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Abstract
Studies concerned with the "adaptations" in bones usually deal with modelling taking
place during the individual's lifetime. However, many adaptations are produced over
evolutionary time. This survey samples some adaptations of bone that may occur over
both length scales, and tries to show whether short- or long-term adaptation is important.
(a) Woven and lamellar bone. Woven bone is less mechanically competent than lamellar
bone but is frequently found in bones that grow quickly. (b) Stress concentrations
in bone. Bone is full of cavities that potentially may act as stress concentrators.
Usually these cavities are oriented to minimise their stress-concentrating effect.
Furthermore, the "flow" of lamellae round the cavities will still further reduce their
stress-concentrating effect, but the elastic anisotropy of bone will, contrarily,
tend to enhance it in normal loading situations. (c) Stiffness versus toughness. The
mineral content of bone is the main determinant of differences in mechanical properties.
Different bones have different mineral contents that optimise the mix of stiffness
and toughness needed. (d) Synergy of whole bone architecture and material properties.
As bone material properties change during growth the architecture of the whole bone
is modified concurrently, to produce an optimum mechanical behaviour of the whole
bone. (e) Secondary remodelling. The formation of secondary osteones in general weakens
bone. Various suggestions that have been put forward to account for secondary remodelling:
enabling mineral homeostasis; removing dead bone; changing the grain of the bone;
taking out microcracks. (f) The hollowness of bones. It is shown how the degree of
hollowness is adapted to the life of the animal.