INTRODUCTION

For many years, the application of resin to enamel, which has been etched with phosphoric acid, has produced a well-sealed interface that is sufficiently strong to retain most resin composite restorations. In recent years, self-etching resins containing acidic methacrylates have been introduced in an attempt to eliminate the steps of etching and rinsing. These methacrylates are intended to produce simultaneous demineralization of the enamel surface and infiltration of the resin into the resulting surface topography and produce resin tags and micromechanical bonds comparable to those evident with conventional etching. Self-etching resins 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. Clinical data is limited to the evaluation of postoperative sensitivity in Class I or II resin composite restorations, which could indicate a poor marginal seal, and to observation of the enamel margins of Class I and II and non-retentive Class V resin composite restorations in which poorly-sealed margins may discolor or become carious. Representative clinical studies have not demonstrated significant differences in the marginal discoloration of non-retentive Class V restorations between self-etching primers, self-etching adhesives and etch-and-rinse adhesives (Burrow & Tyas, 2003; Turkun, 2003; van Dijken, 2004) or among these three classes of adhesive in marginal discoloration or postoperative sensitivity in Class I and II restorations (Denehy & others, 2002; Perdigão, Geraldeli & Hodges, 2003; Yip & others, 2003).

Unfortunately, the strength of the enamel bond cannot be adequately evaluated in clinical studies of Class I, II and V restorations, because it is largely retained through mechanical retention or dentin adhesion, and the results would likely not generalize to clinical situations highly dependent on enamel adhesion, such as non-retentive Class IV restorations. Laboratory studies of bond strength to ground enamel demonstrate that self-etching resins produce lower bond strengths to ground enamel than etch-and-rinse adhesives, with self-etching adhesives lower than self-etching primers (Inoue & others, 2001; De Munck & others, 2003, 2005b). This effect is more pronounced on unground enamel (Pashley & Tay, 2001; Perdigão & Geraldeli, 2003). All of the studies previously cited include just one or two self-etching products, thus, many materials have not been evaluated.

This study compared the microtensile bond strength to ground enamel of three self-etching adhesive systems to a self-etching primer system and an etch-and-rinse adhesive system.

METHODS AND MATERIALS

The extracted teeth were collected in accordance with human subjects' regulations at the Medical College of Georgia. Twenty-five third molars stored in water saturated with thymol at 4°C were used within three months of extraction. Based on a previous study that showed no difference between facial and lingual enamel bond strengths (Shono & others, 1997), 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 (Ecomet 3, Buehler, Ltd, Lake Bluff, IL, USA). These flat areas were prepared in a plane estimated to be parallel to the underlying dentino-enamel junction.

Four commercial self-etching resin systems were selected for the study: Adper Prompt L-Pop (3M ESPE, St Paul, MN, USA), iBond GI (Heraeus Kulzer, Inc, Armonk, NY, USA), Xeno III (Caulk/Dentsply, Milford, DE, USA) and Clearfil SE Bond (Kuraray Co, Ltd, Osaka, Japan). The first three are self-etching adhesives, while the fourth is a self-etching primer to which a layer of adhesive is applied. The etch-and-rinse system Adper Scotchbond Multi-Purpose (3M ESPE) was used as a control. Each adhesive system was applied according to the manufacturer's instructions and is listed in Table 1. For the self-etching resins, in order to completely evaporate solvents, air drying was continued for approximately 15 seconds past a “dried” appearance of the resin layer. The sample size was five teeth for each adhesive system.

Table 1 Manufacturer, Instructions/Technique, Composition, and pH for Each Adhesive System

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Following application of the adhesives, approximately 6-mm thick cores of one of the same manufacturer's hybrid resin composites were built incrementally on the flattened etched enamel of each tooth. The resin composites used were Z 250 (3M ESPE), Venus (Heraeus Kulzer), TPH Spectrum (Caulk/Dentsply) and AP-X (Kuraray). Increment thickness was limited to 2 mm, and curing was accomplished from all directions using a fast halogen light source (VIP, BISCO, Inc, Schaumburg, IL, USA) for a total of 5 minutes of curing per specimen. Light output of 600 mW/cm² was verified throughout the study, using the unit's built-in radiometer.

After storage in distilled water at 37°C for 24 hours, the restored teeth were hemisected occluso-gingivally along the mesio-distal axis. Each half-tooth was then sectioned facio-lingually into serial slabs approximately 0.9-mm thick using a slow-speed water-cooled diamond saw (Isomet, Buehler, Ltd). Each slab was then sectioned perpendicular to the flat-ground enamel into resin composite, enamel and dentin beams approximately 0.9 × 0.9 mm in cross section according to the “non-trimming” version of the microtensile test (Shono & others, 1999). Each tooth yielded 5 to 10 beams for bond strength evaluation.

