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Acta Odontológica Latinoamericana

versão On-line ISSN 1852-4834

Acta odontol. latinoam. vol.23 no.2 Buenos Aires set. 2010



The effect of carbamide peroxide bleaching agents on the microhardness of dental ceramics


Sheila P. Passos, Aleska D. Vanderlei, Susana M. Salazar-Marocho, Sarina M.B. Azevedo, Vanessa Z.C. Vasquez, Estevão T. Kimpara

Department of Dental Materials and Prosthodontics São Paulo State University at São José dos Campos, Brazil.

CORRESPONDENCE Dra.Sheila Pestana Passos. Av. Engenheiro Fracisco Jose Longo, 777 Sao Dimas. Sao Jose dos Campos Sao Paulo – Brazil. CEP: 122450 000. e-mail:


This study examined the effect of 10% and 16% carbamide peroxide bleaching agents on the surface microhardness of micro-particulate feldspathic ceramics (VM7 and VM13, Vita Zahnfabrik). Forty specimens (8-mm diameter, 2-mm thickness) were divided into four groups (n=10): G1- VM7 + 10% Whiteness, G2- VM7 + 16% Whiteness, G3- VM13 + 10% and G4- VM13 + 16% Whiteness. The home-use bleaching agents were applied for 8 hours on 15 days, and the specimens were stored in distilled water at 37°C. The Vickers hardness number (HV) was determined for each specimen. Data were analyzed by the Wilcoxon and Mann- Whitney tests (p<0.05). The microhardness values before exposure were: g1- 433 (57); g2- 486 (22); g3- 509 (28); g4- 518 (24), and after exposure: G1- 349 (32); G2- 496 (95); G3- 519 (38); G4- 502 (81). G2 exhibited a higher and significant difference than G1 in VM7 groups, and the effect of bleaching concentration was shown to be significant by the Mann-Whitney test. And for VM13, both the Wilcoxon and Mann-Whitney tests showed no significant differences. When using 10% carbamide peroxide, the microhardness of VM7 ceramic was affected, and there were no effect on the microhardness between VM7 and VM13 ceramics when 16% carbamide peroxide was used.

Key words: Ceramics; Hardness; Tooth-bleaching agent.


Efeito do peróxido de carbamida na microdureza superficial de cerâmicas dentais

Este estudo examinou o efeito do agente clareador peroxido de carbamida a 10% e a 16% na microdureza superficial de ceramicas feldspaticas micro-particuladas (VM7 e VM13, Vita Zahnfabrik). Quarenta corpos-de-prova (8 mm de diametro, 2 mm de espessura) foram divididos em quatro grupos (n=10): G1- VM7 + 10% Whiteness, G2- VM7 + 16% Whiteness, G3- VM13 + 10% e G4- VM13 + 16% Whiteness. Os agentes clareadores foram aplicados por 8 horas durante 15 dias e os cp foram armazenados em agua destilada a 37°C. A dureza Vickers (HV) de cada cp foi determinada. Os dados foram analisados pelos testes de Wilcoxon e Mann-Whitney (p<0.05). Os valores da dureza antes da exposicao ao agente clareador foram: g1- 433 (57); g2- 486 (22); g3- 509 (28); g4- 518 (24), e depois da exposicao: G1- 349 (32); G2- 496 (95); G3- 519 (38); G4- 502 (81). G2 exibiu diferenca significante e microdureza maior comparado ao G1 nos grupos da VM7 e o efeito da concentracao do clareador foi significante, apresentados atraves dos testes Mann-Whitney. Para VM13, ambos testes, Wilcoxon e Mann-Whitney, nao apresentaram diferenca significante. Quando o peroxido de carbamida a 10% foi avaliado, a microdureza da ceramica VM7 foi afetada, e nao houve diferenca na microdureza entre as ceramicas VM7 e VM13 quando o peroxido de carbamida a 16% foi utilizado.

Palavras chave: Ceramicas; Dureza; Agentes clareadores.



