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 Table of Contents  
Year : 2023  |  Volume : 15  |  Issue : 2  |  Page : 168-173

Influence of adhesive application method and thermocycling on the bonding performance of different adhesive systems to dentin

1 Department of Restorative and Dental Materials, National Research Centre, Giza, Egypt
2 Department of Conservative Dentistry, Faculty of Oral and Dental Medicine, Future University, Cairo, Egypt

Date of Submission08-Nov-2022
Date of Decision09-Jan-2023
Date of Acceptance13-Jan-2023
Date of Web Publication28-Apr-2023

Correspondence Address:
Dr. Lamiaa Mahmoud Moharam
Department of Restorative and Dental Materials, National Research Centre, Giza
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jioh.jioh_235_22

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Aim: To assess the effect of multiple layers application of different adhesive systems on the microshear bond strength (μSBS) of dentin with and without thermocycling. Materials and Methods: Two hundred forty sound human premolars had their enamel surfaces removed to expose even surfaces of dentin. The teeth were mounted in acrylic resin blocks and then assigned arbitrarily into four main groups (n = 60) representing the investigated adhesives (a self-etch [SE] adhesive, a total-etch [TE] adhesive, and a multimode [MM] adhesive used in TE and SE modes). Individually, the main groups were equally alienated into three subgroups (n = 20), each representing the number of the applied adhesive layers (one layer [1L], two layers [2L], and three layers [3L]). Then, each respective subgroup was divided subsequently into two equal divisions with n = 10 each according to thermocycling (with and without thermocycling). Each occlusal surface received three composite microrods. Prepared specimens were reserved in distilled water at temperature of 37°C until the μSBS test was performed after 24 h or after thermocycling for 3000 cycles at 5°C–55°C water bath. Results: A three-way analysis of variance (ANOVA) disclosed that multilayer application, adhesive type, and thermocycling showed a significant statistical impact on μSBS. The 1L, 2L, and 3L groups showed a statistically significant difference between the groups. The 3L groups showed the highest μSBS, whereas 1L groups recorded the least μSBS. Within the adhesive groups, a statistically significant difference was evident. The highest μSBS was recorded for TE, whereas SE recorded the least μSBS. Thermocycling (TC) has a statistically significant effect on μSBS and “no-thermocycling” (T0) groups recorded higher μSBS than thermocycled groups. Conclusion: Multilayer application of the tested adhesives had the potential to increase dentin μSBS.

Keywords: Bond Durability, Multilayer Application, Multimode Universal Adhesives, Self-Etch Adhesives, Total-Etch Adhesives

How to cite this article:
Moharam LM, Afifi RH. Influence of adhesive application method and thermocycling on the bonding performance of different adhesive systems to dentin. J Int Oral Health 2023;15:168-73

How to cite this URL:
Moharam LM, Afifi RH. Influence of adhesive application method and thermocycling on the bonding performance of different adhesive systems to dentin. J Int Oral Health [serial online] 2023 [cited 2023 Nov 30];15:168-73. Available from:

  Introduction Top

Restorations’ durability depends on exposure of adhesive–tooth interface to different stimuli in the oral environment.[1] Although enamel bonding is considered durable and reliable, dentin bonding is still more challenging, which could be owed mainly to its heterogeneous structure.[2]

Recent technologies of dental adhesives aim to decrease clinical application time and reduce technique sensitivity.[3] Novel dental adhesives were launched to the market under the term “multimode” (MM) or “universal” adhesives, to be applied with either total-etch (TE) or self-etch (SE) mode.[4] Their multiapproach competence permits applying the “selective enamel-etching” technique, which merges enamel TE technique merits with simplified dentin SE approach,[3] enabling clinicians to choose the proper strategy for their use in different conditions.

Inherent dentin wetness presents an obstacle for proper and durable bonds. Therefore, dentin bonding systems combine hydrophilic monomers[5] for immediate high dentin bond strength.[6] Some TE adhesives contain water and hydrophilic monomers, acting as semipermeable membranes allowing dentinal fluid transduction, accelerating resin–tooth interfaces degradation, and significantly compromising long-term bond durability.[7] Single-step SE and universal adhesives comprise water that is essential for hydrophilic monomers’ ionization.[8] However, residual water within adhesives reduces monomers conversion degree leading to inferior mechanical properties compared with the two-step SE adhesives.

