ARTÍCULOS ORIGINALES

Effect of storage temperature on pH of in-office and at-home dental bleaching agents

 

Andrea Freire, Lucí Regina Panka Archegas, Evelise Machado de Souza, Sérgio Vieira

School of Dentistry, Pontifical Catholic University of Parana, Curitiba, Parana, Brazil.

CORRESPONDENCE Sergio Vieira Rua Imaculada Conceicao, 1155 Prado Velho - Zip Code: 80215-901 Curitiba - Parana - Brazil Business phone number: 55 41 3271-1637 Fax: 55 41 3271-1405 e-mail: s.vieira@pucpr.br

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ABSTRACT

The aim of this study was to determine the pH of several commercially available in-office and at-home dental bleaching products stored at room temperature and refrigerated. The products were divided into in-office (n=09) and at-home (n=12) bleaching gels and submitted to two different temperatures, namely, room temperature (23°C±1°C) and refrigeration temperature (4°C±1°C). The pH was measured using a portable pH meter with a direct electrode, which was calibrated with standard buffer solutions at pH 4.0 and 7.0 and recalibrated for each new product. The pH of the dental bleaching products tested ranged from 2.39 ± 0.10 to 6.52 ± 0.09, and was found to vary significantly both with temperature and between bleaching agents. Most of the bleaching gels were found to be acidic, especially the in-office bleaching products, and the refrigerated at-home gels had the highest pH values. Thus, we conclude that the storage temperature affected the pH of the products tested.

Key words: pH; Dental bleaching; Storage temperature.

RESUMO

Efeito da temperatura de armazenamento no pH de géis clareadores dentais

O clareamento de dentes vitais e baseado no contato direto do agente clareador com a superficie do esmalte por um longo periodo de tempo. Desta forma, este produto pode causar efeitos adversos na estrutura do esmalte humano. O baixo pH e a alta concentracao de acido pode levar a alteracoes morfologicas, sendo sugestivo da ocorrencia de processo erosivo. O objetivo deste estudo foi determinar o pH de diversas marcas comerciais de produtos para clareamento dental, armazenados em temperatura ambiente e refrigerados. Os produtos foram divididos em geis para uso em consultorio (n=09) e uso caseiro (n=12) e submetidos a duas diferentes temperaturas, tal como ambiente (23°C±1°C) e refrigerada (4°C±1°C). O pH dos produtos foi mensurado utilizando um pHmetro portatil com eletrodo direto, calibrado com solucoes padroes de pH 4,0 e 7,0 e recalibrado a cada novo produto. O pH de todos os geis clareadores testados variou de 2,39 ± 0,10 a 6,52 ± 0,09. Houve uma diferenca significante entre agentes clareadores e temperatura de armazenamento. Os maiores valo res de pH foram atribuidos aos produtos para clareamento caseiro e refrigerados. Os resultados deste estudo demonstraram haver grande variabilidade entre as marcas comerciais disponiveis, com a maioria apresentando um pH acido, especialmente os produtos para clareamento em consultorio. Os maiores valores de pH foram atribuidos aos geis refrigerados para uso caseiro. Desta forma, a temperatura de armazenamento afeta o pH dos produtos testados.

Palavras chave: pH; Clareamento dental; Temperatura de armazenamento.

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INTRODUCTION

One of the most important interactive communication skills a person possesses is the smile¹. Dental esthetics, including the color of the teeth, are thus of great importance to patients². It has been reported that 28% of adults in the UK are dissatisfied with the appearance of their teeth³ and that 34% of adults in the USA are dissatisfied with the present color of their teeth⁴. In addition, in a survey of 3215 subjects, 50% of respondents felt that they had some kind of tooth discoloration⁵.
Esthetic dentistry has introduced corrective procedures to treat discolored teeth, such as dental bleaching, direct resin restoration and the use of laminate veneers and crowns⁶. The treatment of dental discoloration by different bleaching techniques has attracted a large amount of interest from the profession, as these techniques are non-invasive and easy to carry out⁷. Bleaching agents are available for in-office or athome therapy⁶. At-home dental bleaching typically uses a relatively low concentration of whitening agent and is applied to the teeth via a custom-made mouthguard that is used for at least 2 weeks, whereas in-office bleaching generally employs whitening agents at higher concentrations for a shorter period. In-office treatments can result in significant whitening after only one session^7,8 but may require multiple treatment appointments for optimum whitening⁹. Contemporary bleaching systems are primarily based on hydrogen peroxide (HP) or one of its precursors, notably carbamide peroxide (CP)⁷. As bleaching of vital teeth involves direct contact between the whitening agent and the enamel surface for a long period, many in vitro studies have evaluated the potential effects of bleaching agents on human enamel^10-15. The acidic properties of the bleaching agents can lead to changes in the mineral content of enamel⁸, and a number of studies investigating the effects of pH report that low pH and high acid concentrations may cause morphological alterations in enamel suggestive of an erosive process^16,17.
Although most manufacturers recommend that bleaching agents should be kept refrigerated to increase their shelf-life, there is a lack of studies investigating the impact of storage temperature on the chemical properties of bleaching agents. Therefore, the aim of this study was to determine the pH of several commercially available in-office and at-home dental bleaching products stored at room temperature or refrigerated.

