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Operative Dentistry, 2005, 30-5, 608-616
The Effect of 10%
Carbamide Peroxide,
Carbopol and/or Glycerin on
Enamel and Dentin Microhardness
RT Basting • AL Rodrigues, Jr • MC Serra
Clinical Relevance
Changes in enamel and dentin microhardness may be related not only to carbamide
peroxide, but also to the presence of other components in bleaching agents, such as car-
bopol and glycerin. Carbopol and its associations may cause alterations in microhard-
ness compared to Opalescence. None of the treatment agents or associations evaluated
was inert for dental microhardness, although glycerin seemed to affect enamel and
dentin to a lesser degree.
SUMMARY
This study evaluated the effects of 10% car-
bamide peroxide, carbopol and glycerin and
their associations on microhardness over time on
enamel and dentin. Eight treatment agents were
evaluated: a commercial bleaching agent con-
taining 10% carbamide peroxide (Opalescence
10% Ultradent), 10% carbamide peroxide, car-
bopol, glycerin, 10% carbamide peroxide + car-
bopol, 10% carbamide peroxide + glycerin, car-
bopol + glycerin and 10% carbamide peroxide +
carbopol + glycerin. Three hundred and twenty
human dental fragments, 80 sound enamel frag-
ments (SE), 80 demineralized enamel fragments
(DE), 80 sound dentin fragments (SD) and 80 dem-
ineralized dentin (DD) fragments, were exposed
to the treatment agents (n=10). These agents
were applied onto the surface of the fragments
eight hours a day for 42 days. After eight hours,
they were washed from the dental fragment sur-
faces after five back-and-forth movements with a
soft bristle toothbrush under distilled and deion-
ized running water. During the remaining time
(16 hours per day), the fragments were kept in
individual vials in artificial saliva. After the 42-
day treatment period, the specimens were kept
individually in artificial saliva for 14 days. Knoop
microhardness measurements were performed at
baseline, after eight hours, and 7, 14, 21, 28, 35
and 42 days, and 7 and 14 days post-treatment
(corresponding to 49 and 56 days after the initial
treatment agent applications). The non-paramet-
ric Kruskal-Wallis analysis showed significant
differences among the agents at each time inter-
val, except at baseline for sound and demineral-
ized enamel and dentin. For SE, SD and DD, there
was a decrease in microhardness values during
treatment with all agents. There was a tendency
towards lower microhardness values after treat-
ment with carbopol and its associations for
*Roberta Tarkany Basting, DDS, MS, ScD, professor,
Department of Restorative Dentistry, Dentistry Center
Research Center São Leopoldo Mandic, Campinas, SP, Brazil
Antonio Luiz Rodrigues, Jr, DDS, MS, ScD, professor,
Department of Social Medicine, School of Medicine of
Ribeirão Preto (FAEPA-HCRP), University of São Paulo
(USP), Ribeirão Preto, SP, Brazil
*Mônica Campos Serra, DDS, MS, ScD, professor, Department
of Restorative Dentistry, School of Dentistry of Ribeirão Preto
(FORP), University of São Paulo (USP), Ribeirão Preto, SP,
Brazil
*Reprint request: Avenida do Café, s/nº, CEP: 14040-904, Ribeirão Preto –
SP, Brazil; e-mail: rbasting@yahoo.com or mcserra@forp.usp.br
609
Basting, Rodrigues Jr & Serra: The Effect of 10% Carbamide Peroxide, Carbopol and/or Glycerin
sound tissues. DD showed low microhardness
values during and after treatment with CP and
its associations. For DE, there was an increase in
microhardness values during treatment with all
agents and in the post-treatment phase. The
baseline microhardness values were not recov-
ered during the 14-day post-treatment phase.
Opalescence 10%, carbamide peroxide, carbopol,
glycerin and their associations may change the
microhardness of sound and demineralized dental
tissues, even in the presence of artificial saliva.
INTRODUCTION
Bleaching procedures with 10% carbamide peroxide
agents have been used as a simple and effective tech-
nique for the removal of intrinsic and extrinsic stains
(Haywood, 1994; Haywood, 2000). The clinical protocol
employs a bleaching agent in a tray for two to eight
hours during the day or night for two to six weeks of
treatment (Haywood & Heymann, 1989; Haywood,
2000; Ritter & others, 2002).
