Current measures of skeletal health rely heavily on the quantification of bone mass, oversimplifying the complex and multifaceted nature of bone tissue. Though diminished bone quantities do correlate with increased fracture incidence, bone mass is increasingly proving to be only one of a multitude of factors influencing the lifelong integrity of the skeleton. Bone quality, a multifaceted approach to bone health, considers all the components of bone, from nanostructure to macrostructure, and their interactions to develop a holistic approach to bone health. At the microstructural level, linear microcracks, a form of cortical microdamage, act as a toughening mechanism of the bone and are thought to be a major contributor to fracture risk. Known to contribute to the formation of fatigue fractures, where accumulated damage within the cortex results in whole bone failure, current understanding of in vivo levels of microdamage is limited. Additionally, work has focused predominantly on cyclic loading and fatigue, without considering the role cortical level damage may play in failure from a traumatic load.

This study lays out detailed methods for the preparation and analysis of linear microcracks in cortex of the human rib. Revised criteria for counting and measuring cracks is detailed in order to reduce the subjectivity of such measurements, with the aim of improving intra- and inter-observer error. These methods are then applied to a sample of 30 individuals, 15 females and 15 males, ranging in age from 40–99 years of age. Paired 6th ribs were required from each individual, one to act as a baseline of in vivo microdamage prevalence and the other to be used in experimental testing. Using the control ribs, in vivo microdamage was assessed for the influences of age and sex. Neither of these demographic variables proved to have any influence on microcracks in the study sample. The test ribs were placed in an experimental fixture where they were impacted to failure in anterior-posterior displacement, creating a simulated two-dimensional bending scenario. From each test, structural properties were calculated, as well as microcrack data collected from histological sections at the sites of fracture. Analysis of this data showed a tendency towards decreased structural response when cortical microdamage was greater, however, few of the relationships reached a level of statistical significance. Linear microcrack initiation and propagation may be rate dependent, where the loads used in this experimental scenario were too fast to allow for a microdamage response. Additionally, while linear microcracks do not appear to significantly influence rib structural response in dynamic loading, they may be essential for characterizing bone's material response and should be further explored. Teasing out the role of microcracks in bone's mechanical response is a preliminary step in fully defining the parameters of bone quality, which is essential to improving methods of fracture risk assessment and targeted treatments.