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Effect of bleaching on subsurface micro-hardness of composite and a polyacid modified composite
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 198–203
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Effect of bleaching on subsurface micro-hardness of composite and a polyacid modified composite Christian Hannig a,b,∗ , Sebastian Duong b , Klaus Becker b,c , Edgar Brunner d , Elke Kahler d , Thomas Attin b,c a
Department of Operative Dentistry and Periodontology, University of Freiburg, Hugstetter St. 55, D-79106 Freiburg, Germany ¨ Department of Operative Dentistry, Preventive Dentistry and Periodontology, University of Gottingen, ¨ Robert-Koch-Str. 40, 37075 Gottingen, Germany c Clinic for Preventive Dentistry, Periodontology and Cariology, University of Zurich, ¨ ¨ Plattenstr. 11, 8032 Zurich, Switzerland d Department of Medical Statistics, University of Gottingen, ¨ Humboldtallee 32, 37075 Freiburg, Germany b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To investigate the influence of different bleaching techniques on subsurface phys-
Received 5 September 2005
ical properties of composite and polyacid modified composite tested via determination of
Accepted 10 January 2006
micro-hardness. Methods. Specimens of Tetric Flow, Tetric EvoCeram and Compoglass were light cured (2.5 mm thickness) and stored in artificial saliva for 2 weeks (n = 12/group). The samples
Keywords:
were only removed for application of the following bleaching agents in a humid atmosphere:
Composite
Either Vivastyle (1 h/d), Whitestrips (30 min/d), sodium-perborate–water mixture (once for
Bleaching
72 h), Simply White (1 h/d), or Opalescence XtraBoost (1st and 5th day for 15 min) were
Micro-hardness
applied on the surfaces of the samples. Untreated specimens served as negative controls,
Restoration
samples treated with ethyl alcohol for 1 h acted as positive controls. After the bleaching
Polyacid modified resin composite
period, samples were cross-sectioned and the micro-hardness (Knoop) of different subsurface levels (0.1 mm–2.0 mm) was determined. Results. All bleaching techniques significantly reduced the Knoop-hardness of the restoratives compared to untreated controls. Thereby, bleaching significantly affected not only superficial but also the deep layers of the specimens: in superficial layers (0.1 mm, 0.2 mm) lowest micro-hardness values amounted to 69.5% and 76.3% of the respective untreated controls (Compoglass/Vivastyle). In deeper subsurface levels, the lowest hardness was observed with Opalescence/Tetric EvoCeram (0.3 mm: 78.3%; 0.4 mm: 80%; 0.5 mm: 80.5%; 1.0 mm: 84.2%; 2.0 mm: 84.4%). Significance. Bleaching with the tested bleaching agents softens the adhesive restorative materials examined. Due to the fact that subsurface layers are also affected, polishing of the surface may not suffice for re-establishing the physical properties of the surface of the fillings. © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗ Correspondence to: Department of Operative Dentistry and Periodontology, University of Freiburg, Hugstetter St. 55, D-79106 Freiburg, Germany. Tel.: +49 761 270 4957; fax: +49 761 270 4762. E-mail address: [email protected] (C. Hannig). 0109-5641/$ – see front matter © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2006.01.008
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 198–203
1.
199
Introduction
There is an increasing demand for home bleaching in western societies for achieving a whiter smile. Besides custom made conventional tray systems based on individually crafted trays, products such as Whitestrips [1–3] or paint-on-bleaching regimens for the purpose of home bleaching are adopted [4]. In this context, it has to be regarded that many of these newer products are available as over-the-counter-products. Unintended application of the bleaching products on existing restorations by the patients cannot be excluded, especially if bleaching is not performed and monitored by a dentist. Thereby it has to be regarded that the prognosis and longevity of restorations depends not only on the mechanical, but also the physical properties of the filling material [5]. Effect of bleaching on dental restorative materials in general has been reviewed recently [6]. Due to their organic matrix, composite resin materials especially are more prone to chemical alteration compared to inert metal or ceramic restorations [7–12]. However, impact of bleaching on surface micro-hardness of composites is described controversially in the literature. Decreases as well as increases in surface micro-hardness induced by bleaching have been found, whereas other studies revealed no significant alteration in the micro-hardness [13–18]. Scanning electron microscopic studies and profilometric investigations indicated an increase in surface roughness and porosities on the composite surfaces after incubation with bleaching materials [13,14,19]. With polyacid modified resin based composites (compomers), surface degradation and softening have also been observed after bleaching regimens [20,21]. Polishing of the composite restorations after bleaching therapy is recommended in order to remove the altered and softened surface structures [5,6]. Generally, the subsurface deterioration of composites induced by peroxides or other chemical compounds in bleaching agents seems conceivable, due to their ability to diffuse and their high chemical reactivity [22–24]. However, it has not been investigated to what depth softening of the materials occurs, so that it is uncertain if polishing of the surface suffices to remove the softened layers. The aim of the present study was, therefore, to evaluate subsurface effects of bleaching agents on current composite filling materials by the evaluation of micro-hardness at different levels in composite specimens treated with different bleaching systems.
Fig. 1 – Set-up of the mold and the lantern slide during fabrication of the specimens.
For fabrication of the specimens, molds (diameter: 7 mm, height: 2.5 mm) with persistent holes were prepared from Palavit G (Heraeus Kulzer, Hanau, Germany). Each mold was placed on a special lantern slide (Schott, Mainz, Germany). The lantern slide (1.0 mm thickness) is permeable to light of wavelength between 380 nm and 2400 nm to a degree of 90%. Composites were filled in the cavities of the molds against the lantern slide and light polymerized for 20 s (Spektrum 800, DeTrey/Dentsply, Konstanz, Germany) through the lantern slide at a standardized distance of 1 mm (Fig. 1). Following this, the lantern slide was removed and the specimens light cured for another 40 s. Light intensity of the polymerization device was checked with an integrated photometer after six polymerization steps showing that intensity was constant at a level of 550 mW/cm2 . The top surface of the composite samples was wet ground and polished up to grain size 4000. About 0.1 mm was removed during the grinding procedure. Bleaching agents were applied to the polished surfaces only after storage in artificial saliva. The number of composite specimens was n = 252, each experimental group contained 12 specimens of the respective filling material.
2.2.
