Bonding effectiveness of self-etch adhesives to dentin after 24 h water storage.

PURPOSE: This study evaluated the immediate bonding effectiveness of five self-etch adhesive systems bonded to dentin.
MATERIALS AND METHODS: The microtensile bond strength of five self-etch adhesives systems, including one two-step and four one-step self-etch adhesives to dentin, was measured. Human third molars had their superficial dentin surface exposed, after which a standardized smear layer was produced using a medium-grit diamond bur. The selected adhesives were applied according to their respective manufacturer's instructions for μTBS measurement after storage in water at 37°C for 24 h.
RESULTS: The μTBS varied from 11.1 to 44.3 MPa; the highest bond strength was obtained with the two-step self-etch adhesive Clearfil SE Bond and the lowest with the one-step self-etch adhesive Adper Prompt L-Pop. Pretesting failures mainly occurring during sectioning with the slow-speed diamond saw were observed only with the one-step self-etch adhesive Adper Prompt L-Pop (4 out of 18).
CONCLUSIONS: When bonded to dentin, the self-etch adhesives with simplified application procedures (one-step self-etch adhesives) still underperform as compared to the two-step self-etch adhesive Clearfil SE Bond.

INTRODUCTION

In clinical practice, self-etch adhesives are currently widely used. They are based on the use of acidic functional monomers that can simultaneously demineralize and prime dentin. Self-etch adhesives eliminate the rinsing phase, reducing significantly clinical application time, technique sensitivity, and risk errors during application.[1] Self-etch adhesives can be classified into two-step self-etch adhesive which involve the application of an additional layer of solvent-free hydrophobic resin creating stronger adhesive layers, and one step self-etch adhesive which contain hydrophilic monomers, water, and volatiles solvents.[2]

As many adhesive are available on the dental market, laboratory-screening tests remain indispensable in providing data that, to a certain degree, predict clinical effectiveness. Currently, the microtensile bond strength test (μTBS) is the best screening method for a quantitative bonding effectiveness measurements.[3]

The performance of one-step self-etch systems to dentin appears to be material-dependent.[4]

Therefore, the current study evaluated mechanically different self-etch adhesives bonded to dentin. A two-step self-etch (Clearfil SE Bond) adhesive, served as “gold-standard” adhesive as it have been shown to be excellent performer in clinical and laboratory studies.[2567]

The hypothesis tested was that simple-to-use adhesives (one-step self-etch or all-in-one adhesive) have a similar bonding effectiveness to dentin as control gold-standard two-step self-etch adhesives.

MATERIALS AND METHODS

Selection of adhesives and tooth preparation

Five adhesives, including one two-step self-etch and four one-step self-etch adhesives, were chosen [Table 1]. Fifteen sound human molars (3/adhesive), gathered following informed consent approved by the Commission for Medical Ethics of the University, were used. The teeth, stored in 0.5% chloramine, were used within 3 months of extraction. Flat dentin surfaces were prepared by removing the coronal tooth part with an Isomet low-speed diamond saw (Isomet 1000, Buehler, Lake Bluff, IL, USA). Then, a standardized smear layer was prepared using a regular grit diamond bur (842, Komet, Lemgo, Germany) mounted in the MicroSpecimen Former (University of Iowa, Iowa City, IA, USA). The dentin surfaces were verified for the absence of enamel and/or pulp tissue exposition using a stereomicroscope (Wild M5A, Heerbrugg, Switzerland).

Table 1

Composition and instructions for use of the adhesives studied

Bonding procedures

Three teeth were used per adhesive. The adhesives were applied strictly following the manufacturer's guidelines [Table 1]. A composite build-up was then made using a single resin composite (Z100, 3M ESPE, St Paul, MN, USA), which was applied in five increments with a height approximately 1 mm and light cured for 40 s with an Optilux 500 light-curing device (Demetron, Kerr, Danbury, CT, USA) with a light output of not < 550 mW/cm.[2]

