INTRODUCTION

In orthodontics, bonding with light-cured adhesive resins is popular because the extended working time and precise bracket placement can only be accomplished with “on-demand polymerization.” Light-curing units (LCUs), therefore, have become an important part of the orthodontist‘s’ armamentarium. To date, the most popular light-curing unit has been the quartz-tungsten-halogen (QTH)–based unit.1 In these LCUs, halogen bulbs generate light when electric energy heats a small tungsten filament to high temperatures. Selective filters screen the wavelengths so that only blue light is emitted.2

Despite their popularity, QTH units have several shortcomings. First, halogen bulbs have a limited effective lifetime of approximately 40–100 hours.3 In addition, their bulbs, reflectors, and filters degrade over time because of the large quantity of heat produced during duty cycles.3 This results in a reduction of curing effectiveness. To overcome drawbacks of QTH units, Mills et al4 proposed the use of solid-state light-emitting diodes (LEDs) for the polymerization of light-cured adhesives.1 LED technology uses junctions of doped semiconductors to generate light. These units avoid the use of heat-generating halogen bulbs and have about 10,000 hours of life with little if any degradation of the output.15 Furthermore, LED units are portable and are resistant to shock and vibration.

There has been an increasing interest in LED units because of their longer life span and consistent light output. In addition, manufacturers claim that new generation LED units provide faster monomer conversion than that achieved with conventional LCUs. By speeding up the curing process, LED units may save chair time for doctor and patient.

On the other hand, speeding the curing process may well result in an increase in the amount of microleakage. Polymerization shrinkage of the adhesive material may cause formation of microleakage-promoting microgaps between the adhesive material and the enamel surface, which may initiate white spot lesions under the bracket surface area.6 In restorative dentistry, microleakage is defined as the seeping and leaking of fluids and bacteria between the tooth-restoration interface.7 It is well documented that microleakage increases the likelihood of recurrent caries and postoperative sensitivity.7 From the orthodontic point of view, it is possible to interpret this fact as the likelihood of formation of white spot lesions at and under the adhesive-enamel interface.

The null hypotheses tested in this study were: (1) the type of LCU used (QTH or LED) would not affect the amount of microleakage observed beneath brackets, and (2) the bracket type used (ceramic or metal) would not influence the amount of microleakage observed beneath brackets.

MATERIALS AND METHODS

Forty freshly-extracted noncarious premolars were collected and stored frozen (at 4°C) in deionized water until experiments took place (a maximum of 1 month). The enamel was checked under a transillumination unit (Pluraflex HL 150, Litema, GSD, Germany) for the absence of cracks and developmental defects. Teeth were cleaned off debris and further polished with pumice and rubber cups. Then, teeth were randomly assigned into four groups of 10 teeth each. The groups received the following bracket bonding procedures:

Group 1: A metal bracket (Ormco Series 2000, 1st and 2nd bicuspid w/hook, Optimesh XRT-based, 0.18-inch slot size, Ormco, Orange, Calif) was bonded to the tooth with Transbond XT (3M Unitek, Monriva, Calif) and cured with an LED curing unit (SmartLite, Dentsply/Caulk, Milford, Del) for 20 seconds (10 seconds each from mesial and distal margins). The materials and application procedures used for bracket bonding are presented in Table 1.

TABLE 1. Materials and Application Procedures

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Group 2: A ceramic bracket (Mystique, NSB base with acrylic central base, 0.18-inch slot size, GAC, Bohemia, NY, USA) was bonded to the tooth with Transbond XT (3M Unitek, Monriva, Calif, USA) and cured with the same LED curing unit (SmartLite, Dentsply/ Caulk, Milford, Del, USA) as with group 1.

Group 3: A metal bracket (Ormco Series 2000, 1st and 2nd bicuspid w/hook, Ormco, Orange, Calif) was bonded with Transbond XT and cured with a QTH unit (Hilux, Benlioglu, Turkey) for 40 seconds (20 seconds each from mesial and distal margins).

Group 4: A ceramic bracket (Mystique, NSB base with acrylic central base, 0.18-inch slot size, GAC, Bohemia, NY) was bonded to the tooth with Transbond XT and cured with the same QTH curing unit (Hilux, Benlioglu, Turkey) as with group 3.

