Fatigue strength of human cortical bone: age, physical, and material heterogeneity effects.

Human bone is highly heterogeneous at the microscale and this heterogeneity has been thought to relate to some fracture scenarios. The fatigue strength of cortical bone has been shown to relate to its inner architecture (osteonal or fibrolamellar) and some physical characteristics in the past, but never to the heterogeneity of its microstructure. The present study examines for the first time, the fatigue strength of human bone for six individuals of various ages in three main modes (tension, compression, and shear), the salient physical characteristics of these same specimens and the elastic mismatch that is present at a microstructural level by performing microhardness measurements on osteons and their surrounding matrix areas. The results showed that the ratio of hardness values in osteons and interstitial areas adversely affects fatigue strength with an effect more potent if not equal to the other usual factors such as "age" or features, material density, porosity, and mineral content, which are measured in a homogenized (averaged over a cross section) manner at a macrostructural (large scale) level. It has been known that fatigue microcracks localize in regions between osteons and interstitial matrix and in hypermineralized matrix areas. The present results indicate that, perhaps, heterogeneity not only influences the initiation of microcracks, but also their eventual growth and coalescence into larger cracks, which are detrimental for the integrity of the material.