Effect of storage temperature of 10% carbamide peroxide gel on bleaching efficacy and enamel surface roughnessABSTRACT
Objective: To evaluate the effect of different storage temperatures of a 10% carbamide peroxide gel on bleaching efficacy and enamel surface roughness.
Methods: Bovine incisor specimens (4 x 4 x 3mm) were randomized and allocated to five study groups (n=10), namely, bleaching with bleaching gel (Whiteness Perfect 10%, FGM) without prior storage (WP, positive control), gels stored for three months at temperatures of 25ºC (WP3 m 25 oC), 35ºC (WP3 m 35 oC), and 41ºC (WP3 m 41 oC) , and specimens kept in remineralizing solution (NT, negative control). Color analyses (ΔEab, ΔE00 e ΔWId) and roughness (Ra, µm) were performed considering the initial time (T1) and 24 hours after the fourteenth day of bleaching (T2).
Results: There were no significant differences in color variation between the groups stored at different temperatures (WP3 m 25 oC, WP3 m 35 oC, WP3 m 41 oC) and the nonstorage group (WP) (p>0.05). All gels showed significantly greater color changes than did the NT group (p<0.05). For Ra, there were no significant differences between the bleached groups and the NT group (p>0.05).
Conclusion: Regardless of temperature, the 10% carbamide peroxide gel demonstrated stability after 3 months of storage, considering its bleaching efficacy and absence of changes in enamel surface roughness.
Indexing terms
Carbamide Peroxide; Dental bleaching; Product Storage; Tooth bleaching agentes
RESUMO
Objetivo: Avaliar o efeito de diferentes temperaturas de armazenamento de um gel de peróxido de carbamida 10% na eficácia clareadora e rugosidade do esmalte dental.
Métodos: Espécimes de incisivos bovinos (4 x 4 x 3 mm) foram randomizados e alocados aos cinco grupos do estudo (n=10), sendo eles: clareamento com gel clareador (Whiteness Perfect 10%, FGM) sem armazenamento prévio (WP, controle positivo), géis armazenados por três meses nas temperaturas de 25 ºC (WP3 m 25 ºC), 35 °C (WP3 m 35 ºC), 41 °C (WP3 m 41 ºC) e espécimes mantidos em solução remineralizante (NT, controle negativo). Foram realizadas análises de cor (ΔEab, ΔE 00 e ΔWId) e rugosidade (Ra, µm) considerando o tempo inicial (T1) e 24 horas após o décimo quarto dia de clareamento (T2).
Resultados: Não houve diferenças significativas entre os grupos armazenados nas diferentes temperaturas (WP3 m 25 ºC, WP3 m 35 ºC, WP3 m 41 ºC) e o grupo sem armazenamento (WP) quanto à variação na cor (p>0,05). Todos os géis apresentaram alteração de cor significativamente superior ao grupo NT (p<0,05). Para Ra, não houve diferenças significativas entre os grupos clareados e o grupo NT (p>0,05).
Conclusão: Independente da temperatura, o gel de peróxido de carbamida 10% demonstrou estabilidade após 3 meses de armazenamento, considerando sua eficácia clareadora e ausência de alterações na rugosidade do esmalte dental.
Termos de indexação
Peróxido de Carbamida; Clareamento dental; Armazenamento de produtos; Clareadores dentários
INTRODUCTION
Dental bleaching gels contain Hydrogen Peroxide (HP) or one of its precursors, Carbamide Peroxide (CP), in their composition [1]. Bleaching agents act through a redox reaction, promoting the formation of oxygen free radicals, which diffuse into the dental structure to the dentin by cleaving complex organic molecules, chromophores, into smaller molecules, resulting in substrate bleaching [2,3].
Commonly used techniques include the in-office technique, which involves the use of high concentrations of HP (35% to 40%), with or without association with the at-home technique, mainly based on CP use (in concentrations of 10% to 20%) [1], which is a precursor of HP, as it decomposes into HP and urea when in contact with water [4]. CP 10% is a commonly marketed concentration for at-home treatment due to its bleaching efficacy, which is similar to that of more concentrated CP gels, but with the main advantage of presenting a lower intensity of dental sensitivity [5]. However, despite the satisfactory results obtained, it is an unstable material that may lose its efficacy over time and upon exposure to light, heat, and environmental changes, especially when improperly stored [6].
