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Effect of in-office tooth bleaching on the microhardness of six dental esthetic restorative materials
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 153–158
available at www.sciencedirect.com
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Effect of in-office tooth bleaching on the microhardness of six dental esthetic restorative materials b ¨ ¨ Olga Polydorou a,∗ , Jurgen Schulte Monting , Elmar Hellwig a , Thorsten M. Auschill a a
Department of Operative Dentistry and Periodontology, Dental School and Hospital, Albert-Ludwigs-University, Freiburg, Germany Department of Medical Biometry and Statistics, Institute of Medical Biometry and Statistics, Albert-Ludwigs-University, Freiburg, Germany b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The aim of this in vitro study was to evaluate the effect of the in-office bleaching
Received 25 August 2005
technique on the microhardness of six dental esthetic restorative materials.
Accepted 4 January 2006
Methods. Four composite resins (a hybrid, a flowable, a micro-hybrid and a nano-hybrid), an ormocer and a ceramic were tested, after the use of an in-office bleaching product. Fourteen specimens of each composite and the ormocer were fabricated and randomly divided into
Keywords:
two groups of seven samples each. One group was polished and the other group remained
In-office bleaching
unpolished. For the ceramic, seven polished samples were fabricated. Two samples of each
Microhardness
group were used as negative controls. The specimens were bleached for 15, 30 and 45 min.
Composite resins
Five Knoop microhardness measurements were made on each sample, for each of the follow-
Ceramic
ing periods tested: before bleaching, after 15, 30 and 45 min of bleaching, 24 h and 1 month
Ormocer
after the bleaching procedure. Data were analyzed by the repeated measures analysis of
Dental materials
variance with three between factors and one within. Results. The differences in the microhardness values between the bleached and the control samples for the composites and the ceramic, were not statistically significant (hybrid: p = 0.264; flow: p = 0.584; micro-hybrid: p = 0.278; nano-hybrid: p = 0.405; ceramic: p = 0.819). For the ormocer, although bleaching did not have any significant effect on the unpolished samples (p = 0.115), it caused an increase on microhardness of the polished samples. Significance. Bleaching with 38% hydrogen peroxide does not reduce the microhardness of the restorative materials tested. Therefore, no replacement of restorations is required after bleaching. © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Esthetics, by definition, is the science of beauty, that particular detail of an animate or inanimate object that makes it appealing to the eye [1]. Several factors may alter the appearance of smiles, including alterations in the form, texture, position and color of the teeth [2]. Discolored teeth can be treated
∗
with various restorative techniques, such as direct composite veneers, indirect porcelain veneers, ceramic crowns or even with bleaching. Tooth bleaching has been routinely used since the late 1870s [3]. Bleaching techniques may be classified by whether they involve vital or non-vital teeth or by whether the procedure is performed in-office or has an at-home component. The
Corresponding author. Tel.: +49 761 270 4757; fax: +49 761 270 4762. E-mail address: [email protected] (O. Polydorou). 0109-5641/$ – see front matter © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2006.01.004
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d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 153–158
use of bleaching for improving the esthetics of natural dentition has widened only after the introduction of home bleaching systems in the 1990s [4]. The latter created a resurgence of bleaching, primarily because of its relative ease of application, the lower cost, its general availability to all socio-economic classes of patients, the safety of the materials used and the high percentage of successful treatments [5]. With the home or nightguard vital bleaching technique, the patients apply bleaching solutions, most of which contain 10–15% carbamide peroxide, to their teeth in custom-fitted splints for a few hours per day. Over the past few years, in-office tooth bleaching systems employing the use of strong oxidizing agents have been re-introduced. The advantages are that treatment is totally under the dentist’s control, the soft tissues are generally protected from the process and it has the potential for bleaching quickly in situations in which it is effective [6,7]. Very often in the daily clinical practice, tooth-colored restorations exist in the teeth that are planned to be bleached. The effects of such strong oxidizing agents on the physicomechanical properties of restorative materials have, however, not been widely studied. While strength itself is of great importance in determining a material’s service performance, bulk properties are only part of the story. The surface behavior of a restoration, for example, is relevant to considerations such as its ability to be polished, its retention of that smooth surface, i.e. to withstand scratching, as well as to withstand the stress due to an opposing cusp, and these aspects are not easily modeled by a simple axial loading test. To obtain data about surface hardness, a number of techniques are available.
Hardness is defined as the resistance of a material to indentation [8]. As hardness is related to a material’s strength, proportional limit and its ability to abrade or to be abraded by opposing dental structures/materials [9], any chemical softening resulting from bleaching might have implications on the clinical durability of restorations. Although there are several reports on the effect of home bleaching systems on composites [10–13], little is known about the effects of the in-office bleaching technique on the restorative materials [14,15]. No data exist in the literature on the effects of bleaching systems on micro-hybrid and nano-hybrid composite resins, and ormocers. Because of the plethora of new composite restorative materials which are introduced for esthetic restorations nowadays and the lack of adequate literature data, as long as the effect of the in-office bleaching procedures on the microhardness of the composite materials is concerned, there is need of more research in this field. Therefore, the aim of this in vitro study was to evaluate the effect of the in-office bleaching technique on the microhardness of six dental esthetic restorative materials.
2.
Materials and methods
An in-office bleaching product and six clinically used esthetic restorative materials were selected for this in vitro study. The restorative materials were four composite resins (a hybrid, a flowable, a micro-hybrid and a nano-hybrid), an ormocer and a porcelain, representing the commonly used categories of the
Table 1 – Materials used Material
Type ®
Tetric Ceram
Hybrid composite resin
Tetric Flow®
Flowable composite resin
Enamel Plus HFO
Micro-hybrid composite resin
Filtek® Supreme
Nano-hybrid composite resin
Definite®
Ormocer
Vitablocs® Mark II for Cerec®
Ceramic
Opalescence® Xtra® BoostTM
Chemically activated bleaching agent
a
Manufacturers’ data.
