Effects of Exposure to Cola-Based Soft Drink on Bleaching Effectiveness and Tooth Sensitivity of In-Office Bleaching: A Blind Clinical Trial

Introduction

Dental bleaching is undoubtedly one of the most common clinical procedures in Dentistry probably because dental appearance is a determining factor in the attractiveness of a face and tooth color is considered the most important factor with regard to dental aesthetics.¹ As it is an effective, conservative, and low-cost procedure,² it should be considered the first choice for discolored teeth.³ When dealing with vital teeth, dental bleaching can be achieved through a variety of methods, such as in office (performed by the professional), at home (supervised by the professional), or both methods (combined or jump-start technique).⁴

Notwithstanding the effectiveness of the whitening technique, laboratory studies have reported alterations in enamel surface, such as an increase in roughness, demineralization potential,^5,6 and permeability,⁷ consequently affecting the results of the bleaching treatment. Coloring and acidic pH agents, such as cola-based drinks, coffee, and wine, have been demonstrated to stain the dental structures and also affect the expected end result, as well as the longevity of the bleaching treatment.^8–13

These findings support professionals’ recommendations to their patients to avoid ingesting coloring agents in their diets during their bleaching treatments.¹⁴ However, when dealing with in vivo conditions, all dietary restrictions can be restricted, especially soft drinks.

Recently, a study¹⁵ revealed that bleaching agents, even in low concentrations, could result in changes in the dentinal collagen structure. Taking in consideration that dentin consists of a network of tubules, which transport fluids, ions, and molecules, it is unknown whether the changes could alter the retention of chromogenic molecules in dentin as well the sensitivity with intake coloring and acidic pH agents, as soft drink. One randomized clinical trial¹⁶ demonstrated that coffee consumption does not have an effect on color changes during a bleaching treatment. However, coffee solutions have a higher pH than cola-based soft drink.^11,13 Observe that, several in vitro studies showed that cola-based soft drinks are able to induce a similar or even higher color change than coffee during dental bleaching. However, to extent of author’s knowledge, no clinical studies have been performed evaluated the effect of cola-based soft drink during bleaching treatment.

The cola-based soft drinks are widely intaken in Western world^17,18 and several negative adverse effects are related to health indicators such especially the calcium homeostasis and bone turnover.^19,20 In a Dentistry viewpoint, the intake of acidic beverages between meals, such as the cola-based soft drinks demonstrated a significative relation to erosion and, consequently with some degree of dentine hypersensitivity.²¹

It was to remember that, the success of bleaching treatment depends of the effectiveness of whitening as well the absence or reduced bleaching-induced tooth sensitivity,^22,23 and considering the growing consumption soft drinks around the world, this clinical trial evaluated whether the effectiveness of bleaching, as well as tooth sensitivity, is affected by exposure to cola-based soft drinks during in-office bleaching treatments using 35% hydrogen peroxide. The null hypotheses are that the exposure to cola-based soft drink during the bleaching treatment will not affect the (1) the effectiveness of bleaching, (2) the absolute risk of bleaching-induced tooth sensitivity and (3) the intensity of bleaching-induced tooth sensitivity.

Materials And Methods

This study was approved by the research ethics committee of the Ceuma University (UNICEUMA), according to protocol number. 1.422.841 (24/02/2016). It has also been registered in the registry of clinical trials Rebec under RBR-2nz5s2 number. All the volunteers signed a free informed consent form. This study was conducted in full accordance with the World Medical Association Declaration of Helsinki.

Study Design, Setting, and Location of Data Collection

This study was conducted in accordance with the Consolidated Standards of Reporting Trials statement.¹⁵ This was a single-blind (evaluators), parallel design with an equal allocation rate between groups, and it took place within the dental clinics of Local University from January 2017 to September 2017.

Recruitment

The participants who took part in this study were patients seeking dental treatment at the dental clinics at the university. A total of 58 participants were examined in a dental chair after dental prophylaxis with pumice and water to check whether they met the study’s eligibility criteria.

Eligibility Criteria

Patients included in this clinical trial were between 18 and 40 years old and had good general and oral health. Participants were recruited by means of local advertisements. The participants were required to have caries-free maxillary and mandibular anterior teeth, without restorations on the labial surfaces. The central incisors had to be shade A2 or darker, judged by comparison with a value-oriented shade guide (VITA Classical, VITA Lumin, VITA Zahnfabrik, Bad Sackingen, Germany). Participants who had undergone previous tooth-whitening procedures; presented anterior restorations’ were pregnant or lactating; and/or had severe internal tooth discoloration (eg, tetracycline stains, fluorosis, pulpless teeth), bruxism habits, or any other pathology that could cause sensitivity (eg, recession, dentin exposure) were excluded from the study because they would not be immediately eligible for a cosmetic treatment such as bleaching, given that the other restorative needs would need priority attention. Using the criteria described in the “Tooth Sensitivity (TS) Evaluation“ section, the patients were asked about previous experiences with TS the week before the bleaching therapy began. Patients with TS equal to or greater than mild were also excluded from the study.

