An In-Vitro Acidic Media Simulation of GERD and Its Effect on Machine-Milled Ceramics’ Optical Properties

Abstract

Background: Gastroesophageal reflux disease (GERD) exposes restorative materials to gastric acid, which may compromise their esthetic and optical properties. Limited evidence exists regarding the performance of different CAD/CAM ceramics under acidic challenges. Methods: Forty CAD/CAM ceramic discs were prepared (n = 10 per group): high-translucency zirconia (Z; Ceramill Zolid Gen-X), lithium disilicate (E; IPS e.max CAD), zirconia-reinforced lithium silicate (S; VITA Suprinity), and hybrid ceramic (C; Cerasmart 270). Specimens were immersed in simulated gastric acid (0.06 M HCl, pH 1.2) at 37 °C for 96 h. Color difference (ΔE) and translucency parameter (ΔTP) were recorded before and after immersion using a spectrophotometer. Data were analyzed using one-way ANOVA with Tukey’s post hoc test (α = 0.05). Results: All materials exhibited changes in color and translucency after acidic immersion. Group Z demonstrated the lowest ΔE values, indicating the best color stability, whereas group C showed the highest ΔE and a significant reduction in ΔTP. Groups E and S revealed moderate but clinically acceptable changes. Intergroup differences were statistically significant (p < 0.05). Conclusions: Exposure to simulated gastric acid as in (GERD) resulted in measurable alterations in the optical properties of CAD/CAM ceramic materials. The extent of color change and translucency loss differed among the materials tested. High-translucency zirconia (Z) exhibited the greatest stability, while hybrid ceramic (C) showed the most pronounced changes. Zirconia-reinforced lithium silicate (S) and lithium disilicate (E) demonstrated moderate alterations, falling between these two extremes.

