Do dental bleaching sessions prior to orthodontic treatment change the bond strength of esthetic brackets?

ABSTRACT Objective: The aim of this experimental in vitro study was to evaluate whether dental bleaching performed before orthodontic treatment change the shear bond strength (SBS) of monocrystalline and polycrystalline esthetic brackets. Methods: Sixty (60) bovine incisors teeth were used and randomly divided into the following six groups (n=10): SCP (without bleaching/polycrystalline brackets); SCM (without bleaching/monocrystalline brackets); 1CP (one bleaching session/polycrystalline brackets); 1CM (one bleaching session/monocrystalline brackets); 3CP (three bleaching sessions/polycrystalline brackets); and 3CM (three bleaching sessions/monocrystalline brackets). The brackets were bonded seven days after the bleaching sessions. The samples were submitted to the SBS test in a universal testing machine (Instron model 4411) at 1 mm/min crosshead speed. The two-way analysis of variance (ANOVA) and the Tukey tests were performed at a 5% level of significance. After the mechanical test, samples were evaluated to determine the adhesive remnant index (ARI). Results: The SBS values were significantly higher for the monocrystalline brackets, when compared with the polycrystalline type (p< 0.0001), and significantly higher with three bleaching sessions than without bleaching (p< 0.0436). The ARI showed predominance of failures between the bracket and resin for all the groups (score 3). Conclusion: Three dental bleaching sessions increased the SBS values. Monocrystalline brackets showed higher SBS values than the polycrystalline type.

Perciano SG, Carneiro DPA, Santos PR, Correr AB, Vedovello SAS, Valdrighi HC Do dental bleaching sessions prior to orthodontic treatment change the bond strength of esthetic brackets?

INTRODUCTION
Dental bleaching is an easy and conservative therapy for whiting teeth. 1,2 Dental bleaching can be performed at-home or in-office, [3][4][5] and the most common used substances are carbamide and hydrogen peroxide in various concentrations. 3,4 Patients willing to have white and aligned teeth have associated dental bleaching with orthodontic treatment, 6 undergoing the procedure before or after orthodontic treatment. 7 Adhesive used for bracket bonding may undergo degradation or solubility due to the presence of bleaching agents. 8 Various methods have been proposed to avoid problems related to reduction in bond strength after bleaching, such as application of antioxidant, desensitizing agents, 2,4 sodium ascorbate, tocopherol acetate, retinol acetate, 9 or increase post-bleaching time interval for bracket bonding. 1,5,8 Patients who are concerned about esthetics during orthodontic treatment have the option of using monocrystalline or polycrystalline esthetic brackets. 10 Both types are composed of aluminum oxide, 11,12 with different manufacturing process. Polycrystalline ceramic brackets are composed of polycrystalline alumina, made up of aluminum oxide crystals fused at high temperatures, allowing various brackets to be molded simultaneously.
Perciano SG, Carneiro DPA, Santos PR, Correr AB, Vedovello SAS, Valdrighi HC Do dental bleaching sessions prior to orthodontic treatment change the bond strength of esthetic brackets? 5 Polycrystalline brackets are more common and popular, due to the quality of the material and the relative ease of production. In turn, monocrystalline ceramic brackets consist of a mass cast at a high temperature (2,100 o C) forming a single aluminum oxide crystal that will result in the fabrication of a single bracket, thus making the production process more expensive. However, this form of milling presents lower incorporation of impurities, making the brackets stronger and more translucent. This difference may favor the transmission of light and influence the polymerization of the resin used for bonding the brackets, therefore generating an influence on the shear bond strength (SBS). 13 Concern is related to the SBS of brackets bonded after bleaching procedures, [6][7][8][9]14 and few studies have compared the retention of different types of esthetic brackets used nowadays.
Based on the literature, studies of SBS after bleaching treatments refer to a single session of dental bleaching. [1][2][3][4]6,8,9,15 In an endeavor to approximate clinical reality, in which the dentist commonly performs more bleaching sessions, the proposal of the present study was to test the following hypoth- bracket (monocrystalline or polycrystalline) may influence the SBS. Therefore, the aim was to evaluate, in vitro, the SBS of esthetic brackets (monocrystalline or polycrystalline) after one and three dental bleaching sessions.

MATERIAL AND METHODS
The present experimental in vitro study was approved by the Perciano SG, Carneiro DPA, Santos PR, Correr AB, Vedovello SAS, Valdrighi HC Do dental bleaching sessions prior to orthodontic treatment change the bond strength of esthetic brackets?

DISCUSSION
The initial hypothesis of this study, that one and three in-office bleaching sessions would reduce the bond strength, was  It is believed that the differences in the present results, when compared with the literature, was due to the methodological differences, because studies normally compare the group without bleaching and groups submitted to a single dental bleaching session. However, the present research intended to approximate the clinical reality, in which in-office dental bleaching is commonly performed in more than one session, to attain the expected result. Previous studies that compared the surface roughness after dental bleaching have suggested that bleaching promoted changes in the superficial layers of enamel. [17][18][19][20] Roughness then justified the SBS increase in the groups exposed to three bleaching sessions, suggesting that more bleaching sessions promoted irregularities in the enamel, thus increasing the mechanical retention of resin on the tooth surface.
When the types of esthetic brackets (monocrystalline and polycrystalline) used in this study were compared, regardless of the condition, the monocrystalline brackets showed is advantageous, because the ideal is to remain all the material on the tooth surface, thus protecting the enamel. 24,25 Finally, the authors suggest that new clinical studies should be carried out with the objective of evaluating the long-term retention of esthetic brackets after dental bleaching, since the results of this study express an in vitro setting.

CONCLUSION
After three sessions, dental bleaching increased shear strength of aesthetic brackets. The monocrystalline brackets presented the highest SBS values. Overall responsibility:

SGP.
The authors report no commercial, proprietary or financial interest in the products or companies described in this article.