Tissue heterogeneity, composite architecture and fractal dimension effects in the fracture of ageing human bone

The mechanical characteristics of human bone, especially those relating to age, are of immense interest to everyone. A great amount of information has already been accumulated on the macromechanical/phenomenological aspects of bone behaviour and while some aspects, such as stiffness and strength, have been attributed to effects at the architectural/compositional level, some others like toughness have been related to events at the molecular/biophysical level. It is not always easy to unravel the intimate relationship between the architectural and remodelling changes at the macroscale to the biophysical/chemical effects occurring at the ultrastructural level. There is however, the mesostructural level (fractography), which is commonly overlooked or has been approached in a purely qualitative manner. In this article we concentrated primarily on the variation of toughness of ageing bone with age and then examined the fracture profile morphology of the various samples by fractal analysis. The results show that the way bone actually fractures, in either slow/ductile or fast/brittle fracture, has an underlying connection to the architectural status of each individual and the way ageing bone changes as a ‘material’ as well as a ‘collection’ of heterogeneous elements and structures. Of course, fracture morphology cannot simply and uniquely be described by one fractal dimension, but fracture nevertheless is determined by the intrinsic architecture of the bone structure and its material.