Sample Size Calculation

The primary outcome of this study was color changes in the participants’ teeth, which were evaluated with a spectrophotometer (Vita Easyshade, Vident, Brea, CA, USA). Previous studies^24–26 have reported that 2 bleaching sessions with the product Whiteness 35% HP Maxx, a predecessor of Whiteness HP Automixx 35 (FGM Dental Products, Joinville, SC, Brazil), produced a whitening effect of about 8 (ΔE). To detect a difference of 3.5 (ΔE), which is clinically perceptible when comparing pairs of teeth per study group, with a power of 80% and an alpha of 5%, a minimum sample size of 22 patients per group was required. The sample size was calculated on the website www.sealedenvelope.com. The present study was powered to detect a significantly high effect.

Allocation Concealment, Experimental Groups, and Blinding

Patients who met the inclusion criteria were asked about their daily cola-based soft drink (Coca-Cola, São Paulo, Brazil) consumption. Those who did not drink cola-based soft drinks were allocated to the control group. No other dietary restrictions were placed on the participants in the control group.

The patients who had reported drinking cola-based soft drinks at least twice a day every day were placed in the experimental group. No dietary restrictions were placed on the participants in the experimental group. Apart from their daily Coca-Cola intake (2 to 3 glasses daily), these patients were instructed to make mouth rinses with instant Coca-Cola (50 mL) for 30 s, 4 times a day (8 am, 12 am, 4 pm, and 8 pm). They were instructed to perform one of the rinses immediately after each bleaching session while still in the office. For this procedure, participants received bottles of Coca-Cola, and were instructed to wait at least 15 mins after their Coca-Cola rinses before washing the mouth with clean water, brushing their teeth, or eating.

The aim of this procedure was to increase the exposure of bleached teeth to Coca-Cola. As a measure of adherence to the experimental protocol, participants were given a diary in which they were asked to take note of the number of Coca-Cola mouth rinses performed daily. They were instructed about the importance of the procedure and to report any time they forgot or were unable to perform mouth rinses. Participants and operators could not be blinded to the study groups. However, the evaluator who performed the color assessments was blinded to the treatments.

Study Intervention

All subjects received dental prophylaxis 2 weeks before the bleaching protocol began. The gingival tissue of the teeth to be bleached was isolated using a light-cured resin dam (Top Dam, FGM Dental Products). The bleaching procedure was performed using a 35% HP gel (Whiteness HP Automixx 35, FGM Dental Products) according to the manufacturer’s instructions (Table 1) for 2 sessions, three 15 min applications per session, with an interval of 7 days between each session. At this time, all participants were instructed to brush their teeth regularly using toothpaste without desensitizing or bleaching agents. Regarding oral hygiene, all participants were instructed to brush their teeth regularly and were asked to refrain from using whitening toothpaste and mouthwash containing peroxides.

Table 1 Product and Composition and Application Regimen

Color Evaluation

Teeth color was recorded before the bleaching procedure for the baseline and 30 days after the end of the bleaching procedure. For this purpose, we used 1 objective (Vita Easyshade spectrophotometer, Vident) and 2 subjective instruments (value-oriented shade guides from VITA Classical and VITA Bleachedguide 3D, VITA Zahnfabrik). Color evaluation was completed in a room under artificial lightning conditions without interference from outside light. For all measurements, the color of the middle third of the central incisors was checked.

For the objective evaluation, a preliminary impression of the maxillary arch using high-putty silicone Coltoflax (Vigodent, Rio de Janeiro, RJ, Brazil) was taken, and a window on the labial surface of the silicon guide was created by a metal device with a 6-mm radius to standardize the area for color evaluation with the spectrophotometer. The color was determined using the parameters of the digital spectrophotometer, on which the following values were indicated: L*a*and b* parameters were recorded. L* represents the value from 0 (black) to 100 (white), and a* and b* represent the shade, where a* is the measurement along the red-green axis and b* is the measurement along the yellow-blue axis. The difference between baseline and each recall period (ΔE*) was calculated by using the following formula: ΔE = [(ΔL*)² + (Δa*)² + (Δb*)²]1/2. For the subjective evaluation, the 16 tabs of the shade guide (VITA classical, VITA Zahnfabrik) were arranged from lightest to darkest as follows: B1, A1, B2, D2, A2, C1, C2, D4, A3, D3, B3, A3.5, B4, C3, A4, C4. Color changes were calculated from the beginning of the active phase to the individual recall times by calculating the change in the number of shade guide units (ΔSGUs), which occurred toward the lighter end of the value-oriented list of shade tabs. Two calibrated blind examiners participated in this evaluation, and in the event of disagreement between the examiners, a consensus was reached.

