Original articleAge-related changes in the collagen network and toughness of bone
Introduction
Over the years, loss of bone mineral or bone mass has been considered the major cause of age-related bone fractures.36, 38 However, the large overlap in bone density that exists between healthy individuals and patients who sustain bone fractures suggests that low bone density is not the only reason for the weakening of bone.37 A recent study reported that the risk of bone fracture for older women (average 75 years old) is about 7%, whereas such a risk is only 1% for much younger individuals (average 45 years old), although they have a similar bone density level.23 In addition, it has been found that, although elderly black Gambian women also experience loss of bone mass, they rarely suffer osteoporotic bone fractures as compared with their white counterparts.2
Bone is a natural composite comprising mineral (mainly hydroxyapatite), organic (mostly type I collagen), and water phases.25, 28 Thus, the biomechanical properties of bone are dependent on the quality and spatial arrangement of these constituents.35 Recent studies have shown that the mineral predominantly contributes to bone stiffness,42, 43 whereas the quality of collagen matrix may predominantly determine the toughness of bone.10, 12, 15, 47, 49 In addition, it was found that osteoporosis is not just a simple loss of bone mass, but involves significant changes in the biochemical and physical properties of the collagen network.27 Thus far, only a few studies on the age-related changes in collagen and their correlation with the toughness of bone have been reported in the literature.47, 49 Although these studies have demonstrated the involvement of collagen in age-related changes in bone quality, the underlying mechanisms are still not clear.
The hypothesis of this study is that the mechanical integrity of the collagen network in human bone deteriorates with age, and such adverse changes correlate with the decreased toughness of aged bone. To test this hypothesis, we examined age-related changes in collagen molecular structures, the mechanical integrity of the collagen network, and the mechanical properties of bone. Finally, we attempted to explore the correlation of age-related changes in the collagen network with the toughness of bone.
Section snippets
Materials and methods
Thirty fresh frozen human cadaveric femurs were acquired from the Musculoskeletal Transplant Foundation (Edison, NJ) and a local tissue bank, ranging between 19 and 89 years of age. All samples were screened carefully to avoid the influence of any bone-related pathologies. These samples were divided into three age groups: young adults (19–49 years old); middle-aged (50–69 years old); and elderly (>70 years old). Each group included ten samples (n = 10). Eight men and two women were included in
Results
ANOVA analyses indicated that age had significant effects on the mechanical integrity of bone (Table 1). Although neither the ultimate nor yield strength (σs and σy) of bone showed any significant difference for the young and middle-aged groups, they decreased significantly for the elderly group. In contrast, aging exhibited little effect on the elastic modulus (E) of bone. The work to fracture (Wf) of bone decreased with increasing age. Interestingly, it was observed that the resilience of
Discussion
To test the hypothesis that the mechanical integrity of the collagen network in bone deteriorates with age, and such changes correlate with the age-related decrease in the toughness of bone, we measured the mechanical properties of the demineralized bone and bone samples (n = 30) acquired from a broad range of ages (19–89 years), and determined their compositional and microstructural characteristics. The experimental results of this study indicate that the mechanical integrity of the collagen
Acknowledgements
This study was supported by grants from the Whitaker Foundation (RG-99-0440), NIAMS/NIH (R03 AR46428), and the San Antonio Area Foundation.
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