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It has been established that the ability of erbium lasers to ablate hard dental tissue is due primarily to the laserinitiated subsurface expansion ofthe interstitial water trapped within the enamel and that by maintaining a thin film of water on the surface of the tooth, the efficiency of the laser ablation is enhanced. It has recently been suggested that a more aggressive ablative mechanism, designated as a hydrokinetic effect, occurs when atomized water droplets, introduced between the erbium laser and the surface of the tooth, are accelerated in the laser's field and impact the tooth's surface. It is the objective ofthis study to determine ifthe proposed hydrokinetic effect exists and to establish its contribution to the dental hard tissue ablation process. Two commercially available dental laser systems were employed in the hard tissue ablation studies. One system employed a water irrigation system in which the water was applied directly to the tooth, forming a thin film of water on the tooth's surface. The other system employed pressurized air and water to create an atomized mist of water droplets between the laser hand piece and the tooth. The ablative properties of the two lasers were studied upon hard inorganic materials, which were void of any water content, as well as dental enamel, which contained interstitial water within its cystalline structure. In each case the erbium laser beam was moved across the surface of the target material at a constant velocity. When exposing material void of any water content, no ablation of the surfaces was observed with either laser system. In contrast, when the irrigated dental enamel was exposed to the laser radiation, a linear groove was formed in the enamel surface. The volume of ablated dental tissue associated with each irrigation method was measured and plotted as a function of the energy within the laser pulse. Both dental laser systems exhibited similar enamel ablation rates and comparable ablated surface characteristics. The results of the study suggest that, although the manner in which the water irrigation was introduced differed, the mechanism by which the enamel was removed appeared basically the same for both dental laser systems, namely rapid subsurface expansion of the interstitially trapped water. It is the conclusion of this study that if the proposed hydrokinetic effect exists, it is not effective on hard materials, which are void of water, and it does not contribute in any significant degree in the ablation of dental enamel.

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This study was performed in order to evaluate the safety of pulpal and dentinal tissue when exposed to the 2.94 micron pulsed Er:YAG laser radiation for the clinical procedures ofcaries removal, cavity preparation and laser etching prior to acid etching and restoration. This paper discusses the histological changes observedin dentin and pulpal tissue induced by the Erbium laser and the dental drill as perfonned in the clinical setting at five test sites. Carious teeth were treated in-vivo and then extracted at various periods ranging from immediately to 1 year post treatment. Following extraction, the specimens were evaluated by a double-blind histological examination of dentin and pulp tissue for comparison of results. Specimens were ranked and scored according to the absence or severity of the histopathologic changes observed and the results subjected to statistical analysis (ANOVA) to determine if significant differences existed between treatment groups (laser vs. drill) and/or procedure. Conclusions: Analysis ofthe results indicate that there were no significant differences attributable to method oftreatment. Etching by either method resulted in no discernible change. Cavity preparation and caries removal resulted in mild to moderate changes in the tissues.

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