Effects of Thermocycling and Light Source on the Bond Strength of Metallic Brackets to Bovine Teeth

Orthodontic brackets are routinely bonded by using light-cured materials. With them, the clinician has control of working time, more accurate bracket placement, easy removal of excess and immediate insertion of the orthodontic archwire (1). Different light sources are currently available for photoactivation (2-9) and they should provide adequate polymerization the light-activated materials. Quartztungsten-halogen (QTH) and light-emitting diodes (LEDs) use irradiation times of 20 to 40 s (10,11), while Effects of Thermocycling and Light Source on the Bond Strength of Metallic Brackets to Bovine Teeth


IntroductIon
Orthodontic brackets are routinely bonded by using light-cured materials.With them, the clinician has control of working time, more accurate bracket placement, easy removal of excess and immediate insertion of the orthodontic archwire (1).
On the other hand, bracket-bonding failure sometimes during the stages of treatment due to heavy forces produced by an archwire.In addition, light-cured materials are subjected to thermal changes in the oral cavity.Thermocycling has been used to determine if temperature variations produced stresses in the lightcured materials that might influence on the bond strength (14).In Orthodontics, thermocycling regimens between 500 and 6,000 cycles have been used (14)(15)(16)(17)(18).
However, literature is still not conclusive about the effectiveness of different light-curing units, especially after thermocycling.This study evaluated the effects of thermocycling and different light sources (QTH, LED, PAC or laser) on the bond strength of metallic orthodontic brackets to bovine tooth enamel using an adhesive resin.The hypotheses tested were that there are (1) significant differences in the bond strength among the light sources, and (2) significant differences after thermocycling.

MAtErIAL And MEtHodS
One hundred sixty bovine mandibular incisors were collected, embedded in autopolymerizing acrylic resin (Clássico Produtos Odontológicos, São Paulo, SP, Brazil) in polyvinyl chloride tubes (Tigre, Joinvile, SC, Brazil), with the buccal face of parallel to the tube height, with the cementoenamel junction located 3 mm above the acrylic resin.The buccal face of all teeth was cleaned with a rotational brush (Gaúcha Fornituras, Porto Alegre, RS, Brazil) and nonfluoridated pumice-water slurry (S.S. White, Petrópolis, RJ, Brazil) for 10 s, rinsed with air-water spray for 10 s and dried with air for 10 s.
The middle third of the buccal face of all teeth were etched using 35% phosphoric acid gel (3M ESPE, St. Paul, MN, USA) for 20 s, rinsed with air-water spray for 20 s, dried with air for 20 s, and the teeth were divided into 8 groups (n=20), according to the light source used and whether or not they were subjected to a thermocycling regimen (Table 1).One layer of a primer (3M Unitek, Monrovia, CA, USA) was applied on the etched area of buccal face of the teeth.After that, stainless steel standard maxillary incisor brackets (Synergy; Rocky Mountain Orthodontics, Denver, CO, USA) were positioned and firmly bonded with Transbond XT light-cured bonding resin (3M Unitek).A microbrush was used to remove excess.
After the bonding procedures, all specimens (groups 1 to 8) were stored in distilled water at 37º C for 24 h.After this period, the specimens of groups 5 to 8 were subjected to a thermocycling regimen in a thermal cycler (MSCT 3; Marnucci ME, São Carlos, SP, Brazil) totalizing 1,500 cycles in distilled water between 5 and 55°C with 30-s dwell time in each bath and transfer time of 10 s between baths.
Shear bond testing was performed in a mechanical testing machine (Model 4411; Instron, Canton, MA, USA) with a knife-edged rod at a crosshead speed of 1.0 mm/min until failure.A mounting jig was used to align the bracket-tooth interface parallel to the testing device.Bond strength values were calculated in MPa and analyzed by two-way ANOVA and Tukey's test (α=0.05).
After debonding, a stereomicroscope (Olympus Corp, Tokyo, Japan) was used to analyze the tooth and bracket surfaces under ×8 magnification.The Adhesive Remnant Index (ARI) was used to classify the failure modes (19): 0, indicates that no bonding resin on the tooth; 1, indicates that less than half of the bonding resin remained on the tooth; 2, indicates that more than half of the bonding resin remained on the tooth and 3, indicates that all bonding resin remained on the tooth, with a distinct impression of the bracket mesh.

