The evaluation of shear bond strength of denture teeth to CAD/CAM dentures acrylic after aging in gastric acid

Abstract

The aim of this in vitro study is to compare the shear bond strengths of denture base resins obtained by different methods with different artificial teeth and to investigate their resistance to bonding by gastric acid in patients with gastroesophageal reflux disease. Samples were prepared using two different manufacturing methods for denture base resins (heat-polymerization and CAD/CAM) and three different types of artificial teeth (acrylic, Microfiller Reinforced Polymer Matrix Composite and CAD/CAM milled). CAD/CAM-milled denture base was combined with three different artificial teeth, while heat-polymerized base was combined with acrylic teeth. Each base-tooth combination was randomly divided into 2 subgroups (n = 14). One subgroup was immersed in artificial gastric acid and the other in artificial saliva solution. Shear bond strength testing was performed using a universal testing machine. The results obtained were analysed using two-way ANOVA and multiple comparisons were examined using Tukey's test. A significance level of p < 0.050 was used.

The heat-polymerized acrylic base and acrylic teeth groups showed the highest shear bond strength (p < 0.001). There is no significant difference between the shear bond strengths of artificial gastric acid and artificial saliva groups (p = 0.152).

CAD/CAM milled denture base resin bonded to different types of prefabricated teeth show similar shear bond strength. The bond strength of acrylic teeth bonded to a heat-polymerized acrylic base is higher. Gastric acid does not affect the shear bond strength.

1. Introduction

Nowadays, with the evolution of clinical trends, it is known that the frequency of fabricating removable or fixed dentures made of acrylic, which is known to be a more flexible material, has increased due to various failures experienced with rigid materials in implant-supported restorations [1,2]. Due to the lack of proprioceptive sensation around the implant, the force transmitted to the denture tooth can be 7–10 times higher than that of a tissue-supported denture [3]. This can lead to fractures in the dentures or separation of the teeth and the base [4]. The proliferation of acrylic supported dentures in clinical practice has increased the importance of providing a higher bond between the teeth and the base material.

Debonding of implant or tissue-supported removable dentures is a common complication and is known to have several causes. These include the chemical composition of the tooth and base materials used and the manufacturing methods. The denture base material has a significant influence on the bond strength of the tooth/denture base complex. By selecting the appropriate tooth/denture base combination and following the correct manufacturing principles, dentists can provide more durable dentures [5,6]. The most commonly used denture bases are made of polymethylmethacrylate (PMMA). Despite polymerization shrinkage, the traditional method of manufacturing PMMA (type 1) bases using hot water bath method is still considered the traditional method [7]. In recent years, with the development of CAD/CAM technologies, the production of denture bases from prepolymerized acrylic blocks, as opposed to traditional methods, has come to the fore [8,9]. These bases, produced by milling, do not undergo polymerization shrinkage. These materials, which are less porous than traditional denture bases, reduce the number of appointments and can be electronically archived thanks to the production method [6,10]. Dentures are usually made from PMMA. Although acrylic resins have a similar chemical structure, their physical and mechanical properties can differ depending on the manufacturing process [11]. Traditionally, PMMA teeth are bonded to the denture base by chemical and micromechanical bonding. However, due to their poor mechanical properties, manufacturers continue to introduce new denture tooth designs, often promising better wear resistance, improved mechanical properties or better esthetics. The new generation of dentures are made with organic, inorganic and nanocomposite ingredients. This has been reported to improve bonding to traditional denture base resins [12,13]. Denture manufactured using CAD/CAM technology are usually bonded to cavities milled into the base surface [14,15]. Although this process is more aesthetically pleasing than the monolithic subtractive method, the rate of artificial tooth separation from the denture base is higher [16].

Materials that remain in the oral environment for long periods of time are exposed to many chemical and mechanical effects that can adversely affect the properties of the materials [[17], [18], [19]]. In the oral cavity, acidic drinks, foods and some drugs are extrinsic acid sources, while gastric juice is an intrinsic acid source with a pH between 1 and 1.5 [20].

In patients with gastro-oesophageal reflux disease (GERD), gastric juice, which is a highly acidic source, reaches the oral cavity via the oesophagus [21]; in eating disorders such as anorexia and bulimia nervosa, gastric juice reaches the oral cavity via vomiting [22,23].

