Impact of erosion and aging simulation on chairside materials

This study investigates the influence of acid exposure and thermocycling simulating erosion tooth wear, on the optical properties and surface roughness of chairside materials. Resin‐ceramic, lithium disilicate, premium zirconium oxide and resin composite material comprise the materials tested. To simulate the dental erosion and aging, specimens from each material were immersed in hydrochloric acid, while the thermocycling procedure included 10,000 cycles. The translucency, the color differences and the surface roughness were calculated. The materials phase composition was tested using X‐ray diffraction analysis to evaluate T‐M phase transformation. The CIEDE2000 color difference and the translucency parameter were tested different significantly among groups. Data were statistically analyzed via independent samples t‐test, and paired samples t‐test. The thermocycling procedure and the exposure to the acid solution had different effect on the surface roughness of CAD/CAM materials. The present result demonstrated the negative effect the acid exposure has on the zirconia material in terms of color difference. However, no color differences over the threshold of acceptability were recorded after the thermocycling procedure. Both polymer materials exhibit an increase of the surface roughness when they were immersed in acid but they did not display an increase in roughness when they were thermocycled.


| INTRODUCTION
Dental erosion is a dental pathology that affects an increasing number of people. Erosive tooth wear (ETW) has a high prevalence rate of >30% among adults (Bartlett et al., 2008;Salas et al., 2015). External factors like acidic food consumption and exposure to atmospheric acid, as well as internal factors like gastric acid reflux, can lead to dental erosion (Fernández et al., 2022). The causes of ETW include gastroesophageal reflux disease (GERD), which has become more prevalent due to changes in diet (Shimazu et al., 2018), as well as bulimia nervosa, chronic alcoholism, and hyperemesis gravidarum. The presence of an acidic environment with a pH level lower than 5.5 is essential for the occurrence of dental erosion (Lussi et al., 1993), while gastric fluids possess a pH level of about 2.7, indicating a strong possibility that dentin can dissolve in cases of acid reflux (Shimazu et al., 2018). During ETW, non-carious lesions are mainly detected at the palatal surface of the anterior upper teeth and are usually restored with resin composite, resin computer-aided design/computer-aided manufacturing (CAD/CAM), or lithium disilicate materials. The occlusal surfaces of the posterior lower molars are also affected and, in many cases, are restored with zirconia materials owing to their high flexural strength (Schlueter & Luka, 2018). However, acidic products could interact with restorative materials and alter their optical properties resulting in discoloration. Several studies have been conducted to evaluate the effects of acid exposure and UV aging on various dental materials. Findings from these studies suggest that both acid exposure and UV aging can cause alterations in the optical properties of resin composites and CAD/CAM materials (Alnasser et al., 2019;Choi et al., 2021;Kim et al., 2022). Furthermore, studies evaluating the surface roughness of several CAD/CAM materials after acid exposure have reported an increase in surface roughness and topographic alterations resulting from the simulated erosive challenge. Additionally, research has revealed that ceramics and resin composite restorative materials exhibit greater resilience against material loss resulting from acid exposure, as compared to GICs (Wan Bakar & McIntyre, 2008).
Importantly, aging and acid exposure are known factors that can increase the surface roughness of dental materials, which may lead to the accumulation of bacterial biofilm and consequently increase the risk of periodontal diseases (Abusleme et al., 2000).
Tooth erosive wear have been treated with direct or indirect nanohybrid composites (Milosevic & Burnside, 2016), with resin nanoceramics or glass ceramics CAD/CAM materials (Oudkerk et al., 2020) with satisfactory results in the short and long term (Oudkerk et al., 2020).