rESuLtS
The shear bond strength mean values are shown in Table 2.No significant differences (p>0.05) in bond strength were found when the conditions without and with thermocycling were compared for any of the light sources.There were no significant differences (p>0.05) in bond strength among the light sources, irrespective of performing or not thermocycling.There was a predominance of ARI scores 1 in all groups (Table 3).

dIScuSSIon
The first hypothesis tested in this study was rejected, as no significant differences were observed among the light sources regardless of the thermocycling condition.These findings are in agreement with those of previous studies, which also found no significant differences among different light sources (2,5,9,(20)(21)(22).However, a recent study has found significant differences among LED, QTH and PAC units (4).According to Rueggeberg (23) photoactivation is dependent on the radiant exposure, which is the product of irradiance and exposure time.In this study, PAC (14.4 J/cm 2 ) and laser (10 J/cm 2 ) showed lower radiant exposure during photoactivation than LED (60 J/cm 2 ) and QTH (32 J/cm 2 ).However, the laser was effective for photoactivation probably because its emission spectrum is concentrated on the absorption peak of camphorquinone (at 468 nm).In relation to the PAC, according to Gonçalves et al. (9), the absence of differences might be explained by the fact that a very thin resin layer is necessary for bracket bonding and thus the differences in energy dose were probably not great enough to influence the bond strengths.
The durability of the bond between bracket/ bonding resin and teeth in clinical use must be evaluated.Thermocycling is used to determine if temperature variations might influence on the bond strength.Various types of thermocycling methods, such as artificial ageing, have been employed to determine the durability of bracket bonding but have not used longterm water storage (18).According to De Munck et al. (24), a decrease in the bond strength could be caused by hydrolytic degradation of the interface components.
The second hypothesis tested was also rejected, as no significant differences were found between thermocycling and water storage for any of the curing conditions.These results agree with those of previous studies, which found no significant difference in bond strength after thermocycling (14,(16)(17)(18).It may be speculated that a larger number of cycles is necessary to permit accelerated simulation (16).According to Reynolds (25), bond strength values between 6 to 8 MPa are adequate for orthodontic applications under clinical conditions, which means that all groups in the present study had clinically acceptable bond strengths to resist forces during orthodontic treatment.
The results of ARI scores indicated that the majority of failures on debonding let less than half of the bonding resin on the tooth (scores 1).This is clinically advantageous because there would be less adhesive to remove from the tooth surface after debonding.
In summary, the present s t u d y d e m o n s t r a t e d t h a t thermocycling and light source were not decisive factors for the bond strength of brackets to tooth surfaces.Although the light sources had a similar performance with respect to bond strength, the use of high-intensity light-curing units is   The ARI scale has a range between 0 and 3. 0, indicates that no bonding resin on the tooth; 1, indicates that less than half of the bonding resin remained on the tooth; 2, indicates that more than half of the bonding resin remained on the tooth and 3, indicates that all bonding resin remained on the tooth, with a distinct impression of the bracket mesh.
recommended to obtain effectiveness of polymerization of the bonding resin and care should be taken during bonding procedures, irrespective of the light source used.Future studies should also be carried out using regimens with a larger number of thermal cycles.
Same uppercase letters in the same row and lowercase letters in the same column indicate no statistically significant difference (p>0.05).

Table 1 .
Groups according to light sources or thermocycling.

Table 2 .
Bond strength mean values (in MPa) and standard deviations.

Table 3 .
Frequency distributions of the Adhesive Remnant Index (ARI) scores.