Storage in water, artificial saliva, or thermocycling are common approaches for artificial aging. They simulate various conditions and clarify degradation mechanisms of resin–tooth bond.[9] Bonding quality of simplified all-in-one adhesives is enhanced by several techniques such as prolonged air-drying time, warm air drying, hydrophobic layer addition, and multiple coating of adhesives. Still, literature focusing on SE and universal adhesives long-term bond durability is limited.[10] Immediate bonding features have been typically assessed in literature. Yet, bond durability is more clinically relevant.[11]

Thus, this study assessed the outcome of multiple coating of the tested adhesive systems on immediate and thermocycled (TC) dentin microshear bond strength (μSBS). The following null hypotheses were proposed: (1) adhesive application modes would have no effect on the bond durability; (2) the tested adhesive systems had no effect on bond durability with multilayer application; and (3) thermocycling would have no effect on the bond durability of different adhesive systems with multilayer application.

  Materials and Methods Top

Experimental design

This study was approved by the Medical Ethics Committee of the National Research Centre in Egypt with reference number 4615052021. Two hundred and forty extracted human premolars were arbitrarily assigned to four main groups with (n = 60) each, representing different adhesive systems investigated in the study (Solobond M [SM] [control group], OptiBond, and MM Scotchbond Universal [SBU] in TE [SBU MM-TE] and SE [SBU MM-SE] modes). The main groups were each alienated into three subgroups of (n = 20) each representing applied adhesive layers (one [1L], two [2L], and three layers [3L]). Then each subgroup was further alienated into two equal divisions with (n = 10) each according to thermocycling (TC and no thermocycling [T0]). R statistical package was used for sample size calculation (version 2.15.2; The R Foundation for Statistical Computing, Vienna, Austria). The one-way analysis of variance (ANOVA) disclosed that 10 samples had the proper sample size, which showed that within the different study groups, there was a mean difference with a two-sided significance level of 5% and 80% power.

Teeth selection and preparation

Two hundred and forty extracted sound human premolars for orthodontic purposes were cleaned from debris and then kept in a solution of 0.1% thymol at 4°C till usage. The roots of the selected teeth were sectioned 2 mm beyond the cementum–enamel junction, then the crowns were implanted partly in acrylic resin that was chemically cured[6] (Acrostone, Acrostone Dental Factory, Cairo, Egypt) using molds of split-Teflon (1-cm height × 2-cm diameter) encircled with plastic rings for stabilization. Occlusal surfaces were wet-ground with a double-side cutting diamond disk at low speed, perpendicular to the long axis of the teeth, and exposing flat superficial dentin surfaces. Using 600–1000 grit silicon-carbide papers, the specimens were wet-ground in a circular motion for 1 min to standardize smear layer production.[4] Stereomicroscope (Olympus BX 60, Olympus Optical Co. LTD, Tokyo, Japan) was used to inspect the specimens carefully to eliminate any enamel residues or other flaws.

Adhesive application protocols

[Table 1] presents different materials, their composition, manufacturers, and the investigated application protocols investigated. As per the manufacturers’ commendations, the tested adhesives were applied to the prepared dentin surfaces and then photo-cured with light-emitting diode light-curing device (Elipar S10, 3M ESPE, St. Paul, Minnesota) of power ≥1000 mW/cm2, which was checked periodically using a portable radiometer (Demetron 100, Kerr Corporation, Orange, California).
Table 1: Materials, composition, and application protocols of the different adhesives

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Resin composite buildups

Flowable composite microrods buildup (Filtek Z350 XT Flowable Composite, 3M ESPE) were constructed using Tygon tubes (0.8-mm internal diameter × 1-mm height). Each microrod was photo-cured for 20 s as per manufacturer’s instructions. Sharp blade was used to remove the Tygon tubes. Each occlusal table received three composite microrods. Then, the specimens were kept in distilled water at 37°C in tight-seal vessels[10] till μSBS testing. After 24 h, half of the specimens were tested, whereas the rest of the specimens were TC for 3000 cycles (THE-100 SD Mechatronic Thermocycler, Munich, Germany) in a bath of cold water at 5°C for 30 s followed by hot water bath at 55°C for 30 and 10 s dwell time at 37°C.

