Effect of microstructure on the mechanical properties of Haversian cortical bone

The mechanical properties of cortical bone have been extensively studied at the macrostructural scale. However, knowledge of the macroscopic mechanical properties is not sufficient to predict local phenomena, such as damage or bone remodeling, both of which are dependent on local mechanical behavior. The objective of this study is to quantify the mechanical properties of cortical bone at several length scales, with emphasis on the microstructure of Haversian systems. Samples of mature bovine cortical bone, with a Haversian microstructure, were obtained from the posterior area of the mid-femoral diaphysis. A nanoindentation technique was used to measure the local Young's modulus. The distribution of the bone mineral content was obtained by backscattered electron imaging using a scanning electron microscope. A novel compression device employing microextensometry techniques was developed to quantify local strains. Digital image correlation was performed on the microstructure imaged by optical microscopy during compression tests. This study demonstrated that the local Young's modulus and strain were heterogeneous at the scale of an osteon. For both properties, the ratio between the maximum and minimum values was approximately two. The local Young's modulus and bone-mineral content were reasonably correlated (r2 = 0.75; P < 0.0001), but this was not the case for the distribution of local strains versus bone mineral content (r2 = 0.395; P < 0.0001). Hence, local strains cannot be described simply in terms of the bone mineral content, as the Haversian canal and osteonal microstructure have a major influence on these properties. In conclusion, the microstructure must be considered in evaluating the local strain and stress fields of cortical bone.