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. Prior to 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 thorough rinsing with distilled water, the samples were air-dried and embedded in epoxy resin (Struers, Copenhagen, Denmark). In each sample, four parallel longitudinal sections were made through the labial surface (a total of 40 sections per group) using a low-speed diamond saw (Isomet, Buehler, Lake Bluff, Ill) in the buccolingual direction.

All sections were examined by two calibrated investigators under a stereomicroscope (Wild Type 308700, Heerbrug, Switzerland) at standard magnification (16×) in a blinded fashion. Microleakage was determined by direct measurement using an Ultra-Cal IV digital caliper with an inherent accuracy of 0.02 mm (Ted Pella Inc, Redding, Calif).

Each section was scored for microleakage at the incisal and gingival levels along both interfaces (bracket-adhesive interface and adhesive-enamel interface). Scoring was made according to the following criteria (Figure 1):

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Score 0: No dye penetration between the bracket-adhesive or adhesive-enamel interface.

Score 1: Dye penetration restricted to 1 mm into the bracket-adhesive or adhesive-enamel interface.

Score 2: Dye penetration into the inner half (2 mm) of the bracket-adhesive or adhesive-enamel interface.

Score 3: Dye penetration into 3 mm of the bracket-adhesive or adhesive-enamel interface.

In cases of disagreement between scoring, consensus was obtained by using the greater score. The schematic explanation of scoring is shown in Figure 1. Figures 2 and 3 demonstrate individual examples of scoring.

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RESULTS

Microleakage was observed in all groups. Tables 2 and 3 demonstrate the microleakage scores of the adhesive-tooth and adhesive-bracket interfaces, respectively. Ceramic brackets cured with either LED or QTH LCUs (Groups 2 and 4) exhibited lower microleakage scores compared to those observed under metal brackets (P < .05). When the LED curing unit was used for photopolymerization, both adhesive interfaces under the ceramic brackets displayed significantly less microleakage than those under metal brackets. When the QTH curing unit was used, ceramic brackets displayed significantly less microleakage than did metal brackets in the bracket-adhesive interface along both gingival (P = .001) and incisal (P = .002) margins. However, although it was less than that found in metal brackets, the microleakage in the tooth-adhesive interface was not significantly different. The type of LCU (QTH or LED) did not result in a significant difference in the amount of microleakage observed beneath the bracket.

TABLE 2. Comparison of Microleakage Scores between Adhesive-Tooth Interfaces^a

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TABLE 3. Comparison of Microleakage Scores between Adhesive-Bracket Interfaces^a

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For metal brackets, the type of LCU did not significantly affect the amount of microleakage, except for gingival margins in the tooth-adhesive interface. When both bracket systems were compared, the use of LED under metal brackets resulted in the most dramatic difference in terms of microleakage. Overall, ceramic brackets cured with LED units yielded the best combination.

DISCUSSION

The potential of white spot lesion formation has become a particular clinical problem ever since direct-bonded orthodontic brackets were introduced.8 Enamel demineralization and white spot lesions occur during and sometimes remain after orthodontic treatment.910 It is reported that an average of two of the three teeth bonded with either of the bonding material were affected by some form of enamel opacity after orthodontic treatment, the most common type identified being a diffuse opacity.

O'Reilly and Featherstone11 and Øgaard et al12 have shown that visible white lesions can develop within 4 weeks. Although microleakage-oriented caries is a well documented entity in the restorative dentistry literature, the potential of caries adjacent to and beneath orthodontic brackets still remains as an underestimated threat to the permanent tooth, especially with regard to long-term fixed therapy. In the present study, adhesive-bracket and adhesive-tooth interfaces were scored separately. The adhesive-tooth interface is the critical one regarding the occurrence of white spot lesion, and the adhesive-bracket interface may have a role in bracket failure caused by bond degradation.

CONCLUSIONS

• Microleakage was observed along all bonding interfaces, regardless of the type of bracket or LCU used. More effort should definitely be made regarding prevention of enamel demineralization during long-term orthodontic treatment.

• The tested LED curing unit may provide a reduction in chair time but may cause more leakage between adhesive-bracket interface when metal brackets are used.

• Regardless of the type of curing unit used, ceramic brackets exhibited lower microleakage scores than metal brackets.

• Bonding of ceramic brackets with an LED-compatible adhesive resin and photopolymerized with a LED curing unit may significantly reduce the amount of microleakage that will occur beneath the bracket.