MATERIALS AND METHODS

Nine in-office bleaching gels and 12 at-home bleaching agents were studied (Table 1). All were fresh and had not passed the expiry date. The pH of each product was measured using a portable pH meter with a direct electrode (Quimis, model Q400BD, Diadema, Sao Paulo, Brazil), which was initially calibrated using pH 4.0 and pH 7.0 standard buffer solutions (LabMaster Ltd., Pinhais, Parana, Brazil) and then recalibrated before each new product was tested. All products were tested in triplicate at both room temperature (23°C ± 1°C) and refrigeration temperature (4°C ± 1°C). The following measures were taken to ensure the reliability of the test: the bleaching agents were placed in disposable analyzer cups to allow adequate contact with the tip of the pH electrode; care was taken not to introduce air bubbles into the sample; the products were kept in contact with the pH electrode until the pH value had stabilized; and the electrode was thoroughly washed in a stream of water before each sample was tested and then rinsed with distilled water and dried with absorbent paper.

Table 1: Bleaching agents and their manufacturers

Statistical analysis consisted of two-way analysis of variance (ANOVA) of the values obtained after normality of the data had been accepted by the Shapiro- Wilk test. As significant differences were found, the mean values of each group were compared using the Tukey HSD test for multiple comparisons. The Kruskal-Wallis test was used to compare different products, and the comparisons were considered statistically significant for p-values less than 0.05.

RESULTS

The pH of all the products tested ranged from 2.39 ± 0.10 (highly acidic) to 6.52 ± 0.09 (neutral) (Fig. 1 and 2). Significant differences in the variables tested were found between in-office and at-home products (p=0.021) and between products stored at room temperature and those stored at refrigeration temperature (p=0.002). Higher pH values were found both for at-home bleaching products and for gels that were kept refrigerated (Fig. 3). The most acidic at-home and in-office bleaching products at both storage temperatures were Mix Day (Villevie, Dentalville do Brasil Ltd., SC, Brazil) and Polaoffice (SDI Ltd, Victoria, Australia), respectively.

Fig. 1: Mean pH of the different products used for at-home dental bleaching.

Fig. 2: Mean pH of the different products used for in-office dental bleaching.

Fig. 3: Variation in pH according to the bleaching technique and storage temperature.