Tooth Sensitivity Evaluation

Patients were instructed to fill out a form to record their perception of TS immediately after, 1 hr after, 24 hrs after, and up to 48 hrs after each session. The patients should fill out the form any time they felt pain. It was also explained to them that if they did not feel any TS, the intensity in the pain scales would be zero. In case of pain, the patients should report their TS intensity, using a 5-point numeric rating scale (NRS) using the following criteria: 0 = none, 1 = mild, 2 = moderate, 3 = considerable, and 4 = severe, and the visual analog scale (VAS). This scale employs a 10-cm horizontal line with the words “no pain“ at one end and “worst pain“ at the opposite end.

The median (NRS) and the average (VAS) of bleaching-induced TS intensity as experienced by each patient were calculated throughout the bleaching treatment period. The overall percentage of patients with TS, as well the total number of days on which patients experienced TS, was also evaluated.

Statistical Analysis

The analysis followed the intent-to-treat protocol and involved all participants who were randomly assigned in their respective groups (Figure 1). The median and interquartile range of color change in ΔSGU, as well as the means and standard deviations of color change in ΔE between baseline and 30 days after bleaching, were calculated. To assess whether the bleaching therapies were effective, data from both groups were compared using the Mann–Whitney test for the ΔSGU data and the Student’s t-test for ΔE.

Figure 1 Flow diagram of the clinical trial, including detailed information on the participants.

The absolute risks of bleaching-induced TS for both groups were compared by means of the Chi-square test (α = 0.05, test for proportion of dependent data ratio). The relative risk and the confidence interval (CI) for the effect size were also calculated.

The bleaching-induced TS intensity data set for both the VAS and NRS scales were plotted in histograms and inspected for normal distributions. The VAS data were considered to have a normal distribution, and the groups were compared using a Student’s t-test. Otherwise, because the NRS data did not have a normal distribution, the groups were compared using the sign test and Wilcoxon signed-rank test (α = 0.05).

Results

A total of 58 participants were examined according to the eligibility criteria (Figure 1), but only 44 participants remained for the clinical trial. The main reasons for the exclusions were shade (incisors lighter than A2) and the presence of anterior restorations. The mean age (years) of the participants and the baseline SGUs are described in Table 2. One can observe comparable data among treatment groups.

Table 2 Baseline Characteristics of the Participants Included in This Clinical Trial, for Both the Study Groups

Adherence to Protocol and Loss to Follow-Up

All participants made mouth rinses with 50 mL of Coca-Cola for 30 s, 4 times of day and of 44 participants, 4 forgot to do the mouth rinse at 8 am 1 day. None of the patients discontinued the intervention, and all participants attended the recall visits during the bleaching treatment. Figure 1 depicts the flow diagram with detailed information on the participants in the different phases of the study design.

Color Change

After 30 days, a significant whitening effect was detected by the three colors measurement tools for both the evaluated groups (Table 3). However, no statistically significant difference was observed between the study groups (Table 3; p > 0.08).

Table 3 Means and Standard Deviations of the Color Measured by Vita Classical and Vita Bleached Guide (ΔSGU) and by Easyshade Spectrophotometer (ΔE) Between Baseline versus 30 Days After Bleaching for Two Study Groups*

Tooth Sensitivity

The absolute risks of bleaching-induced TS were 32% (control group; 95% CI 16–52) and 27% (experimental group; 95% CI 13–48), and there was no significant difference between the 2 groups (risk ratio: 0.8 [95%IC 0.34–2.14] p = 0.74; Figure 2). Regarding bleaching-induced TS intensity, the present results showed higher, significant scores in the VAS and NRS values after 1 hr of bleaching (p < 0.05; Table 4). However, bleaching-induced TS intensity decreased in the post-treatment period between 1 and 24 hrs, similar to values obtained during treatment. A significant reduction in bleaching-induced TS was observed until 48 hrs for both scales (p > 0.05; Table 4).

Table 4 Intensity of Tooth Sensitivity in VAS (Average ± SD) and NRS (Median/Interquartile Range) in Several Evaluation Times, as Well as, Statistical Evaluation

Figure 2 Comparison of the number of patients who experienced tooth sensitivity (TS) at least once during the bleaching treatment in both the study groups.