The popularity of zirconia ceramics has increased since the introduction of multi-layered translucent zirconia used for monolithic restorations (Zarone et al., 2019). Dental clinicians promote the use of zirconia ceramics owing to their superior biocompatibility, favorable mechanics, and esthetic properties compared with conventional zirconia (Denry & Kelly, 2008). The fabrication time for zirconia restorations at the laboratory or dental clinic has been shortened since the establishment of CAD/CAM technology, which is an effective and predictable method for creating dental restorations with extended longevity (Aziz et al., 2019;Fasbinder et al., 2020). As zirconia material has evolved from the traditional tetragonal zirconia that is partially stabilized by 3% yttria (3Y-TZP) to the more esthetically translucent monolithic zirconia with a higher Y 2 O 3 content and an increased cubic phase, manufacturers have managed to increase the translucency of zirconia materials (Tabatabaian, 2019). Along with the higher cubic phase, clinicians have managed to overcome the difficulty of achieving adhesion between dental tissues and zirconia ceramics with the help of new adhesion techniques and resin cement containing adhesive monomers that can promote chemical adhesion through the reaction between acidic functional and oxide groups on the zirconia surface (Chen et al., 2014;Quigley et al., 2021). The above resulted in the clinical use of zirconia for esthetic anterior cases (Naveau et al., 2019) or as a material for anterior veneers (Lawson et al., 2021). Along with the use of multi-layered translucent monolithic zirconia ceramics, several other materials are included in the armamentarium of a CAD/CAM dental clinic. IPS e.max CAD is one of the most used dental materials with a survival rate of 83.5% after 10 years (Rauch et al., 2018) and 96.4% after 5 years (Nejatidanesh et al., 2018). However, the common failures of IPS e.max CAD include fractures (3.8%) and secondary caries (7.7%) (Spitznagel, Boldt, & Gierthmuehlen, 2018). Moreover, manufacturers have recently introduced resin-matrix ceramics (RMCs) and polymerinfiltrated ceramic network (PICN) materials to combine the advantages of polymer materials and ceramics. These advantages, compared with resin composite used for direct restorations, include esthetic appearance (Pulgar et al., 2019), increased edge chipping resistance (Hampe et al., 2021), increased mechanical (Furtado de Mendonca et al., 2019;Goujat et al., 2018), and superior chemical properties (Mourouzis et al., 2020). Several studies have found that the restorations achieved with the above materials showed a survival rate of 97% after 3 years of use (Lu et al., 2018) with restoration fracture (3.8%) being the only common failure (Spitznagel, Scholz, et al., 2018).
Several studies have investigated the impact of bleaching agents (Mourouzis et al., 2013), oral hygiene solutions (Buyukkaplan et al., 2017) or acidic drinking products (Theocharidou et al., 2021), on the surface characteristics of dental materials. However, no evidence has supported the impact of external factors such as the combination of thermocycling and acid exposure on CAD/CAM materials. Testing the optical properties of dental materials after thermocycling and acid exposure is important because these challenges can cause changes in color and translucency that could affect the esthetic outcome of the restoration. Thermocycling can cause microcracks and delamination of the material, leading to changes in color, while acid exposure can result in surface roughness, loss of translucency, and color changes.
By testing the optical properties of dental materials after thermocycling and acid exposure, researchers can evaluate the material's ability to withstand these challenges and maintain its esthetic properties over time. This information can then be used to make more informed decisions about material selection for dental restorations, ensuring that patients receive the most durable and esthetically pleasing restorations possible. Furthermore, these tests can also help to identify the optimal processing and finishing techniques for dental materials, which can affect their durability and esthetic properties.
To our knowledge, no study has investigated alterations to the optical properties and surface characteristics of new CAD/CAM materials after thermocycling and immersion in acidic solutions. The processes of thermocycling and acid exposure are capable of simulating intraoral conditions in vitro by inducing variations in temperature and acidity within an aqueous environment. Accordingly, the aims of this study were: to evaluate the optical properties (translucency and color differences) of representative CAD/CAM materials and examine their surface roughness alterations after thermocycling and immersion in an acidic solution that simulates the aging and dental erosion found intraorally among patients. Three null hypotheses were set for this study: the first null hypothesis (H01) stated that the CAD/CAM materials would not demonstrate color changes at the threshold of acceptability after acid exposure; second null hypothesis (H02) stated that the CAD/CAM materials would not demonstrate color changes after undergoing thermocycling; and third null hypothesis (H03) stated that the tested materials would not have surface roughness changes after undergoing thermocycling or acid exposure.

