| Abstract|| |
Background: Bleaching products may show some side effects on soft and hard tissues and restorative materials in the oral cavity. This study evaluated the effect of carbamide peroxide gel with and without fluoride ions on the microhardness and surface roughness of tooth-colored restorative materials.
Materials and Methods: In this in-vitro study, 76 cubic specimens (4 mm 3 × 4 mm 3 × 3 mm 3 ) were fabricated from 4 aesthetic A3-shade restorative materials. These materials consisted of two composite resins and two glass ionomers. The specimens made from each material were treated with the following surface treatments: 1. Control group: The specimens were not bleached and were stored in normal saline. Group 2. Fluoridated 20% carbamide peroxide gel, treated 3 h a day for 4 weeks. Group 3. Treated 1 h a day with fluoride-less 22% carbamide peroxide for two weeks. From each group, three other specimens were selected to be evaluated in terms of changes in surface roughness, under scanning electron microscopy (SEM).
Results: In this study, fluoridated 20% carbamide peroxide gel increased the microhardness of the four aesthetic restorative materials. The fluoride-free carbamide peroxide 22% reduced the microhardness of the four used materials, which this change was significant for Vitremer and Amelogen. SEM analyses showed changes in surface roughness of glass ionomer specimens.
Conclusion: The effect of bleaching on the microhardness of restorative materials is material dependent. Before the application of bleaching systems on the glass ionomer materials, the application of a protective barrier should be considered.
Keywords: Bleaching, composite resin, glass ionomer, microhardness
|How to cite this article:|
Alaghehmand H, Esmaeili B, Sheibani SA. Effect of fluoride-free and fluoridated carbamide peroxide gels on the hardness and surface roughness of aesthetic restorative materials. Indian J Dent Res 2013;24:478-83
The esthetic appearance of a person's smile is influenced by the color, shape, and position of the teeth. Bleaching is the most minimally invasive method to obtain an optimally esthetic result in natural teeth.  With the introduction of at-home or in-office bleaching techniques in previous decades, there has been an increased demand for peroxide based bleaching agents. Bleaching agents improve the appearance of discolored teeth by degrading peroxide molecules to free radical molecules. These free radicals break large color molecules by redox reactions. 
|How to cite this URL:|
Alaghehmand H, Esmaeili B, Sheibani SA. Effect of fluoride-free and fluoridated carbamide peroxide gels on the hardness and surface roughness of aesthetic restorative materials. Indian J Dent Res [serial online] 2013 [cited 2019 Jul 18];24:478-83. Available from: http://www.ijdr.in/text.asp?2013/24/4/478/118397
Several studies have shown the influence of bleaching materials on the hardness , and erosion of enamel.  Others have suggested application of calcium and fluoride gel , or casein phosphopeptide-amorphous calcium phosphate (CPP-ACP)-containing paste  for enamel demineralization and improvement of enamel hardness. The application of fluoridated bleaching materials has shown to be associated with increased demineralization in enamel surfaces. 
However, bleaching materials and techniques may, however, have a different influence on restorative materials. Chemical softening resulting from bleaching may affect the microhardness of restorative materials and the clinical durability of tooth-colored restorations.  In one study, a high-concentration carbamide peroxide-containing home bleaching system significantly influenced the hardness of different restorative materials.  Some of the researches have shown the effect of bleaching materials on surface roughness and staining susceptibility of tooth-colored restorative materials. , On the other hand, some studies have shown no significant differences in surface roughness between the bleached and the control groups for tooth-colored restorative materials. , Müjdeci et al. evaluated the effect of 10-16% carbamide peroxide gel on the hardness of composite resins and observed increases in the level of hardness of all groups.  The purpose of this study was then to evaluate the effect of fluoridated high-concentration carbamide peroxide-containing home bleaching systems on microhardness and surface roughness of two composite resins and two glass ionomers.
| Materials and Methods|| |
Opalescence PF (potassium nitrate & fluoride) (Ultradent product Inc, USA), a home bleaching system containing 20% carbamide peroxide, potassium nitrate and fluoride, and Nite white ACP (Philips oral health, Netherland), a home bleaching system containing 22% carbamide peroxide, potassium nitrate and amorphous calcium phosphate, were used in the present study. Esthetic restorative materials selected for the study included two nanocomposites (Filtek P60, Amelogen Plus) and two Light cure resin modified glass ionomer (Vitremer, Fuji II LC). Materials, contents and their manufacturers are listed in [Table 1].
