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In Vitro Action of Various Bleaching Agents on the Microhardness of Human Dentin

Jesus Djalma PECORA (
Antonio Miranda CRUZ-FILHO
Manoel Damião de SOUSA-NETO
Ricardo Gariba SILVA (

Faculdade de Odontologia Ribeirão Preto University of São Paulo
Trabalho publicado no Brazilian Dental Journal   5(2):129-134, 1994
Ribeirão Prelo, SP, Brasil


The authors verifieda decrease in human dentin microhardness after application of the following bleaching agents for 72 hours: sodium perborate + water, sodium perborate + 3% hydrogen peroxide, sodium perborate + 30% hydrogen peroxide, Endoperox (crystallized hydrogen peroxide), Proxigel (carbimide peroxide), and 30% hydrogen peroxide. Sodium perborate + water and sodium perborate + 3% hydrogen peroxide caused less of a decrease in dentin microhardness than Endoperox, Proxigel and 30% hydrogen peroxide which caused the greatest decreases in dentin microhardness. Sodium perborate + 30% hydrogen peroxide fell into a statistically intermediate position.

Key words: bleaching agents, dentin microhardness, Endodontics.


Dental surgeons seek to discover solutions and techniques capable of bleaching teeth whose color has been altered after endodontic treatment and also of bleaching teeth with live pulp which have changed color due to tetracycline ingestion or an excess of fluoride during the calcification phase.

Intracoronary bleaching of endodontically treated teeth has been considered to be efficient; however, recent evidence suggests that this procedure can result in significant complications. Cervical resorption of roots has been attributed to the toxic nature of 30% hydrogen peroxide, with can leak through the dentinal tubules and reach periodontal tissues (Harrington and Natkin, 1979; Friedman et al., 1988; Pécora et al., 1991; Dahlstrom, 1993).

Seali et al. (1981) reported that 30% hydrogen peroxide caused destructive changes of pulp tissue favoring internal resorption in vital teeth. Lado et al. (1983) showed that 30% hydrogen peroxide denatures human dentin, rendering this tissue immuologically unacceptable. Bowles and Thompson (1986) reported pulp reactions when bleaching vital teeth with the use of hydrogen peroxide with or without heat. Such reactions ranged from simple reversible inflammatory reactions to necrosis. They reported that pulp enzymes are inhibited in the presence of hydrogen peroxide, especially when associated with heat.

Saquy et al. (1992) verified that the association of sodium perborate and 30% hydrogen peroxide increased dentin permeability and the addition of heat increased the action of this solution, significantly increasing tissue permeability. Rotstein et al. (1992) detected that 30% hydrogen peroxide caused alteration in the dental structure of both cementum and dentin. Canepa et al. (1993) reported that the association of sodium perborate and 30% hydrogen peroxide with heat caused decreased dentin microhardness, and that this was even more notable with repeated application of these bleaching agents. Dahlstrom (1993) verified that hydrogen peroxide is capable of generality highly reactive hydroxyl radicals which can destroy connective tissue components, collagen and hyaluronic acid.

A high percentage of teeth which are bleached present color regression over a period of up to five years (Howell. l98L Lado et al., 1983; Friedman et al., 1988).

The present study investigates the action of various bleaching agents, during 72 hors, on human dentin microhardness.


A total of 36 extracted human maxillary central incisors were used. They were sectioned at the cemento-enamel junction to separate the crown from the root. The cervical part of the root was sectioned transversely to prepare a 1-mm thick disk. These 36 disks were placed in rapid polymerization acrylic resin blocks so that one surface of the dentin disk was exposed to the external surface. After polymerization of the resin, these blocks were smoothed with 400, 500 and 600 sandpaper in order to obtain a smooth and uniform dentin surface. Running water was used during this process to avoid injuring the dentin. The smoothness of the dentin surface was verified with the use of a 30X magnifier.

The dentin samples were placed in a recipient containing deionized distilled water at 370C until time for microhardness readings. Dentin microhardness was measured with a Vickers hardness instrument (Wolpert, Germany) with a load of 5O grams for 15 seconds. The samples were placed in [this instrument so that the readings were always made in the same region.

Dentin microhardness was first read on normal dentin, without the application of a bleaching agent. Ten measurements for each sample were made. A bleaching agent was then applied on the dentin and this was stored on a Petri dish which was placed in an incubator an 3 70C for 72 hours. This sample was then washed with deionized distilled water to remove the bleaching agent and microhardness was then measured. Ten readings were also made of these samples to obtain a mathematical mean of microhardness. These same steps were carried out wit1u all of the bleaching agents studied: sodium perborate (NaBO2.H202.3H20; Reagen) ± water, sodium perborate + 3% hydrogen peroxide, sodium perborate + 30% hydrogen peroxide, Endoperox (Septodont), carbimide peroxide (Proxigel Inodon) and 30% hydrogen peroxide (Vetec).


The mean values of microhardness of each dentin disk before and after application of the bleaching agents are shown in Table 1. Data were submitted to analysis which verified the normally of the samples. Analysis of variance was carried out for independent data (solutions) and dependent data (treatment, before and after). Analysis of variance showed a statistically significant difference at the level of 1% between bleaching agents and treatments (before and after). Microhardness was greater before the application of bleaching agents. The Tukey lest was applied to show differences among bleaching agents. Sodium perborate + water and sodium perorate + either 3% or 30% hydrogen peroxide were statistically similar. Endoperox, Proxigel and 30% hydrogen peroxide were statistically similar and different from the previous group. Sodium perborate + 30% hydrogen peroxide showed a microhardness between the two other groups.

