Associations between the spatial distribution of bone density in the vertebra and intervertebral disc health
The association between age-related vertebral fractures (VFx) and disc degeneration (DD) is not clear, despite the high prevalence of both conditions. Load is transferred to the vertebra by the adjacent intervertebral discs, and degenerative changes within the disc alter how the net force is distributed over the interface between vertebra and disc, known as the vertebral endplate (EP). The ability of the vertebra to resist fracture depends not only on the magnitude of the net force, but also on the distribution. Multiple lines of evidence suggest that the ability of the vertebra to withstand the distribution of applied force depends on the spatial distribution of bone mineral density (BMD) within the vertebra. First, the strength and stiffness of a region of bone in the vertebra are highly correlated with the BMD of that region. Second, changes in the spatial distribution of regional BMD have been associated with aging and DD. Thirdly, some of these observed changes have been replicated in computer models bone adaption with in the presence of progressive DD, suggesting that bone adaption is occurring in response to the altered force distribution associated with DD, and that maladaptation could elevate the risk of fracture. Notably, the current clinical method of identifying patients at risk of fracture is to use an average measure of BMD for the entire vertebra. The lack of consideration of the spatial distribution of BMD may explain why the clinical method used at present does not adequately identify those at risk of fracture. The possible relationship among spatial distribution of BMD, DD, force distribution across the endplate, and vertebral strength suggests that characterizing the spatial distribution of BMD within a vertebra could add to the understanding of why some vertebra are more likely to fracture. This project sought to determine if an association exists between the spatial distribution of vertebral BMD and disc health in order to provide an improved perspective of the clinical sequelae of DD and to improve the ability of clinicians to identify those who would benefit most from intervention. This study found evidence that the distribution of bone in the vertebral body and EP depend on the health of the adjacent disc. The distribution of pressure in discs favors the anterior most portion of the disc in anteriorly flexed postures and the density in the anterior most portion of the EP appears to respond to this shift, suggesting that bone is adapting to loading patterns associated with certain postures more than others. This study also found association between reduced regional disc height and altered distribution of trabecular density which was positive in the nuclear region and negative in the annular region. In some cases there was a lack of association between disc height and density distribution that may indicate maladaptation and thus increased risk of fracture. This study, being cross-sectional could not identify whether the observed alterations in density and degeneration initiated in the vertebra or the disc. However, this study contributes to the understanding of the relationship between the distribution of vertebral density and the functional properties of the adjacent disc that may ultimately improve the clinician's ability to predict VFx.