Variability in the nanoscale deformation of hydroxyapatite during compressive loading in bovine bone

High-energy synchrotron X-ray diffraction is used to study in situ elastic strains in hydroxyapatite (HAP) for bovine femur cortical bone subjected to uniaxial compressive loading. Load-unload tests at room temperature (27°C) and body temperature (37°C) show that the load transfer to the stiff nanosized HAP platelets from the surrounding compliant protein matrix does not vary significantly (p<0.05) with temperature. This emphasizes that the stiffness of bone is controlled by the stiffness of the HAP phase, which remains unaffected by this change in temperature. Both the extent of hysteresis and the residual value of internal strains developed in HAP during load-unload cycling from 0 to -100 MPa increase significantly (p<0.05) with the number of loading cycles, indicative of strain energy dissipation and accumulation of permanent deformation. Monotonic loading tests, conducted at body temperature to determine the spatial variation of properties within the femur, illustrate that the HAP phase carries lower strain (and thus stresses) at the anterio-medial aspect of the femur than at the anterio-lateral aspect. This is correlated to higher HAP volume fractions in the former location (p<0.05). The Young's modulus of the bone is also found to correlate with the HAP volume fraction and porosity (p<0.05). Finally, samples with a primarily plexiform microstructure are found to be stiffer than those with a primarily Haversian microstructure (p<0.05).