Fatigue Microdamage in Bovine Cortical Bone Imaged by Micro-Computed Tomography Using a Barium Sulfate Contrast Agent
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Fatigue Microdamage in Bovine Cortical Bone Imaged by Micro-Computed Tomography Using a Barium Sulfate Contrast Agent Huijie Leng, Xiang Wang, Glen L. Niebur and Ryan K. Roeder Department of Aerospace and Mechanical Engineering, University of Notre Dame Notre Dame, IN 46556, U.S.A. ABSTRACT Accumulation of microdamage during fatigue can lead to increased fracture susceptibility in bone. Current techniques for imaging microdamage in bone are inherently destructive and twodimensional. A non-destructive, three-dimensional technique is needed to measure the spatial density of microdamage accumulation. Therefore, the objective of this study was to image microdamage accumulation in cortical bone during fatigue using micro-computed tomography (micro-CT) with a barium sulfate (BaSO4) contrast agent. Bovine cortical bone beams were loaded under four-point bending fatigue. Two symmetric notches were machined on the tensile surface in order to generate damage at the stress concentrations during loading. Specimens were loaded to a specified number of cycles or until one notch fractured, such that the other notch exhibited accumulated microdamage just prior to fracture. Microdamage ahead of the notch was stained by precipitation of BaSO4 and imaged using micro-CT. Reconstructed images showed a distinct region of bright voxels around the notch tip or along propagating cracks due to the presence of BaSO4, which was verified by backscattered electron imaging and energy dispersive spectroscopy. The stained region exhibited a characteristic kidney shape perpendicular to the notch tip, which was correlated to principal strain contours calculated by finite element analysis. The area of stained regions was positively correlated with the number of loading cycles. INTRODUCTION Healthy bone is crucial to overall human health. Bone provides structural support for the body; protects soft tissues and organs; produces red blood cells; stores minerals; and transmits muscular forces during movement [1]. Microdamage accumulates in cortical bone in vivo due to repetitive loading and is observed in the form of microcracks or diffuse damage [2,3]. In vitro studies have shown that microdamage accumulation has a detrimental effect on the mechanical properties of cortical bone [4,5]. However, in vivo, microdamage is repaired by, and may signal, bone remodeling [6-8]. Thus, excessive accumulation of microdamage prior to remodeling can lead to increased fracture susceptibility. However, the mechanistic role of microdamage in bone fragility is not yet well understood, in part, due to our limited capabilities for imaging and measuring microdamage accumulation. Current techniques, such as UV or light microscopy using fluorescent stains [9-12], backscattered electron imaging (BEI) using a lead-uranyl acetate stain [13], laser scanning confocal microscopy (LSCM) using fluorescent stains [14,15] and serial imaging using fluorescent stains [15], require the preparation of many histologic sections which are inherently destructive and two-dimension
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