INTRODUCTION

The continuous development of adhesive-dentistry technology has led orthodontists to adopt these innovations and add them to their armamentarium. Tooth-conserving and time-saving adhesive methods of retaining orthodontic attachments are replacing traditional methods and procedures. Although the advantages are revolutionary, a significant caries risk under and in the vicinity of the multibonded appliances is of concern.12

It is reported that an average of two of the three teeth bonded with either of the bonding materials were affected by some form of enamel opacity after orthodontic treatment, with the most common type identified as a diffuse opacity. The recorded opacities covered an average of less than one-third of the labial surface34 and may occur in 2–96% of orthodontic patients.5 Bacteria in the dental plaque surrounding orthodontic appliances produce organic acids and lead to enamel demineralization.16 Demineralization (decalcification) occurs when the pH of the oral environment favors diffusion of calcium and phosphate ions out of enamel.6

The references cited above focus mostly on decalcifications and white spots around the brackets, not under the brackets.3–6 Although the area around the brackets is critical, the area under the brackets also needs attention. James et al7 were the first to point out increased risk of decalcification caused by microleakage around orthodontic brackets. 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.7

Although there are not many studies on the microleakage and its carious effects on the enamel, there have been many efforts to overcome the demineralization process around the orthodontic brackets. Fluoride is known to inhibit lesion development during fixed appliance treatment and to enhance remineralization after treatment.18 Daily use of a fluoride rinse combined with oral hygiene instruction can lead to a significant reduction in decalcification,6 the cariostatic effect of topical fluoride treatment resulting primarily from calcium fluoride formation.8 Unfortunately, patient cooperation with home-use topical fluoride agents and maintenance of optimum oral hygiene levels is frequently inadequate.9 As a result, the arrival of fluoride-releasing adhesive systems for bracket bonding has attracted considerable interest, offering a means of fluoride delivery adjacent to bracket-enamel interface and independent of patient cooperation.1011 However, the ability of these materials to reduce decalcification clinically remains equivocal.10–12

Remineralization by release of fluoride is important, but the antibacterial effect is another important property because inactivation of bacteria means a direct strategy to eliminate the cause of dental caries.13 Antimicrobial agents such as chlorhexidine have been suggested to prevent decalcification caused by fixed appliances for such circumstances.14–16 Nowadays, bioactive adhesive systems with antibacterial effects or intensive remineralization ability are considered to be beneficial and capable of producing superior clinical performances.17 Recently, a fluoride-releasing antibacterial bonding agent has been developed by combining the physical advantages of dental-adhesive technology and antibacterial effect.1819

The purpose of this in vitro study was to determine and compare the microleakage under both ceramic and metal brackets bonded with a recently developed fluoride-releasing, antibacterial, light-cured self-etch adhesive system and an acid-etching, conventional, light-cured adhesive system.

MATERIALS AND METHODS

Forty caries-free and intact human premolars readily available and extracted for orthodontic purposes were collected, randomly separated into four equal groups, and stored in distilled water. Teeth were cleaned and polished with pumice and rubber cups for 10 seconds and received the following surface-preparation and adhesive-application procedures according to the manufacturer's directions (Table 1):

• Group 1. An acid-etching adhesive system + metal bracket: Transbond XT Primer (3M Unitek, Monrovia, Calif) + Transbond XT Light-Cure Adhesive paste + metal bracket (Ormco Series 2000; first and second bicuspid with hook, part No. 303–1511, lot No. 05F788F, Ormco, Orange, Calif)

• Group 2. An acid-etching adhesive system + ceramic bracket: Transbond XT Primer + Transbond XT Light-Cure Adhesive paste + ceramic bracket (Mystique, reference kit No. 00–531–10, lot No. 1104, GAC International Inc, Bohemia, NY)

• Group 3. A fluoride-releasing, antibacterial, self-etching adhesive system + ceramic bracket: Clearfil Protect Bond (Kuraray Dental, Osaka, Japan) + Transbond XT Light-Cure Adhesive paste + ceramic bracket

• Group 4. A fluoride-releasing, antibacterial, self-etching adhesive system + metal bracket: Clearfil Protect Bond + Transbond XT Light-Cure Adhesive paste + metal bracket

