Effect of Dentin Surface Pretreatments and Thermocycling on the Shear Bond Strength of Resin Cement: An In Vitro Study

Abstract

The objective of this in vitro study was to investigate the effects of dentin pretreatment protocols and thermocycling on the shear bond strength (SBS) of a self-adhesive resin cement (Maxcem elite chroma) on dentin. A total of 168 extracted human third molars were randomly divided into four main groups according to dentin pretreatment: no treatment, 10% polyacrylic acid, Optibond universal, and Scotchbond universal plus. Half of these were subjected to thermocycling (5000 cycles; 5–55 °C). Composite resin rods were bonded using the self-adhesive resin cement, and SBS was measured with a universal testing machine. Two-way ANOVA showed that dentin pretreatment and thermocycling significantly affected SBS, with significant interaction between factors (p < 0.001). The highest SBS was observed in the Optibond universal group (18.71 ± 0.43 MPa), while the lowest SBS occurred in the 10% polyacrylic acid-treated group after thermocycling (2.69 ± 0.39 MPa). Thermocycling significantly reduced SBS in all groups. These results indicate that pretreatment with a compatible universal adhesive improves bond durability, whereas 10% polyacrylic acid pretreatment adversely affects bonding performance.

1. Introduction

Resin cements are low-viscosity composite materials that exhibit superior mechanical properties and esthetics compared to water-based cements [1,2]. They are frequently utilized as luting agents for various dental restorations, including all-ceramic and porcelain-fused-to-metal (PFM) crowns and bridges, indirect composite restorations, full metal restorations, prefabricated posts, and orthodontic appliances. Resin cements can be classified into three categories: (1) Conventional resin cements (etch-and-rinse): These require the application of an etchant, primer, and adhesive bonding agent to the tooth surface. This multi-step system is considered complex and technique-sensitive. (2) Adhesive resin cements (self-etch or etch-and-dry): These eliminate the etching and rinsing steps; however, the tooth structure is still modified by an acidic primer or a self-etch adhesive bonding agent. (3) Self-adhesive resin cements: These adhere to the tooth structure independently, without the need for separate etching, priming, or bonding agents [3].

Recently, manufacturers have incorporated various functional monomers into self-adhesive resin cements to enhance their adhesion to tooth structures and a wide range of restorative substrates [4]. These materials may be used either alone or in combination with an additional adhesive system, such as etch-and-rinse or self-etch adhesives. Accordingly, some manufacturers describe these materials as universal self-adhesive resin cements [5].

In this context, the term self-adhesive refers to resin cements that are designed to bond directly to tooth substrates without the need for separate etching, priming, or bonding steps. In contrast, the term universal does not indicate a distinct adhesive mechanism, but rather describes the material’s versatility and broad clinical applicability, including bonding to enamel, dentin, ceramics, metals, and resin composites, as well as compatibility with different adhesive strategies. Therefore, so-called universal self-adhesive resin cements remain self-adhesive by definition, while their designation as universal reflects their expanded clinical indications and manufacturer-described versatility rather than a separate cement classification [6].

This versatility allows universal self-adhesive resin cements to be applied across a wide range of restorative procedures, contributing to simplified clinical workflows and the potential for reliable and durable bonding outcomes in contemporary dental practice.

Therefore, self-adhesive resin cement is increasingly popular in dental clinics nowadays due to its simplification of use and acceptable bond strength. Without the etching step, the potential for post-operative sensitivity is dramatically reduced [7]. However, there are several studies that have reported that the bond strength of this type of cement was lower than that in etch-and-rinse and self-etch systems [8,9,10,11]. In an effort to overcome this limitation, various surface conditioning techniques have been explored to enhance bond strength, including phosphoric acid etching and the application of diverse primers prior to cementation [12].

In dentin, the use of strong acid etching (e.g., 35–37% phosphoric acid) is known to reduce bond strength. This occurs because viscous cements cannot fully penetrate the denuded collagen fibrils to achieve complete polymerization [13,14], which may also result in nano-leakage [15]. Furthermore, the bond strength of self-adhesive resin cements to dentin relies more on chemical adhesion than micromechanical interlocking [16]. This chemical bond is obtained through the chelation of acidic functional groups with calcium ions from the hydroxyapatite of the dental substrate [8,17]. Thus, removing residual hydroxyapatite from the collagen network via strong acid etching can have an adverse effect on bond strength [15,18]. Therefore, phosphoric acid pretreatment should be avoided in dentin [19]; it remains highly recommended for enamel [20]. In enamel, acid treatment removes the smear layer, demineralizes the surface, and increases the surface energy [20,21], allowing resin monomers to flow into the created microporosities and polymerize to form retentive resin tags.

