INTRODUCTION

Tooth bleaching represents the most common elective dental procedure for treatment of discolored teeth,¹ and according to Dutra and others,² an estimated more than 1 million Americans whiten their teeth annually, driving nearly $600 million in revenues for dental offices. A great part of the success of the bleaching therapy is that this procedure is a very conservative, simple, and low-cost procedure.³

Unfortunately, tooth sensitivity (TS) is the most frequently reported side effect associated with bleaching, particularly with in-office bleaching protocols that employ relatively high concentrations of hydrogen peroxide.^4-6 Even though the sensitivity will be only transient and will be resolved a few days after the end of the bleaching procedure, it is an unpleasant experience^7,8 and can be severe and irritating enough to lead patients to withdraw from treatment in some cases.⁹

This adverse effect has motivated clinicians and researchers to develop strategies for the prevention of bleaching-induced TS.¹ Several approaches, such as administration of analgesics, anti-inflammatories, antioxidants, and corticosteroids,^10-13 have failed to minimize this side effect caused by bleaching products. The most useful and effective agent for the management of bleaching-induced TS is potassium nitrate associated or not associated with sodium fluoride when compared with a placebo group.^14-17

A recent systematic review¹⁸ showed that usually a 3%-5% potassium nitrate gel was applied for 10-30 minutes prior to the in-office bleaching procedure in a tray or directly in the office.^14-17 Only recently, as a response to the demand for an alternative way of bleaching, a new product has become available on the market: a prefilled disposable tray containing potassium nitrate (UltraEZ, Ultradent Products Inc, South Jordan, UT, USA). Prefilled disposable tray systems are comfortable and have a low cost, as the professional does not need to fabricate a custom bleaching tray (impression, model buildup, tray fabrication, etc), and the procedure can be done at home¹⁹ or in the office in the waiting room immediately before in-office bleaching. However, to the extent of our knowledge, no clinical studies have been performed comparing potassium nitrate–based products delivered via different methods, which was the main objective of the present study.

Therefore, the aim of this equivalent clinical trial was to compare bleaching-induced TS between the application of a prefilled disposable tray containing a desensitizer agent prior to in-office bleaching and a conventionally delivered tray system with a desensitizer agent. The following null hypotheses were tested: 1) the preventive use of desensitizer agents in different trays will not affect the absolute risk and intensity of bleaching-induced TS, and 2) the preventive use of desensitizer agents in different trays will not affect the color change after bleaching.

METHODS AND MATERIALS

Study Intervention

To maintain the allocation concealment before starting the bleaching procedure, a custom-fitted tray was made for all patients. For this purpose, an alginate impression of each subject's maxillary and mandibular arch was made and filled with dental stone. To produce study models, block-out material to the labial surfaces of teeth was not applied. A 1-mm soft vinyl material, provided by the manufacturer, was used to fabricate the custom-fitted tray for the desensitizer gel. The excess material on the labial and lingual surfaces was cut 1 mm from the gingival junction.

Before each bleaching session, subjects were instructed to wear the conventional-delivered tray system containing 5% potassium nitrate and 2% sodium fluoride desensitizing gel (Desensibilize KF 2%) or the prefilled disposable tray containing less than 5% potassium nitrate and less than 1% sodium fluoride desensitizing gel (UltraEZ, Ultradent Products Inc) for 15 minutes according to the randomization of patients.

Immediately after tray removal, subjects were instructed to wash the tray and brush their teeth as usual before the in-office bleaching was performed. For this purpose, the gingival tissue was isolated with a light-cured resin dam (Opal Dam, Ultradent Products Inc). The 40% hydrogen peroxide gel Opalescence Boost (Ultradent Products Inc) was applied in two 20-minute applications (manufacturer's directions) to all maxillary incisors, canines, and premolars of the same patient (Table 1). After seven days, this procedure was repeated using the same protocol. All participants were instructed to brush their teeth at least three times a day using fluoridated toothpaste (Colgate, Colgate-Palmolive, SP, Brazil).

Table 1 Products, Composition, and Application Regimens

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RESULTS

A total of 110 participants were examined in a dental chair to determine if they met the inclusion and exclusion criteria. A total of 79 patients were included in this clinical study (Figure 1). The mean age (years) of the participants and the baseline SGU are described in Table 2. One can observe comparable data among treatment groups by ensuring the comparability of the baseline features. None of the patients discontinued the intervention or presented adverse effects during the intervention. No medication and/or desensitizer were necessary to be prescribed or applied to the participants from this study for the relief of bleaching-induced TS.