Dental bleaching has been one of the most soughtafter treatments in restorative dentistry, because it is considered a conservative alternative for recovering esthetic characteristics. For brightening discolored teeth, the use of hydrogen peroxide or peroxide releasing agents, such as carbamide peroxide or sodium perborate, has been a popular treatment1. The bleaching agents available are usually based on 6-20% and 25-40% peroxide gels for home and in-office whitening, respectively2.
Carbamide peroxide solutions immediately dissociate on contact with tissue or saliva3 into hydrogen peroxide and urea. These products degrade into oxygen and water, and into ammonia and carbon dioxide, respectively4. For home treatment, bleaching times depend on the length of time per day that the patient applies the bleaching solution.3 On the other hand, in-office bleaching uses higher-concentration solutions applied for shorter periods of time, since these products are capable of producing more peroxide radicals, hence accelerating the process5. The effects of the bleaching agents may result in changes in physical properties, surface morphology, and color of different restorative materials, and these effects have been investigated in several in vitro studies, simulating the clinical situation as closely as possible1. Thus, it would be impossible for bleaching agents to remain in the oral environment, because they may reduce microhardness of dental enamel 6-8, the main reasons for this reduction being the effect of the peroxide on the organic matrix of the enamel and the action of the bleaching agents present in the gels. Tooth bleaching is not believed to create macroscopically visible defects, but microscopic alterations could themselves cause undesirable effects.2 Several studies have assessed the effects of peroxide carbamide, such as microstructural changes and decreased hardness5,9, on dental restorative materials4,9,11,12. Furthermore, it has been observed that 10% carbamide peroxide reduced the microhardness of feldspathic dental ceramics9. However, the authors found no study of the effect of carbamide peroxide on the surface texture of dental ceramics. Some studies have demonstrated that there was no significant difference in the effect of the bleaching agents on any restorative material3,4,13,14.
Few studies have been conducted on the effects of this agent on the microhardness of various esthetic restorative dental materials. Only Turker and Biskin9, Turker and Biskin4, Butler, et al.9. and Moraes, et al.2 have evaluated the effect of carbamide peroxide agents on the surface of ceramics. In addition, there is no report in the literature regarding the influence of highly concentrated solutions on ceramics. Swift13 and Haywood3 reported that night-guard vital bleaching techniques have no significant effect on the color or physical properties of porcelain or other ceramic materials and amalgam or gold. Significant changes on the ceramic surface caused by the action of bleaching agents may cause changes in the mechanical properties of the material. Therefore, the purpose of this study was to evaluate the influence of 10% and 16% carbamide peroxide bleaching agents on the surface microhardness of two ceramics. The null hypotheses to be tested are that: (1) 10% carbamide peroxide bleaching agent does not influence the microhardness VM7 ceramic surface; (2) 10% carbamide peroxide bleaching agent does not influence the microhardness VM13 ceramic surface; (3) 16% carbamide peroxide bleaching agent does not influence the microhardness VM7 ceramic surface; (4) 16% carbamide peroxide bleaching agent does not influence the microhardness VM13 ceramic surface.


Table 1 shows the brand names, material types, main compositions, manufacturers and batch numbers of the products used in the current study.

Table 1: The brand names, material types, main components, manufacturers, and batch numbers of the products used in the current study.

Forty discs measuring 4x4 mm (diameter and height) were made with the ceramic materials listed in Table 2. All the specimens were handled in accordance with the manufacturer’s instructions, using a specific program for the ceramic firing (Vacumat 40 VITA, Zahnfabrik-Germany). After firing, the specimens were included in acrylic resin and finished with a polishing machine (Labpol 8-12, Extec, USA), using abrasive papers Nos. 600, 800 and 1200 (3M, St. Paul, USA) to remove any irregularities and create a flat surface. All the ceramic blocks were cleaned in an ultrasonic bath for 5 min with distilled water, and glaze fired according to the manufacturer’s instructions. The discs were divided into 4 experimental groups of 10 discs each, according to the ceramic (VM7 and VM13) and the bleaching agent concentration (10% and 16% / Whiteness, FGM Gel) (Table 3).

Table 2: Experimental groups.

Table 3: Means (± standard deviation) of HV values in experimental groups. Confidence level (95%) of the means and t-test of the matched samples*.

The samples were then taken to the hardness tester (Digital Microhardness Tester FM, Future-Tech) for Vickers hardness (VH) analysis at 50-gf loading for 15 seconds. Three indentations were performed on each sample, and the mean was calculated. Once the initial microhardness was determined, the peroxide carbamide (Whiteness – FGM 10% and 16%) was applied on the exposed surface of the ceramic materials for 8 hours, according to the manufacturer’s specifications. During the time interval between the gel applications, the test specimens were stored in distilled water at 37oC, simulating a conventional dental bleaching treatment. The process was repeated daily for 15 days. The final microhardness was measured after the last bleaching agent application, following the same process as for the initial reading. Statistical analysis was performed using Statistics 8.0 for Windows (Analytical Software Inc, Tallahassee, FL, USA). The microhardness means obtained for each specimen were analyzed by the Wilcoxon and Mann- Whitney tests (p<0.05) in order to determine the significant differences between initial and final microhardness of the specimens. P-values less than 0.05 were considered statistically significant in all tests.


The data obtained and the comparisons are summarized in Table 3. Wilcoxon showed that after bleaching VM7 using 10% carbamide peroxide, surface microhardness decreased significantly (G1: p=0.002); and for 16% carbamide peroxide, the microhardness exhibited no difference (G2: p=0.743). The effect of bleaching concentration differed significantly according to the Mann-Whitney test (p=0.021). Bleaching of VM13 exhibited no significant difference in the ceramic microhardness, for both carbamide peroxide concentrations, accord ing to the Wilcoxon (G3: p=0.191; G4: p=0.511) and Mann-Whitney tests (p=0.319).
After bleaching the ceramics, VM7 showed the lowest surface microhardness value when the 10% bleaching agent was used, compared to all groups. No difference was found among the other groups after bleaching agent exposure.