| MATERIALS AND METHODS
This study included materials most commonly used for anterior and posterior restorations in cases of ETW rehabilitation (Alnasser et al., 2019;Sulaiman et al., 2015). Resin-ceramic CAD/CAM blocks Vita Enamic (Vita Zahn Fabrik), lithium disilicate materials IPS e.max CAD, and premium zirconium oxide material IPS e.max ZirCad MT Multi (Ivoclar Vivadent), were the evaluated CAD/CAM materials. The manufacturers, types, and compositions of the materials used in this study are listed in Table 1. The control group included a universal resin composite Mosaic (Ultradent).

| Specimen preparation
Forty (n = 40) square-plate specimens were prepared for each evaluated material, generating a total of 160 square-plate specimens, which were divided into two groups: thermocycling (TC) and acid exposure (AE) groups. Commercially available CAD/CAM blocks of IPS e.max CAD and Vita Enamic materials were vertically sectioned along the longitudinal axis using a low-speed precision microtome (IsoMet 1000; Buehler) with a 0.3 mm diamond-coated blade, under water coolant. The resulting specimens had dimensions of 14 mm Â 12 mm Â 1 mm. For the IPS e.max ZirCAD Multi MT material, the initial dimensions of the specimens were 19 mm Â 17 mm Â 1.5 mm before the sintering procedure. After completion of the sintering process, the specimens were reduced in size to 14 mm Â 12 mm Â 1 mm using the same sectioning and cooling procedure. The specimen thickness was evaluated using a The number of specimens for all groups was determined using statistical calculations after a pilot study. The pilot study's statistical analyses were performed using GPower 3.1.9.2 for Mac and the following statistical tests were performed: one-way analysis of variance (ANOVA) within factors, an err prob = 0.05, power (1 À b err prob) = 0.80, and three repetitions.

| Optical properties measurements
The baseline measurements for the optical properties of all specimens from the materials tested were calculated with a scientific optical spectrophotometer (Shimandzu UV-2401PC Series, UV-VIS). To allow repeatable measurements, a customized 3D-printer holder of black and white color was fabricated to hold the specimens in the same position inside the spectrophotometer. The spectrophotometer was calibrated with a standardized and pressed powder of barium sulfate according to the manufacturer's instructions. Each specimen was measured thrice consecutively; L*, a*, and b* parameters were recorded, and a mean value of the readings was calculated.
The translucency (TP 00 ) was calculated using the CIEDE 2000 (1:1:1) color difference formula (Hampe et al., 2021): where where ΔL was the changes in lightness, Δ C = difference in chroma, Δ Η = difference in hue; and Rt, S L , S C , and S H are weighting factors.
The three K terms were additional weighting factors that were set to

| Surface roughness measurement
The average surface roughness of each specimen was measured using a three-dimensional optical non-contact surface profilometer Bruker Contour GT (Bruker Cooperation) and with the help of an observational software programme Vision 64 (Bruker Cooperation, Tucson, AZ, USA) at Å $ magnification. Each specimen was measured in three selected areas and the surface roughness parameters Sa (average surface roughness) and Sz (sum of the largest profile peak and largest profile valley) were recorded. The first measurement was made from the center of the specimen and the other two at different random points from the center.

