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 Table of Contents  
Year : 2018  |  Volume : 15  |  Issue : 3  |  Page : 192-198

Microtensile bond strength of self-etching adhesives to enamel

Department of Restorative Dentistry, Faculty of Dentistry, Cairo University, Giza, Egypt

Date of Submission28-May-2018
Date of Acceptance16-Jun-2018
Date of Web Publication10-Oct-2018

Correspondence Address:
Ahmed Z Elhoshy
Associate Professor of Restorative Dentistry, Faculty of Dentistry, Cairo University, Cairo
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/tdj.tdj_20_18

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It is uncertain whether single-step self-etching adhesives form bond to enamel as reliable as those of etch-and-rinse adhesives. This study compared the microtensile bond strengths to ground enamel of three one-step self-etching adhesive systems, a self-etching primer system and an etch-and-rinse adhesive system.
Materials and methods
Human enamel was ground flat with 320-grit silicon carbide paper. The self-etching adhesives Clearfil Tri S Bond (Kuraray), Futurabond NR (Voco) and Hybrid Bond (Sun-Medical), the adhesive with a self-etching primer Clearfil SE Bond (Kuraray) and the etch-and-rinse adhesive Admira Bond (Voco) were applied as directed, followed by a core of the same manufacturers' hybrid resin composite. A microtensile bond strength evaluation was performed after 48 h of water storage, using untrimmed beams ~1.0 ± 0.1 mm2 in cross-sectional area at a cross-head speed of 0.5 mm/min.
There were no pretest failures in any group, and failures were predominately adhesive or mixed. Adhesion to enamel of Futurabond NR and Clearfil SE Bond was not significantly different from Admira Bond, while Clearfil Tri S Bond and Hybrid Bond demonstrated significantly lower bond strengths (one-way analysis of variance, Tukey–Kramer multiple-comparison test, P < 0.05).
Clinical significance
Bond strength to ground enamel of self-etching adhesive systems was dependant on the type of adhesive and some materials showed bond strength that was not different than that of etch-and-rinse system.

Keywords: enamel, microtensile, self-etch adhesives

How to cite this article:
Elhoshy AZ, Aboelenein K. Microtensile bond strength of self-etching adhesives to enamel. Tanta Dent J 2018;15:192-8

How to cite this URL:
Elhoshy AZ, Aboelenein K. Microtensile bond strength of self-etching adhesives to enamel. Tanta Dent J [serial online] 2018 [cited 2019 May 26];15:192-8. Available from: http://www.tmj.eg.net/text.asp?2018/15/3/192/243073

  Introduction Top

Studies on the adhesion of self-etching primers on enamel are recent, and results are not as consistent as those reported for the adhesion of the same products to dentin. Some authors have reported that the bond strength of self-etching primers is inferior to that obtained with adhesive systems, which utilize.

Phosphoric acid as a surface conditioner [1],[2],[3],[4],[5]. Conversely, other studies that tested composite-to-enamel bond strength with self-etching adhesive systems have reported values as high as 20–30 MPa [6],[7],[8] being in the same range as those reported on phosphoric acid-etched enamel. In studies of laboratory microleakage, self-etching adhesives were found to be approximately equal along dentin margins to those with separate etchant, but less effective along enamel margins [9],[10],[11]. The reduced effectiveness in adhesion to enamel of some self-etching products has been attributed to the relative acidity of the resins, with strong self-etchant said to produce more effective enamel etch than mild products [12].

The self-etching primer can simultaneously cause both acid conditioning and priming in one step. These adhesive systems eliminate separate steps of acid etching and water rinsing, thus simplifying bonding procedures. Such simplification may reduce the technical errors that often follow the use of total-etch adhesives, such as an over-etching, over-wetting, or over-drying of the prepared tooth surface.

Self-etching adhesives are of two types: self-etching primers, which are applied prior to an adhesive layer, and self-etching adhesives, which combine these two steps. Whether these newer products produce the same superior margins along enamel walls as their etch-and-rinse predecessors is of critical importance. Representative clinical studies have not demonstrated significant differences in the marginal discoloration of nonretentive Class V restorations between self-etching primers, self-etching adhesives and etch-and-rinse adhesives [13],[14],[15].

The objective of the present study was to compare the resin–enamel bonds made with three types one-step self-etching adhesives, one two-step self-etching primer with these of a total-etch adhesive, using the microtensile bond test, scanning electron microscopy (SEM) and fractography. The null hypothesis was that there is no difference between the self-etching materials and total-etch adhesive in their bond strengths to ground enamel.