There are many studies investigating the destructive effects of gastric acid entering the oral cavity on the tooth surface and indirect restorative materials, but studies investigating the effects of the associated acid on the bond between denture bases and teeth are limited [[24], [25], [26], [27]].

The aim of this study was to investigate the bond strength of denture base materials and teeth produced by different manufacturing techniques with simulated gastric acid and to determine the base-teeth combination that should be recommended for patients with gastro-oesophageal reflux disease and eating disorders.

The null hypothesis is that exposure to simulated gastric acid will have no effect on the bond strength of tooth and base materials produced by different manufacturing techniques.

2. Materials and methods

In our study, two different materials were used as base materials; the first group was prepared with heat-polymerized (H) and the second group was prepared with CAD/CAM prepolymerized (C) material. The teeth used are of 3 different contents; CAD/CAM prepolymerized material (D), composite teeth with Microfiller Reinforced Polymer Matrix (V) and acrylic artificial teeth (A). The contents and manufacturer information are shown in Table 1.

Table 1.

Materials and manufacturer information.

Brand	Material Type	Composition	Manufacturer
IvoBase CAD (C)	CAD/CAM Prepolymerized acrylic resin base	PMMA	Ivoclar Vivodent, Liechtenstein
SR Triplex Hot (H)	Heat polymerized acrylic resin base	PMMA	Ivoclar Vivodent, Liechtenstein
Eray (A)	Acrylic artificial teeth	PMMA (cross-linked)	Eraylar- ostim, Ankara, Turkey
Ivotion Dent (D)	CAD/CAM Prepolymerized acrylic resin teeth	Double-cross linked polymethyl methacrylate	Ivoclar Vivodent, Liechtenstein
Vita Physiodens (V)	Artificial teeth	MRP matrix composite	VITA Zahnfabrik, Bad Säckingen,Germany

The prepolymerized base was bonded with prepolymerized (CD), composite (CV) and heat-polymerized (CA) artificial teeth. The heat-polymerized base group was bonded with heat-polymerized acrylic (HA) artificial teeth (control group). In order to detemined the number of specimens of the test groups, statistical power analysis was performed with the G Power program (G ∗ Power 3.1 software; Heinrich Heine University, Düsseldorf, Germany) with a margin of error of 5 %, a sensitivity of 0.4 and a power of 85 %, and the number of specimens was determined as 112 (n = 14) in total. As a result of the literature review, the dimensions of the specimens were determined as 10 mm in diameter and 2 mm in height for the denture base and 5 × 5 mm in length and 2 mm in height for the artificial teeth [28]. The bases and teeth combinations are shown in Fig. 1.

Fig. 1.

Base and teeth combinations.

STL (Standard Triangle Language) files were designed in SolidWorks (SolidWorks 2021, Massachusetts, USA) with dimensions of 10 mm Ø, 2 mm for the heat-polymerized base specimens and 5x5x2 mm for the tooth specimens. The duplicates of the designed specimens were obtained from CAD/CAM wax block (Bilkim, Izmir, Turkey) using a milling machine (Coritec 350I, imescore, Eiterfeld, Germany). Wax duplicates imitating teeth were embedded in a dental stone. The wax was then removed. Acrylic dentin resin (Eraylar-ostim, Ankara, Turkey) dough was prepared by mixing 23 g powder and 10 ml liquid according to the manufacturer's instructions and applied to the resulting gap. It was then cured in water at 110 °C, 15 min, 4 bar pressure and heat-polymerized tooth samples were obtained. The base wax samples obtained by milling were combined with the heat-polymerized acrylic tooth samples and fixed on the dental stone. The heat-polymerized acrylic base resin dough (SR Triplex, Ivoclar Vivodent, Liechtenstein) was prepared according to the manufacturer's instructions. The acrylic dough was applied to the gaps after wax removal and cured in a flask (MD-135, Meta Dental, Ankara, Turkey) in 100 °C water for 45 min. H-A samples were obtained. STL files were designed with dimensions of 10 mm Ø, 25 mm high for prepolymerized acrylic base samples and 5x5x18 mm for prepolymerized tooth samples. The CAD/CAM prepolymerized base (IvoBase CAD, Ivoclar Vivodent, Liechtenstein) and CAD/CAM prepolymerized teeth blocks (Ivotion Dent, Ivoclar Vivodent, Liechtenstein) with homogeneous content were milled to the designed dimensions using a milling machine (Coritec 350i, imes-icore, Eiterfeld, Germany). The positioning of the designs on CAD/CAM prepolymerized blocks is shown in Fig. 2(A and B). The 10 mm Ø diameter 25 mm high base specimens and 5x5x18 mm teeth specimens were placed in a sectioning device (IsoMet 1000, Censico International Pvt. Ltd., Uttar Pradesh, India). They were cut at 500 rpm under water cooling using a cutting blade (Buehler IsoMet Diamond Wafer-ing Blade, Series 15 LC, No:11–4276, Illinois, USA). 2 mm thick prepolymerized base and teeth specimens were prepared.

