Comparative assessment of the resin-dentin interface using three different universal adhesives: An ex vivo study

Abstract

Background:

Universal self-etch adhesives are increasingly used in restorative dentistry, but comparative evidence about their penetration and interface quality in dentin under simulated clinical conditions is limited.

Objectives:

To evaluate and compare the resin-dentin interface, depth of penetration, and hybrid layer thickness using three universal self-etch adhesives Single Bond Universal, Tetric N Bond Universal, and Solare Universal Bond under simulated pulpal pressure.

Materials and Methods:

Forty-five caries-free premolars, extracted for orthodontic reasons, were randomly assigned to three groups. Standardized Class I cavities (1 mm depth into dentin) were restored using each adhesive (15 teeth/group) and corresponding composites. Adhesives were mixed with 0.1% rhodamine B, applied according to manufacturer instructions, and light-cured in two layers. After extraction and storage, specimens were sectioned and examined under confocal laser scanning microscopy for hybrid layer thickness and resin tag penetration. Data were analyzed with ANOVA and post hoc Tukey tests (P < 0.05).

Results:

Tetric N Bond Universal showed the highest mean resin tag penetration (697.6 ± 130.0 µm) and hybrid layer thickness (528.7 ± 117.2 µm), statistically different from Single Bond Universal (634.8 ± 71.7 µm tag, 478.3 ± 85.5 µm hybrid layer) and Solare Universal Bond (644.4 ± 92.9 µm tag, 512.7 ± 105.5 µm hybrid layer). No significant difference was found between Single Bond Universal and Solare Universal Bond. All groups demonstrated clinically acceptable interfaces with well-formed hybrid layers and resin tag penetration.

Conclusion:

Among the tested adhesives, Tetric N Bond Universal exhibited superior hybrid layer thickness and resin tag penetration. This may relate to its specific chemical composition and hydrophilic-hydrophobic monomer balance. Further research with larger samples and additional performance outcomes is warranted.

INTRODUCTION

The advancement of dental adhesive technology has fundamentally transformed restorative dentistry, particularly in the ability to reliably bond composite resins to dentin. The hybrid layer is a resin monomers infiltrated collagen scaffold. It establishes the basis of dentin adhesion, which is the micromechanical adhesion. Its ideal thickness and even infiltration is vital to close tubules, counteract the stresses of polymerization, and eliminate hydrolytic degradation with time. Different etching plans such as total etch, self-etch, and selective etch have the ability to vary the depth of the hybrid layer through the control of demineralization. Properly hybridized layers are associated with high durability and reduced leakage rates. Dentin bonding remains a complex challenge due to the substrate’s heterogeneous composition, high organic and water content, and the persistence of the smear layer following cavity preparation, all of which can interfere with adhesive penetration and long-term bond integrity.[1] Dental adhesive systems are generally categorized into two types: Etch-and-rinse and self-etch adhesives. Etch-and-rinse adhesive systems require the preliminary application of phosphoric acid. These systems are available in either three-step versions comprising separate etching, priming, and bonding stages or two-step versions, where the primer and adhesive are combined into a single component. In such total-etch approaches, hybrid layer formation depends on superficial dentin demineralization by the acid, which exposes a collagen matrix subsequently infiltrated by hydrophilic resin monomers. Self-etch adhesives involve two steps or a single step, depending on whether the acidic primer and adhesive resin are provided separately or combined in one solution. These systems simplify the bonding procedure, reduce application time, and minimize technique sensitivity compared with traditional multi-step etch-and-rinse adhesives.[2]

The introduction of universal self-etch adhesives represents a significant milestone, offering clinicians the versatility to use both etch-and-rinse and self-etch modes with simplified application protocols.[3,4] These adhesives are formulated with functional monomers like 10-methacryloyloxydecyl dihydrogen phosphate (MDP), allowing simultaneous micromechanical interlocking and chemical bonding, which supports effective adhesion even in challenging clinical scenarios.[5] In addition to the hybrid layer, resin tags, which protrude into dentinal tubules, improve retention by blocking fluid pathways and redistributing occlusal loads. Adequate tag length enhances sealing and reduces the risk of sensitivity. They also contribute to the increase in the time of restoration, preventing bacterial invasion and water uptake at the interface.[6]

Despite these advancements, distinct variations in composition including hydrophilicity, solvent content, and monomer chemistry can influence each product’s clinical performance and durability over time. For example, recent developments like 2-hydroxyethyl methacrylate (HEMA) - free adhesives contribute to improved hydrophobicity, potentially reducing water sorption and enhancing durability.[7]

This study was designed to evaluate and compare the interfacial characteristics specifically hybrid layer thickness and resin tag penetration created by three prominent universal adhesives under the simulated clinical conditions. Single Bond Universal (3M ESPE), Tetric N-Bond Universal (Ivoclar Vivadent), and Solare Bond Universal (GC Corporation) were selected due to their popularity and the limited comparative data on their resin-dentin interfacial characteristics under ex vivo conditions.

