Radiolucent device to apply compression and flexion for the study of vertebral failure mechanisms
Naylor, Charlotte Chia Ch'i
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Vertebral fracture is the most commonly occurring clinical outcome of osteoporosis. Measurement of bone mineral density (BMD) in the spine is the primary method of assessing fracture risk. The accuracy of BMD measurements is limited by their inability to characterize variations in bone strength resulting from numerous factors, including the spatial distribution and microstructure of the bone tissue within the vertebra, the geometry of the vertebra, and the loading conditions. A more complete evaluation of fracture risk rather than identification of high-risk patients solely by low BMD is desirable. However, a lack of understanding of the mechanisms of fracture has beleaguered the development of more accurate predictors. Experimental approaches that integrate mechanical loading with three-dimensional imaging, via micro-computed tomography (µCT), can provide new insight into the initiation and propagation of fracture. Although these approaches have primarily used compressive loading, vertebrae are commonly under both compression and flexion during daily activities. The overall goal of this study was to create a device to apply a combination of compression force and flexion moment to a vertebra in a manner that is compatible with µCT imaging. The condition of compatibility dictated strict constraints on the size of the device and materials used. An important focus was maintenance of the imposed rotation and displacement throughout the progression of loading and imaging for a single test as well as across tests. The final design was able to easily apply the necessary forces without drift of position or damage to the apparatus for three samples as well as to allow the necessary data acquisition. Images obtained with f-1CT and force data collected through a load cell confirmed fracture of the vertebra and achievement of the desired displacements. An important addition to the device will be to incorporate a method measuring the non-uniform distribution of load across the boundaries of the vertebra. This device will be used in experiments aimed at a better understanding of mechanisms of failure in the vertebra, and the evaluation of the accuracy of computer models for use in a promising, non-invasive, clinically feasible method for diagnosis of fracture risk.
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