A study of human and bovine trabecular bones using nanoindentation and quantitative backscattered electron imaging

Abstract

Osteoporosis is a disease with which bone progressively loses density and becomes fragile, leading to high risk of bone fracture. It is a major health issue in Australia, especially in the elderly population. Early awareness about the disease can help find effective medication and reduce bone fractures with appropriate lifestyle changes. In medical diagnosis, bone mineral density (BMD) measurement is used. However, there are increased evidences to demonstrate that BMD measurement alone may be insufficient to detect the pathological changes in bone that caused by the disease. Therefore, it is essential to gain a more complete understanding of the bone properties and their effect on osteoporosis. Bone is a hierarchical composite material, which mainly consists of mineral apatite and organic matrix. Previous studies found that osteoporosis or aging could cause microstructural changes in trabecular bones of a proximal femur, the most common site for osteoporosis, and an increased degree of anisotropy was observed in osteoporotic bone properties. To identify the risk of fracture occurrence in the proximal femur as well as to prevent bone fracture, it is important to understand the relationship between bone mineral composition and bone mechanical properties. This thesis thus focused on the characterisation of the mechanical properties and mineral composition of trabecular bone, in order to address several key issues relating to the causes of osteoporosis, including: how does trabecular bone respond to different loading directions ? how do bone cysts (which are the implication of pathological scenario due to the trabecular fracture) affect the mechanical properties and the mineral composition of trabeculae ? how does the mineral content affect the mechanical properties of trabeculae across different trabecular types and pathology scenarios ? To answer those questions, trabecular bone samples harvested from bovine and human femoral heads were prepared for testing. Nanoindentation was first used to characterise the mechanical behaviour of trabeculae and atomic force microscopy (AFM) was employed to examine the bone surface tomography. Quantitative backscattered electron imaging (qBEI) was utilized for determining the bone mineral density distribution and the mineral content. The results showed that at microscopic scale longitudinal trabeculae were stiffer and harder than transverse trabeculae, because those bones were more highly mineralized. Longitudinal trabeculae also exhibited greater resistance to plastic deformation. The difference in mechanical properties between longitudinal and transverse trabeculae is partially attributed to the difference in their mineral compositions. To further understand the mechanical behaviour of trabeculae, longitudinal and transverse trabeculae were tested in two orthogonal directions. Results showed that both longitudinal and transverse trabeculae had greater elastic modulus in axial direction than in radial direction. Because the axial loading direction is axially aligned with the primary loading direction of a proximal femur, the mechanical anisotropy could be due to the orientation preference of trabecular bone at the lowest level of hierarchy, i.e. collagen and mineral level. This study also discovered that bone cysts in the proximal femur could significantly affect the mechanical properties of longitudinal and transverse trabeculae. The abnormal trabeculae, i.e. the bones with cysts, were stiffer and harder than the normal bone, i.e. the bone without cysts. The significant difference in modulus and hardness between the normal and abnormal trabeculae was observed often in the lower calcium concentration groups. The abnormal trabeculae exhibited less capability for containing elastic and plastic deformation. These findings suggest that trabeculae are more brittle under certain pathological conditions, such as bone cysts. In summary, fragility of a proximal femur is believed to be related to mineral composition, stiffness and hardness of its bone tissue. The alternation of the bone organic matrix might be responsible for the mechanical behaviour of the bone under pathological scenario. Therefore, a stiffer bone matrix can increase the brittleness of a bone material, hence, or the fragility of bone tissues. This study also indicates that organic phases, i.e. collagen, could affect, or at least partially, the mechanical behaviour of trabecular bone tissues. As a consequence, the study of bone mechanical property and fracture associated with osteoporosis should take into account for the contribution of both organic phase and mineral composition.