Infrared laser ablation of dental enamel: Influence of an applied water layer on ablation rate and peripheral damage

Studies have shown that a water spray may augment the laser ablation rate of dental hard tissues in addition to reducing heat accumulation. However, the mechanism of augmentation is controversial and poorly understood. The influence of an optically thick applied water layer on the ablation rate was investigated at wavelengths in which water is a primary absorber and the magnitude of absorption varies markedly. Water was manually applied with a pipette (∼5-10 μl drops) and troughs were cut in enamel blocks using a laser scanning system. Q-switched and free running Er:YSGG (2.79 μm) and Er:YAG (2.94 μm), free running Ho:YAG and 9.6 μm TEA CO₂ laser systems were investigated. The addition of water increased the rate of ablation and produced a more desirable surface morphology during enamel ablation with all the erbium systems. Ablation was markedly more efficient for the Q-switched erbium lasers than for the longer free-running laser systems when a water layer was added. Although, the addition of a thick water layer reduced the rate of ablation during CO₂ laser ablation, the addition of the water removed undesirable deposits of non-apatite mineral phases from the crater surface. There was extensive peripheral damage after irradiation with the Ho:YAG laser with and without added water without effective ablation of enamel. The results of this study suggest that water augments the ablation of dental enamel by aiding in the removal of loosely attached deposits of non-apatite mineral phases from the crater surface, thus producing a more desirable crater surface morphology. The non-apatite mineral phases interfere with subsequent laser pulses during erbium laser irradiation reducing the rate of ablation and their removal aids in maintaining efficient ablation during multiple pulse irradiation.