Microtensile bond strength testing

After 24-h storage in water, the resin–dentin-bonded specimens were sectioned with a water-cooled diamond saw (Isomet 1000, Buehler Ltd, Lake Bluff, IL, USA) in both the X and Y directions to obtain rectangular sticks (six to seven) from the central part of the coronal dentin surface. The dimensions of the sticks were then measured by means of a digital caliper (CD-15CPX, Mitutoyo, Kanagawa, Japan) from which the cross-sectional area was calculated (approximately 0.9 mm2). The nontrimmed microspecimens were fixed to a modified microtensile bond testing jig[8] with cyanoacrylate glue (Model Repair II Blue, Dentsply-Sankin, Tokyo, Japan) and tested in tension at a crosshead speed of 1.0 mm/min using an LRX testing machine (Lloyd, Hampshire, UK) equipped with a load cell of 100 N. The bond strength values were calculated in MPa by dividing the imposed force (in N) at the time of fracture by the bond area (in mm2).

The failure modes were evaluated with a stereomicroscope (Wild M5A) at a magnification of up to ×50 and categorized as “interfacial,” “cohesive,” (dentin or composite) or “mixed.” The data were statistically evaluated by one-way ANOVA and Tukey's multiple comparisons test at a significance level of α = 0.05.

RESULTS

The μTBS of the five self-etch adhesives to dentin are shown in Table 2. The mean μTBS values ranged from 11 to 44.3 MPa. The lowest value was obtained for the one-step self-etch adhesive (Adper Prompt L-Pop) and the highest was obtained with the two-step self-etch adhesive Clearfil SE Bond.

Table 2

Microtensile bond strength and failure analysis

Pretesting failures (mainly failures during sectioning with the slow-speed diamond saw) occurred only with the one-step self-etch adhesive Adper Prompt L-Pop (4 out of 18). No pretesting failures were recorded for any other adhesive tested. Failure analysis showed interfacial, cohesive, and/or mixed fractures, depending on the adhesive tested [Table 2]. A general trend was observed: Specimens that presented with lower bond strength (Adper Prompt L-Pop, Xeno III, and G-Bond) failed more at the resin–dentin interface (interfacial failure). On the other hand, specimens with higher bond strengths (Clearfil SE Bond and Clearfil S3 Bond) failed more cohesively in dentin or resin.

DISCUSSION

In the current study, the bonding effectiveness of five self-etch adhesives to dentin was comparatively investigated. The hypothesis that simple-to-use adhesives (one-step self-etch or all-in-one adhesive) have a similar bonding effectiveness to dentin as control gold standard two-step adhesives were rejected as one-step self-etch adhesives revealed a significantly lower bond strength [Table 2]. As the mean μTBS is statistically lower in caries-affected dentin than in normal dentin for self-etch adhesives, this study was conducted on sound dentin.[9]

The μTBS test was used in this study to mechanically assess the strength of the resin–dentin interface complex. Today, the μTBS is one of the most commonly used methodologies[1011] since it has several advantages:[10] multiple specimens can be obtained from a single tooth, the stress is more uniformly distributed during loading across the interface.[3] Many modifications to this μTBS method have been proposed in the literature,[12] especially for the final preparation of the interface area. In the current study, the authors opted for nontrimmed μTBS specimens that combine a good stress distribution at the interface[13] with a minimal amount of processing; the 1 mm × 1 mm resin–dentin sticks are cut out of the restored tooth and directly transferred to the universal testing machine.

Only the central dentin portion that is located directly above the pulp was used in the current study to minimize any regional variation between the periphery and central dentin substrate.[14] This, however, reduced the number of specimens available for testing (6–7, instead of up to 30 in other studies[12]) but was thought to increase the validity of the results. As revealed by Vanajasan et al., the origin of dentin, site of bonding, and bonding area had significant influence on bond strength of one-step and two-step self-etch adhesive.[15]

As they are simple-to-use, self-etch adhesives are clinically most attractive, because they do not need a rinse step, thus avoiding the technique-sensitive drying of etched dentin. Furthermore, previously acid etching dentin do not significantly increase the bond strength of one-step self-etch adhesive to dentin.[16]

