Validation of immature ovine bone models using digital image correlation

Data on paediatric bone properties is very limited due to the scarcity of specimen in this particular age range, and due to ethical complexities regarding experimental testing on paediatric cadavers. A recently validated technique for adults using CT-based finite element analysis (FEA), has the potential to provide a deeper understanding of immature bone fracture mechanism without the need of extensive paediatric cadaveric testing. This thesis aimed to investigate the validity of a CT-based FEA approach for immature bones, using lamb bones as surrogate to children’s bones, by comparing strain results against 3D digital image correlation (3D-DIC) data during four-point bending tests.
In this thesis, two experimental set-ups were designed to run four-point bending tests on lamb femur specimens using 3D-DIC to capture full-field surface strain. The first set of experiments applied bending load directly onto the femurs, whereas the second set of experiments embedded the bones with the load applied to the embedding materials instead. The four-point bending tests were replicated using an image-based FEA approach. The FE geometries were segmented from either QCT or µCT images. The material properties were derived from the CT image’s attenuations using existing empirical findings relating elastic modulus to bone mineral density. Various model set-ups and boundary conditions have been investigated in order to determine the best ones that matched the experimental results.
Through assessing and comparing results obtained in the experimental tests and the FEA, the improved FE approach was successfully validated using lamb bones. Furthermore, it was concluded that the cortical region of the diaphysis (mineralised bone) of infants and toddlers can be modelled with adequate accuracy using just a homogeneous mesh of isotropic elastic modulus. For children of three years and above, anisotropy of the bone may need to be considered to appropriately replicate the properties of lamellar bone. Further studies are required in order to better characterize the empirical relationship between elastic modulus and mineral density in immature bone, as well as the cortical and trabecular bone material properties at various developmental ages. The periosteum plays an important role in the fracture behavior of immature bone; thus, it is important to separately characterize the material properties of the periosteum. For a complete representation of bone in vivo, both the periosteum and the mineralizing bone should be integrated into the FE model.