How does human bone resist fracture?

The fracture of bone is clearly a major health concern, especially for the elderly. Medical therapies to reduce the possibility of bone fracture to date have principally centered on treating the loss in bone mass (bone mineral density) that accompanies aging (i.e., addressing the loss in bone quantity). However, it is now known that there is an additional, perhaps more significant, effect of the degradation in the inherent properties of bone (i.e., a loss in bone quality) with age. To address this issue, we review here the structure and properties of bone, focusing on its strength and fracture resistance from the perspective of the multidimensional hierarchical nature of its structure. We show that bone derives its resistance to fracture from a multitude of deformation and toughening mechanisms at many of these size-scales, ranging from the nanoscale structure of its protein molecules to its macroscopic physiological state.