INTERRELATIONSHIPS BETWEEN DENSITOMETRIC, GEOMETRIC, AND MECHANICAL-PROPERTIES OF RAT FEMORA - INFERENCES CONCERNING MECHANICAL REGULATION OF BONE MODELING

A compensation for differences in bone material qualtiy by bone geomet ric properties in femora from two different strains of rats was previo usly shown by us. A feedback mechanism controlling the mechanical prop erties of the integrated bones was then proposed, in accordance with F rost's mechanostat theory. Evidence of such a system is now offered by the finding of a negative correlation between the modeling-dependent cross-sectional architecture (moment of inertia) and the mineral-depen dent stiffness (elastic modulus) of bone material in the femoral diaph yses of 45 normal Wistar rats of different sexes, ages, and sizes. The strength and stiffness of the integrated diaphyses were found to depe nd on both cross-sectional inertia and body weight, not on bone minera l density. These findings are interpreted as supporting the hypothesis that the architectural efficiency of diaphyseal cross-sectional desig n resulting from the spatial orientation of bone modeling during growt h is optimized as a function of the body weight-dependent bone strain history, within the constraints imposed by bone stiffness. Results sug gest a modulating role of biomass, related to the system set point det ermination, and explain the usually observed lack of a direct correlat ion between mineral density and strength or stiffness of long bones in studies of geometrically inhomogeneous populations.