When both groups were compared, higher, significant scores in the VAS (1 to 24 hrs) and in the NRS (1 hr after and 1 to 24 hrs) were observed for the experimental group in comparison with the control group (p < 0.05; Table 4).

Discussion

Patients are often instructed by their dentists to stay on a white diet during active bleaching treatment and to refrain from smoking, as well as drinking coffee, red wine, and colored soft drinks. All these directions are based on previous in vitro studies that reported that staining susceptibility increased when the bleached enamel was exposed to colored drinks such as red wine, coffee and cola-based soft drink, especially after the bleaching treatment.^8–11 Thus, the alterations promoted by the bleaching agents in the enamel surface could favor greater retention of coloring agents.

However, regarding the change of color, the mouth rinses with Coca-Cola did not affect the effectiveness of tooth bleaching in comparison with the control group. Both the protocols demonstrated significant whitening after 2 bleaching sessions. Thus, the results oppose the idea that acidic beverages containing coloring agents, as cola-based soft drinks, that cause extrinsic stains reduce bleaching potential^11–13 and it lead us to accept our first null hypothesis.

It is worth mentioning that this study associated objective and subjective methods of color assessment. Although the common sense that, spectrophotometer gives more accurate results,²⁴ this instrument is yet not currently used in clinical practice. On the other hand, shade guide units are the most used tools for color evaluation in the clinicians’ armamentarium.²⁴ Also, recently Pecho et al (2016)²⁷ indicate that instrumental shade determination should be accompanied by experienced human visual assessment to better visual perception of color change. This is probably, the main reason that both methods, subjective and objective are frequently used in clinical trial evaluating bleaching materials and techniques,^16,24,28,29 as well as, in the present study.

In agreement with the results obtained in this study, Rezende et al 2013¹⁶ and De Geus et al 2015²⁸ clinically demonstrated that exposure to coffee or smoking, respectively, during at-home bleaching with 10–16% carbamide peroxide did not affect the effectiveness of dental bleaching.^16,28 Additionally, the role of a white diet, with restrictions on coffee, tea, red wine, and dark fruits, was not significantly associated with higher bleaching effectiveness.¹⁴ These studies did not evaluate the effect of cola-based soft drinks, which have a lower pH than coffee, tea, and red wine.^11,13,28 In fact, it was previously demonstrated that cola-based soft drinks stain bleached enamel similar or even more than coffee, possibly because a lower pH increases the porosity¹¹ provided by the potential of demineralization on the tooth surface^11,13 and increases the retention of coloring agents.

However, the experimental conditions tested in existing in vitro studies varied greatly in, for instance, the time of exposure to coloring agents.^9,11,30 Because of this, they were unable to simulate a clinical condition. For example, in Pirollo et al 2014¹¹ after the bleaching procedure (35% HP), exposure to a cola-based soft drink increased from 10 mins to 72 hrs. In the present study, the time of bleached tooth exposure to Coca-Cola was determined by considering the time it took to swallow liquid.³¹ Considering that this probably did not take longer than 2 to 5 s, 4 daily mouth rinses performed for 30 s each would represent excessive intake of the beverage. Furthermore, clinical situations that imitate real-life conditions, such as patients brushing their teeth normally and human saliva-replacing materials lost by the tooth structure during bleaching^32,33,34 could attempt to reproduce laboratory setting, but they certainly will not able to completely simulate the cascade of events of the oral environment, as in a clinical trial.

Another aspect related to color, which is worthwhile to explore, is that substances able to stain extrinsically, such as cola-based soft drink,^11–13 are compounds constituted of chromogenic polyphenols. These compounds are believed to bind to proteins, such as the pellicle or bacteria on the surface of the teeth.³¹ Actually, these macromolecular chains practically incapable of passing through human enamel, which acts as a semipermeable membrane that allows the passage of low molecular weight molecules.³⁵

Moreover, it is known that the bleaching process results from oxidation of the organic tissues of human dentin, and not of the enamel structure.³⁶ Furthermore, the extrinsic stains by adsorbed pigments and biofilm are prone to be efficiently removed by professional dental prophylaxis. In this study, after 30 days of the bleaching procedure, maintenance of the effectiveness of bleaching was observed during the patient follow-ups.

In terms of TS, when it was compared to the absolute risk of sensitivity, no significant difference was observed between the study groups. The literature hypothesizes several factors that can be associated with TS, among them the pH of bleaching gels,²⁶ dentin thickness,³⁷ and the concentration of bleaching agents.³⁸ However, even though the current study evaluated the presence and absence of Coca-Cola mouth rinses, the bleaching agent and protocol were the same for both study groups. Doing it like this helps us to understand the similar risk of sensitivity, leading to reject the second null hypothesis.

