Introduction

Currently, teeth whitening is considered a minimally invasive and aesthetic procedure for patients demanding to whiten their teeth, following the aesthetic standards of modern Western societies^1,2. Intrinsic and extrinsic factors can cause the chromatic alteration of the teeth, the former being related to the staining of the internal dental structure due to systemic diseases, consumption of certain medications -like tetracycline- dental trauma, pulpal necrosis; while the latter is related to chromogenic agents deposition on the enamel surface, such as chlorhexidine, pigments from the consumption of beverages or foods, smoking, biofilm, among others^3-6.

Since it was first described by Haywood and Heymann⁷ using a 10% carbamide peroxide-based antiseptic solution, a wide variety of chemicals are used as active principles for tooth bleaching, such as sodium perborate, carbamide peroxide, and hydrogen peroxide^6,8. Teeth whitening can be performed in two different methods: i) In-office procedures using hydrogen peroxide-based gels in high concentrations, ranging from 35% to 40%^5,9,10, with application time varying according to the manufacturer, dental condition, indications, and administered in single or multiple doses. ii) Supervised whitening, popularly known as at-home whitening, which uses low-concentrated hydrogen peroxide (typically up to 10%) or carbamide peroxide (between 16% and 22%) gel. This requires using at-home trays filled with the gels at predefined daily periods (30 minutes to 8 hours) for 1 to 4 weeks^4,11. The patient performs the application of the agent, so the success of the treatment depends on his/her own, who must use the gel according to the dentist’s recommendations. Nowadays, studies show that no technique is better than any other in terms of whitening results¹².

These gels work as a result of the degradation of their components that release highly reactive free radicals that, in order to stabilize themselves, bind to the organic molecules responsible for the pigmentation of dental tissues, breaking the carbon double bonds and changing their light reflection pattern, which culminates in the whitening of the dental structure^12,13. The low molecular weight of hydrogen peroxide facilitates its diffusion through enamel and dentin^14-16, releasing reactive oxygen which, being unstable, seeks to react with other free or weakly bound substances^13,17. The thickness of enamel and dentin, the number of whitening sections performed, and the frequency of application, composition, and peroxide concentration in the gels are some of the factors that influence the penetration of oxygen-derived free radicals into the pulp chamber^15,16. Still, these factors can also influence the occurrence of bleaching-induced dentin sensitivity and gingival irritation^18,19, which are the most common side effects of the whitening technique.

Peroxide solutions are usually more stable in acidic environments and may cause an erosive tooth wear effect on the surface of dental tissues and increase tooth sensitivity after treatment. To avoid these effects, the most concentrated gels are presented in a two-phase system, in which one phase contains the acidic and stable peroxide solution and, in the other, a thickening agent associated or not with remineralizing substances (calcium, fluorides, etc.) in an alkaline medium. When mixed, the final solution becomes thixotropic with a pH near neutral, and the higher the pH of the gels, the greater their degradation, which results in a higher release of free radicals²⁰. However, during peroxide degradation, there is a continuous release of free radicals, with an increase in the number of hydrogen ions that can cause the solution to return to a more acidic environment²¹ and become erosive again. Thus, it is questioned whether gels with neutral or alkaline pH can become acidic during the whitening process.

Therefore, this study aimed to evaluate the stability of 35% to 40% hydrogen peroxide bleaching gels used in office-based therapies alone and in contact with the enamel. The null hypotheses tested were that i) the pH of the gels tested will not change during the application period indicated by the manufacturer; ii) there will be no difference in the pH of the gels tested when placed over enamel.

Materials and methods

This was an observational and experimental in vitro study conducted with hydrogen peroxide gels and bovine substrate (n=3). The tested gels were Opalescence^TM Boost^TM 40% (OpB) (Ultradent Products Inc., Utah, USA), Whiteness HP Blue 35% (WHPB) (FGM Dental Group, Santa Catarina, Brazil), and Whiteness HP 35% (WHP) (FGM Dental Group, Santa Catarina, Brazil). The composition and characteristics of each are described in Table 1.

Each gel was prepared by mixing the active principle with the thickener, following the indications recommended by the manufacturer. The mixture was placed inside a 15 mm diameter glass tube with a thickness of up to 8 mm to ensure total coverage of the electrode, which was placed in contact with the gel immediately after handling (Figure 1). The pH measurement of the gels was recorded and analysed with the aid of a contact pH meter (PG2000, Gehaka, São Paulo, Brazil) using an electrode sensitive to pH variation, which was calibrated using buffer solutions with pH values of 4, 7 and 10. The pH was measured at 5-minute intervals until completing the exposure time indicated by the manufacturer, which was 20 minutes for the OpB, 40 minutes for the WHPB and 15 minutes for WHP (Table 1).