1. Introduction

Dental erosion may arise from endogenous acids of gastric origin, which represent an important intrinsic etiological factor. Dental erosion is thought to be one of the primary causes of tooth wear, in which the acids from non-bacterial sources cause the loss of tooth structure [1]. These erosive acids can have an external source, like citrus fruits and acidic drinks, or an inherent source, like stomach acid [2]. Unfortunately, upon exposure to gastric acids, the degree of damage to tooth surfaces is greater because gastric acid possesses a much lower pH and demonstrates stronger erosive potential compared with commonly consumed dietary acids [3]. The stomach is considered the primary intrinsic source of acid. The pH of gastric content has been reported to be as low as 1, indicating its high acidity. Gastric reflux is defined as the backward movement of gastric contents from the stomach into the mouth without voluntary control. The regurgitated fluids contain multiple chemical, organic, and enzymatic components such as trypsin, bile acids, hydrochloric acid, and undigested food particles [3,4]. Regurgitation and vomiting can cause stomach contents to ascend into the oral cavity and are frequently linked to what is known as gastroesophageal reflux disease (GERD) [4]. GERD is a widely encountered gastrointestinal disorder in which gastric contents reflux into the esophagus without voluntary control. It is a relatively widespread ailment globally, with adult prevalence rates in different nations ranging from 21% to 56%. Furthermore, it is estimated that 25–40% of individuals in the United States suffered from symptoms of GERD to varying degrees. Additionally, heartburn or reflux symptoms occur in approximately 45–85% of pregnant women during the first and second trimesters. GERD has several recognized oral manifestations, including burning mouth symptoms, enamel erosion accompanied by dentinal sensitivity, decreased vertical dimension due to structural loss, and compromised dental esthetics [5,6]. Current evidence demonstrates a significant correlation between GERD and dental erosion, suggesting a higher incidence of erosive wear in patients with GERD. Because some of the affected patients are unaware that GERD occurs, dentists are often one of the first specialists to detect the condition through accompanying tooth erosion [7]. When compared to carbonated drinks, the high acidity of the stomach contents that enter the intraoral ecosystem is likely to induce more tooth wear [8,9]. The acidic environment associated with GERD is caused by the stomach’s hydrochloric acid reflux, in contrast to dental caries, where demineralization results from the acidic environment created by plaque bacteria. In addition to the lingual sides of the maxillary teeth, the erosive force may also affect the occlusal and other dental surfaces. where it is typically most noticeable [9,10]. The increased patients’ desire for esthetic restorations turned out to be an integral point of clinical concern. Since dental ceramics are regarded as chemically inert restorative materials, they are frequently utilized as tooth-colored, biocompatible restorations. The most biomimetic natural tooth replacements are ideal for use with dental ceramics. In addition to their better composition, microstructure, and favored optical properties. However, the durability of dental ceramics can be affected by various factors. These factors may include extreme temperature fluctuations and frequent exposure to highly erosive or acidic agents [11]. Exposure of dental ceramics to acidic agents or erosive media can lead to progressive surface degradation [12,13]. This degradation is related to the differing stability between the crystalline and glassy phases; the glassy component is more prone to chemical attack, prompting the selective release of alkaline ions [13,14]. As degradation progresses, clinical consequences may include enhanced wear on antagonistic teeth, surface discoloration, increased roughness, and a heightened tendency for plaque accumulation [15]. ISO 6872 recommends evaluating the chemical solubility of ceramic materials by exposing them to a 4% acetic acid solution at 80 °C for a duration of 16 h [16]. In comparison, Sulaiman et al. simulated more severe clinical conditions by immersing ceramic specimens in hydrochloric acid (pH 1.2) for 96 h at 37 °C, approximating long-term oral exposure [17]. Conversely, El Sokkary et al. employed 4% acetic acid aging, which also demonstrated adverse effects on the surface and mechanical properties of CAD/CAM ceramic materials [18]. A lot of studies focused on how color differences (ΔE) and the translucency parameter (ΔTP) of indirect (CAD/CAM) ceramic restorations are affected by multiple factors [5,6,9,14]. Delta E (ΔE) represents the difference between two colors within the Lab* color space. Since these values are derived mathematically, the choice of color difference formula must be considered when making comparisons. For example, in case of Color Verifier software alone, three formula options are available, each producing slightly different outcomes. In the CIE Lab* model, frequently applied in color science, color difference is quantified as the Euclidean distance between coordinates in a three-dimensional space, emphasizing the relative distance between points rather than their specific locations. Translucency (ΔTP) refers to a material’s capacity to transmit light [5]. In dental material studies, translucency is frequently quantified using the translucency parameter (ΔTP), which serves as a standardized comparative index. It is determined as the CIELAB color difference (ΔEab) of a specimen when placed over standardized black and white backgrounds of equal thickness [6,9,14]. While the CIEDE2000 system has been introduced to improve perceptual accuracy, the CIELAB color space and its ΔEab formula remain the most commonly applied in translucency studies within the dental field [14]. In a recent investigation, Anna et al. demonstrated that both monolithic zirconia and lithium disilicate ceramics underwent noticeable changes in color and ΔTP following aging and acidic challenges, with lithium disilicate exhibiting greater alterations [19]. Similarly, Morsi and Wahba examined the influence of hydrothermal aging on zirconia translucency across different thicknesses. Their findings indicated that translucency increased as thickness decreased and that e.max CAD consistently showed greater translucency than high-translucency zirconia, although all materials experienced ΔTP reduction due to low-temperature degradation [20]. According to Mourouzis, the influence of thermocycling and acidic immersion on surface roughness was not uniform across materials, with CAD/CAM ceramics and resin polymer restoratives responding differently to these conditions. Lithium disilicate, premium zirconium oxide, resin-ceramic, and resin composite materials were tested. The negative effect of acid exposure on the zirconia material is displayed in terms of colour difference with less surface roughness. Both polymer materials exhibit an increase in surface roughness and colour difference when immersed in acid, but not all specimens displayed an increase in roughness or colour difference after thermocycling [21]. A recent investigation evaluated how thermocycling and simulated gastric acid exposure influence the surface roughness of different ceramic materials. Specimens underwent either 10,000 thermocycles in distilled water or were immersed in a gastric acid substitute for ninety-one hours. Under acidic conditions, zirconia exhibited the greatest resistance to surface and microstructural alterations, with lithium disilicate showing moderate stability, whereas zirconia-reinforced lithium silicate experienced the most pronounced degradation under both aging protocols. These findings contrast with those reported by Mourouzis regarding the influence of thermocycling [22]. Likewise, a separate study evaluated how simulated gastric acid influences the optical properties of various CAD/CAM restorative materials, including translucent zirconia (Ceramill Zolid), lithium disilicate (IPS e.max CAD), a resin-based nanoceramic (Cerasmart), and PEEK veneered with a high-impact composite layer. Following 96 h of immersion in hydrochloric acid (pH 1.2) at 37 °C, Ceramill Zolid and Cerasmart showed the smallest color alterations, IPS e.max CAD displayed a moderate shift, while veneered PEEK exhibited the most pronounced discoloration [23]. In the study conducted by Makkeyah et al., the surface characteristics and color stability of two widely utilized CAD/CAM ceramics, (IPS e.max CAD) lithium disilicate and (VITA Suprinity) zirconia-reinforced lithium silicate, were assessed following immersion in orange juice, artificial saliva or a phosphoric acid-based carbonated drink. An increase in surface roughness and a decrease in color stability were observed for IPS e.max CAD and VITA Suprinity, respectively. SEM and energy-dispersive X-ray analyses indicated dissolution of the glassy matrix and subsequent exposure of the silicate crystal phase [24].