Main compositiona Silinized barium glass, ytterbium trifluoride, Ba–Al-fluorosilicate glass, highly dispersed silicon dioxide, spheroid mixed oxide, bis-GMA, UDMA, TEGDMA, catalysts, stabilizes and pigments Silinized barium glass, ytterbium trifluoride, Ba–Al-fluorosilicate glass, highly dispersed silicon dioxide, spheroid mixed oxide, bis-GMA, UDMA, TEGDMA, catalysts, stabilizes and pigments Glass fillers, highly dispersed silicone dioxide, bis-GMA, diurethandimethacrylate, 1,4-butandioldimethacrylate Combination of aggregated zirkonia/silica cluster filler, bis-GMA, UDMA, TEGDMA and bis-EMA Polymerizable ormocer-matrix, inorganic fillers, initiators, stabilizers and pigments Fine particle feldspar ceramic 38% hydrogen peroxide
Manufacturer
Batch numbers
Ivoclar Vivadent AG, Schaan, Liechtenstein
F09282
Ivoclar Vivadent AG, Schaan, Liechtenstein
F09820
GDF mbH, Rosbach, Germany
2002003428
3M ESPE, Seefeld, Germany
3AG
Denstply DeTrey GmbH, Konstanz, Germany
0212000524
¨ VITA, Bad Sackingen, Germany Ultradent® Products, Inc., S. South Jordan, UT, USA
7627 6428
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Table 2 – Testing periods of the bleaching agent Bleaching agent
Testing periods T1
T0 Opalescence® Xtra® BoostTM (38% H2 O2 )
Before bleaching
T2
After 15 min of bleaching
T3
After 30 min of bleaching
esthetic restorative materials, used today. The materials, the product names and the manufacturers are listed in Table 1. For the preparation of samples, the color A3 or the color corresponding to A3 was used for every material. Transparent thermoforming discs with 2 mm thickness were used for the production of the composite and ormocer samples. Holes of diameter 4.5 mm were drilled into the discs. The discs were positioned on a transparent plastic matrix strip lying on a glass plate. They were then filled with the restorative materials. The samples were built up in one increment (2 mm). After inserting the materials into the discs, a transparent plastic matrix strip was put over them and on top of this a glass slide was placed in order to flatten the surface. Every sample was light cured for 40 s, once, using a halogen curing light (Elipar® Highlight, ESPE 3M, Seefeld, Germany). The porcelain samples were made out of Vitablocs Mark II using the Cerec 3D system (Vita, ¨ Bad Sackingen, Germany). A total of 77 samples, 14 of each of the composite materials and the ormocer and 7 of the porcelain, were fabricated and randomly divided into two groups with seven samples in each. The samples of one group were polished and the other remained unpolished. For the porcelain, only polished samples were prepared. Unpolished samples of porcelain were not tested because of the fact that they are not used in clinical practice. Polishing of the samples was done with medium, fine and superfine Sof-Lex disks (3M ESPE) on a slow-speed
T4
After 45 min of bleaching
T5
24 h after T 3
1 month after T 3
hand-piece in accordance with the manufacturer’s instructions. From each group (n = 7) two samples were randomly selected as negative controls. All samples were stored in distilled water at room temperature for 24 h before any procedure. The bleaching procedure was followed on the top surface of each sample. Bleaching took place for 15, 30 and 45 min. At the end of every bleaching procedure, the treated specimens were washed under running distilled water, using a soft toothbrush and then were placed in fresh distilled water until the next application or until the end of the time period and during the post-bleach 1 month period. The distilled water was renewed every 7 days to minimize the effect of the monomer leaching into the storage medium. For the microhardness measurements, a Knoop microhardness tester (Leitzt Miniload 2, Ernst Leitzt GmbH, Wetzlar, Germany) was used, with a 300 g load for the porcelain samples and a 50 g load for the composite and ormocer samples. The loading time was 30 s for all groups. Five microhardness measurements were obtained on the top surface of each sample, on the following time periods: before bleaching (T 0), after 15 min (T 1), after 30 min (T 2), after 45 min (T 3) of bleaching, 24 h (T 4) and 1 month (T 5) after the end of the bleaching procedure (Table 2). All statistical analyses were carried out at a significance level of 0.05. The “Repeated Measures Analysis of Variance
Table 3 – Mean Knoop hardness values (KHN) and standard deviations (S.D.) for the composite resins, in every testing period, after bleaching with Opalescence® Xtra® BoostTM Material
Polishing Bleaching ®
Tetric Ceram Tetric Ceram® Tetric Ceram® Tetric Ceram® Tetric Flow® Tetric Flow® Tetric Flow® Tetric Flow® Filtek® Supreme Filtek® Supreme Filtek® Supreme Filtek® Supreme Enamel Plus Enamel Plus Enamel Plus Enamel Plus
− −a +b +b −a −a +b +b −a −a +b +b −a −a +b +b a
− +d −c +d −c +d −c +d −c +d −c +d −c +d −c +d c
T0
T1
T2
T3
T4
T5
31.57 (1.64) 29.75 (8.54) 36.26 (4.18) 35.33 (6.15) 26.44 (0.93) 25.01 (1.68) 25.68 (1.29) 24.80 (2.50) 38.18 (8.57) 44.76 (6.16) 40.34 (3.12) 42.98 (4.37) 45.92 (1.28) 46.11 (8.85) 43.57 (0.17) 45.69 (1.53)
35.17 (3.09) 26.22 (5.91) 30.58 (1.62) 31.41 (7.15) 25.69 (0.07) 26.39 (2.53) 24.21 (1.24) 26.16 (2.92) 38.41 (0.63) 40.21 (4.25) 45.10 (6.11) 36.84 (7.01) 37.94 (2.62) 38.71 (6.28) 45.00 (6.67) 36.62 (2.97)
32.97 (5.27) 24.68 (4.01) 34.01 (0.30) 33.81 (8.13) 26.53 (6.72) 25.42 (1.89) 24.59 (0.75) 25.86 (1.67) 42.20 (3.02) 36.86 (3.88) 47.22 (5.35) 39.18 (8.70) 36.54 (2.10) 39.72 (3.03) 46.04 (9.59) 38.10 (2.19)
34.99 (0.27) 27.57 (3.39) 39.16 (0.26) 31.50 (4.51) 23.18 (0.10) 22.63 (1.30) 24.75 (2.55) 22.46 (1.97) 42.16 (1.50) 37.87 (2.66) 46.37 (5.50) 37.41 (7.01) 40.00 (3.22) 39.02 (4.03) 46.92 (0.31) 46.70 (4.44)
31.15 (3.59) 27.26 (4.16) 31.41 (1.41) 32.07 (6.75) 21.92 (2.18) 24.81 (2.69) 21.81 (3.71) 24.67 (0.68) 42.10 (2.44) 38.94 (2.28) 44.72 (5.78) 39.43 (6.94) 37.96 (9.01) 42.35 (3.64) 44.05 (2.26) 39.47 (4.94)
37.63 (4.83) 29.65 (4.85) 29.72 (0.01) 28.96 (7.14) 23.27 (4.31) 24.73 (1.92) 24.58 (0.15) 22.59 (2.74) 40.22 (5.55) 39.81 (2.66) 41.77 (1.20) 35.61 (4.23) 41.68 (3.69) 41.83 (5.75) 43.94 (0.34) 37.73 (6.52)
Bleaching with 38% hydrogen peroxide did not exert any statistically significant effect on microhardness, for all the composite resins tested. Polishing of the composite samples did not have any significant influence on the effect of bleaching on microhardness. a b c d
Without polishing. With polishing. Without bleaching. With bleaching.
