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Table of Contents   
ORIGINAL RESEARCH  
Year : 2013  |  Volume : 24  |  Issue : 1  |  Page : 87-92
Effect of desensitizer application on shear bond strength of composite resin to bleached enamel


Dental Materials Research Center and Department of Operative Dentistry, Isfahan University of Medical Sciences, Isfahan, Iran

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Date of Submission21-Jul-2010
Date of Decision16-Aug-2010
Date of Acceptance11-Oct-2010
Date of Web Publication12-Jul-2013
 

   Abstract 

Objective: Tooth sensitivity is common after vital tooth bleaching. The aim of this study is to evaluate the effect of a desensitizing agent on shear bond strength of composite resin to bleached enamel; and determine whether a delay of one or two weeks in bonding procedure is sufficient subsequent to bleaching/desensitizer regimen.
Materials and Methods: Buccal enamel surfaces of ninety-six human sound molars were prepared and divided into eight groups. The surfaces of specimens in Group 1 as negative control group were bonded by composite resin using the single bond adhesive. Specimens in Groups 2-4 were bleached with an at-home bleaching agent (Daywhite ACP). Relief ACP desensitizing gel alone was applied in Group 5. In Groups 6-8, specimens were bleached same as in Group 2 and relief ACP desensitizing gel was applied same as inGroup 5 subsequent to each bleaching session. Composite cylinders were bonded after 24 h, 7 days and 14 days in Groups 2-4, respectively, and also in Groups 6-8, respectively. The shear bond strengths of the cylinders were tested and data was analyzed using two-way ANOVA and Tukey test (α = 0.05).
Results: The results showed that bleaching and bleaching/desensitizer regimens significantly reduced the bond strength of composite resin to enamel. However, desensitizer alone did not reduce bond strength. No statistically significant differences were found between bleaching and bleaching/desensitizer regarding bond strength.
Conclusion: Bleaching or bleaching/desensitizer treatment significantly decreases bond strength of composite resin to enamel. In both regimens, adhesive bonding is recommended after two weeks.

Keywords: Bleaching, bond strength, desensitizer, enamel, sensitivity

How to cite this article:
Khoroushi M, Ghazalgoo A. Effect of desensitizer application on shear bond strength of composite resin to bleached enamel. Indian J Dent Res 2013;24:87-92

How to cite this URL:
Khoroushi M, Ghazalgoo A. Effect of desensitizer application on shear bond strength of composite resin to bleached enamel. Indian J Dent Res [serial online] 2013 [cited 2020 Oct 28];24:87-92. Available from: https://www.ijdr.in/text.asp?2013/24/1/87/114961
Tooth discoloration is a great esthetic problem. Its etiologic factors are varied and complex but are usually classified as intrinsic, extrinsic, or internalized in nature. [1] Today, in-office or at-home, vital bleaching has gained popularity among dental practitioners and patients as a simple, safe, conservative, effective, and predictable method to lighten teeth, with acceptable outcomes. [2] Vital tooth bleaching uses oxygenating agents, such as hydrogen peroxide and its derivatives, to diffuse through enamel and dentin. These materials decompose into perhydroxyl anions (HO 2− ) and free radicals to react with organic chromophores. Therefore, the enamel and dentin are lightened. [3]

Although bleaching treatments clearly result in a lighter tooth color, they also cause some complications such as tooth sensitivity, [3] possible changes in tooth structure, [4],[5] decreases in microhardness, [4],[6] microleakage of restorations, [7],[8] external root resorption, [1] and decreased bond strength of resins. [9] Clinicians and researchers are all interested in determining whether any of the changes in the enamel surface also result in alterations of its adhesive characteristics to bonding materials. Several authors have addressed the problem, reporting a significant reduction in bond strength of resin materials and tooth fracture resistance subsequent to bleaching. [10],[11]

It has been reported that patients undergoing vital bleaching techniques might experience tooth sensitivity as a side effect. [12],[13],[14] Earlier studies have attempted to identify factors involved in sensitivity; however, only a history of sensitive teeth and more than one application per day have been reported to have any correlation. [15],[16] Age, sex, exposed dentin or cementum, cracks, pulp size, allergies and decay were not considered as factors involved in sensitivity. [15],[16]

At present, history of tooth sensitivity is not considered as a contraindication for vital bleaching. There are several methods to reduce tooth sensitivity after or during bleaching. Decreases in wearing time and frequency of application, temporary interruption of whitening procedure and use of active ingredients such as potassium nitrate, [16],[17] or fluoride, [18],[19],[20] and amorphous calcium phosphate (ACP) [21] are the methods commonly used by clinicians, which has prompted manufacturers to incorporate them into their whitening products or offer them as separate products, referred to as desensitizers.