For this reason, various types and concentrations of mild acid agents have been tested to condition the dentin surface, particularly polyacrylic acid (PAA) [22]. This acid is widely used in restorative dentistry, as it is a component of the liquid phase in glass ionomer cements (GICs) and also serves to prepare the dentin substrate [23]. It has consistently demonstrated satisfactory bond strength between GICs and dentin substrates [24]. Numerous studies indicate that using 10–25% PAA can partially remove the smear layer and create a layer rich in calcium and phosphate ions. This layer can form a chemical bond with phosphoric acid methacrylate in the resin cement, resulting in enhanced bond strength [25,26,27,28,29]. Notably, the bond strength of resin cement containing phosphoric acid methacrylate, such as glycerophosphate dimethacrylate (GPDM), may be improved when dentin is pretreated with PAA. Thus, the pretreatment of dentin with 10% PAA was included into our study.

Pretreating dentin with an adhesive is another common method used to enhance performance. El-Guindy et al. [30] suggested that the observed increase in bond strength following adhesive application may be attributed to the presence of acidic monomers. These monomers partially dissolve the smear layer and demineralize the dentin surface to create microporosities. Because these adhesives have a lower viscosity than self-adhesive resin cements, they exhibit superior flowability, allowing them to penetrate these pores effectively before polymerization. Furthermore, chemical bonding occurs between phosphate ester monomers and the calcium in hydroxyapatite, which further strengthens the interface.

Similarly, Atalay et al. compared the shear bond strength of two recently marketed universal self-adhesive resin cements, G-CEM ONE and RelyX universal, with and without their respective proprietary adhesive bonding agents. The study found that applying an adhesive agent prior to cementation improved the bond to the tooth substrate significantly [31].

To simulate intraoral conditions, thermocycling was employed as a commonly used artificial aging method for dental materials. This process followed the temperature range of 5–55 °C as defined in ISO/TS 11405:2015 [32]. ISO/TS 11405:2015 provides general guidance for adhesion testing but does not specify detailed thermocycling parameters for artificial aging protocols. Therefore, the present study used a thermocycling regimen of 5000 cycles between 5 °C and 55 °C, which is widely used to simulate thermal stresses encountered in the oral environment in in vitro adhesion studies.

Currently, no studies have investigated the effects of alternative dentin pretreatment modalities on the shear bond strength (SBS) between dentin and this specific self-adhesive resin cement (Maxcem elite chroma) under both thermocycled and non-thermocycled conditions. Furthermore, there is limited information regarding the effect of dentin pretreatment involving GPDM-containing adhesive systems and their corresponding self-adhesive resin cements.

GPDM is a functional acidic monomer incorporated in certain universal adhesive systems and resin cements. Its molecular structure contains phosphate groups capable of ionically interacting with calcium ions in dentin hydroxyapatite, while its methacrylate groups participate in copolymerization with the resin matrix. This dual functionality allows GPDM to act as a molecular link between dentin and resin, potentially contributing to both chemical bonding and micromechanical retention. The effectiveness of this interaction is influenced by the condition of the dentin surface and the availability of calcium ions following surface pretreatment [33].

For these reasons, this study evaluates whether dentin pretreatment with 10% PAA or universal adhesives improves the SBS of a self-adhesive resin cement. Identifying the most effective surface pretreatment is crucial for optimizing the clinical performance and longevity of adhesive restorations. By comparing these strategies, this study aims to provide a better understanding of their influence on the bonding performance of self-adhesive resin cements.

Therefore, the objective of this study was to evaluate the effects of alternative pretreatment modalities to enhance the dentin/self-adhesive resin cement (Maxcem elite chroma) by using 10% polyacrylic acid, Optibond universal adhesive, Scotchbond universal plus adhesive, or no treatment. There were two hypotheses in this study: The first null hypothesis stated that there was no significant difference in the SBS of dentin treated with a 10% polyacrylic acid solution, Optibond universal adhesive, Scotchbond universal plus adhesive, or no treatment before being luted with Maxcem elite chroma. Consequently, the first alternative hypothesis suggested that a significant difference did exist in the SBS of dentin under these identical treatment conditions. The second null hypothesis asserted that 5000 cycles of thermocycling did not significantly affect the SBS across the same pretreatment dentin conditions. The second alternative hypothesis indicated that 5000 cycles of thermocycling did significantly impact the SBS under the same pretreatment conditions.

To evaluate these hypotheses, SBS was measured using a standardized in vitro methodology under controlled conditions, and the data were statistically analyzed to determine differences among the treatment groups and to assess the effect of thermocycling on bond durability.

2. Materials and Methods

The experimental part of this study received ethical approval from the Human Research Ethics Committee of the Faculty of Dentistry, Chulalongkorn University (HREC-DCU), under research code 121/2023. The approval number is 135/2023, certified on 5 January 2024.

The materials utilized in this study are detailed in

Table 1. Materials used in this study.

