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Failure prediction of cancellous bone using morphological data of trabeculae structure
Trabecular architecture has been considered as an important determinant of osteoporosis and other pathological conditions in bone. The microstructure of the trabecular network is often measured using measures such as bone volume fraction (BV/TV), trabecular number (Tb.N), trabecular thickness (Tb.Th), separation (Tb.Sp), and surface-to-volume ratio of bone (BS/BV). These parameters have been examined for different anatomical sites and physiological conditions such as aging, bone disease, and recently, remodeling caused by chemical treatments. A related parameter, the degree of anisotropy of trabecular bone, was observed to highly correlate with increase in fracture risk of the hip. Trabecular connectivity based on topological measurements has also been useful for predicting changes in mechanical properties with bone loss. Microcomputed tomography (micro-CT) was introduced to explore the three-dimensional (3D) architecture of bone. Due to its high resolution (as small as a few micrometers), micro-CT can obtain precise 3D images at the microlevel of trabecular structure. For in vitro micro- CT studies, different voxel sizes have been used, ranging from 8 to 120 microns. Although high resolution is achievable using micro-CT, scanning large specimens such as a whole vertebral body may require use of a spatial resolution corresponding to a voxel size greater than 100 microns. Because 100 microns is in the order of typical trabecular thickness, partial volume effects will cause errors when computing the morphological parameters for trabecular bone. Ding and Hvid found that the magnitudes of stereological parameters were strongly dependent on voxel size for
voxel size larger than 100 microns. Ruegsegger et al. indicated that the morphological values could be corrected to a specific resolution up to 200 microns voxel size based on the monotonic dependence of morphological parameters on image resolution. However, the effect of scanning and reconstruction voxel size has not been considered separately in previous studies. The separate and combined effects of these voxel sizes on morphological variables need to be evaluated. Scanning voxel size is a measure of the quality of the raw data images and determines the best level of detail that can be resolved in the image. The raw data can be appropriately reconstructed at any reconstruction voxel size that is larger than (or equal to) the scanning voxel size. Reconstruction voxel size is the actual voxel size chosen for the 3D image reconstruction. It is 22 ordinarily assumed that a larger reconstruction voxel size will decrease the accuracy of the image. Nevertheless, mainly to avoid large computational costs, images may have to be coarsened using greater reconstruction voxel
sizes than scanning voxel size in some applications such as micro-CT-based, large-scale finite element models (note that choosing a reconstruction voxel size smaller than the scanning voxel size cannot improve the quality of the reconstructed image because the raw data do not support the smaller reconstructed voxels). The contribution of microstructure to the mechanical properties of cancellous bone has been widely accepted, and to this end, many indices have been devised to further describe the influence of changes in bone microstructure onto its mechanical properties. However, these methods present an average number for the entire specimen and do not take into consideration general inhomogeneities and local variations in bone microstructure. Therefore, it is imperative that the weakest link in cancellous bone is identified, and its contribution to the failure properties of whole bone evaluated.
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