Özer and Oğuz recently investigated the effects of acidic drinks and simulated gastric acid on optical and surface characteristics of various monolithic CAD/CAM materials. Lithium disilicate, feldspathic ceramic, and (Lava Ultimate) a resin nano-ceramic. Specimens were polished using three different approaches (mechanical, glaze, and mechanical followed by glaze) and subsequently immersed in either an acidic beverage, simulated gastric acid, or artificial saliva. Gloss measurements revealed that simulated gastric acid caused a more pronounced decrease in gloss than artificial saliva, regardless of the polishing technique utilized. The authors concluded that glaze application is essential to preserve the esthetic appearance of monolithic CAD/CAM restorations in patients with frequent acidic beverage intake [25]. The present study provides new contributions by including a direct comparison between three clinically relevant CAD/CAM ceramic classes—feldspathic glass-ceramic, zirconia-reinforced lithium silicate, and a hybrid nano-ceramic resin-matrix material—under identical experimental conditions. Furthermore, a standardized polishing protocol was applied across all material groups to eliminate finishing-related variability. These methodological considerations enhance the clinical applicability of the findings and support more informed material selection for patients at a high risk of gastric acid exposure.

Aim: The objective of this investigation was to examine how simulated gastric reflux conditions affect the optical properties (color stability and translucency) of four CAD/CAM ceramics—high-translucency zirconia, lithium disilicate, zirconia-reinforced lithium silicate, and hybrid ceramic—after immersion in an acidic medium.

Null Hypothesis: The null hypothesis stated that acid exposure would not result in statistically significant variation in ΔE or TP among the tested ceramic groups.

2. Materials and Methods

2.1. Study Design and Grouping of Specimens

A total of 40 machine-milled ceramic disks were used in the current study. The specimens were divided into four main groups (Z, E, S and C); each group contained 10 specimens according to the CAD/CAM ceramic materials that were used.

The materials tested included high-translucency zirconia (Ceramill Zolid Gen-X, Amann Girrbach, Koblach, Austria), lithium disilicate (IPS e.max CAD, Ivoclar, Schaan, Liechtenstein), zirconia-reinforced lithium silicate (VITA Suprinity, VITA Zahnfabrik, Bad Säckingen, Germany), and hybrid ceramic (Cerasmart 270, GC Corp., Tokyo, Japan). The chemical composition and selected mechanical properties of the tested materials are summarized in

Table 1. Chemical composition and key mechanical properties of the tested CAD/CAM materials.