Nineteen cubic specimens (a total of 76 specimens) were fabricated from each restorative material (A3 shade) (according to the manufacturer instructions), using a 4 mm 4 mm × 3 mm Heliotest Teflon mold. The molds were slightly overfilled with the materials and then covered with a mylar strip (Hawe Neos Dental, Gentilino, Switzerland) and pressed with a glass slab to remove voids and excess material. They were then polymerized according to the manufacturer's instructions using a halogen light cure unit (Astralis7, IvoclarVivadent, Austria) with in light intensity of 750 mW/Cm 2 . Following removal of the Mylar strip, the specimens were stored in saline normal at 37°C for 1 week. The specimens were polished using a sequence of 600-800-1200-2500 grit silicon carbide paper (Buehler, lake Bluff, IL, USA) and 1 μm polishing paste (Ultradent diamond polish mint, Ultradent product Inc, USA). The polished specimens were cleaned in distilled water in an ultrasonic cleaner.
From each restorative material, one specimen was selected as the "surface roughness control specimen" and was stored in normal saline during the experiment, like the other specimens. Each type of restorative material was then randomly divided into two groups (n = 9), and the specimens were treated with either Opalescence PF or Nite white ACP. The bleaching agents were painted onto the top surface of each specimen according to the manufacturer's instructions at room temperature and then stored at 37°C during the bleaching period. The Opalescence PF gel was applied 3 h/day for 4 weeks and the Nite White gel was applied twice per day (30 min each) for 2 weeks. For this purpose, first the specimens were brought out of the normal saline, then were air-dried for 5 s and afterward the bleaching agent approximately 1 mm thin was placed on the specimens and after the bleaching time was over they were rinsed for standardized time of 1 min with distilled water to remove the bleaching materials and were stored back in normal saline in 37°C.
From each groups, six specimens were selected for microhardness testing. Specimens before and after bleaching were evaluated using a digital microhardness tester (Vickers hardness testing machine, MH1 model, standardized with ASTM, NIST and DIN, Kupa Pajuhesh Corporation, Iran). For all specimens, a 500-g load was applied through the Vickers indenter at crosshead speed of 30 mm/sec with a dwell time of 10 seconds. Three indentations were made at different points on each specimen, and an average value was obtained from these three measurements.
Scanning electron microscopy
From each group, three specimens were selected with the control specimen to be subjected to electron microscopy for morphology and surface roughness evaluations. The specimens were dried, gold sputter coated and observed using a scanning electron microscope (JXA-8230, Jeol Ltd, Tokyo, Japan).
The collected Vickers microhardness values comparisons were done before and after bleaching were performed using parametric statistical tests ANOVA, Tukey test at a statistical significance level of 0.05.
| Results|| |
Mean Vickers hardness values and standard deviation of the tested composites and glass ionomers before and after bleaching with opalescence PF and Nite white ACP are shown in [Table 2] and [Table 3].
|Table 2: Descriptive statistics of the microhardness (in VHN) of four restorative materials before and after bleaching with Opalescence PF|
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|Table 3: Descriptive statistics of the microhardness (in VHN) of four restorative materials before and after bleaching with Nite White|
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Using the opalescence PF, a statistically significant increase in hardness compared to before bleaching was found only for the P60 composite (P = 0.031) and not in other groups. The home bleaching system Nite white ACP reduced the microhardness values of all four restorative materials. This reduction was statistically significant for Vitremer glass ionomer (P = 0.006) and Amelogen composite (0.035). Nite white ACP did not produce any statistically significant effect on the surface microhardness of the Fuji IILC (P = 0.056), and Filtek P60 (P = 0.110). The differences between microhardness values of all restorative materials before and after bleaching with the two agents are presented in [Figure 1].