Ten percent carbamide peroxide seems to be effective
and safe (Curtis & others, 1996; Ritter & others, 2002)
and has the American Dental Association acceptance
seal for some brands (Haywood, 1993; Haywood &
Robinson, 1997). The addition of carbopol and glycerin
as thickening agents improves adherence of the bleach-
ing agent to the surface of dental structure, allowing for
a prolonged time for the release of carbamide peroxide
(Haywood, 1994; McCracken & Haywood, 1996).
Because the bleaching of vital teeth involves direct
contact of the whitening agent with the outer surface of
enamel and dentin in areas of defects, abfraction or
abrasion lesions, exposed root surfaces and marginal
areas between teeth and restorations, many studies
have evaluated the potential effects of these agents on
superficial micromorphology, changes in mineral con-
tent and microhardness. Scanning electron microscopic
evaluations have reported porosities and erosion on
enamel (Ben-Amar & others, 1995; Bitter, 1998; Bitter
& Sanders, 1993; Ernst, Marroquin & Willershausen-
Zonnchen, 1996; Flaitz & Hicks, 1996; Josey & others,
1996; Shannon & others, 1993; Smidt, Weller & Roman,
1998; Zalkind & others, 1996) and dentin (Zalkind &
others, 1996). In vitro studies have also reported some
alterations in mineral content and both enamel and
dentin microhardness after exposure to 10% carbamide
peroxide (Attin & others, 1997; Basting, Rodrigues &
Serra, 2003; Freitas & others, 2002; Oliveira & others,
2003; McCracken & Haywood, 1995, 1996; Pécora &
others, 1994; Rodrigues & others, 2001; Rotstein & oth-
ers, 1996; Smidt & others, 1998; Seghi & Denry, 1992).
Changes in enamel and dentin microhardness may be
related not only to the acidic pH of the bleaching
agents, which is responsible for a prolonged storage
time of the product, but also to the presence of other
components in commercial bleaching agent products.
McCracken and Haywood (1995) verified a significant
decrease in microhardness in the outer 25.0 µm of
enamel surface after treatment with a product contain-
ing carbopol. Basting and others (2003) also reported a
significant decrease in enamel surface microhardness
when using a placebo agent with carbopol and glycerin
with a neutral pH, even in the presence of artificial sali-
va. Freitas and others (2002) showed the same behav-
ior for this product in dentin. However, no in vitro stud-
ies evaluated the effects of bleaching agents on dem-
ineralized dental tissues. Bleaching agents have possi-
bly been applied to active carious lesions in enamel and
dentin.
In an in situ study, Basting, Rodrigues and Serra
(2001) observed significant differences in enamel micro-
hardness after treatment with 10% carbamide peroxide
bleaching agent and a placebo containing carbopol and
glycerin. The sound and demineralized enamel submit-
ted to the 10% carbamide peroxide bleaching agent
showed significantly lower microhardness values than
that submitted to a placebo agent. However, no differ-
ences were found between the sound and demineralized
dentin treated with bleaching or placebo agents, but
slightly higher microhardness values for dentin
exposed to a bleaching product.
However, the isolated effects of carbopol, glycerin and
10% carbamide peroxide, and even the combined effects
of those components on the microhardness of sound and
demineralized enamel and dentin tissues, are also
unknown.
This study evaluated in vitro the effects of 10% car-
bamide peroxide, carbopol, glycerin and their associa-
tions on the microhardness of sound and demineralized
enamel and dentin tissues and compared their values
with those of a 10% carbamide peroxide commercial
bleaching product at different time intervals.
METHODS AND MATERIALS
Experimental Design
The factors under study were:
Treatment agents (eight levels): Opalescence 10%
Ultradent, 10% carbamide peroxide; carbopol, glycerin,
10% carbamide peroxide + carbopol, 10% carbamide
peroxide + glycerin, carbopol + glycerin, and 10% car-
bamide peroxide + carbopol + glycerin.
