Characterization of the Effects of X-ray Irradiation on the Hierarchical Structure and Mechanical Properties of Human Cortical Bone

Bone comprises a complex structure of primarily collagen, hydroxyapatite and water, where each hierarchical structural level contributes to its strength, ductility and toughness. These properties, however, are degraded by irradiation, arising from medical therapy or bone-allograft sterilization. We provide here a mechanistic framework for how irradiation affects the nature and properties of human cortical bone over a range of characteristic (nano to macro) length-scales, following x-ray exposures up to 630 kGy. Macroscopically, bone strength, ductility and fracture resistance are seen to be progressively degraded with increasing irradiation levels. At the micron-scale, fracture properties, evaluated using insitu scanning electron microscopy and synchrotron x-ray computed micro-tomography, provide mechanistic information on how cracks interact with the bone-matrix structure. At sub-micron scales, strength properties are evaluated with insitu tensile tests in the synchrotron using small-/wide-angle x-ray scattering/diffraction, where strains are simultaneously measured in the macroscopic tissue, collagen fibrils and mineral. Compared to healthy bone, results show that the fibrillar strain is decreased by w40% following 70 kGy exposures, consistent with significant stiffening and degradation of the collagen. We attribute the irradiation-induced deterioration in mechanical properties to mechanisms at multiple length-scales, including changes in crack paths at micron-scales, loss of plasticity from suppressed fibrillar sliding at sub-micron scales, and the loss and damage of collagen at the nano-scales, the latter being assessed using Raman and Fourier Transform Infrared spectroscopy and a fluorometric assay. Bone is a natural composite of organic, mineral and water assembled in the form of a complex hierarchical structure [1]. At the molecular level, it comprises a network of polymetric proteins, primarily type I collagen, with hard and stiff mineral nanoparticles of hydroxyapatite that reinforce it. At sub-micron levels, the collagen forms fibrils (w100 nm diameter) with mineral platelets assembled and periodically spaced on the inside and on the fibril surface; at the micron-scale such fibrils are twisted together to form collagen fibers. At even coarser scales on the order of 10 to 100 s mm, human bone's characteristic structure consists of osteons, which are concentric layers of lamellar bone, w100 mm in diameter, that contain a central, longitudinal, tubular cavity (Haversian canal), blood vessels, and nerves, and which represent a functional unit by which the physical and biological homeostasis of bone is actively maintained. The stiffness, strength and toughness properties 1 of bone develop from this multi-scaled, hierarchical structure [1e4], which spans from nanometer to macroscopic dimensions (Fig. 1) [5,6]; indeed, …