Fig. 2.

A, Positioning the prepared base designs on the CAD/CAM base block; B, Positioning of the prepared tooth designs on the CAD/CAM tooth block.

Microfiller Reinforced Polymer matrix (MRP) composite (Vita Physiodens posterior, VITA Zahnfabrik, Bad Säckingen,Germany) molars were fixed to an acrylic plate using sticky wax (Polywax, Bilkimya, Izmir, Turkey) and cut with a sectioning device (IsoMet 1000, Censico International Pvt. Ltd., Uttar Pradesh, India) under water cooling at 500 rpm to obtain straight surfaces. All surfaces were fixed to the acrylic block with sticky wax until a size of 5x5x2 mm was obtained and the cutting process was repeated. The 5x5x2 mm section was obtained from the dentin section located in the center of the teeth. According to the manufacturer's information, the MRP composite structure is contained in all layers in the teeth [29].

To prepare CV, CD, CA combinations, prepolymerized base samples were fixed to the glass using cythacrylate adhesive. Tooth specimens were placed in the centre of the base specimens and moulds were made by applying type A silicone (Presigum; President Dental, Germany). After the mold was removed, the bonding surfaces were alumina sandblasted with 100 μ Al₂O₃ at a distance of 10 mm for 5 s at a pressure of 2 bar with a device (Basic quattro, Renfert, Germany) in accordance with the adhesive bond manufacturer's instructions and the sandblasting residues were cleaned with oil-free compressed air.

The adhesive bond (Ivotion Bond, Ivoclar Vivadent AG, Liechtenstein) was prepared according to the manufacturer's instructions and applied to the bonding surfaces. The moulds were then assembled, fixed and cured in a pressure polymerization unit (Vertex, Multicure, Henry Schein Inc., Australia) at a temperature of 50 °C and a pressure of 4 bar. And CV, CD, CA samples were obtained, shown in Fig. 3A, B.

Fig. 3.

Dimensions of the specimens: A, tooth specimen bonded to acrylic base (top view); and B, tooth specimen bonded to acrylic base (side view).

Artificial gastric acid solution (GA) was prepared from 37 % hydrochloric acid (HCl) containing 0.06 M (0.113 % solution in distilled water) with a pH value of 1.2 [25] Artificial saliva (AS) solution (pH = 6.8) prepared according to DIN 53160-1 standard (Artificial Saliva, Testonic, Istanbul, Turkey) was used. The samples in each group were randomly divided into two subgroups. One subgroup was immersed in GA, and the other subgroup was immersed in AS, with a depth sufficient to cover them by at least 5 ml. The immersion times represented 8 years (96 h) of GA contact, taking into account that GERD had 3 meals and 1 snack per day and the contact time of gastric acid with the denture was 30 s. The duration of overnight wear was not taken into account for those who did not remove the denture at night [11]. All samples were immersed in an incubator (Nüve Incubator, Istanbul, Turkey) at 37 °C for 96 h. They were then washed with distilled water for 1 min and dried. The samples were then embedded in 12 mm diameter polyvinyl chloride (PVC) rigid plastic pipes using autopolymerized acrylic resin (S.C repair acrylic, Imicryl, Konya, Turkey). All samples were immersed in distilled water at 37 °C for 24 h before testing [30]. The specimens were then placed in a Universal Testing Machine (Devotrans, D.V.T. GPE 5 KN YBS, Turkey) for shear bond strength (SBS) testing without being kept in a dry environment. The blade of the testing machine was placed perpendicular to the junction of the base and the artificial tooth. The testing machine applied force at a crosshead speed of 1 mm/min until the connection failed to break. The area of the joint interface was the area of the artificial tooth adhering to the base, 5 mm² × 5 mm² = 25 mm². The SBS was calculated using the following formula [28].

$$\text{Shear}\text{Bond}\text{Strength}\left(\text{MPa}\right)=\text{Load}\left(\mathrm{N}\right)/\text{Area}\left({\text{mm}}^2\right)$$