Time (nine levels): baseline, 8 hours, and 7, 14, 21, 28,
35 and 42 days of treatment, and 7 and 14 days post-
treatment period (corresponding to 49 and 56 days after
the beginning of the bleaching treatment).
The experimental units consisted of 320 dental slabs:
80 sound enamel slabs; 80 demineralized enamel slabs;
80 sound dentin slabs and 80 demineralized dentin
slabs. Ten dental fragments of each dental tissue (n=10)
610
Operative Dentistry
were randomly and evenly assigned to the eight differ-
ent treatment agents. The effects of the different treat-
ment agents on enamel were not compared to dentin,
neither were the effects of sound tissues compared to
the demineralized ones.
Three repeated measurements of Knoop microhard-
ness were taken from the surface of each specimen at
each time interval.
Dental Fragments Preparation
This study had the approval of the FORP/USP Ethical
Committee Guidelines in accordance with the National
Health Council (Conselho Nacional de Saúde, 2003).
Seventy-seven non-erupted third molars were used.
Immediately after extraction for reasons other than the
experiment, the teeth were kept in 0.1% thymol. They
were sectioned with double-faced diamond discs (KG
Sorensen, Barueri, SP, Brazil) at a low motor speed
(Kavo do Brasil, Joinville, SC, Brazil), dividing the root
from the coronary portion to obtain 320 dental slabs
with 3 mm x 3 mm x 2 mm (160 enamel slabs and 160
dentin slabs). In the root, the apical third was discard-
ed and only the cervical region was used. Care was
taken not to leave the dental fragments dehydrated for
long periods. Those slabs that presented stains or
cracks after observation under stereomicroscope loupe
(Meiji Techno EMZ Series, Saitama, Japan) at 30x were
discarded.
The dental fragments were embedded individually in
a self-curing polyester resin in a polyvinylchloride ring
mold 2.0-cm in diameter, with the external surface of
the enamel or dentin exposed. The molds were removed
and the external surfaces of the dental fragments were
leveled by a water-cooled mechanical grinder
(Maxgrind/Solotest, São Paulo, Brazil). These proce-
dures were conducted to form parallel planar surfaces
for the Knoop microhardness tests. For the enamel sur-
faces, aluminum oxide discs of 400, 600 and 1000 grit
were used sequen-
tially
(Carborundum/3M
do Brasil Ltda,
Sumaré, Brazil)
with water cooling.
Polishing was per-
formed using polish-
ing cloths (Top, Gold
and Ram, Arotec Ind
e Com Ltda, Cotia,
Brazil) and diamond
pastes of 6, 3, 1 and
1/4 µm (Arotec Ind e
Com Ltda) with
mineral oil cooling
(Red lubricant,
Arotec Ind e Com
Ltda). For the dentin
fragments, only aluminum oxide discs were used in a
sequential granulation of 600, 1000 and 1200 grit
(Carborundum/3M do Brasil Ltda, Sumaré, Brazil)
with water cooling. Between each sequential disc, the
dental fragments were immersed in an ultrasonic dis-
tillated water bath for 10 minutes.
Dental Slabs Preparation
To obtain 80 demineralized enamel slabs and 80 dem-
ineralized dentin slabs, caries-like lesions were gener-
ated by a dynamic model of demineralization and rem-
ineralization cycles similar to the model proposed by
Featherstone and others (1986) and modified by
Delbem and Cury (2002).
The enamel and dentin fragments were submitted to
cycles of de-remineralization. The group that made up
the sound group of each dental tissue was not submit-
ted to the de-remineralization cycles; instead, the spec-
imens were stored in a humid environment.
Specification of the Treatment Agents
The treatment agents are presented at Table 1 accord-
ing to composition and manufacturer.
A 10% carbamide peroxide commercial bleaching
agent (Opalescence 10% Ultradent, South Jordan, UT,
USA) was used as a control as it is accepted as safe and
effective by the American Dental Association (ADA). It
contains 10% carbamide peroxide and amounts of car-
bopol and glycerin not specified by the manufacturer.
The flavor tested was “regular.”
The treatment agents evaluated include 10% car-
bamide peroxide (CP), carbopol (C) and glycerin (G).