|Figure 1: The differences between the three microhardness values (in VHN) before and after bleaching. Total = average of the four restoration materials bleached with each of the bleaching materials|
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The Tukey test showed that between the two glass ionomers (before and after bleaching), Fuji II LC microhardness values were significantly higher than Vitremer (P = 0.000). Also between the two composites (before and after bleaching), the results of P60 were significantly higher than Amelogen (P = 0.000).
According to the Tukey test, the effects of the two bleaching agents on the microhardness values of the Vitremer glass ionomer (the differences between before and after microhardness values for each bleaching material 0.041 – (–3.191) = 3.233 Vickers hardness number) was not significantly significant (P = 0.193). Also the effects of the two bleaching agents on the microhardness values of Fuji II LC glass ionomer was not significantly different (P = 0.325, the difference between the discrepancies = 2.866 VHN). However, the effects of the bleaching gels on the microhardness values of Amelogen composite were significantly different (P = 0.008, difference between the discrepancies = 4.875 VHN). Also there was a significant difference (4.625 VHN, P = 0.014) between the effects of the bleaching materials on the microhardness values of the P60 composite.
Scanning electron micrographs were taken at ×500 magnification from the glass ionomer specimens, while the magnification of composite electron micrographs was ×1000. On the surface of the bleached glass ionomer specimens, morphological alterations including small fractures, shallow dimpling and etched surfaces were observed (compared with the control specimens), which these alterations were more visible on the surfaces of Vitremer specimens. However, composite resins did not show any considerable changes after bleaching [Figure 2], [Figure 3], [Figure 4] and [Figure 5].
|Figure 2: 500× electronn micrographs of Vitremer glass ionomer specimens. (a) After bleaching with Nite White. (b) Control. (c) After bleaching with Opalescence PF|
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|Figure 3: 500× electronn micrographs of Fuji II LC glass ionomer specimens. (a) After bleaching with Nite White. (b) Control. (c) After bleaching with Opalescence PF|
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|Figure 4: 1000× electronn micrographs of Amelogen composite specimens. (a) Control. (b) After bleaching with Nite White. (c) After bleaching with Opalescence PF|
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|Figure 5: 1000× electronn micrographs of P60 composite specimens. (a) Control. (b) After bleaching with Nite White. (c) After bleaching with Opalescence PF|
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| Discussion|| |
Carbamide peroxide is a composition containing hydrogen peroxide which is decomposed to roughly one-third hydrogen peroxide and two thirds urea. The hydrogen peroxide is decomposed again and produces free radical perhydroxyl, which is a highly active free radical with a high oxidizing potential, and is able to affect both pigment macromolecules as well as resin matrices.  A reduction in microhardness due to organic matrix erosion may enhance the roughness of restorations and may decrease their wear resistance. Peroxides have been claimed to reduce oxidative cleavage of polymer chains, and thereby unreacted double bonds are expected to be the most vulnerable parts of the polymers.  Current bleaching agents also include doping agents such as carbopol, which may reduce the destructive effect of free radicals by attaching to them.  In the present study, the effects of carbamide peroxide gels with (Opalescence PF) and without (Nite white ACP) fluoride ions on the surface microhardness and roughness of two nanocomposites (Filtek P60, Amelogen Plus) and two Light cure resin modified glass ionomer (Vitremer, Fuji II LC) was assessed for the first time. A significant reduction in microhardness after of the application of Nite white ACP was observed in Vitremer glass ionomer and Amelogen composite and a significanwase after the application of Opalescence PF was observed in the P60 composite. Free radicals induced by peroxides may impact the resin-filler interface and cause a filler-matrix debonding. This fact may lead to the reduction of microhardness and displacement of inorganic filler particles in the composite surfaces. Reduction of microhardness was observed in using of Nite white ACP. Inconsistent to the findings of the present study, Malkondu and et al.  observed a reduction in the surface microhardness due to the inorganic filler loss on the surface of composite after using Opalescence PF [Figure 4] and [Figure 5]. In 2008, Hao Yu et al. assessed the effect of Opalescence PF 15% on the hardness of three composite resins and a conventional glass ionomer in an insitu environment, and reported that the hardness of the conventional glass ionomer increased while the hardness of the composites did not change significantly.  Mortezavi et al. in 2008 showed that Opalescence PF (20% carbamide peroxide) and Opalescence Quick (35% carbamide peroxide) did not have any significant effects on the surface roughness of microhybrid (Point 4) and nanofilled (Filtek Supreme and Premise) composites.  Our results also confirm theirs regarding Opalescence PF ineffectiveness on surface roughness of tooth-color restorative materials.
