Quantifying the nature and consequences of morphological changes in trabecular bone structural units (BSU)

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Age-related fractures are a serious health concern in Canada. Many women and men are affected every year, resulting in significant impairment, reduced quality of life, and even death. The economic burden on the Canadian health care system is large and is expected to increase as the population ages. The forearm, hip, and vertebrae are the skeletal sites where most age-related fractures occur. These locations are characterized by a high-proportion of cancellous bone, a tissue which can be described as a 3D network of plates and rods called trabeculae. The size, distribution, and orientation of the trabeculae -referred to as the architecture- changes throughout our lifetimes as our bodies adapt to meet our current loading needs through a process called remodeling. For example, decreased activity or metabolic changes with age can result in bone loss, which means fewer and smaller trabeculae. Bone mineral density (BMD) tests are a common way to diagnose this bone loss; unfortunately, BMD only indicates how much bone is present and not the quality of the tissue itself. As a result, there is a large overlap in BMD values between those that do and do not experience a fracture. More information about bone tissue is needed to improve predictions of fracture risk. The remodeling process occurs in small steps where existing tissue is resorbed and new tissue is created. Over time, the accumulation of each step results in trabeculae that are a patchwork of little regions called bone structure units (BSU) -also known as hemiosteons or trabecular packets- separated from each other by highly mineralized layers of cement line. The resulting patchwork can be compared to a wall of irregularly shaped bricks where each brick (BSU) is separated from each other by a layer of mortar (cement line). Previous work, in our lab and elsewhere, has shown that BSU get smaller with age and deteriorating trabecular architecture. These changes are expected to decrease the mechanical and failure behaviour of the bone, increasing someone's fracture risk; however, they remain under-studied. The proposed research has three goals. The first is to determine the connection between the changes in BSU geometry and the changes in the trabecular architecture at different locations in the human skeleton. The second goal is to identify the mechanical consequences of the changes in BSU geometry by creating extended finite element method (XFEM) models of idealized BSU to simulate how different shape and size distributions hasten or slow crack propagation. Finally, the third goal is to determine whether similar changes occur in rats so that controlled experimental studies can be performed in the future to discover how various factors affect BSU geometry. Successfully completing these three goals will benefit the health and safety of Canadians by improving our understanding of the role that trabecular BSU play in age-related fractures and could improve fracture-risk assessment.