## 4 Discussion  
The potential applications of Nd:YAG laser irradiation for treating dental hypersensitiviy has been well demonstrated through many investigations over the last years [12]. When considering that dental erosion and dentine hypersensitivity are often related, it would be interesting if the laser could control the progression of dentine erosion as well as to treat dentine hypersensitivity at the same time. However, few studies have focused on evaluating the effects of the Nd:YAG laser to avoid the progression of erosive lesions [13; 14; 15]. In light of this, the present study evaluated the effects of Nd:YAG laser irradiation and topical fluoride as methods to control dentine erosion based on the optical attenuation coefficients that were calculated from OCT images. Several laser energy densities, either associated with fluoride application or not, were evaluated. Fluoride treatment it is a well-established solution for preventing and controlling dental erosion. Its protective effect results from the formation of CaF2 like material on tissue surface, which increases the dentine resistance to acids. Our findings indicate that fluoride alone was not able to avoid the progression of dentine erosion that is induced by acid challenge. However, it is important to notice that the dentine specimens were submitted to a single application of fluoride instead of daily applications, as recommended by medical guidelines [22; 23; 24; 25]. Besides, many authors agree that acid drinks can easily remove the CaF2 layer. Thus, the benefits of its implementation alone would be quickly neutralized [26; 27]. Increases in demineralization were also observed in the specimens from groups L1, FL1, L3, and FL3, indicating that laser irradiation under these conditions was not able to control dentine erosion, regardless of fluoride application. The positive effects promoted by laser irradiation are dependent on the parameters used, mostly on the energy density. The progression of dentine erosion in samples from groups L3 and FL3 probably occurred due to the low energy that was used to irradiate the samples, which did not induce sufficient chemical and morphological changes to improve the resistance of dentine to acid challenge. On the other hand, high energy density may induce the formation of craters on the dentine surface, which also leads to the progression of dentine erosion [28]. [PERSON] and coworkers evaluated the formation of craters on dentine specimens that were irradiated by Nd:YAG laser with different energy conditions (50, 100, and 150 mJ). According to their results, 50 mJ does not produce crater formation. On the other hand, craters on dentine surface were evidenced on samples irradiated with 100 and 150 mJ [28]. Thus, it is possible that the high power employed to irradiate the specimens from L1 and FL1 (79.5 mJ) also induced craters on dentine samples, enabling acid diffusion into the dentine tubules and, consequently, the progression of erosive lesions. No significant differences were obtained for the OAC values of dentine samples from L2 and FL2 at day 10 and 15, indicating that laser irradiation under this condition did not induce the formation of craters on dentine tissue and promoted sufficient chemical and morphological changes to avoid the progression of dentine erosion. Conceptualization, A.C.C.A. and D.M.Z. ; methodology, M.C.D.-M.; software, A.Z.F. ; formal analysis, C.A.L. ; investigation, M.C.D.-M; data curation, C.A.L. ; writing--original draft preparation, C.A.L and M.C.D.-M.; writing--review and editing, C.A.L. ; supervision, A.Z.F., D.M.Z. and A.C.C.A. ; funding acquisition, A.C.C.A. and D.M.Z.