Microshear bond strength testing

Acrylic-mounted specimens were attached to the lower jig of the universal testing machine (Instron, Model 3345, Instron Instruments, Buckinghamshire, UK). A 0.8-mm orthodontic wire loop was attached to the machine’s upper jig around each composite microrod. At 1-mm/min crosshead speed, the μSBS test was performed until each rod’s failure. Using computer software (BlueHill Universal, Instron Testing Software, Buckinghamshire, UK), the maximum force was calculated in MPa.

Statistical analysis

For each group in each test, the standard deviation and mean values were calculated. Kolmogorov–Smirnov and Shapiro–Wilk tests were used to explore the data for normality, and a parametric (normal) distribution was shown. For comparison between more than two groups in nonrelated specimens, one-way ANOVA followed by Tukey post hoc test was used. To compare between two groups in related specimens, paired sample t test was used. For testing the interactions between different variables, three-way ANOVA was used. The level of significance was set at P ≤ 0.05. IBM SPSS Statistics version 20.0 for Windows was used to perform the statistical analysis.

  Results Top

The interaction between the different variables demonstrated by three-way ANOVA showed that multilayer application, adhesive type, and thermocycling revealed a significantly statistical effect on dentin μSBS (P < 0.001). [Table 2] and [Figure 1] show μSBS results for different groups. Different adhesives’ effect on μSBS with multilayer application regardless thermocycling showed statistically significant difference between tested adhesives (optiBond-all-in-one (OB), SM, SBU MM-SE, and SBU MM-TE), and SM recorded the highest values, whereas OB recorded the least values. Regarding multilayer application effect on μSBS for each adhesive with and without thermocycling, a statistically significant difference was found between 1L, 2L, and 3L, where the highest values were found in 3L, whereas 1L recorded the least values. The effect of thermocycling of different tested adhesives on μSBS with multilayer application showed a significant statistical difference between T0 and TC. The highest values were recorded for T0, whereas the least values were recorded for TC.
Table 2: Mean and standard deviation values of microshear bond strength of the different tested groups

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Figure 1: Bar chart showing the effect of application modes and thermocycling on μSBS of different adhesives to dentin. μSBS = microshear bond strength

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  Discussion Top

The modern SE and MM dental adhesives were developed to guarantee more reliable bonding process. Yet, the two-bottle TE adhesive system remains the gold standard for bond durability. The μSBS results of this study revealed a statistically significant between 1L, 2L, and 3L groups for the adhesives with and without thermocycling. Thus, the first null hypothesis was rejected. Increased adhesive layer thickness showed a significant increase in bond strengths, developing more consistent bond to dentin. Although oxygen-inhibition might cause inadequate adhesive polymerization with adhesive application in very thin layers,[12] balanced polymerization stresses at composite–tooth interface were developed on adhesive application in thick layers.[13] Moreover, the greater the adhesive thickness, the more elastic it becomes, due to resin deformation that suppresses such stresses.[14] Multilayer adhesive application might seal nonpolymerized oxygen-inhibition layer, enhancing polymerization via increasing monomer conversion, thus developing stronger dentin bond.[15] Our results showed that thermocycling and adhesive type had statistically significant effect on μSBS. Though, the later could be adhesive-dependent. Hence, the second and third null hypotheses were rejected as well. In this study, SM (TE) adhesive recorded the highest μSBS with 3L application, which could be owed to its compositional acetone solvent, which might facilitate deeper monomer penetration into dentin, enhancing micromechanical retention.[16] As per manufacturer instructions, SM (TE) adhesive was left undisturbed for 30 s for each layer before light-curing. Therefore, more time was allowed for functional monomer infiltration into dentin and more solvent evaporation, leading to even adhesive layer development.[17] These results agreed with Chowdhury et al.[17] as an increased dentin bond strength with application of consecutive coatings of TE adhesives was reported. However, MM adhesive SBU in TE or SE mode showed better bond strength than OB (SE) adhesive. This could be owed to methacryloyloxydecyl dihydrogen phosphate (MDP) content of SBU, showing high-intensity nano-layering, in addition to its nanofillers content, which might contribute to thicker adhesive layer production. These results agreed with Hirokane et al.[18] who reported that the double-layer application of the tested MM adhesives had a positive effect on dentin SBS. In this context, Deren et al.[19] recommended 2–3L SBU application and then polymerizing the last layer for dentin bond strength enhancement, which was owed to enhanced MDP concentration.

