Bone response to in vivo mechanical loading in C3H/HeJ mice

Bone, being sensitive to mechanical stimulus, adapts to mechanical loads in response to bending or deformation. Although the signal/receptor mechanism for bone adaptation to deformation is still under investigation, the mecha nical signal is related to the amount of bone deformation or strain. Adapta tion to changes in physical activity depends on both the magnitude of incre ase in strain above average daily levels for maintaining current bone densi ty and the Minimum Effective Strain (MES) for initiating adaptive bone form ation. Given the variation of peak bone density that exists in any human po pulation, it is likely that variation in levels for MES is, to a considerab le degree, inherited and varies among animal species and breeds. This study showed a dose-related periosteal response to loading in C3H/HeJ mice. The extent of active formation surface, the rate of periosteal bone formation, and area of bone formation increased with increasing peak periosteal strain . In these mice, the loaded tibia consistently showed lower endocortical fo rmation surface and mineral apposition rate than the nonloaded bones at eve ry load level. Although periosteal expansion is the most efficient means of increasing moment of inertia in adaptation to bending, a dose response inc rease in endocortical formation would have been predicted. Our characteriza tion of the mouse bone formation response to increasing bending loads will be useful in the design of experiments to study the tibial adaptive respons e to known loads in different mouse breeds.