Their associations were also tested: 10% carbamide per-
oxide + carbopol (CP + C), 10% carbamide peroxide +
glycerin (CP + G), carbopol + glycerin (C + G) and 10%
carbamide peroxide + carbopol + glycerin (CP + C + G).
These products were freshly obtained and/or prepared
in a dispensing pharmacy. The consistency of the asso-
Treatment Agents Composition Manufacturer
Opalescence 10% (OPA) 10% carbamide peroxide; Ultradent Products Inc, South Jordan,
carbopol; glycerin; flavoring* UT, USA
10% carbamide peroxide (CP) 10% carbamide peroxide Proderma – Pharmacy, Piracicaba, Brazil
Carbopol (C) Carbopol Proderma – Pharmacy, Piracicaba, Brazil
Glycerin (G) Glycerin Proderma – Pharmacy, Piracicaba, Brazil
10% carbamide peroxide + 10% carbamide peroxide + Mixed formula, Proderma – Pharmacy,
carbopol (CP + C) carbopol Piracicaba, Brazil
10% carbamide peroxide + 10% carbamide peroxide + Mixed formula, Proderma – Pharmacy,
(CP + G) glycerin Piracicaba, Brazil
Carbopol + glycerin (C + G) Carbopol + glycerin Mixed formula, Proderma – Pharmacy,
Piracicaba, Brazil
10% carbamide peroxide + 10% carbamide peroxide + Mixed formula, Proderma – Pharmacy,
carbopol + glycerin carbopol + glycerin Piracicaba, Brazil
(CP + C + G)
*The manufacturer does not indicate the percentage of each component.
Tabl e 1: Composition and Manufacturer of Each Treatment Agent
611
Basting, Rodrigues Jr & Serra: The Effect of 10% Carbamide Peroxide, Carbopol and/or Glycerin
ciation of carbamide peroxide + carbopol + glycerin and
carbopol + glycerin was similar to the commercial product.
Application of Treatment Agents
Prior to treatment, an individual tray for each speci-
men was manufactured from a 1.0-mm thick flexible
ethyl vinyl acetate polymer (Bio-Art Equipamentos
Odontológicos Ltda, São Carlos, Brazil) placed in a vac-
uum-forming machine (P7, Bio-Art Equipamentos
Odontológicos Ltda).
Both the sound and demineralized enamel and dentin
fragments were exposed to the treatment agents eight
hours a day for 42 days. A syringe was used to apply
0.02 mL of each agent to each specimen. The specimens
were individually covered with a tray and soaked in
individual closed vials with 13.5 mL of artificial saliva
(pH 7.0) at 37°C ± 2°C.
After eight hours, the treated specimens were taken
out of the storage media and the trays removed. The
treatment agents were removed from the dental frag-
ment surfaces by making five back-and-forth move-
ments with a soft bristle toothbrush (Oral B 35/Gillette
do Brasil Ltda, Manaus, Brazil) to remove the viscous
film that formed on the fragment surfaces, which was
then washed under distilled and deionized running
water for five seconds.
During the remaining daily time (16 hours per day),
the fragments were kept in individual vials with 13.5
mL of artificial saliva (pH 7.0) at 37.0°C ± 2.0°C. The
artificial saliva was changed every two days and con-
sisted of a remineralization solution proposed by
Featherstone and others (1986) and modified by
Delbem and Cury (2002). It contained calcium hydrox-
ide, phosphoric acid, potassium chloride, buffering
agent and deionized and distilled water.
This cycle was repeated for 42 days, corresponding to
the maximum clinical period for a bleaching treatment
of six weeks as recommended by Haywood and
Heymann (1989).
Post-treatment Phase
After the 42-day treatment period, the specimens were
kept in their individual vials with 13.5 mL of artificial
saliva (pH 7.0) at 37.0°C ± 2.0°C during 14 days. The
artificial saliva was also changed every two days.
Thus, the possible remineralizing effects of the artifi-
cial saliva on the microhardness of sound and dem-
ineralized dental fragments could be evaluated.