Although the mechanism responsible for the bleaching-induced TS is unclear, it is hypothesized that, during bleaching treatment, the HP increases the permeability of the enamel and diffuses through the dentin, reaching the pulp chamber.^25,26 Thus, chemical products resulting from the degradation of HP penetrate the pulp chamber²³ and lead to the activation of nociceptive sensors and transient inflammatory reactions with higher intensity after the procedure,^25,39,40 but this is usually reduced within 48 hrs.⁴¹ In fact, it could explain the significantly higher bleaching-induced TS scores after 1 hr of bleaching, followed by a continuous reduction in these values to up 48 hrs, for both the study groups.

Although the exposure to Coca-Cola did not affect bleaching effectiveness, higher and significant scores for bleaching-induced TS intensity were detected in comparison to patients who did not drink Coca-Cola, leading to reject the third null hypothesis. In contrast to Rezende et al 2013,¹⁶ whose findings found that coffee did not affect TS, it should be considered that Coca-Cola has a more acidic pH (≅ 2.53) than coffee (≅ 4.46).^11–13,42 A well-known fact in the literature is that the erosive potential of Coca-Cola affects not only mineral loss and dental enamel^19,43 but also, in epidemiological and clinical evidences, calcium homeostasis and bone turnover^19,44 even in a short time period.¹⁹ It is worth mentioning that, although the evaluation of the erosive potential was not within the scope of the present study, dental erosion was not observed during the follow-ups of this study, especially for patients who ingested Coca-Cola.

During the bleaching process, the permeability of the enamel surface with HP diffusing through the dentin had increased.⁴⁴ This idea that the bleaching process occurs in organic tissues³⁶ has been reaffirmed by recent studies that suggested that the HP bleaches the dentin by oxidizing dentin phosphoproteins (the main noncollagenous protein in dentin),^15,45 may be due to the denaturation of collagen and its effects on amide I and III in dentin.¹⁵ Certainly, a reduction in the amount of minerals in enamel also promoted the exposure of acid and carbonated beverages such as Coca-Cola and could intensify the diffusion of HP through dentin, especially given that the HP is more readily dispersed within deeper regions than the outer portions due to a reduction in mineral content and density^6,45 and, probably, would activate more nociceptive sensors with inflammatory reactions in dental pulp.

Actually, interesting studies aiming to find associated factors for dental hypersensitivity demonstrated a positive trend between TS and dietary acid intake.^21,46 O’Toole and Bartlett²¹ demonstrated that dental hypersensitivity is common among patients who consumed acidic drinks 2 or more times a day, as well as acidic drinks between meals without considering the erosive tooth wear, as occurred in the current study. Although the dental hypersensitivity and bleaching-induced tooth sensitivity cannot be confused.²³ These previous findings help confirm our hypothesis that Coca-Cola in mouth rinses is a potential factor for the increase in TS intensity.

Although only the Coca-Cola was evaluated and the commercial products show particularities not completely clear in the composition hindering an extrapolation, the consumption of other carbonated and acidic beverages should seem with caution, since that it was related to be one of the potential factors for the sensitivity in this study. Also, it is worth mentioning the limitations of the experimental design of this study. Unfortunately, the participants were not randomized to the study groups because this procedure means that all patients have the same chance to be allocated in any group. As in one of the groups, the patients need to intake routinely Coca-Cola, it would not be ethically correct submitting patients which do not intake routinely Coca-Cola to intake 2 to 3 glasses daily during 2 weeks, as well as, described in previous clinical trials evaluating the effect of coffee or smoking during in bleaching treatment.^16,28 In spite of this, these findings are stronger than those obtained in laboratory conditions. In addition, dietary restrictions were only utilized in the control group. If differences were detected between groups, we would not be able to discriminate between the effects of Coca-Cola exposure and those of other staining foods in the patient’s diet. Another limitation of the study is that the participants were predominantly young people with good oral hygiene, without restorations. These results may not be applicable to the elderly population and those with poor oral hygiene conditions.

Future clinical trials are needed to better evaluate the influence of other components of the diet in whitening effectiveness and TS associated to bleaching therapies, especially in the elderly populations.

Conclusion

Although the exposure to cola-based soft drinks during the in-office bleaching treatment did not affect the effectiveness of bleaching, a higher intensity of tooth sensitivity was related by patients who ingested cola-based soft drinks during the bleaching period. This means that the exposition to cola-based soft drinks should be avoided, mainly when higher concentrated hydrogen peroxide, as indicated for in-office bleaching treatment was used.