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Table 4 – Mean Knoop hardness values (KHN) and standard deviations (S.D.) for the ormocer, in every testing period, after bleaching with Opalescence® Xtra® BoostTM Material
Polishing Bleaching − −a +b +b
− +d −c +d
a
®
Definite Definite® Definite® Definite®
c
T0
T1
T2
T3
T4
T5
47.47 (3.30) 44.22 (4.55) 33.15 (11.16)* 40.69 (6.79)*
41.12 (1.74) 38.71 (7.14) 25.83 (4.37)* 31.86 (4.38)*
37.75 (2.89) 35.00 (3.56) 25.02 (1.97)* 29.27 (4.36)*
36.06 (4.37) 38.63 (7.09) 24.50 (1.75)* 31.89 (3.36)*
37.35 (1.21) 36.66 (4.31) 24.23 (1.91)* 34.38 (5.10)*
41.21 (3.08) 40.23 (2.92) 28.48 (2.47)* 37.96 (4.95)*
The polished bleached samples of the ormocer were found to be statistically significantly harder as compared to the polished unbleached samples, in every time period tested. a b c d ∗
Without polishing. With polishing. Without bleaching. With bleaching. Statistically significant difference between the bleached and unbleached samples, for the same time period (T 0, T 1, T 2, T 3, T 4 and T 5).
with 3 between factors and 1 within factor” was used for the evaluation of the results. The within factor was “Time”.
3.
Results
A total number of 2425 indentations were made, implying the same number of microhardness measurements. Tables 3–5 show the mean Knoop hardness values (KHN) and the standard deviations (S.D.) for each time period tested and for all groups of restorative materials after bleaching with 38% hydrogen peroxide. According to the repeated measures analysis of variance, bleaching with 38% hydrogen peroxide did not produce any statistically significant effect on the microhardness of the composite resins and the ceramic (Tetric Ceram® : p = 0.264; Tetric Flow® : p = 0.584; Enamel Plus: p = 0.278; Filtek® Supreme: p = 0.405; Ceramic: p = 0.819). For all the composite materials, polishing did not exert any significant influence on the effect of bleaching on the microhardness (Tetric Ceram® : p = 0.273; Tetric Flow® : p = 0.945; Enamel Plus: p = 0.278; Filtek® Supreme: p = 0.103). Both polished and unpolished groups of all the composite resins tested (Tetric Ceram® , Tetric Flow® , Enamel Plus and Filtek® Supreme), showed the same behavior during the bleaching procedures. As long as Definite® is concerned, no significant difference (p = 0.115) was found between the bleached unpolished samples and the control unpolished samples. However, the polished samples that were bleached with 38% hydrogen peroxide were found to be statistically significantly harder (p = 0.0135) compared to the polished unbleached samples.
According to the “Univariate Tests of hypotheses for within subject effects”, no statistically significant interaction was found between the duration of the bleaching application and the effect of bleaching on the microhardness, for all the restorative materials tested (Tetric Ceram® : p = 0.854; Tetric Flow® : p = 0.584; Enamel Plus: p = 0.069; Filtek® Supreme: p = 0.634; Definite® : p = 0.807; Ceramic: p = 0.083).
4.
Discussion
The materials used in the present study were stored in distilled water for 24 h in order to allow for post-irradiation hardening of the composites and the ormocer [16,17]. Clinically relevant bleaching regimens that followed manufacturers’ recommendations were adopted for the current research. The bleaching product selected for this study contained 38% hydrogen peroxide and this represents the product with the highest hydrogen peroxide concentration available on the market. It was applied to the surface of the samples for 15, 30 and 45 min, representing the clinical conditions of the in-office bleaching procedure [7]. This was in contrast to several other bleaching studies in which materials were exposed continuously to bleaching products for several days to simulate cumulative effects over a period of time [18,11]. Yap and Wattanapayungkul [19] also used small exposure times (30 min) using light activation simultaneously. The bleaching agent used in this study did not have any significant effect on the microhardness, for all materials tested, except ormocer. This coincides with the results of Yap and Wattanapayungkul [19] who also concluded that the effect of
Table 5 – Mean Knoop hardness values (KHN) and standard deviations (S.D.) for the ceramic, in every testing period, after bleaching with Opalescence® Xtra® BoostTM Material Ceramic Ceramic
Polishing Bleaching a
+ +a
− +c
b
T0
T1
T2
T3
T4
T5
226.96 (14.03) 193.91 (23.92)
189.81 (8.32) 176.57 (35.31)
184.5 (17.23) 177.24 (21.44)
179.14 (17.58) 191.24 (21.07)
163.46 (9.85) 216.25 (27.58)
104.33 (16.84) 93.4 (8.20)
For the ceramic, no statistically significant difference was found in microhardness between the bleached and the control samples, in every time period tested. a b c
With polishing. Without bleaching. With bleaching.