TABLE 1. Materials and Application Procedures*

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Specimens were stored in distilled water for 4 weeks at 37°C, after which thermal cycling in deionized water was performed at 5 ± 2°C to 55 ± 2°C for 500 cycles with a dwell time of 30 seconds and a transfer time of 10 seconds. Before dye penetration, the apices were sealed with sticky wax and the specimens were coated with two consecutive layers of nail varnish up to 1 mm from bracket margins. Specimens were then immersed in 0.5% basic fuchsin solution (Wako Pure Chemical Industry, Osaka, Japan) for 24 hours. After a thorough rinsing with distilled water, the samples were air dried and embedded in epoxy resin (Struers, Copenhagen, Denmark). Four parallel longitudinal sections were made through the occlusal surfaces with a low-speed diamond saw (Isomet, Buehler, Lake Bluff, IL) in the bucco-lingual direction.

Two blinded, calibrated researchers examined all the sections under a stereomicroscope (Wild Type 308700, Heerbruug, Switzerland) at 16× magnification. Each section was scored from both incisal and gingival margins to the brackets between both the bracket-adhesive interface and the adhesive-enamel interface. Scoring was made according to the following criteria (Figure 1): 0 = no dye penetration between the bracket-adhesive or the adhesive-enamel interface, 1 = dye penetration restricted to 1 mm of the bracket-adhesive or adhesive-enamel interface, 2 = dye penetration into the inner half (2 mm) of the bracket-adhesive or adhesive-enamel interface, 3 = dye penetration into 3 mm of the bracket-adhesive or adhesive-enamel interface.

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The microleakage score of each tooth regarding bracket-adhesive and adhesive-tooth interfaces was obtained by calculating the mean score of incisal and gingival scores. Statistical evaluation of microleakage scores among the test groups was performed by Kruskal-Wallis test and Mann Whitney U-test with Bonferroni correction with significance set at P = .05.

RESULTS

All the groups exhibited microleakage between either the adhesive-enamel interface or the bracket-adhesive interface. Although not statistically significant, metal brackets generally represented higher microleakage scores compared with ceramic brackets between a bracket-adhesive-tooth complex regardless of the adhesive system used.

When adhesive-enamel interface microleakage was considered, there was no statistical significance between the overall evaluation; however, a significant difference was observed between group 3 (Clearfil Protect Bond + ceramic bracket) and group 4 (Clearfil Protect Bond + metal bracket) only at the gingival side (P = .022) (Table 2).

TABLE 2. Comparison of the Microleakege Scores Between Adhe sive and Enamel Surfaces From Incisal-Gingival Sides and the Over all Evaluation by Kruskal-Wallis Test and Mann-Whitney U-Test With Bonferroni Correction^a

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The evaluation of microleakage between the adhesive and the bracket revealed that metal brackets (groups 1 and 4) showed statistically more microleakage compared with ceramic brackets (groups 2 and 3) (P < .05) regardless of the bonding agent from all aspects (Table 3). Figures 2 and 3 show no leakage and leakage under metal brackets, and Figures 4 and 5 show no leakage and leakage under ceramic brackets.

TABLE 3. Comparison of the Microleakege Scores Between Bracket and Adhesive Surfaces From Incisal-Gingival Sides and the Overall Evaluation by Kruskal-Wallis Test and Mann-Whitney U-Test With Bonferroni Correction^a

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DISCUSSION

For the restorative dentistry clinics, microleakage is the seeping and leaking of fluids and bacteria between the tooth or restoration junction and interface. Microleakage increases the likelihood of recurrent caries and postoperative sensitivity.20 From the orthodontic point of view, microleakage presents the likelihood of formation of white-spot lesions on the enamel at the adhesive-enamel interface. Expansion and contraction occur when the teeth are heated and cooled by the ingestion of hot or cold foods. If the coefficient of thermal expansion for a restorative material does not match that of the teeth, the teeth and material expand and contract at different rates. Repeated expansion and contraction of teeth and restorative materials at different rates results in fluids being sucked in and pushed out at the margins of a restoration. This phenomenon is called percolation.20 In this research, thermocycling and aging procedures were carried out to mimic percolation, as our hypothesis was based on the microleakage after some service life in the mouth.

The linear thermal coefficients of expansion of enamel and ceramic or metal brackets and the adhesive systems do not match closely (ie, for resin composites α = 20–55 ppm/°C, stainless steel bracket for 316L stainless steel α = 16 ppm/°C, and enamel α = 12 ppm/°C).2122 Metal brackets contract and expand more than ceramic brackets, enamel, or the adhesive systems, producing microgaps between the bracket and the adhesive system and causing leaking of oral fluids and bacteria beneath the brackets.

