Adaptation to differential loading: comparison of growth-related changes in cross-sectional properties of the human femur and humerus.

Changes in long bone cross-sectional geometry during growth can be influenced by biological and mechanical factors. Here, we assess relationships between cross-sectional geometric properties and length of the human humerus and femur during postnatal growth to test the hypothesis that loading history plays an important role in the development of adult bone morphology. A skeletal sample including 83 paired humeri and femora from individuals between birth and age 30 was examined. Midshaft cross-sectional geometric properties were determined based on computed tomographic scans and the two bones were compared by examining growth trajectories and scaling relationships between the cross-sectional properties and bone length. The growth trajectories for both bones were similar in many respects and showed that increase in length ceased by age 20, whereas increase in cross-sectional properties continued into the third decade of life. When compared to bone length, the cross-sectional geometric properties of the femur and humerus were similar early in postnatal life, but increased at a greater rate in the femur particularly during the first decade of life, leading to divergent adult morphologies. A beam model was developed to predict maximum midshaft strains in each bone as a function of age. The moment acting on the femur was estimated from an analysis of gait in children and the moment acting on the humerus was chosen so that the magnitude of the maximum midshaft strains in the two bones was equivalent in adulthood. With this model, the maximum midshaft strains for the femur were predicted to be higher than for the humerus during the first decade of life. These data support the concept that load history plays an important role in accretion of bone mass during postnatal growth.