Application of bleaching techniques
Samples were stored in artificial saliva (pH 6.8) according to Schemehorn et al. [25] for 2 weeks and removed for the application of the bleaching systems only. Adoption of bleaching techniques was performed according to manufacturers’ instructions. In order to simulate clinical procedure, bleaching was performed in a wet chamber (37 ◦ C). Remnants of bleaching gel were removed with running water after each single application. The following bleaching agents were applied:
2.
Materials and methods
2.2.1. Vivastyle (Vivadent, Liechtenstein, # GL1020), 10% carbamide peroxide)
2.1.
Sample preparation
This gel usually used for charging individually crafted bleaching trays was adopted for 1 h per day for 14 days. During each application, 0.1 ml was applied per specimen.
Three different adhesive filling materials were used in the study: a composite resin (Tetric EvoCeram, Ivoclar/Vivadent, Schaan, Liechtenstein # F38345), a flowable composite (Tetric Flow, Ivoclar/Vivadent, Schaan, Liechtenstein, # GM1097) and a polyacid modified resin composite (Compoglass, Ivoclar/Vivadent, Schaan, Liechtenstein, # G11795).
2.2.2.
Sodium perborate (Merck, Germany)
This peroxide releasing reagent, usually used for internal bleaching was mixed with distilled water (2 g/ml) and applied to the specimens for 72 h in a wet chamber. Until determina-
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tion of micro-hardness, samples were stored in artificial saliva for 11 days. During the application, 0.1 mg was applied per specimen.
2.2.3. H2 O2
Table 1 – Knoop-micro-hardness of untreated controls in the respective subsurface levels of the materials Compoglass, Tetric EvoCeram and Tetric Flow (MV (S.D.)) Depth (mm)
Opalescence XtraBoost (Ultradent, # 68R3), 38%
Compoglass
This in-office-bleaching product was adopted twice (1st and 5th day) for 15 min each. During each application, 0.1 ml was applied per specimen.
0.1 0.2 0.3 0.4 0.5 1.0 2.0
2.2.4. Simply White (Colgate, # 3327AM), 5.9% H2 O2 (Lot #3327AM) This paint on bleaching product was applied 1 h a day for 14 days. Each of the specimens’ surfaces was charged with 0.1 ml gel.
2.2.5.
Whitestrips (Blend-a-med, # 3319BT2B), 6.5% H2 O2
Strips were adopted twice a day for 30 min each for 14 days.
2.2.6.
Positive control: ethyl alcohol (96%)
Ethanol was applied for 1 h (0.1 ml per sample).
2.2.7.
Negative control
Non contaminated specimens served as controls.
2.3.
Determination of Knoop-hardness
The molds containing the composite samples were cross¨ sectioned with a water-cooled saw (Exakt, Neumunster, Germany) in order to achieve coplanar centrepieces for determination of micro-hardness (Knoop). Micro-hardness was determined at the following distances from the light cured, polished and bleached surface: 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, 1.0 mm, 2.0 mm. With each specimen, three measurements were performed at each level. Determination of Knoop-hardness was performed according to DIN standard 52333 (ASTM D 1470). The diamond was loaded with 2 N [26].
2.4.
Knoop-micro-hardness of controls
Statistics
Statistical evaluation was performed for selected levels (0.1 mm, 0.3 mm and 1.0 mm).
Tetric EvoCeram
64.3 (2.5) 63.0 (4.8) 60.5 (3.4) 60.3 (2.9) 57.5 (3.9) 56.9 (3.9) 55.4 (3.2)
68.4 (3.9) 64.8 (4.4) 64.7 (5.1) 63.2 (5.5) 62.9 (3.8) 61.0 (4.1) 60.2 (4.2)
Tetric Flow 49.7 (4.4) 47.5 (3.8) 47.2 (3.6) 47.9 (3.0) 47.6 (3.2) 49.2 (3.7) 48.3 (2.5)
n: 36 measurements per depth and material.
The statistical design underlying the measurements of the Knoop-hardness in the present trial was a repeated measures design, i.e. the same samples are repeatedly observed at several depths. The impact of depth and bleaching materials on the hardness of the probes was examined by parametric repeated measures ANOVA for each of the three restorative materials separately, analogous to the methods described in [27]. If any hypothesis was rejected in the ANOVA analysis (the level of significance was set at 5%), subsequent post-hoc analyses were performed by using the Bonferroni–Holm correction as an appropriate adjustment of the significance level [28]. Statistics were calculated using the software package SAS 9.1 (SAS Institute Inc., Cary, NC, USA) [27].
3.
Results
All bleaching systems affected Knoop-hardness of the filling materials at all the levels evaluated. Absolute values of micro-hardness are given for controls in Table 1. It is noteworthy that all composites featured a different Knoop-hardness in the controls. Lowest values were observed with the flow-material. The highly filled composite Tetric EvoCeram and the polyacid modified resin composite Compoglass displayed comparable micro-hardness-values. As expected, in deeper layers, micro-hardness was lower compared to the outer surface which was nearest to the polymerization device. Due to the differing micro-hardness at certain
Table 2 – Mean micro-hardness (%) of Tetric EvoCeram in the subsurface levels after treatment with different bleaching agents and ethyl alcohol as related to hardness of controls in the respective levels Micro-hardness (percentage of controls)
Contaminated with Sodium perborate Whitestrips Vivastyle Simply White Opalescence Xtra Boost Ethyl alcohol
0.1a
0.2a
0.3a
0.4a
0.5a
1.0a
2.0a
73.7 (3.8) 75.5 (5.2) 80.6 (6.2) 73.6 (7.9) 72.9 (4.4) 73.8 (9.1)
81.9 (5.3) 83.3 (4.1) 84.1 (7.0) 79.2 (5.7) 77.7 (3.4) 81.8 (4.3)
85.3 (7.1) 84.0 (4.6) 85.9 (7.0) 79.4 (6.4) 78.3 (3.9) 82.2 (8.8)
89.5 (7.8) 87.2 (7.4) 89.8 (7.2) 83.3 (4.2) 80.0 (4.3) 83.1 (10.6)
88.3 (5.6) 88.3 (5.2) 90.8 (7.5) 82.5 (4.8) 80.5 (4.3) 81.5 (5.7)
88.6 (8.5) 90.5 (6.4) 93.7 (9.1) 86.1 (7.4) 84.2 (5.3) 83.6 (7.5)
90.5 (5.1) 91.3 (4.5) 93.7 (6.4) 85.1 (5.5) 84.4 (4.3) 85.8 (6.0)
n: 36 measurements per depth and material (MV (S.D.)). a
Depth (mm).