Type	Name (Lot Number)	Manufacturer	Composition	Application Method
Self-adhesive resin cement	Maxcem elite® chroma (9651612)	Kerr, Orange, CA, USA	HEMA, GDMA, UDMA, TEGDMA, GPDM, TMBHP, fluoroaluminosilicate glass, barium glass filler, fumed silica (69 wt%)	1. Apply automixed cement onto the tooth substrate and position the resin composite rod with a constant load of 1000 g. 2. Cure with light for 20 s.
Polyacrylic acid	Dentin Conditioner (2303111)	GC Corporation, Tokyo, Japan	10% polyacrylic acid	1. Apply the acid to the tooth surface using the disposable applicator brush, employing agitating motion for 10 s. 2. Rinse thoroughly.
Universal adhesive	Optibond universal adhesive (9243902)	Kerr, Orange, CA, USA	GPDM, GDMA, HEMA water, acetone, ethanol, initiators	1. Apply the adhesive to the tooth surface using a disposable applicator brush with agitating motion for 20 s. 2. Air-thin for 5 s from a distance of 10 mm, until the adhesive no longer moves. 3. Light-cure for 20 s.
	Scotchbond universal plus adhesive	3M Oral Care, St. Paul, MN, USA (9446762)	Bis-GMA, 10-MDP, HEMA, Vitrebond copolymer, ethanol, water, initiators, fillers	1. Apply the adhesive on the tooth surface using the disposable applicator brush with agitating motion for 20 s. 2. Air-thin for 5 s from a distance of 10 mm until the adhesive no longer moves. 3. Light-cure for 20 s.

Abbreviations: HEMA, 2-Hydroxyethyl methacrylate; GDMA, Glycerol dimethacrylate; UDMA, urethane dimethacrylate; TEGDMA, triethylene glycol dimethacrylate; GPDM, Glycerol phosphate dimethacrylate; TMBHP, 1,1,3,3-Tetramethylbutyl Hydroperoxide; Bis-GMA, bisphenol A-glycidyl methacrylate; 10-MDP, methacryloyloxydecyl dihydrogen phosphate.

, which includes information about their composition and manufacturers. Additionally, Figure 1 and Figure 2 illustrate the experimental flowchart and the specimen preparation stage, respectively. All bonding procedures were performed by a single trained and calibrated operator to minimize operator-related variability.

2.1. Tooth Preparation

One hundred and sixty-eight extracted human third molars were collected for this study. The teeth were cleaned with running water. Any adherent tissue and debris were removed. The specimens were stored in 1% chloramine-T trihydrate solution for no more than 1 week and then were stored in distilled water (ISO 3696, grade 3) [34] at 4 °C in the refrigerator. The distilled water was changed every 2 months until the specimens were tested (ISO/TS 11405:2015).

The teeth were sectioned horizontally 1 mm below the central groove using a slow-speed saw (Isomet, Buehler Ltd. Lake Bluff, IL, USA) under water cooling. Then, the teeth were inspected under a light stereomicroscope at 40× magnification (Olympus SZ61, Olympus Optical Co., Tokyo, Japan) to ensure that they did not have any pulp cavity exposure, dental caries, or cracks. The sectioned teeth were embedded in self-cured acrylic resin in cylindrical polyvinylchloride rings 2 cm in diameter. Each specimen was polished with P120 silicon carbide paper (Wetordry abrasive sheet, Maplewood, 3M, MN, USA) under water coolant on a polishing machine (Nano 2000 grinder-polisher with a FEMTO 1000 polishing head, Pace Technologies, Tucson, AZ, USA), ending with P400 paper to obtain a flat dentin surface. The samples were then cleaned with spraying water for 10 s at a pressure of 0.27 MPa from a distance of approximately 10 mm using a triple syringe and were stored in a container with 100 percent humidity until use.

2.2. Scanning Electron Microscopy (SEM) Evaluation

Eight teeth were randomly selected for evaluation using SEM to examine various surface treatment conditions (n = 2 for each condition). The conditions tested included no surface treatment, the application of 10% polyacrylic followed by rinsing, the application of Optibond universal adhesive, and the application of Scotchbond universal plus adhesive. Detailed procedures for all treatments are outlined in Table 1. The treated specimens were stored in a clean plastic container until analysis, which was conducted using SEM at a magnification of 5000× to assess surface morphology (SEM, FEI Company, Versa 3D, Hillsboro, OR, USA).

2.3. Composite Specimen Preparation

A light-cured resin composite (Filtek™ Z350 XT, 3M ESPE, St Paul, MN, USA), shade A3, was used to prepare specimens by loading the composite resin into a silicone mold with an inner diameter of 3 mm and a height of 5 mm using an incremental technique. Each layer was light-activated for 40 s with a light-emitting diode (LED) light-curing unit (Demi, SDS Kerr Corporation, Orange, CA, USA) with a radiance of 1450 mW/cm². The air abrasion machine (Basic Quattro, Renfert, Hilzingen, Germany) blasted 50 μm aluminum oxide particles (Korox, Bego, Bremen, Germany) at the surface of one end of the resin composite rod for 15 s at a distance of 1 cm and a pressure of approximately 0.25 MPa. After that, the composite resin rods were cleaned in distilled water with an ultrasonic cleaner (Ultrasonic cleaner VI, Yoshida Dental Trade Distribution Co., Tokyo, Japan) for 10 min.

