Original Article
Bond Strength of Metallic Brackets After Dental Bleaching Virna Cavalcante Patuscoa; Gil Montenegrob; Marcos Augusto Lenzac; Adilson Alves de Carvalhoa ABSTRACT Objective: To test the hypothesis that there is no difference between the action of at-home and in-office vital bleaching on the shear bond strength of metallic brackets bonded with composite resin 24 hours after bleaching. Materials and Methods: Forty-five human upper premolars were randomly divided into three groups: (1) control, (2) 10% carbamide peroxide at-home bleached, and (3) 35% hydrogen peroxide in-office bleached. Twenty-four hours after bleaching the teeth were pumiced, bonded with metallic brackets, and stored in distilled water. One day after bonding the shear bond strength of the brackets was determined. Results: The mean shear bond strength of Group 1 (control) and Group 2 (carbamide peroxide bleached) were not statistically different. Group 3 (hydrogen peroxide bleached) had a significantly lower mean shear bond strength than Group 1 and Group 2. Conclusions: The hypothesis is rejected. Use of 10% carbamide peroxide bleaching does not significantly alter shear bond strength values. On the other hand, use of 35% hydrogen peroxide bleaching significantly reduces these values and diminishes the amount of resin remnant on the tooth surface after bracket debonding. (Angle Orthod. 2009;79:122–126.) KEY WORDS: Dental bleaching; Bond strength; Metallic bracket; Composite resin
INTRODUCTION Because of the development of glass ionomer and resin composite materials, orthodontics practitioners have been able to solve a problem of bracket attachment to teeth by substituting a substance that satisfactorily adheres to dental enamel for metal bands.1 This adhesion results from the micromechanical retention of an adhesive to the irregular surface of an acidconditioned enamel and to the retaining elements of the bracket base. For a successful bonding three conditions may be considered: (1) the enamel and its preparation, (2) the shape of the bracket base, and (3) the bonding material.2 Various factors can influence the bond strength of brackets bonded to enamel, such a Postgraduate student, Department of Orthodontics, Federal University of Goiaˆnia, Goiaˆnia, Brazil. b Professor, Department of Cosmetic Dentistry, University of Planalto Central, Goiaˆnia, Brazil. c Professor and Chairman, Department of Orthodontics, Federal University of Goiaˆnia, Goiaˆnia, Brazil. Corresponding author: Dr Virna Cavalcante Patusco, SHIS QI 07 conjunto 17 casa 15 Lago Sul, Brası´lia-DF Brazil 71615-370 (e-mail:
[email protected])
Accepted: November 2007. Submitted: July 2007. 2009 by The EH Angle Education and Research Foundation, Inc.
as dental prophylaxis, acid etching, and dental bleaching.1 Various products are currently used to bleach teeth.3,4 Bleaching can be done internally on a nonvital tooth or externally on a vital tooth.5 One procedure for external tooth bleaching is carried out by a clinician in a dentist’s office by applying a high concentration of hydrogen peroxide solution activated by heat or light. This technique demands adequate isolation of soft tissues.3,6–11 Another method of external bleaching is done by the patient at home using a low concentration of carbamide peroxide solution.3,6,12 A 10% carbamide peroxide solution is decomposed into urea, ammonia, carbonic acid, and hydrogen peroxide at a low concentration of 3.0% to 3.5%.3,11,13,14 This composition makes the product more stable and less acidic,13 enabling it to be applied directly to oral tissues with the aid of a tray. Much research has been done to evaluate the effect of bleaching gels on the surface of enamel and the subsequent effect on the bond strength of composite resin. The results are quite controversial. After evaluating the effect of aqueous solutions of hydrogen peroxide at concentrations of 3% to 35%, some studies reported an increase of enamel porosity, a loss of mineral content, and a loss of prismatic form5,15–18 plus a DOI: 10.2319/072507-345.1
BONDING STRENGTH OF METALLIC BRACKETS AFTER DENTAL BLEACHING