S = L/A (S; shear bond strength, L; load separating the specimen, A; area of bond surface)

Specimens were examined under ×3 magnification using a Zumax binocular magnifier (Zumax, Su-zhou, China) and failure types were classified: adhesive, cohesive and mixed failures. Adhesive failure refers to a complete separation between the denture base resin and the tooth; cohesive failure describes a complete failure that occurs within the denture base resin or within the tooth. Mixed failure describes a combination of both adhesive and cohesive failures. One sample was randomly selected from each group, and the samples were placed in a SEM (Hitachi VP-SEM SU1510, Tokyo, Japan) and imaged at 100x magnification (20.0kv). Statistical analysis of the data was performed using Minitab v15. Normality was analysed using the Shapiro-Wilk test. As the data were normally distributed, two-way ANOVA was used. Multiple comparisons were analysed using the Tukey test. Descriptive statistics of quantitative variables were presented as mean ± standard deviation. The significance level was set at p < 0.050.

3. Results

The heat-polymerized acrylic base and acrylic teeth groups exhibited the highest shear bond strength. The prepolymerized base and prepolymerized teeth groups exhibited the lowest shear bond strength value (p < 0.05). SBS result of gastric acid groups and artificial saliva groups were compared and no significant difference was found (p > 0.05).

The base materials have statistically significant differences in bond strength (Table 2) (F = 44.046; p < 0.001).

Table 2.

Descriptive of bond strength according to material and solution and multiple comparison results.

Material	Solutions		Sum
	Gastric Acid	Artificial Saliva
CD	5.64 ± 2.16	6.01 ± 2.0	5.83 ± 2.05c
CV	7.53 ± 2.17	8.23 ± 1.81	7.88 ± 1.99b
CA	6.72 ± 3.08	7.19 ± 3.18	6.95 ± 3.08bc
HA (control)	12.53 ± 3.16	13.82 ± 2.77	13.18 ± 2.99a
Sum	8.11 ± 3.73	8.81 ± 3.88	8.46 ± 3.81

∗Mean±s.deviation; a-c: There is no difference between main effects with the same letter.

The bond strengths of CD, CV and CA groups are exhibited in Table 2. The bond strength of the HA group are significantly higher than those of the CD, CV and CA groups (Table 2).

The main effect of the material variable on bond strength was statistically significant, as exhibited in Table 3 (F = 44.046; p < 0.001).

Table 3.

Comparison of bond strength according to material and solution.

	Sum of Square	DF	Mean Square	F	p	${\eta }^2$
Material	889.520	3	296.507	44.046	<0.001	0.560
Solution	14.030	1	14.030	2.084	0.152	0.020
Materiala Solution	3.560	3	1.187	0.176	0.912	0.005

^aDF: Degrees of freedom; F: Two-Way Analysis of Variance Test Statistics;: Partial Eta Squared; R2 = 56.44 %; Adjusted R2 = 53.51 %.

All specimens were analysed for failure mode and are exhibited in Fig. 4.

Fig. 4.

All specimens are failure modes.

In the HA-AS and HA-GA groups, no adhesive failure mode was observed, and the highest cohesive failure mode was found to belong to these groups.

Large porosity and irregular particles in the Ivotions bond structure were observed in the SEM image of the CD-GA sample (Fig. 5A), the SEM image of the CV-GA sample exhibits a compact and regular structure. (Fig. 5B), the SEM image of the CA-GA sample exhibits large porosity and irregular particles where it is bonded (Fig. 5C), The mix failure image of the HA-GA sample exhibits the incompletely polymerized beads. (Fig. 5D).

Fig. 5.

A, CD-GA sample - Adhesive failure, at ×100 magnification. The black arrow exhibits porosity and irregular areas within the adhesive agent; B, CV-GA Cohesive failure at ×100 magnification. The black arrow shows regular structure in the base; C, CA-GA sample - Mix failure at ×100 magnification. The black arrow exhibits the large porosity; D, HA-GA sample - Mixed failure, at ×100 magnification. The black arrow exhibits the incompletely polymerized beads.

4. Discussion

This in vitro study investigated the effect of artificial gastric acid on the bond strength of various teeth-base combinations. Exposure to artificial gastric acid did not affect the bond strength of teeth-base materials produced by different fabrication techniques and the null hypothesis was accepted.