Material/(Group)	Type/Category	Main Composition (% by Weight)	Flexural Strength (MPa)	Elastic Modulus (GPa)	Manufacturer
Ceramill Zolid Gen-X (Z)	5Y-TZP high-translucency zirconia	ZrO2 + HfO2 + Y2O3 (>99%), trace Al2O3	1000 ± 100	210	Amann Girrbach, Koblach, Austria
IPS e.max CAD (E)	Lithium disilicate glass-ceramic	SiO2 (57–80%), Li2O (11–19%), K2O, P2O5, ZrO2, ZnO, Al2O3	360 ± 60	95	Ivoclar Vivadent, Schaan, Liechtenstein
VITA Suprinity (S)	Zirconia-reinforced lithium silicate (ZLS)	SiO2 (56–64%), Li2O (15–21%), ZrO2 (8–12%), P2O5, K2O, Al2O3	420 ± 50	70	VITA Zahnfabrik, Bad Säckingen, Germany
Cerasmart 270 (C)	Resin-matrix hybrid ceramic	Bis-MEPP + UDMA + inorganic fillers (71 wt% SiO2–Ba glass)	270 ± 30	30	GC Corp.,Tokyo, Japan

, based on manufacturer information. Milling was performed using a five-axis CAD/CAM unit (Ceramill Motion 2, Amann Girrbach GmbH, Koblach, Austria) under standardized dry-milling conditions recommended by each manufacturer to ensure consistent surface quality.

• Regarding group Z, Ceramill Zolid Gen-X white is a high-translucency zirconia material with a chemical composition primarily composed of ZrO₂ (zirconia), Y₂O₃ (yttria), HfO₂ (hafnium oxide), and other oxides. The yttria content is crucial for stabilizing the zirconia’s tetragonal phase, which contributes to its strength and durability.
• Regarding group E, IPS e.max CAD HT A3, a dental CAD/CAM material, is a lithium disilicate glass-ceramic block with a high translucency level.
• Regarding group S, VITA Suprinity PC A3, a popular dental material, is a zirconia-reinforced lithium-silicate glass ceramic. Its composition includes silica (SiO₂) 56–64%, lithium oxide (Li₂O) 15–21%, zirconia (ZrO₂) 8–12%, and lanthanum oxide (La₂O₃) 0.1%. Additionally, it contains pigments and other various components.
• Regarding group C, Cerasmart 270 is a hybrid ceramic CAD/CAM block that combines the properties of high-strength ceramics and composites. It is designed to provide fast and long-lasting indirect restorations. Specifically, Cerasmart 270 A3 is a version with shade A3, which is a common shade used in dentistry. This hybrid block features a Full Coverage Silane Coating (FSC) with enhanced nanofiller technology, providing superior physical and aesthetic properties.

2.2. Specimen Preparation

Forty disks were milled from the four different materials; 10 disks per group were milled into cylinders with dimensions (10 × 8 × 1.5 mm³) each, using a precision cutting machine (Isomet 4000, Buehler, Lake Bluff, IL, USA) operating at 2500 rpm under water cooling. To compensate for dimensional reduction during sintering, the zirconia specimens were initially milled at a size 20% larger than the final intended dimensions. A digital caliper was used to confirm thickness (Thermo Fisher Scientific, Waltham, MA, USA). Disks were ultrasonically cleaned in distilled water. Zirconia samples were sintered at 1450 °C using a Ceramill Therm furnace (Amann Girrbach AG, Koblach, Austria) according to the manufacturer’s instructions. IPS e.max CAD and VITA Suprinity discs underwent crystallization in a Programat P310 furnace (Ivoclar Vivadent AG, Schaan, Liechtenstein) following the recommended firing schedules. White Zolid Gen-X zirconia discs were shade-characterized prior to sintering by immersion in Ceramill FX A3 coloring liquid for 10 s and were then dried at 80 °C for 60 min. Cerasmart discs required no crystallization or sintering; they were only cleaned and surface-polished. The polished surface was standardized using a Robinson brush in combination with Pearl Surface Z polishing paste. All specimens were polished using a standardized protocol consisting of a sequential application of silicon carbide abrasive papers (grits: 600, 800, 1200, and 2000) under constant water irrigation. Polishing was performed using a semi-automatic polishing device (Program P30, Ivoclar Vivadent AG, Bendererstrasse Schaan, Liechtenstein), applying a pressure of approximately 2 N for each grit size. Each polishing step was carried out for 20 s at 300 rpm. Following polishing, each specimen was cleaned for ten minutes in an ultrasonication device within a distilled water bath (Silfradent, Santa Sofia, Forli-Cesena, Italy). This was done before being allowed to air dry on absorbent paper. Following ultrasonic cleaning, a tiny scale (AXIS Sp. Z.O.O., 80–125 Gdańsk, Pomorskie, Poland, ul. Kartuska 375B) was used to weigh each ceramic disk specimen both before and after it was immersed in a gastric-like acidic solution. All specimen fabrication, surface finishing, and testing procedures were carried out by the same calibrated operator to minimize procedural variability and ensure consistency across all experimental groups.

