As the average age of the population increases, so does the incidence of age-related bone fracture. Some of the age related increase in fracture is due to bone mass loss, however, a significant part remains unexplained. Bone is a composite material, made up of organic (primarily collagen), mineral (primarily hydroxyapatite) and fluid (primarily water) phases;​ these three phases in combination form a stiff and strong material. There is little change in mineralization with aging;​ nevertheless bone becomes weaker, implicating the organic phase.

Immediately prior to mechanical failure, bone has been observed to whiten (i.e. stress-whiten) in a manner visually similar to polymer crazing, a known toughening mechanism. Stress-whitening is known to occur in normal bone and contribute to the remarkable toughness of bone. However, the underlying mechanism of stress-whitening remains unclear.

This dissertation was focused on how the organic phase makes bone tough. Specifically, it was hypothesized that stress-whitening, a known toughening mechanism, arose from the organic phase. By better understanding the origin of stress-whitening, and more generally bone toughness, better treatments and diagnostic capabilities can be developed to treat and predict bone fracture risk.

The importance of the organic phase was first probed with finite element analysis of trabecular bone cores. The sensitivity of apparent bone fragility to intrinsic damage properties was investigated. The intrinsic damage parameters investigated were attributable to either the mineral or organic phase. Intrinsic damage properties, attributable to the organic and inorganic phase both had profound nonlinear effects on the apparent tissue level mechanical response. Apparent yield strength and toughness varied strongly (1200% and 400%, respectively) with relatively small changes in the intrinsic damage behavior. These findings confirmed the importance of intrinsic damage properties providing a theoretical link between changes in the organic phase and bone fragility.

The nature of stress-whitening within bone was thoroughly investigated by mechanical loading of demineralized bone specimens. It was established that stress-whitening occurred in demineralized bone matrix. The presence of stress-whitening in demineralized bone contradicted previous suggestions that stress-whitening observed in whole bone tissue was caused by the mineral. Stress-whitening was not permanent nor was the process affected by repeated loading suggesting that stress-whitening was not causing permanent damage as previously thought.

Demineralized bone matrix when less solvated was stiffer and stress-whitened less, both suggesting brittle mechanical behavior. Decreasing hydration/solvation of the demineralized bone simulates the effects of decreased hydration in aged bone matrix. Interestingly, demineralized bone specimens also became whiter without the presence of mechanical loading, when immersed in a solution that was less hydrating of the collagenous matrix (the specimens solvent-whitened). The stress-whitening and solvent-whitening variation with Hansen hydrogen bonding potential (δ_h) was consistent with a common mechanism causing stress-whitening, solvent-whitening and solvent-based optical clearing. The stress-whitening observed was consistent with increased Mie scattering caused by a reversible non-damaging collagen fibril densification through expulsion of the solvent driven by externally applied stress leading to changes in the optical properties of the demineralized bone.

Direct measurement of refractive index of thin sections of demineralized bone matrix during compression was done using a surface force apparatus. Refractive index was linearly related to deformation of the specimens. Similar to stress whitening, refractive indices were lower and less sensitive to deformation when immersed in a lower δ_h solvent. Changes in refractive index were ascribed primarily to displacement of fluid out of the specimen (reduced volume fraction of free solvent) and secondarily to load induced changes in the refractive index of the collagenous phase (densification of the collagen fibrils by expulsion of solvent).

Stress-whitening may contribute to bone toughness independent of mineral content. Age related changes in bone fragility may be related to measurable changes in matrix optical properties. Understanding bone quality, particularly the stress-induced toughening mechanism investigated here, may lead to new therapeutic targets or possibly to non-invasive methods to assess bone quality.