With the popularisation of digital workflows, CAD/CAM systems have begun to replace traditional methods in the fabrication of complete dentures [9,31]. It is known that teeth and tissues are exposed to much greater forces in implant-retained complete dentures [3]. Due to the very low repair rate of implants-retained complete dentures, monolithic complete dentures were more often preferred by patients [32]. According to the results of this study, subtractive manufacturing using CAD/CAM systems, which offers advantages such as fewer appointments and faster production, needs to be developed to be an alternative to conventional methods [8,9].

Acrylic artificial teeth are often used due to their chemical bonding to the denture base, high impact resistance, and good strength [33]. For chemical bonding to occur, two factors are necessary: physical contact and a chemical reaction between the base resin and the dental resin to form a polymer network. For this interpenetrating polymer network to form, the polymers must be dissolved [34]. Acrylic monomers need to wet and swell the bonding site during the dough phase. The more cross-linked agents and residual monomers present, the less swelling effect of the monomers This can be explained by the high monomer absorption capacity of the acrylic surface [35]. Limited monomer diffusion leads to residual monomer and the appearance of incompletely polymerized beads, Fig. 5D shows incompletely polymerized beads. The chemical bonding reaction between the PMMA base and the acrylic tooth is assumed to occur faster due to the high compatibility of their components. The diffusion rate observed in the heat-polymerized base and tooth is higher compared to cross-linked teeth, leading to a strong bond [30,36]. In accordance with this information, the high bond strength (13.18 ± 2.99 MPa) of the heat-polymerized control group in our study is consistent with the high rate of cohesive failure observed.

When Han et al. compared the SBS of conventional and CAD/CAM prepolymerized blocks, they found that the SBS of the conventional method was higher than that of the teeth bonded to the prepolymerized base, although the results were comparable. The difference in the adhesive agent content (META and 4-META) used by Han et al. may have contributed to the different SBS values observed with the prepolymerized base [30]. Choi et al., similar to our results with Ivotion bond, found that the bond strength of the prepolymerized base was lower than that of the conventional group. Many studies have demonstrated that use of MMA bond with a PMMA base increases the bond strength, but high amounts can increase the residual monomer ratio, which negatively affects the bond. The high residual monomer content in the adhesive bond resulted in a lower SBS value [[37], [38], [39]].

Bonding depends on the presence of free monomers. The initiator of heat-polymerized acrylic resins, benzoyl peroxide, is activated at 80 °C and activates the release of free radicals, resulting in the formation of free monomers. Free monomers allow the artificial tooth to integrate into the basic polymer chains during polymerization. In case of insufficient free monomers, incomplete polymerization leads to less cross-linking, making bond formation more difficult and can lead to a decrease in bond strength. The Ivotion bond agent contains autopolymerized acrylic resin. The diffusion of autopolymerized acrylic resins into the artificial tooth surface is limited, resulting in lower bond strength than heat-polymerized acrylic resins [3,40]. It is known that autopolymerized resin polymer beads swell as the temperature increases and the highest polymer bead formation is observed at 70 °C. Increasing the temperature is recognized to increase the adhesion strength by increasing the diffusion rate and adhesive failures tend to become cohesive failures after 50 °C [41]. According to the manufacturer's instructions, Ivotion bond polymerizes at 50 °C, and higher temperatures are not recommended. The low polymerization temperature of the bond may complicate monomer diffusion and affect bonding with the artificial tooth, which might have led to the prepolymerized group showing lower bond strength and a higher adhesive failure rate compared to the control group [39,40,42].

In the study by Helal et al., which used a similar test method and adhesive agent to our study, the SBS of acrylic teeth (7.39 MPa) for the prepolymerized base block subjected to airborne abrasion was very close to our CA group, but significantly lower than our CV group. This difference is thought to be due to the variation in filler content of the composite teeth used [3,28].

With the development of the dental industry, acrylic teeth with low wear resistance have been replaced by cross-linked acrylic teeth with higher resistance [43]. Cross-linked, double cross-linked (DCL), and highly cross-linked polyacrylic resins, as well as inorganic micro-filled polymer teeth homogeneously dispersed in the matrix, have been developed [10,44]. Conventional cross-linked acrylic teeth wear more than acrylic teeth with filler content [38,45]. Prepolymerized resin blocks produced at high temperatures and pressures exhibit high wear resistance and hardness. DCL teeth have superior hardness properties compared to conventional and cross-linked teeth [46,47]. For the connection of plastic polymers, monomers must be present in a solvent. This solvent swells the polymer surface to be connected and allows the diffusion of the solvent; the quality of the connection depends on the diffusion rate of the solvent. High cross-linkers can hinder the monomer from diffusing into the matrix, decreasing the diffusion rate and adversely affecting the bond. The high cross-linked structure of the prepolymerized teeth group in our study may have made monomer diffusion difficult due to its highly cross-linked structure [43,[48], [49], [50]].