Several studies have shown that ceramic brackets produce significantly stronger bond strength compared with conventional metal brackets.23–25 Increased bond strength with ceramic brackets resulted in bond failure at the enamel surface rather than at the bracket-adhesive interface, resulting in more enamel fractures after debonding.25–28 This 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. Similarly, the weaker bond strength of metal brackets may be attributed to relatively more microleakage between the bracket and the adhesive.

Several factors affect the bond strength of brackets, such as the adhesive system used, composite composition, photopolymerization type, and exposure time. Although not evidence based in orthodontics, microleakage may also contribute to the bond strength. In restorative-dentistry literature, numerous studies address the effect of microleakage on durability of bond strength2930; however, James et al7 could not demonstrate any correlation between microleakage and bond strength.

Microleakage scores obtained from the incisal and gingival margins of the brackets demonstrated significant differences, implying increased microleakage in the gingival side. This may be related with the surface curvature anatomy, which may result in relatively thicker adhesive at the gingival margin.

Studies in restorative dentistry have demonstrated that curing composites causes polymerization shrinkage and microleakage.3132 Polymerization shrinkage also varies from composite to composite and depends on the percentage of filler, the diluents, the percentage of the monomer conversion in the specific composite resin, and the photopolymerization type.3334 In restorative dentistry, composite resin is placed in volume of cavity preparation, and curing can create excessive shrinkage and gap formation along the composite-preparation interface. In contrast, orthodontic adhesive layers are very thin, and there is adhesive at the edges of the bracket to absorb some shrinkage. Because the bracket is free floating, the shrinkage can pull the bracket closer to the enamel.35 Therefore, in orthodontic applications, polymerization shrinkage and subsequent microleakage is less of a concern than it is in restorative dentistry.7

Miyazaki et al36 showed that polymerization shrinkage increases as the filler content decreases. With a filler content of about 10%, Clearfil Protect Bond is considered a filled adhesive. Filled low-viscosity resins are thought to have a strain capacity sufficient to relieve stresses between the shrinking composite restoration and the rigid tooth substrate, thereby improving the conservation of bond.37 However, no statistical significance was observed between the microleakage values of conventional adhesive system and the Clearfil Protect Bond.

The antibacterial effects of adhesive systems indicate the inhibition of caries formation, especially along the enamel margins.3839 Various attempts were made to minimize white-spot lesion formation during orthodontic treatment, including the use of adhesive systems containing fluoride or an antibacterial agent.40–42 However, if microleakage beneath brackets cannot be impeded, inactivation of bacteria caused by microleakage will be a direct strategy to eradicate the cause of white-spot lesions and therefore caries formation.

The second experimental adhesive system, Clearfil Protect Bond, is a recently developed self-etching adhesive system containing 12-methacryloyloxydodecyl-pyridinium bromide (MDPB), which is an antibacterial monomer incorporated in antibacterial adhesives. It causes an electrical imbalance in the bacterial cell wall, leading to cell wall destruction and, ultimately, bacterial death.43–45 It has been reported that MDPB copolymerizes with other monomers after curing, and the antibacterial agent is covalently bonded to the polymer network. The immobilized agent does not leach out from the material but acts as a contact inhibitor against the bacteria that attach to the surface.13 Findings concerning in vitro antibacterial activity, bonding ability, cytotoxicity, and pulpal response of MDPB-containing self-etching primer or adhesive have been published.46–49

Antibacterial activity of MDPB may not extend around the bracket, unlike its fluoride-release effect, but it is effective when bacteria contact the surface after microleakage. This technology is a safe mechanism that allows controllability of the antibacterial agent, MDPB,50 ensuring the immobilization of the antibacterial molecule at the site of therapeutic importance, which is the enamel surface under the bracket in our circumstances. However, further studies are essential to test both the in vivo properties of this material and the effectiveness of the antibacterial and fluoride-releasing effects on reduction of the incidence of white-spot lesions. Other factors such as photopolymerization type, filler content, and composite resin type should also be evaluated for microleakage in orthodontics as in restorative dentistry.

CONCLUSIONS

• All the brackets exhibited some amount of microleakage regardless of the adhesive and bracket type, highlighting the importance of microleakage beneath brackets.

• Metal brackets exhibited more microleakage than did ceramic brackets, particularly at the bracket-adhesive interface.