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

Many studies of laser-induced thermal decomposition of dental enamel have demonstrated a reduction in the rate of acid dissolution, size of artificial caries-like lesions, and acid reactivity. Additionally, studies have correlated the loss of carbonate from dental enamel with a reduction in acid dissolution. Dental mineral consists of hydroxyapatite with many substitutions, the major one being carbonate ( approximately 3-5% by weight), which markedly affects acid reactivity. The principle objective 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 microm with 2-microsec and 100-microsec pulses and at lambda = 10.6 microm with 2-microsec pulses. Carbonate loss was calculated from infrared spectra as a function of depth and compared to numerical simulations of the maximum temperature rise.

Carbonate loss was initiated at temperatures greater than 400 degrees C, but was complete only after repeated irradiation of the surface above the melting threshold. Carbonate loss of dental enamel irradiated at 9.6 microm with a 100-microsec pulse and at 10.6 microm with a 2-microsec pulse was greater than that of enamel irradiated at 9.6 microm with a 2-microsec pulse. The depth of carbonate loss in dental enamel irradiated with a 2-microsec pulse was greater for lambda = 10.6 microm than for lambda = 9.6 microm.

The depth of modification is consistent with the presented model 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.