INTRODUCTION

Since the introduction of carbamide peroxide for at-home bleaching,¹ this technique has been considered the most conservative treatment for vital discolored teeth when compared with other treatment modalities, such as veneers, crowns, or composite bonding.² High levels of satisfaction and effectiveness of at-home bleaching have been reported,^3–5 making this therapy the first choice for vital tooth bleaching. Perdigão and others⁶ reported that the popularity of tooth whitening has increased with the advent of patient-applied, peroxide-based whitening agents, as well as increased media influence.

Although at-home bleaching has increased dramatically in popularity, in-office bleaching products are still in demand⁷ for several reasons. First, some patients do not adapt well to the at-home protocol because they prefer not to use a bleaching tray; second, patients sometimes require faster results and cannot wait for two to three weeks to see the results of their treatment; and finally, some patients need to be closely monitored due to the presence of extensive tissue recession or deep, unrestored abfraction lesions.²

The use of hydrogen peroxide in high concentrations (35%-38%) applied by a dental professional allows the patient to obtain visible results even after only one clinical appointment. This has been responsible for the increase of in-office bleaching treatment in the last few years. The results of a published survey of general practitioners showed that 33% of dentists in the United States use in-office bleaching.⁸

Usually the hydrogen peroxide is applied and left undisturbed on tooth surfaces for five to 15 minutes and this procedure is repeated three to five times at each clinical appointment, depending on the bleaching gel brand. The reason for the gel being refreshed every five to 15 minutes is the fast degradation rate of hydrogen peroxide. However, an earlier study⁹ evaluating a hydrogen peroxide gel with low concentration demonstrated that after one hour in contact with the tooth structures, the average of the total remaining active hydrogen peroxide was 32.23%. Given that the degradation kinetics of bleaching gels seems to be independent of the initial concentration, one may suppose that a 35% hydrogen peroxide gel might have a similar degradation rate.¹⁰

Thus, considering that the gel retains substantial activity after one hour, one may speculate that a single application of the gel for a prolonged period might yield similar bleaching outcomes. This approach would lead to less chair-time and would reduce the costs associated with in-office bleaching because less material would be spent per patient. Moreover, the risk of occasional soft tissue burns would be reduced because the material would be handled only once.

Recently, two clinical studies compared the bleaching efficacy of a single 45-minute versus three 15-minute applications.^11,12 Whereas Marson and others¹¹ reported good results in terms of bleaching efficacy, comfort, sensitivity, and patient satisfaction, Matis and others¹² demonstrated that three 15-minute applications were more effective than a single 40-minute application.

Because there is ongoing controversy on this topic, the aim of this randomized clinical study was to evaluate the bleaching outcomes and tooth sensitivity of a 35% hydrogen peroxide gel applied in three 15-minute or one 45-minute applications.

MATERIALS AND METHODS

This clinical investigation was approved by the scientific review committee and by the committee for the protection of human subjects of the university's institutional review board. A total of 30 undergraduate students who had anterior teeth with a shade of C2 or darker as judged by comparison with a value-oriented shade guide (Vita Lumin, Vita Zahnfabrik, Bad Säckingen, Germany) were enrolled in a double-blind, controlled clinical trial. All participants received dental screening and dental prophylaxis two weeks before bleaching started and signed an informed consent form before the study began.

Shade Evaluation

The 16 tabs of the shade guide are arranged from highest (B1) to lowest (C4) value (Table 1). Although this scale is not linear in the truest sense, the changes were treated as though they represented a continuous and approximately linear ranking for the purpose of analysis. Shade changes were calculated from the start of the active phase to the individual recall times by calculating the change in the number of shade guide units (Δ SGU) that occurred toward the lighter end of the value-oriented list of shade tabs.

