The effect of home bleaching on surface of zinc phosphate cement: SEM study
‫ دراسة بالمجهر الماسح اإللكتروني‬:‫تأثير تبييض األسنان المنزلي على سطح اسمنت فوسفات الزنك‬
Short Title:
Bleaching and Zinc phosphate cement
ABSTRACT
Objective: was to microscopically evaluate the effect of different concentrations of
Carbamide Peroxide (CP) home bleaching agents on the surface of Zinc Phosphate Cement.
Methods: Forty Teflon rings of 10 mm internal diameter and 2mm thickness were loaded
with Zinc Phosphate Cement. After 15 minutes, excess cement was removed. The specimens
were equally divided into four groups; one control group and three experimental groups
exposed to 10%, 16% and 22% CP agents for 4 hours/day for two weeks. Specimens were
examined under a scanning electron microscope. The surface micromorphology of the
specimens were carefully evaluated and compared. This study was conducted in 2012 at the
College of Dentistry Research Center (CDRC), King Saud University Results: the plate-like
crystals were evident on the surface of specimens of the control group, but appeared as
multiple flat irregular crystals with rounded edges when specimens were treated with 10%,
16% and 22% CP bleaching agents. CP bleaching agents reduced the number of zinc oxide
particles in the experimental more than the control group specimens and the matrix of the
cement was eroded at some areas of the experimental groups. As the concentration of CP was
increased, the number of the zinc oxide particles was decreased. Conclusions: There was an
evidence of morphological changes on the surface of zinc phosphate cement when exposed to
10%, 16% and 22% CP home bleaching agents.
KEYWORDS: Carbamide peroxide (CP), 10% CP, 16% CP, 22% CP, Zinc phosphate
cement
1
‫ملخص البحث‬
‫الغرض من البحث هو التقييم المجهري لتأثير التركيزالمختلف لمادة بيروكسيد الكربامايد المستعملة في تبييض األسنان‬
‫ مم بإسمنت‬2 ‫ مم وسمكها‬10 ‫ ملئت أربعون حلقة تيفلون قطرها الداخلي‬:‫ الطرق‬.‫المنزلي على سطح اسمنت فوسفات الزنك‬
‫ قسمت العينات بالتساوي إلى أربعة مجموعات أحدها استعمل‬.‫ دقيقة‬15 ‫ أزيل الزائد من اإلسمنت بعد‬.‫فوسفات الزنك‬
‫ ساعات‬4 ‫ بيروكسايد الكاربامايد لمدة‬%22 ‫ و‬%16 ،%10 ‫للمقارنة معها وتم تعريض المجموعات األخرى التجريبية إلى‬
.‫ تم تقييم ومقارنة الشكل السطحي الدقيق للعينات‬. ‫ فحصت العينات تحت المجهر الماسح اإللكتروني‬.‫في اليوم ألسبوعين‬
‫ وجدت ألواح من‬:‫ النتائج‬.‫ جامعة الملك سعود‬-‫ في مركز البحوث بكلية طب األسنان‬2012 ‫عمل هذا البحث في العام‬
‫البلورات على أسطح العينات في المجموعة المستعملة للمقارنة معها ولكنها بدت كبلورات مسطحة متعددة األشكال حوافها‬
‫ قللت مركبات بيروكسايد‬.‫ من مركبات بيروكسايد الكاربامايد‬%22 ‫ و‬%16 ،%10 ‫مستديرة عند عالجها باستعمال‬
‫الكاربامايد من رقائق أوكسيد الزنك في عينات المجموعات التجريبية أكثر من عينات مجموعة المقارنة كما وجد تآكل‬
‫ قل‬،‫ لوحظ أنه كلما زاد تركيز مركبات بيروكسايد الكاربامايد‬.‫للقالب الغشائي في بعض المناطق في المجموعات التجريبية‬
،%10 ‫ اتضحت تغييرات شكلية لسطح فوسفات الزنك عند تعريضه لمركبات‬:‫ االستنتاج‬.‫عدد رقائق أوكسيد الزنك‬
.‫ بيروكسايد الكاربامايد المستعملة في تبييض األسنان المنزلي‬%22 ‫ و‬%16
INTRODUCTION
Bleaching is considered as a conservative treatment of discolored teeth. Different methods
of Tooth bleaching have been introduced. They are in-office bleaching where a high
concentration of Hydrogen Peroxide (HP) of 35 to 50%; and at-home bleaching of lower
concentrations of Carbamide Peroxide (CP) of 5 to 22%. Haywood1 (2000) stated that CP
products can be used in concentrations up to 35%. The night-guard vital (at-home) bleaching
method is the most popular method. A 10% solution of CP is roughly 3% hydrogen peroxide
and 7% urea.1,2 The hydrogen peroxide is considered as the active ingredient.2 The urea raises
the hydrogen ion concentration (pH) of the solution.3
Bleaching products were proven to be efficient in teeth whitening, but their effect on dental
restorative materials is contradictory.4 Swift5 (1997) stated that bleaching can be used with
little fear of damage to existing restorations. However, it remains unclear how far bleaching
may result in significant deterioration of restorations under clinical conditions.
