Ultrasonic assessment of the determinants of human cortical bone elasticity: Relative contributions of Haversian porosity and mineralized matrix stiffness

At the mesoscale (a few millimeters), cortical bone can be described as a two-phase material which consists in a relatively hard matrix and pores. The mesoscopic elastic properties are hence essentially determined by the stiffness of the matrix and the pores morphology. The goal of the study was to assess the relative contributions of tissue elasticity and porosity to mesoscopic elasticity. Measurements were conducted on human femoral cortical bone (21 specimens taken from 10 women donors aged from 66 to 98 years). A 50-MHz scanning acoustic microscope was used to assess the cortical porosity and evaluate the bone matrix elasticity. A contact ultrasonic method based on wave velocity and apparent tissue density measurements was applied to determine the bone mesoscale stiffness coefficients. The mesoscale stiffness was highly correlated to the cortical porosity whereas the mean tissue elasticity (reflected by acoustic impedance values) did not explain the mesoscopic stiffness variations. This work suggests that, for the elderly population, the elastic properties of the mineralized matrix do not undergo large variations among different samples and the cortical porosity accounts for most of the variations of mesoscale elasticity.