Introduction

Adhesive dentistry has transformed restorative procedures, highlighting minimally invasive techniques that preserve tooth structure. While direct composite resin restorations offer aesthetic and conservative options, their long-term success depends on achieving a durable bond to dentin, a substrate more challenging than enamel due to
its composition and structure [1].

Dental adhesive systems have evolved throughout several generations, and have been broadly categorized as total-etch and self-etch methods [2]. Total-etch systems typically involve a separate etching stage using phosphoric acid, followed by rinsing, priming, and bonding. Two-step total-etch systems combine a primer and adhesive [3]. Self-etch systems incorporate acidic monomers in a primer or adhesive, simplifying the procedure by combining etching and priming steps, and often bonding as well [4]. All procedures are combined into a single application in one-step self-etch adhesive systems [5]. Universal adhesives are a newer category designed for use with total-etch, self-etch, or selective-etch strategies [6]. However, the effectiveness of these diverse adhesive systems is significantly challenged by the varying conditions of dentin substrate, particularly when dealing with carious lesions. This necessitates a detailed understanding of how adhesives perform on compromised dentin [7].

Affected dentin (an inside layer of carious area, some bacterial content, minor modifications in collagen fibrils, and potential for remineralization) must maintain in deep cavities to prevent pulp exposure, while infected dentin (the outermost layer of carious area, distinguished by the existence of bacteria, demineralized dentin, and denatured collagen fibrils) must be removed [8].

Current management of caries often involves selective removal, leaving caries-affected dentin (CAD) in the cavity to preserve pulpal health [9]. However, CAD presents structural and compositional alterations compared to sound dentin (SD), including decreased mineral content, augmented porosity, and alterations in collagen matrix [10]. These changes can negatively impact the success of adhesive systems [11].

The quality of dentin bonding is influenced by the type of adhesive, caries removal method, and properties of the remaining dentin [12]. Techniques for creating in vitro carious lesions in dentin have been developed to investigate adhesion in lab-created clinical situations. Accord­ing to investigations, bond strengths in artificial dentin afflicted by caries are comparable to those in CAD [13].

Artificial caries models, such as pH cycling or bacterial culture, are valuable in vitro tools for assessing adhesive performance under controlled conditions, mimicking aspects of natural caries progression [14]. Simulating different depths or severities of lesions in artificial models can provide insights into how adhesive performance is affected by the extent of demineralization [15]. The pH-cycling technique offers the benefit of mimicking a dynamic carious process where demineralization is greater than remineralization; however, creating standardized natural carious lesions for in vitro studies is a complicated task [16].

It could be expected that the bond strength in CAD is decreased by total-etch adhesives due to their tendency to cause deeper dentin demineralization, thicker hybrid layers, and incompletely impregnated regions under the hybrid layer [17]. However, the self-etch adhesives were not able to dissolve and eliminate acid-resistant mineral deposits within the dentinal tubules of CAD due to their higher pH.

Objectives

The current study compared the shear bond strength (SBS) of a two-step total-etch (Single Bond 2) and one-step self-etch (Scotchbond Universal) adhesive systems applied to sound and CAD. The null hypotheses were: (1) the SBS of SD would be significantly greater than artificial CAD in both tested adhesive systems; and
(2) the bond strength would not differ significantly between the two assessed adhesive systems used on SD and CAD.

Material and methods

This study used a two-step total-etch (Single Bond 2)
and one-step self-etch (Scotchbond Universal) adhe­sives. Nanofilled resin composite (Filtek™ Z35XT) served as the restorative material, and 34% phosphoric acid etchant (Scotchbond Universal Etchant) was employed for the total-etch procedure. The materials used in the current research are shown in Table 1.

Sample size calculation

G*Power 3.1.9.2 software was employed to calculate the necessary sample size. Based on earlier findings from Nicoloso et al. [17], and given 80% power, an effect size of 47.91, and assumption of a two-sided 5% significant threshold for comparisons, a sample size of 25 teeth per subgroup was calculated and employed in this study.

Grouping

Based on dentin condition (i.e., sound or caries-affect­ed), 100 molars with exposed coronal dentin surface were allotted into two main experimental groups (n = 50). Then, these groups were subdivided into two subgroups (n = 25 per subgroup), with a total of four subgroups, according to the type of adhesive as follows: Subgroup 1 (SB-SD): Single Bond 2 applied to SD; Subgroup 2 (SB-CAD): Single Bond 2 applied to caries-
affected dentin; Subgroup 3 (SU-SD): Scotchbond Universal applied to SD; Subgroup 4 (SU-CAD): Scotchbond Universal applied to CAD.

