Introduction
Carious dentin is composed of two layers, namely an outer soft layer of caries-infected dentin, where active caries progression occurs, and an inner relatively hard layer of caries-affected dentin (CAD), which is free of bacteria and has the potential for remineralization [1,2]. According to the principles of minimally invasive dentistry and selective caries removal, the goal is to eliminate the highly infected and denatured superficial layer while preserving the deeper, remineralizable CAD [3-5]. This approach is especially crucial in deep lesions near the pulp, where it helps reduce the risk of pulpal exposure and allows the CAD to recover and remineralize over time [6]. However, the long-term success of this strategy depends on the effective sealing of the remaining carious tissue. Adequate sealing prevents nutrient supply to residual bacteria, thereby arresting further progression of the lesion [7].
CAD, which is slightly softened but bacteria-free, exhibits morphological, chemical, and physical properties that differ significantly from sound dentin. The reduced mineral content and denaturation of organic components in CAD can compromise bonding strength and durability. In addition, incomplete infiltration of adhesive monomers and the formation of irregular, defective hybrid layers can further reduce bonding performance [8]. Consequently, the development of a reliable adhesive system for CAD has emerged as a key factor in improving the clinical success of dental treatments.
Scotchbond Universal Plus (SBP; 3M ESPE, St. Paul, MN, USA) is a recently introduced universal adhesive that can be applied in total-etch, selective-etch, and self-etch modes. It contains a radiopaque resin that is completely free of bisphenol A-glycidyl methacrylate (Bis-GMA) and bisphenol A (BPA) derivatives, offering radiopacity similar to dentin while maintaining low viscosity for improved flowability and handling. According to the manufacturer, SBP provides equally high bond strength to both CAD and sound dentin, with high marginal adaptation and a void-free hybrid layer, thereby effectively sealing the CAD [9]. However, limited studies have directly evaluated the bonding performance of SBP specifically on CAD, which underscores the need for further investigation.
The aim of this study was to evaluate the bonding performance of SBP to CAD. In particular, this study sought to compare the shear bond strength (SBS) between CAD and sound dentin using the self-etch mode.
Materials and Methods
1. Shear bond strength measurement
1) Preparation of bovine tooth specimens
Extracted sound bovine mandibular incisors stored frozen at below -20°C for less than one month were used. After thorough surface cleaning, the roots were removed using a high-speed handpiece. The teeth were then stored in 0.1% thymol solution, and any teeth with caries, fractures, or cracks were excluded from the study.
Each tooth was embedded in resin within a plastic mold measuring 2.0 cm in inner diameter and 2.0 cm in height, ensuring that the labial surface was exposed (Fig. 1). To prevent heat generation during embedding and to maintain dentin moisture, the embedded specimens were immediately covered with gauze moistened with distilled water and subsequently stored in cold distilled water after initial setting. Dentin surfaces were exposed using a precision cutting machine (Accutom-50, Struers, Ballerup, Denmark), and then polished sequentially using LaboPol-5 (Struers) and 600, 1200, 2000, and 4000 grit silicone carbide paper (R&B Inc., Daejeon, South Korea) at 200 rpm for 60 seconds per grit. All polishing procedures were performed under water cooling, and the standardized protocol ensured uniform surface smoothness and consistency [10].
2) Group classification
Forty specimens were randomly divided into four groups (n = 10) based on the dentin substrate condition and the type of adhesive used, as summarized in Table 1. The tested adhesives were SBP and Single Bond Universal (SBU; 3M ESPE), both applied in self-etch mode. The self-etch mode was selected because it minimizes addi-tional demineralization and collagen collapse in CAD, thereby preserving the remineralizable dentin matrix and reducing technique sensitivity in clinical practice [8]. In addition, it was chosen to standardize the adhesive application protocol and eliminate variability associated with separate etching and rinsing steps, improving the reproducibility of bonding to CAD. Twenty specimens were demineralized using a pH-cycling model to create artificial CAD, while the remaining teeth were kept sound and stored in distilled water during the demineralization process. Subsequent bonding and restoration procedures were then carried out.