In general, higher temperatures are known for their potential to reduce the bleaching efficacy of CP gel [7,8]. In some previous studies, storage of gels containing CP 10% and CP and 16% CP at 35°C after 12 months was able to reduce bleaching effectiveness [9], and gels stored at refrigerated temperature (5°C) maintained their ideal viscosity thus, it is recommended to avoid the presence of heat in the environment used to store the gel [10].
The main reported detriment would be the direct influence on the chemical stability of the formulation, with consequent compromise of the physicochemical properties of the product, such as pH, cohesiveness, adhesiveness, and even possible degradation of the bleaching agent. Thus, storage at temperatures between 5°C and 25°C is recommended [8]. This is because changes in gel properties can impact the decrease in pH of the product, which can compromise enamel morphology, and increase roughness, and mineral loss [11,12].
However, in addition to the scarcity of available studies that have evaluated the direct effects of storage temperature on color after bleaching [9,10], no studies have analyzed the effects of storage temperature on roughness. Therefore, this study aimed to store 10% CP for three months at room temperature (25°C) and high temperatures (35°C and 41°C) and analyze the bleaching efficacy of the product and enamel roughness. The study hypotheses suggested that temperature and gel storage time would not interfere with (1) dental bleaching efficacy or (2) enamel roughness.
METHODS
Design and allocation of specimens
The experimental units of the study were forty freshly extracted bovine incisors, free of cracks or stains, which were selected for the evaluation of bleaching efficacy and roughness using the commercial gel Whiteness Perfect 10% − FGM (WP), before and after storage at different times (baseline and 3 months) at various temperatures (25 ± 2°C, 35 ± 2°C, or 41 ± 2°C). The chemical composition, manufacturer, and batch number of the products are listed in Chart 1.
Seventy-five specimens were prepared, representing a 50% margin above the fifty samples in the study. This procedure was important to allow block randomization in the study by discarding samples with L* values that deviated significantly from the mean [13]. Thus, the selected samples were then distributed to the five study groups (n=10) through stratified random sampling, ensuring statistical similarity, and evaluated according to the bleaching treatment (Table 1). Prior pH calibration was also performed using a digital pH meter (mpA-210, MS-Tecnopom Instrumentação®), with the electrode tip positioned in the bleaching gel, to measure pH values after the storage period in triplicate.
Fabrication of samples
Extracted bovine teeth were cleaned, disinfected in a 0.1% thymol solution, those with cracks and stains were discarded and selected for cutting the specimens with diamond discs (KG Sorensen, Serra, ES, Brazil) attached to the handpiece (Kavo, Campinas, SP, Brazil), starting from the crown and root section, and making samples measuring 4 mm × 4 mm in width and 3 mm in thickness (1 mm of enamel and 2 mm of dentin), measured with a digital caliper (DC 500, Mitutoyo, Suzano, SP, Brazil). Dentine and enamel regularization were performed using silicon carbide (SiC) sandpapers (600-grit, 1200-grit, 2500-grit, and 4000-grit) (Buehler, Lake Bluff, IL, USA) and felts (1 µm and ¼ µm), along with polishing pastes of the same grit. Debris removal from each sandpaper or felt change was carried out in an ultrasonic bath (Marconi, Piracicaba, SP, Brazil), with distilled water for 10 minutes (Marconi, Piracicaba, SP, Brazil).
Pigmentation of samples
For 6 consecutive days [14], the samples were stained within a container containing a solution of 100 ml of black tea (Matte Leão – Fazenda Rio Grande) in a bacteriological incubator at 37°C. Prior to this process, lateral marking was made on the specimen with a #1012 spherical diamond tip, enabling the identification of the initial position of the sample in color/rugosity measurements; dentin protection with colorless varnish (Impala, Guarulhos, SP, Brazil) was also performed. Extrinsic pigmentation was removed with prophylaxis and repolishing using the felts and pastes used in the initial finishing and polishing, maintaining a statistically equal initial average roughness. For color stabilization, the samples were stored in artificial saliva for 14 days, with daily solution changes [15].
Storage of gels
The whitening gel of the WP3m35°C and WP3m41°C groups was stored in incubators containing the exact predetermined temperatures (35°C and 41°C), and the group stored at room temperature (25°C) was kept in a cabinet protected from heat and climatic changes with an embedded digital thermometer and maintained at 25°C. The WP group was not stored.