  Materials and Methods Top

Microtensile bond strength

Twenty-five third molars stored in water saturated with thymol at 4°C were used within 1 month of extraction. The occlusal halves of the facial and lingual surfaces of each tooth were ground flat using 320-grit silicon carbide paper under running water at 60 rpm on a polishing machine (Buehler Ltd, Lake Bluff, Illinois, USA). These flat areas were prepared in a plane estimated to be parallel to the underlying dentinoenamel junction (DEJ).

Four commercial self-etching resin systems [Table 1] were selected for the study: Futurabond NR (Voco, Cuxhaven, Germany), Clearfil Tri S Bond (Kuraray Medical, Japan), Hybrid Bond (Sun-Medical, Japan.) and Clearfil SE Bond (Kuraray Medical, Japan.). The first three are one-step self-etching adhesives, while the fourth is a self-etching primer to which a layer of adhesive is applied. The etch-and-rinse system Admira Bond (Voco) was used as a control. Each adhesive system was applied according to the manufacturer's instructions as follow:
Table 1: Composition of the adhesive systems used in the study

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Admira Bond

The enamel surface was etched for 15 s with 37% phosphoric acid, rinsed with water spray. Excess water was removed with air blast for 3 s leaving the dentin moist. Admira Bond was applied with disposable brush, thinned with mild air for 2–3 s and light cured for 20 s using a halogen light source (Vivadent, Schaan, Liechtenstein). The output of the light curing unit was regularly checked (500 mW/mm2).

Clearfil SE Bond

The primer was applied for 20 s, blown with mild air. The bond was applied, thinned with a gentle stream of air and light cured for 20 s.

Clearfil Tri S Bond

The primer was applied for 20 s, blown with mild air. The bond was applied, thinned with a gentle stream of air and light cured for 20 s.

Hybrid Bond

Hybrid Bond was dispensed into the mixing well. A Hybrid Bond brush was dipped into the solution, stirred shortly and then applied to the surface for 20 s. The adhesive was thinned with gentle blast of air for 5 s and light cured for 20 s.

Futurabond NR

One drop of liquid A and one drop of liquid B was mixed in the mixing palette for 5 s. The material was then applied to enamel surface, massaged for 20 s, dried with air for 5 s and light cured for 20 s. The sample size was five teeth for each adhesive system.

Following application of the adhesives, ~6-mm thick cores of one of the same manufacturer's hybrid resin composites were built incrementally on the flattened enamel of each tooth. The resin composites used were Grandio (Voco), Clearfil APX (Kuraray Medical) and Pecalux (Sun-Medical). Increment thickness was limited to 2 mm, and each increment was cured for 20 s using the light source (Visulux curing unit).

After storage in distilled water at 37°C for 24 h, the restored teeth were hemisectioned occluso-gingivally along the mesio-distal axis. Each half-tooth was then sectioned facio-lingually into serial slabs ~0.9-mm thick using a slow-speed water-cooled diamond saw (Voco). Each slab was then sectioned perpendicular to the flat-ground enamel into resin composite, enamel and dentin beams ~0.9 × 0.9 mm in cross-section according to the 'nontrimming' version of the microtensile test [16]. Each tooth yielded six to eight beams for bond strength evaluation.

After 24 h of additional storage in distilled water at 37°C, the dimensions of each beam were measured with a digital calliper (Absolute Digimatic Model CD 6' CS; Mitutoyo Corp., Kanagawa, Japan) accurate to ±5 μm. Each slab was attached to the set-up by their lateral sides and placed in a universal testing machine (Instron Corp., High Wycombe, UK), then the tensile load will be applied at cross-head speed of 0.5 mm/min until failure occurs.

The types of failure were observed at 2.5 × magnification and categorized as adhesive, cohesive or mixed. In addition to microtensile testing, fractured interfaces from each group with high-bond, average-bond and low-bond strengths were selected for SEM (Philips, Eindhoven, the Netherlands) microscopic evaluation.

