Influence of water on ablation of teeth by the Er:YAG laser

Abstract

We investigated how water affects the ablation of hydroxyapatite (HAP) by the Er:YAG laser. Ablation of HAP requires that water be both internal and external to the tooth. The wavelength of the Er:YAG laser is 2.94 μm, which almost reachs the absorption spectra peak for water. Therefore, the Er:YAG laser has two characteristics. One is photo-ablation by laser absorbed by water molecules inside and outside the tooth. The other is the photo-thermal effect from heat. We used extracted teeth that had been dried in an electric furnace (dried teeth) and the teeth that had been kept in physiological saline solution immediately after extraction (wet teeth). Quantiative analysis of the moisture content was done by thermal desorption spectroscopy and Karl Fischer's method. Laser irradiation of the enamel and dentin of the test teeth was done with the laser tip 0.1 mm from the dental surface for 1 sec at 3 Hz (3 pulses) and 2 sec at 5 Hz (10 pulses). The pulse width was 10 ns, and the radiation energy was 200 mJ. The external conditions during irradiation were either a spray of light water (normal distilled water), a spray of heavy water, or no spray (air). Light water absorbs photons emitted by the Er:YAG laser, while heavy water does not. Scanning electron micrographs were taken of the ablated tooth surface, and the area and depth of the tested surfaces were measured. We also recorded the number of cracks on the surface of the ablated teeth. We found that water in the tooth affected the ablated enamel to a greater extent than it did the dentin. We also found that the hydroxyl radicals of the HAP increased temperature and contributed to photo-ablation. Permeated external water improved ablation efficiency by supplying water to the dried surface of the ablated tooth.