Self-etch adhesives differ not only because of the number of clinical steps (two- and one-step self-etch adhesives) but also because of the wide interaction intensity that can be observed. A clear correlation between the pH of self-etch primer and the depth of interaction with dentin was observed as reported by De Munck et al.[17] Today, four categories can be distinguished as follows:[17] (1) strong self-etch adhesives have a pH lower than 1 and an interfacial micromorphology (3–4 μm deep fully demineralized hybrid layers) that is very similar to that of etch and rinse adhesives; (2) they are intermediately strong self-etch adhesives with a pH of around 1.5. These adhesives have a hybrid layer of about 1–2 μm, wherein at the bottom part, some hydroxyapatite is preserved; (3) they are mild self-etch adhesives with a pH of around 2. The hybrid layer is < 1 μm thick and is only partially demineralized; and (4) recently, a category of ultra-mild self-etch adhesives was added for self-etch adhesives that come with a primer pH higher than 2.5.[18]

According to De Munck et al.,[17] these adhesives do not remove the smear layer and interact with smear layer covered dentin only up to a few hundredth of a nanometer. As interaction with intact dentin is almost nonexistent, the micromechanical resistance of the interface complex is very dependent on impregnation and stabilization of the smear layer. As a result, the μTBS is far more dependent on the preparation of the surface.[7] In light of bond durability, mild and ultra-mild self-etch adhesives have some unique properties since not all hydroxyapatite is removed from the interaction zone, much calcium is available for additional chemical interaction with specific adhesive functional monomers.[19]

A two-step self-etch adhesive Clearfil SE Bond served as “gold standard” adhesive as it have been shown to be excellent performer in clinical and laboratory studies.[20212223] The clinical performance of this adhesive in randomized Class V studies is excellent[22] and even approaches that of the gold standard three-step etch and rinse adhesive OptiBond FL.[8] These excellent in vitro and in vivo results must be ascribed to their unique two-fold bonding mechanism related to the mild self-etch approach, which comprises a micromechanical bonding component through the formation of a 1 μm thick hybrid layer that may provide resistance to acute debonding stress (as imposed during μTBS testing). In addition, this adhesive contains the functional monomer 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP), which enables an intensive and stable chemical bond with hydroxyapatite.[21] Such primary chemical interaction improves the resistance to hydrolytic breakdown,[10] and thus, clinically keeps the restoration margins sealed for a longer period. Furthermore, the high degree of conversion of two-step self-etch adhesives resists water aging and improves the initial bond strength and durability of the resin–dentin bond.[1]

Adper Prompt L-Pop is a strong one-step self-etch adhesive. This adhesive scored the lowest bonding effectiveness of all the adhesives tested (11.1 MPa), and it was also the only adhesive for which pretesting failures were recorded (4 out of 18 specimens). This poor in vitro performance is corroborated with several clinical Class V studies that report less favorable in vivo performance for this strong self-etch adhesive.[621]

Xeno III is a typical intermediately strong self-etch adhesive. Although its μTBS was the third lowest of all the adhesives tested, clinically, up to 10% of the Class V restorations debonded after only 2 years of clinical service, as reported in a randomized Class V study[23] comparatively with the 100% and 94% retention rate obtained with a three-step etch and rinse adhesive OptiBond FL, respectively, at 5 and 7 years.[8]

Furthermore, both of the ultra-mild self-etch adhesives that were tested, G-Bond and Clearfil S3 Bond, contain functional monomers with additional chemical bonding potential, suggesting a similar two-fold bonding mechanism. Short-term clinical evaluation of these adhesives in Class V restorations suggests, however, adequate bonding performance to this lightly prepared, highly mineralized dentin tissue.[2]

Long-term clinical follow-up and adhesion of self-etch adhesives to different substrates, such as carious dentin, should be investigated in the future research.

CONCLUSIONS

The bonding effectiveness of current commercial self-etch adhesives is variable. The highest μTBS obtained in the current study was with the two-step self-etch adhesive Clearfil SE Bond which remains the gold standard. Ultra-mild adhesive Clearfil S3 Bond and intermediately strong adhesive Xeno III are well performed as they combine optimal bonding effectiveness with a simplified application protocol.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.