The bleaching effect was material and concentration dependent.  In this study two similar bleaching materials with carbamide peroxide base and similar concentration were used. The recommended application protocol of the bleaching agent Opalescence PF is much more than Nite white ACP. In spite of this, Opalescence PF did not have the destructive effects on specimens.
Turker et al. assessed the surface roughness with scanning electron microscopy (SEM), and surface profilometry. In their study, although no statistically significant difference was observed between the microfilled composite and the control group, SEM showed shallow pits on the surface of these specimens. These pits were cleare in specimens bleached with Nite White (16% carbamide peroxide, pH = 5.5) and Rembrant (10% carbamide peroxide, pH = 6.7) compared with Opalescence (10% carbamide peroxide, pH = 6). Their study showed that bleaching agents had a considerable effect on the surface roughness of resin-modified glass ionomer, but had a weak effect on the surface of microfilled composite.  This suggests that the pH value is an important factor for the rate of reaction in the bleaching process. In the present study, similar to the results of Turker et al., no substantial changes were observed on the surface of the composites while the bleached glass ionomers showed obvious surface changes (compared to control) such as cracks, shallow pits, and etched patterns, which these changes were more visible on the surface of Vitremer glass ionomer, compared with Fuji II-LC. Ionization of buffered hydrogen peroxide in the pH range of 9.5 to 10.8 produces more perhydroxyl HO 2 free radicals. The result is a 50% greater bleaching effect in the same period as other pH levels.  Carbamide peroxide decomposes first into hydrogen peroxide and urea, which further continues to decompose into CO 2 and ammonia. Ammonia is a strong base that then offers an elevated pH environment, one that is more favorable for producing stronger free radicals. Opalescence PF has an average pH value of 6.5 and Nite white has an average pH value of 5.5. that; however, fewer than and has lower elevated pH environment that may affect much more on the surface microhardness and roughness.  As previously mentioned, the difference between the two types of bleaching systems on microhardness may be due to the different application period, the diffusion ability of the materials, and moreover, the elevated pH environment.
| Conclusion|| |
Based on the results of this study, it is concluded that the effect of bleaching on the microhardness of restorative materials is material dependent and the application times used for bleaching did not have the destructive effects on specimens. Also, before the application of bleaching systems on the glass ionomer materials, the application of a protective barrier should be considered. Further research is necessary to evaluate whether the home bleaching materials will have any impact on the bonding strength between tooth-colored materials and enamel and dentin.
| Acknowledgment|| |
We would like to thank Dental Materials Research Center and Research Council of Babol University of Medical Sciences. Also, we wish to thank Farzan Research Institute for Research and Technology for technical assistance.
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Department of Esthetic and Restorative Dentistry, Dental Materials Research Center, Dental School, Babol University of Medical Sciences, Babol
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3]