Introduction

In recent years, notable advancements have been made in dental adhesive systems. However, the adhesive interface remains the most vulnerable aspect of tooth-composite restorations [1, 2]. This has created a grow­ing demand for simplified adhesives with enhanced performance, prompting the emergence of universal adhesives [3]. When applied to conditioned dentin, the osmotic pressure is believed to drive interstitial fluid from the dentinal tubules toward the adhesive interface. Moisture infiltration into adhesives is difficult to avoid, with considerable uncertainty surrounding the extent and effect of moisture penetration on bonding quality [4]. Moisture contamination, whether from saliva, blood, or water during bonding, has been shown to compromise adhesive performance, reducing resin tag formation and depth due to decreased resin penetration into dentinal porosities [5-7].

Newly developed universal adhesives exhibit both micromechanical and chemical bonding potential, particularly those incorporating 10-MDP and M-TEG-P monomers, offering better hydrolytic stability than traditional adhesives. However, clinical data on their long-term efficacy remain limited [8]. The longevity of composite restorations is directly linked to the strength and integrity of adhesive bond [9-11].

Moisture control poses significant clinical challenges, especially in posterior teeth or difficult to isolate regions [12]. Heterogeneous wetting of the dentin surface can lead to unpredictable bonding, affecting both etch-and-rinse and self-etch adhesive performance [13]. Over-wet dentin impedes adhesive polymerization and collapses the collagen matrix, limiting resin infiltration into demineralized dentin [13].

Equally important is the long-term integrity of hybrid layer. Degradation due to enzymatic activity and water sorption reduces bond strength over time [14]. As universal adhesives vary in composition and recommended application protocols, their long-term behavior under different moisture conditions warrants further investigation [15].

Thermocycling simulates intraoral temperature variations, which affect bond strength due to differences in thermal expansion between adhesive and tooth structures [16-18]. With temperatures in the oral cavity ranging from 0°C (e.g., ice cream) to 60°C (e.g., hot food), adhesives should be evaluated both after 24-hour water storage and post thermocycling [19].

During function, compressive, tensile, and shear forces are transferred from the enamel to dentin. In restored teeth, these forces are distributed more complexly to the adhesive interface, where high bond strength is critical to resist failure [17]. Measuring shear bond strength (SBS) remains a primary tool for assessing the performance of adhesive systems [10]. Additionally, adhesive remnant index (ARI) is used to quantify residual adhesive following debonding, providing insights into failure modes.

Therefore, this study aimed to investigate the bond strength of universal adhesives under moisture and dry conditions. The null hypothesis was that thermocycling and moisture variation would not significantly affect SBS or ARI outcomes.

Objectives

This in vitro study aimed to evaluate the SBS and ARI scores of two universal adhesives, including Yamakin Aquabond and One Coat Bond, under varying dentin moisture conditions and after artificial aging using thermocycling.

Material and methods

Materials used in the study were as follows: (1) Yamakin TMR-Aquabond0 (Yamakin Co., Japan; Lot 132421); (2) One Coat Bond 7 Universal (Coltene Whaledent, Switzerland; Lot N04496); (3) Composite DX Universal (GC Kalore, Japan; Lot BJBDJF); and (4) LED curing unit (Apoza Led 10W).

Sample size calculation

Based on prior studies [20, 21], with 80% power and a 0.05 a level, a sample size of 7 per subgroup was calculated. Adding 15% to compensate for potential sample loss, 64 teeth were allocated across 8 subgroups (n = 8 per group).

Study setting and design

This in vitro study was conducted in the Operative Laboratory of the Faculty of Dentistry, Al-Azhar University, Assiut Branch, Egypt.

Sixty-four freshly extracted maxillary premolars were collected, stored in 0.5% chloramine-T at 4°C for one week, then placed in distilled water until further use [3]. A randomized, double-blind design was employed, in which one investigator performed the random allocation of samples, and another investigator, blinded to the group assignments, carried out the outcome assessment independently.

Inclusion and exclusion criteria, and group allocation

Inclusion criteria [22]: Extracted human maxillary premolars with intact, sound enamel, and dentin surfaces.

