INTRODUCTION

In routine orthodontic practice it is essential to obtain a reliable adhesive bond between an orthodontic attachment and tooth enamel. To simplify the orthodontic bonding procedure and reduce chair time, self-etching primers are manufactured to combine two or more bonding steps.1

In late 2000, a new self-etching primer, Transbond-Plus Self-Etching Primer (TSEP: 3M-Unitek, Monrovia, Calif) was developed especially for orthodontic bonding. It includes methacrylated phosphoric acid esters, which will both etch and prime the enamel surface before bonding. TSEP has been experimentally tested in several in vitro tests1–5 and revealed promising adhesive bonding results.34

Although tooth-conserving and time-saving adhesive methods of retaining orthodontic attachments are replacing traditional methods and procedures, a significant caries risk under and in the vicinity of the multibonded appliances is of concern.67 According to Arhun et al,8 investigators in the literature910 were interested in decalcifications and white spots around the brackets, not under the brackets, and they pointed out that although the area around the brackets is critical, the area under the brackets also needs attention.

The polymerization shrinkage of the adhesive material may cause gaps between the adhesive material and enamel surface and lead to microleakage, thus facilitating the formation of white-spot lesions under the bracket surface area.11 Gap formation contributes to microleakage, permitting the passage of bacteria and oral fluids from the oral cavity.12 It is well documented that microleakage increases the likelihood of recurrent caries and postoperative sensitivity.11 From the orthodontic perspective, it is possible to interpret this fact as supportive of the formation of white spot lesions between the bracket and enamel interface. In the orthodontic literature, James et al11 noted the increased risk of decalcification caused by microleakage around orthodontic brackets. Then Arhun et al8 assessed microleakage of a tooth-adhesive-bracket complex when metal or ceramic brackets were bonded with a conventional and an antibacterial adhesive. They found that metal brackets caused more leakage between the adhesive-bracket interface, which may lead to lower clinical shear bond strength and white-spot lesions. Arikan et al13 investigated different light-curing units and bracket types for microleakage observed beneath brackets and concluded that ceramic brackets cured with light-emitting diode units were the best combination, demonstrating the lowest microleakage scores.

So far, to our knowledge, no studies have been reported investigating the effect of an orthodontic self-etching primer system on microleakage under brackets. The aim of this study was to compare the in vitro microleakage of orthodontic brackets (metal and ceramic) between enamel-adhesive and adhesive-bracket interface at the occlusal and gingival sides produced by self-etching primer systems with that of a conventional acid-etching system. For the purposes of this study, the null hypothesis assumed that there were no statistically significant differences between the microleakage of an enamel-adhesive-bracket complex at the occlusal and gingival margins and enamel prepared by TSEP and by a conventional-etching method.

MATERIALS AND METHODS

RESULTS

Comparisons of the microleakage scores between occlusal and gingival sides for enamel-adhesive and adhesive-bracket interfaces are shown in Table 1. Microleakage was observed in all groups except Group 1 at the occlusal side between enamel-adhesive, and all groups at the occlusal side between adhesive-bracket interfaces. In all groups the gingival sides exhibited higher microleakage scores compared with those observed in the occlusal sides for both adhesive interfaces. For enamel-adhesive interface, ceramic brackets showed higher microleakage on the gingival than on the occlusal side (P < .01). When the self-etching primer was used for metallic brackets (P < .05) and conventional etching was used for ceramic brackets (P < .001), less microleakage was determined on the occlusal side. Therefore, the null hypothesis was rejected in part.

Table 1.  Descriptive statistics and comparison of the microleakage scores between occlusal and gingival sides for enamel-adhesive and adhesive-bracket interface^a

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Statistical comparisons of the microleakage scores among four groups at the occlusal and gingival sides between enamel-adhesive and adhesive-bracket interfaces are shown in Table 2. According to statistical analysis, in both adhesive interfaces all groups displayed similar microleakage under the occlusal side of the brackets, and scores were not statistically different (P > .05). Statistically significant differences were found at the gingival sides between enamel-adhesive (P < .001) and adhesive-bracket (P < .01) interfaces among all investigated groups. Therefore, the null hypothesis was rejected. Brackets bonded with TSEP (Groups 2 and 4) revealed statistically higher microleakage between enamel-adhesive interfaces at the gingival side. When microleakage at the adhesive-enamel interface was considered, metal brackets bonded with TSEP (Group 2) showed the highest scores and ceramic brackets bonded with TSEP (Group 4) showed the lowest scores at the gingival side.

Table 2.  Multiple comparisons of the microleakage scores between groups for occlusal and gingival sides in enamel-adhesive and adhesive-bracket interface

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DISCUSSION

Enamel etching with phosphoric acid created an etch pattern characterized by a deep and uniform demineralization area. These demineralized areas were infiltrated by the resin of the priming solution, producing resin tags penetrating into demineralized surface.14 Compared with phosphoric acid, TSEP produced a uniform and more conservative etch pattern with regular adhesive penetration and a less aggressive enamel demineralization.14 When any problem exists during bonding process, it is possible for seeping and leaking of fluids and bacteria to occur between enamel-adhesive interfaces, and we can interpret these as the potential white-spot lesion on the enamel surface. A review of the literature indicated that no studies have compared the microleakage of metallic and ceramic brackets bonded with orthodontic composites to enamel that have been prepared with conventional acid etching and TSEP procedures.