Several investigators reported evidences of surface changes of restorative materials when
exposed to home bleaching agents with different CP concentrations. Surface roughness of
tooth-colored restorative materials after application of 10% and 15% CP was reported.6,7
However, Turker and Biskin8 (2003) found that surface roughness of tooth-colored restorative
materials was slightly changed by 10% and 16% CP concentrations. Color changes of
composite restorations treated by 10% CP were also reported.9 In 2010, Prabhakar et al10
reported reduction in hardness but not bond strength of composite restorative materials when
exposed to 10% and 22% CP. Mujdeci and Gokay11 (2006) found no adverse effect of 10%
CP and 14% HP on microhardness of tooth-colored restorative materials. Softening of the
composite after exposing it to CP was also reported.6 Ulukapi et al12 (2003) reported a
negative effect of 10% CP on the marginal adaptation of composite restoration, but not of
amalgam restoration.
Home bleaching agents of 10% and 16% CP significantly decreased microhardness of
porcelain13, but no significant changes in surface roughness were reported for the feldspathic
porcelain.8
2
When 10% CP bleaching solutions are used, methacrylate provisional restorative materials
undergo discoloration.14 However, Bis-acryl composite resin provisional material and
polycarbonate provisional prefabricated crowns were not discolored by 10% CP solutions.14
In cemented cast restorations, a microscopic cement-filled gap exists at the restoration
margin. It is usually exposed to the oral environment and may not be visible to the naked eye.
When a cast restoration is subjected to a vital tooth bleaching agent, the cement-filled gap is
exposed to the bleaching agents. Resin cements showed no evidence of changes in surface
integrity when exposed to 10% CP agents.15 However, Resin-Modified Glass Ionomer
cements demonstrated a significant increase in surface roughness when exposed to 10%
CP.8,15,16 In 1992, Jefferson15 et al reported an eroded matrix of the Zinc Phosphate Cement
and Glass Ionomer Cement when exposed to 10% CP.
The effect of different concentrations of home bleaching agents on dental luting cements
has not been extensively investigated. Limited information exists on their effect on the surface
of Zinc Phosphate Luting Cement.
The purpose of this study was to microscopically evaluate the effect of different
concentrations of CP home bleaching on the surface of Zinc Phosphate Cement.
METHODS
Forty Teflon rings of 10 mm internal diameter and 2mm thickness were prepared. Each
ring was placed on a celluloid strip seated on a glass slab. Zinc Phosphate Cement (Zinc
Phosphate Cement, HENRY SCHEIN INC, Melville, NY, 11747, USA) was mixed on a glass
slab at room temperature following the manufacturer's directions and loaded into the rings.
Another celluloid strip was placed on top of the loaded ring and another glass slab was seated
on it. After 15 minutes, the ring containing the set cement was cleaned of excess cement. All
specimens were immersed in 15 ml artificial saliva at 37oC for 24 hours. The specimens were
then equally divided into four groups, each containing ten specimens. One group was used as
a control. The other three experimental groups were repeatedly exposed to three different
concentrations of Carbamide Peroxide home bleaching agents (10%, 16% and 22%)
(NITEWHITE ACP, Discus Dental, LLC, CA 90232, USA) following the manufacturer's
directions. The bleaching agents were applied daily for 4 hours for a period of two weeks.
