INTRODUCTION

Many researchers have studied adhesion to enamel.1 Although different modalities have been tested,2–4 at present, phosphoric acid etching seems to be the most frequently used method of enamel surface preparation. One of the potential disadvantages of etching with phosphoric acid is that the acid causes demineralization of the most superficial layer.5–8 To control excessive enamel loss, maleic and polyacrylic acids have been used as alternatives for phosphoric acid. The use of polyacrylic acid has been found to result in a reduction in bond strength.9–11

A unique characteristic of some new bonding systems in operative dentistry is that they combine conditioning and priming agents into a single acidic primer solution for simultaneous use on both enamel and dentin.1213 Combining conditioning and priming into a single treatment step results in improvement in both time and cost-effectiveness for clinicians as well as for patients. In a self-etching primer, the active ingredient is a methacrylated phosphoric acid ester. The phosphoric acid and the methacrylate group are combined into a molecule that etches and primes at the same time. The phosphate group on the methacrylated phosphoric acid ester dissolves the calcium and removes it from the hydroxyapatite. But rather than being rinsed away, the calcium forms a complex with the phosphate group and gets incorporated into the network when the primer polymerizes. Agitating the primer on the tooth surface serves to ensure that fresh primer is transported to the enamel surface. Etching and monomer penetration to the exposed enamel rods are simultaneous. In this manner, the depth of the etch is identical to that of the primer penetration. Three mechanisms act to stop the etching process. First, the acid groups attached to the etching monomer are neutralized in a similar way, as is phosphoric acid, by forming a complex with the calcium from the hydroxyapatite. Second, as the solvent is driven from the primer during the airburst step, the viscosity rises, slowing the transport of acid groups to the enamel interface. Finally, as the primer is light cured and the primer monomers are polymerized, transport of acid groups to the interface is stopped.14

These new systems were also found to be effective when bonding the brackets to the enamel. In recent laboratory studies on acidic primers by Bishara et al,15–17 two different acidic primers for restorative dentistry and/or adhesives for restorative dentistry and orthodontics were tested. The results indicated that both self-etching primer systems resulted in significantly reduced bond strength values. It was suggested that more research was needed to determine whether currently available orthodontic adhesive systems could be used with an acidic primer.16

Recently, a new acidic primer (Transbond Plus, 3M Unitek, Monrovia, Calif) to be used for orthodontic purposes was introduced. The purpose of this study was to determine the efficacy of three different self-etching primers on the shear bond strength (SBS) of orthodontic brackets and the bracket/adhesive failure mode in comparison with conventional 37% phosphoric acid etching.

MATERIALS AND METHODS

Eighty noncarious human maxillary and mandibular premolars, extracted for orthodontic purposes, were used in this study. Teeth with hypoplastic areas, cracks, or gross irregularities of the enamel structure were excluded from the study. The teeth were stored in distilled water after extraction. The water was changed weekly to avoid bacterial growth. The sample was randomly divided into four groups of 20 each. Each tooth was mounted vertically in a self-cure acrylic so that the crown was exposed. The buccal enamel surfaces of the teeth were pumiced, washed, and dried before surface preparation.

Enamel surfaces were prepared, and the brackets were bonded to the teeth according to one of the following protocols:

Transbond Plus group

The orthodontic self-etch primer Transbond Plus (TBP) is predosed so that it is used for single applications. For activation, the two components, ie, acid and the primer, were squeezed together, and the resulting mix was applied directly onto the tooth surface. The area to be etched was rubbed with the solvent for three seconds and was gently air-dried for two seconds. Table 1 describes the principle ingredients of the self-etching primers as supplied by the manufacturers.

TABLE 1. Principle Ingredients of the Three Self-Etching Primers as Provided by the Manufacturers

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RESULTS

SBS comparisons

The descriptive statistics for the SBSs of the four groups tested are presented in Table 2. As can be seen from the table, the highest bond strength values were produced by the TBP group, followed by the AE, CSE, and EP3 groups. The ANOVA and Duncan's multiple range test revealed that TBP (16.0 ± 4.5 MPa), when compared with the other conditioning methods, yielded significantly higher bond strength values. CSE (11.5 ± 3.3 MPa) produced bond strength values that are statistically comparable to those of acid etching (13.1 ± 3.1 MPa). The use of EP3 for enamel conditioning resulted in the lowest mean SBS value (9.9 ± 4.0 MPa). The survival graphs (Figure 1) indicate that half of the brackets remain bonded at 18 MPa in the TBP group, at 14 MPa in the control group, at 11 MPa in the SEB group, and at 8 MPa in the EP3 group. The survival curves of the last three groups are almost parallel to each other, which indicates a uniform decrease in the survival of brackets with increasing stresses. On the other hand, in the TBP group, the curve is less steep, and this indicates a lower correlation of bond failure with the increasing shear stress values.

TABLE 2. Descriptive Statistics and the Results of the Duncan's Multiple Range Test Comparing the Shear Bond Strengths (in MPa) of the Four Groups Tested

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ARI comparisons

The results of the chi-square comparisons indicated that there were significant differences between the four groups tested (Table 3) (P ≤ .001). There was a greater frequency of ARI scores of 1 with the TBP and AE groups, which indicated that failures were mainly in the adhesive-bracket interface. Differences in ARI scores were not statistically different between the TBP and AE groups. In the CSE group, the failures were mostly cohesive within the resin (ARI score = 2, 3, and 4). Scores of the CSE group were statistically different from that of the AE and TBP groups. In the EP3 group, there was a higher frequency of ARI scores of 4 and 5, which indicated that less residual adhesive remained on the enamel than on the bracket base after debonding. The ARI scores of the EP3 group were statistically different from those of the other three groups.