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Table 3 – Mean micro-hardness (%) of Tetric Flow in the subsurface levels after treatment with different bleaching agents and ethyl alcohol as related to hardness of controls in the respective levels Micro-hardness (percentage of controls)
Contaminated with Sodium perborate Whitestrips Vivastyle Simply White Opalescence Xtra Boost Ethyl alcohol
0.1a
0.2a
0.3a
0.4a
0.5a
1.0a
2.0a
79.2 (7.1) 87.2 (5.1) 83.9 (6.0) 85.3 (5.0) 84.6 (4.8) 74.7 (4.0)
85.1 (8.7) 94.3 (3.8) 91.7 (6.0) 92.7 (3.6) 91.1 (4.8) 79.3 (4.8)
84.7 (7.5) 97.4 (3.5) 95.4 (5.6) 95.5 (4.2) 91.7 (2.5) 82.8 (5.4)
84.8 (6.2) 96.8 (4.1) 94.1 (6.4) 94.7 (6.2) 92.5 (2.9) 82.4 (5.6)
86.4 (7.1) 98.6 (6.0) 96.4 (5.4) 96.6 (6.5) 94.5 (6.0) 82.7 (5.9)
83.5 (8.5) 99.9 (4.3) 94.0 (7.6) 94.2 (5.2) 91.1 (5.4) 81.9 (6.2)
87.6 (8.0) 99.8 (4.1) 93.6 (6.9) 93.0 (4.2) 93.7 (4.2) 81.1 (7.9)
n: 36 measurements per depth and material (MV (S.D.)). a
Depth (mm).
Table 4 – Mean micro-hardness (%) of Compoglass in the subsurface levels after treatment with different bleaching agents and ethyl alcohol as related to hardness of controls in the respective levels Micro-hardness (percentage of controls)
Contaminated with Sodium perborate Whitestrips Vivastyle Simply White Opalescence Xtra Boost Ethyl alcohol
0.1a
0.2a
0.3a
0.4a
0.5a
1.0a
2.0a
77.8 (12.8) 75.0 (3.7) 69.5 (8.3) 73.4 (5.5) 72.0 (11.7) 71.8 (7.6)
83.7 (5.3) 80.1 (5.0) 76.2 (6.5) 79.4 (9.0) 78.9 (11.1) 74.4 (4.9)
92.5 (10.7) 86.6 (4.8) 85.4 (5.0) 83.7 (5.3) 83.0 (9.0) 79.4 (8.3)
97.4 (10.1) 89.2 (4.1) 83.7 (5.0) 85.3 (5.3) 83.0 (9.0) 80.5 (8.9)
97.2 (7.7) 92.8 (6.2) 89.3 (4.1) 89.5 (8.2) 88.3 (7.3) 84.0 (6.6)
97.5 (7.4) 90.1 (5.7) 92.1 (4.6) 92.3 (7.6) 90.4 (8.3) 84.1 (4.4)
92.8 (5.8) 95.0 (3.6) 93.3 (4.5) 91.0 (6.1) 92.1 (7.5) 85.8 (5.1)
n: 36 measurements per depth and material (MV (S.D.)). a
Depth (mm).
levels of the control-specimens, micro-hardness of the experimental groups is given in percent of the respective controls (Tables 2–4). The most distinct interference of bleaching agents with the filling materials was observed in the superficial layers as expected, but also deeper layers were significantly affected in most groups. For the composite Tetric EvoCeram, the strongest impact of bleaching was observed with Opalescence Xtra Boost, even more pronounced than for the positive control ethyl alcohol (Table 2). Vivastyle yielded the least distinct effect. Apart from Vivastyle at the level of 1 mm, all bleaching agents reduced the Knoop-hardness of Tetric EvoCeram significantly compared with untreated samples. With the exception of Vivastyle, no significant differences were recorded between the influence on the samples treated with ethyl alcohol and the bleached specimens (Table 5). Effects on Tetric Flow were least pronounced compared with Tetric EvoCeram and Compoglass. The most distinct effects were detected in positive controls (ethyl alcohol) followed by sodium perborate (Table 3). Only at 0.1 mm depth was the impact of all bleaching agents on micro-hardness significant when compared with controls. At 0.3 mm and 1 mm, only sodium perborate and Opalescence reduced Knoophardness significantly when compared with controls. Apart from sodium perborate, all bleaching agents had a significantly lower impact on micro-hardness of Tetric Flow, compared with ethyl alcohol (Table 6). The slightest reduction in micro-hardness in the Compoglass group was observed with sodium perborate. Ethyl alcohol
reduced micro-hardness of Compoglass to the greatest extent (Table 4). With some exceptions, the specimens treated with certain bleaching agents featured significantly lower Knoophardness with respect to negative controls (Table 7): There was no significant difference between Compoglass treated with sodium perbotate (0.3 mm; 1 mm) and the respective negative controls (Table 7). Similar micro-hardness was observed in bleached Compoglass samples and in positive controls, apart from speci-
Table 5 – Statistical evaluation of the influence of the bleaching agents on Tetric EvoCeram
Control vs. sodium-perborate Control vs. Opalescence Control vs. Simply White Control vs. Vivastyle Control vs. Whitestrips Ethyl alcohol vs. sodium-perborate Ethyl alcohol vs. Opalescence Ethyl alcohol vs. Simply White Ethyl alcohol vs. Vivastyle Ethyl alcohol vs. Whitestrips
0.1 mm
0.3 mm
1 mm
<0.0001 <0.0001 <0.0001 <0.0001 <0.0001 1.0000 1.0000 1.0000 0.0167 1.0000
<0.0001 <0.0001 <0.0001 <0.0001 <0.0001 1.0000 1.0000 1.0000 1.0000 1.0000
0.001 <0.0001 <0.0001 0.2881 0.0113 0.7763 1.0000 1.0000 0.0059 0.1783
Knoop-hardness after treatment with the different bleaching agents was compared with negative controls and positive controls (ethyl alcohol) at the selected levels 0.1 mm, 0.3 mm and 1.0 mm. Displayed p-values are adjusted for multiple testing according to Bonferroni–Holm (p < 0.05).