2.4. Sample Size Calculation

The sample size was calculated using the G*Power program (version 3.1.9.7, Heinrich-Heine Düsseldorf University, Düsseldorf, Germany). Based on the pilot investigation (n = 6 per group), a total of 145 samples were determined (effect size f = 0.35, α error probability = 0.05, power (1 − β error probability) = 0.95, numerator degrees of freedom = 3, number of groups = 8). Consequently, a minimum of 18.125 samples were required for each group. In this study, 20 samples per group were utilized to accommodate a 10% error margin, resulting in 160 samples allocated for SBS testing. Additionally, there were 2 samples designated for SEM evaluation for each surface treatment. Therefore, the total number of samples amounted to 168.

The components and the method of applying surface treatment agents used in this study are shown in Table 1. After the treatment process, self-adhesive resin cement (Maxcem elite chroma, Kerr Corporation, Orange, CA, USA) was applied through a hole in the adhesive tape and the composite rod was then placed onto the dentin surface with 1000 g constant load for controlling the cement thickness (according to ISO/TS 11405:2015). The curing light from a light-curing unit (Demi, Kerr Corporation, Orange, CA, USA) with a radiance of 1450 mW/cm² was shone on the cement for 2 s; then, the surrounding excess cement was removed, and subsequently, light was applied for 20 s in four different directions onto the resin–dentin interface for complete polymerization. After 24 h of storage in distilled water at 37 °C, a computer-generated random sequence was used to allocate samples into experimental groups, ensuring minimal selection bias.

2.5. Surface Treatment and Bonding Process

An adhesive tape with 80 µm thickness and a 2.38 mm hole was placed over the dentin surface of each specimen, and then the specimens were randomly divided into 8 groups based on the following dentin surface treatment protocols: Group 1 (N), no treatment + Maxcem elite chroma; Group 2 (NT), no treatment + Maxcem elite chroma + thermocycling; Group 3 (PAA), 10% polyacrylic acid and rinse + Maxcem elite chrom; Group 4 (PAAT), 10% polyacrylic acid and rinse + Maxcem elite chroma + thermocycling; Group 5 (OU), Optibond universal adhesive + Maxcem elite chroma; Group 6 (OUT), Optibond universal adhesive + Maxcem elite chroma + thermocycling; Group 7 (SUP), Scotchbond universal plus adhesive + Maxcem elite chroma and Group 8 (SUPT), Scotchbond universal plus adhesive + Maxcem elite chroma + thermocycling.

2.6. Artificial Aging by Thermocycling

Group 2 (NT), Group 4 (PAAT), Group 6 (OUT), and Group 8 (SUPT) were placed into a thermocycler unit (Thermocycler THE 1100/1200, SD-Mechatronik GMBH, Westerham, Germany). Specimens underwent 5000 cycles between 5 °C and 55 °C in water with a 30 s dwell time and a 10 s transfer time.

2.7. Shear Bond Strength Testing

The notched-edge SBS testing was chosen because a crosshead with a notched edge provides more accurate force distribution, leading to more precise measurements. This setup more accurately represents adhesive shear strength in comparison to peel strength, which is usually assessed using a crosshead with a straight edge. Additionally, notched-edge SBS testing can effectively simulate clinical stresses and is well-suited for self-adhesive resin cements, as it reduces the high rate of pre-test failures often seen with microtensile testing.

The notch edge SBS test was carried out using a universal testing machine (Universal testing machine; EZ-S 500N, Shimadzu corporation, Kyoto, Japan) with a notched-edge crosshead blade at a crosshead speed of 1 mm/min, following ISO 29022:2013 [35], until failure, and the bond strength data were calculated in MPa. Means and standard deviations were recorded for each group tested.

All procedures were performed under normal room-temperature conditions, and then each specimen was analyzed to identify the failure modes.

2.8. Failure Mode Analysis

The fracture areas were investigated using a 40× magnification stereomicroscope (SZ61, Olympus Corporation, Tokyo, Japan). The failure mode was modified from Matinlinna [36] and Klasiri [37], and was classified into 3 types after fracture. (a) Adhesive failure was identified when less than 40% of the resin cement was visible on the tooth’s surface. (b) Cohesive failure was identified when at least 60% of the resin cement was exposed on the tooth’s surface. (c) Mixed failure, encompassing both cohesive and adhesive failures, was identified when more than 40% but less than 60% of the resin cement was visible on the tooth surface. The percentage of area in failure mode was measured using ImageJ software, Version 1.54p (http://imagej.net/ij/download.html (accessed on 16 April 2025)). The area of resin cement on the tooth surface was measured twice by two independent operators. The mean of the measurement area was then divided by the total bonded surface area (πr², r = 2.38/2) and multiplied by 100. This calculation yielded a percentage that shows how much resin cement is still present on the tooth surface.

