Water Content Measured by Proton-Deuteron Exchange NMR Predicts Bone Mineral Density and Mechanical Properties

NMR was used to measure matrix water content in normal and hypomineralized cortical bone. Water content showed an inverse relationship with mineral content, suggesting it could serve as a surrogate measure for the bone's degree of mineralization. Introduction: So far, true bone mineral density (DMB; degree of mineralization of bone) can not be measured nondestructively. Materials and Methods: Here, a new technique combining ¹H nuclear magnetic resonance (NMR) spectroscopy and deuterium isotope exchange was used to measure water content in cortical bone from two groups of rabbits: a control group and a group fed a low-phosphorus (P) diet to induce hypomineralization of the bone matrix. Results: NMR-derived water content was higher in the P-depleted group and showed an inverse relationship with mineral content (measured gravimetrically and by ³¹P NMR). Hypomineralized bone was found to be weaker than normal bone as demonstrated by mechanical testing. More importantly, the data showed a strong inverse correlation between water content and bone mechanical properties, which indicates that water content could be predictive of the bone's mechanical competence. Conclusions: Water content could potentially serve as a surrogate measure for the bone's degree of mineralization, and this technique could be used to study other disorders of mineral homeostasis known to alter the mineralization state of the matrix. Although the method presented here is not suitable for in vivo measurements of bone water content, the authors have previously shown that ¹H NMR images of bone can be acquired; thus, noninvasive quantification of bone water may be feasible.