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Table 2 Baseline Characteristics of the Participants Included in This Clinical Trial

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Tooth Sensitivity

A total of 26 patients (absolute risk: 67%, 95% CI: 51% to 79%; Table 3) reported pain in the conventional-delivered tray group. Thirty-three patients (absolute risk: 84%, 95% CI: 70% to 93%; Table 3) reported pain in the prefilled disposable tray group. No significant difference was observed between the risks of TS of the two study groups (p=0.37; Table 3).

Table 3 Comparison of the Number of Patients Who Experienced Tooth Sensitivity (TS) at Least Once During the Bleaching Regimen in Both Groups Along With Absolute Risk and Risk Ratio^a

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The TS intensity of both bleaching protocols was statistically similar (p=0.38), and the overall TS intensity at different assessment points is reported in Table 4. In all time assessments, the mean difference ranged from −0.01 to −0.03 and was not clinically important (Table 4). Most of the TS complaints occurred within the first 24 hours after bleaching and moved closer to zero after 24 hours.

Table 4 Tooth Sensitivity Intensity (Means ± Standard Deviations) at the Different Assessment Points for Both Study Groups and the Statistical Comparison Along With the Effect Size (95% Confidence Interval) as Well as the p-Value of the Pairwise Comparison^a

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Color Change

The intra- and interexaminer kappa values were 0.95 to 0.88 for Vita Classical and 0.91 to 0.91 for Vita Bleachedguide, respectively. A whitening of approximately 7 to 9 SGUs and a ΔE of approximately 12 were detected for both groups 30 days after bleaching (Table 5). No statistically significant difference was observed between the study groups (p>0.12). For all color measurements, the mean difference ranged from −0.2 to 1.9 and was not clinically important (Table 5).

Table 5 Color Change in Shade Guide Units (SGU) and ΔE (Means ± Standard Deviations) Between Baseline and 30 Days After Bleaching for the Two Treatment Groups Along With the Effect Size (95% Confidence Interval) as Well as the p-Value of the Pairwise Comparison^a

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DISCUSSION

The results of the present study showed that no significant difference in terms of absolute risk and TS intensity was observed for a desensitizer gel containing nitrate potassium and fluoride applied in a conventional tray or in a prefilled disposable tray.

Regarding both substances used, the exact action mechanism of potassium nitrate and sodium fluoride for reducing bleaching-induced TS is not well understood. It is likely that fluoride prevents TS due to deposition of fluoride crystals in the exposed dentinal tubules.^21-23 However, in the present study, patients presenting teeth with visible cracks were excluded, mainly because enamel surface cracks or craze lines are potential factors in increasing bleaching-induced TS,²⁴ and this helps to explain the lack of preventive action of fluoride when applied alone before bleaching.²⁵

On the other side, it is likely that potassium ions are the active component, and potassium nitrate works by reducing dentinal sensory nerve activity due to the depolarizing activity of the K+.^21,26-28 However, for this purpose, it was required that the potassium nitrate could be transported within the pulp chamber, preferably before hydrogen peroxide penetration.

Although the more useful protocol for preventing bleaching-induced TS is the topical application of substances containing potassium nitrate,^18,19,29 only recently have two papers shown that potassium nitrate penetrates the pulp chamber.^1,30 According to these papers, the transport within the tooth may be facilitated by the low molecular weight (101.10 g/mol) and water solubility of potassium nitrate gel.^1,30 Kwon and others³⁰ showed that this penetration occurs within the first five minutes after application, similar to hydrogen peroxide penetration,^31,32 and this is why nitrate potassium desensitizer gel needs to be applied before or together with the bleaching application.

Several factors may be involved in the potassium nitrate penetration, and one of them is material viscosity. The company that markets the prefilled disposable tray also produces the same product in syringes, and this is the most used product when applied in conventional-delivered tray systems.^19,27 However, according to Kwon and others,¹ UltraEZ (Ultradent Products Inc) in a syringe is the most viscous product when compared with other products in the market, and this characteristic negatively affects the penetration of the nitrate potassium. Unfortunately, no information is available regarding the viscosity of Desensitizer KF2, although the manufacturer indicated that this is a low-viscosity gel, probably similar to other gels available in the market.¹

Another factor that must be taken into account is the amount of gel applied. The proper dosage of potassium nitrate for maximum efficacy is still unknown, but it is expected that within certain parameters, a higher dosage of gel means a higher amount of gel inside the pulp chamber.