Single Bond Universal (marketed as Scotchbond Universal) was the first commercially available universal adhesive and remains widely used among clinicians, incorporating 10- MDP and a methacrylate-modified polyalkenoic acid copolymer. Tetric N-Bond Universal features a balanced resin matrix of hydrophilic, hydrophobic, and amphiphilic monomers. Solare Universal Bond, a newer acetone-based system, contains two active monomers 4-methacryloxyethyl trimellitate anhydride (4-MET) and a phosphoric acid monomer.

No previous research has directly compared these three universal adhesives within such ex vivo models, limited literature data are available on Solare Bond Universal.

Innovations in microscope technology have improved adhesive dentistry research. Traditionally, researchers examine the resin-dentin interface using optical microscopes and scanning electron microscopy. The confocal microscope is distinguished by capturing thin optical slices from laser-penetrated surfaces of whole samples, avoiding distortions from manual cutting and providing greater clarity of the resin-dentin bond.[8,9]

This study utilized confocal laser scanning microscopy (CLSM) for nondestructive quantification of hybrid layer thickness and resin tag penetration under simulated pulpal pressure and vital pulp conditions, which has been used previously only in a few studies on adhesive systems.[10,11]

MATERIALS AND METHODS

The study protocol was approved by the Institutional Ethical Committee and institutional research board (IEC reg No: ECR/269/indt/TG/2016).

Forty-five intact, caries free human premolars (age 15–25 years), scheduled for orthodontic extraction, were selected. Tooth vitality was confirmed using pulp vitality tests for all the selected teeth. Patients and their parents received information in their native language about the procedures, potential discomfort associated with restorations, and assurance that the study would not alter their orthodontic treatment plan. Written informed consent was obtained from all the participants.

On the day of the procedure, teeth were scaled and polished. Under rubber dam isolation, standardized Class I cavities (1 mm into dentin) were prepared on the occlusal surfaces of the teeth by a single operator.

Sample size calculation

The sample size of 15 teeth per group (total 45) was determined a prior using G*Power software (version 3.1.9.7, Heinrich Heine University Düsseldorf, North Rhine-Westphalia, Germany), assuming a medium effect size of 0.35 (from prior adhesive studies), α = 0.05, power = 0.80–0.90, and one-way ANOVA for three groups. This ensures adequate statistical power to detect the differences in hybrid layer thickness and resin tag penetration while aligning with ex vivo dental research standards.

Group allocation

Teeth were randomly divided into three groups equally (n = 15 each):

• Group I: Single Bond Universal (3M ESPE)

• Group II: Tetric N Bond Universal (Ivoclar Vivadent)

• Group III: Solare Universal Bond (GC Corporation).

Restorative protocol

To produce strong fluorescence for visualizing bonding agent distribution at the resin-dentin interface under CLSM, Rhodamine B dye (0.01% wt) was mixed with 5 mL of each universal adhesive in the study groups, following the protocol by Bim Júnior et al.[12]

Cavities were gently blot-dried. Bonding agents were applied using a scrubbing motion with microbrushes for 20 s, followed by air thinning of excess material. These were light cured for 30 s using a Bluephase LED unit, after which a second layer of bonding agent was similarly applied, excess removed by air, and composite incrementally placed to restore the cavities. The restorations were then subjected to finishing and polishing with burs. Teeth were subsequently extracted without trauma and stored in distilled water until they were sectioned.

Sectioning and confocal laser scanning microscopy analysis

Each tooth specimen’s crown was sectioned vertically to the cementoenamel junction in the buccolingual plane, aligned parallel to the tooth’s long axis, using a low-speed, water cooled diamond saw to prevent thermal damage and ensure precise cuts suitable for CLSM. Subsequently, the sections were polished sequentially with abrasive discs of decreasing grit sizes to achieve an optically smooth surface with minimal roughness, optimizing clarity for CLSM imaging of resin dentin interfaces [Figure 1].

Figure 1.

