Raman Spectroscopic Changes in Teeth Interiors When Externally Bleached In-Vitro

Despite widespread clinical application and research study, the mechanisms of tooth discoloration through peroxide bleaching are not completely understood.    Objective:  The location and magnitude of bleach effects on teeth were examined in vitro by comparing fluorescence and Raman Total Spectral Intensity changes in subsurface tooth enamel,  DEJ and dentin in teeth bleached externally – as part of a kinetic study to determine maximum changes associated with saturated external bleach  activity.    Methods:  Extracted human premolars were analyzed for external CIELAB (L*a*b*) color with a Fuji-1000CCD and stratified into treatment groups for kinetic study.  Teeth were subsequently subjected to bleaching with whitening strips (Crest^® Whitestrips™ Supreme containing 14 % hydrogen peroxide) in a cycling protocol including topical bleach exposure alternating with storage in pooled human saliva.     Teeth were analyzed in cross section by nondestructive confocal laser scanning microscopy coupled with a Raman spectrometer (excitation He/Ne laser at 632 nm).   Combined line scans at discrete intervals were measured from the enamel surface through central DEJ and dentin with magnitude of fluorescence change in the form of the Raman Total Spectral Intensity.  Results: Raman Spectra showed no changes in mineral components associated with bleaching throughout the kinetic process.  The Raman Total Integral (50-3600 cm-1) ranged from 1x10⁶-1x10⁷ and: 1) increased from surface enamel to DEJ (3x increase); 2) decreased in subsurface enamel in ordered fashion from 0-100 % external bleach color change; 3) for DEJ was only decreased from 50-100 % external bleaching;  4) was unaffected in dentin with external color change from 0-100 % maximum.  Conclusion:   Peroxide bleaching of teeth is associated with changes in fluorescence in dental enamel.  At typical clinical levels of consumer whitening acceptance only subsurface enamel appears affected.  At more intense levels of bleaching DEJ changes RSTI properties.  External bleaching does not appear to significantly oxidize coronal dentin.