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 153–158
in-office tooth bleaching on hardness was material-dependent and that no significant difference was observed between the control and the bleached groups for all the composite materials tested. Taher [20] reported that in-office bleaching did not have any statistically significant effect on the surface hardness of the ormocer, which is in agreement with the results of this study. In the literature, there are only a few studies concerning the effect of bleaching agents on the microhardness of various esthetic restorative materials, and most of them have tested home bleaching agents. Bailey and Swift [10] reported a reduction in Knoop hardness of the composite resins after home bleaching. Turker and Biskin [12] showed that the microhardness of the composite materials decreased or increased depending on the bleaching agents that were used. Campos et al. [2] found the microhardness of composite materials to be unchanged after home bleaching procedures. Such wide variations in data suggest that some toothcolored restorative materials may be more susceptible to alteration and some bleaching agents are more likely to cause those alterations [21]. The latter may be attributed to the differences in pH values between the bleaching agents [22]. Fortunately, the pH of most current bleaching agents is close to neutral. The pH of the Opalescence® Xtra® BoostTM used in the present study is after mixing and before use, 7.0. The bleaching product used in the present study is a 38% hydrogen peroxide bleaching agent. Hydrogen peroxide can form several different active oxygen species depending on temperature, pH, light, co-catalysts, presence of transitional metals and other conditions [23]. Hydrogen peroxide is an oxidizing agent and has the ability to produce free radicals, HO2 − and O− . The perhydrohyl free radical HO2 − is very reactive. It may break up large macromolecular stains into smaller stain molecules. It is also thought to attach to the molecular stain in the inorganic structure as well as to the protein matrix [3]. The free radicals eventually combine to form molecular oxygen and water. Some aspects of this chemical process might accelerate the hydrolytic degradation of composites described by Soderholm [24]. Chemical softening of composite resins is believed to occur in vivo, contributing to wear of the resin in both stress-bearing and non-stress-bearing areas [25,26]. Softening is caused by chemicals with solubility parameters similar to those of the resin matrix. The bis-GMA resin polymer can be softened by chemicals with solubility parameters in the range of 18.2–29.7 (MPa)1/2 [27]. A wide variety of solvents have solubility parameters within this range [28]. In the present study, no statistically significant difference in microhardness was found between the bleached and the unbleached samples of the ceramic. This is in contrast with the findings of Turker and Biskin [12] who concluded that the bleaching agents that they used decreased the microhardness of feldspatic porcelain. In another study by the same authors, the surface composition of these samples was measured by “Energy Dispersive X-ray Microanalysis”. This revealed reductions in the feldspatic porcelain surface SiO2 content of 4.82 and 4.44% for the Nite-White (16% carbamide peroxide) and Rembrandt (10% carbamide peroxide) bleaching agents, respectively. The SiO2 forms the matrix [9,29] and would thus affect the surface hardness. This small amount of released SiO2 was not considered to be of clinical signifi-
157
Fig. 1 – Mean KNH values of the four groups of Definite® in each time testing period. Definite® (a) samples without polishing and without bleaching, (b) samples without polishing and with bleaching, (c) samples with polishing and without bleaching and (d) samples with polishing and with bleaching. The KNH values of (c) (control group) were statistically significantly lower that those of (d), while between (a), (b) and (d), no statistically significant difference was found. cance. The difference in the bleaching agents used and consequently of their pH may be the reason for the difference in the results. Additionally, it must be mentioned that a reduction in the microhardness of the porcelain samples was observed on both bleached and unbleached samples. The reduction was the same independent of bleaching. Further research is necessary to identify the reason for this change in microhardness. Furthermore, another aim of this study was to evaluate if the effect of bleaching on microhardness was influenced by polishing. For the composite resins, it was found that the effect of bleaching was not influenced by the surface treatment. It is known from the literature that the hardness of the celluloid strip-opposed surfaces of the composite materials is lower than that of the polished surfaces for the first few days after light curing [30]. However, the same study showed that there is no difference in microhardness between polished and unpolished samples, even 6 days after light curing [30]. The effect of polishing procedures on surface hardness appears to be material and technique-dependent [16,31]. For the ormocer, the results of the present study showed that sample polishing has a significant effect on the bleaching and also on microhardness. The polished samples of the ormocer that were bleached were harder compared to the polished unbleached samples. No clinical problem comes out of this result. Higher microhardness values of a material mean nothing but better clinical performance [8,9]. However, it must be mentioned that this increase is caused by the fact that the polished control group had lower KNH values at T 0 and during the whole procedure (Fig. 1). Within the limits of the present study, it can be summarized that no replacement of the tested restorative materials is required after in-office bleaching.
5.
Conclusions
On the basis of the results of this study, it can be concluded that: (1) thirty-eight percent hydrogen peroxide did not cause any significant reduction in microhardness of the six dental esthetic restorative materials;
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(2) the application times used for bleaching in the present study, representing the bleaching periods used for in-office bleaching, were harmless for the restorative materials used; (3) polishing of the composite resins did not have any influence on the effect of bleaching on the microhardness; (4) further research is necessary to explain the effect of polishing on the microhardness of Definite® ; (5) there is no sufficient reason to indicate the replacement of restorations, except the cases that have esthetic involvement.