| Thermocycling procedure
After the baseline measurements, 20 specimens from each material were artificially aged with a thermocycling procedure of 10,000 cycles in deionized water solution at 5 -55 C, with a transfer time of 5 s and dwelling time of 30 s. In total, 10,000 cycles corresponded to 1 year of clinical use (Gale & Darvell, 1999). The temperature was controlled with an accuracy of 0.1 C. Following the thermocycling procedure, all the specimens were rinsed under running water, blotted, and dried with tissue paper. The translucency parameter (TP 00 ), and the CIEDE 2000 were remeasured using the same protocol and equipment as baseline measurements.

| Exposure to acidic conditions
A method of simulating gastric acid in laboratory studies to replicate the clinical situation is not well established. However, in this study published protocol for simulating acid exposure was used (Alnasser et al., 2019). After baseline measurements, to simulate intraoral dental erosion under laboratory conditions, 20 specimens from each material were immersed in HCL (pH = 2) for 91 h, representing gastric acid exposure for approximately 12 months (Doerr et al., 1996). These times were calculated on the assumption that a bulimic patient purges three times a day, for an average of 5 min per purge. Therefore, on average, teeth would be exposed to gastric acid for 15 min a day (Backer et al., 2017;Hetherington et al., 1994;Sulaiman et al., 2015;Theocharidou et al., 2021).
All specimens were immersed in glass vessels filled with 10 mL of 5% HCL (pH = 2) and incubated at 37 C for 91 h, which is an equivalent of 12 months of acid exposure. After 91 h, each specimen was rinsed with deionized water and blotted dry. The optical properties were re-evaluated using the scientific spectrophotometer and the surface roughness was examined using the optical 3D profilometer.

| Scanning electron microscopy and energydispersive X-ray spectroscopy (SEM-EDS)
The surface of the specimens was analyzed and characterized before and after thermocycling and acid exposure using a JEOL JSM 6390LV scanning electron microscope (SEM). Elemental analysis was performed using energy-dispersive X-ray spectrometry

| X-ray diffraction analysis
The phase composition of the materials tested, before, and after acid exposure and thermocycling procedures was evaluated using X-ray diffraction (XRD) analysis with the specimens at baseline conditions from each group. XRD analysis was performed using a diffractometer with Ni filters and Cu K radiation (PW1710, Philips). Diffraction patterns were obtained at 20 -75 at a scan speed of 0.008 /min. XRD patterns were analyzed using the Rietveld refinement method, allowing for the calculation of the cubic phase amount of the zirconia ceramics using the entire diffraction pattern.

| Statistical analysis
The values of TP 00 , CIEDE 2000 , Sa, and Sz before and after each experiment were checked for normality and homogeneity assumptions using the Shapiro-Wilk and Levene's tests respectively. While normality was not rejected in any case, homogeneity of variance was rejected in some cases. An independent samples t-test was used to compare each material at baseline and after the thermocycling procedure or acid exposure. The correlation of translucency after thermocycling and acid exposure among the materials was also checked using Pearson's correlation statistical test. A paired samples t-test was used to compare the surface roughness parameters (Sa and Sz) between the materials before and after the thermocycling procedure or acid exposure. All analyses were conducted using the IBM Statistics software (version 27.0), and the statistical significance for each test was set at p < .05.

| RESULTS
The mean values and standard deviations of the color differences (CIEDE 2000 ) and surface roughness for all the materials before and after the thermocycling procedure and acid exposure are presented in Tables 2 and 3, respectively. 3.1 | Color differences