Sampling and preparation workflow for universal adhesive application and confocal laser scanning microscopy analysis. (a) Group I specimens (Single Bond Universal), (b) Group II specimens (Tetric N-Bond Universal), (c) Group III specimens (Solare Universal Bond), (d-f) sectioning of samples and polishing for confocal microscopy evaluation

The specimens were dehydrated through a graded ethanol series, beginning with 25% for 20 min, followed by 50% for 20 min, 75% for 20 min, 95% for 30 min, and finally 100% for 60 min. The processed samples were then examined using a confocal scanning electron microscope (Zeiss LSM T-PMT) equipped with an oil immersion objective and operated with a 543 nm laser source specific for Rhodamine B.

The system configuration was standardized and maintained at identical settings throughout the entire investigation. Each resin-dentine interface was examined comprehensively, and five optical images were randomly captured in fluorescent mode. Resin tags were selected based on their clarity, depth, and continuous extension from the surface into the tooth structure [Figure 2].

Figure 2.

Confocal laser scanning microscopy images showing rhodamine B–labeled resin penetration along the resin–dentin interface for Single Bond Universal (left), Tetric N Bond Universal (middle), and Solare Universal Bond (right). Yellow arrows indicate representative sites where resin tag penetration depth (µm) and angle (°) to the dentin interface were measured

These photomicrographs were examined and thickness of the hybrid layer and length of resin tag measured under Image Browser software (zeiss, Germany) in micrometers. The findings were tabulated and mean derived.

Statistical analysis

The mean values per group were compared using the one-way ANOVA and post hoc Tukey HSD tests, with significance at P < 0.05.

RESULTS

The statistical analysis demonstrated significant intergroup differences in both resin tag penetration and hybrid layer thickness. Post hoc comparisons indicated that Tetric N Bond Universal (Group II) exhibited significantly higher values for both parameters compared to the other two adhesive systems (P < 0.05) [Tables 1 and 2]. No statistically significant difference was observed between Single Bond Universal and Solare Universal Bond, although solar bond universal exhibited superior mean resin tag penetration and hybrid layer thickness [Figure 3].

Table 1.

Mean comparison of depth of penetration of resin tag among different groups

	n	Mean	SD	SE	95% CI for mean		Minimum	Maximum	ANOVA
					Lower bound	Upper bound
Group I	15	634.7853	71.65290	18.50070	595.1053	674.4654	506.39	758.30	F=1.677 P=0.036
Group II	15	697.5707	130.02962	33.57350	625.5627	769.5787	505.76	965.33
Group III	15	644.4167	92.90113	23.98697	592.9697	695.8636	470.91	843.58
Total	45	658.9242	102.65952	15.30358	628.0819	689.7666	470.91	965.33

SD: Standard deviation, SE: Standard error, CI: Confidence interval

Table 2.

Mean comparison of thickness hybrid layer among different groups

	n	Mean	SD	SE	95% CI for mean		Minimum	Maximum	ANOVA
					Lower bound	Upper bound
Group I	15	478.3633	85.45881	22.06537	431.0378	525.6888	402.47	696.10	F=0.926 P=0.042
Group II	15	528.7227	117.22186	30.26655	463.8074	593.6380	419.07	883.31
Group III	15	512.7360	105.53063	27.24789	454.2951	571.1769	414.75	726.79

SD: Standard deviation, SE: Standard error, CI: Confidence interval

Figure 3.

Mean comparison of depth of penetration of resin tag and hybrid layer among different groups

DISCUSSION

In this study, premolars scheduled for orthodontic extraction were collected from the individuals aged between 15 and 25 years. With increasing age, dentinal thickness tends to increase, whereas the density of odontoblasts and pulp fibroblasts decreases. Arola and Reprogel[13] reported that aging leads to a higher rate of both damage initiation and propagation within dentin, accompanied by a reduction in dentinal tubule diameter, which consequently decreases dentin permeability. Variations in dentin substrate such as inter-tooth differences and age-related changes in tubule diameter and mineralization can influence the depth of material penetration. To minimize these variables, teeth from patients within the 15–25-year age range were selected for the present study.

Noncarious premolars were selected for this study because caries causes the pulp to either completely block dentinal tubule lumens or reduce their diameter, helping prevent bacteria and toxins from reaching the pulp. Hybrid layers formed in caries-affected dentin are typically thicker yet more porous compared to those in sound dentin. Yoshiyama et al.[14] reported that hybrid layers measure <1 µm in thickness in normal dentin but range from 6 to 8 µm in caries-affected dentin.