Table 1:  Arrangement of Vita Classic Shade Guide (Adapted From Browning and Others¹⁷)

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Two calibrated evaluators (L.Y.T. and D.H.) recorded the shade of each participant's teeth at baseline and weekly intervals. The measurement area of interest for shade matching was the middle third of the facial surface of maxillary anterior teeth, according to American Dental Association (ADA) guidelines.¹⁵ Five participants who were not included in the sample because they were used in the pilot study participated in the training phase of this study. The two examiners scheduled these patients for bleaching and evaluated their teeth against the shade guide weekly. Before beginning the study evaluation the two examiners were required to agree on the shade to the extent of at least 85% (κ statistic).

RESULTS

Of the 30 participants who began the study, all completed the study. The level of agreement between the two evaluators by means of κ statistics was 92%. The statistical analyses revealed that significant differences were observed for the treatment modality (Student t-test, p<0.05) and clinical sessions (McNemar's test, p<0.005).

The means and standard deviations of Δ SGU are shown in Table 2. The mean tooth color at baseline was similar for the two groups (Student t-test, p>0.05). However, a significantly faster bleaching occurred for the 3 × 15-minute group after the first and second bleaching sessions (McNemar's test, p<0.05). After the bleaching sessions, the participants from the 1 × 45-minute group showed a final color change of 5.8 SGUs, whereas participants from the 3 × 15-minute group showed a final color change of 6.9 SGUs (Student t-test, p<0.05) (Table 2).

Table 2:  Tooth Shade at Assessment Points for the Two Treatment Groups

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With regard to the prevalence of tooth sensitivity, only two participants from the 3 × 15-minute group did not experience tooth sensitivity after the bleaching treatment (Table 3). Most of the participants reported tooth sensitivity immediately and up to 24 hours after the bleaching protocol. In a small percentage of patients (17%) this sensitivity persisted on the second day after treatment. Figure 1 depicts the percentage of participants according to the level of reported sensitivity for both groups. It was observed that no severe sensitivity was reported by any of the participants. A similar report of considerable intensity was observed for both treatment modalities (40%). Moderate sensitivity was reported by 46.7% of the participants from the 1 × 45-minute group, whereas this figure was lower for the 3 × 15-minute group (26.7%).

Table 3:  Comparison of the Number and Percentage (%) of Patients Who Experienced Tooth Sensitivity During the Bleaching Regimen

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The intensity of sensitivity was significantly higher in the 1 × 45-minute group when compared with the 3 × 15-minute group (p=0.04). In terms of means and standard deviations, the intensity of tooth sensitivity was significantly higher in the group receiving the 1 × 45-minute (2.43 ± 1.1) application than that observed in patients receiving the 3 × 15-minute application (1.86 ± 0.9; p=0.04) (Table 4).

Table 4:  Means and Standard Deviations of the Intensity of Tooth Sensitivity During the Bleaching Regimen as Well as the Respective Medians and the Minimum (Min) and Maximum (Max) Scores

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DISCUSSION

A value-oriented shade guide was used for visual identification of the color change throughout the study period. The selection of this method was based on the fact that it is an easy, fast, and satisfactory method that has been used in several investigations.^16–18 The validity and reliability of this color evaluation method was confirmed by a recent study.¹⁹ The authors reported that despite the subjectivity of the method, visual assessment of tooth color using the Vitapan Classical shade guide is a valid method, with good reliability for differentiating between dark and light colors.¹⁹

The results of this study indicated that the application of a 35% hydrogen peroxide gel for 45 minutes decreased the bleaching efficacy and slightly increased overall patient sensitivity when compared with three 15-minute applications. Three studies that evaluated the degradation kinetics of hydrogen peroxide and carbamide peroxide gels for at-home bleaching under clinical conditions were found in the literature.^9,10,20 They demonstrated that both bleaching agents have similar overall degradation kinetics, although hydrogen peroxide was shown to have a faster degradation rate. Although there is still 50% of the active ingredient after two hours when carbamide peroxide is used, this is the amount available after 20 minutes for at-home hydrogen peroxide gels.^9,10,20 Although a significant amount of active ingredient can be retrieved from the tooth surfaces, this may not be sufficient to sustain the same degree of bleaching obtained in the first 15 minutes, and therefore, this might well be the reason why the participants in the 1 × 45-minute group showed less color change after two appointments.

