Bone fractures affect over 1.5 million people a year in the United States and can lead to a decrease in life expectancy and quality of life. While some fractures occur due to a single overloading event such as a fall, many fractures develop over time. Insufficiency fractures are one type of fracture that develop over time and typically occur in regions of the skeleton dominated by cancellous bone. In cancellous bone, the accumulation of microdamage results in a loss of biomechanical performance and is believed to contribute to fracture incidence. However, relatively little is known about the how microdamage accumulates in cancellous bone and the aspects of cancellous bone structure that influence the development of microdamage.

While the development of microdamage is driven by stresses and strains at the tissue-level, the complex microarchitecture of cancellous bone prevents the direct measurement of tissue-level stresses/strains. Additionally, naturally forming stress concentrations called resorption cavities form on the surface of cancellous bone. Finite element models can be used to calculate the tissue-level stress/strain in cancellous bone. According to finite element models, the largest stresses/strains will occur at the surface of cancellous bone and the stresses around resorption cavities will be higher than other surfaces of the bone. However, finite element models are created from three-dimensional images of the bone and the images are not typically obtained at resolutions capable of examining resorption cavities. Additionally, the material properties of cancellous bone are not homogeneous and may influence the location of microdamage formation. The oldest and stiffest tissue is found near the center of trabeculae away from the locations that experience the highest stresses. Therefore, first, we characterized the size and location of resorption cavities. Next, we explored the spatial relationship between microdamage and resorption cavities by developing three-dimensional spatial correlation techniques and determining the spatial relationship between microdamage and resorption cavities. Finally, we examined how well tissue-level strains measured from finite element models predicted the location of microdamage.

The size and location of resorption cavities suggest that they can generate large stress concentrations in cancellous bone. However, microdamage preferentially formed away from resorption cavities, and the majority of microdamage was located distant from the surface of trabeculae. Additionally, reductions in biomechanical performance during fatigue loading were explained primarily by the largest microdamage sites. Hence, only microdamage sites larger than a certain size appear to influence the mechanical performance of cancellous bone following cyclic loading. Furthermore, when using finite element models, regions of cancellous bone displaying the greatest principal tissue strains were able to predict the location of the largest and most biomechanically relevant microdamage sites.

Together, the current work suggests that losses in biomechanical performance following damage accumulation in cancellous bone are due to a few large Together, the current work suggests that losses in biomechanical performance following damage accumulation in cancellous bone are due to a few large microdamage sites that form near the center of trabeculae. Furthermore, microdamage accumulation is poorly related to location stress concentration due to microgeometry, suggesting that other factors such as tissue heterogeneity may be more influential in determining microdamage accumulation.