Exclusion criteria [22]: Teeth with caries or restorations, structural enamel defects or hypoplasia, cracks, fluorosis, abfractions, demineralization, and developmental anomalies (e.g., amelogenesis imperfecta).

Specimens were divided into two main adhesive groups, with 32 samples in each group: Groups G1-G4: Yamakin, and groups G5-G8: One Coat Bond. Each group was subdivided by: Moisture condition: dry vs. moist (n = 16 each), and aging condition: immediate vs. thermocycled (n = 8 each).

Subgroups were classified as follow: G1: Yamakin/dry/immediate; G2: Yamakin/dry/thermocycled; G3: Yamakin/wet/immediate; G4: Yamakin/wet/thermocycled; G5: One Coat/dry/immediate; G6: One Coat/dry/thermocycled; G7: One Coat/wet/immediate; G8: One Coat/wet/thermocycled.

Ethics approval and consent to participate

Ethical approval for this study was obtained from the Institutional Ethics Committee of Al-Azhar University Assiut Branch, Egypt (approval number: AUAREC20250004-13). Informed consent was acquired from all subjects involved in the study. As the study included extracted teeth, verbal consent was considered sufficient and was explicitly approved by the Ethics Committee. All procedures were conducted in strict accordance with the ethical principles outlined in the Declaration of Helsinki, which emphasizes the respect for human tissue, privacy, and responsible use of biological specimens [22].

Sample preparation

Occlusal enamel was removed with a water-cooled diamond saw to expose a flat superficial dentin surface, standardized at a depth of approximately 1.0 ± 0.2 mm from the dentin-enamel junction (DEJ). Samples were mounted in PVC tubes with acrylic resin. Surfaces were polished with a 600-grit silicon carbide paper under running water to obtain a smooth and uniform dentin surface suitable for bonding procedures. Samples with visible gaps or air bubbles were excluded [23].

Bonding protocol

Two universal adhesives were used in their corresponding groups: Yamakin TMR AQUABOND0 (Yamakin Co., Japan; Lot 132421) and One Coat Bond 7 Universal (Coltene Whaledent, Switzerland; Lot N04496). For dry bonding condition, the adhesive was applied with micro brush for 20 seconds, airdried for 2-5 seconds, and then light cured (10 s, 1400 mW/cm²). Regarding moist bonding condition, a total of 2.5 µl of distilled water was applied to the dentin surface using a calibrated micropipette, and gently spread with a sterile micro brush to create a uniform, standardized moist dentin condition prior to bonding procedure [23].

Composite build-up

Composite DX Universal (Kalore GC, Japan; Lot BJBDJF) was placed in 2 mm increments using a Teflon mold (3 mm diameter × 2 mm height), and then light cured with LED curing unit (Apoza Led 10W) for 10 seconds [24].

Aging protocol

Immediate group (G1, G3, G5, and G7) samples were stored in deionized water for 24 hours before testing. Using an SD mechatronic thermocycler (Germany), aged groups (G2, G4, G6, and G8) underwent 10,000 thermocycles (5-55°C), with 30-second immersions and 10-second stay times [25].

Measurements of SBS and failure mode

Specimens were measured for SBS using a universal testing machine [23]. Specimen-containing acrylic block was fastened to the lower fixed head of Instron type 3345 universal testing apparatus. The testing machine’s top movable head was equipped with a uni-beveled chisel with a 0.5 mm wide blade. The chisel blade was positioned as near to the composite/dentin interface as feasible, and crosshead speed was set at 1.0 mm/min until the specimen failed. BlueHill 3 (Instron, England) machine software was used to determine the SBS in MPa, dividing the force required for failure (Newton) by the surface area (mm²), as follows [20]:

SBS (MPa) = Load (N)/Area (mm²)

Failure mode was analyzed using XCAM 1080P8MPB stereo microscope ToupTek Photonics (Zhejiang, China), and scored with ARI index. The ARI score was determined according to Artun and Bergland as follows [26]: 0 = no adhesive left on the tooth; 1 = less than half of the adhesive left on the tooth; 2 = more than half of the adhesive left on the tooth; 3 = all adhesive left on the tooth.

