Elastic and physicochemical relationships within cortical bone.

The purpose of this study was to examine the relationships that exist between the elastic properties and the physicochemical properties of cortical bone in two groups of experimental animals. The animal model was the immature mutant dwarf rat, and the groups consisted of rats treated and not treated with recombinant human growth hormone (rhGH). The objective was to establish and broaden the quantifiable link between the three-dimensional form and function of bone beyond the typical unidirectional measures. This study was based on previously reported work that refined the ultrasonic elasticity technique for use with small specimens (<1.0 mm) and determined that the administration of rhGH can counter the degenerative effects produced by hormone-suppressed downregulation on the elastic and physicochemical characteristics of cortical bone. Ultrasonic wave propagation and density measurements were used previously to determine the three-dimensional (orthotropic) material properties of rat femoral cortical bone. X-ray powder diffraction, microscopic, morphometric, and biochemical analysis techniques have been used to describe physicochemical properties, including mineral crystal size, cortical porosity, mineral and nonmineral content, and microstructural characteristics. In this study, mathematical relationships between the local physicochemical (independent variable) and elastic (dependent variable) properties were formulated via linear and nonlinear regression analyses. In general, apparent density was found to have the highest level of correlation with most of the longitudinal and shear moduli (R(2) = 0.300 to 0.800). Concomitantly, mineral crystal width and cortical porosity offered the best correlations with the Poisson's ratios (R(2) up to 0.600). Wilcoxon t tests verified a significant decrease in the elastic properties in dwarf rat cortical bone after rhGH treatments (p < 0.05). Physicochemical measures of bone quality (density, crystal size) generally decreased while measures of bone quantity (cortical area, moments of inertia) generally increased (p < 0.05) after rhGH treatments. Some mineral and nonmineral properties were unchanged. This study presents a quantifiable link between cortical bone elasticity and its composite construction as measured across two dramatically different experimental groups.