Morphological interface with enamel

The enamel of five additional teeth was also ground as previously described, and the etching component of each adhesive system was applied as directed. The adhesive was applied to the prepared enamel following the manufacturer's instructions but without the final photo polymerization, so that the resin could be eliminated for observation of the enamel surface. One hour after application of the adhesive, the enamel specimens were sonicated for 15 min, and then placed in 95% alcohol for 10 min and 100% alcohol three for times 10 min. Dehydration was completed by 12 h in an acetone bath. After drying, the specimens were sputter-coated with gold. These enamel specimens and the selected fractured beams were examined at different magnifications in a SEM operated at 15 kV.

Bond strength was calculated and data obtained for the five subgroups were statistically analyzed using a one-way analysis of variance, followed by a Tukey–Kramer multiple-comparison test at a 5% confidence level.

  Results Top

Bond strength

The bond strength of the tested adhesives is presented in [Table 2]. Adhesion to ground enamel of the Futurabond NR and Clearfil SE Bond self-etching systems was not significantly different from Admira Bond, while the two self-etching adhesive Clearfil Tri S Bond and Hybrid Bond systems demonstrated significantly lower bond strengths, with no significant differences among them (P < 0.05).
Table 2: Microtensile bond strength of the tested adhesives to enamel

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Morphological interface with enamel and failure patterns

SEM evaluation showed that conventional phosphoric acid etching produced the greatest topographical changes in the enamel surface [Figure 1]. Among the self-etching resins, Hybrid Bond and Futurabond NR showed topographical changes in the enamel surface similar to total-etch system [Figure 2] and [Figure 3]. Clearfil SE bond and Clearfil tri S bond showed the least changes in enamel surface [Figure 4].
Figure 1: Morphological changes in the enamel surface produced by conventional phosphoric acid etching: dissolution of either enamel prism cores or boundaries and creates microporosity on the enamel surface.

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Figure 2: Morphological changes in the enamel surface produced by Futurabond DC which were similar to those produced by total-etch system.

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Figure 3: Topographical changes in the enamel surface produced by Hybrid Bond.

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Figure 4: Clearfil SE bond showed the least changes in enamel surface.

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There were no pretest failures in any group. Fractured interfaces in general appeared to be adhesive (56.5%) [Figure 5]a, [Figure 5]b with remnants of resin composite attached to the tooth structure. Mixed type of failures was also evident in 27% of specimens [Figure 6]. In addition, cohesive failures within composite or enamel were seen in 8% of specimens [Figure 7], while cohesive failure at DEJ was seen in16.5% of specimens. Complete results are presented in [Table 3]. For all the adhesive systems, there was an obvious correlation between bond strength and microscopic fracture pattern.
Figure 5: Fractured surface of SE Bond specimen. Failure occurred primarily adhesively at resin–enamel interface (a) with remnants of resin composite attached to the tooth structure (b).

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Figure 6: Mixed type of failures; adhesive at interface and cohesive in composite.

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Figure 7: Fractured surface of Futurabond NR specimen. Failure occurred cohesively in composite.

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Table 3: Failure patterns of the tested adhesives to ground enamel

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  Discussion Top

In the present study, the specimens were slabs or beams that had a uniform dimension throughout their length. The top of the beam was resin composite, followed by the resin–enamel bond; beneath that was enamel, the DEJ and dentin. Thus, each slab was made up of many different structures. Since, they all had the same macroscopic cross-sectional area, the weakest material or structure should have failed first. The cohesive strength of enamel has been reported to vary from 11 to 25 MPa depending on the orientation of the prisms with respect to the tensile loading [17]. The highest strength (25 ± 10 MPa) was found when enamel prisms were stressed parallel to the prism orientation, as was done in the current study. The cohesive strength of the DEJ has been reported to be 50 MPa [18]. The cohesive strength of dentin varies with proximity to the pulp and direction of tubules, but has been reported to be 80 ± 14 MPa when stresses are perpendicular to the long axis of tubules [19]. Finally, the strength of resin–enamel bonds measured by the microtensile method has been reported to vary from 33 to 44 MPa for total-etch adhesive systems and from 21 to 37 MPa for self-etch adhesives or related products [16],[20]. Thus, enamel is the weakest structure in such a bonded assembly. The expectation was that most of the failures would be in enamel.

However, the failure patterns reported were different according to the tested adhesives. For all materials, the frequent mode of failure was adhesive at the interface (56.5%). This was followed by mixed adhesive/cohesive failure (27%), cohesive in enamel and or composite (8%) then cohesive at DEJ (16.5%). Failure at DEJ was reported in 26% of Admira Bond and 20% of Futurabond NR. For other adhesives, this failure mode account for 15, 11 and 9.5%, respectively.