Microhardness Testing
Microhardness measurements were taken before ini-
tial exposure to the treatment agents (baseline) after 8
hours and 7, 14, 21, 28, 35 and 42 days, and 7 and 14
days post-treatment (corresponding to 49 and 56 days
after initial application of the treatment agents). A
Knoop indenter was used; the long axis of the diamond
was kept parallel to the dentinal surface in a micro-
hardness testing machine (Future Tech, FM-1e, Tokyo,
Japan). Three microhardness measurements were
taken on each specimen at different time intervals. A
load of 25.0 grams was used for the enamel fragments
and a load of 10 grams was used for the dentin for five
seconds.
Statistical Analysis
Knoop microhardness responses were statistically
evaluated by Kruskal-Wallis test, followed by pair-wise
multiple comparison (Conover, 1971), according to den-
tal slab tissue/type (enamel or dentin, sound or dem-
ineralized). Means were obtained after triplicates were
averaged. The response value at each time was then
subtracted from its respective baseline mean, yielding
the ultimate response value.
RESULTS
Statistical analysis showed significant differences
among the agents at each time interval, except at base-
line, for sound and demineralized enamel and dentin.
Table 2 and Figure 1 show the means and results of
the pair-wise multiple comparisons for the sound
Time Treatment Agents
(hours)
OPA CP C G CP + C CP + G C + G C + G + CP
8 -71.4 d -32.6 g -200.8 a -34.1 f -205.3 a -73.8 e -113.1 b -84.6 c
168 -103.2 b -50.4 d -297.8 a -83.9 c -303.3 a -140.3 b -306.5 a -87.9 c
336 -83.8 c -26.1 c -314.0 a -71.2 d -310.0 a -83.0 c -302.6 a -89.2 b
504 -106.9 c -29.8 d -317.7 a -44.8 d -311.7 b -108.9 c -325.3 ab -95.1 c
672 -140.4 b -14.3 e -316.7 a -54.6 d -310.7 a -106.6 c -324.7 a -97.4 c
840 -107.8 c -4.9 f -318.4 a -56.5 e -309.0 ab -70.7 d -305.0 b -107.7 c
1008 -120.3 b -7.0 e -317.9 a -27.5 d -305.4 a -99.7 c -313.4 a -109.6 b
1176 -90.2 b 18.9 d -301.7 a -27.1 c -308.8 a -60.2 c -301.0 a -94.8 b
1344 -76.7 c 1.5 ed -298.4 a -52.7 e -298.7 a -44.3 d -303.4 a -82.0 b
*Equal letters horizontally indicate mean values that are not significantly different.
Tabl e 2: Means and the Results of Pair-wise Comparisons of the Knoop Microhardness Difference Values for Sound Enamel
Slabs
612
Operative Dentistry
enamel fragments. There was a decrease in enamel
microhardness values over time for all agents evaluat-
ed. Lower microhardness values were obtained after
treatment with C, C + G and CP + C, even eight hours
after application and during the post-treatment period.
An increase in microhardness values above baseline
was observed when using CP during the post-treatment
phase.
For the demineralized enamel frag-
ments, there was an increase in micro-
hardness values during treatment
with all the agents and in the post-
treatment phase (Table 3 and Figure 2).
There was a decrease in microhard-
ness values for sound dentin frag-
ments after treatment with all agents;
however, lower values were obtained
with the use of OPA, C, CP + C, CP +
G and CP + C + G (Table 4 and Figure 3).
The demineralized dentin fragments
showed a decrease in microhardness
values during the treatment period for
all agents, mainly after the application
of CP, CP + G, C + G and CP + C + G.
However, these fragments showed an
increase in microhardness values dur-
ing the post-treatment phase, which
was not observed for the other dental
tissues (Table 5 and Figure 4).
DISCUSSION
Although research has been conducted
to evaluate the effects of bleaching
agents on enamel and dentin (Attin &
others, 1997; Ben-Amar & others,
1995; Bitter, 1998; Bitter & Sanders,
1993; Ernst & others, 1996; Flaitz &
Hicks, 1996; Josey & others, 1996;
McCracken & Haywood, 1995, 1996;
Nathoo, Chmielewski & Kirkup, 1994;
Pécora & others, 1994; Rodrigues &
others, 2001; Rotstein & others, 1996;
Seghi & Denry, 1992; Shannon & oth-
ers, 1993; Smidt & others, 1998;
Zalkind & others, 1996), it does not
consider the isolated effects of each
component of these products, which
may adversely affect dental hard tis-
sues. Different brands of 10% car-
bamide peroxide bleaching agents
present different effects on enamel
and dentin, and this variation may be
related to the composition of each
product (Basting & others, 2003;
McCracken & Haywood, 1996).
