Fracture and Ageing in Bone: Toughness and Structural Characterization

The development of a mechanistic understanding of the increase in fracture risk in human bone with age is essential to public health. This represents a challenge for fracture mechanics as bone has a complex, hierarchical structure with characteristic features ranging from nanometer to macroscopic dimensions, and is thus much more complex than most engineering materials. In this study, we review ex vivo fracture experiments which quantitatively assess the effect of age on human cortical bone in the proximal-distal orientation, i.e. along the long axis of the bone. Specifically, cortical bone is seen to exhibit rising crack-growth resistance with crack extension; the toughness is consequently evaluated in terms of R-curves, measured in bones taken from a wide range of age groups (34-99 years). Both crack-initiation and crack-growth toughnesses were determined and were found to deteriorate with age; the initiation toughness decreases some 40% over the 65 years of ageing, while growth toughness is effectively eliminated over the same age range. The reduction in crack-growth toughness is considered to be associated primarily with a degradation in the degree of extrinsic toughening, in particular involving crack bridging in the wake of the crack. This explanation is supported by an examination of the micro-/nanostructural changes accompanying the process of ageing, performed using deep-UV Raman spectroscopy, computed X-ray tomography and optical/electron microscopy.