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¨ [13] Yalcin F, Gurgan S. Effect of two different bleaching regiments on the gloss of tooth colored restorative materials. Dent Mater 2005;21:464–8. [14] Attin T, Hannig C, Wiegand A, Attin R. Effect of bleaching on restorative materials and restorations—a systematic review. Dent Mater 2004;20:852–61. [15] Jung CB, Kim HI, Kim KH, Kwon YH. Influence of 30% hydrogen peroxide bleaching on compomers in their surface modifications and thermal expansion. Dent Mater J 2002;21:396–403. [16] Yap AU, Lye KW, Sau CW. Surface characteristics of tooth-colored restoratives polished utilizing different polishing systems. Oper Dent 1997;22:260–5. [17] Wan AC, Yap AU, Hastings CW. Acid–base complex reactions in resin-modified and hybridglass ionomer cements. J Biomed Mater Res 1999;48:700–4. [18] Monaghan P, Lim E, Lautenschlager E. Effects of home bleaching preparations on composite resin color. J Prosthet Dent 1992;68:575–8. [19] Yap AU, Wattanapayungkul P. Effects of in-office tooth whiteners on hardness of tooth-colored restoratives. Oper Dent 2002;27:137–41. [20] Taher NM. The effect of bleaching agents on the surface hardness of tooth colored restorative materials. J Contemp Dent Pract 2005;6:18–26. [21] Swift Jr EJ, Perdigao J. Effects of bleaching on teeth and restorations. Compend Contin Educ Dent 1998;19: 815–20. [22] Price RB, Sedarous M, Hiltz GS. The pH of tooth-whitening products. J Can Dent Assoc 2000;66:421–6. [23] Feinman RA, Madray G, Yarborough D. Chemical, optical, and physiologic mechanisms of bleaching products: a review. Pract Periodontics Aesthet Dent 1991;3: 32–6. [24] Soderholm KJ. Influence of silane treatment and filler fraction on thermal expansion of composite resins. J Dent Res 1984;63:1321–6. [25] Wu W, Toth EE, Moffa JF, Ellison JA. Subsurface damage layer of in vivo worn dental composite restorations. J Dent Res 1984;63:675–80. [26] van Groeningen G, Jongebloed W, Arends J. Composite degradation in vivo. Dent Mater 1986;2:225–7. [27] Wu W, McKinney JE. Influence of chemicals on wear of dental composites. J Dent Res 1982;61:1180–3. [28] Brandrup J, Immergut E. Polymer handbook. 3rd ed. New York: John Wiley and Sons; 1989. [29] Philips RW. Science of dental materials. Philadelphia, PA: W.B. Saunders Co.; 1982. [30] Park SH, Krejci I, Lutz F. Hardness of celluloid strip-finished or polished composite surfaces with time. J Prosthet Dent 2000;83:660–3. [31] Yap AU, Sau CW, Lye KW. Effects of finishing/polishing time on surface characteristics of tooth-colored restoratives. J Oral Rehabil 1998;25:456–61.
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Effect of in-office tooth bleaching on the microhardness of six dental esthetic restorative materials b ¨ ¨ Olga Polydorou a,∗ , Jurgen Schulte Monting , Elmar Hellwig a , Thorsten M. Auschill a a
Department of Operative Dentistry and Periodontology, Dental School and Hospital, Albert-Ludwigs-University, Freiburg, Germany Department of Medical Biometry and Statistics, Institute of Medical Biometry and Statistics, Albert-Ludwigs-University, Freiburg, Germany b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The aim of this in vitro study was to evaluate the effect of the in-office bleaching
Received 25 August 2005
technique on the microhardness of six dental esthetic restorative materials.
Accepted 4 January 2006
Methods. Four composite resins (a hybrid, a flowable, a micro-hybrid and a nano-hybrid), an ormocer and a ceramic were tested, after the use of an in-office bleaching product. Fourteen specimens of each composite and the ormocer were fabricated and randomly divided into
Keywords:
two groups of seven samples each. One group was polished and the other group remained
In-office bleaching
unpolished. For the ceramic, seven polished samples were fabricated. Two samples of each
Microhardness
group were used as negative controls. The specimens were bleached for 15, 30 and 45 min.
Composite resins
Five Knoop microhardness measurements were made on each sample, for each of the follow-
Ceramic
ing periods tested: before bleaching, after 15, 30 and 45 min of bleaching, 24 h and 1 month
Ormocer
after the bleaching procedure. Data were analyzed by the repeated measures analysis of
Dental materials
variance with three between factors and one within. Results. The differences in the microhardness values between the bleached and the control samples for the composites and the ceramic, were not statistically significant (hybrid: p = 0.264; flow: p = 0.584; micro-hybrid: p = 0.278; nano-hybrid: p = 0.405; ceramic: p = 0.819). For the ormocer, although bleaching did not have any significant effect on the unpolished samples (p = 0.115), it caused an increase on microhardness of the polished samples. Significance. Bleaching with 38% hydrogen peroxide does not reduce the microhardness of the restorative materials tested. Therefore, no replacement of restorations is required after bleaching. © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Esthetics, by definition, is the science of beauty, that particular detail of an animate or inanimate object that makes it appealing to the eye [1]. Several factors may alter the appearance of smiles, including alterations in the form, texture, position and color of the teeth [2]. Discolored teeth can be treated
∗
with various restorative techniques, such as direct composite veneers, indirect porcelain veneers, ceramic crowns or even with bleaching. Tooth bleaching has been routinely used since the late 1870s [3]. Bleaching techniques may be classified by whether they involve vital or non-vital teeth or by whether the procedure is performed in-office or has an at-home component. The
Corresponding author. Tel.: +49 761 270 4757; fax: +49 761 270 4762. E-mail address: [email protected] (O. Polydorou). 0109-5641/$ – see front matter © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2006.01.004
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d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 153–158
use of bleaching for improving the esthetics of natural dentition has widened only after the introduction of home bleaching systems in the 1990s [4]. The latter created a resurgence of bleaching, primarily because of its relative ease of application, the lower cost, its general availability to all socio-economic classes of patients, the safety of the materials used and the high percentage of successful treatments [5]. With the home or nightguard vital bleaching technique, the patients apply bleaching solutions, most of which contain 10–15% carbamide peroxide, to their teeth in custom-fitted splints for a few hours per day. Over the past few years, in-office tooth bleaching systems employing the use of strong oxidizing agents have been re-introduced. The advantages are that treatment is totally under the dentist’s control, the soft tissues are generally protected from the process and it has the potential for bleaching quickly in situations in which it is effective [6,7]. Very often in the daily clinical practice, tooth-colored restorations exist in the teeth that are planned to be bleached. The effects of such strong oxidizing agents on the physicomechanical properties of restorative materials have, however, not been widely studied. While strength itself is of great importance in determining a material’s service performance, bulk properties are only part of the story. The surface behavior of a restoration, for example, is relevant to considerations such as its ability to be polished, its retention of that smooth surface, i.e. to withstand scratching, as well as to withstand the stress due to an opposing cusp, and these aspects are not easily modeled by a simple axial loading test. To obtain data about surface hardness, a number of techniques are available.
Hardness is defined as the resistance of a material to indentation [8]. As hardness is related to a material’s strength, proportional limit and its ability to abrade or to be abraded by opposing dental structures/materials [9], any chemical softening resulting from bleaching might have implications on the clinical durability of restorations. Although there are several reports on the effect of home bleaching systems on composites [10–13], little is known about the effects of the in-office bleaching technique on the restorative materials [14,15]. No data exist in the literature on the effects of bleaching systems on micro-hybrid and nano-hybrid composite resins, and ormocers. Because of the plethora of new composite restorative materials which are introduced for esthetic restorations nowadays and the lack of adequate literature data, as long as the effect of the in-office bleaching procedures on the microhardness of the composite materials is concerned, there is need of more research in this field. Therefore, the aim of this in vitro study was to evaluate the effect of the in-office bleaching technique on the microhardness of six dental esthetic restorative materials.