For acid-etch systems, the primary mechanism of adhesion to enamel has long been micromechanical, when applied on enamel, phosphoric acid causes a selective dissolution of either enamel prism cores or boundaries and creates microporosities on the enamel surface ranging in depth from 5 to 50 μm [21]. Unlike phosphoric acid conditioning, the action of self-etch systems on enamel has been reported to produce ill-defined surface structures [7],[22],[23]. This was suggested to be further augmented by secondary bonds, which have formed between hydrophilic resins and enamel.

The results of this study are not in accordance with earlier ones 1–5 in that single-phase self-etching resin do not produce as good a bond with enamel as etch-and-rinse adhesives. These studies also reported little correlation between the pH of the adhesive and bond strength.

The bond strength of Admira Bond was the highest followed by Futurabond NR and Hybrid Bond. However, there were no significant differences in bond strengths between these materials. Clearfil SE Bond and Clearfil Tri S Bond exhibited bond strengths values significantly less than that of other materials, but the bond strength of the two materials were not significantly different.

Futurabond NR is a self-etching enamel dentin bonding agent. Liquid A contains special polyfunctional adhesive monomers (methacryl phosphorus acid ester and carbonic acid modified methacrylic ester). These phosphoric and carboxylic acid esters are essential for the adhesive bond as they acidify/condition the tooth structure. As acids only react in aqueous conditions, the water for this reaction is contained in liquid B of Futurabond NR. Mixing liquid A and liquid B activates Futurabond NR to produce a pH value of 1.4 [24]. Applying and working-in the bonding agent mobilizes the smear layer left after preparation and penetrates down to the tooth structure. It then solubilizes the hydroxyapatite of the tooth structure to create a retentive pattern on the enamel surface and demineralize the dentin analogous to conventional etching with phosphoric acid. Chemical bonding takes place also on the surface of the tooth structure due to complexation of the calcium by the adhesive monomers.

Hybrid Bond is a one-step self-etching adhesive. Hybrid Bond contains 4-META as an active monomer component. In an aqueous environment this monomer is converted to the dicarboxylic acid 4-MET, the etching component of Hybrid Bond with pH around 1 [25]. This high acidity resulted in rather aggressive demineralization. Enamel prism cores or boundaries were exposed and nearly hydroxyapatite crystallites were dissolved. Consequently, the underlying mechanism of bonding of Hybrid Bond is primarily diffusion-based, like that of the total-etch approach. Further, the hydrophilic end of 4-MET offers the advantages of forming ionic bonds to the calcium in apatite [26]. This bonding mechanism which seems to encapsulate the apatite crystals with a relatively hydrophobic methyl methacrylate bonding resin may explain the excellent performance of this material.

Clearfil SE Bond is a two steps, while Clearfil tri S bond is one-step mild self-etching adhesive. The primer of Clearfil SE Bond and S Tri Bond contains 10-MDP as functional monomer dissolved in HEMA and water to result in a pH around 2. These two materials produced the minimal changes in surface topography resulting in a very superficial resin infiltration. This ineffective demineralization might explain the lower bond strength of these materials to enamel. In a recent study [27] evaluating the enamel etching capacity of some contemporary self-etching adhesives, Clearfil SE Bond gave low proportions of calcium and phosphate ions loss and was considered unsatisfactory. Similar findings were also reported by Grégoire and Ahmed [28] and Di Hipólito et al. [29].

In the present study, only adhesives and resin composites from the same manufacturer were used. This eliminated the risk of incompatibility between any of the adhesives and a single resin composite. Like all studies that evaluate microtensile bond strength to enamel, this study is limited by adhesives approaching and exceeding the tensile strength of enamel. This produces failures at other sites in the specimens instead of the interface being studied, particularly through the enamel itself and along the DEJ, which occurred about 24.5% for the materials that demonstrated the highest bond strengths. The SEM evaluation of fractured interfaces is also limited by this, as there is little to be gained from observing cohesive fractures through tooth structure.

  Conclusion Top

Bond strength to ground enamel of self-etching adhesive systems are dependant on the type of adhesive system, some of the new adhesive systems showed bond strength values comparable to that of etch-and-rinse systems.

There was no correlation between bond strength and morphological changes in enamel surface.

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Conflicts of interest

There are no conflicts of interest.

  References Top

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]

  [Table 1], [Table 2], [Table 3]


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