Intrinsic bone tissue properties in adult rat vertebrae: modulation by dietary protein.

Bone strength depends on bone mass, geometry, microarchitecture, and intrinsic tissue quality. Whether intrinsic bone tissue properties could be influenced by changes in dietary protein is not known. To address this issue, nanoindentation tests were performed on the lateral, anterior, and posterior site of L5 vertebral bodies in adult female rats fed a normal protein containing diet and in ovariectomized (OVX) rats receiving an isocaloric low protein diet with or without isocaloric essential amino acids supplements. The tissue properties varied significantly between the different sites (anterior, posterior, lateral), suggesting possible effects of heterogeneous stress distribution on the vertebrae in vivo. Isocaloric low protein intake associated with ovariectomy led to significant decreases of indentation modulus, hardness, and dissipated energy on the posterior vertex. Axial compression tests of adjacent vertebral bodies were correlated with the indentation results. Correlations between macroscopic mechanical data obtained by axial compression of vertebral body, and intrinsic tissue properties measured by nanoindentation test suggest that postelastic behavior strongly varied with material fragility detected on the tissue level. Macroscopic stiffness however may be dominated by bone geometry changes and less by variations of intrinsic bone tissue properties. Combining parameters of tissue properties and bone mineral density was highly predictive of vertebral body ultimate strength. Besides geometry and microarchitecture, intrinsic bone tissue properties are important determinants of the mechanical competence of rat vertebrae. Changes in intrinsic tissue properties could thus contribute to the increased bone fragility found in protein undernutrition.