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 Table of Contents  
ORIGINAL ARTICLE
Year : 2016  |  Volume : 8  |  Issue : 2  |  Page : 59-65

An in vitro evaluation of microhardness of different direct resin-based restorative materials on using 10% carbamide peroxide gel as a bleaching agent


Department of Conservative Dentistry and Endodontics, Panineeya Institute of Dental Sciences and Research Centre, Hyderabad, Telangana, India

Date of Web Publication13-Oct-2016

Correspondence Address:
Chaitanya Byragoni
Department of Conservative Dentistry and Endodontics, Panineeya Institute of Dental Sciences and Research Centre, Road No. 5, Kamalanagar, Dilsukhnagar, Hyderabad - 500 060, Telangana
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2249-4987.192193

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  Abstract 

Purpose: This in vitro study was done to evaluate the effect of a 10% carbamide peroxide bleaching agent on the micro hardness of three types of direct resin-based restorative materials.
Materials and Methods: Fifteen disk-shaped specimens (5.0 mm diameter and 2.0 mm depth) of each material, including a micro hybrid resin composite (Z250), a nanofilled resin composite (Z350), a hybrid resin composite (Z100), were fabricated and then polished with medium, fine, and superfine polishing discs. After being polished, specimens were cleaned with distilled water for 2 min in an ultrasonic bath to remove any surface debris and then stored in distilled water at 37ΊC for 24 hours. Specimens from each material were divided into three groups (n=5). One group was selected as a control group (non treated with bleaching agent). The other two groups were treated with bleaching agent for 14 days (group A) and followed by immersion in artificial saliva for 14 days (group B). The top surfaces of the specimens in the different groups were also subjected to the Vickers hardness test with a load of 300 g and 15-second dwell time. Data were analyzed witha one-way analysis of variance and Turkey's HSD test (α = 0.05).
Results: There was a general reduction of Vickers hardness numbers (VHN) values of treated groups compared with the control group for each material used, but this reduction was minimal, with no significant difference between groups in Z250, whereas the other two materials (Z350, Z100) showed a significant reduction of VHN of treated groups compared with the control group. Conversely, the findings showed no significant difference between treated groups A and B in all materials used.
Conclusion: A 10% carbamide peroxide bleaching agent had an adverse effect on the micro hardness of nanofilled and hybrid types of resin-based composite materials compared with the micro hybrid type.

Keywords: Carbamide peroxide, microhardness, microhybrid, nanohybrid


How to cite this article:
Solomon RV, Byragoni C, Jain A, Juvvadi Y, Babu R. An in vitro evaluation of microhardness of different direct resin-based restorative materials on using 10% carbamide peroxide gel as a bleaching agent . J Oral Res Rev 2016;8:59-65

How to cite this URL:
Solomon RV, Byragoni C, Jain A, Juvvadi Y, Babu R. An in vitro evaluation of microhardness of different direct resin-based restorative materials on using 10% carbamide peroxide gel as a bleaching agent . J Oral Res Rev [serial online] 2016 [cited 2019 Jun 25];8:59-65. Available from: http://www.jorr.org/text.asp?2016/8/2/59/192193


  Introduction Top


An observable difference in the color of the teeth is often the first evidence of variation from normal in human dentition. Tooth discoloration is a common dental finding, associated with clinical and esthetic problems. It has different etiology, appearance, composition, location, severity, and firmness in adherence to the tooth surface. [1] In general, tooth discoloration has been classified as intrinsic, extrinsic types sometimes it may be a combination of both. There are several treatments options for discolored teeth such as direct and indirect veneers, micro- and macro-abrasion, esthetic crowns, and bleaching, among these bleaching, is the most conservative, effective, and accessible treatment option for discoloration. Bleaching was first used in the 1870s, and its use widened after the introduction of home bleaching techniques as these are simpler, less expensive, less complicated, and requires less in-office time and these were introduced in the year 1989. [2],[3]

