Modeling the modification depth of carbon dioxide laser-treated dental enamel

Background and Objectives: Many studies of laser-induced thermal decomposit ion of dental enamel have demonstrated a reduction in the rate of acid diss olution, size of artificial caries-like lesions, and acid reactivity. addit ionally, studies have correlated the loss of carbonate from dental enamel w ith a reduction in acid dissolution. Dental mineral consists of hydroxyapat ite with many substitutions, the major one being carbonate (similar to 3-5% by weight), which markedly affects acid reactivity. The principle objectiv e of the present work was to determine the precise depth of modification, i .e., thermally induced decomposition of dental enamel (carbonate loss), at the predicted optimum laser irradiation parameters. Study Design/Materials and Methods: Bovine enamel blocks were irradiated at lambda = 9.6 mu m with 2-mu sec and 100-mu sec pulses and at lambda = 10.6 mu m with 2-mu sec pulses. Carbonate loss was calculated from infrared spe ctra as a function of depth and compared to numerical simulations of the ma ximum temperature rise. Results: Carbonate loss was initiated at temperatures greater than 400 degr ees C, but was complete only after repeated irradiation of the surface abov e the melting threshold. Carbonate loss of dental enamel irradiated at 9.6 mu m with a 100-mu sec pulse and at 10.6 mu m with a 2-mu sec pulse was gre ater than that of enamel irradiated at 9.6 mu m with a 2-mu sec pulse. The depth of carbonate loss in dental enamel irradiated with a 2-mu sec pulse w as greater for lambda = 10.6 mu m than for lambda = 9.6 mu m. Conclusion: The depth of modification is consistent with the presented mode l that incorporates the absorption depth and thermal relaxation time/pulse duration. However, repeated irradiation is required for complete removal of carbonate, depending on absorption depth and pulse duration. (C) 1999 Wile y-Liss, Inc.