## 4 Discussion
In this study, the working pH was 1.2 and the immersion time was 18 h at 37 \({}^{\circ}\)C. In the literature, there are studies that considered that the simulated gastric acid exposure of CAD-CAM materials for 7.5 h, represent one month of gastric acid exposure, 45 h--represent six months of exposure and 91 h--represent one year of exposure. [PERSON] et al. used CAD-CAM materials exposed to simulated gastric acid for 6 and 18 h, and they calculated that these times represent 2 and 8 years of exposure of dental structure to vomiting [27]. [PERSON] et al. exposed monolithic zirconia to acid solution for 96 h, thus simulating over 10 years of dental structure exposure to vomiting [29]. Evaluating these studies, we can say that, in the literature, there is no clear consensus regarding the method of gastric acid simulation and the equivalent time for replication for an in vivo model. According to ISO standard 6872, which refers to the solubility test for dental materials, the use of 4% acetic acid and an exposure time of 16 h at 80 \({}^{\circ}\)C is the equivalent with 2 years of clinical exposure [50]. It is well known that gastric juice has a demineralization effect on enamel, dentin, and cement [28]. Due to its very low pH, it can dissolve the glassy matrix of ceramic materials [31]. In the present study, it was demonstrated that, in the case of feldspar ceramic, nanoceramic resin, hybrid ceramic, and leucite-reinforced glass ceramic, scanning electron microscopy revealed observable changes in the surface topography after simulated gastric acid exposure for all tested monolithic materials. In the case of Triluxe Forte, the pores and grooves present before immersion were larger after, but there were less irregularities on the material surface. Regarding the Cerasmart material, exposure to simulated gastric acid solution led to increased surface roughness. This phenomena could be explained by the continued leaching of the particles in the presence of the simulated gastric acid solution. The Enamic and Empress CAD materials became more porous with a lot of irregularities. This could be due to the dissolution of the ceramic part contained in the monolithic material or other components [30]. Following analyses on the surface morphology after exposure to simulated gastric acid solution, we can say that Triluxe Forte displayed areas of possible degradation, due to the formed crater-like grooves (Figure 4B). In addition, the porosity of the surfaces leads to an accumulation of bacterial plaque and the wear of antagonistic natural teeth. The appearance of pores denotes that exposure to an acidic element will influence the surface of the exposed material [51]. The impact of the simulated gastric acid solution on the optical properties of all four evaluated monolithic materials showed that there were no differences among the specimens, except for Triluxe Forte, which underwent a slight discoloration after exposure to simulated gastric acid. Passing light through a translucent material is reduced by the scattering of small-sized particles, such as filler particles and porosity voids, which can influence color perception and appearance of dental ceramics [52]. In the present study the translucency of the tested materials increased insignificantly after simulated gastric acid exposure and the color values had undergone minor changes, which do not affect the perceptibility and the acceptability thresholds. [PERSON] et al. obtained the same results, stating that all the ceramic materials tested promoted a color change of \(\Delta E<1\), classified as clinically undetectable [30]. The color of a restoration can be affected by a rough surface that reflects an irregular and diffuse pattern of light [53]. According to the results obtained (Table 4), at baseline, leucite-reinforced glass-ceramic showed the lowest surface roughness, while hybrid ceramic showed the highest surface roughness.