Bleaching refers to lightening and whitening of discolored vital or nonvital teeth using oxidizing materials, such as hydrogen peroxide, carbamide peroxide, and sodium perborate. These materials can penetrate the tooth structure, emitting reactive oxygen molecules, which dissolves and release chromogens. Bleaching generally has an approximate lifespan of 1-3 years; however, the change may be permanent in some situations. [4]

Carbamide peroxide (10%) is the most widely used home bleaching agent and is approved by American Dental Association. [2] Carbamide peroxide breaks down into one-third of hydrogen peroxide and two-thirds of urea. According to current scientific data, the time required to achieve the greatest whitening effect depends directly on the duration of treatment and the concentration of the applied gel but does not depend on the type of gel. [5]

Various studies have shown that home bleaching is safe in regards to its effect on tooth structure and oral soft tissues; however, there are some concerns about its effect on dental restorative materials. [6] Data from laboratory studies have shown increased mercury release from dental amalgams exposed to carbamide peroxide solutions for periods ranging from 8 h to 14-28 days. The amount of mercury released varied with type of amalgam and bleaching agent and it ranging from 4 to 30 times higher than in saline controls. It has been suggested that bleaching may increase the solubility of glass-ionomer and other cements. Furthermore, the bond strength between enamel and resin-based fillings was reduced in the first 24 h after bleaching. After 24 h, there was no difference in the strengths of composite resin bonds to bleached and nonbleached enamel. Following bleaching, hydrogen peroxide residuals in tooth structures inhibit the polymerization of resin-based materials and thus reduce bond strength. Therefore, tooth-bleaching agents should not be applied before restorative treatment with resin-based materials. [6],[7]

Chemical softening resulting from bleaching agents may affect the physical and mechanical properties including the durability of composite restorations. Studies that showed the relationship between bleaching and the surface microhardness of composite resins reported contradictory results. Few authors reported a decrease in superficial and deeper surface hardness of bleached composite resins following different bleaching regimens, while  others concluded that exposure of packable composites to 10% of carbamide peroxide gel has no significant effect on the surface hardness. [5],[8],[9]

Aim

The aim of the study to evaluate the effect of 10% carbamide peroxide bleaching agent on the microhardness of three types of direct resin-based restorative materials, and also to evaluate role of saliva in inhibition of microhardness reduction.


  Materials and Methods Top


Materials used in this study are:

  • Filtek Z 250 (Micro hybrid) (3M ESPE, St Paul, MN, USA)
  • Filtek Z 350 (Nano filled) (3M ESPE)
  • Filtek Z100 (hybrid) (3M ESPE)
  • Bleaching agent 10% carbamide peroxide (Opalescence) [Figure 1].
    Figure 1: 10% carbamide peroxide

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Specimen fabrication

For each resin-based composite [Figure 2], 15 disk-shaped specimens (4 mm diameter Χ 5 mm depth) were fabricated in shade A2. Cylindrical rubber molds were positioned on a transparent plastic matrix strip lying on a glass plate [Figure 3]. The resin-based composite materials were placed in 2.0 mm increments up to 5 mm depth. After the materials were inserted into the mold, a transparent plastic matrix strip was put over them [Figure 4], and a glass plate was secured to flatten the surface. Every specimen was light polymerized for 40 s, by means of a halogen light (Eli par 2500, 3M ESPE) within the range of 480-520 mW/cm 2 [Figure 5] and verified with a curing radiometer The specimens were polished with medium, fine, and superfine polishing discs (Sof-Lex, 3M ESPE) on a slow-speed handpiece rotating in one direction. After being polished, specimens were subjected to ultrasonic cleaning with distilled water for 2 min to remove any surface debris [Figure 6]. All specimens were stored in distilled water at 37΀C for 24 h. Specimens from each material were divided randomly into three groups (five/group) [Figure 7]:
Figure 2: Composite resin tubes

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Figure 3: Rubber molds

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Figure 4: Placing matrix strip

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Figure 5: Halogen light curing

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Figure 6: Ultra sonic bath

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Figure 7: Composite specimens

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Control group

Immersed for 14 days in artificial saliva, with no bleaching treatment [Figure 8]:

  • Group A: [Figure 9] Treated with a 10% carbamide peroxide bleaching agent for 14 days.
  • Group B: [Figure 10] Treated with 10% carbamide peroxide bleaching agent for 14 days and then immersed in artificial saliva for 14 days.
Figure 8: Control group (composite resin specimens immersed in artificial saliva)

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Figure 9: Group A (composite resin specimens treated with 10% carbamide peroxide)

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Figure 10: Group B (composite resin specimens treated with 10% carbamide peroxide and immersed in artificial saliva)

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Control group

The specimens in the control group were stored in artificial saliva for 14 days at 37΀C and no bleaching, followed by immersion in distilled water for 24 h at 37΀C in preparation for the microhardness test. The artificial saliva was replaced daily.

Bleaching procedure

Bleaching agent on the top surfaces

At the end of every bleaching application, the tested specimens were washed and placed in fresh artificial saliva for 16 h at 37΀C until the next application.

The artificial saliva was replaced daily in Groups A and B.

Vickers hardness test

After the specimens were dried for the Vickers hardness test was administered in a Micro-Vickers hardness tester (Model HVD-1000C series automatic Micro-Vickers hardness tester).

The specimens were placed on the platform, with the surface being tested facing the diamond indenter. A load of 300 g was applied to the surface for a 15 s dwell time.

Statistical analysis

Statistical calculations were performed with SPSS version 16.0 software (SPSS Inc., Chicago, IL, USA).

Data were subjected to one-way analysis of variance (ANOVA) and Turkey's honest significant difference multiple comparisons test, with the probability for statistical significance set at α = 0.05.


  Results Top


One-way ANOVA showed a significant difference between the behaviors of the Nanohybrid and hybrid resin-based composite materials under the different conditions of at-home bleaching in Groups A and B compared with the control group (P = 0.0001) [Table 1].
Table 1: Vickers hardness numbers means and standard deviations of different resin-based composite materials under different conditions of bleaching compared with control group and the statistical analysis


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Vickers hardness number (VHN) mean values of Filtek Z250 were minimally reduced in the different bleaching Groups (A and B) compared with that of the control group, but with no significant difference (P = 0.830 and P = 0.676, respectively).

VHN mean values of Filtek Z350 in Groups A and B were significantly reduced compared with that of the control group (P = 0.0001), whereas there was no significant difference between the VHN mean values of Groups A and B (P = 0.892).

VHN mean values of Filtek Z100 in Groups A and B were significantly reduced compared with that of the control group (P = 0.0001), whereas there was no significant difference between the VHN mean values of Groups A and B (P = 0.083) [Table 2] and [Chart 1 [Additional file 1]]. Micro-Vickers hardness tester microscopic images are shown in [Figure 11].
Table 2: Intra group comparison of Vickers microhardness values


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Figure 11: Micro-Vickers hardness tester microscopic images

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  Discussion Top


The demand for esthetic dentistry has dramatically grown in the past, and so there has been rapid development of new nonrestorative treatment for discolored teeth. Bleaching is a conservative treatment option for discolored teeth. Scientific basis for bleaching vital teeth is sound enough as the enamel and dentin are porous tissues. Clinicians all over the world have incorporated the vital bleaching into their profession and started using oxygenating agents such as carbamide peroxide or hydrogen peroxide for effective bleaching. [10],[11] A 10% of carbamide peroxide is the most commonly used bleaching agent in at-home bleaching technique.

The bleaching gel used in this study contains 10% of carbamide peroxide. This type of gel is unstable and degrades. It induces oxidative cleavage of polymer chains and then leads to chemical softening of dental materials. [8],[11]

The mechanism of action of bleaching agents on tooth structures apparently is due to oxidation of enamel and dentin molecules causing changes in color. There is a need for prolonged contact of the agent with the dental structure to allow the oxidation process to take place. This contact also occurs between the agent and preexisting restorations, with the latter being exposed to the same condition. Similarly, any chemical softening resulting from bleaching has implications on the clinical durability of restorations. [10] An organic matrix composite resins make these materials more prone to chemical changes than other restorative materials. [6]

Carbopol (carboxy polymethylene) that is added to thicken the gel, improve adherence to the tooth surface and prolong the release of oxygen and slows chemical reaction. [12]

Environmental temperature influenced the effects of bleaching on surface and subsurface microhardness of restorative materials. Bleaching at increased temperatures showed greater softening effects on the surface and subsurface layers of dental materials. [13] Hence, in this study, all the specimens were stored at 37΀C.