MDP has phosphoric-acid functional group, which chemically interacts with hydroxyapatite (HA) crystals, developing calcium-carboxylate and calcium-phosphate stable salts with partial surface decalcification. Moreover, the concept of adhesion–decalcification states that as long as the calcium salts of the acidic monomer are less soluble, the adhesion to the HA-based substrate will be more stable and stronger.[20] Consequently, Krawczyk-Stuss et al.[21] demonstrated substantial bond durability improvement due to MDP/dentin chemical interaction.

Our results revealed that TE mode of SBU showed better results than SE mode. These findings agreed with Saito et al.[22] who concluded that the SE bonding mode of the tested MM universal adhesives was quite comparable with the one-step SE adhesives. However, it was not up to the level of the two-step or the three-step adhesive systems. Thus, the authors estimated that universal adhesive functional monomers had a significant role in developing HA and exposed collagen fibril chemical interaction. Furthermore, due to the hydrophobic interaction of MDP with collagen, it was found that MDP is able to accomplish a steady interaction with the collagen.[23] These findings were contradicted by Hanabusa et al.[24] and Isolan et al.[25] They conveyed an analogous dentin bond strength with the universal adhesive in both TE and SE strategies. Such contradictions could be owed to the fact that there was no aging performed in their studies, as well as only immediate bond strengths were evaluated.

Although SE adhesive (OB) showed the least μSBS among tested adhesives, 2L and 3L application enhanced its bond strength. This could be related to its composition. OB incorporates water, HEMA, ethanol, and acetone, causing intense water sorption and performing as semipermeable membrane that allows water passage, thus affecting its mechanical characteristics.[5],[7] Moreover, the high hydrophilicity of the universal adhesives, which leads to water entrapment within the hybrid layer, and inadequate solvent elimination can adversely affect its polymerization.[26]

Additionally, thermocycling creates stresses at tooth–restoration interface, due to differences in the modulus of thermal expansion of polymeric materials. As water penetration into adhesive–dentin interface plasticizes adhesives, the hot water during thermocycles accelerates resin hydrolysis.[27] A significant reduction in one-step SE adhesive bond strength was reported even after short-term water storage.[28] In our study, bond strength reduction following thermocycling was greater in 1L groups, suggesting that multilayer application might have enhanced bond durability. Moreover, mild universal adhesives, such as SBU, are less reliant on adhesion strategy and more stable after aging[29] in compliance with our study. These findings agreed with Itoh et al.[30] who reported declined μTBS after 6-m water storage for OB, owed to adhesive water sorption.

Finally, one can advocate multilayers application of different adhesive systems to create a reliable and durable bond to the tooth structure. Further research is recommended to assess the impact of aging on the multilayer application of the different adhesive systems to different tooth substrates at various clinical conditions.


Under limitations of the present in vitro study, it could be concluded that the application of different adhesive systems in multilayers had the potential to increase the dentin μSBS. Meanwhile, dentin bond strength and durability might be adhesive-dependent. The MM universal adhesive bonding performance was quite comparable with the gold standard TE adhesives. Furthermore, a significant negative effect was shown for thermocycling on the bond strength of dentin, which was enhanced with adhesive multilayer application.



Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

Authors’ contributions

All the article authors had contributed in an equal manner from writing to publishing.

Ethical policy and institutional review board statement

The Medical Ethics Committee of the National Research Centre in Egypt has approved this study with reference number 4615052021.

Patient declaration of consent

Not applicable.

Data availability statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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  [Figure 1]

  [Table 1], [Table 2]


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