Influence of the hydration state on the ultrashort laser ablation of dental hard tissues.

Since about 40 years, laser-based surgical tools have been used in medicine and dentistry to improve clinical protocols. In dentistry, femtosecond lasers have been claimed to be a potential ablation tool. It would, however, be good to perform a more fundamental investigation to understand ablation interaction mechanisms and possible side effects, depending on different specific components of the target tissue. The goal of this study is to show the changes of ablation characteristics in the femtosecond regime at different levels of structural water within dental hard tissues. Thirty human teeth samples were split into three hydration groups and subdivided into dentin and enamel groups (n = 5). The specimens were irradiated using a 70-fs Ti:sapphire laser (with a 1-kHz repetition rate and a 801-nm wavelength output). Ablation was performed using five different power levels and three exposure times. The results clearly show an inversely proportional dependence of the ablation threshold to the hydration level of the tissues. A known mathematical model was adapted in order to include the influence of the changes on the relative fractional composition of dental hard tissues. This analysis was consistent with the experimental results regarding the ablation threshold. High thermal and mechanical damages were observed as a high repetition rate had been applied. Macroscopic images and scanning electron microscopy images were used to preliminarily analyze both the thermal and mechanical damage thresholds, and their variations according to the hydration level present. By manipulating the hydration states, the modifications in the proportions of the molecules that build dental hard tissues clearly shift, and therefore, the characteristics of a plasma-induced ablation change.