substantial reduction in resin composite bond strength.13,19–23 On the other hand, Sterrett et al14 and Haywood et al24,25 found no alteration on enamel except normal morphologic variations. Other studies3,4,16,26 also identified no significant alterations on composite bond strength. Because of the progress made in cosmetic dentistry, dental bleaching has become quite frequent, and many patients seeking orthodontic treatment are interested in having their teeth bleached. Despite its popularity, the variety of conclusions4,5,12–14,23 published concerning its effects on enamel and on the bond strength of composite resins suggests the need for more work. The purpose of this study was to investigate the effect of 10% carbamide peroxide at-home bleaching and 35% hydrogen peroxide in-office bleaching activated by a laser on the shear bond strength of metallic brackets bonded with composite resin to human upper premolars 24 hours after bleaching. The null hypothesis of this study is that there is no difference between the actions of at-home and in-office vital bleaching on the shear bond strength of metallic brackets bonded with composite resin 24 hours after bleaching. MATERIALS AND METHODS Forty-five extracted human upper premolars were stored in a solution of 0.1% thymol for a week and then in distilled water at room temperature. The criteria for tooth selection were intact buccal enamel surface and no pretreatment with chemical agents such as alcohol, formalin, hydrogen peroxide, or any other form of bleaching agent. The bonding area of each tooth was located in the most central area of the middle third of the buccal surface. The teeth were embedded in plastic tubes with self-cure acrylic resin. A mounting jig was used to align the bonding area of the buccal surface perpendicular to the tube base. The teeth were randomly divided into three groups of 15 specimens each: Group 1, control group; Group 2, carbamide peroxide bleached group; and Group 3, hydrogen peroxide bleached group. The buccal surfaces of the teeth were cleaned with a pumice/water paste in a rubber cup on a slow-speed handpiece for 5 seconds, washed for 10 seconds, and dried for 10 seconds using an air water syringe. In the control group specimens the bonding area was etched with 37% phosphoric acid for 30 seconds, washed for 10 seconds, and air dried for the same amount of time. A layer of self-cured orthodontic adhesive (Concise, 3M, Sumare´, Sa˜o Paulo, Brazil) was used to bond orthodontic metallic brackets (Roth Monobloc with hook, Morelli, Sorocaba, Sa˜o Paulo, Brazil) to the tooth bonding area. All excess resin was removed from
123
around the bracket. The bracket was pressed firmly until it adhered to the tooth. The teeth were stored in distilled water in a sealed container at room temperature for one day before debonding. In the carbamide group specimens a layer of bleaching gel (Whiteness Perfect, FGM, Joinville, Santa Catarina, Brazil) approximately 1-mm thick was applied daily for 4 hours onto the bonding area for 14 days. This replicates the instructions usually given to the patient in the dental office. After each daily bleaching session, the gel was washed away using an air water syringe for 5 seconds and stored in distilled water in a sealed container at room temperature. After the 14-day bleaching period, the teeth were stored as above for 24 hours before bonding. The teeth were then pumiced, bonded, and stored as before. In the hydrogen group the bleaching gel (Whiteness HP, FGM, Joinville, Santa Catarina, Brazil) was mixed according to the manufacturer’s instructions and applied to the tooth buccal surface in a layer approximately 1-mm thick. The gel was left standing for 2 minutes and then exposed to a diode emission light (3Light, Clean Line, Taubate´, Sa˜o Paulo, Brazil) for 40 seconds. After its color turned from crimson to colorless the gel was left standing for another three minutes. The teeth were cleaned with gauze and the bleaching procedure above was repeated four times, replicating the in-office bleaching. Then, the mixture was washed away and the teeth were stored in distilled water at room temperature for 24 hours before bonding. The teeth were then pumiced, bonded and stored as before. Twenty-four hours after bonding, the shear bond strength of the bonded brackets was determined with a universal testing machine (DL 2000, Emic, Sa˜o Jose´ dos Pinhais, Sa˜o Paulo, Brazil) with a crosshead speed of 0.5 mm/minute and a 50-kg load cell. The plastic tube was held in a metallic base, and the chiseledge plunger mounted in the movable crosshead of the machine was positioned at the upper surface of the bracket. The force required to remove the bracket was measured in kilogram-force (kgf). After debonding, the enamel surface and the bracket base were examined under ⫻15 magnification with a stereomicroscope (RZ Trinocular Zoom, Meiji, Santa Clara, Calif). The Adhesive Remnant Index (ARI) was used to evaluate the amount of adhesive remaining on the tooth, set according to the criteria of Table 1. Statistical Analysis Descriptive statistics, including the mean, standard deviation, and minimum and maximum values, were calculated for the shear bond strength of the three
