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Bone as a Structural Material:
Origins of its Fracture Resistance and Biological Degradation
Robert O. Ritchie
Materials Sciences Division, Lawrence Berkeley National Laboratory, and
Department of Materials Science and Engineering, University of California, Berkeley
Abstract:
The age-related deterioration in the quantity of bone and its architecture and resultant fracture
properties, coupled with increased life expectancy, are responsible for increasing incidences of bone
fracture in the elderly segment of the population. In order to develop effective treatments, an
understanding of the mechanisms underlying the structural integrity of bone, in particular its inherent
fracture resistance is essential. Here we examine the origins of the toughness of human cortical bone in
terms of the contributing micro-mechanisms and their characteristic length scales in relation to its
hierarchical structure. It is shown that at length-scales at or below a micrometer or so, the toughening
mechanisms in bone are primarily intrinsic, and include mechanisms such as fibrillar sliding at the
collagen fibril (i.e., ~100 nm) and collagen fiber (~1 μm) levels. These are essentially “plasticity”
mechanisms that operate ahead of a growing crack, e.g., by forming a plastic zone to blunt the crack tip.
At length-scales above a micrometer or so, the toughening mechanisms are primarily extrinsic, and are
associated with crack deflection/twist and crack bridging. In terms of measured fracture toughness of
bone, the latter mechanisms are particularly potent; they affect the growth rather than the initiation of
cracks and as such lead to resistance-curve toughening behavior. There is also the process of
microcracking, which in addition to serving as an intrinsic “plasticity” mechanism and possibly
signaling the remodeling of bone, acts principally to motivate the extrinsic deflection and bridging
mechanisms, which in turn results in the marked anisotropic fracture behavior. In this context, realistic
short-crack measurements of the crack initiation and growth toughnesses are used to evaluate the
effects of aging and certain drug treatments (e.g., steroids, bisphosphonates) in bone, and are combined
with structure characterization using UV Raman spectroscopy, transmission electron microcopy, 2-D in
situ fracture tests in an environmental scanning electron microscope and 3-D ex situ examination of
crack paths using synchrotron x-ray computed tomography, to determine the microstructural features
that underlie the toughness of bone and how this can degrade with biological factors.
Society of Engineering Science ▪ 47 th Annual Technical Meeting 8