Whitening treatment
During the experimental procedure, 0.2 g of the commercial gel Whiteness Perfect 10% FGM was applied to the specimens for 4 hours daily for 14 days, according to the manufacturer’s instructions. Throughout this time, the samples were stored in an incubator (37°C ± 2) inside a closed container, containing moist gauze. After 4 hours, the samples were washed, dried with delicate absorbent tissue paper (Kleenex, MMC Brasil Indústria e Comércio Ltda), and stored in artificial saliva at 37°C ± 22°C until the whitening procedure of the following day [10].
The WP group allowed for an evaluation of the whitening efficacy of the gel immediately after purchase under factory storage conditions. On the other hand, the specimens from the WP3m25°C, WP3m35°C, and WP3m41°C groups were whitened after a storage time of 3 months for the gels at 25°C, 35°C, and 41°C; moreover, while these two groups underwent whitening treatment for 14 days, another 10 specimens were stored in artificial saliva (NT group), the composition of which was based on other studies (13,14): Ca 1.5 mmol/L; P 0.9 mmol/L, KCL, and Tris buffer solution 0.1 mol/L, with daily changes of this solution, thus serving as the negative control of the study.
Tooth color measurement
To measure the color before treatment (T1) and 24 hours after the fourteenth day of treatment (T2), a reflectance spectrophotometer (Konica Minolta CM-700d) was used. The specimens were placed in a Teflon device inside a light chamber (GTI Graphic Technology Inc.) using the “daylight” option. Three readings were taken for each specimen at different positions, which were repositioned at 120° relative to the base, using the surface previously marked with a #1012 spherical diamond tip (KG Sorensen) as the initial reference position [15]. The CIE Lab color space was employed, which comprises three coordinates: L*, a*, and b*, which were input into the formulas ΔE, ΔE00, and ΔWId, respectively.
Roughness measurement (Ra)
With the assistance of a Surftest SV 2100 roughness measuring machine (Mitutoyo, SP, Brazil), three Ra readings were obtained for the samples at T1 and T2, which were repositioned at 120° relative to the base, with the initial reference position being the surface marked by the #1012 spherical diamond tip. The ANSI parameters were: a cutoff length of 0.25 mm, a 1.25 mm reading length, and a speed of 0.1 mm/s [15].
Statistical analysis
Initially, descriptive and exploratory analyses of the data were conducted. The roughness data were analyzed using a mixed-effects generalized linear model for repeated measures over time. Nonparametric Kruskal-Wallis and Dunn tests were employed for analyzing color variation data (ΔE, ΔE00, and ΔWId). All analyses were performed using the statistical software R (R Core Team, 2023) at a significance level of 5%.
RESULTS
Regarding the color results, all the whitened groups (WP, WP3m25°C, WP3m35°C, WP3m41°C) showed significantly greater variation than did the NT group (p>0.05), as presented in Table 2. There were no differences when comparing groups stored at different temperatures (WP3m25°C, WP3m35°C, WP3m41°C) and the commercial gel without storage (WP) (p>0.05). Only NT showed significantly less variation than the other groups (p<0.05). For Ra, there was no significant difference between groups (p>0.05) or between time points (p>0.05), as presented in Table 3.
DISCUSSION
This study evaluated the efficacy of a whitening gel containing 10% CP stored at three different temperatures (25°C, 35°C, and 41°C) and under different storage conditions (no previous storage and after 3 months of storage). The determined temperatures were based on previous study [9]. The study hypotheses were confirmed, as the enamel color change and roughness were not affected by the storage time or temperature of the gel.
There are reports from other authors about possible chemical and physical changes in whitening gels after adverse storage conditions [8,11,16]. In the case of home whitening gels, patients are responsible for storing these products. The authors described changes in the microhardness [17], roughness [12], and whitening efficacy [9,10] of dental enamel.
Among the hypotheses raised, the effect of temperature increase on gel pH stability has been mentioned [16], and storing whitening gels under refrigeration is suggested due to sensitivity to light, heat, or environmental adversity [6]. According to a previous study [11], maintaining products at high storage temperatures promotes the dissociation of some of their components, resulting in a higher concentration of H+ ions and pH reduction. This altered pH is also reported for its potential effects on enamel surface roughness [18], alteration in its active content, and texture properties [8]. However, in the present study, there was no significant change in roughness or impairment in gel whitening efficacy, even after 3 months of storage at the study temperatures.
