In Vitro Action of Various Carbamide Peroxide Gel Bleaching Agents on the Microhardness of Human Enamel

Elilton Cavalcante PINHEIRO JÚNIOR 1
Rivail Antonio Sérgio FIDEL 1
Antonio Miranda da CRUZ FILHO 2
Ricardo Gariba SILVA 2
Jesus Djalma PÉCORA 2
1Disciplina de Endodontia, Faculdade de Odontologia, Universidade do Estado do Rio de Janeiro, Rio de Janeiro, RJ, Brasil 2Disciplina de Endodontia, Departamento de Odontologia Restauradora, Faculdade de Odontologia de Ribeirão Preto, Universidade de São Paulo, Ribeirão Preto, SP, Brasil

Braz Dent J (1996) 7(2): 75-79 ISSN 0103-6440

| Introduction | Material and Methods | Results | Discussion | Conclusions | References |

The authors verified a decrease in human enamel microhardness after application of the following carbamide peroxide gel bleaching agents for 8 hours daily for 1 week: 10% Nite White®, 16% Nite White®, Opalescence®, Karisma Alpha® and Perfect Smile®. Statistical analysis showed that these agents caused a decrease in enamel microhardness. Nite White® (16%) was the most effective in reducing enamel microhardness and Opalescente® the least effective. Nite White® (10%), Karisma Alpha® and Perfect Smile® fell into a statistically intermediate position.

Key Words: bleaching agents, enamel microhardness, carbamide peroxide gels.


Dental surgeons seek to discover solutions and techniques capable of bleaching vital teeth whose color has been altered due to tetracycline ingestion or an excess of fluoride during the calcification phase. A thermocatalytic technique with hydrogen peroxide, a microabrasive technique with hydrochloridric acid or other acid and the nightguard technique have been indicated for bleaching vital teeth. After the introduction of the nightguard technique for bleaching vital teeth with 10% carbamide peroxide-based gel, several brands have emerged on the market. Because it is a technique with simple home application, its use is being abused, principally in Latin America, where this product can be easily bought, including on TV. When indicating the nightguard technique, Haywood and Heyman (1989) advised that this technique ought to be performed by a dental professional. They also stated that no long-term clinical results had been reported to date. In a review of the literature on the effects of solutions and techniques of bleaching vital teeth on oral tissues, Powell and Bales (1992) suggested that when using high concentrations the professional must prevent accidental exposure of buccal mucosa, maintain pulpar vitality, and keep time and temperature to a minimum. Recent research has shown interest in verifying the harmful effects of carbamide peroxide on dental structures, dental pulp, buccal mucosa and the general health of the patient. Histologically, Kwong et al. (1993) observed a moderate inflammatory pulpar response in teeth extracted after application of carbamide peroxide gel for two weeks. Cherry et al. (1993) showed that ingestion of different concentrations (10 to 35%) of brands of carbamide peroxide gel used for bleaching of teeth resulted in toxic effects in adult female rats. After 2 h, mean respiration per min of the rats that ingested Quik Start® (35% carbamide peroxide) decreased from 169 to 55 and body temperature decreased from 38.4 to 34oC. They also observed partial eye closure, blood in the urine and incontinence. Three of 22 animals died within 49 h of gastric hemorrhaging, and 8 of 10 rats stopped ovulation. Animals receiving carbamide peroxide (10 or 15%) exhibit similar but milder symptoms. Seghi and Denry (1992) observed that enamel treated with bleaching gel exhibited a small but significant decrease in abrasion resistance and report that this was most likely due to alteration of the organic matrix of the enamel and the chemical action of hydrogen peroxide. Shannon et al. (1993) studied the effect of three 10% carbamide peroxide bleaching agents on enamel microhardness and surface morphology. They found that there was no statistical significance in microhardness values of all treated specimens after two weeks of application. Scanning electron microscopic evaluation revealed significant surface alterations in enamel topography for slabs treated with bleaching solutions for 4 weeks. The most severe alterations were found in slabs exposed to the lower pH solutions. To date, studies of the action of carbamide peroxide gels used in bleaching vital teeth on enamel microhardness has not been conclusive. The purpose of this investigation was to examine the effect of four carbamide peroxide-based gels on enamel microhardness after 8 hours of daily exposure for one week.

Material and Methods

A total of 25 recently extracted human maxillary central incisors were used. The buccal surfaces of the crowns of these teeth were removed and cut with double-faced steel disks to form rectangular blocks. These blocks of enamel were placed in previously prepared acrylic disks so that the enamel was exposed to the environment with the dentin on the inner part of the acrylic resin. The disks were divided into groups of 5 and after planning they were polished with 400, 500 and 600 sandpaper in order to obtain a smooth and uniform enamel surface. Running water was used during this process to avoid enamel injuries. This procedure was carried out seeking to place the most uniform pieces of enamel in the disks. The enamel samples were placed in a recipient containing artificial saliva at 37oC until microhardness measurements before the application of bleaching agents. Enamel microhardness was measured with a Vickers hardness instrument (Wolpert, Germany) with a load of 200 g for 15 sec. The samples were placed in this instrument so that the readings were always made in the same region. A total of 10 measurements were made for each specimen. The five groups were submitted to five different brands of carbamide peroxide: 16% Nite White®, 10% Nite White® (Discus Dental Inc.), Opalescence® (Ultradent Products Inc.), Karisma Alpha® (Confi-Dental Products Co.) and Perfect Smile® (Perfect Smile Inc.) for 8 hours daily at 37oC and 95% relative humidity in artificial saliva. The specimens were then washed for 5 min in tap water and re-immersed in artifical saliva for 16 hours at 37oC . This cycle was repeated for one week. At the end of the experiment, 10 measurements of microhardness for each specimen were made.


