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International Journal of Contemporary Dental and Medical Reviews (2018), Article ID 041218, 1-5 Pages

R E S E A R C H B R I E F

Hybrid layer evaluation of all-in-one adhesive with single and multiple applicationsSwathi Aravelli1, Nimeshika Ramachandruni2, Swetha Kasam1

1Department of Conservative Dentistry and Endodontics, Malla Reddy Dental College for Women, Hyderabad, Telangana, India, 2Department of Conservative Dentistry and Endodontics, Malla Reddy Dental College for Women, Hyderabad, Telangana, India

AbstractBackground: Earliest commercial versions of self-etch bonding systems were hydrophobic, which did not allow them to adapt to hydrophilic dentin. The adhesive must be hydrophilic in nature as it should be able to diffuse and penetrate in an aqueous environment. The tested self-etching adhesive (Tetric N-bond) contains acid monomers, often mixed with water, to make the adhesive systems sufficiently acidic to form hybridized complex. Single layer application of self-etch adhesive results in a layer that is too thin for successful photopolymerization. Aim: The purpose of this study was to evaluate the effect of single and multiple applications of all-in-one self-etch adhesive on hybrid layer. Materials and Methods: Tetric N-Bond self-etch all-in-one adhesive was used in the study. Fifteen extracted human mandibular molars mounted in self-cure acrylic resin with the occlusal enamel removed were cross-sectioned to obtain 1 mm thick dentin disks and then divided into three groups.• Group 1: Single layer of adhesive was applied and light cured for 10 s;• Group 2: Two layers of adhesive were applied with intermediate curing between each

layer;• Group 3: Two consecutive layers of adhesive were applied with light curing after application of two layers. Resin composite build-ups were made and hybrid layer evaluation was done using scanning electron microscope.Results: There is no significant relationship between increase in number of adhesive layers and thickness of hybrid layer. There is no significant relationship between curing each layer and curing of two consecutive layers of adhesive. Conclusion: No significant relationship exists between increase in number of adhesive layers and thickness of hybrid layer. Clinical significance: Self-etching adhesive systems allow practitioner to place restorations in a more simplified manner with decreased postoperative sensitivity. After application, these products create very thin coatings, which may be oxygen inhibited and hence poorly polymerized which can be overcome by simple changes in bonding technique, such as applying two layers of all-in-one adhesives.

Keywords: All-in-one adhesive, consecutive applications, hybrid layer

Correspondence Dr. Swathi Aravelli, Department of Conservative Dentistry and Endodontics, Malla Reddy Dental College for Women, Hyderabad, Telangana, India. Phone: +91-9866021672. E-mail: drswathi.endo@gmail.com

Received 28 December 2018; Accepted 30 May 2019

doi: 10.15713/ins.ijcdmr.134

How to cite the article: Aravelli S, Ramachandruni N, Kasam S. Hybrid layer evaluation of all-in-one adhesive with single and multiple applications, Article ID: 041218, 2019. doi: 10.15713/ins.ijcdmr.134

Introduction

The goal in adhesive dentistry is to achieve an adequately strong bonding of the restorative resin to the tooth structure so that there are optimum retention, minimal microleakage, and, hence, better color stability and clinical longevity of the restoration.[1]

Self-etching system follows a trend toward simplification. They promote interdiffusion of the adhesive through the smear layer, providing procedures that are more reliable and less sensitive than the conventional adhesive techniques.[2]

Self-etch adhesive systems did not improve bonding effectiveness to dentin in spite of their purported reduction in technique sensitivity.[3] To offset the limitations of self-etching adhesives, altered bonding protocols that increase resin-dentin bond quality were suggested; those are the multiple applications of adhesive[4] or increased substrate contact time of the acidic primers.[5]

The purpose of this study is to determine the effect of single and multiple coatings with intermittent curing and curing of consecutive layers of all-in-one adhesive on hybrid layer formation.

Aravelli, et al. Hybrid layer of all-in-one adhesive

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Materials and Methods

Sample preparation for scanning electron microscope (SEM) analysis

Fifteen freshly extracted human mandibular molars stored in distilled water were used. These teeth were mounted in blocks of acrylic resin and the occlusal third of crown was removed using slow-speed micromotor handpiece equipped with a diamond impregnated disk under water coolant. Subsequently, remaining surface was polished with 180 grit, 240 grit, and 600 grit silicon carbide sandpaper until no enamel remained. Teeth were then cross-sectioned to obtain 1 mm thick dentin disks. Outer surface of each disk was polished with wet 600 grit silicon carbide sandpaper to create a uniform smear layer. The samples were randomly divided into three groups of five each. Specimens were treated with adhesive as mentioned above in three groups.

Group 1 (single layer of adhesive)Single layer of Tetric N-bond self-etch adhesive (Ivoclar Vivadent, Schaan, Liechtenstein) was applied to the exposed dentin surface using a fully saturated applicator tip of adhesive for 30 s and gently air dried for 3 s then light cured for 10 s using blue phase C8 light-emitting diode unit (Ivoclar Vivadent, Schaan, Liechtenstein) at a light intensity of 800 mW/cm2.

