Very few quantitative comparisons between mechanical test behavior of cortical bone and microscopic evidence of damage have been reported. In this study, the hypothesis that age-related degradation of mechanical properties in human cortical bone is associated with increases in damage in the form of microcracks was investigated. The initial modulus and yield stress were 6% (not significant) and 10% (p = 0.05) lower, respectively, in specimens from elderly femora than in specimens from young adult femora. However, both groups showed a 34% decrease in modulus after being loaded to 1% strain. Microcracks were observed in cement lines and between lamellae and were parallel to the loading direction. There were 50% more cracks in longitudinal sections of tested specimens than in controls from elderly femora; however, there were no more cracks in tested specimens than in controls from young adult femora. In addition, there were twice as many cracks in controls and three times as many cracks in tested specimens from elderly femora than in those from young adult femora (p < 0.01). A microstructurally based model was developed which supported the mechanical test results and indicated that damage began to develop at about 1500 με. The results suggest that older bone may have reduced mechanical properties due to the presence of more microcracks, and that older bone is more susceptible to developing microcracks at a given strain level. However, the mechanical test data indicate that specimens from young adult femora also sustained some kind of damage as a result of mechanical loading, which requires further investigation.
Keywords: Cortical bone; Damage; Aging; Mechanical properties; Model