2.
Materials and methods
An in-office bleaching product and six clinically used esthetic restorative materials were selected for this in vitro study. The restorative materials were four composite resins (a hybrid, a flowable, a micro-hybrid and a nano-hybrid), an ormocer and a porcelain, representing the commonly used categories of the
Table 1 – Materials used Material
Type ®
Tetric Ceram
Hybrid composite resin
Tetric Flow®
Flowable composite resin
Enamel Plus HFO
Micro-hybrid composite resin
Filtek® Supreme
Nano-hybrid composite resin
Definite®
Ormocer
Vitablocs® Mark II for Cerec®
Ceramic
Opalescence® Xtra® BoostTM
Chemically activated bleaching agent
a
Manufacturers’ data.
Main compositiona Silinized barium glass, ytterbium trifluoride, Ba–Al-fluorosilicate glass, highly dispersed silicon dioxide, spheroid mixed oxide, bis-GMA, UDMA, TEGDMA, catalysts, stabilizes and pigments Silinized barium glass, ytterbium trifluoride, Ba–Al-fluorosilicate glass, highly dispersed silicon dioxide, spheroid mixed oxide, bis-GMA, UDMA, TEGDMA, catalysts, stabilizes and pigments Glass fillers, highly dispersed silicone dioxide, bis-GMA, diurethandimethacrylate, 1,4-butandioldimethacrylate Combination of aggregated zirkonia/silica cluster filler, bis-GMA, UDMA, TEGDMA and bis-EMA Polymerizable ormocer-matrix, inorganic fillers, initiators, stabilizers and pigments Fine particle feldspar ceramic 38% hydrogen peroxide
Manufacturer
Batch numbers
Ivoclar Vivadent AG, Schaan, Liechtenstein
F09282
Ivoclar Vivadent AG, Schaan, Liechtenstein
F09820
GDF mbH, Rosbach, Germany
2002003428
3M ESPE, Seefeld, Germany
3AG
Denstply DeTrey GmbH, Konstanz, Germany
0212000524
¨ VITA, Bad Sackingen, Germany Ultradent® Products, Inc., S. South Jordan, UT, USA
7627 6428
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d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 153–158
Table 2 – Testing periods of the bleaching agent Bleaching agent
Testing periods T1
T0 Opalescence® Xtra® BoostTM (38% H2 O2 )
Before bleaching
T2
After 15 min of bleaching
T3
After 30 min of bleaching
esthetic restorative materials, used today. The materials, the product names and the manufacturers are listed in Table 1. For the preparation of samples, the color A3 or the color corresponding to A3 was used for every material. Transparent thermoforming discs with 2 mm thickness were used for the production of the composite and ormocer samples. Holes of diameter 4.5 mm were drilled into the discs. The discs were positioned on a transparent plastic matrix strip lying on a glass plate. They were then filled with the restorative materials. The samples were built up in one increment (2 mm). After inserting the materials into the discs, a transparent plastic matrix strip was put over them and on top of this a glass slide was placed in order to flatten the surface. Every sample was light cured for 40 s, once, using a halogen curing light (Elipar® Highlight, ESPE 3M, Seefeld, Germany). The porcelain samples were made out of Vitablocs Mark II using the Cerec 3D system (Vita, ¨ Bad Sackingen, Germany). A total of 77 samples, 14 of each of the composite materials and the ormocer and 7 of the porcelain, were fabricated and randomly divided into two groups with seven samples in each. The samples of one group were polished and the other remained unpolished. For the porcelain, only polished samples were prepared. Unpolished samples of porcelain were not tested because of the fact that they are not used in clinical practice. Polishing of the samples was done with medium, fine and superfine Sof-Lex disks (3M ESPE) on a slow-speed
T4
After 45 min of bleaching
T5
24 h after T 3
1 month after T 3
hand-piece in accordance with the manufacturer’s instructions. From each group (n = 7) two samples were randomly selected as negative controls. All samples were stored in distilled water at room temperature for 24 h before any procedure. The bleaching procedure was followed on the top surface of each sample. Bleaching took place for 15, 30 and 45 min. At the end of every bleaching procedure, the treated specimens were washed under running distilled water, using a soft toothbrush and then were placed in fresh distilled water until the next application or until the end of the time period and during the post-bleach 1 month period. The distilled water was renewed every 7 days to minimize the effect of the monomer leaching into the storage medium. For the microhardness measurements, a Knoop microhardness tester (Leitzt Miniload 2, Ernst Leitzt GmbH, Wetzlar, Germany) was used, with a 300 g load for the porcelain samples and a 50 g load for the composite and ormocer samples. The loading time was 30 s for all groups. Five microhardness measurements were obtained on the top surface of each sample, on the following time periods: before bleaching (T 0), after 15 min (T 1), after 30 min (T 2), after 45 min (T 3) of bleaching, 24 h (T 4) and 1 month (T 5) after the end of the bleaching procedure (Table 2). All statistical analyses were carried out at a significance level of 0.05. The “Repeated Measures Analysis of Variance