Clinically, most dental restorations are carried out under local anesthesia, which can influence dentin permeability. Under normal conditions, a slight positive pulpal pressure causes outward fluid movement through exposed dentin. According to Pitt Ford et al.,[15] 2% lidocaine without a vasoconstrictor does not reduce pulpal blood flow, thereby decreasing intrapulpal pressure and minimizing outward fluid flow. Hence, in the present study, local anesthesia was not administered until the composite restoration was completed.

Tetric N Bond universal showed the thickest hybrid layer and the longest resin tags compared to all other groups.

Kulkarni et al.[16] evaluated Single bond universal and Tetric N-Bond Universal adhesive systems. Single bond universal has consistently shown lower bond strength values as compared to Tetric N-Bond Universal.

Tetric N Bond Universal demonstrated superior performance in hybrid layer formation and resin tag penetration compared to the other adhesives tested.[17] This enhanced performance can be attributed to its well-balanced composition of hydrophilic and hydrophobic monomers, the presence of high-quality MDP, and an optimized solvent system. Effective resin penetration is crucial for ensuring long-term bond durability and preventing microleakage.[18] While all three adhesives created clinically acceptable interfaces, the distinct advantages of Tetric N Bond Universal suggest it may provide greater durability in clinical environments challenged by vital pulp conditions and physiological pulpal pressure. Similar comparative in vitro studies, such as that by Jayasheel et al.[19] have also reported higher bond strengths with certain universal adhesives, supporting the potential clinical advantages of these materials.

Solare Bond Universal performed slightly better than Single Bond Universal, but inferior to Tetric N Bond.

Solare Bond Universal, an acetone-based adhesive with a vapor pressure of 184 mm Hg at 20°C, may enable superior penetration of its acid monomer compared to ethanol-based Single Bond Universal. Acetone may also lower the surface tension of water, promoting even spreading across moist surfaces, and act as an effective “water chaser” by displacing residual moisture while transporting the resin primer until equilibrium is reached. It has been suggested that acetone may excel at delivering resin into demineralized tooth surfaces due to this water-displacing property, as evidenced by Jacobsen and Söderholm,[20] Gwinnett and Matsui,[21] and Kanca,[22] which could explain its potentially enhanced bonding agent penetration over Single Bond Universal.

HEMA-containing universal adhesives typically enhance dentin wettability and monomer diffusion, leading to thicker hybrid layers (often 4–6 μm) and more extensive resin tag formation with lateral branches compared to HEMA-free systems, which produce thinner interfaces (around 0.8–1 μm) but offer better long-term stability due to reduced water uptake and collagen degradation. In the present study, Tetric N-Bond Universal, which combines containing HEMA with Bis-GMA and D3MA in a balanced monomer matrix outperformed both Solare Bond Universal (HEMA-free, relying on 4-MET, MDP, and MDTP for hydrophobic bonding) and Single Bond Universal (HEMA-containing with Vitrebond copolymer). This study’s results challenge the general trend, since the HEMA-inclusive Single Bond showed inferior performance likely due to excessive water retention, phase separation, and hydrolysis risks despite its hybrid layer visibility being akin to that of earlier adhesives. This discrepancy highlights that factors, beyond HEMA content such as optimized monomer ratios, filler distribution, application mode, and dentin moisture control may drive superior shear bond strength and durability, with Solare’s design providing commendable wettability and stability despite thinner tags, underscoring the need for formulation balance over a simplistic focus on HEMA presence.[23]

It is important to note that acetone based adhesives like Solare Universal Bond require careful handling due to their volatility and limited shelf life.

The pH, solvent chemistry, and the particular functional monomers and their proportions in dental adhesives are the significant factors of adhesives. Optimal requirements of surface moisture needed to obtain a good dentin bonding depend on the type of solvent and the quantity of water used; the research studies show that acetone type of solvents require more accurate wet-bonding than ethanol solvents. Due to the volatility of acetone, even when the acetone is used under wet or dry conditions, high dentin bond strengths may be obtained through surface agitation. Single Bond Universal also employs solvent mixture of ethanol and water at the optimum level to reduce the viscosity to increase wetting and penetration of the solution on the moist and dry dentin and its Vitrebond copolymer increases its resistance to degradation due to humidity to ensure consistency in performance.[23,24,25]

Study limitations include the relatively small sample size and use of ex vivo models only, highlighting the need for future research on nanoleakage, bond strength, and aging effects to better understand long term performance.

CONCLUSION

Tetric N Bond Universal demonstrated the highest depth of penetration and hybrid layer thickness among the tested universal adhesives. Its improved performance suggests a clinical advantage in restorative procedures requiring durable dentin bonds. Nevertheless, all three adhesives resulted in satisfactory interface quality under simulated pulpal pressure.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.