Before each exposure to the CP bleaching gels, the specimens were rinsed with saline for 2
minutes and dried using Laboratory Wipes (Kimwipes EX-L Laboratory Wipers, KimberlyClark Corporation, Roswell, GA 30076, USA). At the end of each exposure, the CP bleaching
gel was first wiped off using wipes then the specimen was rinsed well with saline. All
specimens were stored in artificial saliva at 37oC for 24 hours to be ready for the next
exposure to CP gel. During the exposure to CP gels, the specimens were stored in a humid
atmosphere at 37oC. All specimens were stored in distilled water at 37oC at all times except
during CP treatment period. All specimens were prepared, gold coated to be ready for
examination under a scanning electron microscope (JOEL, JSM-T330A Scanning Electron
Microscope, JOEL Ltd, 1-2 Musashino 3-Chrome Akishima, Tokyo 196-85558, Japan)). The
surface micromorphology of the specimens were carefully evaluated and compared. This
study was conducted in 2012 at the College of Dentistry Research Center (CDRC), King Saud
University.
3
RESULTS
The zinc oxide particles in the specimens of the control group were tightly packed (Fig1).
They had a plate-like crystalline appearance with well-defined surfaces and edges. The
particles were formed on all the surface of the cement masking the matrix.
Fig. 1. Scanning electron microscope image (X500 magnification) of zinc phosphate cement
of the control group. The tightly packed zinc oxide with a plate-like crystalline
appearance and well-defined surfaces and edges are formed on all the surface of the
cement masking the matrix.
In specimens that were exposed to 10% CP gels, the plate-like crystalline particles were
missing (Fig2). Instead, multiple flat, irregular deposits were seen covering a large cement
surface area. The particles had rounded edges. Some of them were fused together. The matrix
of the cement was slightly eroded at few areas. Small oxide particles were seen embedded
within the cement matrix. Several cracks were observed at some areas due to drying of
specimens during their preparation for SEM examination. Some pores are evident.
4
Fig.2. Scanning electron microscope image (X500 magnification) of zinc phosphate cement
after application of 10% CP agent. Multiple flat, irregular zinc oxide particles are
covering a large cement surface area. They have rounded edges and some of them
were fused together. The matrix of the cement is slightly eroded at few areas with
oxide particles embedded within it. Several cracks and pores are observed.
Specimens of the 16% CP gel group showed slight erosion of the matrix (Fig3). More flat
and irregular crystals with rounded edges were observed on the surface of the cement with
some of them fused together. Small particles were seen embedded within the cement matrix.
There were some pores.
5
Fig.3. Scanning electron microscope image (X500 magnification) of zinc phosphate cement
after application of 10% CP agent. Slight erosion of the matrix is evident. Particles are
more flat and irregular with rounded edges and some of them are fused together. Small
particles were seen embedded within the cement matrix with evidence of some pores.
Specimens that were exposed to 22% CP gels showed little more erosion of the surface of
the matrix of the cement. Fewer flat and irregular crystals were observed on the surface of the
cement (Fig4). The particles had rounded edges. Some of them fused together. They were also
embedded within the matrix. Few pores were evident.
6
Fig.4. Scanning electron microscope image (X500 magnification) of zinc phosphate cement
after application of 10% CP agent. Slightly more erosion of the matrix of the cement is
evident. Fewer flat and irregular crystals are observed on the surface of the cement. The
particles had rounded edges with some of them fused together. Some of them are
embedded within the matrix. Few pores are evident.
DISCUSSION
Zinc phosphate cement has powder and liquid components. The powder is composed of
90% zinc oxide and 10% magnesium oxide and other pigments like silica oxide and bismuth
oxide. The liquid is composed of concentrated orthophosphoric acid, 40% water, 2.5%
aluminium phosphate and 5% zinc phosphate. Both components are mixed together to form a
matrix of amorphous glass-like non-crystalline tertiary zinc phosphate which binds unreacted
particles of zinc oxide together.15 The aluminium content is confined only to the matrix of the
cement and it aids in moderating the reaction. 15 The amorphous glass-like non-crystalline
component is stable under relatively dry conditions (relative humidity less than 30%).
However, in the presence of excess moisture or high humidity after complete hardening of the
cement, the unstable surface layer of the cement transforms to the crystalline form, hopeite
(Zn3(PO4)2.4H2O).17,18 However, Servais and Cartz17 (1971) stated that the hopeite crystals at
the surface grow from zinc oxide particles situated at the surface that act as nuclei. The
hopeite crystals are held weakly to the cement surface.17 After one or two weeks at 100%
relative humidity large crystals of hopeite can be seen freely at the surface.17 The rate of
growth of the hopeite crystals was found to be dependent on the temperature conditions; at
37oC, the rate of growth was found distinctly greater. 17 In the present study, all specimens
were stored at all times in artificial saliva at 37oC. So the hopeite crystals may have formed on
the surface of the set cement especially in the presence of artificial saliva. However, since
they are weakly held to the cement surface, they were most probably lost by drying the
surfaces of the cement specimens during their preparation for SEM examination. In 2010,
7
Boston and Jefferies19 stated that exposure to 36% HP bleaching gel could reduce the water
content near the cement surface through osmotic effect. Therefore, the formation of the
hopeite crystals were further reduced due to the osmotic effect of the CP bleaching gels on the
surface of the cement.