The chemistry of carbamide peroxide bleaching
agents is based on its ability to generate free radicals,
which are highly reactive. The free radicals of hydrogen
peroxide are non-specific, extremely unstable and can
potentially react not only with the pigmented carbon
rings, but also with other organic molecules to achieve
stability (Goldstein & Garber, 1995). Thus, changes in
the chemical or morphological structure of a tooth must
Figure 1: Linear diagram of the means of Knoop Microhardness Number (KHN) differences for
each treatment agent over time for sound enamel slabs.
Figure 2: Linear diagram of the means of Knoop Microhardness Number (KHN) differences for
each treatment agent over time for demineralized enamel slabs.
613
Basting, Rodrigues Jr & Serra: The Effect of 10% Carbamide Peroxide, Carbopol and/or Glycerin
be of concern when using bleaching
techniques as a treatment for whiten-
ing teeth. Although some studies have
reported no significant changes in
dental microhardness when using
short-term regimens of carbamide
peroxide (Nathoo & others, 1994;
Potocnik, Kosec, Gaspersic, 2000;
Seghi & Denry, 1992; Shannon & oth-
ers, 1993), others observed a decrease
in enamel and dentin microhardness
when using these bleaching agents for
two weeks or more, even with the use
of artificial saliva or fluoride solutions
(Attin & others, 1997; Basting & oth-
ers, 2003; Freitas & others, 2002;
McCracken & Haywood, 1995;
Oliveira & others, 2003; Rodrigues &
others, 2001; Smidt & others, 1998).
In this study, a decrease in micro-
hardness for sound enamel and dentin
was found even after eight hours of
treatment with all agents. Although
the remineralizing effect of saliva was
expected during the 16-hours of
immersion in artificial saliva, a slow,
continuing decrease and maintenance
of low values of enamel and dentin
microhardness was observed through-
out the experimental phase.
Some of the thickening agents in
saliva substitutes generally use car-
bopol, carboxymethylcellulose or other
polymers (Christersson, Lindh &
Arnebrandt, 2000; Van der Reijden &
others, 1997). In this study, the artifi-
cial saliva used was supersaturated in
minerals and no salivary proteins
were considered (Featherstone & oth-
ers, 1986), but its remineralization
effect was observed during the post-
treatment period.
Polymers used as thickening agents
for saliva substitutes largely inhibited
further demineralization, except car-
bopol, which causes demineralization,
especially in a remineralization solu-
tion. Carbopol completely inhibited hydroxyapatite
crystal growth because of its high calcium-binding
capacity (Van der Reijden & others, 1997). Carbopol
was not added as an ingredient to the artificial saliva
used in this study, but it was evaluated alone or in com-
bination with glycerin and carbamide peroxide. A
decrease in microhardness values for sound enamel
and dentin during treatment with almost all agents
containing carbopol was observed, showing a continu-
ing demineralization of enamel and dentin at neutral
pH. In a microhardness evaluation comparing the
effects of two 10% carbamide peroxide bleaching agents
with and without carbopol on enamel, McCracken and
Haywood (1995) showed a significant decrease in
microhardness in the outer 25 µm of the enamel sur-
face after treatment with the product containing car-
bopol. This difference was related not only to the pH
Figure 3: Linear diagram of the means of Knoop Microhardness Number (KHN) differences for
each treatment agent over time for sound dentin slabs.
Figure 4: Linear diagram of the means of Knoop Microhardness Number (KHN) differences for
each treatment agent over time for demineralized dentin slabs.
614
Operative Dentistry
level of the products, but also to the presence of car-
bopol. Probably, the neutralizing effect of saliva in the
mouth and the combination of carbopol with other com-
ponents of bleaching agents may reduce its negative
effect on dental microhardness, although other formu-
lations may be developed for reducing the hazardous
effects of this product on dental mineral content.