Fluoride as an ion or combined with other elements, such as sodium fluoride, potassium nitrate, and more recently ACP, are being used in such products. [21],[22] Additionally, some multipurpose materials, such as CPP-ACP, are recommended in case of tooth sensitivity and for tooth remineralization. [23] It is well documented that potassium nitrate reduces dental sensitivity by decreasing the ability of nerve fibers in the dental pulp to repolarize after an initial depolarization due to pain sensation. Fluoride is added to potassium nitrate because it is believed that fluoride aids in the alleviation of sensitivity by blocking dentinal tubules, thus reducing fluid flow to pulp chamber. It is also believed that ACP reduces sensitivity by blocking tubules and surface defects with hydroxyapatite crystals which form from the interaction of ACP and carbonate. [22]

A few studies have shown that the inclusion of these agents in bleaching materials does not affect bleaching efficacy. [17],[21],[24] Furthermore, there are some concerns about the effect of these desensitizing agents on tooth structure and composition. According to the recent studies, fluoridation increases microhardness in bleached teeth. [4],[6],[24] Some studies have demonstrated that inclusion of sodium fluoride into bleaching agents improves enamel remineralization through an increase in inorganic crystals containing fluoride. [25],[26]

Application of such different desensitizing agents with whitening agent or using them separately by patients before or subsequent to bleaching may affect bonding of resin to tooth structure. Metz et al., reported a decrease in bond strength subsequent to bleaching/desensitizer regimen up to 14 days. [21] Moule et al., reported that combined use of carbamide peroxide and CPP-ACP significantly reduces bond strength. [23] A study by Tόrkkahraman et al., showed that beaching and desensitizer application, including 3% potassium nitrate and 0.11% fluoride ion, significantly decreased shearing bond strengths of orthodontic brackets on human enamel. [22] Chaung et al., reported that treatment with 0.37% fluoridated carbamide peroxide maintained the bond strength as effectively as the unbleached enamel. They pointed out that additional fluoride in bleaching agents may facilitate subsequent restorative treatment by inhibiting enamel demineralization. [27]

It seems changes in enamel structure and composition, and also enamel bond strength induced by these bleaching agents, especially by the newer agents and/or different protocols for rehardening/desensitizing, need more evaluations. Furthermore, as the most acceptable recommendation is to delay any restorative bonding after bleaching to solve the transient adhesion problem, [1] the delay in bonding at least for one-three weeks after the last bleaching session is recommended. But it is not clear whether the same regimen in postponing bonding procedure is enough subsequent to the bleaching/desensitizing protocols. Therefore, the aim of this in vitro study have been to evaluate the effect of bleaching and/or desensitizer application on shearing bond strength of composite resin to enamel; and determine whether a delay of one or two weeks in bonding procedure is sufficient subsequent to bleaching/desensitizing regimen. A newly-formulated desensitizing gel, named "relief ACP", was evaluated.


   Materials and Methods Top


Subsequent to approval by the University Human Research Ethics Board, 120 extracted human molars were collected and stored in 0.2% thymol solution at 4°C. Then, ninety-six intact extracted human permanent molars without any caries or visible defects were chosen and stored in 0.2% thymol solution at room temperature. Before the experiment, the teeth were stored in distilled water for 24 h for the complete removal of thymol residues. The roots were separated from the crowns approximately 2 mm apical to the cemento-enamel junction using a low-speed diamond saw under water cooling. Each tooth was individually embedded in auto-polymerizing acrylic resin. The teeth were placed, with their buccal surfaces upward, parallel to the ground and cleaned with fluoride-free pumice using a plastic brush mounted on a low-speed rotary instrument. The specimens were kept in distilled water at 37°C except during the bleaching, desensitizing, bonding, and testing procedures.

All the teeth were randomly assigned to 8 groups of 12 teeth each:

Group 1: (Negative Control): Neither bleaching agent nor desensitizer was applied.

Groups 2, 3 and 4: The teeth were bleached with 9.5% hydrogen peroxide (Daywhite ACP, Discus Dental, USA). The bleaching gel was placed in direct contact with the prepared enamel surfaces for 2 h. To this end, plastic rings were prepared and fixed on enamel surfaces using cellophane; the rings were 5 mm in diameter and 2 mm in height. Then the bleaching gel was injected into the wells produced. After the rings were filled and it was made sure that no bubbles were present, they were covered with the second layer of cellophane to protect the bleaching gel from environmental contaminants and air. After 2 h, the gel was washed away and the same procedure was repeated twice daily for five consecutive days. During the bleaching period, the specimens were kept in an incubator at 37°C at relative humidity.

In specimens in Groups 2, 3 and 4 on the 6 th , 12 th and 19 th days (after 24 h, one week and two weeks), composite cylinders were bonded using single bond (3M ESPE; St Paul, MN, USA) and Z100 composite resin (3M ESPE), respectively. Before bonding, the teeth were incubated in distilled water at 37°C.

Group 5: Only relief ACP desensitizing gel (Relief ACP, Discus Dental, USA) was placed on the enamel surfaces of the teeth for half an hour according to manufacturer's instructions. The gel was washed away and the same procedure was repeated twice daily for five consecutive days. All the teeth were rinsed with water before bonding.