In this study, the plate-like zinc oxide particles were evident on the surface of specimens of
the control group. But they were missing when specimens were treated with 10%, 16% and
22% CP bleaching agents. In fact, they appeared as multiple flat irregular crystals with
rounded edges. They covered most of the surface of the cement. Similar findings were
reported by Jefferson et al15 after using 10% CP bleaching.
In this study, CP bleaching agents reduced zinc oxide particles more than the control group.
This was supported by the study of Boston and Jefferies19 where 36% HP was used. However,
they found that the mean roughness, hardness and modulus of zinc phosphate exposed to 36%
hydrogen peroxide agents were not statistically different from the unexposed control samples.
In the present study, CP agents were used at much lower concentrations. Therefore, the effect
of CP on zinc phosphate cement was minimal.
The solubility of zinc phosphate cement increases when the pH of the medium is lowered.15
This may result in an increase in solubility of zinc oxide particles of the cement. In 2009,
Londono et al16 stated that solubility of zinc phosphate cement as a result of the bleaching
process might be due to the low pH and oxidizing effect of home bleaching agents. In their
study, they used 38% HP and 20% CP bleaching agents. Haywood1 (2000) stated that a 10%
CP does not etch like phosphoric acid. This may be due to the effect of urea in the CP which
raises the hydrogen ion concentration (pH) of the solution.2 In the present study, as the
concentration of CP was increased, the number of the zinc oxide particles was slightly
decreased, but no major changes of zinc phosphate cement were observed.
In this study, the matrix of the cement was slightly eroded. This finding is supported by
Jefferson et al15 (1992) who used EDAX (Energy-Dispersive X-ray) Microanalysis of the zinc
phosphate cement and reported a decrease in aluminum content of zinc phosphate cement
when treated with 10% CP. Since aluminum is confined to the matrix, then the matrix is
eroded by the 10% CP bleaching gel.15 Boston and Jefferies19 (2010) also reported occasional
loss of matrix surrounding the zinc oxide particles when using 36% HP. However, they found
no decrease in hardness or increase in roughness as might be expected.19 They concluded that
their findings do not indicate the need for caution when using a high concentration of (36%)
of HP bleaching agent in the presence of restorations luted with zinc phosphate cement. 19 In
the present study, the matrix was not extensively lost because of the low concentrations of CP
agents used. Therefore, there is no major problem of using 10%, 16% and 22% CP home
bleaching agents in the presence of restorations luted with zinc phosphate cement.
In this study, the porosity of the set cement was evident at few areas. This porosity arises
from the excess water present when the cement is mixed which separates into globules
because it is unable to diffuse out of the cement before has hardened (in about 8 minutes).
Subsequently this excess water diffuses out of the cement leaving pores.
Intraoral conditions like load cycling due to mastication and parafunctional activities, toothbrushing and food abrasion, thermal cycling, exposure to a wide range of dietary liquids,
smoking, oral hygiene products in addition to saliva may have additional effects on the
surface of the zinc phosphate cement that is exposed to CP agents. Further researches are
needed to study the effect of those factors on the integrity of the zinc phosphate cement after
exposure to bleaching agents.
8
Within the limitations of this study, the following conclusions were drawn:
1. The plate-like crystals were evident on the surface of specimens of the control group and
appeared as multiple flat irregular crystals with rounded edges when specimens were
treated with 10%, 16% and 22% CP home bleaching agents.
2. As the concentration of CP bleaching agent is increased, the number of the zinc oxide
particles was decreased and a little more erosion of matrix was observed.
3. There is no major problem of using 10%, 16% and 22% CP home bleaching agents in the
presence of restorations luted with zinc phosphate cement.
ACKNOWLEDGEMENT
The author would like to acknowledge King Saud University, Riyadh, Kingdom of Saudi
Arabia for its financial support of the research as well as the College of Dentistry Research
Center (CDRC) for technical support of the research.
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