Meeting Abstract
Bone plasticity can result in changes in cortical thickness or distribution in the limb skeleton following exposure to different loading environments. However, the relative contributions of muscular contraction and ground reaction force (GRF) to an observed phenotype are not fully understood. We examined the influence of loading environment on femur cross-sectional geometry and mineral apposition in male outbred mice. Four-week-old mice were divided into 4 treatment groups, each exposed to a different source of bone strain for 21 days: downward jumping to increase GRF, swimming to increase muscle loading, wheel running to mimic the combination of muscle and GRF in normal locomotion, and sedentary controls. Mice were injected with fluorochrome bone labels during the last week of treatment. Undecalcified femoral mid-diaphyseal sections were photographed using light and fluorescent microscopy. Images were analyzed for cortical growth patterns and cross-sectional properties (cross-sectional area, moments of inertia). The impact group had significantly greater cross-sectional area, moments of inertia, and polar moments of inertia than controls, indicating increased strength in torsion and bending. Fluorochrome labeling indicated the greatest mineral apposition was posterolaterally in all groups, with additional cortical drift medially or posteriorly characterizing each loading regime. These results demonstrate that impact loading is more effective at increasing bone cross-sectional properties than swimming or running. Future analyses of whole bone morphometrics will further elucidate the roles of GRF and muscle-induced forces in long bone modeling in the context of voluntary locomotion.