Table 3 – Mean Knoop hardness values (KHN) and standard deviations (S.D.) for the composite resins, in every testing period, after bleaching with Opalescence® Xtra® BoostTM Material
Polishing Bleaching ®
Tetric Ceram Tetric Ceram® Tetric Ceram® Tetric Ceram® Tetric Flow® Tetric Flow® Tetric Flow® Tetric Flow® Filtek® Supreme Filtek® Supreme Filtek® Supreme Filtek® Supreme Enamel Plus Enamel Plus Enamel Plus Enamel Plus
− −a +b +b −a −a +b +b −a −a +b +b −a −a +b +b a
− +d −c +d −c +d −c +d −c +d −c +d −c +d −c +d c
T0
T1
T2
T3
T4
T5
31.57 (1.64) 29.75 (8.54) 36.26 (4.18) 35.33 (6.15) 26.44 (0.93) 25.01 (1.68) 25.68 (1.29) 24.80 (2.50) 38.18 (8.57) 44.76 (6.16) 40.34 (3.12) 42.98 (4.37) 45.92 (1.28) 46.11 (8.85) 43.57 (0.17) 45.69 (1.53)
35.17 (3.09) 26.22 (5.91) 30.58 (1.62) 31.41 (7.15) 25.69 (0.07) 26.39 (2.53) 24.21 (1.24) 26.16 (2.92) 38.41 (0.63) 40.21 (4.25) 45.10 (6.11) 36.84 (7.01) 37.94 (2.62) 38.71 (6.28) 45.00 (6.67) 36.62 (2.97)
32.97 (5.27) 24.68 (4.01) 34.01 (0.30) 33.81 (8.13) 26.53 (6.72) 25.42 (1.89) 24.59 (0.75) 25.86 (1.67) 42.20 (3.02) 36.86 (3.88) 47.22 (5.35) 39.18 (8.70) 36.54 (2.10) 39.72 (3.03) 46.04 (9.59) 38.10 (2.19)
34.99 (0.27) 27.57 (3.39) 39.16 (0.26) 31.50 (4.51) 23.18 (0.10) 22.63 (1.30) 24.75 (2.55) 22.46 (1.97) 42.16 (1.50) 37.87 (2.66) 46.37 (5.50) 37.41 (7.01) 40.00 (3.22) 39.02 (4.03) 46.92 (0.31) 46.70 (4.44)
31.15 (3.59) 27.26 (4.16) 31.41 (1.41) 32.07 (6.75) 21.92 (2.18) 24.81 (2.69) 21.81 (3.71) 24.67 (0.68) 42.10 (2.44) 38.94 (2.28) 44.72 (5.78) 39.43 (6.94) 37.96 (9.01) 42.35 (3.64) 44.05 (2.26) 39.47 (4.94)
37.63 (4.83) 29.65 (4.85) 29.72 (0.01) 28.96 (7.14) 23.27 (4.31) 24.73 (1.92) 24.58 (0.15) 22.59 (2.74) 40.22 (5.55) 39.81 (2.66) 41.77 (1.20) 35.61 (4.23) 41.68 (3.69) 41.83 (5.75) 43.94 (0.34) 37.73 (6.52)
Bleaching with 38% hydrogen peroxide did not exert any statistically significant effect on microhardness, for all the composite resins tested. Polishing of the composite samples did not have any significant influence on the effect of bleaching on microhardness. a b c d
Without polishing. With polishing. Without bleaching. With bleaching.
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d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 153–158
Table 4 – Mean Knoop hardness values (KHN) and standard deviations (S.D.) for the ormocer, in every testing period, after bleaching with Opalescence® Xtra® BoostTM Material
Polishing Bleaching − −a +b +b
− +d −c +d
a
®
Definite Definite® Definite® Definite®
c
T0
T1
T2
T3
T4
T5
47.47 (3.30) 44.22 (4.55) 33.15 (11.16)* 40.69 (6.79)*
41.12 (1.74) 38.71 (7.14) 25.83 (4.37)* 31.86 (4.38)*
37.75 (2.89) 35.00 (3.56) 25.02 (1.97)* 29.27 (4.36)*
36.06 (4.37) 38.63 (7.09) 24.50 (1.75)* 31.89 (3.36)*
37.35 (1.21) 36.66 (4.31) 24.23 (1.91)* 34.38 (5.10)*
41.21 (3.08) 40.23 (2.92) 28.48 (2.47)* 37.96 (4.95)*
The polished bleached samples of the ormocer were found to be statistically significantly harder as compared to the polished unbleached samples, in every time period tested. a b c d ∗
Without polishing. With polishing. Without bleaching. With bleaching. Statistically significant difference between the bleached and unbleached samples, for the same time period (T 0, T 1, T 2, T 3, T 4 and T 5).
with 3 between factors and 1 within factor” was used for the evaluation of the results. The within factor was “Time”.
3.
Results
A total number of 2425 indentations were made, implying the same number of microhardness measurements. Tables 3–5 show the mean Knoop hardness values (KHN) and the standard deviations (S.D.) for each time period tested and for all groups of restorative materials after bleaching with 38% hydrogen peroxide. According to the repeated measures analysis of variance, bleaching with 38% hydrogen peroxide did not produce any statistically significant effect on the microhardness of the composite resins and the ceramic (Tetric Ceram® : p = 0.264; Tetric Flow® : p = 0.584; Enamel Plus: p = 0.278; Filtek® Supreme: p = 0.405; Ceramic: p = 0.819). For all the composite materials, polishing did not exert any significant influence on the effect of bleaching on the microhardness (Tetric Ceram® : p = 0.273; Tetric Flow® : p = 0.945; Enamel Plus: p = 0.278; Filtek® Supreme: p = 0.103). Both polished and unpolished groups of all the composite resins tested (Tetric Ceram® , Tetric Flow® , Enamel Plus and Filtek® Supreme), showed the same behavior during the bleaching procedures. As long as Definite® is concerned, no significant difference (p = 0.115) was found between the bleached unpolished samples and the control unpolished samples. However, the polished samples that were bleached with 38% hydrogen peroxide were found to be statistically significantly harder (p = 0.0135) compared to the polished unbleached samples.
According to the “Univariate Tests of hypotheses for within subject effects”, no statistically significant interaction was found between the duration of the bleaching application and the effect of bleaching on the microhardness, for all the restorative materials tested (Tetric Ceram® : p = 0.854; Tetric Flow® : p = 0.584; Enamel Plus: p = 0.069; Filtek® Supreme: p = 0.634; Definite® : p = 0.807; Ceramic: p = 0.083).
4.