In this study, the dye-penetration method was chosen to determine microleakage of the bonded specimens. This is the most commonly used method to assess microleakage of dental materials.15 It is easy to perform, fast, and economical, but the shortcoming of the technique is the subjectivity of reading the specimens.16 In our study all specimens were evaluated by the two operators at two times to evaluate measurement error. The inter- and intraexaminer Kappa scores for assessing microleakage were high; all values were greater than 0.75.

Microleakage, however, may not be similar on the other sides on a bonded tooth, although studies on restorative dentistry have assumed that one-side assessment is representative of the whole tooth.17 Airlocks in the marginal gap, leaching of water-soluble tracers during processing, and failure of only a few sections to allow interpretation of the full pattern, limit dye-penetration tests to low reproducibility and precision. It is important to note that the assessments in the present study were made by four parallel longitudinal sections through the occlusal and gingival surfaces in the bucco-lingual direction according to Arhun et al8 between enamel-adhesive and adhesive-bracket interfaces. Mean scores for the four occlusal sections give the occlusal microleakage score and mean scores for the gingival sections give the gingival microleakage score.

In vitro, microleakage is commonly assessed to detect bond failure at the enamel sealant interface through dye penetration. This failure can be due to polymerization shrinkage or different linear coefficients of thermal expansion from tooth hard substances and resin materials.18 Thermal cycles are widely used to simulate temperature changes in the mouth, generating successive thermal stresses at the tooth-resin interface. In restorative dentistry, Kubo et al19 investigated the microleakage of self-etching primers after thermal and flexural load cycling and found that the marginal integrity of self-etching primers did not deteriorate even after thermal cycles (5000–10,000 cycles) and flexural loads. Similarly, several other workers have shown that an increase in the number of thermal cycles was not related to an increase in microleakage of restorations.182021 Therefore, thermocycling was not performed in this study.

Polymerization shrinkage varies by adhesive composition, that is, percentage of filler, the diluents, or the percentage of the monomer conversion in the specific composite resin22 and curing type.23 Polymerization shrinkage of the adhesive material may cause microleakage-promoting micro gaps between the adhesive material and the enamel surface, which may initiate microleakage and possible white-spot lesions under the bonding area.11 However, from an orthodontic perspective this condition is different. Adhesives at the edges of the bracket can absorb some shrinkage,8 and this shrinkage can pull the bracket closer to the enamel by the free floating of the bracket. In contrast to the thick composite resin put in a prepared cavity in restorative dentistry, polymerization shrinkage and the subsequent microleakage is less of a concern in orthodontic adhesives because a thin layer is used.

Arhun et al8 found that microleakage scores obtained from the incisal and gingival margins of the brackets demonstrated significant differences, implying increased microleakage on the gingival side. They interpreted these differences as related to the surface curvature anatomy, which may result in relatively thicker adhesive at the gingival margin. However, the authors did not explain which visible light-curing device was used and how. In the present study a conventional quartz tungsten halogen curing device with a 10-mm diameter light tip was used for 40 seconds from the occlusal surface.

Results of this study indicated that porcelain brackets showed statistically significantly more microleakage at the gingival side than the occlusal side in the enamel-adhesive interface. We also observed no microleakage at the occlusal sides in the adhesive-bracket interface. Our findings were similar with Arhun et al,8 but our interpretation was different. We thought that lower or no microleakage scores at the occlusal side than at the gingival side may be related to the curing method in that we applied light from occlusal.

The presence of resin tags prepared at the enamel surface by acid etching is an important factor to fight leakage.18 A deeper etching pattern ensures the possibility of better resin penetration, but it does not guarantee a sealant-enamel interface that is free of microleakage or better sealant retention.18 This is supported by an in vivo study where no differences were found between sealants applied over a self-etching adhesive or H₃PO⁴ etched teeth after 24 months.24 However, several in vitro studies do not advocate the use of self-etching adhesives on intact enamel because of significantly lower bond strengths, greater microleakage, and an etching pattern that is not deep enough to obtain good penetration of bonding resin.2526 Our results were consistent with the literature for microleakage under brackets at the enamel-adhesive interface.8 Both metallic and ceramic brackets bonded with TSEP showed significantly higher microleakage than when the conventional acid-etch method was used.

Several studies in the literature2728 indicate that ceramic brackets produce stronger bonds than the metallic orthodontic brackets. Arhun et al8 reported that the increased strength and difficulty in debonding for ceramic brackets may be attributed to the close adhesion of the ceramic bracket to the adhesive in the absence of microleakage. Similar to the opinion of Arhun et al,8 we thought incomplete polymerization of the adhesives under metallic brackets may explain this difference, because investigators2930 indicated a number of factors that affect the final degree of cure of a resin. These included the chemical structure of the dimethacrylate monomer and the polymerization conditions, that is, atmosphere, temperature, light intensity, photoinitiator concentration, filler type, shade of adhesive resin, and the reflective characteristics of adhesive resin bulk.30 Yoon et al31 explained this incomplete polymerization of the cure by decreases from the top surface inward.

Further studies are necessary in orthodontics to investigate the correlations between microleakage and shear bond strength, different bonding materials, and curing devices. Moreover, a study should be designed to investigate the reason for the difference in the amount of microleakage between the gingival and occlusal sides of the orthodontic brackets.

CONCLUSIONS

• Gingival sides in all groups exhibited higher microleakage scores compared with those observed in occlusal sides for both adhesive interfaces.

• Enamel-adhesive interfaces exhibited more microleakage than did the adhesive-bracket interfaces.

• Brackets bonded with self-etching primer system revealed significantly higher microleakage at the enamel-adhesive interface of the gingival side.