Accumulation of in-vivo fatigue microdamage and its relation to biomechanical properties in ageing human cortical bone

Bone matrix accumulates microdamage in the form of microcracks as a result of everyday cyclic loading activities. In two very recent studies, which used conventional histological stains and light microscopy techniques, the amount of this in-vivo microdamage in the cortices of long bones has been shown to increase with age. These articles have suggested that in-vivo microcracks may have an effect on the material properties of the tissue. However, a precise quantitative relationship between the number of microcracks and the mechanical properties of these same bones has not been produced before, and in particular the way the microcracks may affect the stiffness, the strength or possibly the toughness of the tissue. This article presents an examination of the in-vivo microdamage in human bones by the use of laser scanning confocal microscopy, which offers better discrimination and allows examination of the cracks in-situ. Quantification of in-vivo fatigue microcracks was performed by counting the microcrack numerical density and surface density in specimens for which we have previously derived a full set of mechanical properties as a function of age. It is shown that bone microdamage relates more to the toughness (measured by three different measures) of ageing bone tissue than to its stiffness and strength. The result allows us (i) to re-evaluate the fragility of ageing human bone and put more emphasis on its energy-related resistance to fracture than perhaps on its stiffness or strength and also (ii) to understand more fully the causal relationship and interactions between microcracks and tissue toughness.