DISCUSSION

The increasing importance of tooth whitening for patients and consumers is reflected in the dramatic rise in the number of tooth-whitening products and procedures, with a concomitant rise in the number of studies of the efficacy of such products². In this study, the pH values of twenty-one commercially available tooth-bleaching products varied from 2.39 ± 0.10 (highly acidic) to 6.52 ± 0.09 (neutral). This variation in pH could be a result of the different formulations used by each manufacturer, as bleaching agents can contain stabilizers such as phosphoric or other inorganic acids that allow them to be stored for prolonged periods¹⁸.
The present study also demonstrated that at-home bleaching products have higher pH values than inoffice products. One possible explanation for these findings could be related to higher concentrations of hydrogen peroxide in the in-office formulations, a feature that may have a more adverse effect on the mineral content and surface structure of dental tissues¹⁹. Hegedus et al.¹⁴ (1999) evaluated the effect of bleaching agents on enamel surface using an atomic force microscope and showed that differences in groove depth were caused by differences in hydrogen peroxide concentrations. According to the results of an in vitro study²⁰, hydrogen peroxide at concentrations of 35% and 38% cause significantly more loss of (Ca+2) from the enamel surface than 10% carbamide peroxide. Therefore, 35% and 38% hydrogen peroxide can potentially cause dental erosion²⁰. Clinically, increased porosity allows the bleaching agent to penetrate more easily through enamel and dentin, explaining the dental sensitivity observed during its use²¹. In vitro studies have shown that the rate of enamel demineralization is related to pH^22,23. When thin sections of human enamel were exposed to solutions with different pH values, the rate of mineral loss increased linearly with time and inversely with pH. In solutions with pH values of 4.3 and 5.0, lesions were found with well-defined surface layers, whereas in solutions with pH 6.0, lesions were produced with no apparent surface layers²². In the present study, the only at-home whitening gel that exhibited a pH of less than 5.0 at both temperatures was Mix Day. The in-office bleaching gels Polaoffice, Polaoffice Red, Opalescence Xtra and Mix One were also found to have pH values lower than 5.0. There are several reports in the literature describing in detail the detrimental effects of peroxide- containing tooth-bleaching products on enamel surface^10,12-15. The results of these studies are dependent on the methodology and materials used as well as the pH of the products analyzed.
The formulation used by manufacturers can cause enamel demineralization depending upon the amount of thickening agents, such as carbopol and glycerin^10,11. The addition of these components improves adherence of the bleaching agent to the surface of the dental structure, allowing more time for the release of peroxide¹⁸. A significant decrease in the microhardness of enamel surface was reported when a placebo agent was used with carbopol and glycerin¹¹. In an in vitro study¹⁰, the isolated effects of carbopol, glycerin and 10% carbamide peroxide on dental tissues were evaluated. The authors concluded that 10% carbamide peroxide, carbopol, glycerin and their associations reduced the amount of sound enamel and dentin microhardness and caused dentin to demineralize. Carbopol and its associations caused changes in microhardness, while glycerin appeared to affect enamel and dentin to a lesser degree. When making judgments on the effects of highly acidic or basic substances, it is important to consider the exposure time and how often the product is used²⁴. At-home products are used for 2-8 hours every day during two, three or more weeks, while in-office products are only applied for about 45 minutes approximately once a week but may require multiple treatment appointments. Any solution with a pH value lower than 5.5 may cause erosion, particularly if it is applied for long periods and repeated over time²⁵. The protective effect of saliva plays a major role in moderating the extent of erosion in the mouth¹⁷. The pH of saliva, plaque and bleaching agent may rise as a result of conditions in the oral environment. Additionally, the presence of peroxidases, other enzymes and saliva can decrease the harmful effects of hydrogen peroxide¹¹. Thus, individuals with low or reduced salivary flow are susceptible to erosive tooth damage26. However, remineralization by saliva may not always be complete, leaving areas susceptible to further decalcification¹⁹, plaque retention¹⁹ and consequent staining. Therefore, in view of the enamel demineralization that could occur at low pH, it might be prudent to apply topical fluoride gel at the end of the bleaching procedure to help remineralize dental tissues^19,27,28, although this treatment does not induce better resistance against erosion of enamel²⁸.
This study also demonstrated that lower pH values were found when the products were stored at room temperature. This finding can be attributed to the effect of chemical equilibrium, which is related to the concentration of reactants in the product. Le Chatelier’s Principle states that if a condition, such as temperature, changes, the position of the equilibrium will shift in a direction that best reduces the stress, and a new equilibrium position will be reached²⁹. In addition, pH value decreases when the concentration of H+ ions increases²⁹. Keeping the products at higher temperatures can induce dissociation of some components, leading to a higher concentration of H+ ions and a consequent reduction in pH. This has major implications for storage of the product as its shelf-life is reduced, and it is therefore recommended that bleaching gels be kept refrigerated, both in dental offices and dental stores. Additionally, a higher concentration of oxygen ions (O-2) is found in a solution with acid pH, whereas in a solution with a basic pH there is a higher concentration of perhydroxyl ions (HOO-), which allow a more effective oxidation-reduction reaction than oxygen and prevent the teeth coming into contact with a potentially demineralizing acid solution30. The results of our study underline the need for manufacturers to reformulate their bleaching gels and for dental stores to provide better storage conditions and product information. Since several bleaching gels studied were found to be acidic and the storage temperature affected the pH of the tested products.

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