2.9. Statistical Analysis

IBM SPSS. V29.0.1 (SPSS Inc., Chicago, IL, USA) was used to evaluate the quantitative data from the eight separate groups at a 95% confidence level. Normality was assessed using the Shapiro–Wilk test (p < 0.05). The equality of variation was then examined using Levene’s test. The data had equal variance and a normal distribution, according to the findings. Therefore, the data were analyzed using a two-way ANOVA, and the differences between groups were then determined using a post hoc Bonferroni test (p < 0.05).

3. Result

3.1. SEM Evaluation

The SEM images at 5000× magnification demonstrated that the no-treatment group displayed elongated grooves from polishing with silicon carbide paper, accompanied by a smear layer on all surfaces. Conversely, the 10% polyacrylic acid group exhibited the elimination of the smear layer and the exposure of dentinal tubules on all surfaces. Both universal adhesive groups exhibited smooth surfaces coated with bonding agents (Figure 3).

3.2. Shear Bond Strength Values

The results of the two-way ANOVA analysis (

Table 2. Two-way ANOVA to investigate the effect of dentin pretreatments and thermocycling on shear bond strength of Maxcem elite chroma self-adhesive resin cement.

Source	Type III Sum of Squares	df	Mean Square	F	Sig.	Partial Eta Squared
Corrected Model	3845.908	7	549.415	1282.589	<0.001	0.983
Intercept	13,942.943	1	13,942.943	32,549.253	<0.001	0.995
Surface_treatment	3392.063	3	1130.688	2639.546	<0.001	0.981
Thermocycling	389.719	1	389.719	909.784	<0.001	0.857
Surface_treatment interaction Thermocycling	64.126	3	21.375	49.900	<0.001	0.496
Error	65.111	152	0.428
Total	17,853.962	160
Corrected Total	3911.020	159

) indicated that dentin pretreatments (factor 1) and thermocycling (factor 2) had a statistically significant impact on the SBS values (p < 0.001). Additionally, a statistically significant interaction between these two factors was detected (p < 0.001). Effect sizes were classified as small (η² = 0.01), medium (η² = 0.06), and large (η² = 0.14), per Cohen [38]. In this study, the two-way ANOVA revealed significant main effects for both the dentin pretreatment factor and the thermocycling factor, as well as a significant interaction between these factors. The dentin pretreatment factor demonstrated a statistically significant influence on SBS, F (3, 152) = 2639.546, p < 0.001, with a large effect size (η² = 0.981). The thermocycling factor also demonstrated a significant effect, F (1, 152) = 909.784, p < 0.001, resulting in a large effect size (η² = 0.857).

The mean shear bond strength values and modes of failure are presented in Table 3. The highest bond strength was recorded in the OU group (18.71 ± 0.43 MPa), while the lowest was noted in the PAAT group (2.69 ± 0.39 MPa).

In the non-thermocycling condition, the groups were ranked based on their shear bond strength (SBS) from highest to lowest, as follows: OU group (18.71 ± 0.43 MPa), SUP group (11.08 ± 0.69 MPa), N group (9.75 ± 0.73 MPa), and PAA group (4.04 ± 0.28 MPa). There were significant differences observed (p < 0.001). In the thermocycling condition, the groups were ranked by SBS from highest to lowest as follows: OUT group (13.78 ± 0.97 MPa), SUPT group (8.01 ± 0.87 MPa), NT group (6.61 ± 0.55 MPa), and PAAT group (2.69 ± 0.39 MPa). There were significant differences noted (p < 0.001). The SBS of the specimens significantly decreased (p < 0.001) after undergoing 5000 thermocycles under the same treatment condition. Moreover, the reduction in SBS after thermocycling of four treatment conditions (no treatment, 10% polyacrylic acid, Optibond universal adhesive, and Scotchbond universal plus adhesive) ranged from 26.35% to 33.42%.

Table 3. Mean shear bond strength along with the standard deviation (MPa) and percentages (%) of failure mode.

Group	Mean SBS ± SD (MPa)	SBS Reduction After Thermocycling (%)	Failure Mode Percentage (%)
			Adhesive	Mixed	Cohesive
Group 1: No treatment (N)	9.75 ± 0.73 A		16 (80)	4 (20)	0
Group 2: No treatment and thermocycling (NT)	6.61 ± 0.55 B	32.21%	15 (75)	5 (25)	0
Group 3: 10% polyacrylic acid and rinse (PAA)	4.04 ± 0.28 C		20 (100)	0	0
Group 4: 10% polyacrylic acid, rinse and thermocycling (PAAT)	2.69 ± 0.39 D	33.42%	20 (100)	0	0
Group 5: Optibond universal adhesive (OU)	18.71 ± 0.43 E		11 (55)	9 (45)	0
Group 6: Optibond universal adhesive and thermocycling (OUT)	13.78 ± 0.97 F	26.35%	13 (65)	7 (35)	0
Group 7: Scotchbond universal plus adhesive (SUP)	11.08 ± 0.69 G		17 (85)	3 (15)	0
Group 8: Scotchbond universal plus adhesive and thermocycling (SUPT)	8.01 ± 0.87 H	27.71%	16(80)	4 (20)	0

The different superscript letters indicate significant differences at p < 0.05.