A total of 500 microhardness values were submitted to statistical analysis which verified the normality of the samples (250 before and 250 after application of carbamide peroxide agents). Analysis of variance was carried out for independent data (solutions) and dependent data (treatment, before and after) and showed a statistically significant difference at the level of 1% between carbamide peroxide gels and treatment (before and after). Microhardness was greater before the application of the bleaching agents. The Tukey test was applied to show differences among the carbamide peroxide gels. Nite White® (16%) was the most effective in reducing microhardness and Opalescente® the least effective. Nite White® (10%), Karisma Alpha® and Perfect Smile® fell into a statistically intermediate position. Figure 1 is a graph of interaction of dependent factors (enamel microhardness before and after treatment) with independent factors (bleaching agents) and shows the great reduction of enamel microhardness with all agents studied.


All carbamide peroxide-based gels tested significantly reduced the microhardness of human enamel. A load of 200 g, which was established in a pilot study seeking the least load necessary to obtain a well-defined indentation, was used on the enamel in the Vickers hardness instrument. The difference in our results compared to those of Seghi and Denry (1992) and Shannon et al. (1993) were only due to the application of different loads. We cannot state that 10% carbamide peroxide was directly responsible for the large reduction in microhardness. We believe that this reduction was due to two factors: a) the action of peroxide on the organic matrix of the enamel as reported by Covington et al. (1990), and b) acidic agents or chelating agents present in the bleaching gels. We must point out that only the manufacturer of Perfect Smile® reported the complete formula of the product (10% carbamide peroxide, carbopol, triethanolamine glycerin, citric acid, EDTA, sodium benzoate, flavor and artificial sweetener). The remaining manufacturers of the bleaching agents studied cited only the presence of carbamide peroxide, carbopol, thickening agent, glycerin, flavor and artificial sweetener. These formulas are very elementary and not sufficient for a product that ought to have a good shelf-life. Without preservatives, these products would easily decompose or become contaminated since without acidic agents carbamide peroxide decomposes rapidly freeing oxygen and becoming inefficient soon after fabrication. If manufacturers do not clearly reveal the chemical composition of their products, they should not be available commercially. Seghi and Denry (1992) report that all of the currently available bleaching agents contain some form of hydrogen peroxide as the whitening agent. The chemistry of this agent is based primarily upon its ability to generate free radicals in most solvents, especially water. The hydroxyl radicals lack one electron, are extremely electrophilic and unstable, and will attack most other organic molecules to achieve stability, generating other radicals. The exact mechanism of tooth bleaching is not totally known. Our findings, as well as those of other researchers, especially the toxic effect by ingestion of carbamide peroxide observed by Cherry et al. (1993), justify that gels used in the nightguard technique ought to be investigated before being used indiscriminately by the general population. Because the gels used in this study act on the microhardness of enamel, their use or indication ought to be the strict responsibility of the dentist and not advertised and sold freely without protecting the health of the patient.


1. Carbamide peroxide bleaching agents tested in this study decreased human enamel microhardness.
2. 16% Nite White® was the most effective in reducing enamel microhardness.
3. Karisma Alpha®, 10% Nite White® and Perfect Smile® decreased enamel microhardness in a statistically similar manner and more than Opalescence®.


Cherry DV, Bowers Jr DE, Thomas L, Redmon AF: Acute toxicological effects of ingested tooth whiteners in female rats. J Dent Res 72: 1298-1303, 1993

Covington JS, Friend GW, Lamoreaux WJ: Carbamide peroxide tooth bleaching: effects on enamel composition and topography. J Dent Res 69: 175 (Abstract 530), 1990

Haywood VB, Heymann HO: Nightguard vital bleaching. Quint Int 20: 173-176, 1989

Kwong K, Mohammed S, McMillian M, Stokes AN: Evaluation of a 10 percent carbamide peroxide gel vital bleaching agent. NZ Dent J 89: 18-22, 1993

Powell LD, Bales JD: Tooth bleaching: its effect on oral tissues. J Am Dent Assoc 122: 50-53, 1991

Seghi RR, Denry I: Effects of external bleaching on indentation and abrasion characteristics of human enamel in vitro. J Dent Res 71: 1340-1344, 1992

Shannon H, Spencer P, Gross K, Tira D: Characterization of enamel exposed to 10% carbamide peroxide bleaching agents. Quint Int 24: 39-44, 1993

Correspondence: Prof. Dr. Jesus Djalma Pécora, Faculdade de Odontologia de Ribeirão Preto, Universidade de São Paulo, 14040-904, Ribeirão Preto, SP, Brasil. E-mail:

Accepted May 2, 1996
Electronic publication: February, 1997