The amount of bleaching gel present in a prefilled disposable tray is about 60 mg, as per manufacturer descriptions. To the extent of the authors' knowledge, no studies have measured the amount of desensitizer gels inside the trays; however, different studies have measured the amount of bleaching gels, and these quantities range between 500 and 900 mg per application, mainly because the application depends on the patients' subjective interpretation regarding the amount of gel to insert in the tray.^33,34 Once again, a lower effectiveness of prefilled disposable trays is expected.

However, according to Kwon and others,¹ the most important factor in terms of potassium nitrate penetration is the product concentration. Although the exact concentration of nitrate potassium is a manufacturer property, according to the MSDS of each manufacturer, both delivered methods (conventional-delivered tray system and prefilled disposable tray) containing similar concentrations of potassium nitrate and sodium fluoride (Table 1) help to explain the similar results between both products.

The cause of bleaching-induced TS is not completely understood. According to the “hydrodynamic hypothesis,”³⁵ thermal and tactile stimuli are effective for evaluating dental sensitivity when dentin is exposed.³⁶ Since the hydrodynamic theory of dentin sensitivity enjoys wide acceptance as the explanation of dentinal sensation, many authors view bleaching-related pain as a form of dentin sensitivity.³⁷ Major differences distinguish bleaching-related pain from dentin hypersensitivity.

Although pain in bleached teeth can be evoked by thermal or other stimuli, most patients complain of tingling or shooting pain (zingers)⁴ without provoking stimuli. Pain during and following bleaching treatments can affect intact teeth lacking dentin exposure, which is in sharp contrast to dentin sensitivity, in which pain occurs in teeth with exposed dentin. Recently, Markowitz³⁸ showed an alternative hypothesis related to the bleaching-induced TS. According to this author,³⁹ bleaching-induced TS arises as a consequence of peroxide penetrating the tooth structure, causing direct activation of a neuronal receptor and not through the hydrodynamic mechanism.

This fact, along with the current need of reporting patient-centered outcomes, led the authors of the present study to use self-reported pain, as done in clinical trials of bleaching that evaluate TS as the primary outcome.^{5,10,12,15,19,24,29,39} However, we cannot rule out the fact that there are other methods described in the literature to evaluate TS that are not employed in the present study, and this can be considered one of the study limitations. Among them, we can cite the Schiff Cold Air and Yeaple Probe Tactile methods, commonly used in studies that evaluate dentin hypersensitivity.^40,41 To the extent of the authors' knowledge, they have not been used in bleaching studies yet, but they need to be included in future bleaching studies to determine if they can add significant information.

Regarding the color evaluation, despite one recent clinical study suggesting that potassium nitrate pretreatment may negatively affect whitening efficacy,¹⁵ the results of the present study showed significant whitening at the end of the bleaching protocol, with the use of three different instruments, which is in accordance with the findings of several other clinical trials that evaluated color change.^3,5,10-18 As both desensitizer gels used were colorless, no significant interaction with tooth color was expected.

In the present study, color measurement was performed only in canines because these teeth are darker than incisors,^42,43 and this allows for more sensitive color change evaluation. An earlier article showed that a significantly stronger overall increase in lightness was observed for canines after treatment when compared with incisors. At the end of the bleaching treatment, teeth become more homogeneous in terms of lightness values.⁴⁴ In agreement with these findings, Ontiveros and others⁴⁵ observed that whitening of canines was 1.4-1.6 times more pronounced that of incisors. A recent study of the literature demonstrated that a higher degree of whitening occurs in teeth with a darker baseline color.³ By using canines as the reference for color evaluation, the recruitment of patients becomes easier. Patients with incisors of color A3 are rare, but patients with canines of color A3 are quite common.

Although canines did not have a flat labial face, the use of custom molds enabled standardization of the area for color evaluation, thereby securing a correct angulation for placement of the spectrophotometer tip in all recall periods of this study. However, it worth mentioning that at least two previously published clinical studies did not show any significant variations in the color measurement with the spectrophotometer when measured in incisors or canines.^44,45

Finally, if the use of a prefilled disposable tray to apply the desensitizer agent is taken into account, this new system has an advantage in comparison with the use of conventional custom trays. The professional does not need to fabricate a personalized custom tray (impression, model buildup, tray fabrication, and so on), and the procedure can be done in the waiting room of the office before dental care or even at home, in the same way that the prefilled disposable trays containing hydrogen peroxide can be used.³⁹

CONCLUSIONS

The use of a prefilled disposable tray containing potassium nitrate and fluoride before the application of the in-office bleaching product did not affect the whitening degree and did not decrease self-reported TS, which was similar to the use of a conventional-delivered tray system.