Table 3. Mean shear bond strength along with the standard deviation (MPa) and percentages (%) of failure mode.

Group	Mean SBS ± SD (MPa)	SBS Reduction After Thermocycling (%)	Failure Mode Percentage (%)
			Adhesive	Mixed	Cohesive
Group 1: No treatment (N)	9.75 ± 0.73 A		16 (80)	4 (20)	0
Group 2: No treatment and thermocycling (NT)	6.61 ± 0.55 B	32.21%	15 (75)	5 (25)	0
Group 3: 10% polyacrylic acid and rinse (PAA)	4.04 ± 0.28 C		20 (100)	0	0
Group 4: 10% polyacrylic acid, rinse and thermocycling (PAAT)	2.69 ± 0.39 D	33.42%	20 (100)	0	0
Group 5: Optibond universal adhesive (OU)	18.71 ± 0.43 E		11 (55)	9 (45)	0
Group 6: Optibond universal adhesive and thermocycling (OUT)	13.78 ± 0.97 F	26.35%	13 (65)	7 (35)	0
Group 7: Scotchbond universal plus adhesive (SUP)	11.08 ± 0.69 G		17 (85)	3 (15)	0
Group 8: Scotchbond universal plus adhesive and thermocycling (SUPT)	8.01 ± 0.87 H	27.71%	16(80)	4 (20)	0

The different superscript letters indicate significant differences at p < 0.05.

3.3. Mode of Failure

In terms of failure mode, only adhesive failures occurred in the PAA and PAAT groups. Conversely, the other groups—the N group (80%), NT group (75%), OU group (55%), OUT group (65%), SUP group (85%), and SUPT group (80%)—showed adhesive failure, including a mix of failure types. All groups showed no evidence of cohesive failure in the resin cement (Figure 4).

4. Discussion

This research evaluates the effects of various pretreatment modalities and the process of thermocycling on the SBS between dentin and self-adhesive resin cement (Maxcem elite chroma). The results indicate that, regardless of thermocycling, the highest SBS was observed in the groups in which Optibond universal adhesive was applied to the dentin. Conversely, the lowest SBS was recorded in the group treated with 10% polyacrylic acid prior to luting with Maxcem elite chroma resin cement, with this difference being statistically significant (p < 0.001). Additionally, the SBS for all groups significantly decreased after undergoing thermocycling (p < 0.001).

Consequently, the first null hypothesis—that there was no difference in SBS when dentin was treated with 10% polyacrylic acid, Optibond universal adhesive, Scotchbond universal plus adhesive, or no treatment before luting with Maxcem elite chroma —was rejected. The second null hypothesis—asserting that thermocycling and non-thermocycling conditions did not significantly impact the SBS values among the same pretreatment dentin conditions—was also rejected. Furthermore, the two-way ANOVA analysis revealed that both dentin pretreatment and thermocycling exerted a large effect size, indicating that both factors are critical determinants of SBS.

In contemporary practice, restorative dentistry has become more popular. Beyond the restoration of severely damaged teeth, intact teeth are frequently treated to meet esthetic demands. The longevity of restorations depends on several factors, including the materials selected, the specific restorative procedure, patient-related parameters, the operator’s experience [39], moisture control techniques, and, most significantly, the adhesive protocols employed [40].

Self-adhesive resin cements (SARCs) were introduced in 2002 as a subgroup to address the demand for materials that are less technique-sensitive and more user-friendly. Due to the clinical advantage of a simplified, single-step application, these cements have become frequently used in dental practice. However, the bonding mechanism of this type of resin cement relies more on chemical bonding than on micromechanical retention due to their limited ability to create a significant hybrid layer [16,41,42].

In the present study, both the N and NT groups (no surface treatment groups) exhibited lower shear bond strength (SBS) values compared to the groups that utilized a universal adhesive (OU, OUT, SUP, and SUPT groups). This difference may be attributed to several factors: (a) The resin cements contain essential components such as multifunctional monomers, cross-linking agents, initiators, and fillers. The phosphate groups in the multifunctional monomers demineralize and infiltrate the dentin surface; however, this demineralization is only superficial, leading to a lower bond strength in comparison to total-etch and self-etch systems [17,43]. Although efforts were made to reduce the initial pH and prolong acidity, the resin cement became more hydrophilic, which ultimately hindered the polymerization process [29]. (b) These cements consist of over 60% filler material, resulting in high viscosity that limits their ability to penetrate dentinal tubules compared to adhesives [9,44]. (c) Upon mixing and exposure to moisture, the cement initially exhibits a low pH, which gradually shifts to neutrality due to a chelating reaction involving the phosphate groups in the acidic monomer, the fillers in the luting cement, and the calcium ions from hydroxyapatite in dental substrates [45]. This neutralization hinders the acidic monomers from effectively dissolving dentin minerals, further reducing bond strength [9]. (d) Moreover, residual water in the smear layer and intrinsic moisture on the tooth surface can compromise bond integrity; the absorption of water by hydrophilic components creates channels for nano-leakage and may inhibit the polymerization of monomers [46]. Additionally, in deep preparations, water blisters that form at the dentin–cement interface can soften the resin, weakening the overall bond strength.