3) Artificial caries induction
To induce artificial carious lesions, a demineralizing solution (2.2 mM CaCl2, 2.2 mM NaH2PO4, 0.05 M acetic acid, pH = 4.5) and a remineralizing solution (1.5 mM CaCl2, 0.9 mM NaH2PO4, 0.15 mM KCl, pH = 7.0) were prepared [11,12]. Each specimen was immersed in the demineralizing solution for 8 hours and in the remineralizing solution for 16 hours. Between solution changes, specimens were rinsed with distilled water. Solutions were freshly prepared for each cycle and monitored with a pH meter. The cycling was performed for 14 days.
4) Adhesive application and composite resin filling
All adhesive and resin composite applications were performed by a single operator in accordance with the manufacturers’ instructions. The adhesive application protocols are summarized in Table 2. A cylindrical metal mold (2.0 mm in diameter, 3.0 mm in height) was fixed onto the dentin surface for each group. The composite resin used was Filtek™ Z-350 (3M ESPE) in A1 shade, and it was cured with Demi™ Plus (Kerr, Orange, CA, USA) at a light intensity of 1,200 mW/cm². The resin was placed into the cylindrical metal mold in two increments of 1.5 mm each, and each increment was light-cured for 20 seconds, following the manufacturer’s instructions. All specimens were stored in distilled water at 37°C for 24 hours prior to SBS testing.
5) Shear bond strength testing
SBS was measured using a universal testing machine (Quro, Yangju, Korea). Each specimen was positioned such that the bonding interface between dentin and composite was aligned parallel to the long axis of the testing device. A crosshead speed of 1 mm/min was applied, and the maximum load at debonding was recorded and converted into SBS (MPa) based on the bonded surface area (Table 3) [13].
6) Statistical analysis
The normality of the SBS data was assessed using the Shapiro-Wilk test. As the data did not follow a normal distribution, non-parametric statistical methods were applied. The Kruskal-Wallis test was used to compare SBS among the groups, followed by Bonferroni correction for multiple comparisons. A Bonferroni-adjusted significance level of p < 0.0083 was applied (0.05/6 comparisons). Statistical analysis was performed using SPSS version 27.0 (SPSS Inc., Chicago, IL, USA).
2. Observation of resin tags and quantitative analysis
Two randomly selected specimens from each group were sectioned perpendicular to the tooth long axis using a diamond disk. The exposed surfaces were etched with 37% phosphoric acid for 1 minute, rinsed with water, and dried for one week. The dried specimens were gold-palladium coated using a sputter coater (Hitachi, Marunouchi, Japan), and examined under a scanning electron microscope (SEM; Hitachi) at 10.0 kV at 1000× and 2000× magnifications.
To quantitatively evaluate the resin-dentin interface, the number and length of resin tags, as well as the thickness of the adhesive and hybrid layers, were measured using ImageJ software (National Institutes of Health, Bethesda, MD, USA). All measurements were performed on SEM images at ×1,000 magnification. The length of resin tags was calculated by measuring all clearly distinguishable tags in the image and averaging the values. The number of resin tags was determined by directly counting all distinct tags within the same image. The thicknesses of the adhesive and hybrid layers were measured using the software’s linear measurement tool.
Results
No statistically significant difference in SBS was observed between SBP and SBU on sound dentin (Table 3, Fig. 2, p= 0.621). However, on artificial CAD, SBP showed significantly higher SBS compared to SBU (Table 3, Fig. 2, p < 0.0083). Regardless of the adhesive used, the SBS to sound dentin was significantly higher than that to CAD (Table 3, Fig. 2). A Bonferroni-adjusted significance level of p < 0.0083 was used for multiple comparisons.