Specimen preparation

One hundred freshly extracted human molars were obtained and sanitized with 0.5% chloramine solution. Inclusion criteria were: (1) intact teeth; (2) non-caries teeth; (3) permanent teeth with complete root formation; (4) teeth free from any developmental and formative anomalies.
Exclusion criteria were: (1) carious teeth; (2) presence of previous restorations, fractures, or cracks; (3) prior endo­dontic treatment; (4) presence of non-carious lesions (attrition, fluorosis).

Soft tissue was removed from teeth with an ultrasonic scaler, and samples were inserted in self-curing acrylic resin up to the cemento-enamel junction. Using a slow-speed diamond saw (Isomet, Buehler) under water irrigation, their occlusal surfaces were cut perpendicular to their long axis to produce smooth dentin surfaces (Figure 1). The prepared dentin surfaces were then polished with a 600-grit silicon carbide paper to create a uniform smear layer. Teeth were divided into two primary groups (n = 50 each), i.e., SD and CAD.

Creation of artificial caries-affected dentin

Caries-like lesions were produced according to a pH cycling protocol of Nicoloso et al. study [17]. Each cycle consisted of 8 hours in a demineralizing solution (2.2 mM CaCl₂, 2.2 mM NaH₂PO₄, 0.05 M acetic acid, pH 4.5) and 16 hours in a remineralizing solution
(1.5 mM CaCl₂, 0.9 mM NaH₂PO₄, 0.15 mM KCl,
pH 7.0). Following each cycle, solutions were replaced, and specimens were washed with deionized water and subsequently blotted dry. This procedure was followed daily for two weeks; the solutions were refreshed every day and pH was verified via a calibrated pH meter [17].

Adhesive application

Based on adhesive system, every primary group was subdivided into two subgroups (n = 25 each). Two dental adhesive systems were utilized in this study, including one-step self-etch adhesive (Scotchbond Universal) and two-step total-etch adhesive (Single Bond 2).
Adhesives were applied according to the manufactu­rer’s guidelines.

Scotchbond Universal

Applied using a self-etch approach, it was exposed to active rubbing for 20 seconds, 5 seconds of air thinning, and 10 seconds of light curing with a LED curing device (Bluephase G2, Ivoclar Vivadent; 1000 mW/cm²).

Single Bond 2

The dentin surface was subjected to acid etching using 34% phosphoric acid (Scotchbond Universal Etchant) for 15 seconds, then the etched dentin surface was rinsed with water for 10 seconds and blotted dry. The adhesive was applied by a disposable microbrush in two coatings with vigorous agitation for 15 seconds, then subjected to air thinning and 10 seconds of light curing.

Composite build-up

A nanofilled resin composite (Filtek Z350 XT, 3M ESPE) was applied using cylindrical Teflon molds (internal diameter: 3 mm; height: 4 mm), positioned perpendicular to the prepared dentin surface in each sample. Composite was packed in two 2-mm layers; every layer was light-cured for 40 seconds using LED curing device. Molds were carefully removed after being cured to prevent interfacial stress. Every sample was kept in distilled water at ambient temperature for 24 hours to permit maximum degree of conversion [18].

Shear bond strength testing

A universal testing equipment (Instron 3345, Norwood, MA, USA) was used to mount the samples. A notched-edge shear blade was aligned at the dentin–composite interface, and loaded at a crosshead speed of 1 mm/min till debonding occurred. The debonding force (Ff) divided by the sample’s cross-sectional area (n), yielded the SBS needed to produce debonding [19]:

SBS = Ff/n.

Statistical analysis

Shapiro-Wilk test was employed to confirm normality assumption with α = 0.05. Homogeneity of variances was evaluated using Bartlett’s test. A two-way analysis of variance (ANOVA) was done to examine primary impacts of adhesive type (Single Bond 2 vs. Scotchbond Universal) and dentin condition (SD vs. CAD) as well as their interaction effect on SBS. At α = 0.05, the significance threshold was established. With signi­ficant main effects detected, post-hoc comparisons were performed using independent t-tests to identify speci­fic group variations. Statistical analyses were done using SPSS version 21 (Statistics Statistical Procedures Companion, Chicago, IL, USA).

Results

Effect of dentin condition

Post-hoc analysis using independent t-tests revealed that specimens bonding to SD exhibited significantly greater SBS than those bonding to CAD in the two adhesive systems (Table 2 and Figure 2). For Single Bond 2,
the variation was statistically significant (t = 23.45, p < 0.0001), with SD showing a mean bond strength of 56.01 ± 6.36 MPa compared to 21.93 ± 3.19 MPa
for CAD. Similarly, for Scotchbond Universal, SD demonstrated significantly higher bond strength (46.55 ± 3.77 MPa) compared to CAD (17.25 ± 2.25 MPa) (t = 32.67, p < 0.0001).

