Anisotropic post-yield behavior of human cortical bone
Age-related bone fractures have been an increasing concern as the life expectancy of humans has been increasing. The increase of fracture incidences in the elderly is associated with the reduction of energy dissipation capacity of aging bone. Therefore, identifying the underlying mechanisms of energy dissipation during post-yield deformation is critical in developing strategies to prevent bone fractures. The purpose of this study was to determine the contribution orientation has on the post-yield properties of bone since it is an anisotropic material. It is hypothesized that while post-yield properties of cortical bone are transversely isotropic, the underlying mechanism of post-yield deformation is independent of loading orientation. To test this hypothesis, forty-eight cylindrical samples were obtained from femurs of eight middle-aged male donors (51.5 +/- 3.3 years old). One set of 24 was tested monotonically and the other set was tested in a progressive loading scheme. After testing, the samples were stained and photographed under a fluorescent microscope to detect microdamage. The data show there are significant differences in energy dissipation between the longitudinal direction when compared with the transverse (circumferential and radial) directions, with values being larger in the longitudinal direction. The data also show that there are no significant orientation differences in the yield strain. These suggest that the underlying mechanism of post-yield deformation of cortical bone in compression is independent of orientation.