TABLE 3. Frequency Distribution of the ARI Scores of the Four Groups Tested^a

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SEM examination

There were no distinct features in the enamel-resin interface to differentiate between the SEM micrographs of the cross-sections of the AE and TBP-treated enamels. Figure 2 shows a cross section from a TBP-treated enamel surface in which the outer enamel surface and TBP layer was not sectioned with the saw but was cracked. An irregular but smooth hybrid layer, 3–4 μm in thickness, covers the enamel prisms. Figures 3 and 4 show the adhesive under the bracket base after dissolution of the tooth structure in acid to reveal and compare the resin tag formation in the AE and TBP-treated enamels, respectively.

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In the AE samples (Figure 3), a distinct honeycombed structure with micro- and macrotag formations is present. In the TBP-treated samples (Figure 4), an irregular tag formation with no apparent indentations of enamel prism or core material is present.

DISCUSSION

The present study evaluated the SBS values produced by three self-etching primers and compared them with those produced by the conventional acid-etching technique. The lowest SBS values were recorded in the two self-etching primer groups (CSE and EP3) designed to be used for restorative purposes. The CSE bonding system has a pH value of 2.0 and is comparatively less acidic than EP3 and TBP, which have pH values of 0.75 and 1.0, respectively. The efficacy of self-etching primers on unground enamel, however, does not depend upon their etching aggressiveness.20 Even with the existing differences in the composition of the three self-etching primers, it is very difficult to find a correlation between the monomer structure and the bond strength.19

The low bond strength values produced by the aggressive self-etchant EP3 in our study were in accordance with the findings of Toledano et al.19 Contradictory to our results, on the other hand, Pashley and Tay,20 comparing the acid-etching, CSE, and Prompt-L-Pop (PLP; restorative version of TBP) groups, found significantly lower microtensile bond strength values with the self-etching primers than with acid etching. But when the self-etching primers were rinsed and replaced with the control resin, the bond strengths in the PLP group were no longer significantly different from those produced by 32% phosphoric acid etching. In the same study, similar to our results, an aggressive etching effect and a thick hybrid layer formation after PLP application were reported.

Bishara et al15 discussed the selective compatibility of early acidic primers with different adhesives and mentioned the need for acidic primers manufactured for orthodontic purposes. TBP, as the only orthodontic acidic primer in the market today, was superior to the other three conditioning methods both in terms of SBS and the time required for application.

It was interesting to observe that the average bond strength in the TBP group was significantly higher than that of the conventional AE technique. Self-etching primer produces a highly porous surface on aprismatic enamel. The etching effect approaches that of phosphoric acid despite the absence of additional gross retentive structures that are produced by differential dissolution of the underlying enamel prisms. One important advantage to simultaneous etching and priming is that the primer penetrates the entire depth of the etch, ensuring an excellent mechanical interlock.

SEM examination of the impressions of the AE (Figure 3) and TBP-treated (Figure 4) enamels showed different surface characteristics. Despite the lack of tag formation, high bond strengths obtained with TBP and the fact that more adhesive remained on enamel after debonding (ARI scores = 1 and 2) implicate the presence of different mechanisms that constitute adhesion. Conversely, several other investigators have concluded that there is no correlation between the bond strength and the tag length and that the adhesive strength of the resin to enamel is mainly attributable to the resin's ability to penetrate between the enamel crystallites and rods.2122

Bishara et al15 reported on tag formation but their results differed from the current study's observations. Using CSE, they found weaker bonds and lesser residual adhesive on enamel after debonding in the acidic primer group than in the phosphoric acid group. According to the authors,15 the thin and less uniform resin tags observed in the SEM photographs of acidic primer–treated enamel were conducive to poor adhesion. Because the acidic primer and the adhesives investigated are different, comparison of their observations with the current one is difficult.

In a recent study, Bishara et al17 evaluated the use of a newly developed restorative acidic primer (PLP) for bracket bonding. The findings indicated that the use of a self-etch primer produced lower but clinically acceptable SBS values. The low bond-strengths may partly be explained with the different application time of PLP. In the restorative version, the manufacturer recommends 15-seconds application times to remove the smear layer on dentin, whereas three seconds are sufficient for enamel to bond orthodontic attachments. Like the present study, this was an in vitro study, and one must exercise care in applying the findings in clinical situations.

CLINICAL CONSIDERATIONS AND CONCLUSIONS

After our encouraging laboratory findings and having in mind all the shortcomings of an in vitro setting, we have bonded 218 attachments (brackets and tubes) on 12 patients since June 2001. To date only four attachments have failed (three mandibular second molars and one lower incisor).

The intermediate steps such as acid etch for 30–60 seconds, wash thoroughly, dry, and then prime are no longer necessary with self-etching primers. TBP is packaged for single-patient use to avoid contamination, is fast to apply, and is cost effective when used for full bonding.

For optimal bonding, we recommend the following procedure:

• Dry the tooth surface.

• Apply TBP. Rub it thoroughly for at least three seconds, and always wet the surface with new solution to ensure monomer penetration. The presence of water in the chemical composition of TBP may necessitate air-drying, but as the operator moves from one side to the other, the solvent evaporates and drying is no longer necessary.

• Bond the bracket with Transbond XT.

At the moment, we do not have enough data on the compatibility of TBP with orthodontic adhesives other than light-cured Transbond. But within the limitations of our laboratory and clinical findings, it is clear that the hybrid layer produced after TBP application on enamel is capable of bonding strongly to enamel surfaces and by no means reduces the bond strength of orthodontic brackets.

Self-etching primers are effective, but only future long-term clinical studies will prove their clinical reliability.