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Multi-scale evaluation of bone combining indentation, in situ XCT mechanics and digital volume correlation

Student thesis: Doctoral Thesis

Bone is an anisotropic and heterogeneous material with hierarchical structure, hence its mechanical behaviour in the sub-microstructure (1-10 μm) affects its performance in the macrostructure. In recent years, the combination of high-resolution X-ray Computed Tomography (XCT) with in situ mechanical testing has been used to analyse bone morphology and mechanics at different resolution and specimen sizes.

The primary aim of this PhD work is to combine high-resolution X-ray computed tomography (XCT) imaging, in situ/ex situ mechanics and digital volume correlation (DVC) on native and regenerated bone tissue, to provide a better understanding of their mechanical behaviour and its relationship to their morphological properties. At first, the effect of laboratory-based XCT imaging irradiation on the bone tissue, at the sub-microstructure, was examined to ensure the reliability of the results in the in situ XCT experiments carried out in this work. Then, cortical bone microstructural morphology was analysed in relation to its performance during plastic deformation and crack initiation, as well as morphological changes associated to the three-dimensional (3D) full-field strain distribution computed via DVC. Finally, bone regeneration was investigated at organ level using in situ XCT imaging for the mineralised tissue; whilst the radiolucent cartilage matrix in the same region was studied through histological analysis.

Indentation was employed to assess the degradation of the mechanical properties of trabecular bone tissue in the sub-microscale (ex situ nanoindentation) as well as to induce plastic deformation at the microscale in cortical bone tissue (in situ XCT indentation). Specifically, nanoindentation was used to obtain the elastic modulus prior and post- XCT irradiation of tissue, on the surface of the specimens; and the damage induced in the whole volume was estimated by comparison with the 3D full-field strain distribution. Moreover, microcrack formation and prevention in the cortical bone was observed in axial and transverse orientation following plastic deformation. In particular, the microstructural morphology of the specimens was correlated to the crack initiation and strain accumulation in the volume. Microcrack formation in the trabecular and cortical bone was identified at regions of high strain accumulation. Alterations in the canal and pore network of cortical bone were associated to the strain accumulation during in situ loading. At organ level, in situ XCT microcompression was used to investigate the mechanical behaviour of the newly formed bone in femoral mid-shaft, where the diaphyseal fracture healing was studied under loading in the apparent elastic level. The strain accumulation in the fracture varied depending on the progress of the healing in the diaphysis, whereas the histological results showed the presence of soft tissues (i.e. cartilage matrix, skin and muscles) that contributed in the overall mechanics of the femur by sustaining the load.

In conclusion, the results of this work show the 3D full-field strain distribution in the microstructure of bone and the contribution of its mechanical properties in the microscale to its performance in the macroscale in mature and regenerated tissue under plastic and elastic deformation respectively.
Original languageEnglish
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Award date2020

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