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Table 6 – Statistical evaluation of the influence of the bleaching agents on Tetric Flow
Control vs. sodium-perborate Control vs. Opalescence Control vs. Simply White Control vs. Vivastyle Control vs. Whitestrips Ethyl alcohol vs. sodium-perborate Ethyl alcohol vs. Opalescence Ethyl alcohol vs. Simply White Ethyl alcohol vs. Vivastyle Ethyl alcohol vs. Whitestrips
0.1 mm
0.3 mm
1 mm
<0.0001 <0.0001 <0.0001 <0.0001 <0.0001 0.2060 0.0003 <0.0001 0.0009 <0.0001
<0.0001 0.0015 0.1795 0.1915 0.8210 1.0000 0.0007 <0.0001 <0.0001 <0.0001
<0.0001 0.0048 0.1625 0.1402 1.0000 1.0000 0.0034 <0.0001 <0.0001 <0.0001
Knoop-hardness after treatment with the different bleaching agents was compared with negative controls and positive controls (ethyl alcohol) at the selected levels 0.1 mm, 0.3 mm and 1.0 mm. Displayed p-values are adjusted for multiple testing according to Bonferroni–Holm (p < 0.05).
Table 7 – Statistical evaluation of the influence of the bleaching agents on Compoglass
Control vs. sodium-perborate Control vs. Opalescence Control vs. Simply White Control vs. Vivastyle Control vs. Whitestrips Ethyl alcohol vs. sodium-perborate Ethyl alcohol vs. Opalescence Ethyl alcohol vs. Simply White Ethyl alcohol vs. Vivastyle Ethyl alcohol vs. Whitestrips
0.1 mm
0.3 mm
1 mm
<0.0001 <0.0001 <0.0001 <0.0001 <0.0001 0.1882 1.0000 1.0000 1.0000 1.0000
0.0714 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 1.0000 0.7186 0.1924 0.0882
1.0000 0.0043 0.0415 0.0318 0.0029 <0.0001 0.1473 0.0235 0.0312 0.1849
Knoop-hardness after treatment with the different bleaching agents was compared with negative controls and positive controls (ethyl alcohol) at the selected levels 0.1 mm, 0.3 mm and 1.0 mm. Displayed p-values are adjusted for multiple testing according to Bonferroni–Holm (p < 0.05).
mens treated with sodium perborate (0.3 mm; 1 mm), Vivastyle (1 mm) and Simply White (1 mm) (Table 7).
4.
Discussion
The present study is the first attempt to evaluate subsurface impact of bleaching regimens on composite materials. The influence of bleaching on adhesive filling materials was evaluated via determination of Knoop-hardness at different subsurface levels of bleached adhesive restorative materials. All bleaching regimens considerably reduced micro-hardness in all layers of the samples. Three different adhesive filling materials were used in the study: a composite resin, a flowable composite and a polyacid modified composite resin in order to evaluate different matrices as well as both high- and low-filled restorative materials, respectively. Ethyl alcohol was chosen as positive control, because the negative effect of alcohol on polymer materials and composite resins is well documented in the literature [8–12]. This is also
true for subsurface alterations in the micro-hardness of resinbased restoratives [7]. Storage of composite specimens in saliva between incubation with the bleaching material was done to simulate the clinical situation [25]. For the purpose of standardization, this intermittent storage was performed with artificial saliva instead of human saliva in the present study. Storage in natural saliva may modify or attenuate the effect of peroxides on the composite matrix [6]. One aspect may be that peroxidase activity in the surrounding saliva and in the pellicle formed on specimens may have an impact on the amount of peroxides [1,29]. Moreover, saliva may help to wash out peroxides from the surface. The use of the bleaching agents was performed as recommended by the manufacturers. Therefore, bleaching agents were applied for different periods so that comparison between the agents should be done with caution. Other investigations have shown that bleaching has an impact on surface micro-hardness of composites [13,14]. These observations were now confirmed in the present study also for deeper layers of adhesive filling material. Several reasons may be assumed for this phenomenon. Hydrogen peroxide is known to have high capacities for oxidation and reduction, generating free radicals [5]. In addition to its reactivity, hydrogen peroxide demonstrates an extensive ability for diffusion [22–24]. Possibly, peroxides induce oxidative cleavage of polymer-chains. Thereby, unreacted double bonds are expected to be the most vulnerable parts of the polymers. The reduced molar mass of the decomposing products may lead to a softening and reduction in micro-hardness. Furthermore, free radicals induced by peroxides may impact the resin-filler-interface and cause a filler-matrix debonding, as discussed elsewhere [5]. This may lead to microscopic cracks resulting in an increase in surface roughness as shown in the scanning electron microscopic pictures [5,6]. Due to this effect on the resin-filler interface, the different amount of fillers in the flowable and highly filled composites may account for the observed different effects of the bleaching agents. It is noteworthy that highly concentrated bleaching materials, such as sodium perborate, induced the softening of filling materials as well as the low concentrated paint-on-products. Several conclusions can be drawn for the clinician. It is to be expected that composite fillings with reduced physical properties at all levels are more prone to abrasion. Therefore, bleaching of extensive restorations involving the occlusal surface should be avoided. Sodium perborate, usually adopted for internal bleaching of root-treated teeth, was also included in the present study. It was also obvious that during internal bleaching the surrounding composite filling will be affected significantly. Due to this fact, the clinician is advised to exchange the whole existing composite filling after internal bleaching. Especially for those teeth with root canal treatment, optimal stabilisation with adhesive restorations is demanded without any impairment of the physical properties of the restorations. Furthermore, based on the present findings, bleaching of anterior composite fillings as recommended in another study for removal of discolorations seems very doubtful [30]. Also, polishing of bleached composite fillings in order to remove softened surface areas as recommended in other
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 198–203
studies [5] does not re-establish the physical properties of the complete filling, due to the significant subsurface softening observed.
5.
Conclusions
• Bleaching softens subsurface layers of the restorative materials examined. • Due to the fact that deeper layers are also affected, polishing probably does not suffice for re-establishing the physical properties of the filling material. • In clinical practice, bleaching of teeth restored extensively with composites should be avoided.
Acknowledgements We would like to thank Dr. M. Reichenbach from Ivoclar/Vivadent, Liechtenstein for supplying the materials and Prof. Dr. A. de Meijere from the chemical faculty of ¨ the University of Gottingen for his comments on oxidative damage of polymers.
