Objectives: The null hypothesis to be tested in this study was that CO2 laser irradiation on enamel surface would affect the microstructures of the dentin and dentin-enamel junction (DEJ). Methods: The buccal enamel surfaces, about 2 mm from the cementum-enamel junction, of thirty freshly extracted human mandibular third molars were irradiated by CO2 laser in a contact mode. Irradiation was applied across a length of 5 mm at a speed of 1 mm/sec in the mesiodistal direction. All irradiations were in continuous mode, and the output powers included 2, 3, 4, 5, 8, 10 watts. After CO2 laser treatment, specimens with a thickness of 2 mm were cut in the buccal-lingual direction and observed a scanning electron microscope (SEM) to examine any structural changes in the enamel, DEJ, and dentin. In addition, heat transfer simulation was applied to calculate the temperature distribution within the specimen after CO2 laser irradiation. Results: Two and three watts of power output only produced superficial structural changes in the enamel. When the laser power output exceeded 3 W, surface ablation of enamel and significant structural changes in the dentin beneath the DEJ were observed. From the outer irradiated enamel surface to the inner dentin structure, distinct zones were observed and tentatively named ablation, melted, grain growth, coalescence, unaffected enamel, burst, lamination, densification, and unaffected dentin zones. In addition, CO2 laser irradiation had a detrimental effect to the DEJ, causing separation of the enamel and dentin along the DEJ. The heat transfer simulation showed that temperature declined rapidly in the enamel while the slower temperature decrease existed in the dentin, indicating that the dentin was prone to storage thermal energy than the enamel. Conclusion: The operation parameters of CO2 laser should be deliberately selected to avoid thermal damage to the dentin and DEJ.

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