Study of Mechanical Properties of Bone by Measuring Load Transfer via High-energy X-ray Diffraction

Synchrotron high-energy X-ray scattering is used to investigate the in situ strains in hydroxyapatite (HAP) platelets and mineralized collagen fibrils in bovine cortical bone. Compressive load-unload tests at room temperature (27°C) and body temperature (37°C) show that the load transfer to the stiff nano-sized platelets from the surrounding compliant protein matrix does not vary significantly 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. Monotonic loading tests in compression and tension, conducted at 37°C, illustrate the spatial variation of properties within a single femur, which is correlated to the mineral content, porosity and microstructure of the samples. The average apparent modulus of HAP and fibrils (EappHAP and Eappfib, respectively), defined as the ratio of applied stress and phase strain, is obtained as 27.5 +/- 6.6 and 18.5 +/- 8.9 GPa, respectively, in compression. These values are significantly higher than the values of 20.0 +/- 5.4 and 4.1 +/- 2.6 GPa obtained for HAP and fibrils, respectively, in tension. The difference between the two types of loading is attributed to greater plastic deformation of collagen in tension, which results in greater strains in the collagen fibril, and concomitant greater load transfer to the HAP.;Increasing synchrotron X-ray doses (5-3880 kGy) affect neither apparent HAP nor fibrillar modulus, up to stresses of -60 MPa (measured during in situ loading and unloading). However, the residual elastic strains in the HAP phase decrease markedly with increased irradiation, indicating damage at the HAP-collagen interface. Analysis of the X-ray diffraction peak widths shows that unit cells of HAP which are under the highest initial residual strains are most able to relax due to irradiation, resulting in a net decrease in the strain distribution (RMS strain). The constancy of apparent moduli is explained by temporary debonding at the HAP-collagen interface (thus reducing the residual strain), followed by rapid re-bonding (so that load transfer capability is not affected).;Bone undergoes creep when subject to constant stresses, where both HAP and fibrillar strains increase linearly with time. This suggests that as bone deforms macroscopically, it behaves as a traditional composite, shedding load from the more compliant, viscoelastic matrix to the reinforcing elastic HAP platelets. However, the opposite behavior is seen when highly irradiated bone and dentin (which has similar structural organization at the nanoscale as bone) are subject to creep stresses: the reinforcing HAP platelets progressively transfer some of their initially acquired elastic strains back to the softer protein matrix during creep. This behavior is explained by the occurrence of damage at the HAP-collagen interface. Systematic investigation of this change in load transfer behavior as a function of radiation dose at -80 MPa shows that the rate of compressive elastic strain accumulation in HAP decreases with increasing dose, until, at ∼115 kGy, it changes sign, indicating that the HAP phase is shedding load during creep deformation. For doses in excess of ∼300 kGy, the rate of HAP elastic strain shedding for crept samples, remain independent of dose, suggesting a saturation of damage and/or stiffening of the collagen matrix due to intermolecular cross-linking. The HAP and fibril strain rates in bone and dentin also increase significantly with stress and temperature.;Finally, cyclic compressive loading tests were carried out on irradiated bones at 37° C with varying mean stresses (-55 to -80 MPa), loading frequencies (0.5 to 5 Hz), and stress ranges (-30 to -110 MPa). The HAP and fibrillar apparent modulus vary in a complex manner with fatigue cycles indicating that load partitioning between the HAP and protein phases evolves with fatigue. Synchrotron micro-computed tomography of some of the specimens showed that cracks are produced during fatigue, and that they mostly occur parallel to osteon lamellae and near Haversian canals.