INTRODUCTION

The advent of self-etching priming adhesives has simplified the use of resin-based bonding systems in clinical practice. The first versions were based on two-bottle systems with a separate etching-priming liquid, followed by the application of an adhesive resin. These systems have been shown to be less technique sensitive1 and have the capability of bonding to various locations on teeth, for example, coronal, radicular and pulp chamber dentin.2–3 Data from clinical trials have also indicated these systems are a good alternative to those systems that have a separate etch, primer and adhesive and have been shown to perform better than the “wet bonding” phosphoric acid etch-based systems.4–5

Self-etching systems act by dissolving the smear layer but leave remnants of the smear plugs and etch the superficial enamel and dentin at the same time. It is this simultaneous etch and infiltration of the resin that is believed to be conducive to forming a well-infiltrated hybrid layer, even to dentin approximating the pulp.6

Recently, manufacturers have further developed self-etching priming resin-based adhesives into a single solution, often referred to as “all-in-one” systems. These adhesives combine the etching, priming and adhesive steps into one process. Although very simple in technique, studies show that these systems may not perform as well as two-step self-etching priming systems. This is thought to be partially due to water in the adhesive, which is needed to maintain its acidity and also the smear layer being incorporated into the adhesive layer.7–9 These systems have also been divided into low pH and high pH systems, and their subsequent mode of adhesion is also thought to vary.6 Van Meerbeek and others6 have stated that low pH systems tend to form a micromechanical bond to dentin; whereas, higher pH systems are believed to incorporate a chemical interaction with calcium in the dentin, in addition to the micro-mechanical bond.

Comparative data on adhesive ability for the recent all-in-one systems is still limited. The aim of this research was to compare the micro-shear bond strengths of one two-step self-etching adhesive with five commercially available all-in-one systems. The null hypothesis is that there is no difference among bond strengths of the systems tested.

METHODS AND MATERIALS

Human molars, which were used within three months after extraction, were stored in water containing Thymol and maintained at 4°C. The teeth were extracted as part of a comprehensive treatment plan and collected after obtaining verbal consent from the patient to retain the teeth for research purposes. Ten teeth were used for the study. The roots were removed from the teeth at the cementoenamel junction, then the crowns were sectioned buccolingually using a diamond blade and stored in water prior to embedding in Type III dental stone. The sectioned surfaces comprised enamel and dentin, which were finished with wet 600-grit silicon carbide paper for approximately 10 strokes to ensure a flat surface and uniform smear layer.

The adhesive systems used are listed in Table 1. The enamel and dentin were bonded according to the manufacturers' instructions. The bonded area was delineated by placing a piece of polyethylene tube 0.7 mm in internal diameter and approximately 1.5 mm high on the uncured resin bonded surface and light cured. The tube was filled with Clearfil APX resin composite (Kuraray Medical, Tokyo, Japan, Batch #1079AA) and cured for 20 seconds. Each of approximately five tubes was bonded to the enamel and dentin of each tooth section. The bonded specimens were stored in water at 37°C for 24 hours.

Table 1 Materials and Manufacturers Used

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The specimens were placed in a jig attached to a universal testing machine, and a loop of stainless steel wire was placed around the resin composite and loaded at a rate of 1 mm/minute until bond failure occurred. The load at failure was recorded, then converted to MPa. Fifteen specimens were tested for each resin-based adhesive for enamel and dentin.

The fracture mode for each of the bonded tubes was also recorded. The fracture pattern was divided into one of three types: a) partial adhesive failure, where remnants of bonding resin remained on the enamel or dentin; b) adhesive failure at the resin-enamel or resin-dentin interface; c) mixed adhesive failure and cohesive failure in enamel, dentin or resin composite.

The mean bond strengths were calculated then analyzed statistically using univariate analysis of variance (ANOVA) followed by multiple comparisons between groups using the Tukey's HSD test with the significance level set at p<0.05.

RESULTS

The mean bond strengths to enamel and dentin are listed in Table 2. The failure modes are listed in Table 3. All systems showed the bond strengths to dentin were greater than to enamel.

Table 2 Mean Microshear Bond Strengths to Enamel and Dentin MPa (±SD); n=15

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Table 3 Fracture Modes to Enamel and Dentin

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Results of the ANOVA showed that both the substrate (enamel or dentin) and material significantly affected bond strengths p<0.05. For materials bonded to enamel, the two-step self-etching-priming material SE Bond was significantly stronger than S3, Adper Prompt L-Pop and One-Up Bond F (p<0.05). One-Up Bond F was significantly weaker than all the other test materials.

When bonded to dentin, SE Bond was significantly stronger than all the other materials tested (p<0.01), and One-Up Bond F was significantly weaker than all the other test materials (p<0.01).

Go! was significantly stronger than S3 and One-Up Bond F Plus (p<0.05) when bonded to enamel. The bond strength to dentin for Go! was not significantly different from S3, G-Bond and Adper Prompt L-Pop.