While the clinically approved denture base material roughness value is 0.2 μm, this plays a critical role in preventing staining, plaque accumulation, biofilm formation, and stomatitis. Gastric acid, with a pH that can drop by 1–1.5, can regularly reach the oral environment due to disorders such as GERD. In particular, gastric acid reaching the oral environment with a pH below 5, along with carbonated drinks, wines, and orange juices containing citric acid, can cause erosion of acrylic teeth and denture bases [17,51,52]. According to the results of this study, there is no significant difference between artificial gastric acid and artificial saliva SBS, adhesive failure was higher in the groups immersed in gastric acid. Tinaztepe et al. showed that artificial gastric acid altered the roughness and hardness of the denture base materials, with the prepolymerized base being affected the least [11]. Low pH causes hydrolysis of ester radicals in PMMA monomers, breaking chemical and physical bonds, thereby softening the material surface and reducing hardness [52,53].

Many studies have shown that organic and inorganic fillers affect the polymer matrix and change the hardness value [43,48,54,55]. Acidic solutions destroy the resin matrix structure, causing the filler structure to separate, which increasing roughness and decreasing hardness. It can be concluded that gastric acid negatively affects bonding by destroying the polymer matrix structure. Since teeth with a high filler ratio are more resistant to acids, it is expected that their bond strength will be less affected [54,55]. Low pH beverages damage the acrylic tooth surface and increase roughness in contact with acrylic artificial teeth. Among acrylic resin artificial teeth, those with microfiller content are known to have the best mechanical properties and wear less [10,56]. This result is supported by the fact that the CV-GA group using Vita Physiodens artificial teeth with microfiller content showed more cohesive failures.

The fact that there was no statistically significant difference in SBS between the groups immersed in gastric acid and artificial saliva is attributed to the changes caused by water absorption in the PMMA matrix. Hydration of the acrylic material has been reported to change its structure, penetrating or bonding to its disordered outer layer and creating a plasticizing or softening effect as it dissolves the bonds formed during polymerization. The effect of this degradation on bonding is inevitable [57]. It has been reported that acids and distilled water disrupt the surface integrity of resins, reduce hardness, and acids may be more effective [58]. It is possible that the artificial saliva solution, with a pH of 6.8, affects the structure of the adhesive bond agent with PMMA resin content less than gastric acid, leading to a lower adhesive failure rate.

The limitations of our study are that enzymes and neutralising compounds present in saliva solution were not stimulated and solutions were prepared with distilled water. The effect of occlusion and forces on the tooth on the bond strength has not been investigated. These factors need to be investigated in future studies.

5. Conclusions

According to the findings, the control group had the highest shear bond strength (13.18 ± 2.99 MPa), followed by the Microfiller Reinforced Polymer matrix composite teeth group bonded to the prepolymerized base (7.88 ± 1.99 MPa), with no significant difference observed compared to the acrylic tooth group. Artificial gastric acid has no effect on the bond strength. The CAD/CAM materials used in the production of dentures need to be improved in terms of bond strength compared to conventional methods. The conventional method can be used safely in clinical practice.

CRediT authorship contribution statement

Neslihan Güntekin: Writing – review & editing, Writing – original draft, Visualization, Software, Resources, Methodology. Burcu Kızılırmak: Validation, Methodology, Investigation, Formal analysis.

Data and code availability statement

Data will be made available on request. For requesting data, please write to the corresponding author.

Ethics declarations

Not applicable.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

A list of abbreviations

CAD/CAM:

SBS

Shear Bond Strength

PMMA

Polymethylmethacrylate

GERD

Gastro-osephageal Reflux Disease

STL

Standard Triangle Language

GA

Gastric Acid

AS

Artificial Saliva

HCL

Hydrochloric Acid

4-META

4metakriloksietil trimetil anhidrat

MRP

Microfiller Reinforced Polymer Matrix

DCL

double cross-linked