Representative SEM images (at 1000× and 2000× magnifications) of the resin-dentin interfaces in all experimental groups are shown in Fig. 3 and Fig. 4. On sound dentin, a greater number and length of resin tags was observed compared to CAD, regardless of the adhesive used. In CAD, SBP exhibited more and longer resin tags than SBU. Additionally, SBP presented a thinner adhesive layer than SBU, regardless of the dentin substrate.
Quantitative evaluation results of the SEM images are presented in Table 4. On sound dentin, both adhesives showed similar resin tag lengths and hybrid layer thickness. However, SBP exhibited a thinner adhesive layer compared to SBU. In CAD, SBP showed longer resin tags and a greater number of tags than SBU, along with a thinner adhesive layer. These findings corroborate the enhanced infiltration capacity of SBP.
Discussion
This study aimed to compare the bonding performance of two adhesives, SBP and SBU, to CAD and sound dentin. The results demonstrated that both adhesives showed relatively higher bond strength to sound dentin than to CAD, and SBP exhibited slightly superior performance compared to SBU on CAD. These findings indicate that bond strength may vary depending on the condition of the dentin substrate and the composition of the adhesive.
Previous studies comparing the bond strength of commercial adhesives to sound dentin have reported inconsistent results. Al-Obaidi and Jsim [14] reported that SBU exhibited higher bond strength than SBP, whereas the study by Alam et al. [15] showed comparable performance between the two adhesives on sound dentin. In the present study, no statistically significant difference was observed between SBP and SBU on sound dentin. This may be attributed to the presence of shared chemical components in both adhesives, such as the functional monomer 10-MDP and polyalkenoic acid copolymer, as well as their similar pH values and identical application modes [9].
The reduced bond strength observed on CAD is primarily attributed to structural degradation and incomplete resin infiltration [16]. CAD is characterized by decreased mineral content and denatured collagen structure, resulting in reduced stability. In addition, occlusion of the dentinal tubules by mineral deposits limits the formation of resin tags [8]. Denatured collagen impedes the infiltration of resin monomers, thereby weakening the adhesive bond. Furthermore, the high moisture content of CAD hinders monomer infiltration, making hybrid layer formation difficult and reducing bonding performance [8,16].
In this study, CAD was simulated using a chemical method, which created a superficially demineralized layer that mimics mineral loss caused by caries [11]. To generate the artificial CAD substrate, the dentin specimens underwent 14 cycles of an 8-hour demineralization process followed by a 16-hour remineralization process. This protocol required a significantly longer remineralization duration compared to conventional models. While the demineralization phase reduced the overall mineral content of the dentin, the subsequent remineralization phase promoted mineral deposition within the dentinal tubules, which impeded resin tag formation [2,12,17]. It has been demonstrated that artificially created CAD exhibits similar bond strength to natural CAD and can form hybrid layers with porous structures [11,12,18].
Various efforts have been made to enhance bond strength to CAD. One study reported that a 15-second pretreatment with sodium hypochlorite effectively dissolved denatured organic material in CAD, thereby significantly improving the microtensile bond strength of self-etch adhesives to this substrate [19]. According to Arrais et al. [20], additional and prolonged acid etching significantly increased the microtensile bond strength to CAD. The use of exogenous cross-linkers, such as flavonoids, as a dentin pretreatment before adhesive application, has been suggested as a promising strategy to enhance the durability of universal adhesive systems applied to CAD [21]. Moreover, modified phosphoric acid containing grape seed extract has been shown to improve bonding characteristics by promoting biomodification during etching and reducing MMP activity within the hybrid layer, offering a potential approach to enhance bonding performance to CAD [22]. However, studies demonstrating improvements in bond strength to CAD using adhesives alone remain limited.
