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

Since clinical measures of bone mineral density do not necessarily predict whether a person will fracture a bone without an intervention, there is a need to find supplementary tools for assessing bone quality. Presently, we hypothesized that measures of mobile and bound water by a Nuclear Magnetic Resonance (NMR) technique are correlated with bone strength and toughness, respectively. To test this, bending specimens from the mid-diaphysis of 18 human femurs were collected from 18 male donors and divided into middle aged and elderly groups. After NMR measurements of each hydrated specimen, an inversion technique was used to convert the free induction decay data into a distribution of spin-spin (T2) relaxation rates. Then, the distribution resolved into three distinct components that likely represent solid hydrogen, water bound to bone tissue, and mobile water that occupy microscopic pores within the bone specimen. The integrated signal intensities of the bound and mobile components were normalized by the wet mass of the specimen. Following NMR measurements, three point bending tests were conducted to determine the modulus of elasticity, flexure strength, and work to fracture of each specimen. Next, the porosity, mineral-to-collagen ratio, and pentosidine concentration were measured. In this sample of human cortical bone, there was no age-related difference in the amount of mobile water, but the decrease in the amount of bound water with increasing age was statistically significant. Moreover, bound water was associated with both strength and work to fracture of bone, while mobile water was correlated with modulus of elasticity and appeared to quantify the level of microscopic pores within bone. On the other hand, bound water was correlated with the concentration of non-enzymatic collagen cross-links. The results of this study indicate that quantifying mobile and bound water with magnetic resonance techniques could potentially serve as indicators of bone quality.