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PATUSCO, MONTENEGRO, LENZA, CARVALHO
Table 1. Adhesive Remnant Index (ARI)
Table 3. Frequency of Adhesive Remnant Index (ARI) Scores of the Three Groups Tested and Result of Pearson’s 2 Testa
Index
Enamel Adhesive Remnant
0 1 2 3
No adhesive on enamel Less than 50% of the adhesive on enamel More than 50% of the adhesive on enamel 100% of the adhesive on enamel
groups of teeth. A comparison of the means of the three groups was made with analysis of variance (ANOVA) and Bonferroni’s test. Pearson’s 2 test was used to determine significant differences in the ARI scores among the groups. The 5% level of significance was adopted for all the statistics tests. RESULTS ANOVA revealed significant differences in the means of the bond strength of the three groups. Bonferroni’s test indicated that the means of Group 1 (control) and Group 2 (carbamide peroxide bleached) were not statistically different. Group 3 (hydrogen peroxide bleached) had a significantly lower mean shear bond strength than Group 1 and Group 2 (Table 2). The result of Pearson’s 2 test revealed significant differences among the three groups (Table 3). The control group showed a slightly lower concentration of resin remnant on the enamel than on the bracket base. The same was true for the hydrogen group, but much less resin remnant was found on the enamel than in the case of the control group. Unlike the control and hydrogen groups, the carbamide group showed a slightly higher amount of residual resin on the enamel in relation to the bracket base. DISCUSSION The ability of hydrogen peroxide to penetrate through enamel and dentin is, in part, a result of the relatively low molecular weight of the peroxide molecule (30 g/mol). All dental bleaching agents ionize and decompose to initiate the chemical bleaching process. Because of its unstable nature, hydrogen peroxide produces several free-radical ions that react with the larger, long-chained, dark-colored molecule, breaking into smaller, lighter-colored structures.11 The goal of external activation in bleaching (ie, light or heat ex-
Groups* ARI
1
2
3
Total
0 1 2 3 Total
5 6 4 0 15
3 1 8 0 12b
10 5 0 0 15
18 12 12 0 42
a ARI scores: 0 indicates no adhesive on enamel; 1, ⬍50% of the adhesive on enamel; 2, ⬎50% of the adhesive on enamel; 3, 100% of the adhesive on enamel. Group 1 indicates control; Group 2, carbamide peroxide bleached; Group 3, hydrogen peroxide bleached. b Three teeth were not used for ARI recording due to enamel fracture at debonding. * Differences among groups were statistically significant (P ⫽ .002).
posure) is to add energy to catalyze the oxidation reaction with the hydrogen peroxide, thus accelerating the bleaching process.9 The heat- and light-activated process immediately produces the rapid stain removal. Many studies have reported controversial results concerning the effects of bleaching gels upon the bond strength of composite resins to enamel.3,4,13,16,19–23,26, Homewood et al26 recently used the same regimen as the control group of this study except for the fact that two carbamide peroxide–bleached groups were stored in phosphate-buffered saline, one for 24 hours and the other for 14 days before bonding. In agreement with this study, they did not find any dramatic alteration in the shear bond strength of brackets bonded 24 hours after bleaching or 14 days afterward. Josey et al16 used a daily nightguard vital bleaching for 10 hours for 1 week and then stored the teeth in artificial saliva for 1 day, 1 week, 6 weeks, and 12 weeks before bonding the brackets. No significant reduction in shear bond strength was shown at the 24hour interval, as was observed in the present study, or at the other storage intervals. Titley et al19 bleached two groups of teeth with 35% hydrogen peroxide for 60 minutes, one group followed by immediate bonding and the other followed by storage in distilled water for 1 day before resin application. In agreement with this study, the shear bond strength values were adversely affected in the 1-day interval
Table 2. Descriptive Statistics and Results of ANOVA and Bonferroni’s Test Groupa
N
Mean
SD
Bonferronib
1—Control 2—Carbamide peroxide bleached 3—Hydrogen peroxide bleached
15 15 15
14.6 14.2 6.8
6.6 5.8 3.2
A A B
a b
Difference among groups was statistically significant (P ⫽ .002). Groups with the same letter do not differ statistically.