Effect of adhesive type

Independent t-test comparisons demonstrated that Single Bond 2 (total-etch) consistently outperformed Scotchbond Universal (self-etch) in both substrate types (Table 3 and Figure 3). On SD, Single Bond 2 achieved significantly higher bond strength (56.01 ± 6.36 MPa) compared to Scotchbond Universal (46.55 ± 3.77 MPa)
(t = 6.26, p < 0.0001). On CAD, Single Bond 2 also showed superior performance (21.93 ± 3.19 MPa) compared to Scotchbond Universal (17.25 ± 2.25 MPa) (t = 5.87, p < 0.0001).

Interaction effect

Two-way ANOVA test revealed significant main impacts among two independent variables and their interaction (Table 4). The main impact of adhesive type on SBS was statistically significant (F = 1377.85, p < 0.0001), indicating that the type of adhesive system significantly
influenced bonding performance. Similarly, dentin condition showed a significant main effect (F = 68.56, p < 0.0001), demonstrating that the substrate condition significantly affected bond strength.

Most importantly, the type of adhesive and dentin condition interacted significantly (F = 7.83, p = 0.0063), indicating that the impact of adhesive type on SBS depends on the dentin substrate condition.

Discussion

Since dentin is still thought to be the weakest link in tooth adhesion, the main aim of contemporary dental adhesive techniques is simple and less complicated bonding [20]. In clinical practice, CAD is recognized to be a more common substrate for bonding. The effectiveness of adhesives applied to CAD may be adversely affected by morphological modifications of the caries process [11].

As the composition and structure of dentin carious lesions can impact the mechanical characteristics of tooth structure and the features of adhesive system, caries modeling is crucial to investigate a number of clinical dental difficulties [14]. Additionally, to lower the likelihood of pulpal exposure and post-operative pulpal discomfort, residual caries remains under restorations after selective (incomplete) caries removal [21].

To achieve standardized artificially created CAD for bond strength testing, the present study used artificial caries fabricated by pH cycling. Studies have demonstrated that while morphological variations exist between artificially created and natural CAD, their bond strength characteristics are comparable. This allows the utilization of laboratory designs in an effort to overcome the significant variation of natural CAD [21, 22]. Particularly, pH cycling mimics the processes of mineral loss and gain, which occur during the oral cavity’s caries cycle via alternating phases of demineralization and remineralization [15].

In this investigation, the SBS approach was used. In a shear bond test, an adhesive is used to bind two materials, and shear stresses are applied until mate­rials are separated. The SBS was determined by dividing the bonded cross-sectional area by the maximum force applied. The test is simple and does not require specialized tools [23]. The current research assessed the effects of dentin condition (sound vs. CAD) and adhesive system (total-etch vs. self-etch) on the SBS.

The findings of the present study revealed that the SBS to SD was substantially higher than the SBS to CAD, irrespective of the type of adhesive used. Therefore, the first part of the null hypothesis was accepted. This confirms that demineralization adversely affects adhesive performance. This can be attributed to a multitude of inherent differences in this substrate compared to SD. One significant factor is the lower mineral content and altered mineral phase of CAD, which provides fewer available bonding sites for adhesive monomers and a less structurally sound substrate for micromechanical retention [24]. The mineral crystals in CAD often have bigger crystalline spaces and broader inter-crystalline spaces, which are dispersed and randomly arranged, thus hindering uniform infiltration of resin. For adhesive bonding to be successful, healthy dentin must have the maximum mineral concentration and undamaged collagen [25].

Furthermore, the collagen network in CAD undergoes degradation and alteration, affecting its ability for proper infiltration by and interaction with resin monomers. While the amino acid composition might not be significantly different, the collagen cross-links can be reduced, leading to a weakened network [26]. The pre­sence of a transparent layer under the carious lesion, characterized by the occlusion of dentinal tubules with mineral deposits, also plays a crucial role in reducing bond strength [12]. These intratubular mineral deposits, often composed of acid-resistant crystals, interfere with resin tag formation, a key component of micromechanical interlocking, and reduce the overall permeability of the dentin [27].

In addition, CAD exhibits an increased water content and decreased permeability compared to SD. This higher water volume resulting from mineral loss can compete with the penetration of hydrophobic resin monomers, while the occluded tubules reduce the diffusion of adhesive components into the dentin [8]. Moreover, the smear layer on CAD is often thicker and more irregular, which can impede resin infiltration, particularly for self-etch adhesives that rely on acidic monomers to penetrate this layer [28].

A meta-analysis have indicated that SD exhibited higher bond strengths than CAD for both total-etch and self-etch adhesive systems [29]. These outcomes are in line with Hass et al. [30], who stated that the bond strength of total-etch and self-etch adhesive systems was reduced on CAD compared to SD.