| Translucency parameter
The translucency of the material specimens differed significantly between the groups following thermocycling and acid exposure F(3,52)  The statistical analysis showed that there was no correlation between the translucency after thermocycling and acid exposure (Pearson's correlation coefficient r = À0.097, p = .473). However, there was a strong positive correlation between the baseline translucency and T A B L E 2 The mean ± standard deviation of color differences (ΔΕ2000), translucency (TP00), Sa, and Sz surface roughness properties for all materials are reported after the thermocycling procedure. T A B L E 3 The mean ± standard deviation of color differences (ΔΕ2000), translucency (TP00), Sa, and Sz surface roughness properties for all materials are reported after the application of acid.  One-way ANOVA reveal that the highest value of Sa and Sz for Vita Enamic was recorded after the thermocycling procedure (0.27 ± 0.03 μm and 9.11 ± 0.58 μm, respectively), whereas that of the resin composite material was documented after the exposure to the F I G U R E 2 X-ray diffraction patterns of all materials at baseline and after acid exposure, (a) IPS e.max CAD, (b) IPS e.max Zircad multi, (c) resin composite, (d) Vita Enamic.
Overall, the material with the highest surface roughness parameters (Sa and Sz) was IPS e.max Zircad material after the thermocycling procedure with an average of 0.92 ± 0.24 μm and 10.67 ± 1.36 μm, respectively. Boxplots of surface roughness (Sa) in μm at baseline before and after exposure to acidic conditions and thermocycling procedures are illustrated in Figure 3.

| SEM-EDS
Representative SEM microphotographs, and EDS analysis of all the materials before and after the thermocycling procedure and exposure to acidic conditions, are presented in Figure 3. Conversely, the polymer materials displayed a smoother surface topography. Topography changes after acid treatment were detected for all materials, which showed an irregular surface as a result of the thermocycling and erosion process. The energydispersive X-ray spectroscopy (SEM-EDS) verified that on the surface of the glass-ceramic materials alumina, silicon, potassium, and sodium were found attached.

