Geometrical, mechanical, and structural adaptation of mouse femora exposed to different loadings

Juvenile mice were utilized as a model system to study the influence of different activity regimens on the change in bone morphology and load-bearing capacity of their femora. Two treatment groups were employed: one exposed to 4g hypergravity (HG) and the other to chronic digging in high ground corn cob litter (HL). A voluntary normal exercise group (NE) served as control. One femur from each animal was used to determine the geometrical properties of the bone; the other femur was used in a load-to-failure experiment. Electron micrographs of femur cross sections were digitized to yield geometrical properties of the bone. Young's modulus and the stress and strain at failure for the bone were determined via a modified Euler beam model. Strain energy was evaluated directly from load-deflection curves obtained during the load-to-failure experiments. Average strain-energy density was then derived from these results. Cross-sectional geometrical properties (diameters, wall thicknesses, area, moments of inertia, and section modulus), mechanical properties (stress at failure and strain energy), and structural properties (bending stiffness, extensional stiffness, and moment at failure) were found to be statistically larger in HG and/or in HL when compared with NE (p < 0.05). Average strain-energy density and strain for HG were much larger than for NE or HL (p ≤ 0.05) and Young's modulus had the trend reversed. The present study indicates that juvenile mouse femora grow, model, and fail in a manner unique to the activity regimen under which they are raised.