Structural alterations and breaking strength of mouse femora exposed to three activity regimens

Mice were used as a model system to correlate loading, changes in cortical and trabecular bone growth and remodeling, and differences in breaking strength of the whole femur. BALB/c mice were raised for 30 days, starting at day 14, under three conditions of loading or exercise: (a) normal exercise control (NE), (b) 4 x normal gravity for a total of one h/day in addition to normal exercises (HG), and (c) chronic digging and burrowing (high-litter cage) (HL). Results indicate that both HG and HL groups significantly increased the cortical cross-sectional area of the mid-shaft of the femur, the moment of inertia in the antero-posterior direction at the mid-shaft, anterior wall thickness of the midshaft cross section, and changed the architecture of trabecular bone in the proximal one-third of the femur. The extent of these structural changes, however, was unique in each group. The HG group (high-intensity, short duration loading) demonstrated the greatest structural changes in the moment of inertia in the antero-posterior direction, which increased by 44% over the control (p less than .001), and in the architecture of trabecular bone. The HL group (lower-intensity, chronic exercise), demonstrated the greatest structural change in cortical bone cross-sectional area, which increased by 27% (p less than .0001). Despite the differences in loading and structural changes in the HG and HL groups, the breaking strength of the whole femur for both groups was approximately 64% greater than the controls (p less than .0001). A mechanical analysis of our data revealed that for our tests the contribution of bending to the total stress on the shaft may be over 50 times greater than that of either compression or tension. Further, the change in the geometry of the cortical bone may account for only approximately 50% of the increase in breaking strength; the remaining increase must be accounted for by changes in material properties and/or placement of trabeculae within the lumen of the shaft.