| X-ray diffraction analysis
The XRD patterns displaying the phase composition for all materials before and after the thermocycling procedure and acid exposure are F I G U R E 3 Scanning electron microscope (SEM), representative topographic images and energy-dispersive spectroscopy (EDS) of IPS e.max CAD, Vita Enamic, Resin composite and IPS e.max Zircad Multi (1, 2, 3, 4), at (a, d, g) baseline, (b, e) after thermocycling, and (c, f, h) after acid exposure. Red arrows indicate topography alterations due to thermocycling or acid exposure. presented in Figure 3. As shown in Figure 3, the IPS Zircad Multi material is composed mainly of cubic and tetragonal phases. The Zirconia material showed a tetragonal phase increase before (51.49%) and after (53.64%) acid exposure. This was observed in the 2 theta 60, 74, and 84 angles. As shown in Figure  who also exposed glass ceramic and resin-ceramic materials to HCL for 46 and 91 h, and explained that the increase in surface roughness of those materials, was due to the grain size detachments detected on the surface of the examined materials. Specifically, when the grain size is large in size, the acid will remove those large fillers and the surface roughness will increase, proportionally to the materials with a lower grain size (Harryparsad et al., 2014). Moreover, consecutive contractions and expansions due to temperature variations of the thermocycling procedure could potentially create fatigue micro-cracks, from which water could penetrate the matrix and hydrolyse the siloxane bond, resulting in the discharge of the fillers and increased surface roughness of the polymer materials (Boussès et al., 2021). On the contrary, crystalline-based zirconia is more surface stable to acidic exposure, and large defects will not be created on the surface of the material (Farhadi et al., 2021). An interesting finding of this study is the increase in the Sz parameter that revealed the presence of deep valleys in the surface of the tested materials. Acidic exposure and thermocycling increased the surface roughness of the resin-ceramic material significantly, compared to the resin composite material. The above finding can be attributed to the effect of aging on the heterogeneous microstructure of the material, creating large defects due to the inconsistent degradation of the ceramic network (86% wt.) and the resistance of the polymer network (14% wt.) of the resin-ceramic material.
Ceramics are considered frontline materials of esthetic rehabilitation of anterior teeth (Malchiodi et al., 2019). Patients with severe long-term chemical-induced wear are concerned about the long-term stability of the restoration color. In this study, all materials except the zirconia material, demonstrated clinically acceptable color differences (<1.8 units) following the thermocycling procedures. However, after acidic exposure, the crystalline-based ceramics (zirconia and lithium disilicate) presented with color differences above the acceptability threshold (>1.8 units) (Paravina et al., 2015). Nonetheless, the surface roughness of the zirconia material IPS e.max Zircad Multi was not affected by any of the aging procedures; however, the procedures altered the color of the material. This can be explained by a previous study (Mourouzis & Tolidis, 2022), in which the surface of the zirconia material was impregnated with residues of materials that adhered to the zirconia lattice during the aging procedures and caused a mismatch between the refractive index of the zirconia (refractive index 2.173) (Backer et al., 2017) and the residues present in the medium, resulting in significant light scattering. The above phenomenon induced color differences above the threshold of acceptance and decreased the level of translucency (Kim, 2020). The above finding can explain the discoloration of the zirconia material after both aging procedures.
In contrast, the resin-ceramic CAD/CAM and composite materials did not present with a color difference before and after acidic exposure and the thermocycling procedure above the acceptability threshold (>1.8 units). Previous studies have reported color differences in the polymer matrix among different components (Liebermann et al., 2021). Specifically, the bisphenol A-glycidyl methacrylate (Bis-GMA) is associated with higher discoloration than the urethan dimethacrylate (UDMA), which is because the hydroxyl groups of the resin monomers that are found in abundance in Bis-GMA can increase water absorption (Topcu et al., 2009). This is evident in the resin- max CAD showed alterations in its structure, microporosities, and striations especially when the material was exposed to acidic conditions. Moreover, IPS e.max Zircad Multi displayed grooves with many impurities attached to the surface while the energy-dispersive X-ray spectroscopy (SEM-EDS) verified that those impurities were alumina, Si, K, and Na attached to the surface. On the contrary, the polymer materials showed a different surface topography with the resin-ceramic CAD/CAM material showing fewer porosities, without craters. The SEM images of the resin composite material showed a uniform surface with some sporadic large craters in some areas of the surface specimens. Those craters were found more often in the specimens after acidic exposure. The latter indicates the dissolution of the glossy matrix and the leaching out of oxide ions (Al-Thobity et al., 2021).
XRD analysis did not reveal any changes in the composition of the CAD/CAM materials before and after the acidic exposure. Moreover, the polymer CAD/CAM material was found to be amorphous with no evidence of crystallization, before and after the acidic exposure. The latter is in accordance with other studies (Hussain et al., 2017;Ramos Nde et al., 2016) and can be justified by the amorphous phase of the inorganic glass.
The limitations of this in vitro study include the fact that the specimens did not experience the complexity of the oral environment, including the forces of mastication and the protective effect of saliva.
Future studies should incorporate more realistic in vitro models that mimic the oral environment and measure more intraoral factors, such as wear, to obtain results that more closely reflect real-world conditions. In addition, the study did not investigate the effect of long-term exposure to acidic environments, which is an important consideration for the longevity of dental restorations. Future studies should include long-term exposure to acidic environments to determine the potential for material degradation over time. Within the limitations of this study, acid exposure significantly affected the optical properties of highly translucent zirconia used for monolithic restorations, resulting in color changes that could be detected by an observer as unacceptable, without affecting the phase composition of the zirconia material.
However, the clinical significance of these color changes is still uncertain, and further research is needed to determine whether they would be visible to dentists or patients and affect the overall esthetic outcome of the restoration. Moreover, lithium disilicate, resin-ceramics CAD/CAM material, and resin composite material exhibited an increased surface roughness after acid exposure, with no color differences over the threshold of acceptability after the thermocycling procedure and acid exposure. However, surface roughness can have an impact on the longevity and durability of the restoration, making it an important factor to consider in material selection. In summary, while this study provides valuable insights into the behavior of different CAD/CAM materials under various conditions, there is still much to be learned about the factors that affect the longevity and durability of dental restorations. Future studies should incorporate more realistic in vitro models and investigate the long-term effects of acidic exposure. These studies will help inform the development of more durable and esthetically pleasing dental restorations.