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Effect of bleaching on subsurface micro-hardness of composite and a polyacid modified composite Christian Hannig a,b,∗ , Sebastian Duong b , Klaus Becker b,c , Edgar Brunner d , Elke Kahler d , Thomas Attin b,c a
Department of Operative Dentistry and Periodontology, University of Freiburg, Hugstetter St. 55, D-79106 Freiburg, Germany ¨ Department of Operative Dentistry, Preventive Dentistry and Periodontology, University of Gottingen, ¨ Robert-Koch-Str. 40, 37075 Gottingen, Germany c Clinic for Preventive Dentistry, Periodontology and Cariology, University of Zurich, ¨ ¨ Plattenstr. 11, 8032 Zurich, Switzerland d Department of Medical Statistics, University of Gottingen, ¨ Humboldtallee 32, 37075 Freiburg, Germany b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To investigate the influence of different bleaching techniques on subsurface phys-
Received 5 September 2005
ical properties of composite and polyacid modified composite tested via determination of
Accepted 10 January 2006
micro-hardness. Methods. Specimens of Tetric Flow, Tetric EvoCeram and Compoglass were light cured (2.5 mm thickness) and stored in artificial saliva for 2 weeks (n = 12/group). The samples
Keywords:
were only removed for application of the following bleaching agents in a humid atmosphere:
Composite
Either Vivastyle (1 h/d), Whitestrips (30 min/d), sodium-perborate–water mixture (once for
Bleaching
72 h), Simply White (1 h/d), or Opalescence XtraBoost (1st and 5th day for 15 min) were
Micro-hardness
applied on the surfaces of the samples. Untreated specimens served as negative controls,
Restoration
samples treated with ethyl alcohol for 1 h acted as positive controls. After the bleaching
Polyacid modified resin composite
period, samples were cross-sectioned and the micro-hardness (Knoop) of different subsurface levels (0.1 mm–2.0 mm) was determined. Results. All bleaching techniques significantly reduced the Knoop-hardness of the restoratives compared to untreated controls. Thereby, bleaching significantly affected not only superficial but also the deep layers of the specimens: in superficial layers (0.1 mm, 0.2 mm) lowest micro-hardness values amounted to 69.5% and 76.3% of the respective untreated controls (Compoglass/Vivastyle). In deeper subsurface levels, the lowest hardness was observed with Opalescence/Tetric EvoCeram (0.3 mm: 78.3%; 0.4 mm: 80%; 0.5 mm: 80.5%; 1.0 mm: 84.2%; 2.0 mm: 84.4%). Significance. Bleaching with the tested bleaching agents softens the adhesive restorative materials examined. Due to the fact that subsurface layers are also affected, polishing of the surface may not suffice for re-establishing the physical properties of the surface of the fillings. © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗ Correspondence to: Department of Operative Dentistry and Periodontology, University of Freiburg, Hugstetter St. 55, D-79106 Freiburg, Germany. Tel.: +49 761 270 4957; fax: +49 761 270 4762. E-mail address: [email protected] (C. Hannig). 0109-5641/$ – see front matter © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2006.01.008
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 198–203
1.
199
Introduction
There is an increasing demand for home bleaching in western societies for achieving a whiter smile. Besides custom made conventional tray systems based on individually crafted trays, products such as Whitestrips [1–3] or paint-on-bleaching regimens for the purpose of home bleaching are adopted [4]. In this context, it has to be regarded that many of these newer products are available as over-the-counter-products. Unintended application of the bleaching products on existing restorations by the patients cannot be excluded, especially if bleaching is not performed and monitored by a dentist. Thereby it has to be regarded that the prognosis and longevity of restorations depends not only on the mechanical, but also the physical properties of the filling material [5]. Effect of bleaching on dental restorative materials in general has been reviewed recently [6]. Due to their organic matrix, composite resin materials especially are more prone to chemical alteration compared to inert metal or ceramic restorations [7–12]. However, impact of bleaching on surface micro-hardness of composites is described controversially in the literature. Decreases as well as increases in surface micro-hardness induced by bleaching have been found, whereas other studies revealed no significant alteration in the micro-hardness [13–18]. Scanning electron microscopic studies and profilometric investigations indicated an increase in surface roughness and porosities on the composite surfaces after incubation with bleaching materials [13,14,19]. With polyacid modified resin based composites (compomers), surface degradation and softening have also been observed after bleaching regimens [20,21]. Polishing of the composite restorations after bleaching therapy is recommended in order to remove the altered and softened surface structures [5,6]. Generally, the subsurface deterioration of composites induced by peroxides or other chemical compounds in bleaching agents seems conceivable, due to their ability to diffuse and their high chemical reactivity [22–24]. However, it has not been investigated to what depth softening of the materials occurs, so that it is uncertain if polishing of the surface suffices to remove the softened layers. The aim of the present study was, therefore, to evaluate subsurface effects of bleaching agents on current composite filling materials by the evaluation of micro-hardness at different levels in composite specimens treated with different bleaching systems.
Fig. 1 – Set-up of the mold and the lantern slide during fabrication of the specimens.
For fabrication of the specimens, molds (diameter: 7 mm, height: 2.5 mm) with persistent holes were prepared from Palavit G (Heraeus Kulzer, Hanau, Germany). Each mold was placed on a special lantern slide (Schott, Mainz, Germany). The lantern slide (1.0 mm thickness) is permeable to light of wavelength between 380 nm and 2400 nm to a degree of 90%. Composites were filled in the cavities of the molds against the lantern slide and light polymerized for 20 s (Spektrum 800, DeTrey/Dentsply, Konstanz, Germany) through the lantern slide at a standardized distance of 1 mm (Fig. 1). Following this, the lantern slide was removed and the specimens light cured for another 40 s. Light intensity of the polymerization device was checked with an integrated photometer after six polymerization steps showing that intensity was constant at a level of 550 mW/cm2 . The top surface of the composite samples was wet ground and polished up to grain size 4000. About 0.1 mm was removed during the grinding procedure. Bleaching agents were applied to the polished surfaces only after storage in artificial saliva. The number of composite specimens was n = 252, each experimental group contained 12 specimens of the respective filling material.
2.2.