Discussion
The materials used in the present study were stored in distilled water for 24 h in order to allow for post-irradiation hardening of the composites and the ormocer [16,17]. Clinically relevant bleaching regimens that followed manufacturers’ recommendations were adopted for the current research. The bleaching product selected for this study contained 38% hydrogen peroxide and this represents the product with the highest hydrogen peroxide concentration available on the market. It was applied to the surface of the samples for 15, 30 and 45 min, representing the clinical conditions of the in-office bleaching procedure [7]. This was in contrast to several other bleaching studies in which materials were exposed continuously to bleaching products for several days to simulate cumulative effects over a period of time [18,11]. Yap and Wattanapayungkul [19] also used small exposure times (30 min) using light activation simultaneously. The bleaching agent used in this study did not have any significant effect on the microhardness, for all materials tested, except ormocer. This coincides with the results of Yap and Wattanapayungkul [19] who also concluded that the effect of
Table 5 – Mean Knoop hardness values (KHN) and standard deviations (S.D.) for the ceramic, in every testing period, after bleaching with Opalescence® Xtra® BoostTM Material Ceramic Ceramic
Polishing Bleaching a
+ +a
− +c
b
T0
T1
T2
T3
T4
T5
226.96 (14.03) 193.91 (23.92)
189.81 (8.32) 176.57 (35.31)
184.5 (17.23) 177.24 (21.44)
179.14 (17.58) 191.24 (21.07)
163.46 (9.85) 216.25 (27.58)
104.33 (16.84) 93.4 (8.20)
For the ceramic, no statistically significant difference was found in microhardness between the bleached and the control samples, in every time period tested. a b c
With polishing. Without bleaching. With bleaching.
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 153–158
in-office tooth bleaching on hardness was material-dependent and that no significant difference was observed between the control and the bleached groups for all the composite materials tested. Taher [20] reported that in-office bleaching did not have any statistically significant effect on the surface hardness of the ormocer, which is in agreement with the results of this study. In the literature, there are only a few studies concerning the effect of bleaching agents on the microhardness of various esthetic restorative materials, and most of them have tested home bleaching agents. Bailey and Swift [10] reported a reduction in Knoop hardness of the composite resins after home bleaching. Turker and Biskin [12] showed that the microhardness of the composite materials decreased or increased depending on the bleaching agents that were used. Campos et al. [2] found the microhardness of composite materials to be unchanged after home bleaching procedures. Such wide variations in data suggest that some toothcolored restorative materials may be more susceptible to alteration and some bleaching agents are more likely to cause those alterations [21]. The latter may be attributed to the differences in pH values between the bleaching agents [22]. Fortunately, the pH of most current bleaching agents is close to neutral. The pH of the Opalescence® Xtra® BoostTM used in the present study is after mixing and before use, 7.0. The bleaching product used in the present study is a 38% hydrogen peroxide bleaching agent. Hydrogen peroxide can form several different active oxygen species depending on temperature, pH, light, co-catalysts, presence of transitional metals and other conditions [23]. Hydrogen peroxide is an oxidizing agent and has the ability to produce free radicals, HO2 − and O− . The perhydrohyl free radical HO2 − is very reactive. It may break up large macromolecular stains into smaller stain molecules. It is also thought to attach to the molecular stain in the inorganic structure as well as to the protein matrix [3]. The free radicals eventually combine to form molecular oxygen and water. Some aspects of this chemical process might accelerate the hydrolytic degradation of composites described by Soderholm [24]. Chemical softening of composite resins is believed to occur in vivo, contributing to wear of the resin in both stress-bearing and non-stress-bearing areas [25,26]. Softening is caused by chemicals with solubility parameters similar to those of the resin matrix. The bis-GMA resin polymer can be softened by chemicals with solubility parameters in the range of 18.2–29.7 (MPa)1/2 [27]. A wide variety of solvents have solubility parameters within this range [28]. In the present study, no statistically significant difference in microhardness was found between the bleached and the unbleached samples of the ceramic. This is in contrast with the findings of Turker and Biskin [12] who concluded that the bleaching agents that they used decreased the microhardness of feldspatic porcelain. In another study by the same authors, the surface composition of these samples was measured by “Energy Dispersive X-ray Microanalysis”. This revealed reductions in the feldspatic porcelain surface SiO2 content of 4.82 and 4.44% for the Nite-White (16% carbamide peroxide) and Rembrandt (10% carbamide peroxide) bleaching agents, respectively. The SiO2 forms the matrix [9,29] and would thus affect the surface hardness. This small amount of released SiO2 was not considered to be of clinical signifi-
157
Fig. 1 – Mean KNH values of the four groups of Definite® in each time testing period. Definite® (a) samples without polishing and without bleaching, (b) samples without polishing and with bleaching, (c) samples with polishing and without bleaching and (d) samples with polishing and with bleaching. The KNH values of (c) (control group) were statistically significantly lower that those of (d), while between (a), (b) and (d), no statistically significant difference was found. cance. The difference in the bleaching agents used and consequently of their pH may be the reason for the difference in the results. Additionally, it must be mentioned that a reduction in the microhardness of the porcelain samples was observed on both bleached and unbleached samples. The reduction was the same independent of bleaching. Further research is necessary to identify the reason for this change in microhardness. Furthermore, another aim of this study was to evaluate if the effect of bleaching on microhardness was influenced by polishing. For the composite resins, it was found that the effect of bleaching was not influenced by the surface treatment. It is known from the literature that the hardness of the celluloid strip-opposed surfaces of the composite materials is lower than that of the polished surfaces for the first few days after light curing [30]. However, the same study showed that there is no difference in microhardness between polished and unpolished samples, even 6 days after light curing [30]. The effect of polishing procedures on surface hardness appears to be material and technique-dependent [16,31]. For the ormocer, the results of the present study showed that sample polishing has a significant effect on the bleaching and also on microhardness. The polished samples of the ormocer that were bleached were harder compared to the polished unbleached samples. No clinical problem comes out of this result. Higher microhardness values of a material mean nothing but better clinical performance [8,9]. However, it must be mentioned that this increase is caused by the fact that the polished control group had lower KNH values at T 0 and during the whole procedure (Fig. 1). Within the limits of the present study, it can be summarized that no replacement of the tested restorative materials is required after in-office bleaching.
5.
Conclusions
On the basis of the results of this study, it can be concluded that: (1) thirty-eight percent hydrogen peroxide did not cause any significant reduction in microhardness of the six dental esthetic restorative materials;
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(2) the application times used for bleaching in the present study, representing the bleaching periods used for in-office bleaching, were harmless for the restorative materials used; (3) polishing of the composite resins did not have any influence on the effect of bleaching on the microhardness; (4) further research is necessary to explain the effect of polishing on the microhardness of Definite® ; (5) there is no sufficient reason to indicate the replacement of restorations, except the cases that have esthetic involvement.
references
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