To overcome the limitations of self-adhesive resin cement systems, various strategies have been employed to enhance bond strength [12]. These include pre-etching with an acid, applying an additional layer of adhesive, or utilizing other surface treatments [47]. Furthermore, even though self-adhesive resin cements are originally designed for single-step use, clinicians have increasingly adapted their use to various etching modes to suit the needs of different clinical situations [48]. Consequently, the present study was designed to evaluate the efficacy of these modified protocols.

Overall, applying a universal adhesive in self-etch mode before luting the restoration with a self-adhesive resin cement was found to increase bonding strength. This can be explained by three main factors: First, the adhesive partially dissolves the smear layer and some minerals on the dentin surface. Second, the viscosity of these adhesives is low, resulting in higher flowability, allowing them to penetrate deeply into dentinal tubules and pores, where polymerization then occurs. Third, the phosphate ester monomers within the universal adhesive react with the calcium hydroxyapatite in the tooth structure to create a stable chemical bond [49].

In this study, dentin surface treatment with adhesives was found to significantly increase shear bond strength (SBS), particularly in the groups pretreated with Optibond Universal (OU and OUT). These groups achieved the highest SBS values among all experimental groups both before and after 5000 thermocycles, outperforming those treated with Scotchbond universal plus adhesive. This enhancement may be due to the hydrophobicity of the cured Optibond universal adhesive layer that blocks dentinal fluid flow from the tubules to the resin cement layer. Furthermore, both Optibond universal adhesive and Maxcem elite chroma contain GPDM monomers, a functional monomer that chemically bonds to dentin calcium via its phosphate group while co-polymerizing with the resin matrix through its methacrylate groups, which could enhance SBS. These findings align with the study by Pheerarangsikul [50], which reported a significant increase in SBS when both the adhesive and the resin cement shared GPDM-based chemistries. This suggests that the synergistic effects of these monomers play a crucial role in improving bond strength, compared to the baseline values obtained with GPDM alone. Therefore, it is advisable for dental practitioners to consider using compatible products from the same manufacturer to optimize adhesive performance. Furthermore, Optibond universal adhesive is a ternary solvent system (water, ethanol, and acetone), which aids in the optimization of its evaporation. This characteristic may be critical, as efficient water removal could enhance the adhesive–resin cement bond strength by preventing hydrolysis, while inadequate evaporation may trap water, resulting in permeable films and weaker, less durable bonds.

For the groups treated with Scotchbond universal plus adhesive (SUP and SUPT), the results indicated lower SBS compared to the OU and OUT groups. Scotchbond universal plus adhesive is known to contain Vitrebond copolymer (VBCP), a proprietary polyalkenoic acid copolymer designed to mitigate the adverse effects of moisture fluctuations [51]. However, Muñoz et al. [52] proposed that VBCP may interfere with the interaction between 10-MDP and the tooth structure through several mechanisms. First, VBCP may compete with MDP for binding sites on hydroxyapatite, potentially reducing the sites available for MDP to establish a stable chemical bond. Second, the high molecular weight of the polyalkenoic acid copolymer could impede the effective approximation of monomers during polymerization, resulting in incomplete polymerization and a weaker bond. Consequently, the SBS in the SUP and SUPT groups was not as high as in the OU and OUT groups, suggesting that the chemical interactions between specific material components play a crucial role in determining overall bond strength.

Regarding dentin surface conditioning, PAA can partially remove the smear layer and create a calcium ion-rich surface. This alteration has the potential to form a chemical bond with the phosphoric acid methacrylate present in resin cement, which may enhance bond strength. This study found that etching dentin with 10% PAA (using an agitation technique for 10 s) resulted in significantly lower SBS compared to all other tested groups. While PAA conditioners typically remove the smear layer and establish a mild demineralization zone that promotes micromechanical retention—especially beneficial for glass ionomers—excessive removal of the smear layer and exposure of dentinal tubules (Figure 3) can pose problems for self-adhesive resin cements. This over-removal may lead to increased dentinal fluid flow. Moreover, Maxcem elite chroma contains hydrophilic monomers such as HEMA and GPDM, which are capable of absorbing more fluid and may interfere with cement polymerization [27,53]. Moreover, the high viscosity of Maxcem elite chroma could not penetrate into the dentinal tubules. These factors led to a lower SBS. Therefore, excessive removal of the smear layer can compromise the bond strength between the resin cement and the tooth structure. For future study, pretreatment of dentin with polyacrylic acid combined with a bonding agent should be performed.