Group 2 (two layers of adhesive with light curing of each layer)Two layers of adhesive were applied in the same manner as described in Group 1. Light curing was done after the application of each layer of adhesive.

Group 3 (two layers of adhesive with light curing after application of the second layer)Two layers of adhesive were applied in the same manner as described in Group 1. Light curing was done after the application of the two layers of adhesive.

After the completion of bonding procedure, 2 mm of composite was applied over these dentin disks and light cured for 20 s; then, samples were stored in distilled water at 37°C for 24 h. These disks were fractured in the center with a chisel for cross-sectional viewing. All specimens were then immersed in 5% hydrochloric acid for 30 s and then washed for 30 s under running water. Sections were then transferred to 70% ethanol and dehydrated in increasing concentration of ethanol for 10 s each. Each specimen was then mounted on an aluminum stub with double-sided carbon conductivity tape, and a thin layer of the gold coat over the samples was done using an automated sputter coater and examined under SEM (Model: JEOL-JSM 5600) at a magnification of ×1000.

Results

Under SEM, very thin hybrid layer (<0.5 μm thick) was observed. It was irregular and non-uniform in thickness with some areas being extremely thin while others were relatively thick in all three groups [Figures 1-3]. These values were subjected to statistical analysis (analysis of variance with post hoc analysis).

There was no significant difference in the mean thickness of hybrid layer among the study groups (P > 0.001) [Table 1]. Post hoc analysis was performed to evaluate the intergroup comparisons which were non-significant [Table 2].

The increase in number of adhesive layers did not appear to affect the thickness of the hybrid layer. There is no significant difference in thickness of hybrid layer between Group 2 and Group 3.

Discussion

The adhesive viscosity and deep dentin demineralization in etch-and-rinse systems, in general, induce the enzyme activation of proteolytic matrix metalloproteinases and affect long-term bond durability. This is unlikely to be seen in self-etch adhesives that etch and penetrate simultaneously, resulting in resin-collagen fixation and consequent long-term bond stability.[6-8]

The rationale behind the selection of Tetric N-Bond self-etch adhesive is that it contains hydrolytically stable methacrylamide monomers instead of the common reactive diluent 2-hydroxyethyl methacrylate (HEMA) which is particularly

Figure 2: Scanning electron microscope image of Group 2

Figure 1: Scanning electron microscope image of Group 1

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Hybrid layer of all-in-one adhesive Aravelli, et al.

unstable in aqueous acid due to the formation of hydrolysis-prone associates. Composition of Tetric N-Bond self-Etch (Ivoclar, Vivadent) includes: Bisacrylamide, water, bismethacrylamide dihydrogen phosphate, amino acid acrylamide, hydroxyl alkyl methacrylamide, highly dispersed silicon dioxide, catalysts and stabilizers.

Salz and Bock[9] compared the adhesive properties and storage ability of methacrylamide monomers to methacrylate-based adhesive formulations and reported that HEMA-free formulations performed more reliably, with the fully acrylamide-based adhesive consistently giving the highest dentin bond strength values.

In the present study, the exposed dentin surface in Group 1 was treated with adhesive according to manufacturer’s instructions. The manufacturer’s instructions for Tetric N-bond self-etch are the application of a single layer for 30 s followed by through air drying and light curing for 10 s. This may result in a layer that is too thin for successful photopolymerization. In Group 2 and Group 3, multiple consecutive coats were applied with light curing after each coat.

The method of light curing after application of each coat was selected, as for simplified adhesive systems, which possess solvents in their composition; the improved adhesive thickness makes it more difficult to volatilize the solvent before light curing, and this result in lower bonding values.[10] Moreover, the effect of repeated light curing to the first coat of bonding resin may be able to increase the conversion of the adhesive resin, enhancing bond strength.

While the micromechanical interlocking is a prerequisite to achieve good bonding (certainly within clinical circumstances), the potential benefit of additional chemical interaction between functional monomers and tooth substrate components has recently regained attention. Additional “primary” chemical interaction is thought to particularly improve bond durability. The way molecules interact with hydroxyapatite-based tissues have been described in the so-called “AD concept” or “Adhesion-Decalcification concept.” More specifically, molecules like

10-methacryloyloxydecyl dihydrogen phosphate or 10-MDP will chemically bond to Ca of HAP forming stable calcium phosphate and calcium carboxylate salts, respectively, along with only a limited surface decalcification effect.[11]

The HEMA-free self-etch adhesive system used in the present study contains MDP monomer which is speculated to have chemical interaction with hydroxyapatite crystals forming stable calcium phosphate and calcium carboxylate salts, respectively, along with only a limited surface decalcification effect (“AD concept”).

The thickness of the hybrid layer and its influence on bonding durability is still uncertain. Some authors have suggested that the dimensions of the hybrid layer may be taken as an indicator of the strain-absorbing capacity of the corresponding interface.[12] This elastic buffer could be of utmost importance for absorbing the stress originated from composite resin polymerization shrinkage.