Application of bleaching techniques
Samples were stored in artificial saliva (pH 6.8) according to Schemehorn et al. [25] for 2 weeks and removed for the application of the bleaching systems only. Adoption of bleaching techniques was performed according to manufacturers’ instructions. In order to simulate clinical procedure, bleaching was performed in a wet chamber (37 ◦ C). Remnants of bleaching gel were removed with running water after each single application. The following bleaching agents were applied:
2.
Materials and methods
2.2.1. Vivastyle (Vivadent, Liechtenstein, # GL1020), 10% carbamide peroxide)
2.1.
Sample preparation
This gel usually used for charging individually crafted bleaching trays was adopted for 1 h per day for 14 days. During each application, 0.1 ml was applied per specimen.
Three different adhesive filling materials were used in the study: a composite resin (Tetric EvoCeram, Ivoclar/Vivadent, Schaan, Liechtenstein # F38345), a flowable composite (Tetric Flow, Ivoclar/Vivadent, Schaan, Liechtenstein, # GM1097) and a polyacid modified resin composite (Compoglass, Ivoclar/Vivadent, Schaan, Liechtenstein, # G11795).
2.2.2.
Sodium perborate (Merck, Germany)
This peroxide releasing reagent, usually used for internal bleaching was mixed with distilled water (2 g/ml) and applied to the specimens for 72 h in a wet chamber. Until determina-
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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 ) 198–203
tion of micro-hardness, samples were stored in artificial saliva for 11 days. During the application, 0.1 mg was applied per specimen.
2.2.3. H2 O2
Table 1 – Knoop-micro-hardness of untreated controls in the respective subsurface levels of the materials Compoglass, Tetric EvoCeram and Tetric Flow (MV (S.D.)) Depth (mm)
Opalescence XtraBoost (Ultradent, # 68R3), 38%
Compoglass
This in-office-bleaching product was adopted twice (1st and 5th day) for 15 min each. During each application, 0.1 ml was applied per specimen.
0.1 0.2 0.3 0.4 0.5 1.0 2.0
2.2.4. Simply White (Colgate, # 3327AM), 5.9% H2 O2 (Lot #3327AM) This paint on bleaching product was applied 1 h a day for 14 days. Each of the specimens’ surfaces was charged with 0.1 ml gel.
2.2.5.
Whitestrips (Blend-a-med, # 3319BT2B), 6.5% H2 O2
Strips were adopted twice a day for 30 min each for 14 days.
2.2.6.
Positive control: ethyl alcohol (96%)
Ethanol was applied for 1 h (0.1 ml per sample).
2.2.7.
Negative control
Non contaminated specimens served as controls.
2.3.
Determination of Knoop-hardness
The molds containing the composite samples were cross¨ sectioned with a water-cooled saw (Exakt, Neumunster, Germany) in order to achieve coplanar centrepieces for determination of micro-hardness (Knoop). Micro-hardness was determined at the following distances from the light cured, polished and bleached surface: 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, 1.0 mm, 2.0 mm. With each specimen, three measurements were performed at each level. Determination of Knoop-hardness was performed according to DIN standard 52333 (ASTM D 1470). The diamond was loaded with 2 N [26].
2.4.
Knoop-micro-hardness of controls
Statistics
Statistical evaluation was performed for selected levels (0.1 mm, 0.3 mm and 1.0 mm).
Tetric EvoCeram
64.3 (2.5) 63.0 (4.8) 60.5 (3.4) 60.3 (2.9) 57.5 (3.9) 56.9 (3.9) 55.4 (3.2)
68.4 (3.9) 64.8 (4.4) 64.7 (5.1) 63.2 (5.5) 62.9 (3.8) 61.0 (4.1) 60.2 (4.2)
Tetric Flow 49.7 (4.4) 47.5 (3.8) 47.2 (3.6) 47.9 (3.0) 47.6 (3.2) 49.2 (3.7) 48.3 (2.5)
n: 36 measurements per depth and material.
The statistical design underlying the measurements of the Knoop-hardness in the present trial was a repeated measures design, i.e. the same samples are repeatedly observed at several depths. The impact of depth and bleaching materials on the hardness of the probes was examined by parametric repeated measures ANOVA for each of the three restorative materials separately, analogous to the methods described in [27]. If any hypothesis was rejected in the ANOVA analysis (the level of significance was set at 5%), subsequent post-hoc analyses were performed by using the Bonferroni–Holm correction as an appropriate adjustment of the significance level [28]. Statistics were calculated using the software package SAS 9.1 (SAS Institute Inc., Cary, NC, USA) [27].
3.
Results
All bleaching systems affected Knoop-hardness of the filling materials at all the levels evaluated. Absolute values of micro-hardness are given for controls in Table 1. It is noteworthy that all composites featured a different Knoop-hardness in the controls. Lowest values were observed with the flow-material. The highly filled composite Tetric EvoCeram and the polyacid modified resin composite Compoglass displayed comparable micro-hardness-values. As expected, in deeper layers, micro-hardness was lower compared to the outer surface which was nearest to the polymerization device. Due to the differing micro-hardness at certain
Table 2 – Mean micro-hardness (%) of Tetric EvoCeram in the subsurface levels after treatment with different bleaching agents and ethyl alcohol as related to hardness of controls in the respective levels Micro-hardness (percentage of controls)
Contaminated with Sodium perborate Whitestrips Vivastyle Simply White Opalescence Xtra Boost Ethyl alcohol
0.1a
0.2a
0.3a
0.4a
0.5a
1.0a
2.0a
73.7 (3.8) 75.5 (5.2) 80.6 (6.2) 73.6 (7.9) 72.9 (4.4) 73.8 (9.1)
81.9 (5.3) 83.3 (4.1) 84.1 (7.0) 79.2 (5.7) 77.7 (3.4) 81.8 (4.3)
85.3 (7.1) 84.0 (4.6) 85.9 (7.0) 79.4 (6.4) 78.3 (3.9) 82.2 (8.8)
89.5 (7.8) 87.2 (7.4) 89.8 (7.2) 83.3 (4.2) 80.0 (4.3) 83.1 (10.6)
88.3 (5.6) 88.3 (5.2) 90.8 (7.5) 82.5 (4.8) 80.5 (4.3) 81.5 (5.7)
88.6 (8.5) 90.5 (6.4) 93.7 (9.1) 86.1 (7.4) 84.2 (5.3) 83.6 (7.5)
90.5 (5.1) 91.3 (4.5) 93.7 (6.4) 85.1 (5.5) 84.4 (4.3) 85.8 (6.0)
n: 36 measurements per depth and material (MV (S.D.)). a
Depth (mm).
