High-energy X-ray diffraction measurement of bone deformation during fatigue

Hard mineralized tissues such as bone are the primary load bearing structures in the body. Its basic building block is an organic matrix - mainly type I collagen, a mineral phase - calcium hydroxyapatite and water. These three components interact to form a very complex structure hierarchically organized from the nano- to the macro-scale [1], which is capable of supporting bodily loads and resisting fracture due to its high strength and toughness. However, despite its high resistance, bone fractures occur due to both impact loads and repetitive or sustained loads exerted over larger periods of time. Cyclic loads are the most direct analog to periodic stresses and strains varying in both intensity and type seen in physiological activities like walking and running [2]. It is thus clinically important to gain a deeper understanding of the mechanisms occurring within this complicated structural hierarchy of bone to ultimately predict and prevent such failures thus leading to improved medical treatments. Fatigue damage has been shown to be characterized by cyclic property degradation [3, 4], caused by accumulation and propagation of microcracks [5, 6]. The formation of microcracks is loading mode dependent [7, 8], which in turn affects the degradation of properties during cyclic loading. The macroscopic behavior has been fairly well understood from the vast number of studies done previously. Still lacking is an understanding of the interaction between the different components of the building block and the subsequent hierarchical levels of bone during such loading, which results in the observed macroscopic behavior. The high-energy X-ray diffraction technique used in this work makes it possible to obtain through-thickness (~ 5 mm in this case) crystal lattice information from the bulk of the sample, compared to other high resolution techniques like nanoindentation which sample only surface volumes up to depths of about 1000 nm [9], or other imaging techniques like scanning and transmission electron microscopy and atomic force microscopy [10], which are good for obtaining surface structural information. With its in situ testing capability [11], this high-energy X-ray diffraction technique has recently found applications in probing individual components of bone [12] and dentin [13] materials. The aforementioned diffraction technique is used in the current work to determine the strains in the HAP and collagen phases of bone during cyclic loading in compression at body temperature, and study their evolution with cycles.