Age‐related factors affecting the postyield energy dissipation of human cortical bone

Abstract

The risk of bone fracture depends in part on tissue quality, not just the size and mass. This study assessed the postyield energy dissipation of cortical bone in tension as a function of age and composition. Specimens were prepared from tibiae of human cadavers in which male and female donors were divided into two age groups: middle aged (51 to 56 years, n ¼ 9) and elderly (72 to 90 years, n ¼ 8). By loading, unloading, and reloading a specimen with rest periods inserted in between, tensile properties at incremental strain levels were assessed. In addition, postyield toughness was estimated and partitioned as plastic strain energy related to permanent deformation, released elastic strain energy related to stiffness loss, and hysteresis energy related to viscous behavior. Porosity, mineral and collagen content, and collagen crosslinks of each specimen were also measured to determine the micro-and ultrastructural properties of the tissue. Age affected all the energy terms plus strength but not elastic stiffness. The postyield energy terms were correlated with porosity, pentosidine (a marker of nonenzymatic crosslinks), and collagen content, all of which varied significantly with age. General linear models suggested that pentosidine concentration and collagen content provided the best explanation of the age-related decrease in the postyield energy dissipation. Among them, pentosidine concentration had the greatest contribution to plastic strain energy and was the best explanatory variable of damage accumulation. ß

Effect of material and structural factors on fracture behaviour of mineralised collagen microfibril using finite element simulation

“…The plateau value that was obtained by our findings is explained by a change in the molecular deformation mechanism from predominantly shear (when 25 β < ) to molecular fracture (when 25 β > ).This plateau value can also be explained by the fact that whenever the number of cross links increases, their stiffness increases up to a threshold value at which the behaviour of all collagen cross-links becomes insensitive to this number of cross links to any further increase. In the macroscopic response of bone, cross-links have been coupled to improve mechanical properties (Banse et al, 2002) and prevent energy absorption by micro-damage formation and may accelerate brittle fracture (Tang et al, 2007;Nyman et al, 2007). The same results were found here for the nano-scale collagen microfibril.…”

“…There are two types of cross-links: enzymatically and non-enzymatically (Seigmund et al, 2008). Considering the macroscopic response of bone, enzymatic cross-linking has been related to improving mechanical properties (Banse et al, 2002) whereas non-enzymatic cross-linking prevents energy absorption by micro damaged formations and may accelerate brittle fracturing Tang et al, 2007;Nyman et al, 2007;Vashishth, 2007). Natural cross-linking gives collagen a high tensile strength and proteolytic resistance (Friess, 1998).…”

Measurements of mobile and bound water by nuclear magnetic resonance correlate with mechanical properties of bone

“…In intramuscular connective tissue, an inverse relationship was found between the level of crosslinking and the level of water binding collagen [30]. Since bone toughness decreases with age [14,15,[31][32][33], we hypothesize that the NMR measure of bound water decreases with age and is directly related to the workto-fracture of cortical bone.…”

“…Lastly, a portion of each broken specimen was embedded in cold mounting epoxy (EPOFIX, Struers, Denmark), and the fracture surface ground with successive grits of silicon carbide paper and then polished with 0.05 micron alumina slurry. As previously described in detail [12,33], porosity (Po) was quantified from binary representations of digital images taken of the polished cross section.…”

The fracture mechanics of human bone: influence of disease and treatment

“…[31][32][33][34] Indeed, compositional factors, such as the crosslinking profile, mineralization distribution, or the collagen content, also have a strong correlation with fracture risk. 35,36 In terms of fracture mechanics, the precise effects of porosity on crack propagation in bone have not been isolated from other aspects of the structure that influence toughness. However, cracks are commonly observed to follow cement lines during fracture mechanics experiments in the presence of normal amounts of bone porosity.…”