Raman Spectroscopic Imaging Markers for Fatigue-Related Microdamage in Bovine Bone

Raman spectroscopic markers have been determined for fatigue-related microdamage in bovine bone. Microdamage was induced using a cyclic fatigue loading regime. After loading, the specimens were stained en-bloc with basic fuchsin to facilitate damage visualization and differentiate fatigue-induced damage from cracks generated during subsequent histological sectioning. Bone tissue specimens were examined by light microscopy and hyperspectral near-infrared Raman imaging microscopy. Three regions were definedtissue with no visible damage, tissue with microcracks, and tissue with diffuse damage. Raman transects, lines of 150−200 Raman spectra, were used for initial tissue surveys. Exploratory factor analysis of the transect Raman spectra has identified spectroscopically distinct chemical microstructures of the bone specimens that correlate with damage. In selected regions of damage, full hyperspectral Raman images were obtained with 1.4-μm spatial resolution. In regions of undamaged tissue, the phosphate ν1 band is found at 957 cm-1, as expected for the carbonated hydroxyapatic bone mineral. However, in regions of visible microdamage, an additional phosphate ν1 band is observed at 963 cm-1 and interpreted as a more stoichiometric, less carbonated mineral species. Raman imaging confirms the qualitative relationship between the Raman spectral signature of bone mineral and the type of microdamage in bovine bone. Two tentative explanations for the presence of less carbonated phosphate in damaged regions are proposed.