The effect of the bleaching agent on dental materials is critically important, because whitening agents may cause deleterious effects, such as decrease in the microhardness5,9,11 and increase in the roughness4,11,15 of some restorative material surfaces. This may increase plaque accumulation or affect esthetics by changing the texture of the ceramic restoration, allowing the accumulation of pigments. The characteristics of the restorative material surface are therefore highly relevant in clinical practice1 and ceramic restorations should be protected before any bleaching treatment16. In this study, the surface microhardness of the veneering porcelain was not affected by the different carbamide peroxide concentrations; however, little is known about the influence of bleaching on ceramics. The bleaching agent used in this study (16% carbamide peroxide) had no significant effect on the surface microhardness of the feldspathic porcelain, as shown in the study by Polydorou et al.17 However, 10% carbamide peroxide decreased the microhardness of the feldspathic porcelain, which was consistent with the findings of Turker and Biskin9.
The effect of the bleaching agent on dental material was investigated in this in vitro study, simulating the clinical situation as closely as possible. In other studies, home bleaching products (10-16% carbamide peroxide) were generally used for 2 or 4 weeks for 4-8 hours a day1; and the same was done in this study. There are few investigations on the effect of bleaching agents on ceramic restorative materials2,4,9. Turker and Biskin9 evaluated the effects of bleaching agents on feldspathic ceramics and observed that 10-16% carbamide peroxide gels (applied for 8 h per day for 3 days) significantly decreased the surface hardness of the ceramic material tested, possibly due to factors such as surface topography or increased material porosity. Additionaly, the low pH of the fluoride gel may result from the hydrofluoric acid, which can lead to the etching of restorations that contain silica such as porcelain9. Turker and Biskin4 report that the surface spectral analyses results indicated a decrease in the SiO2 and K2O2 content of feldspathic ceramics after exposure to carbamide peroxide agents. Since SiO2 forms the matrix, it would thus affect the surface hardness. According to the manufacturer, VM7 ceramic contains more SiO2 content than VM13 ceramic, which contributes to explaining our results. Ceramic compositions are shown in Table 1. Thus, VM7 ceramic showed lower microhardness after exposure to 10% carbamide peroxide in this experiment, while no significant alteration was detected for VM13 ceramic for either bleaching agent concentration. In addition, the depth of the bleaching agent penetration is related to the composition of this matrix, which may explain the results of the microhardness testing. Bleaching therapies with hydrogen peroxide releasing preparations may have a negative effect on restorations and restorative materials, as indicated in numerous in vitro investigations. Polydorou et al.17 observed that clinicians do not need to replace ceramic restorations after at-home bleaching treatment when the restorations are on posterior teeth. On the other hand, Duschner et al.18 reported that surface microhardness testing revealed no significant deleterious effects on the restoration surfaces after they were submitted to bleaching by hydrogen peroxide. Indeed, such alterations cannot be attributed to surface roughness alone, but also to substrate composition, water absorption rate due to permeability alterations, and irregularities left on bleached surfaces, which could promote changes in esthetic characteristics and accumulation of pigments11.
In vitro studies are limited in their attempt to simulate clinical conditions. It has been shown that peroxide levels in bleaching products are depleted depending on the in vivo situation. In this study, the bleaching agents were not diluted or buffered with any water content, such as saliva or distilled water, during bleaching treatments, as in most other studies9,14. The findings of this study require rejection of the first null hypothesis that 10% carbamide peroxide bleaching agent decreases the microhardness of the VM7 ceramic surface, and confirmation of the second, third and fourth null hypotheses that 10% carbamide peroxide bleaching agent does not influence the microhardness of VM13 ceramic surface, and 16% carbamide peroxide bleaching agent does not influence the surface microhardness of VM7 and VM13 ceramics. With the increasing use of ceramic restorations, it is important that dentists be aware of the possible effects of carbamide peroxide on these restorations. However, there are no reports in the literature indicating that bleaching may have any negative impact on restoration surfaces, which could require renewal of the restorations1. This study showed that there was an effect on the surface microhardness of one material, which could result in ceramic failure if treated in vivo. In addition, the microhardness values observed in this study are within the clinically acceptable range, compared to the microhardness values of enamel (}350 HV), and the alterations verified would probably be clinically insignificant. After exposure to the bleaching agent, the ceramic surfaces may be vulnerable to some changes. Nevertheless, further studies are needed to elucidate these aspects more precisely.


Within the limitations of this study, the following conclusions were drawn:
• The microhardness of the VM7 ceramic surface was affected by 10% carbamide peroxide bleaching agent;
• 10% carbamide peroxide bleaching agent did not affect the microhardness of the VM13 ceramic surface, and 16% carbamide peroxide bleaching agent did not affect the surface microhardness of either ceramic.


The authors thank VITA Zahnfabrik (Bad Sackingen, Germany) for providing some of the materials used in this study. We would like to thank Professor Ivan Balducci for helping with the statistical analysis in this study.


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