The influence of tensile and compressive damage on bending fatigue of human cortical bone

Age-related non-traumatic fractures are a major health problem. Historically, only bone mass was considered to predict fracture risk but other factors, including accumulation of microdamage, may also affect bone quality and contribute to bone fragility. The relationship between microdamage and bone fragility is, however, not completely understood. Microdamage formation is a combined function of local strain and tissue properties. Linear microcracks form under compressive loading and run through the cement lines. In contrast, diffuse damage forms under tensile loading as submicroscopic cracks in the interstitial bone. The objective of this study was to understand the effects of damage morphology on bone fragility by determining the relationships between damage morphology, nanoindentation-determined local tissue properties and fatigue behavior of bone obtained from two different age groups. The results of this study demonstrate that the predisposition of bone to form linear microcracks and not diffuse damage is a significant contributor to bone quality and age-related increase in bone fragility. During fatigue loading, younger bone lost local tissue stiffness on the tensile side and had a longer fatigue life. Older bone, in contrast, formed linear microcracks and lost local tissue stiffness on the compressive side and had a shorter fatigue life.