INTRODUCTION

Many of the bleaching systems available today use different concentrations and various application forms as the active bleaching agents.1–11 Vital tooth bleaching with carbamide and/or hydrogen peroxide has been recognized as a conservative and safe procedure for treating tooth discoloration.1213 Some adults who are interested in orthodontic treatment might have also had their teeth bleached or might be interested in bleaching. Since concerns have been raised regarding changes in enamel surface morphology because of the oxidative process produced by the bleaching gel, it seems important to determine whether bleaching would significantly influence the bonding strength of orthodontic bracket adhesives to the enamel surface.

In routine orthodontic practice, it is essential to obtain a reliable adhesive bond between an orthodontic attachment and tooth enamel. To simplify orthodontic bonding and to save chair time, materials that combine two or more steps have been manufactured. To simplify the procedure and to reduce the time spent for orthodontic bonding, self-etching primer (SEP) systems are available in the dental market that use a mixture combining acid and primer into one solution.14

Since the introduction of SEP as an alternative to the traditional acid-etch method, many investigations in operative dentistry have been carried out to evaluate their efficacy. According to White,15 SEPs are easily manipulated and used, resulting in comfort for the patient and decreasing the chair time by 65%.

In late 2000, a new SEP, Transbond Plus Self-etching Primer (3M Unitek, Monrovia, Calif; TSEP) was developed especially for orthodontic bonding. It includes methacrylated phosphoric acid esters, which will both etch and prime the enamel surface before bonding. TSEP has been studied,14–19 and according to several laboratory tests, using the agent before the bonding of brackets showed promising adhesive results.1617

There have been controversial reports regarding the interaction between bleaching agents and bonding materials to enamel. Some authors have reported significant decreases in the mean shear bond strength of orthodontic brackets to bleached enamel compared with unbleached enamel.17–911 However, others found no significant differences in composite bond strengths between bleached and unbleached teeth.2–4

So far, to our knowledge, no studies have investigated the effect of bleaching treatment on the bond strength values of metallic brackets bonded with SEP. The purpose of this in vitro study was to determine the effect of bleaching on the shear bond strength and the adhesive remnant index of metallic brackets bonded with TSEP and orthodontic composite at two time intervals (bleaching immediately before bonding and bleaching 30 days before bonding). The null hypothesis to be tested was that there is no statistical significance in (1) bond strength and (2) failure site location to bleached and unbleached enamel prepared with TSEP between the different time intervals.

MATERIALS AND METHODS

Sixty noncarious mandibular premolars extracted with orthodontic indication were used in this study. Teeth with hypoplastic areas, cracks, or gross irregularities of the enamel structure were excluded. The criteria for tooth selection dictated no pretreatment with a chemical agent such as alcohol, formalin, or hydrogen peroxide or any other form of bleaching. Immediately after extraction, the teeth were scraped of any residual tissue tags and washed under running tap water. The teeth were stored in distilled water, and the water was changed weekly to avoid bacterial growth. The sample was randomly divided into three groups of 20 teeth each. All teeth were mounted vertically in a self-cure acrylic so that the crowns were exposed. The buccal surfaces were cleaned and polished with a rubber cup and slurry with pumice and water, followed by rinsing with a water spray and drying with compressed air.

Specimens were prepared for bracket bonding according to one of the following procedures:

• Group 1. Twenty premolars were separated as the control group; TSEP was gently rubbed onto the surface for approximately 3 seconds with the disposable applicator supplied with the system. Then, a moisture-free air source was used to deliver a gentle burst of air to the enamel.

• Group 2. A commercially available 16% carbamide peroxide bleaching gel (Whiteness Perfect, FGM Dental Products, Joinville, Brazil) was applied with a brush to the enamel surfaces of the embedded teeth in a layer approximately 1-mm thick for 4 hours in 1 day according to the manufacturer's instructions. After completion of the 10 consecutive daily bleaching procedures, the specimens were thoroughly rinsed with a compressed air/water syringe for 30 seconds, air dried, and stored in 250 mL of artificial saliva solution at 37°C. The bonding area was prepared with TSEP as in group 1, before bonding.

• Group 3. This group was treated the same as group 2, except that after bleaching and before bonding, the teeth were stored in artificial saliva for 30 days at room temperature. The artificial saliva was changed every day after the bleaching cycle was completed.

Standard edgewise premolar stainless steel brackets (G&H Wire Company, Greenwood, Ind), with a base surface area of 10 mm², were bonded to the teeth using the standard protocols according to the manufacturer's instructions. After surface preparation with the TSEP, the brackets were bonded with Transbond XT light-cure adhesive paste. Excess resin was removed with an explorer before it was polymerized. Then, a light-emitting diode (Blue Swan Digital, Dentanet, Istanbul, Turkiye) was used for curing the specimens for 20 seconds.

