Investigation of mechanical properties of fragility fractured bone: numerical and experimental approach

Abstract

The human bone has a hierarchical structure which provides mechanical strength to bear the load exerted on the human body during daily activity. The bone fracture is a severe health concern, which occurs due to ageing or pathological conditions such as osteoporosis, type 1 diabetes (T1D), type 2 diabetes (T2D), osteogenesis imperfecta (OI), etc. Most of the cases T1D and OI are found in adolescence, whereas the osteoporosis and T2D are developed with ageing. These disease contracted populations are in the high risk of non-traumatic fracture (fragile fracture). The risk of fragility fracture is predicted with the gold standard clinical tool dual X-ray absorptiometry (DXA), and it is based on the measurement of bone mineral density (BMD). However, in some cases, the BMD fails to predict fragility fracture, such as for the T2D patients, the chance of fragility fracture is high despite normal BMD. Therefore, independent of bone mass, some other factors are dominating to bone fracture. This study aims to elucidate the factors associated with high fragility fracture by characterising the structural, tissue mechanical and apparent mechanical properties of the bone subjected with osteopenia, osteoporosis and T2D. Later these results are used to develop a damage based finite element model (FEM) to predict the trabecular bone strength for different pathological conditions, and XFEM has utilised to understand the role of microstructure on crack propagation behaviour. Further, this study is extended to illustrate the effect of alteration in various measured bone quality parameters due to T2D on hip fracture risk using FEM. Micro-CT, Nanoindentation, uniaxial compression and 3-point bending test were used to characterise the structural and mechanical properties of bone. Further, a nonlinear finite element model was developed to predict trabecular bone’s apparent mechanical strength and effect of alteration in crystal morphology on tissue properties. Furthermore, the XFEM was utilised to illustrate the consequence of an alteration in structural parameters due to ageing in crack propagation behaviour of cortical bone. A multiscale finite element model was developed to understand how different measured bone quality parameters associated with hip fragility fracture. The micro-CT and uniaxial mechanical characterisation results suggest that T2D causes higher degrade in bone structural, apparent stiffness, yield strength, ultimate strength, and toughness compared to OP. In contrast, the nanoindentation results suggest that non-significant difference in material properties between OP and T2D group. The uniaxial compression results also suggest that T2D does not alter the viscoelastic properties of trabecular bone. Furthermore, the damage parameters obtained from uniaxial compression test results indicate that it is independent of the pathological condition. The developed damage based FEM predict bone strength with reasonable accuracy. Besides the cohesive-based nanoscale, FE results suggest that the increase in hydroxyapatite crystal size decrease, the tissue elastic and post-yield properties. The experimental and numerical results for cortical bone reveal that ageing alters cortical bone’s microstructure, such as increased porosity and decreased osteon wall thickness, diminishing the crack toughening mechanism by penetration of crack in osteon and reduce in crack growth toughness. The coupled macro-micro scale FEM quantifies different altered bone quality parameters due to T2D on the femur’s fragility fracture. The model demonstrated that trabecular bone’s altered microstructure due to T2D diminished the post-yield stability of proximal femur and enhanced the chance of fragility fracture. In conclusion, this thesis has described bone’s complex behaviour at different hierarchy using the experimental and numerical technique. It identified the alteration in tissue mechanical properties, microstructural and apparent mechanical properties with a different pathological condition. These Studies are clinically useful as it demonstrates the different factors associated with a fragile fracture in T2D bone, and the model can be utilised to predict the fracture risk.