Although carbamide peroxide was thought to signifi-
cantly change microhardness values for sound dental
tissues due to the release of hydrogen peroxide and
urea, this agent and its association with glycerin
showed slight decreases compared to other agents eval-
uated, probably due to the rise in the hydrogen ion con-
centration (pH) of the solution (Haywood & Heymann,
Time Treatment Agents
(hours)
OPA CP C G CP + C CP + G C + G C + G + CP
8 9.4 d 8.5 d 6.1 ab 5.8 a 6.0 ab 5.9 c 6.3 abc 6.4 bc
168 13.5 c 29.7 h 8.2 a 17.0 f 15.2 d 21.3 g 9.7 b 15.7 e
336 29.5 d 32.3 d 12.4 a 31.2 d 23.5 c 29.2 d 21.8 c 21.2 b
504 40.7 f 46.0 f 17.9 a 40.4 e 24.8 b 35.0 d 34.8 c 20.7 a
672 38.4 c 44.7 d 23.0 a 40.3 c 25.2 a 45.9 e 29.1 b 27.6 a
840 44.8 d 51.4 d 30.4 b 47.0 d 29.3 a 65.7 e 34.8 c 25.4 a
1008 48.6 e 40.5 d 30.3 b 48.7 e 22.9 a 61.5 f 32.8 c 23.3 a
1176 62.8 d 57.1 c 33.2 a 73.2 d 40.8 a 82.6 e 46.4 b 36.9 a
1344 77.5 f 57.3 d 42.5 b 75.8 e 41.2 a 69.9 e 54.7 c 43.1 b
*Equal letters horizontally indicate mean values that are not significantly different.
Tabl e 3: Means and the Results of Pair-wise Comparisons of the Knoop Microhardness Difference Values for Demineralized
Enamel Slabs
Time Treatment Agents
(hours)
OPA CP C G CP + C CP + G C + G C + G + CP
8 -6.6 c -2.2 e -18.0 b -2.3 e -17.0 ab -0.5 e -27.8 a -4.9 d
168 -50.1 e -18.1 f -60.8 c -3.3 h -64.4 b -9.2 g -65.1 a -58.4 d
336 -58.0 e -20.4 f -61.9 c -1.9 h -66.4 b -10.7 g -68.0 a -61.5 d
504 -61.2 e -11.7 f -67.0 c -3.8 g -71.3 a -1.1 g -68.2 b -63.6 d
672 -62.7 d -18.1 e -67.5 c -3.3 f -71.3 a -5.0 f -69.1 b -65.6 c
840 -64.0 e -10.6 f -68.3 c -4.8 g -72.6 a -3.6 g -70.3 b -66.2 d
1008 -67.9 c -14.6 d -70.0 b -4.2 e -73.2 a -5.3 e -71.6 a -70.5 a
1176 -65.6 d -11.3 e -69.7 c -2.3 f -71.9 a -3.6 f -70.2 ab -68.8 b
1344 -60.3 c -4.3 de -68.3 b -3.1 e -70.7 a -5.7 d -68.3 ab -68.4 ab
*Equal letters horizontally indicate mean values that are not significantly different.
Tabl e 4: Means and the Results of Pair-wise Comparisons of the Knoop Microhardness Difference Values for Sound Dentin
Fragments
Time Treatment Agents
(hours)
OPA CP C G CP + C CP + G C + G C + G + CP
8 -4.6 b -5.3 c -5.3 e -2.9 d -3.8 a -5.3 cd -4.9 b -6.0 b
168 -6.9 de -10.2 e -10.2 g -7.0 f -6.2 c -7.1 d -8.7 a -8.1 b
336 -7.0 d -11.3 f -11.3 h -7.9 g -6.2 c -9.5 e -13.2 b -11.2 a
504 -7.7 c -12.6 d -12.6 f -8.5 e -7.3 b -12.0 d -13.3 b -12.8 a
672 -7.2 b -13.7 d -13.7 e -8.7 d -7.9 a -12.7 c -13.5 a -13.6 a
840 -8.1 c -14.1 d -14.1 e -8.5 d -8.8 a -13.8 d -14.3 b -14.3 ab
1008 -8.5 c -15.8 e -15.8 f -9.6 e -8.9 a -15.2 d -16.0 b -16.4 a
1176 -7.4 c -14.1 de -14.1 f -7.6 e -6.7 b -13.0 d -15.3 b -14.3 a
1344 -5.8 b -11.6 c -11.6 f -5.1 c -4.8 a -10.0 d -12.4 a -11.5 a
*Equal letters horizontally indicate mean values that are not significantly different.
