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.
Introduction
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.
Results
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.
Discussion
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.
Conclusions
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®.
References
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: pecora@usp.br
Accepted May 2, 1996
Electronic publication: February, 1997
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