title: Age-related changes in the plasticity and toughness of human cortical bone at multiple length scales.
abstract: The structure of human cortical bone evolves over multiple length scales from its basic constituents of collagen and hydroxyapatite at the nanoscale to osteonal structures at near-millimeter dimensions, which all provide the basis for its mechanical properties. To resist fracture, bone's toughness is derived intrinsically through plasticity (e.g., fibrillar sliding) at structural scales typically below a micrometer and extrinsically (i.e., during crack growth) through mechanisms (e.g., crack deflection/bridging) generated at larger structural scales. Biological factors such as aging lead to a markedly increased fracture risk, which is often associated with an age-related loss in bone mass (bone quantity). However, we find that age-related structural changes can significantly degrade the fracture resistance (bone quality) over multiple length scales. Using in situ small-angle X-ray scattering and wide-angle X-ray diffraction to characterize submicrometer structural changes and synchrotron X-ray computed tomography and in situ fracture-toughness measurements in the scanning electron microscope to characterize effects at micrometer scales, we show how these age-related structural changes at differing size scales degrade both the intrinsic and extrinsic toughness of bone. Specifically, we attribute the loss in toughness to increased nonenzymatic collagen cross-linking, which suppresses plasticity at nanoscale dimensions, and to an increased osteonal density, which limits the potency of crack-bridging mechanisms at micrometer scales. The link between these processes is that the increased stiffness of the cross-linked collagen requires energy to be absorbed by "plastic" deformation at higher structural levels, which occurs by the process of microcracking.
authors.affiliation: Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, USA.
authors.forename: Elizabeth A
authors.initials: EA
authors.lastname: Zimmermann
authors.forename: Eric
authors.initials: E
authors.lastname: Schaible
authors.forename: Hrishikesh
authors.initials: H
authors.lastname: Bale
authors.forename: Holly D
authors.initials: HD
authors.lastname: Barth
authors.forename: Simon Y
authors.initials: SY
authors.lastname: Tang
authors.forename: Peter
authors.initials: P
authors.lastname: Reichert
authors.forename: Bjoern
authors.initials: B
authors.lastname: Busse
authors.forename: Tamara
authors.initials: T
authors.lastname: Alliston
authors.forename: Joel W
authors.initials: JW
authors.lastname: Ager
authors.forename: Robert O
authors.initials: RO
authors.lastname: Ritchie
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000502:physiology*; D001696:Biomechanical Phenomena; D001842:Bone and Bones / Q000502:physiology*; D017127:Glycation End Products, Advanced / Q000032:analysis; D006801:Humans; D008875:Middle Aged; D014057:Tomography, X-Ray Computed
chemical_list: D017127:Glycation End Products, Advanced