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Table 3 – Mean micro-hardness (%) of Tetric Flow in the subsurface levels after treatment with different bleaching agents and ethyl alcohol as related to hardness of controls in the respective levels Micro-hardness (percentage of controls)
Contaminated with Sodium perborate Whitestrips Vivastyle Simply White Opalescence Xtra Boost Ethyl alcohol
0.1a
0.2a
0.3a
0.4a
0.5a
1.0a
2.0a
79.2 (7.1) 87.2 (5.1) 83.9 (6.0) 85.3 (5.0) 84.6 (4.8) 74.7 (4.0)
85.1 (8.7) 94.3 (3.8) 91.7 (6.0) 92.7 (3.6) 91.1 (4.8) 79.3 (4.8)
84.7 (7.5) 97.4 (3.5) 95.4 (5.6) 95.5 (4.2) 91.7 (2.5) 82.8 (5.4)
84.8 (6.2) 96.8 (4.1) 94.1 (6.4) 94.7 (6.2) 92.5 (2.9) 82.4 (5.6)
86.4 (7.1) 98.6 (6.0) 96.4 (5.4) 96.6 (6.5) 94.5 (6.0) 82.7 (5.9)
83.5 (8.5) 99.9 (4.3) 94.0 (7.6) 94.2 (5.2) 91.1 (5.4) 81.9 (6.2)
87.6 (8.0) 99.8 (4.1) 93.6 (6.9) 93.0 (4.2) 93.7 (4.2) 81.1 (7.9)
n: 36 measurements per depth and material (MV (S.D.)). a
Depth (mm).
Table 4 – Mean micro-hardness (%) of Compoglass in the subsurface levels after treatment with different bleaching agents and ethyl alcohol as related to hardness of controls in the respective levels Micro-hardness (percentage of controls)
Contaminated with Sodium perborate Whitestrips Vivastyle Simply White Opalescence Xtra Boost Ethyl alcohol
0.1a
0.2a
0.3a
0.4a
0.5a
1.0a
2.0a
77.8 (12.8) 75.0 (3.7) 69.5 (8.3) 73.4 (5.5) 72.0 (11.7) 71.8 (7.6)
83.7 (5.3) 80.1 (5.0) 76.2 (6.5) 79.4 (9.0) 78.9 (11.1) 74.4 (4.9)
92.5 (10.7) 86.6 (4.8) 85.4 (5.0) 83.7 (5.3) 83.0 (9.0) 79.4 (8.3)
97.4 (10.1) 89.2 (4.1) 83.7 (5.0) 85.3 (5.3) 83.0 (9.0) 80.5 (8.9)
97.2 (7.7) 92.8 (6.2) 89.3 (4.1) 89.5 (8.2) 88.3 (7.3) 84.0 (6.6)
97.5 (7.4) 90.1 (5.7) 92.1 (4.6) 92.3 (7.6) 90.4 (8.3) 84.1 (4.4)
92.8 (5.8) 95.0 (3.6) 93.3 (4.5) 91.0 (6.1) 92.1 (7.5) 85.8 (5.1)
n: 36 measurements per depth and material (MV (S.D.)). a
Depth (mm).
levels of the control-specimens, micro-hardness of the experimental groups is given in percent of the respective controls (Tables 2–4). The most distinct interference of bleaching agents with the filling materials was observed in the superficial layers as expected, but also deeper layers were significantly affected in most groups. For the composite Tetric EvoCeram, the strongest impact of bleaching was observed with Opalescence Xtra Boost, even more pronounced than for the positive control ethyl alcohol (Table 2). Vivastyle yielded the least distinct effect. Apart from Vivastyle at the level of 1 mm, all bleaching agents reduced the Knoop-hardness of Tetric EvoCeram significantly compared with untreated samples. With the exception of Vivastyle, no significant differences were recorded between the influence on the samples treated with ethyl alcohol and the bleached specimens (Table 5). Effects on Tetric Flow were least pronounced compared with Tetric EvoCeram and Compoglass. The most distinct effects were detected in positive controls (ethyl alcohol) followed by sodium perborate (Table 3). Only at 0.1 mm depth was the impact of all bleaching agents on micro-hardness significant when compared with controls. At 0.3 mm and 1 mm, only sodium perborate and Opalescence reduced Knoophardness significantly when compared with controls. Apart from sodium perborate, all bleaching agents had a significantly lower impact on micro-hardness of Tetric Flow, compared with ethyl alcohol (Table 6). The slightest reduction in micro-hardness in the Compoglass group was observed with sodium perborate. Ethyl alcohol
reduced micro-hardness of Compoglass to the greatest extent (Table 4). With some exceptions, the specimens treated with certain bleaching agents featured significantly lower Knoophardness with respect to negative controls (Table 7): There was no significant difference between Compoglass treated with sodium perbotate (0.3 mm; 1 mm) and the respective negative controls (Table 7). Similar micro-hardness was observed in bleached Compoglass samples and in positive controls, apart from speci-
Table 5 – Statistical evaluation of the influence of the bleaching agents on Tetric EvoCeram
Control vs. sodium-perborate Control vs. Opalescence Control vs. Simply White Control vs. Vivastyle Control vs. Whitestrips Ethyl alcohol vs. sodium-perborate Ethyl alcohol vs. Opalescence Ethyl alcohol vs. Simply White Ethyl alcohol vs. Vivastyle Ethyl alcohol vs. Whitestrips
0.1 mm
0.3 mm
1 mm
<0.0001 <0.0001 <0.0001 <0.0001 <0.0001 1.0000 1.0000 1.0000 0.0167 1.0000
<0.0001 <0.0001 <0.0001 <0.0001 <0.0001 1.0000 1.0000 1.0000 1.0000 1.0000
0.001 <0.0001 <0.0001 0.2881 0.0113 0.7763 1.0000 1.0000 0.0059 0.1783
Knoop-hardness after treatment with the different bleaching agents was compared with negative controls and positive controls (ethyl alcohol) at the selected levels 0.1 mm, 0.3 mm and 1.0 mm. Displayed p-values are adjusted for multiple testing according to Bonferroni–Holm (p < 0.05).