Statistical analysis

SPSS v. 22.0 was applied for statistical analysis, while SBS was evaluated using one-way ANOVA and Tukey’s post hoc tests. ARI scores were assessed with Mann-Whitney U test, and statistical significance was set at p < 0.05.

Results

The one-way ANOVA analysis demonstrated a statistically significant difference in the SBS among groups (p < 0.005, η² = 0.861), whereas post hoc analysis revealed as follows:

• G7 subgroup (One Coat Bond/wet/immediate) recorded the highest mean SBS (17.2 ± 1.0 MPa), and was significantly higher than that in all other groups, assigned the superscript “a”.

• G5 subgroup (One Coat Bond/dry/immediate) showed a slightly lower SBS (15.6 ± 0.9 MPa), statistically simi­lar to G8 but significantly higher than all other groups (superscript “ab”).

• G6 subgroup (One Coat Bond/dry/thermocycled) revealed a lower SBS (14.2 ± 1.5 MPa), still signifi­cantly higher than the other groups, except B1 (superscript “b”).

• G8 subgroup (One Coat Bond/wet/thermocycled) showed moderate values (10.5 ± 1.0 MPa), statisti­cally different from top-performing groups (superscript “c”).

• G1 (Yamakin/dry/immediate) and G3 (Yamakin/wet/immediate) had comparable values (6.6 ± 0.7 MPa and 6.3 ± 0.6 MPa, respectively), significantly lower than One Coat Bond groups (superscript “c”).

• G2 (Yamakin/dry/thermocycled) and G4 (Yamakin/wet/thermocycled) had the lowest SBS values (3.9 ± 0.6 MPa and 3.7 ± 0.5 MPa), and both had statistically the lowest values (superscript “d”) (Table 1, Figure 1).

Failure mode results

Failure mode results were evaluated using the ARI and consistent with the SBS findings. Groups that exhi­bited higher SBS values, particularly the One Coat moist bonding group, showed a greater incidence of mixed and cohesive failures, reflecting improved resin infiltration and a more durable adhesive interface. In this group, most specimens recorded ARI scores of 2, indicating partial adhesive remnants on both tooth and bracket surfaces, while some reached score 3, denoting cohesive failures within the resin. In contrast, the dry and thermo­cycled groups, especially the thermocycled Yamakin groups, predominantly demonstrated adhesive failures with ARI scores of 0 or 1, where little-to-no adhesive remained on the tooth surface. This pattern signified weak interfacial bonding and degraded performance following aging. Overall, the ARI findings further corroborated the relative effectiveness of each adhesive system, highlighting that systems with higher SBS scores not only provided stronger bonding but also demonstrated more reliable and cohesive failure patterns under stress.

Discussion

Universal adhesives became popular during the past decade due to their feature of easing bonding procedure with wetness level of dentin [14].

In the present study, all experimental groups showed bond strength values over the clinical limit suggested by Reynolds [27]. Two universal adhesives were selected due to their clinical popularity, differing solvent, and functional compositions (i.e., 10-MDP and M-TEG-P), which influence bonding performance [20]. The results showed that the SBS values of One Coat Bond were significantly higher than those of Yamakin Aquabond under all conditions. Wet conditions of dentin significantly increased bond strength among various groups, and the maximum mean SBS value was obtained for One Coat Bond/wet/immediate subgroup (17.2 MPa). On the other hand, decreased SBS recorded for dry bonding, especially when thermocycling, for Yamakin Aquabond was similar to the over-drying phenomenon. Thermocycling reduced bond strength of both adhesive systems, while Yamakin Aquabond demonstrated a greater decrease in SBS against aging [21].

The superior performance of One Coat Bond can be attributed to the chemical bonding of its 10-MDP monomer, which forms stable and insoluble MDP-Ca salts with hydroxyapatite, resulting in a durable nano-layered interface. In contrast, M-TEG-P in Yamakin Aquabond has fewer acidic groups, leading to lower demineralization, shorter hybrid layer, and weaker ionic bonds. The better bond strength under wet conditions is due to preserved collagen porosity allowing optimal resin penetration. Dry condition collapse of collagen network inhibits monomer penetration. The greater degradation of Yamakin after thermocycling indicates its lower hydrolytic stability because of faster hydrolysis of M-TEG-P bonds compared to stable 10-MDP salts [25].

