UC Davis

Bone tissue “stress-whitens” (changes from translucent to opaque) immediately prior to fracture, yet the underlying mechanisms governing this phenomenon are unclear. Bone is comprised of three major constituents: a mineral phase (mostly hydroxyapatite), an organic phase (mostly collagen type I), and water. Past research has demonstrated that stress-whitening is a property of the collagen component, rather than the mineral component of bone. The hypothesis of the thesis is that stress-whitening is caused by a mechanically-driven change in the microstructure of the collagen component of bone. Label-free Raman microspectroscopy was used to measure changes in the chemical components of demineralized bone before and after tension. We demonstrate that mechanically caused stress-whitening in equine bone is associated with changes in the Raman spectrum. More specifically, the results of the Raman spectral data demonstrate that there is an effect of mechanical loading on the protein structure of bone, specifically as demonstrated by changes in the amide III (1280 cm-1), CH2 (1467 cm-1) and amide I (1671 cm-1) peak intensities. Interpreting these peak changes as related to collagen hydrogen bonding and bone toughness is consistent with an increased crystallization of collagen under mechanical load, which Janet Clark hypothesized as a toughening mechanism of collagenous tissues. The results of this research augment the understanding of the structural mechanisms of bone mechanics and have the potential to supplement the understanding of bone diseases associated with changes in molecular mechanics and disruption of intermolecular hydrogen bonding, such as osteogenesis imperfecta.