Table 1

Figure 1 shows the action of bleaching agents on dentin microhardness after 72 hours. This graph of interaction of dependents factors (dentin microhardness before and after treatment) with independent factors (bleaching agents) shows the position of sodium perborate + 30% hydrogen peroxide, which is closer to the solutions which decrease dentin microhardness to a lesser degree. The great reduction of dentin microhardness 72 hours after the application of Endoperox, Proxigel and 30% hydrogen peroxide is clearly shown in Figure 1.


Dental bleaching can bc performed in both live teeth and in endodontically treated teeth Numerous techniques lave been reported and all use potent oxidizing agents. Bleaching of teeth is nothing more than a chemical process of oxidation-reduction (redox).

Research has shown that dental bleaching can cause serious consequences, such as, external resorption color reversibility and alteration in the collagen structure of dentin (Howell, 1981; Lado el al., 1983; Friedman et al., 1988; Pécora et al., 199 1; Saquy et al., 1992; Canepa et al., 1993).

The present study shows that the action of sodium perborate associated with water or with 3% hydrogen peroxide reduces dentin microhardness slightly. Endoperox (crystallized 35% hydrogen peroxide), Proxigel (carbamide peroxide) and 30% hydrogen peroxide greatly reduced dentin microhardness in comparison to the other group. The association of sodium perborate with distilled water has a pH of 9.8 and the association of sodium perborate with 3% hydrogen peroxide a pH of 8.22.

The association of sodium perborate with water and with 3% hydrogen peroxide are statistically similar. The mixture of sodium perborate with 30% hydrogen peroxide falls in a statistically intermediate position between sodium perborate + water and the agents Endoperox and Proxigel. This intermediate position may be explained by the fact that these solutions react among themselves which lowers the deleterious effect of 30% hydrogen peroxide on dentin. Sodium perborate presents an alkaline pH and hydrogen peroxide an acid pH of 2.0. The mixture of these 2 agents results in a produce with pH 5, which forms water and ionized oxygen. With time, the pH of this mixture becomes alkaline. Thus, the alteration of pH depends on the quantity of each of these agents, being more acid at the beginning if the quantity of hydrogen peroxide is greater than that of sodium perborate. However, with time and as water and oxygen are being formed, the mixture becomes more alkaline.

Since the Tukey test did not allow a definite position for the association of sodium perborate with 30% hydrogen peroxide, the Scheffé test was applied. This test did not alter the position of this mixture in relation to the other bleaching agents studied.

The graph of the interaction of dependent and independent factors (Figure 1) clearly shows that the association of sodium perborate and 30% hydrogen peroxide is displaced closer to the group of sodium perborate and water and sodium perborate and 3% hydrogen peroxide, decreasing dentin microhardness less than the bleaching agents Endoperox, Proxigel (10% carbamide peroxide) and 30% hydrogen peroxide.

Al bleaching agents studied caused a reduce then in dentin microhardness. The results of this study and those of other researchers, in spite of the fact that they on in vitro, show the harmful affects of 30% hydrogen peroxide on human dentin. Thus, researchers must seek an oxidation agent or reducing agent which does not have a deleterious effect on dentin.


1. The bleaching agents tested in this study decreased human dentin microhardness.
2. The application of sodium perborate associated with water or with 3% hydrogen peroxide caused less of a decrease in dentin microhardness than the use of the other bleaching agents studied.
3. Endoperox (crystallized hydrogen peroxide), Proxigel (10% carbamide peroxide) and 30% hydrogen peroxide decreased dentin microhardness in a statistically similar manner and more than sodium perborate with water or with 3% hydrogen peroxide.
4. The association of sodium perborate and 30% hydrogen peroxide falls into an intermediate position between the other two groups.


Bowels WH 1, Thompson LR: Vital bleaching: the effects of heat and hydrogen peroxide on pulpal enzymes. J Endod 12: 108-112. 1986

Canepa R, Sousa Neto MD, Saquy PC. Romani NF, Pécora IE): Estudo "in vitro" da ação do agente clareador (perborato dc sódio + peróxido dc hidrogênio a 30% + calor) sobre a microdureza da dentina. Rev Paul Odont : 18-24. 1993

Dahlstrom S: Complications associated with bleaching of root of filled teeth. Aust Endod News 19:13-14, 1993

Friedman S. Rotstein 1, Libfeld IH Stabholth A, Healing 1: Incidence of external root resorption and esthetic results in 58 bleached pulpless teeth. Endod Dent Traumatol 4: 63-67, 1988

Harrington GW, Natikin E: External resorption associated with bleaching of pulpless teeth. J Endod 5: 344-347, 1979

Howell RA: The prognosis of bleaching root filled teeth. J Endod 1 14: 22-26, 1981

Lado EA, Stanley IIR, Weisman MI: CervicaI resorption in bleaching teeth. Oral Surg 55: 78-80, 1983

Pécora ID, Sousa Neto MD, Costa WF: Apresentação de um método (químico que revela "in vitro" a passagem do peróxido dc hidrogênio a 30% através da dentina radicular. Rev Paul Odonit 13: 34-36, 1991

Rotstein I, Leht Z, Gedalia L: Effects of bleaching agents on inorganic components of human dentin and cementum. 1 Endod 18: 290-293, 1992

Saquy PC, Sonsa Neto MD, Canepa R, Pécora ii): Estudo "in vitro" da permeabilidade dentinária após aplicação dc agentes clareadores. Rev Paul Odont 14: 37-40, 1992

Seali NS, Macintosh E, Taylor AN: Pulpal reactions of bleaching of teeth in dogs. J Dent Res 60: 948-953, 1981
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