Elastic and physicochemical relationships within cortical bone

The purpose of this study was to examine the relationships that exist betwe en the elastic properties and the physicochemical properties of cortical bo ne 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 establi sh and broaden the quantifiable link between the three-dimensional form and function of bone beyond the typical unidirectional measures. This study wa s based on previously reported work that refined the ultrasonic elasticity technique for use with small specimens (<1.0 mm) and determined that the ad ministration of rhGH can counter the degenerative effects produced by hormo ne-suppressed downregulation on the elastic and physicochemical characteris tics of cortical bone. Ultrasonic wave propagation and density measurements were used previously to determine the three-dimensional (orthotropic) mate rial properties of rat femoral cortical bone. X-ray powder diffraction, mic roscopic, morphometric, and biochemical analysis techniques have been used to describe physicochemical properties, including mineral crystal size, cor tical porosity, mineral and nonmineral content, and microstructural charact eristics. In this study, mathematical relationships between the local physi cochemical (independent variable) and elastic (dependent variable) properti es 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). Concomita ntly, mineral crystal width and cortical porosity offered the best correlat ions 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 bon e after rhGH treatments (p < 0.05). Physicochemical measures of bone qualit y (density, crystal size) generally decreased while measures of bone quanti ty (cortical area, moments of inertia) generally increased (p < 0.05) after rhGH treatments. Some mineral and nonmineral properties were unchanged. Th is study presents a quantifiable link between cortical bone elasticity and its composite construction as measured across two dramatically different ex perimental groups. (C) 2000 John Wiley & Sons, Inc.