After 24 hours of additional storage in distilled water at 37°C, the dimensions of each beam were measured with a digital caliper (Absolute Digimatic Model CD 6″ CS, Mitutoyo Corp, Kanagawa, Japan) accurate to ± 5 μm. The beams were then affixed to a Ciucchi device (Kuraray) using Zapit cyanoacrylate glue (Dental Ventures of America, Corona, CA, USA) and tested to failure under tension in a universal testing machine (Vitrodyne V1000, Chatillon, Largo, FL, USA) at a crosshead speed of 0.6 mm/minute. 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, average and low bond strengths were selected for microscopic evaluation. The enamel of five additional teeth was also ground as previously described, and the etching component of each adhesive system was applied as directed, except for the self-etching resins, which were removed with copious absolute ethanol instead of being light cured. These teeth and the selected fractured beams were then dehydrated in alcohol and sputter-coated. The fractured beams were examined at 80× magnification, and the etched enamel surfaces were examined at 5000× magnification in a SEM (Model XL30 FEG, Philips Electronics, Eindhoven, The Netherlands) operated at 10kV.

Bond strength was calculated and data obtained for the five subgroups were statistically analyzed using a one-way ANOVA, followed by a Tukey-Kramer Multiple Comparison Test at a 5% confidence level.

RESULTS

Data for each subgroup were found to be normally distributed. Adhesion to ground enamel of the Clearfil SE self-etching primer system was not significantly different from Scotchbond Multi-Purpose, while the three self-etching adhesive systems demonstrated significantly lower bond strengths, with no significant differ ences among them (p<0.0001). There were no pretest failures in any group. Overall, failures were predominately adhesive or mixed. The incidence of cohesive failures in dentin, enamel or along the dentino-enamel junction was more common with increasing bond strength. Complete results are presented in Table 2.

Table 2 Resin/Enamel Microtensile Bond Strengths

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SEM evaluation showed that conventional phosphoric acid etching produced the greatest topographical changes in the enamel surface (Figure 1). Among the self-etching resins, there appeared to be no correlation between pH and increased surface topography or between microscopic appearance of the etched enamel and bond strength. For all the adhesive systems, there was no obvious correlation between bond strength and microscopic fracture pattern. Fractured interfaces in general appeared to have failed primarily through the unfilled resin layer, with remnants of resin composite attached to the tooth structure for most specimens, even those judged visually to have failed adhesively.

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DISCUSSION

For resin-based adhesives, the primary mechanism of adhesion to enamel has long been micromechanical, with acid etching greatly increasing the surface area, and a stable, well-polymerized resin infiltrated into the surface topography. This can be augmented by secondary bonds, which have formed between hydrophilic resins and enamel.

The results of this study are in accordance with earlier reports, in that single-phase self-etching resins do not produce as good a bond with enamel as etch-and-rinse adhesives. In both this study and in an earlier report (De Munck & others, 2005a), there is little correlation between the pH of the adhesive and bond strength. Since some self-etching resins, such as Prompt L-Pop, appear to adequately etch enamel but yield relatively low bond strengths, it appears that the resin infiltrated into the enamel surface does not form a strong polymer. Studies of such products have demonstrated the presence of water-rich phases that are detrimental to strength and stability (Van Landuyt & others, 2005). Conversely, an adhesive such as Clearfil SE can produce relatively high bond strengths with minimal changes in surface topography compared to etching, which must result from additional secondary bonding. Although promising, it is not known whether this greater reliance on secondary bonds will equal the long-term stability of the micromechanical bonding of conventional etching. The authors believe this should be addressed through clinical studies that focus on enamel bond strength and that the use of self-etching resins alone is contraindicated in clinical situations where maximum enamel adhesion is needed.

For this study, the authors elected to pair only adhesives and resin composites from the same manufacturer for each group of specimens, because this would be the most likely way that resins would be purchased by clinicians. This also eliminated the risk of incompatibility between any of the adhesives and a single resin composite, but dictated that the study was an evaluation of adhesive systems rather than of the adhesives themselves. Like all adhesives and resin composites 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 dentino-enamel junction, which occurred about half of the time for the two materials that demonstrated the highest bond strengths. The value of 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.

CONCLUSIONS

Only the adhesive system that used a self-etching primer demonstrated an enamel bond strength comparable to the etch-and-rinse adhesive system.