Deformation of mineral crystals in cortical bone depending on structural anisotropy

The deformation mechanism of bone at different hierarchical levels has been of wide interest. The important features of bone, its anisotropy and orientation dependent deformation are equally important, which have also gained a long run discussion. Most of the studies are concentrated on protein-rich collagen fibres and matrix, where different deformation mechanisms at the lower length scales are proposed. But in relation to this, how the mineral particles behave depending on their distribution is yet to be revealed in detail. In the present work, we demonstrate mineral crystals deformation and arrangement characteristics on the basis of experimental outcomes. Using X-ray diffraction procedures, we quantified the mineral strains, degree of orientation of the crystallites and their evolution under different applied step-loads in bovine femoral cortical specimens having different alignment with the femoral axis direction. We provide a direct quantitative comparison of these parameters in the specimens having preferential orientations roughly at 0, 30, 45, 75 and 90 degrees with reference to the loading direction. The mineral strains in the compliant specimens, i.e. 0 and 30 degrees oriented specimens were observed to differ with the stiffer specimens, i.e. 75 and 90 degrees oriented specimens, whereas the 45 degrees oriented specimen show almost equal strains at different loads. These were explained by the degree of orientation with reference to the loading direction and the preferential orientation direction of the specimens. On the basis of observed parameters, we describe deformation phenomena of mineral particles to occur in different stages, which consist of redistribution stage, elastic strain stage and inelastic strain stage. These phenomena are expected to occur at different scales and rates depending on the orientation and distribution of crystals.