Consistent with these findings, storage temperatures near 21°C in an in vivo study also did not affect gel whitening effectiveness [10]. In the present study, 37°C and 41°C did not influence whitening efficacy or roughness after three months, a result that differs from the perspective presented by another study [7], whose authors concluded that after storage at 3°C, 20°C, and 40°C, home whitening agents (CP and HP) experienced a decrease in peroxide concentrations after 8 days.
However, these same results are consistent with a previous investigation [9] that evaluated the effects of CP (at 10% and 16%) on tooth color after storage for two different durations (after 3 and 12 months) at three temperatures (10°C, 25°C, and 35°C). In both studies, over the three-month period, there was no change in whitening efficacy. Nevertheles, after 12 months of storage, the authors found that gels stored at 10°C and 25°C were the only ones that did not lose their efficacy, differing from the gel stored at 35°C.
Thus, due to the limited evidence on this topic evaluating other properties of dental enamel or the gel itself, storing the product at room temperature or under refrigeration, and maintaining a limit of 25°C [10] are still recommended. Additionally, consistency is an important parameter for a gel to be properly retained in a tray, and refrigeration appears to favor the maintenance of this characteristic [19].
It is worth noting that many factors can influence results in studies with whitening gels, such as storage method, saliva activity, and exposure time to the whitening product [20], as well as aspects related to tooth color analysis, contact time with the whitening gel, and different peroxide concentrations [21].
pH, may be an important parameter for maintaining Ra without alterations because an acidic gel pH can affect this enamel characteristic [18]. However, the gels in the present study remained close to neutral even when stored at high temperatures. Additionally, remineralization by saliva significantly inhibited demineralization after treatment [22,23]. Another factor is the presence of stabilizers in the commercial formulation, that are capable of delaying peroxide degradation, thus increasing its shelf life [19]. Furthermore, buffering agents maintain the pH as neutral as possible, preventing demineralization of dental enamel [19].
Moreover, urea is a component resulting from the dissociation of CP, along with hydrogen peroxide, which can penetrate the interprismatic regions of enamel to dentin, a factor that could alter the morphology of its surface [24]; however, urea has a neutral pH, suggesting that it can neutralize enamel damage [25]. Sodium fluoride also plays an important role in the whitening gel, which is capable of even recovering hardness alterations in enamel, that is, preventing mineral dissolution [26]. In this regard, a previous study [27] evaluated the effect of a 10% CP whitening agent containing fluoride on the remineralization capacity and prevention of increased roughness and concluded that fluoride has a remineralization effect without affecting whitening efficacy; thus, fluoridated gels, such as the commercial gel in the study, are capable of preventing changes in surface roughness while maintaining whitening efficacy.
Within the limitations of the study, it was possible to verify that the components of the WP whitening gel formulation were able to remain stable for three months, even at high temperatures, promoting effective whitening without altering enamel roughness. The clinical significance of these findings is beneficial because increased enamel roughness can lead to biofilm accumulation, harming the patient’s oral health [28]. Furthermore, it is presumed that the patient would be able to acquire effective whitening treatment, even in the face of a possible need to pause treatment and return later, or for cases where the patient acquired a kit containing more syringes than they would use at that moment and stored it outside the refrigerator until use at another time.
However, these findings are not sufficient to serve as a basis to ensure that there is no damage in storing the product under adverse conditions, as further investigations with other concentrations of the product, different commercial brands, and data on other variables, such as cytotoxicity, texture, and other physical-mechanical and chemical properties of enamel, are needed.
CONCLUSION
The storage of 10% carbamide peroxide gels for three months at higher temperatures (35°C and 41°C) and at room temperature (25°C) promoted effective whitening as much as the gel without storage (factory condition) and did not alter the roughness of the dental enamel.
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How to cite this article
Alves TG, Oliveira LPS, da Silva JA, Vieira-Junior WF, Lima DANL. Effect of storage temperature of 10% carbamide peroxide gel on bleaching efficacy and enamel surface roughness. RGO, Rev Gaúch Odontol. 2026;74:e20260031. http://dx.doi.org/10.1590/1981-86372026003120250100
Data Availability
The research data are available from the corresponding author upon reasonable request.
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Assistant editor
Luciana Butini Oliveira
