Age-related changes in cortical bone toughness: initiation vs. propagation

A number of fracture mechanics studies have characterized bone's resistance to fracture in terms of critical stress intensity factor and critical strain energy release rate measured at the onset of a fracture crack. These studies, although useful, provide a limited insight into fracture behavior of bone as, unlike classical brittle materials, bone is a microcracking solid that derives its resistance to fracture during the process of crack propagation from microfracture mechanisms occurring behind the advancing crack front. The objective of this study was to investigate the efficacy of a propagation approach in identifying age-related changes in bone toughness. Initiation and propagation tests conducted on human cortical bone compact specimens demonstrated different magnitudes of age-related toughness loss in initiation and propagation. Compared to initiation, propagation toughness of cortical bone declined twice as rapidly and reached a near zero value at the 10th decade. These results clearly illustrate that age-related decrease in bone's fracture resistance may be more drastic than currently perceived and that the propagation-based measure may be more sensitive to age-related changes in bone.