BONDING STRENGTH OF METALLIC BRACKETS AFTER DENTAL BLEACHING
bonding group and in the immediate bonding group. There was no statistical difference between the two bleached groups, although the shear bond strength values were higher in the group immersed in distilled water. Uysal et al,6 using a 35% hydrogen peroxide agent either immediately or 30 days before bonding, reported that bleaching significantly reduced the amount of resin on tooth surfaces after debonding. In this study, both the carbamide peroxide–bleached group and the control group, which had no statistical difference between mean shear bond strength, showed quite a similar distribution of resin remnant on the enamel and the bracket base. However, the hydrogen peroxide– bleached group, which had significantly lower mean shear bond strength, had far less resin remnant on the enamel compared with that on the bracket base. On the other hand, Miles et al,23 using a 10% carbamide bleaching gel continuously for a 3-day period either immediately or a week before bonding, reported that most of the resin was left on the enamel; however, this does not explain the substantial decrease of the shear bond strength of the teeth bonded immediately. Studies have examined physical alterations after bleaching as a possible explanation for the change in bond strength. Many authors5,15–18 have related bleaching to considerable changes in enamel structure, such as loss of mineral content, increased porosity, and loss of prismatic form. Only two studies15,16 mention that these changes may affect the retentive qualities of adhesive applied to the enamel surface. Most studies13,16,19–21 state that the quality of composite bond is attributable to the presence of residual peroxide or peroxide-related substances on the enamel surfaces, which might inhibit the polymerization of the composite resin and decrease the number of resin tags. Although Josey et al16 found no significant reduction in shear bond strength at all storage intervals, the lowest mean bond strength was observed 24 hours after the bleaching process, and the maximum bond strength was reached after 6 weeks of storage. Josey et al16 concluded that the hydrogen peroxide had diffused out of the teeth between 1 and 6 weeks. This would account for better shear bond strength after 6 weeks. Other studies19,20 have demonstrated that the reduction in bond strength potential is time dependent and that measurable amounts of peroxide can, with time, be leached from enamel when it is immersed in water. Torneck et al20,21 stated that the shear bond strength potential is markedly reduced when a light-cured resin is applied to bleached enamel shortly after 35% hydrogen peroxide exposure. Exposing the bleached enamel to water 7 days before bonding eliminates the reduction in shear bond strength, and it is emphasized
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that this last period selected was arbitrary and that effects may occur within a shorter time period. Although Miles et al23 mention nothing about hydrogen peroxide residuals, they found better shear bond strength after 7 days. In the Sung et al13 study, the group that was bleached and bonded with an ethanol-based bond agent revealed no statistical decrease in shear bond strength compared with groups bleached and bonded with an acetone-based bond agent, which showed a significant reduction. The authors suggest that the discrepancy in bond strength between the groups may be related to the presence of alcohol in the primer, which interacts with residual oxygen minimizing the inhibitory effects of the bleaching process. The lower shear bond strength values found in the hydrogen peroxide–bleached group of the present study could also be explained by a high concentration of residual peroxide at the enamel surface. The bleaching process is a chemical reaction composed of different factors that determine the rate of the reaction: (1) temperature, (2) concentration of the reactants, and (3) pH value.11 It is known that the in-office bleaching method most often uses 35% to 50% hydrogen peroxide, which is considered a strength 7 to 16 times higher than is used in at-home bleaching techniques.11 The increased temperature and high concentration of the peroxide used in the in-office bleaching to accelerate the reaction and produce an immediate result probably left much residual peroxide on the tooth surface. Besides this, the in-office technique often demands that the steps to bleach the teeth be repeated a few times, as was done in this study, until the desired effect is reached. The repeated laser-energy application on the freshly mixed and repeatedly applied chemicals on the teeth liberate free oxygen radicals, which, in effect, remove much of the dark stains from the surface of the enamel.8 In the Uysal et al4 study the gel was placed only once on the tooth surface, which may explain the insignificant reduction on the shear bond strength of the immediately bleached group. If this procedure had been carried out in vivo, it might not have whitened the teeth satisfactorily. The normal shear bond strength values in the athome bleaching of this study were probably attributable to the lower concentration of the peroxide (10%) and mainly because after each daily bleaching the teeth were stored in distilled water. This might have eliminated the residual peroxide absorbed by the enamel. Therefore, we could have had adequate shear bond strength values in the hydrogen peroxide– bleached group if the teeth had been stored in distilled water for at least 1 week before bonding the brackets.
126 CONCLUSIONS • The use of 10% carbamide peroxide at-home bleaching 24 hours before bonding does not significantly alter shear bond strength values. • The use of 35% hydrogen peroxide in-office bleaching 24 hours before bonding significantly reduces shear bond strength values. • The use of 35% hydrogen peroxide in-office bleaching significantly reduces the amount of resin remnant on the tooth surface after debonding. ACKNOWLEDGMENT We thank the Furnas Centrais Ele´tricas S.A. – Aparecida de Goiaˆnia-Goia´s, Brazil, for its support in developing this study.
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