Furthermore, the lower SBS suggests potential competition for calcium ions. Self-adhesive resin cements rely on acidic functional monomers to bond chemically with calcium in tooth hydroxyapatite; however, residual carboxylate groups from the PAA pretreatment may chelate these calcium ions first. This effectively “competes” with the cement’s monomers, reducing the available sites for the resin cement to form a stable chemical bond. These findings align with previous studies indicating that variations in SBS after dentin pretreatment are heavily influenced by the specific chemical compositions used by different manufacturers [52,53,54]. However, it should be noted that the proposed mechanisms involving calcium ion competition and the potential synergistic effect of GPDM are based on previously reported theoretical and chemical interactions. These mechanisms were not directly investigated in the present study and should therefore be interpreted as speculative explanations rather than definitive conclusions.

In cemented restorations, the immediate functionality of the cement is crucial as oral functions resume immediately after placement. The cement must provide sufficient retention and a reliable seal to prevent microleakage and ensure the restoration’s longevity. Furthermore, the mechanical properties of the cement must be strong enough to withstand forces generated during mastication and other oral activities. Therefore, in the present study, specimens were subjected to 24 h of immersion in water at 37 °C to simulate initial oral conditions, followed by 5000 thermal cycles to simulate aging.

For failure mode analysis, the PAA and PAAT groups exhibited exclusively adhesive failures, whereas the remaining groups showed a combination of adhesive and mixed failures. No cohesive failure within the resin cement was observed in any group. This finding suggests that the cohesive strength of the resin cement exceeded the adhesive bond strength at the dentin–cement interface. Clinically, this indicates that bond failure is more likely to occur at the interface rather than within the cement itself, emphasizing the critical role of dentin surface pretreatment in improving interfacial bonding effectiveness.

For both thermocycling and non-thermocycling conditions, the SBS values varied significantly across the different dentin surface treatments. The groups were ranked by SBS from highest to lowest, as follows: Optibond universal adhesive, Scotchbond universal plus adhesive, no treatment, and 10% polyacrylic acid, with significant differences noted between these groups. These findings indicate that the choice of surface treatment and adhesive significantly influences the bond strength to dentin. Specifically, Optibond universal adhesive provided superior bonding compared to other treatments, whereas the 10% polyacrylic acid treatment applied using the agitation technique weakened the bond when luting with Maxcem elite chroma.

However, the improved bond performance observed with certain universal adhesives should not be interpreted as an endorsement of specific manufacturers. Rather, these findings underscore the importance of chemical compatibility between functional monomers and dentin substrates, which may likewise be achieved by other adhesive systems with comparable compositions. Moreover, it should be noted that the present study was conducted under in vitro conditions; therefore, direct clinical extrapolation of the results should be made with caution, as factors such as intraoral humidity, occlusal loading, and long-term aging may influence bonding performance in vivo.

In conclusion, although thermocycling significantly reduces bond strength across all groups, dentin pretreatment with a compatible universal adhesive markedly improves the bond durability of self-adhesive resin cement, whereas 10% PAA pretreatment negatively impacts bonding effectiveness and is not recommended for clinical use.

It should be acknowledged that this study evaluated only one self-adhesive resin cement and included a universal adhesive produced by the same manufacturer. Consequently, the observed bonding performance may reflect material-specific compatibility. Furthermore, the use of a single resin cement brand and only two universal adhesives represent a limitation that may restrict the generalizability of the findings.

Moreover, potential operator-dependent variability associated with specimen preparation and bonding procedures, as well as the absence of formal intraobserver calibration for failure mode analysis, were not statistically evaluated; therefore, future investigations should incorporate comprehensive reliability analyses to improve reproducibility. In addition, further studies should examine adhesive–cement systems from different manufacturers, with particular emphasis on monomer compatibility and residual dentin surface acidity following polyacrylic acid conditioning, to provide deeper insight into factors influencing the bonding behavior of self-adhesive resin cements.

5. Conclusions

Based on the findings of this study, the following conclusions were drawn:

• The SBS of Maxcem elite chroma resin cement decreased following dentin conditioning with 10% polyacrylic acid.
• Within the tested materials, dentin pretreatment with Optibond universal adhesive consistently resulted in the highest SBS values when used with Maxcem elite chroma resin cement, regardless of thermocycling.
• Thermocycling (5000 cycles) significantly reduced SBS under identical dentin pretreatment conditions.
• For the specific resin cement and adhesive systems evaluated in this study, dentin pretreatment with a compatible universal adhesive demonstrated more favorable interfacial stability than polyacrylic acid pretreatment, which appeared insufficient for optimizing bonding performance.