The failure mode showed little variation among the materials tested, with adhesive failure being the most common type of failure observed.

DISCUSSION

Simplification of the self-etching priming systems has not led to an improvement in bond strengths. The results from this study indicate that the two-step self-etching priming adhesive provided consistently stronger bonds to both substrates, as has been previously reported.8 However, with the exception of One-Up Bond F Plus, the all-in-one adhesives produced remarkably similar bond strengths to both enamel and dentin. Adper Prompt L-Pop, however, showed greater variability when bonded to dentin, which may indicate slightly greater technique sensitivity compared with the other materials tested. This is in variance to the recent study by Perdigão and others,8 who showed Adper Prompt L-Pop bonded well to enamel and dentin; whereas, the current study showed Adper Prompt L-Pop bonded to a lesser extent to enamel than dentin. This may be due to the different methods used. The recent study by Grégoire and Millas10 classified self-etching systems into three broad groups, depending on the pH of the adhesive. The most acidic materials exhibited a pH of less than 1, an intermediate group, of which One-Up Bond F Plus fits and has a pH of 1-1.5 and a third group, with a pH of greater than 1.5, of which Clearfil SE Bond belongs. The pH of G-Bond—2.3 and Clearfil S3—2.4 was reported by Nishitani and others.11 The pH of the latter two materials is much higher than the other systems tested; however, bond strengths were not different from the other materials tested. Therefore, pH alone is not the sole parameter for achieving a good bond.

In their study, Sidhu and others12 showed that G-Bond did not perform as well as the other all-in-one systems they tested. The current study showed that all-in-one systems varied little, other than One-Up Bond-F Plus. However, both the current study and the one conducted by Sidhu and others12 showed the two-step system performed better than the all-in-one systems.

The current study is one of the few studies to investigate the microshear bond strength of all-in-one systems to enamel and dentin. Most other studies have used the microtensile bond test method, which is more complicated and labor intensive for specimen production. In addition, the production of specimens has the potential to induce microcracks at the interface, which can affect bond failure, particularly in enamel.13 When enamel bonding is evaluated, the microshear bond strength test method is considered the more suitable method compared with the microtensile test method. The microshear test is able to bond a number of tubes to the same tooth simultaneously. It is very important, however, to ensure that the wire loop is pushed against the tooth to ensure the shear stress is applied to the adhesive interface between the tooth and resin composite.14 Embedding the teeth in dental stone maintained a moisture reservoir to ensure the tooth sections remain hydrated during specimen preparation and debonding. Orientation of the tooth sections was also different from microtensile bond studies in that the teeth were sectioned buccolingually, which is equivalent to bonding to the sides of cavity walls/margins, rather than the occlusal surface, which is typically done for the microtensile test. This orientation may have had some influence on bond strengths, as several studies indicate that dentin tubule and enamel prism orientation can affect bond strengths.15–18 Care was taken to ensure that the samples bonded to dentin were as close as possible to the dentino-enamel junction to avoid the influence of regional differences in the dentin, such that a close approximation to pulp horns could have increased variation in the results.

Although One-Up Bond F Plus showed lower bond strengths, a recent clinical study using the previous version (One-Up Bond F) showed good results over five years.19 Therefore, attaining very high bond strengths is not necessarily an indicator of clinical success. It would appear that other parameters, such as chemical interaction with the tooth surface and bond stability over the long-term, may be important for the clinical success of bonded restorations.

It is interesting to note that, although a system, such as Adper Prompt L-Pop, has a low pH, it showed no difference in bond strengths to ground enamel compared with the milder etching systems. It would have been anticipated that this system should have etched and bonded well to the enamel surface, but it produced bond strengths equivalent to S3 and was significantly less than the two-bottle system SE Bond. The pH of the adhesive is not the only factor to determine the bond. Functional monomers, the catalyst used, as well as other components in the bonding system, vary among products. All of these factors will influence bonding. However, all systems tested showed better bond strengths to the dentin surface compared with enamel. This could lead to potential problems during curing of the resin composite, as curing contraction could pull the resin composite off the enamel margin, leaving it susceptible to leakage, staining and caries. To overcome this possible problem, particularly to uncut enamel where it has been shown that bond strengths are reduced, the GC Corp for G-Bond recommends the use of phosphoric acid to ensure a good enamel bond.20 Further investigation should be carried out to determine whether the etching of enamel prior to application of self-etching solutions will provide any clinical benefit to retention rates and reduced marginal staining around restorations.

CONCLUSIONS

It seems that bond strengths, which can be attained by all-in-one systems, may provide satisfactory results clinically, although far more data are needed to confirm this. The null hypothesis was partially rejected, since the two-step system demonstrated superior results when bonding to enamel in this laboratory study. However, it is necessary to conduct clinical trials to clearly ascertain performance of the new, all-in-one systems when used to restore teeth.