RESULTS

The descriptive statistics for the shear bond strengths of the various groups tested are presented in Table 1. The results of the ANOVA indicated statistically significant differences among the three groups (P = .000). Thus, the first part of the null hypothesis of this study was rejected. The Tukey test showed that the bond strengths of group 1 (no bleaching, mean: 17.60 ± 7.93 MPa) and group 3 (bleaching 30 days before bonding, mean: 13.95 ± 5.23 MPa) were significantly higher (P > .05) than that of group 2 (bleaching immediately before bonding, mean: 11.45 ± 5.25 MPa). We found no statistically significant differences between groups 1 and 3 (P > .05).

Table 1.  Descriptive Statistics and Results of the Analysis of Var iance Comparing the Shear Bond Strength of the Three Groups Tested

View Fullscreen

The ARI scores for the various groups tested are listed in Table 2. The results of the χ² comparisons indicated that there were significant differences among the three groups (χ² = 33.019, P = .001). Therefore, the second part of the null hypothesis of this study was rejected.

Table 2.  Frequency of Distribution of Adhesive Remnant Index (ARI) Scores (%)

View Fullscreen

DISCUSSION

There is concern that vital bleaching could alter the surface topography of enamel and thus affect the bond strength of adhesives to enamel.2223 Alterations in bond strength might be significant with regard to clinical operative procedures that involve composite resin bonding, such as bonding orthodontic brackets, porcelain veneers, composite veneers, or future composite restorations.3

It has been proposed that residual oxygen from the bleaching agent inhibits resin polymerization.156 Although there are remarkable variations among the recommended postbleaching time periods in different studies (24 hours to 4 weeks), some researchers thought that a delay of at least 2 weeks is needed after bleaching for the tooth structure to regain its prebleaching adhesive properties.10 Uysal et al2 stored their samples in artificial saliva for 30 days and suggested that a bonding delay of a minimum 2 to 3 weeks might be beneficial. In the experimental setup of the present study, the bleached teeth in group 3 were stored in artificial saliva for 30 days before bonding to imitate the conditions of the oral cavity.

Compared with phosphoric acid, TSEP produced a uniform and more conservative etch pattern, with regular adhesive penetration and a less aggressive enamel demineralization.19 It can be inferred from previous laboratory investigations that TSEP can successfully bond orthodontic brackets as well as when phosphoric acid is used with Transbond XT primer.161724 However, a review of the literature indicated that no researchers have investigated the effect of bleaching treatment on the bond strength of metallic brackets bonded with orthodontic composites to enamel that been prepared with TSEP.

We found no statistically significant differences in bond strength between the control group (group 1) and teeth bonded 30 days after bleaching (group 3). Group 2 (bonded immediately after bleaching) showed significantly lower shear bond strength values than the others.

There is an obvious controversy in the literature related to postponing the bonding procedure after bleaching. Previous investigations have demonstrated that immersion of in vitro specimens in distilled water, artificial saliva, or even saline solution results in a complete reversal of the reduced enamel bonds.1358923 Similar to the present results, some authors have shown a significant increase in bond strength for teeth bonded 1 week/30 days after bleaching.125 However, Cacciafesta et al7 found that both bleaching groups (bonding immediately or 1 week after bleaching) have significantly lower shear bond strength values than do unbleached controls. Some of the studies in the literature found no significant differences between the groups bleached at two time intervals.2710 When using TSEP for the enamel surface preparation, the result of the present study is in agreement with those findings assuming that the immersion process is removing the residual oxygen from the bleaching material and that the tooth structure regains its prebleaching adhesive properties.10

Reynolds26 suggested that a minimum bond strength of 6 to 8 MPa is adequate for most clinical orthodontic routine clinical use. All bond strength values of composites used in this study were greater than this minimum requirement and fell within clinically acceptable ranges. However, clinical conditions may significantly differ from an in vitro setting. Moreover, heat and humidity conditions of the oral cavity are highly variable. Because of the probable differences between in vivo and in vitro conditions, a direct comparison cannot be made with the findings of the other studies.

The results of the ARI score comparisons in the current study indicated that there were significant differences among the three groups tested. In groups 1 and 2, there was a higher frequency of ARI scores of 2 to 4, indicating cohesive failures within the resin. In group 3, the failures were shown adhesive (resin/enamel interface) and cohesive characteristics. In the literature, three similar investigations evaluated the ARI scores. Some authors12 have shown a prevalence of cohesive characteristics; however, failures occurred at the enamel-adhesive interface in one study.7 The differences in the mechanical and physical properties of the materials tested in each study (two-step bonding in the previous studies and TSEP in ours) might explain the variability of the results.7

CONCLUSIONS

• The use of a carbamide peroxide bleaching agent immediately before bonding significantly reduces the shear bond strength of teeth bonded with self-etching primer.

• Immersion of bleached teeth in artificial saliva for 30 days before bonding results in a return to control shear bond strength.

• Bleaching and bonding with SEP significantly alters the site of failure during debonding. Especially in group 3 (bleached 30 days before bonding), less residual adhesive remains on the tooth surface.