The thickness of the hybrid layer observed was similar for all the three groups of about 0.5 μm. Takahashi et al.[13] (2010) stated that dual application of all-in-one adhesive system significantly improved the bond strengths of resin cements, but hybrid layer was hardly detected between the resin coating and dentin in each group, even at a high magnification (×7500). Skupien et al.[14] (2010) stated that less aggressive adhesives form a hybrid layer between 0.4 μm and 0.5 μm thick. According to Tay and Pashley[15] (2001), a hybrid layer is considered authentic if it has 0.5 μm of depth in the interdiffusion zone. D’Arcangelo et al.[10] (2009) stated that the ideal adhesive thickness is certainly variable and depends on the adhesive system used. Clinicians should consider the intrinsic properties of each bonding system when using a multilayering technique. If too thick, an adhesive layer would negatively influence the mechanical, physical properties of the restoration with a risk of total cohesive failure in adhesive thickness; therefore, it would be advisable not to apply an excessive number of adhesive coats.

Wei et al.[16] (2009) conducted an in vitro study, in which they evaluated the mechanical properties of the resin-dentin area of single-step self-etch adhesives. The results showed that the hardness of the bonding layer and resin-dentin interface was significantly affected by the double application for each material tested.

Elkassas et al.[17] (2009) evaluated the effect of doubling the adhesive layers of three acetone-based adhesives on the microtensile bond strength and ultramorphological characterization of the resin-dentin interface using SEM. They concluded that doubling the adhesive layer applications significantly improved the bond strength of the two self-etch adhesives (XENO IV and G BOND); however, it had a negative effect on the bond strength of the total-etch adhesive (Prime and Bond NT). The hybrid layer appeared thicker with self-etch adhesives.

Kim et al.[18] (2010) tested the hypothesis that unpolymerized acidic monomers from an aggressive all-in-one self-etching adhesive continued to etch beyond hybrid layers and concluded that an all-in-one adhesive does not etch beyond hybrid layers.

Figure 3: Scanning electron microscope image of Group 3

Aravelli, et al. Hybrid layer of all-in-one adhesive

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However, limitations of the current study include Tetric N-Bond self-etch adhesive used in the current study which was not compared with other adhesives (self-etch and etch and rinse) and relationship of hybrid layer and bond strength also could be evaluated for more clinical significance.

Conclusion

Within the limitations of this in vitro study, no significant relationship exists between increase in number of adhesive layers and thickness of hybrid layer. Light curing of each layer of adhesive or consecutive layers has no significant difference on thickness of hybrid layer.

References

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Table 2: Multiple comparison by post hoc testMultiple comparisons

Comparison of Group 1with Groups 2 and 3Group Group Mean difference Standard error P-valueGroup 1 Group 2 −0.02600 0.02683 0.609 (Non-significant)

Group 3 0.04200 0.02683 0.297 (Non-significant)

Comparison of Group 2 with Groups 1 and 3

Group 2 Group 1 0.02600 0.02683 0.609 (Non-significant)

Group 3 0.06800 0.02683 0.063 (Non-significant)

Comparison of Group 3 with Groups 1 and 2

Group 3 Group 1 −0.04200 0.02683 0.297 (Non-significant)

Group 2 −0.06800 0.02683 0.063 (Non-significant)

Table 1: Comparison of mean thickness values between all three groups by analysis of variance testDescriptive

ThicknessGroup Sample Mean Standard deviation Standard error Minimum MaximumGroup 1 5 0.4200 0.04743 0.02121 0.35 0.48

Group 2 5 0.4460 0.03782 0.01691 0.41 0.50

Group 3 5 0.3780 0.04147 0.01855 0.32 0.43

Total 15 0.4147 0.04882 0.01261 0.32 0.50

Statistical inference Df=2F-value=3.270P-value=0.074 (Non-significant)

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Foxton  RM, et al. Thin resin coating by dual-application of all-in-one adhesives improves dentin bond strength of resin cements for indirect restorations. Dent Mater J 2010;29:615-22.

14. Skupien JA, Susin AH, Angst PD, Anesi R, Machado P, Bortolotto T, et al. Micromorphological effects and the thickness of the hybrid layer a comparison of current adhesive systems. J Adhes Dent 2010;12:435-42.

15. Tay FR, Pashley DH. Aggressiveness of contemporary self  etching systems. I: Depth of penetration beyond dentin smear layers. Dent Mater 2001;17:296-308.

16. Wei S, Shimada Y, Sadr A, Tagami J. Effect of double-application of three single-step self-etch adhesives on dentin bonding and mechanical properties of resin-dentin area. Oper Dent 2009;34:716-24.

17. Elkassas D, Taher HA, Elsahn N, Hafez R, El-Badrawy W. Effect of the number of applications of acetone-based adhesives on microtensile bond strength and the hybrid layer. Oper Dent 2009;34:688-96.

18. Kim J, Mai S, Carrilho MR, Yiu CK, Pashley DH, Tay FR, et al. An all-in-one adhesive does not etch beyond hybrid layers. J Dent Res 2010;89:482-7.

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