Compositional, structural, and phase changes in in vitro laser-irradiated human tooth enamel

Tooth enamel laser irradiated under certain conditions previously has been shown to have reduced subsurface demineralization rates. Identification of these laser-induced changes has bearing on understanding the dissolution rate reduction mechanism; some of these changes, ones that occur in high temperature regions, were studied in this report. X-ray diffraction and infrared spectroscopy were used to identify changes in enamel of extracted intact human teeth subjected to high energy density (approximately 10,000 J/cm2) 10.6 microns wavelength carbon dioxide laser irradiance. The laser irradiance melted the enamel apatite; this solidified melt was composed of minor phases of alpha-tricalcium phosphate, alpha-Ca3 (PO4)2, and tetracalcium phosphate, Ca4(PO4)2O, and a major phase of modified apatite. The apatite modifications, as compared with the original were (1) reductions in contents of water, protein, carbonate, and chloride (or chloride rearrangement); (2) essentially no change in apatite hydroxide content; (3) possible incorporation of oxide replacing some hydroxide ions; and (4) an uptake of traces of carbon dioxide and cyanate. An infrared band at 434 cm-1 that appears in spectra of hydroxyapatite partially dehydroxylated by thermal treatment was assigned to oxide translation. This band was utilized to search for oxide formation in the laser-irradiated tooth enamel.