## 1 Introduction  
The evolution of dentistry has advanced and will continue to advance through the appearance of intelligent materials on the market, designed to contribute to oral health care and, thus, to the improvement of quality of life. To date, many types of dental materials have been generated and are available for clinical applications. Resin based-composites and materials conjugated in cements and metals have been widely produced to replace missing tooth structures and mimic the aesthetic characteristics of tooth structures [1; 2; 3; 4; 5; 6; 7]. Resin based-composites with different inorganic reinforcement particles are commonly commercialized due to their excellent mechanical and physicochemical properties, such as their high strength, bright surface, low surface roughness, resistance to wear/fracture, and adherence to dental tissues [8; 9; 10]. Therapeutic restorative materials are used for minimally invasive and a traumatic restorative dentistry and, in this context, the development of new restorative materials that can mimic the mechanical, physicochemical and aesthetics features of dentin and enamel are necessary [11; 12]. In the recent years, computer-aided design and computer-aided manufacturing (CAD-CAM) systems have allowed monolithic restorations to be made in a single session. CAD-CAM restorative materials have been widely used in dentistry due to their stable quality of materials and lower costs. CAD-CAM technology satisfies aesthetic demands, such as color and translucency reproduction, and even chromatic stability under clinical conditions [13; 14; 15]. While most studies have reported extensive details on the positive characteristics of these materials [16; 17; 18; 19; 20; 21; 22], few studies have investigated the surface behavior of these materials in contact with endogenous (salivary enzymes, gastric juice) or exogenous (acidic beverages) substances. These acidic elements have been correlated with severe dental structure degradation and restoration, most probably due to their acidic pH [23; 24]. Regardless of the origin, these acidic substances attack the surface of the tooth, as well as the surface of CAD-CAM restorative materials, changing their structure over time [25; 26; 27]. It was established that gastric juice produce more severe degradation of dental structures than acidic beverages [28; 29; 30]. Gastric juice comes into contact with the oral cavity as a result of gastroesophageal reflux disease or bulimia nervosa [31; 32; 33]. Even the case of pregnant women with prolonged severe nausea episodes during pregnancy can face the issue gastric juices [23]. Dental erosion, defined as a progressive loss of hard dental tissues due to chemical processes, without the action of bacteria, can be correlated with the intrinsic and/or extrinsic origin of acid. The results of chemical erosion consist in hard, polished, and smooth depression of a tooth's surface [34]. Extrinsic tooth erosion is mainly due to the consumption of low-pH beverages and foods. It may also be due to the use of medications, such as aspirin, which is a low pH drug [35]. All-ceramic restorations that offer biocompatibility and aesthetics may be affected by gastric acid, although less intensely than enamel [31]. Chemical degradation can lead to microstructural changes in surface topography, which can affect light reflection, color perception, and stability [36]. The knowledge of how ceramic materials react to gastric acid can help a dentist in selecting suitable materials for prosthetic restorations for the special patients mentioned above, though not limited to them. Although the literature reports on the effects of acidic elements on the surface of different restorative materials [37; 38; 25; 39; 40; 41], to the best of authors' knowledge, no study has yet to compare, under the same conditions, these four monolithic CAD-CAM materials: feldspathic ceramic, nanoceramic resin, hybrid ceramic, and leucite-reinforced glass ceramic.