Bone Chemical Structure Response to Mechanical Stress Studied by High Pressure Raman Spectroscopy

While the biomechanical properties of bone are reasonably well understood at many levels of structural hierarchy, surprisingly little is known about the response of bone to loading at the ultrastructural and crystal lattice levels. In this study, our aim was to examine the response (i.e., rate of change of the vibrational frequency of mineral and matrix bands as a function of applied pressure) of murine cortical bone subjected to hydrostatic compression. We determined the relative response during loading and unloading of mineral vs. matrix, and within the mineral, phosphate vs. carbonate, as well as proteinated vs. deproteinated bone. For all mineral species, shifts to higher wave numbers were observed as pressure increased. However, the change in vibrational frequency with pressure for the more rigid carbonate was less than for phosphate, and caused primarily by movement of ions within the unit cell. Deformation of phosphate on the other hand, results from both ionic movement as well as distortion. Changes in vibrational frequencies of organic species with pressure are greater than for mineral species, and are consistent with changes in protein secondary structures such as alterations in interfibril cross-links and helix pitch. Changes in vibrational frequency with pressure are similar between loading and unloading, implying reversibility, as a result of the inability to permanently move water out of the lattice. The use of high pressure Raman microspectroscopy enables a deeper understanding of the response of tissue to mechanical stress and demonstrates that individual mineral and matrix constituents respond differently to pressure.