BONE ELASTICITY AND ULTRASOUND VELOCITY ARE AFFECTED BY SUBTLE CHANGES IN THE ORGANIC MATRIX

The mechanical competence of bone can be studied through the measureme nt of the components of its material elasticity, a property which can vary both in magnitude and in dependence upon orientation (anisotropy) . While it is known that the elasticity is largely determined by the m ineral constituents of the bone matric:, it is nonetheless clear that it must be also dependent upon the remaining constituents of bone mate rial. In this work, the influence of organic components on the elastic ity is explored by altering specific constituents of the bone matrix: to varying degrees. This study addresses tno questions: first, are the resulting changes in elasticity strongly or weakly dependent upon dir ection, and second, are they substantially dependent upon the nature a nd magnitude of the induced matrix alteration? To answer these questio ns, we performed different chemical manipulations of the bone matrix a nd measured the changes in elasticity and velocity using the technique of ultrasound critical angle reflectometry. Altering the properties o f the organic matric resulted in substantial and complex changes in th e elasticity of bane, The observed changes were strongly dependent upo n direction, could not be explained by changes in density alone, and v aried strongly with the specific chemical treatment of the matrix. Imm ersion in urea selectively affected protein components of the organic matrix and resulted in reversible changes in velocity and elasticity, while removal of collagen caused anisotropic decreases and removal of all organic matter caused a collapse of all components of the elastici ty. In conclusion, this study confirms that the organic matrix exerts a profound influence on the elasticity and indicates that the measurem ent of elastic properties at multiple directions is necessary in the a ssessment of bone mechanical competence.