However, hydrolytic degradation of the hybrid layer remains a drawback because over time, it deteriorates adhesive dentin interfaces. Therefore, some water is ne­cessary for expansion of the collagen network and to aid in monomer infiltration, but if there is too much water, the phase separation could occur between other hydrophilic components and hydrophobic parts. This may cause resin to poorly penetrate, leading to lack of poly­merization and some voids generation as well as lower adhesion strength, with disorders in interface formed between the resin and dentin layer [28].

The present study showed that the SBS values of One Coat Bond were significantly higher than those of Yamakin Aquabond under all conditions. Our results are similar to that reported by Van Meerbeek et al. [20], who demonstrated a stronger bond strength of some universal adhesives due to better monomer design and superior infiltration. Better performance of One Coat Bond may also be explained by MDP, an amphiphilic phosphate monomer that dissolve the smear layer well and interact with the tooth substrate powerfully [28].

Better bond strength of One Coat Bond in wet dentin is in accordance with recent data, showing a benefit of the wet bonding for hydrophilic resins. Studies by Pashley et al. [22] and Nonato et al. [23] revealed that a wet dentin surface inhibits the collapse of collagen interstices, preserving the porosity required for optimal resin penetration to form a durable hybrid layer. This is especially applicable to clinical environment, because full isolation and dry fields are difficult to achieve. The improved performance of One Coat Bond under wet conditions is consistent with the results reported by Van Meerbeek et al. [20], who underscored the improved performance of universal adhesives owing to their presence of MDP and amphiphilic monomers in wet dentin condition. However, M-TEG-P in Yamakin bond has fewer acidic group (pKa: 1.5-2.0) than 10-MDP (pKa: 1.0) contained in One Coat Bond, leading to lower demineralization of dentin. Shorter/hybrid layer and low wet resin penetration limit calcium chelation and weaken ionic bonds vs. stable nanolayer of 10-MDP salt, named Ca-(10-MDP) salt. This is formed between dentinal calcium and corresponding anion accompanied by a dense network type ‘nano’ oligosilsesquioxane zone on the Ca-(10-MDP) salt, serving as entrapping points for functional monomers tightly cross-linked into resin composite/ dentin interface [20].

Wet conditions of dentin significantly increased bond strength among various groups, and the maximum mean SBS was obtained for One Coat Bond/wet/immediate subgroup (17.2 MPa). This may be attributed to the hydrophilic characteristic of new adhesives allowing its good penetration into wet dentin, maintaining an over expanded collagen mesh and enabling a correct hybrid layer formation [10, 22]. Additionally, universal adhesives are frequently developed to allow application in moist environments simulating a real-life condition, preventing perfect dryness to occur.

On the other hand, decreased SBS values recorded for dry bonding, especially when thermocycling (G2 and G4), for Yamakin Aquabond was similar to the over-drying phenomenon. Perdigão et al. [29] demonstrated that over-dehydration of dentin causes a collapse of the collagen network, thus inhibiting the penetration of adhesive monomers and leaving incompletely polymerized and weakened hybrid layer. This response is commonly more evident for adhesives less capable to withstand moisture changes that most probably occur in Yamakin Aquabond due to its greater reduction under dry and thermocycled conditions [21].

These results underscore the necessity of keeping a judicious dentin hydration during adhesive application, particularly with self-etch or universal system [22]. Although this study demonstrated better results of moist bonding approach on One Coat Bond, some other research reported that too wet dentin may be harmful to bond strength. Mohan and Kandaswamy [13] observed that moisture overloading may induce phase separation in hydrophilic adhesives, affect polymerization, and form voids at the adhesive-dentine interface. This difference may be due to the very specific hydric control in our experimental condition (2.5 µl of distilled water) that has an optimal humid context, but not too much wetness. In addition, the particular solvent system and monomer formulation of One Coat Bond may provide a broader tolerance to moisture changes than other universal adhesives, as proposed by Kumagai et al. [14], who investigated different universal adhesives at diffe­rent moisture conditions. The suboptimal performance of Yamakin Aquabond may result from poor interaction of M-TEG-P with collagen matrix, its highly hydrophi­lic character, and the absence of co-stabilizers, such as HEMA, or an optimized solvent system. Variations in moisture, either excessive wetness or prolonged drying can disrupt adhesive penetration, particularly when the monomer formulation lacks adaptability to dentin heterogeneity. As reported by Perdigão et al. [29], amphiphilicity plays a critical role in modulating resin infiltration, and absolutely hydrophobic monomers without proper balancing may show irregular behavior within the hybrid layer.

