## 4 Discussion
Figure 4: SEM observations: (**A**) intact and polished enamel; (**B**) enamel subjected to acid attack; (**C**) enamel treated with toothpaste A; (**D**) enamel treated with toothpaste A and second acid attack; (**E**) enamel treated with toothpaste B; (**F**) enamel treated with toothpaste B and second acid attack. Comparing the roughness values obtained from the two toothpastes, it can be concluded that there is a statistically significant difference between the mean roughness when using toothpaste A and B after acid attacks. The average roughness found in tooth C is lower than in tooth E. Many studies in the literature confirm the results obtained, while others have reported conflicting results. [PERSON] et al. [12] found that the use of fluoridated toothpastes is effective in regenerating a homogeneous, very compact, thick and uniform surface layer. Indeed, prismatic structure of hydroxyapatite was not observed after the application of fluoridated dendrites. In the same study, he concluded that a non-fluoridated toothpaste does not offer a sufficient degree of remineralization to protect and repair enamel damaged by acid attack. In the 1970s, [PERSON] [18] showed that the use of a 2% sodium fluoride solution provides a protective action against enamel erosion. In vitro, less tooth wear was produced in the presence of fluoride toothpaste than in the presence of fluoride-free toothpaste with an equal formulation. In more recent years, several studies using different fluoride formulations, e.g., sodium fluoride, acidified phosphate fluoride, stannous fluoride, amine fluoride or titanium tetrafluoride have shown a greater protective effect in vitro [19]. In contrast to the present study, [PERSON] and [PERSON] [20] and [PERSON] and [PERSON] [21] found that fluoride cannot reduce dental erosion. In principle, fluoride has a protective effect at a pH higher than the one indicated by the fluorapatite saturation curve at given concentrations of Ca and PO4. According to this study, fluoride at concentrations found in toothpastes are unlikely to have much beneficial effect on erosion, because the amount of acid produced by gastric juices is excessively high. However, it is possible that, if other erosive factors are not excessive, fluoride in solution may provide some protective effect. Results obtained from the present study indicate that there is no statistical difference in protection from a second acid attack; that is, neither toothpaste A nor B offers a sufficient degree of remineralization in the event of a second acid attack. Indeed, from the SEM images provided, it can be seen (Figure 4D,F) that the enamel surface is significantly eroded after a second acid attack. This is confirmed by numerical statistics, which show a considerable increase in roughness of 0.329 \(\upmu\)m for tooth D and 0.313 \(\upmu\)m for tooth F. Fluoride contained in toothpastes (1450 ppm) was not sufficient to prevent a second acid attack. It is reasonable to assume that higher fluoride concentrations could have prevented a second erosive lesion more effectively. This supposition is also confirmed by literature. [PERSON] et al. [22] have shown that treatment with fluoride varnish (2.26%) for 24 h and high concentration fluoride rinses (1.2%) for 48 h applied before acid challenge offer in vitro protection against erosion. This protection is presumed to be due to the precipitation of calcium fluoride-like particles that adhere to tooth surfaces and generate fluorapatite. Thus, a gentle application of fluoride (without destruction of the acquired protective film) before an erosive attack would be more beneficial. Formation of the CaF2-like layer on the tooth surface would act as a \"barrier\" against acid attack. This layer provides some additional minerals to dissolve during an acid attack before underlying enamel is attacked.