2.3. Gastric-like Acidic Solution Preparation

A gastric-like acidic solution was prepared. According to the steps outlined in the previous study, 0.06 M (pH 1.2) hydrochloric acid (HCl) solution was prepared (17). A pH Meter (JENCO USA, model 6173pH, manufactured in China for JENCO Instruments, San Diego, CA, USA) was used to standardize the pH value. The acidic medium was renewed every 24 h to maintain stable pH conditions throughout the immersion period. The specimens were stored in incubator (Memmert GmbH + Co. KG, Schwabach, Germany) at 37 °C for 96 h, positioned with their polished surfaces facing upward, and submerged in 5 cm³ of the simulated gastric acid solution. The specimens were immersed in 0.06 M hydrochloric acid (HCl, pH 1.2) for 96 h to simulate a high-acid challenge environment. Acid-exposure protocol: (Specimens immersed in simulated gastric acid (HCl, 0.06 M; pH 1.2) at 37 °C for 96 h; solution refreshed every 24 h.) was selected based on previous in vitro studies which equate 96 h of continuous exposure at pH 1.2 to approximately 5–10 years of in vivo acid challenge in patients with conditions such as gastroesophageal reflux disease or frequent dietary acid intake (e.g., Schwarz et al., 2021; Li et al., 2022; Gupta and Meyer, 2023) [26,27,28].

2.4. Optical Properties Testing Parameters

2.4.1. Color Differences Determination (ΔE)

Color measurements (L*, a*, b*) were obtained before and after acidic immersion using a calibrated spectrophotometer (JASCO Corporation, Tokyo, Japan). Calibration was performed according to the CIE 1976 Lab* system under the CIE D65 standard illuminant, representing average daylight. Each specimen was aligned with the instrument, and readings were taken using a 4-mm aperture. Before each measurement session, the spectrophotometer was calibrated using the manufacturer-provided white reference tile. All optical measurements were performed in a controlled environment under standardized lighting (D65 daylight illuminant), at a room temperature of 23 ± 1 °C and relative humidity of 50 ± 10% to ensure repeatability.

The color difference (ΔE) between baseline and post-immersion measurements was calculated using the standard CIE Lab* formula:

$$\mathsf{\Delta }E=\sqrt{(\mathsf{\Delta }{L}^{*}{)}^2+(\mathsf{\Delta }{a}^{*}{)}^2+(\mathsf{\Delta }{b}^{*}{)}^2}$$

where ΔL*, Δa*, and Δb* represent changes in lightness, red-green, and yellow-blue chromatic components, respectively.

2.4.2. Translucency Parameter (ΔTP) Values Determination

Translucency was calculated by measuring the CIE L*, a*, b* values of each specimen against standardized white and black backgrounds according to the following formula:

$$TP=\sqrt{(L_W^{*}-L_B^{*}{)}^2+(a_W^{*}-a_B^{*}{)}^2+(b_W^{*}-b_B^{*}{)}^2}$$

where the subscripts W and B represent measurements over white and black backgrounds, respectively.

At each evaluation time point, color (L*, a*, b*) and translucency readings were recorded three times per specimen using the same device and standardized positioning. The average of the three readings was calculated and used for subsequent analysis.

2.5. Statistical Analysis

Data are presented as mean ± standard deviation. Statistical procedures were completed using SPSS (version 20.0; IBM Corp., Armonk, NY, USA). Normal distribution of variables was verified via the Shapiro–Wilk test. One-way ANOVA was applied to evaluate differences in ΔE and ΔTP across groups, and Tukey’s post hoc test was used to conduct multiple comparisons when appropriate. Statistical significance was defined as p < 0.05, with 95% confidence intervals included for each mean.

