morphogenesis and also deformity under various loading conditions. These mechanobiological principles may also help to explain the morphological variations in skeletal structures of different species with different functional activities during development.