However, different results were obtained when the degradation of 35%-38% hydrogen peroxide gels for in-office bleaching was assayed by a potassium permanganate titration method under in vitro conditions.^21,22 The concentration of active hydrogen peroxide measured after 45 minutes of mixing showed that 91%-93% of active hydrogen peroxide was still available for the bleaching procedure. It was speculated that the differences between the in vivo at-home^9,10,20 and in vitro in-office studies^21,22 may depend on the different experimental conditions. Contrary to in vitro conditions,^21,22 the bleaching gel can be washed out by saliva during use before being collected under clinical conditions,^9,10,20 which could decrease the total amount of hydrogen peroxide available for bleaching. Therefore, attempts should be made to evaluate the degradation kinetics of in-office bleaching gels under in vivo conditions.

Although both treatment modalities showed different bleaching rates, both procedures allowed a significant whitening effect. The 1 × 45-minute and the 3 × 15-minute groups allowed bleaching of 5.8 and 6.9 SGUs, respectively, after the two bleaching sessions. These bleaching rates are very close to those that were recently demonstrated in a literature review.²³ Matis and others²³ reported that after two weeks of bleaching with 35% hydrogen peroxide, a change of 4.5 to 9.6 SGUs is expected.

Tooth sensitivity is the most common adverse side effect of bleaching treatment. Although the etiology of this side effect has not yet been fully established, it seems that it results from the peroxide penetrating through enamel and dentin and into the pulp during tooth whitening.^24,25 Within five to 15 minutes after application of a bleaching gel, peroxide penetrates to the pulp, where it irritates nerves and essentially produces a reversible pulpitis.^26–28 Further proof of this passage of peroxide is found in the research that showed color changes in dentin next to the pulp as fast as it occurs next to the dentin-enamel junction.²⁹ This may account for the common occurrence of tooth sensitivity in patients with no gingival recession or other sites of exposed dentin. Moreover, several studies have reported morphologic alterations, reductions in enamel microhardness, and increase in enamel permeability after bleaching,^30–32 factors that may enhance bleaching agent penetration into pulp.

As previously demonstrated, it seems that the pH of the bleaching gel has an impact on enamel microhardness after bleaching therapy. Recently, Borges and others³³ evaluated the influence of two bleaching gels with different pH (acid 3.5 and neutral 6.6) on enamel microhardness. The authors concluded that the enamel microhardness decreased significantly when the most acidic bleaching agent was compared with the neutral formulation.

The hydrogen peroxide gel used in the present investigation has a pH of 7 immediately after mixture. However, after a single 45-minute application, the pH of the gel²² decreases to approximately 5, which can explain the slightly higher tooth sensitivity of the 1 × 45-minute group. A bleaching gel with more acidic features may induce more pulp alterations than a neutral solution does. However, this hypothesis should be further evaluated in detail under clinical conditions.

Similar results in terms of bleaching speed and tooth sensitivity were reported by Matis and others.¹² The authors compared a single 40-minute application and three 15-minute applications of a 36% hydrogen peroxide gel. However, Marson and others¹¹ did not find any difference between 1 × 45-minute and 3 × 15-minute applications. Unfortunately the latter is an abstract and several details of the method, such as the commercial brand name of the bleaching product used and inclusion and exclusion criteria, are not available for the purposes of comparison.

CONCLUSION

Within the limitations of the present investigation it could be concluded that the in-office 35% hydrogen peroxide gel (Whiteness HP Maxx, FGM Dental Products) should be used in three 15-minute applications because this approach favors fast bleaching and reduces the intensity of tooth sensitivity when compared with a single 45-minute application.