Thermocycling used to simulate thermal stress in the oral cavity reduced bond strength of both adhesive systems. On the other hand, Yamakin Aquabond was found to show a greater decrease in the SBS against aging. This may result from its formulation or solvent system, which would be not resistive enough to the thermal fatigue and hydrolytic degradation [7, 8]. In contrast, One Coat Bond showed better long-term stability under thermocycling, exhibiting a more stable hybrid layer formation and chemical bonding interface [21].

The M-TEG-P phosphate esters in Yamakin hydrolyze more quickly than those of 10-MDP stable salts under heating, hence the insufficient resin infiltration leads to exposed collagen, MMP-induced breakdown, and interfacial debonding [29].

The mode of failure, as evaluated by the ARI, revealed a clear correlation with the mechanical data. Groups exhibiting higher bond strength, particularly the One Coat moist bonding group, displayed a greater proportion of mixed and cohesive failures, represented mainly by ARI scores of 2 and 3. These findings indicate deeper resin infiltration and stronger interfacial adhesion to dentin. In contrast, the thermocycled Yamakin groups, which recorded the lowest SBS values, predominantly exhibited adhesive failures corresponding to ARI scores of 0 or 1, signifying weak bonding and limited resin retention on the tooth surface after debonding. This pattern supports the interpretation that moisture preservation at the bonding interface enhances adhesive performance and interfacial durability. These observations are in agreement with the findings of Fernandes de Morais et al. [31], who also reported a positive asso­ciation between higher bond strength values and the predominance of mixed failure modes.

In conclusion, the findings demonstrated strong clini­cal superiority of One Coat Bond under moist dentin conditions simulating physiological intraoral circumstances. The Yamakin adhesive based on M-TEG-P exhi­bited inferior performance after aging, likely as a result of less durable resin infiltration and/or weaker chemical bonding. These findings highlight the necessity of universal adhesives with both hydrolytic stability and thermal resistance, which work well on moist dentin. Therefore, both dentin moisture condition and the specific chemical composition of universal adhesive are critical factors, which influence bond durability; therefore, the study’s null hypothesis was rejected.

Conclusions

Moist dentin conditions enhance universal adhesives performance, resulting in superior SBS and failure resistance in both immediate and aged conditions. The MDP-based adhesive (One Coat Bond) demonstrated superior SBS and more favorable failure modes compared to the M-TEG-P-based adhesive (Yamakin Aquabond), under both immediate and thermocycled conditions. Thermocycling adversely affected the bond strength of both adhesives, with Yamakin Aquabond showing more pronounced degradation.

Disclosures

Author contributions: Conceptualization: K.E.A.E., A.A.G.; Methodology: K.E.A.E., A.A.G.; Investigation and data collection: K.E.A.E., A.A.G.; Formal analysis: K.E.A.E., A.A.G.; Data curation: K.E.A.E.; Writing of original draft: K.E.A.E., A.A.G.; Writing – review and editing: K.E.A.E., A.A.G.; Supervision: A.A.G.; Project administration: K.E.A.E. All authors have read and agreed to the published version of the manuscript.

Funding: This research received no external funding.

Institutional Review Board statement: This study was approved by the Institutional Ethics Committee of Al-Azhar University Assiut Branch, Egypt on January 15, 2025, approval number: AUAREC20250004-13.

Informed consent statement: Informed consent was obtained from all subjects involved in the study. As the study utilized extracted teeth, verbal consent was considered sufficient and was explicitly approved by the Ethics Committee. All procedures were conducted in accordance with the ethical principles outlined in the Declaration of Helsinki.

Data availability statement: All the data generated or analyzed in this study are included in this manuscript. The data generated in this study may be requested from the corresponding author.

Acknowledgments: None.

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

AI use statement: Not applicable.

References