On the other hand, Drobac et al. [31] found that none of the tested adhesives (one-step self-etch and
total-etch) demonstrated a statistically significant variation in SBS between SD and CAD. These variations may be due to differences in methodology, as they used different materials.

The current findings revealed that Single Bond 2
(a two-step total-etch adhesive) produced higher SBS on the sound and CAD in comparison to Scotchbond Universal (a one-step self-etch adhesive). Hence, the second part of the null hypothesis was rejected. This finding agrees with prior research conducted by Drobac et al. [31], showing that total-etch systems generated a greater bond strength on the sound and CAD, owing to more efficient removal of the smear layer and better resin tag formation [31]. A systematic review by Isolan et al. [29] also highlighted that total-etch adhesives
generally achieve higher bond strengths to the sound and CAD compared to self-etch system.

The superior performance of total-etch adhesive systems on CAD is attributed to their capacity to remove smear layers and mineral deposits, which are resistant to acid by phosphoric acid pretreatment, leading to improved resin infiltration and hybrid layer development [32]. Also etching allows resin to enter healthy dentin more deeply, resulting in prolonged resin tags and abundant hybrid layer that enhance micromechanical retention and contribute to higher SBS of total-etch adhesives [33].

In contrast, self-etch adhesives often fail to completely penetrate the deeper and more porous demine­ralized zone of CAD. Their milder acidity is insufficient to eliminate the dense, organic-rich smear layer on CAD, which acts as a barrier to effective infiltration of monomer [34]. Furthermore, self-etch systems exhibit limited ability to dissolve mineral casts within dentinal tubules, resulting in inadequate resin tag development and ultimately lower bond strengths. This has been explained by the acidic monomers’ low pH in relation to 37% phosphoric acid [35]. However, Nicoloso et al. [17] demonstrated no statistically significant variation in dentin bond strength between Scotchbond Universal (in self-etch mode) and Adper Single Bond 2 (total-etch) on artificially created caries dentin. These variations may be due to differences in methodology used, as they employed different artificial caries model.

The significant interaction between adhesive stra­tegy and dentin condition indicates that the bonding effectiveness of each adhesive system is highly substrate-dependent. While Single Bond 2 consistently outperformed Scotchbond Universal on both sound and CAD, the performance gap between the two adhesives narrowed on CAD, as also noted in a study by Aziz and Mahmoud [36]. The statistically significant interaction observed between the adhesive type and dentin condition suggests that the performance of each system is
substrate-dependent [36]. However, the interaction effect was significant, implying that the performance of each adhesive was not uniform across dentin types. It may be attributed to the incomplete infiltration of monomers in CAD, especially for universal adhesives with milder acidity as well as to the complex mixture of ingredients in one-bottle systems, which may compromise their efficacy compared to more structured multistep systems [37].

The limitations of the present study include its laboratory conditions, which cannot fully replicate the complex oral environment, as this could affect bond longe­vity and clinical performance. The pH cycling protocol used to create artificial CAD may not perfectly replicate the characteristics of naturally occurring CAD that can vary significantly. The study evaluated SBS following
24-hour water storage, but it did not consider the potential long-term deterioration of adhesive interface due to hydrolytic or enzymatic processes.

Future research should aim at addressing these limi­tations, for instance, by incorporating long-term aging protocols, using natural CAD, and evaluating bond strength with different testing methodologies in conditions more closely simulating the clinical environment.

Conclusions

According to our results, we can conclude that the bonding to CAD has a negative impact on the SBS compared to SD in the tested adhesive systems. The two-step total-etch adhesive generally provided superior SBS on both sound and CAD conditions in comparison to the one-step self-etch adhesive. Clinicians should consi­der substrate specifications when selecting bonding protocols for restorative procedures.

Disclosures

Author contributions: Conceptualization: R.A.; Metho­dology: R.A.; Questionnaire adaptation: Not applicable; Investigation and data collection: A.G.B.K.; Formal analysis: S.E.N.; Data curation: A.G.B.K., S.E.N; Writing of original draft: H.O.H.; Writing – review and editing: H.O.H., I.E.B.; Supervision: R.A., I.E.B.; Project admini­stration: A.G.B.K. All authors have read and agreed to the published version of the manuscript.

Funding: This research received no external funding.

Institutional Review Board statement: Not applicable.

Informed consent statement: Not applicable.

Data availability statement: The data presented in this study are available within the article. Additional information is available from the corresponding author and can be provided upon reasonable request.

Acknowledgments: None.

Conflicts of interest: The authors declare no conflicts of interest.

AI use statement: No artificial intelligence tools were used in the preparation of this manuscript.

References