3. Results

3.1. Color Difference (ΔE)

Color difference ΔE among the groups (Z, E, S, and C) showed a statistically significant difference (p < 0.05). Conversely, there was no statistically significant difference (p > 0.05) between the E and S groups. Group C (Cerasmart 270) had a statistically significant color change and the least color stability, followed by groups E (IPS e.max) and S (VITA Suprinity). Conversely, group Z (Ceramill Zolid) had the least color change and the best color stability, as shown in

Table 2. L*, a*, b* values (mean ± SD) before and after acidic immersion and resultant color difference (ΔE).

Group	L* (Before)	a* (Before)	b* (Before)	L* (After)	a* (After)	b* (After)	ΔE (Mean)
Z	82.05 ± 1.07	0.99 ± 0.86	19.99 ± 0.58	81.79 ± 0.17	1.20 ± 1.98	19.01 ± 0.51	1.31
E	80.05 ± 1.82	1.00 ± 1.60	19.00 ± 1.13	78.99 ± 1.46	1.10 ± 1.61	17.41 ± 1.43	2.02
S	81.04 ± 1.07	0.99 ± 0.86	19.99 ± 0.58	81.79 ± 0.77	1.00 ± 1.16	18.99 ± 0.58	1.95
C	80.15 ± 1.07	0.92 ± 0.76	18.79 ± 0.18	74.09 ± 0.17	0.85 ± 1.01	16.19 ± 0.38	2.59

Notes: One-way ANOVA across groups for ΔE: significant (α = 0.05). Z showed the least color change, C the greatest. (ΔE) = Color difference, SD = standard deviation.

.

3.2. Translucency Parameter (ΔTP)

After immersion in a gastric-like acidic solution, ceramic material’s translucency parameter (ΔTP) showed a statistically significant difference for each individual group before and after immersion. Only group C (Cerasmart 270) showed a statistically significant decrease in translucency (ΔTP 0.72 ± 0.19 before then 0.53 ± 0.11 after immersion), while groups Z (Ceramill Zolid: ΔTP 0.53 ± 0.15 before then 0.84 ± 0.13 after immersion), E (IPS e.max: ΔTP 0.43 ± 0.05 before then 0.85 ± 0.21 after immersion), and S (VITA Suprinity: ΔTP 0.41 ± 0.04 before then 0.83 ± 0.16 after immersion) experienced a significant increase in translucency. When compared (ΔTP) among groups, there was no statistically significant difference among groups Z, E and S (p < 0.0004). By contrast, there was a statistically significant difference between groups (Z & C), (E & C) and (S & C) where p value 0.312. These findings are shown in

Table 3. Translucency parameter (ΔTP) before and after acidic immersion.

Group	ΔTP Before (Mean ± SD)	ΔTP After (Mean ± SD)	Among-Groups P
Z	0.53 ± 0.15 a	0.84 ± 0.13 d	<0.0004 *
E	0.43 ± 0.05 b	0.85 ± 0.21 d	-
S	0.41 ± 0.04 b	0.83 ± 0.16 d	-
C	0.72 ± 0.19 c	0.53 ± 0.11 e	0.00312 *

In each column, groups sharing the same superscript letter did not differ significantly, while groups marked with different letters showed statistically significant differences (Tukey’s post hoc test, α = 0.05). Within each group, the comparison between baseline and post-immersion measurements revealed significant changes (α = 0.05). * means statistical significant changes (α = 0.05).

.

4. Discussion

This study evaluated the influence of simulated gastric acid exposure on the optical behavior of four CAD/CAM ceramic materials. Gastric fluid is considered the most potent endogenous acidic source in the oral environment, and the regurgitation of gastric contents in gastroesophageal reflux disease (GERD) can lead to erosion of both natural dentition and restorative materials [4,9]. To simulate the impact of gastric acid on optical stability, ceramic specimens were intentionally left unglazed, as glazed layers are often lost due to clinical wear within months of function. Moreover, previous reports have indicated that a well-executed polishing protocol can produce smoother surfaces that resist staining as effectively as, or even better than, glazed surfaces [29,30]. Standardized specimen thickness was ensured following the approach of Rani et al. to minimize variations in light transmission and color interpretation [31].