SBP utilizes a BPA-free, radiopaque resin in place of Bis-GMA, which is present in SBU, resulting in a more flowable formulation [9]. Bis-GMA is a high-molecularweight monomer with low volatility and polymerization shrinkage, rapid curing, and two hydrophobic functional groups. Despite its advantages, its high viscosity and low reactivity necessitate the use of diluent monomers [23,24]. Replacing the highly viscous Bis-GMA appears to reduce the viscosity of SBP. According to the study by Alam et al. [15], the viscosity of SBP was reported to be less than half that of SBU, and the adhesive layer of SBP was approximately twice as thin as that of SBU. In this study, SEM images and quantitative measurements also revealed that SBP consistently exhibited a thinner adhesive layer than SBU, regardless of the dentin substrate, suggesting its lower viscosity. Lowviscosity adhesives can spread more evenly and flow more easily, which likely accounts for the thin adhesive layer observed in SBP. The low viscosity of SBP may have facilitated its infiltration into the dentin, as evidenced by the presence of more numerous and longer resin tags in CAD compared to SBU, as seen in the SEM images.
Studies have shown that the infiltration ability of resin monomers and the uniformity of the hybrid layer significantly influence the bond strength of dentin adhesives [25]. Adequate monomer infiltration is essential for the formation of a stable resin-dentin interface, contributing to strong micromechanical interlocking and enhanced durability. Low-viscosity adhesives enable monomers to penetrate more deeply into the dentinal tubules and collagen network, thereby reinforcing micromechanical retention and forming a more robust hybrid layer. Such infiltration is critical for establishing a strong bond with dentin, ultimately improving bond strength and durability. Improved flowability of the adhesive resin helps eliminate voids and blisters under pressure and promotes the compaction of resin into the dentinal tubules, facilitating the formation of resin tags [26,27].
Previous studies have suggested that a shear bond strength above 17 - 20 MPa is considered clinically acceptable for resin-dentin adhesion [28,29]. In the present study, all groups exhibited bond strength values exceeding this threshold, indicating that the bonding performance of both adhesives, particularly SBP on CAD, may be sufficient for clinical application.
However, this study was conducted in an in vitro environment, and the chemically induced CAD used in the experiment cannot fully replicate the complex biochemical and physical conditions of the oral cavity [12,30]. Chemical methods of caries induction are not standardized in terms of the degree of demineralization or the duration of demineralization-remineralization cycles, making them subject to controversy. Moreover, they fail to reproduce various biochemical changes that occur during natural caries progression, such as activation of bacterial-derived enzymes and modification of dentinal amino acids [12,30-32]. Another limitation is that this study did not incorporate thermocycling, and therefore was unable to evaluate the long-term durability of the adhesives under intraoral temperature fluctuations. Thermocycling may influence long-term bonding performance by promoting microcrack formation due to differences in the coefficients of thermal expansion at the adhesive interface, as well as water absorption and hydrolytic degradation. Thus, future studies should include thermocycling protocols to assess the durability of adhesive systems more comprehensively [33,34]. Lastly, while the standardized smear layer preparation used in this study aimed to enhance the reproducibility of the experimental substrate, it should be noted that the characteristics of smear layers can vary considerably in clinical settings, warranting cautious interpretation of the results [31,35].
Conclusion
This study compared the bonding performance of SBP and SBU on sound dentin and artificial CAD. The results demonstrated that SBP and SBU showed comparable SBS on sound dentin. However, on artificially created CAD, SBP exhibited significantly higher bond strength than SBU. This finding may be attributed to the lower viscosity of SBP, which enhances its infiltration into dentinal tubules and contributes to the formation of a more uniform hybrid layer. Notably, SBP employs a radiopaque resin without Bis-GMA and BPA, which likely contributes to its lower viscosity, improved spreading behavior, and optimized surface contact, thereby enabling the formation of a thinner and more stable adhesive layer. This facilitates deeper penetration of resin monomers into dentinal tubules, enhancing resin tag formation and hybridization, and ultimately improving bond durability.
The findings of this study suggest that SBP offers relatively reliable bonding performance to CAD and has potential for improving the durability of adhesion to CAD. However, considering the limitations of in vitro studies, future clinical investigations using naturally CAD are necessary to validate the long-term durability and clinical effectiveness of SBP.
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