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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 ) 198–203
Table 6 – Statistical evaluation of the influence of the bleaching agents on Tetric Flow
Control vs. sodium-perborate Control vs. Opalescence Control vs. Simply White Control vs. Vivastyle Control vs. Whitestrips Ethyl alcohol vs. sodium-perborate Ethyl alcohol vs. Opalescence Ethyl alcohol vs. Simply White Ethyl alcohol vs. Vivastyle Ethyl alcohol vs. Whitestrips
0.1 mm
0.3 mm
1 mm
<0.0001 <0.0001 <0.0001 <0.0001 <0.0001 0.2060 0.0003 <0.0001 0.0009 <0.0001
<0.0001 0.0015 0.1795 0.1915 0.8210 1.0000 0.0007 <0.0001 <0.0001 <0.0001
<0.0001 0.0048 0.1625 0.1402 1.0000 1.0000 0.0034 <0.0001 <0.0001 <0.0001
Knoop-hardness after treatment with the different bleaching agents was compared with negative controls and positive controls (ethyl alcohol) at the selected levels 0.1 mm, 0.3 mm and 1.0 mm. Displayed p-values are adjusted for multiple testing according to Bonferroni–Holm (p < 0.05).
Table 7 – Statistical evaluation of the influence of the bleaching agents on Compoglass
Control vs. sodium-perborate Control vs. Opalescence Control vs. Simply White Control vs. Vivastyle Control vs. Whitestrips Ethyl alcohol vs. sodium-perborate Ethyl alcohol vs. Opalescence Ethyl alcohol vs. Simply White Ethyl alcohol vs. Vivastyle Ethyl alcohol vs. Whitestrips
0.1 mm
0.3 mm
1 mm
<0.0001 <0.0001 <0.0001 <0.0001 <0.0001 0.1882 1.0000 1.0000 1.0000 1.0000
0.0714 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 1.0000 0.7186 0.1924 0.0882
1.0000 0.0043 0.0415 0.0318 0.0029 <0.0001 0.1473 0.0235 0.0312 0.1849
Knoop-hardness after treatment with the different bleaching agents was compared with negative controls and positive controls (ethyl alcohol) at the selected levels 0.1 mm, 0.3 mm and 1.0 mm. Displayed p-values are adjusted for multiple testing according to Bonferroni–Holm (p < 0.05).
mens treated with sodium perborate (0.3 mm; 1 mm), Vivastyle (1 mm) and Simply White (1 mm) (Table 7).
4.
Discussion
The present study is the first attempt to evaluate subsurface impact of bleaching regimens on composite materials. The influence of bleaching on adhesive filling materials was evaluated via determination of Knoop-hardness at different subsurface levels of bleached adhesive restorative materials. All bleaching regimens considerably reduced micro-hardness in all layers of the samples. Three different adhesive filling materials were used in the study: a composite resin, a flowable composite and a polyacid modified composite resin in order to evaluate different matrices as well as both high- and low-filled restorative materials, respectively. Ethyl alcohol was chosen as positive control, because the negative effect of alcohol on polymer materials and composite resins is well documented in the literature [8–12]. This is also
true for subsurface alterations in the micro-hardness of resinbased restoratives [7]. Storage of composite specimens in saliva between incubation with the bleaching material was done to simulate the clinical situation [25]. For the purpose of standardization, this intermittent storage was performed with artificial saliva instead of human saliva in the present study. Storage in natural saliva may modify or attenuate the effect of peroxides on the composite matrix [6]. One aspect may be that peroxidase activity in the surrounding saliva and in the pellicle formed on specimens may have an impact on the amount of peroxides [1,29]. Moreover, saliva may help to wash out peroxides from the surface. The use of the bleaching agents was performed as recommended by the manufacturers. Therefore, bleaching agents were applied for different periods so that comparison between the agents should be done with caution. Other investigations have shown that bleaching has an impact on surface micro-hardness of composites [13,14]. These observations were now confirmed in the present study also for deeper layers of adhesive filling material. Several reasons may be assumed for this phenomenon. Hydrogen peroxide is known to have high capacities for oxidation and reduction, generating free radicals [5]. In addition to its reactivity, hydrogen peroxide demonstrates an extensive ability for diffusion [22–24]. Possibly, peroxides induce oxidative cleavage of polymer-chains. Thereby, unreacted double bonds are expected to be the most vulnerable parts of the polymers. The reduced molar mass of the decomposing products may lead to a softening and reduction in micro-hardness. Furthermore, free radicals induced by peroxides may impact the resin-filler-interface and cause a filler-matrix debonding, as discussed elsewhere [5]. This may lead to microscopic cracks resulting in an increase in surface roughness as shown in the scanning electron microscopic pictures [5,6]. Due to this effect on the resin-filler interface, the different amount of fillers in the flowable and highly filled composites may account for the observed different effects of the bleaching agents. It is noteworthy that highly concentrated bleaching materials, such as sodium perborate, induced the softening of filling materials as well as the low concentrated paint-on-products. Several conclusions can be drawn for the clinician. It is to be expected that composite fillings with reduced physical properties at all levels are more prone to abrasion. Therefore, bleaching of extensive restorations involving the occlusal surface should be avoided. Sodium perborate, usually adopted for internal bleaching of root-treated teeth, was also included in the present study. It was also obvious that during internal bleaching the surrounding composite filling will be affected significantly. Due to this fact, the clinician is advised to exchange the whole existing composite filling after internal bleaching. Especially for those teeth with root canal treatment, optimal stabilisation with adhesive restorations is demanded without any impairment of the physical properties of the restorations. Furthermore, based on the present findings, bleaching of anterior composite fillings as recommended in another study for removal of discolorations seems very doubtful [30]. Also, polishing of bleached composite fillings in order to remove softened surface areas as recommended in other
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 198–203
studies [5] does not re-establish the physical properties of the complete filling, due to the significant subsurface softening observed.
5.
Conclusions
• Bleaching softens subsurface layers of the restorative materials examined. • Due to the fact that deeper layers are also affected, polishing probably does not suffice for re-establishing the physical properties of the filling material. • In clinical practice, bleaching of teeth restored extensively with composites should be avoided.
Acknowledgements We would like to thank Dr. M. Reichenbach from Ivoclar/Vivadent, Liechtenstein for supplying the materials and Prof. Dr. A. de Meijere from the chemical faculty of ¨ the University of Gottingen for his comments on oxidative damage of polymers.
references
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