There is currently no consensus in the literature regarding the optimal concentration or immersion duration required to simulate gastric erosion in vitro [17]. ISO 6872 recommends immersion in 4% acetic acid at 80 °C for 16 h to evaluate chemical solubility [16,32]. However, Tanweer et al. demonstrated that this exposure simulated decades of intraoral aging [33]. In the present study, hydrochloric acid at pH 1.2 was selected based on the methodology of Sulaiman et al. and Elsukkary et al., who confirmed that HCl more closely resembles gastric acidity in chemical behavior [17,18]. The 96-h immersion period at 37 °C was chosen to approximate extended clinical exposure patterns. A spectrophotometer was used to evaluate ΔE and ΔTP values before and after immersion, given its demonstrated precision and repeatability in color analysis [34,35,36].

The null hypothesis was rejected, as statistically significant differences in both color change (ΔE) and translucency (ΔTP) were observed among the materials. Although ΔE values varied, the changes remained within clinically acceptable limits. The hybrid ceramic (Cerasmart) exhibited the greatest color shift, likely due to water sorption associated with its resin matrix, which increases susceptibility to pigment uptake [37,38]. The zirconia (Zolid Gen-X) group showed the least color alteration, followed by IPS e.max CAD and VITA Suprinity, which aligns with previous findings that materials with higher crystalline content demonstrate better color stability in acidic environments.

For translucency (ΔTP), groups Z, E, and S exhibited increased translucency after immersion, which may be attributed to surface smoothing and changes in surface topography that modify light reflectance [39,40,41]. In contrast, the hybrid ceramic group demonstrated a reduction in translucency, consistent with the resin phase being more prone to hydrolytic degradation and microstructural alteration [42]. These results corroborate previous investigations showing that acid exposure can negatively influence both surface roughness and mechanical integrity of CAD/CAM ceramics, leading to changes in their optical performance [43,44]. The differing responses among the tested materials are largely attributable to their distinct microstructural compositions, crystalline fractions, and polymer content [45,46,47,48,49,50].

Clinical considerations should be emphasized, particularly for restorations placed in patients with GERD or high dietary acid exposure. Materials with higher crystalline reinforcement demonstrated greater resistance to acid-induced deterioration. Therefore, clinicians should prioritize appropriate material selection, provide counseling regarding dietary and behavioral acid control, and consider shorter recall intervals for surface maintenance and monitoring of esthetic changes.

Limitations and Future Directions: This in vitro study focused on hydrochloric acid exposure and did not evaluate other erosive acids such as citric or lactic acid, which may produce different effects. Additionally, the model did not incorporate mechanical fatigue or thermal cycling, which play important roles in the intraoral aging of restorations. Future investigations should incorporate combined thermo-mechanical loading and extended aging protocols to more closely reproduce clinical conditions. Surface characterization using SEM or AFM, along with evaluation of mechanical properties such as flexural strength, would provide a more comprehensive understanding of material degradation. Finally, well-designed clinical trials are necessary to validate these findings in GERD patients and other high-erosion risk populations.

5. Conclusions

• Exposure to simulated gastric reflux conditions as in GERD produced notable alterations in the optical behavior of CAD/CAM ceramics. High-translucency zirconia (Z) showed superior color stability, whereas the hybrid ceramic (C) was the most vulnerable to discoloration and translucency reduction.
• Among the tested materials, zirconia-reinforced lithium silicate (S) and lithium disilicate (E) demonstrated moderate changes in optical properties, showing values between the highest- and lowest-performing materials. Overall, all tested materials remained within the perceptibility threshold, indicating that CAD/CAM ceramics are generally resistant to short-term acidic degradation.
• These findings carry clinical relevance for patients with gastroesophageal reflux disease (GERD) or other conditions associated with chronic intraoral acid exposure. Material selection should be made with consideration for long-term esthetic stability, particularly in patients at high erosive risk. High-translucency zirconia and lithium silicate-based ceramics may offer improved resistance to acid-induced optical deterioration, whereas hybrid ceramics may require more frequent monitoring or maintenance. Evaluation of patient-specific risk factors and appropriate recall intervals are therefore recommended to preserve the esthetic longevity of ceramic restorations.