Tabl e 5: Means and the Results of Pair-wise Comparisons of the Knoop Microhardness Difference Values for Demineralized
Dentin Fragments
615
Basting, Rodrigues Jr & Serra: The Effect of 10% Carbamide Peroxide, Carbopol and/or Glycerin
1989). Urea is capable of penetrating into enamel and
affecting not only the surface, but also the interpris-
matic regions of enamel. The increase in enamel perme-
ability may cause structural changes (Arends & others,
1984; Goldberg & others, 1983) due to the dissociation of
H-bonds between the CO and NH groups (Goldberg &
others, 1983). It denatures proteins and causes confor-
mational changes, although the increased porosity of
the outer enamel surface shown by Hegedüs and others
(1999) may be caused mainly by nascent oxygen when
released in the inner structure. When using 10% car-
bamide peroxide on sound dental tissues for seven days,
Zalkind and others (1996) showed moderate morpholog-
ical changes in the dentin surface, but none in enamel.
Rotstein and others (1996) also showed an increase in
the calcium levels of enamel following treatment with
10% carbamide peroxide, although there was a decrease
in the calcium/phosphorus ratio and potassium levels of
dentin. Changes in the levels of these minerals may
indicate damage to the organic component of the
matrix, especially in dentin, due to the higher organic
concentration.
Glycerin also presented slight decreases in microhard-
ness for sound enamel and dentin, similar to the effect
of carbamide peroxide. It could act as an adsorbed layer
barrier to artificial saliva and to a remineralizing effect.
For demineralized enamel fragments, treatment with
all agents and daily immersion in artificial saliva con-
tributed to a remineralization process shown as an
increase in microhardness values. However, microhard-
ness decreases were observed for demineralized dentin.
Haywood and Robinson (1997) have advocated the use
of carbamide peroxide bleaching agents for initial caries
lesions, mainly root caries, as the caries progression is
retarded or stopped during bleaching. For demineral-
ized dentin, the effect is a high decrease in microhard-
ness values that could increase the depth of lesion for-
mation, and bleaching should not be indicated as a com-
mon procedure. Although the post-treatment period
seems to allow for an increase in microhardness values,
probably due to a mineral deposition on dentin through
a remineralization process, immersion of the fragments
in artificial saliva did not provide recovery of the base-
line values.
The demineralizing effects of agents, other than car-
bamide peroxide contained in bleaching agents, may
play a role. As a general trend, 10% carbamide peroxide,
carbopol, glycerin and their association seem to
decrease sound enamel and dentin microhardness and
demineralized dentin. Carbopol and its associations
cause severe alterations in microhardness compared to
Opalescence, which is a commercial brand available on
the market. None of the agents evaluated were inert for
dental microhardness, although glycerin seemed to
affect enamel and dentin to a lesser degree. Thus, these
results may be an advice warning to manufacturers to
re-formulate the composition of some bleaching prod-
ucts or provide a better agent that does not cause enam-
el or dentin demineralization. The damage to sound and
demineralized enamel and dentin in this experiment
does not, however, necessarily imply demineralization
in vivo, but should be kept in mind in further research.
CONCLUSIONS
Ten percent carbamide peroxide, carbopol, glycerin and
their association decreased sound enamel and dentin
microhardness and demineralized dentin. Carbopol and
its associations caused alterations in microhardness,
although glycerin seemed to affect enamel and dentin to
a lesser degree.
Acknowledgement
The authors are grateful for the financial support received from
FAPESP (Foundation for Research Support of São Paulo State).
Grants: 01/08774-8.
(Received 10 May 2004)
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