Artificial saliva comprised of sodium chloride (NaCl) 0.4 g, Potassium chloride (KCl) 0.4 g, calcium chloride (CaCl2.H2O) 0.795 g, sodium dihydrogen phosphate (NaH2PO4.H2O) 0.69 g, sodium sulfide (Na2S.9H2O) 0.005 g, and distilled water 1000 ml. The pH was adjusted to. It has been reported that the substances present in the composition of saliva may act as accelerators in degrading carbamide peroxide and may minimize its adverse effects by means of the salivary remineralizing potential. In contrast, the findings of this study showed that the use of artificial saliva as a storage medium during and after the bleaching procedure had no benefit, and the reduction in microhardness of the different resin-based composite materials was not inhibited. [14]

Hardness of material is defined as its resistance to permanent surface indentation or penetration, and this property is related to material strength, ductility, elastic stiffness, plasticity, strain, toughness, viscoelasticity, viscosity.

The ability of material to abrade or to be abraded by opposing dental structures, materials, or any chemical softening has implications for clinical durability of dental restorations. [15],[16],[17]

The presence of a higher amount of TEGDMA in the Z350 and the absence of TEGDMA in Z250 explains the low resistance of Z350 to bleaching agents. The incorporation of TEGDMA diluent monomers in the resin matrix may make the resin matrix less resistant to bleaching agents and may increase the softening of resin composite material. In contrast, the reduction in microhardness of Z350 (Nano filled) compared with Z100 (hybrid), which might be related to the presence of high molecular weight of the resin matrix and the lower filler content in Z350 (59.5%) compared with Z100 (66%). [14]

Filtek Z250 consists of 60% by volume of inorganic filler loading (3MESPE, Germany) and may be high enough to be closely packed together to resist to the oxidation and degradation of resinous matrix, hence resist the softening effect of the bleaching agents.

Another factor that may affect the integrity of resin composite surface hardness is the degree of which the filler is bonded to the resin matrix. In this study, the bonding of the inorganic fillers to the resin matrix in Filtek Z250 (micro hybrid) is adequate to resist the effect of bleaching treatment. [18]

Nanofilled composites release more TEGDMA (monomer) than micro hybrid composites. Nanofilled composites may present higher degradation in the oral environment than hybrid ones. This happens as the result of water sorption which leads to monomer elution. [19]

In addition, the theoretically larger total surface area of nanofiller particles allows more water to accumulate at the filler particle-polymeric matrix interfaces, thus increasing water sorption. Water may accumulated at the aggregated zirconia/silica cluster filler-organic matrix interface in the nanofilled composite can create paths for water diffusion toward the inside of aggregates, where microvoids are probably present due to lack of 5-20 nm sized primary particles being impregnated in the polymeric matrix. [20]

Based on the findings of this current study, bleaching agents should not be used indiscriminately in the patient's mouth, and the teeth that have extensive tooth-colored restorations should not be exposed to bleaching agents or at least protected by placing stoppers on the tray/prevent overlapping of tray on the gingiva. Finally, patients should be informed that the physical properties of tooth-colored restorations might be affected by the bleaching procedure, and the restorations might be softened. [21] This could potentially predispose to increased adherence of cariogenic bacteria, surface wear rate, stain absorption, and potential marginal/edge strengths of these restorations and that they may need to be replaced.


  Conclusion Top


Within the limitations of this in vitro study, the following conclusions were drawn:

A 10% of carbamide peroxide bleaching agent had an adverse effect on the microhardness of hybrid and nanofilled resin-based composite materials compared with the micro hybrid type.

The microhardness reduction in the different resin-based composite materials after bleaching was not inhibited by the use of artificial saliva storage media during and after the bleaching procedure.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11]
 
 
    Tables

  [Table 1], [Table 2]



 

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