Detection of fatigue microdamage in whole rat femora using contrast-enhanced micro-computed tomography

Microdamage in bone tissue is typically studied using destructive, two-dimensional histological techniques. Contrast-enhanced micro-computed tomography (micro-CT) was recently demonstrated to enable non-destructive, three-dimensional (3-D) detection of microdamage in machined cortical and trabecular bone specimens in vitro. However, the accumulation of microdamage in whole bones is influenced by variations in the magnitude and mode of loading due to the complex whole bone morphology. Therefore, the objective of this study was to detect the presence, spatial location, and accumulation of fatigue microdamage in whole rat femora in vitro using micro-CT with a BaSO4 contrast agent. Microdamage was detected and observed to accumulate at specific spatial locations within the cortex of femora loaded in cyclic three-point bending to a 5% or 10% reduction in secant modulus. The ratio of the segmented BaSO4 stain volume (SV) to the total volume (TV) of cortical bone was adopted as a measure of damage. The amount of microdamage measured by micro-CT (SV/TV) was significantly greater for both loaded groups compared to the control group (p<0.05), but the difference between loaded groups was not statistically significant. At least one distinct region of microdamage, as indicated by the segmented SV, was observed in 85% of loaded specimens. A specimen-specific finite element model confirmed elevated tensile principal strains localized in regions of tissue corresponding to the accumulated microdamage. These regions were not always located where one might expect a priori based upon Euler-Bernoulli beam theory, demonstrating the utility of contrast-enhanced micro-CT for non-destructive, 3-D detection of fatigue microdamage in whole bones in vitro.