Figure 1 Schematic drawing of the gel positioning over the crowns 

After the readings of each gel, the electrode was washed with distilled water to remove the excess gel and dried with absorbent paper. Then, it was recalibrated, and new measurements were made with the gels in contact with bovine tooth enamel. For that, three bovine crowns were cleaned with pumice and water, and one was used for each gel. The buccal surface was flattened in a polishing device (DP-10, Panambra Industrial e Técnica SA, São Paulo, Brazil) using aluminium oxide abrasive paper (EXTEC^® Aluminium Oxide #600, Extec Corp., Connecticut, USA), to obtain a flat and standardised area, improving the contact area of the gel with the enamel. A barrier was built over the enamel surface with condensation silicone putty (Zetaplus, Zhermack SpA, Rovigo, Italy) and sealed with a gingival barrier to prevent the gel from flowing (Figure 1). This allowed the studied gels to be placed on the crown up to 8 mm thick to ensure complete coverage of the electrode. The pH measurement was performed every 5 min as described above. The pH measurement data were analysed qualitatively. The Mann-Whitney U test was used to compare the values of the gel isolated and in contact with the enamel. The statistical tests had a significance level of 5% (p<0.05).

Results

The pH values for the evaluated time of each gel are shown in Figure 2. The WHP gel showed a significant reduction in pH after 15 minutes, from 7.3 to 6.0 and 7.3 to 5.9, when measured alone or in contact with enamel, respectively. The OpB and WHPB gels maintained a stable and alkaline pH during the times evaluated (values higher than 7.0), with less than 0.5 variation along the test. The median pH values and the interquartile range are shown in Table 2. Although the Mann-Whitney U Test presented a significant difference between the values alone and in contact with enamel for the OpB and WHPB gel, the difference was clinically insignificant, and both remained alkaline along the tested period.

Figure 2 Quantitative variation of the pH for each gel for the time analyse 

Discussion

High-concentration hydrogen peroxide-based whitening gels are widely used as an in-office tooth whitening treatment²⁰. Our study compared the pH stability of different products for professional use with a high hydrogen peroxide concentration with and without bovine tooth enamel contact. The results show values from a slightly acidic pH (6.0 - 5.9) to an alkaline pH (8.5-8.7) without and with enamel contact, respectively (Figure 2). The pH of whitening gels is an issue of concern for dentists, as an acidic gel (pH 5.5 to 6.5) can cause enamel dissolution^22-24, increasing enamel roughness and wear and the risk of tooth sensitivity^10,24-26.

Modern whitening gels have pH levels closer to neutral, and the lower the concentration of hydrogen peroxide, the more alkaline the gel tends to be²⁵. Mostly, the information regarding the pH of the whitening gels can be found in the safety sheet of the products (as described in Table 1) but maintaining the pH during the gel application over enamel is of utmost importance to avoid erosive wear of the tooth structure. In this study, the gels tested presented a stable behavior in terms of pH both alone and in contact with the enamel. Both OpB and WHPB, had an alkaline or near neutral pH (7.0) after mixing, with excellent stability after the manufacturer’s recommended use time, even in contact with the enamel. Therefore, in clinical use, the rate of peroxide degradation and free radical formation tends to be slower, being less likely to cause morphological changes on the enamel surface²⁷. A lower degradation rate promoted by the use of gels with neutral or alkaline pH can also be a lower risk for tooth sensitivity, as shown in several studies^25,28,29. Still, both gels have agents in their composition to help the reduction of sensitivity and/or detrimental effects on tooth structure, such as fluorides or calcium. Some clinical studies indicate that the presence of these agents could increase the saturation of the bleaching gel and minimise the loss of minerals³⁰. The addition of small amounts of calcium effectively reduces the negative effects on enamel hardness and roughness³¹ without altering the effectiveness of whitening since they do not reduce the penetration of the gel through the enamel prisms¹⁶.

On the other hand, the WHP gel went from neutral to slightly acidic in the first 5 min and remained so until the final time of 15 min recommended by the manufacturer (Figure 2). For this gel, the manufacturer recommends up to 3 applications of 15 min each, for a total of 45 min session time (Table 1), possibly due to this pH drop, to avoid or minimise side effects. One of the possible explanations for the decrease in pH is the absence of other products, such as neutralizers, fillers, or even fluorides and calcium, which are intended to reduce the deleterious effects on the enamel, such as reduced microhardness and increased roughness³². The minimum value achieved for WHP gel was about 5.99 alone and 5.85 in contact with the enamel, considered close to the critical value (5.5) for enamel dissolution^22-24, so effects on microhardness or roughness are not expected to be critical.

The behavior of bleaching gels, as well as their stability and pH control, are directly influenced by the type of thickener used, contributing to greater peroxide decomposition, and resulting in lower pH stability⁵. However, information on the type and amount of thickener used in the tested gels has not been found and was also not provided by the manufacturer in the product package inserts. Therefore, the first hypothesis is rejected because the pH of the WHP gel presented a decrease in pH values, going from a neutral pH (7.3) to an acid pH (around 6.0) in the two conditions tested. The second hypothesis is accepted as the pH of the gels tested in both conditions, alone and in contact with bovine tooth enamel, did not show significant differences.

Conclusion

The pH of the whitening gels was slightly affected by the interaction with the dental enamel and the time of clinical use. Despite being an observational study with a small sample size, it was possible to check the pH stability after handling and contact with the dental substrate. The gels Opalescence^TM Boost^TM and Whiteness HP Blue showed higher pH stability during clinical use time, with minimal pH decrease only, remaining stable. The Whiteness HP^TM presented a significant decrease from the beginning to the end (15 min), both alone and in contact with the enamel, going from neutral to slightly acidic.