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 inf luenced by biological and mechanical factors, Here, we assess relation ships between cross-sectional geometric properties and length of the h uman humerus and femur during postnatal growth to test the hypothesis that loading history plays an important role in the development of adu lt bone morphology. A skeletal sample including 83 paired humeri and f emora from individuals between birth and age 30 was examined. Midshaft cross-sectional geometric properties were determined based on compute d tomographic scans and the two bones were compared by examining growt h trajectories and scaling relationships between the cross-sectional p roperties 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-sec tional geometric properties of the femur and humerus were similar earl y in postnatal life, but increased at a greater rate in the femur part icularly during the first decade of life, leading to divergent adult m orphologies, A beam model was developed to predict maximum midshaft st rains in each bone as a function of age, The moment acting on the femu r was estimated from an analysis of gait in children and the moment ac ting on the humerus was chosen so that the magnitude of the maximum mi dshaft strains in the two bones was equivalent in adulthood, With this model, the maximum midshaft strains for the femur were predicted to b e higher than for the humerus during the first decade of life, These d ata support the concept that load history plays an important role in a ccretion of bone mass during postnatal growth.