Groups 6, 7 and 8: The teeth were bleached with 9.5% hydrogen peroxide (Daywhite ACP, Discus Dental, USA). The bleaching gel was placed in direct contact with the prepared enamel surfaces for 2 h. Therefore, the teeth were bleached in the same manner as for Groups 2-4, except for the fact that after each session of bleaching, relief ACP desensitizing gel (Relief ACP, Discus Dental, USA) was placed on the enamel surfaces of the teeth for half an hour according to manufacturer's instructions (in the same manner as described for Group 5). All the teeth were rinsed with copious amounts of water, and bonding of composite resin was performed as described for the other corresponding groups.

Bonding procedure

Before bonding, the facial surfaces of all the teeth were thoroughly rinsed with water and dried with oil- and moisture-free compressed air. A piece of adhesive tape with a 2.5-mm-diameter hole was closely adapted to the center of the flat buccal enamel of each tooth to demarcate the bonding area. Single bond (3M ESPE) was used as bonding agent as follows:

The enamel surface was etched with 35% phosphoric acid for 15 sec and rinsed with water spray for 10 sec leaving the tooth moist. Two consecutive coats of the adhesive were applied with a fully-saturated microbrush tip, gently dried for 2 to 5 sec, and then light-cured for 10 sec. A halogen light-curing unit (Coltolux 2.5, Coltene AG, Feldwiesenstrasse Altstδtten/Switzerland) with 480 mW/cm 2 output was used to cure the adhesive. A plastic mold with a circular hole of 3 mm in diameter and 4 mm in depth was placed over the hole in the adhesive tape. Resin composite (Z-100, 3M ESPE) was placed in 2-mm increments and cured in the mold to form cylindrical posts perpendicular to the enamel surface. Each increment was cured for 40 sec. The specimens were stored in distilled water at 37°C for 24 h; then were subjected to 500 thermal cycles between 5°C and 55°C in water baths, with a dwell time of 30 sec and a transfer time of 12 sec [Table 1].
Table 1: Summary of the experimental design for the study groups


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The shear bond strengths were measured with a Dartec Universal Testing Machine (Dartec, HC10, Dartec Ltd, Stourbridge, UK). A metal knife-edge chisel, aligned 0.2 mm away from the bonded interface, was used as the loading device. The distance from the shearing rod to the enamel surface was maintained using a spacer of two celluloid matrices. The load was applied in a displacement-controlled mode at a crosshead speed of 1 mm/min until failure occurred. In order to measure bond strength in MPa, the resultant force in Newton was divided into the cross-section of the bonded area. The bond strength data were statistically analyzed by two-way ANOVA and a post hoc Tukey test using SPSS 11.5 (α = 0.05).


   Results Top


Shear bond strengths in MPa (means ± SD) for the specimens in study groups are given in [Table 2]. Analysis of variance indicated a significant difference between the groups (P < 0.001) [Table 2]. The highest values of bond strengths were measured in Group 1. The bond strengths in Groups 2 and 3 were significantly lower than those in Group 1 (P < 0.001, and P < 0.001, respectively). No significant differences were observed between Groups 1, 4, 5 and 8 (P < 0.05).
Table 2: Shear bond strength mean ± SD in MPa (n=12)


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Two-way ANOVA revealed that the bond strengths were influenced by "delay in bonding procedure" (F = 181.169, P < 0.001); however, it was not influenced by "desensitizer application" (F = 13.356, P > 0.05) [Figure 1]. The interaction of two factors, "desensitizer application" and "delay in bonding procedure", was not significant (P = 0.784).
Figure 1: Shear bond strength variations in the study groups (MPa) according to the two variables: "time of bonding" and "desensitizer application"

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Multiple comparisons by Tukey test demonstrated significantly higher bond strengths in Groups 1, 4, 5 and 8 compared to other Groups (P < 0.001). Group 5 exhibited bond strength values similar to Group 1 (P = 0.723).

Regarding failure mode, the highest and the lowest percentages of adhesive fractures were observed in groups 2 and 1, respectively [Table 3]. The number of enamel cohesive fractures was higher in Group 5. Significant differences were found among the study groups in terms of fracture modes (P < 0.05) [Table 3].
Table 3: Different fracture modes in the study groups (N)


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   Discussion Top


In the present study, the enamel bond strength underwent changes by the whitening gel. A large number of reports have elaborated on the relationship between bleaching agents and the bond strength of composite materials to enamel subsequent to bleaching. [8],[11],[21],[28],[29],[30],[31] Some authors have suggested that residual bleaching agents influence the bonding process and are responsible for decreased bond strength. [11],[28] Others speculated that the decrease in bond strength might be attributed to any of the changes in enamel surface and/or composition. [32],[33]

In the present study, relief ACP as a triple-action gel, consisting of 5% potassium nitrate, 0.22% sodium fluoride and 0.375% ACP, was used as a desensitizing agent. Furthermore, the whitening agent used in this study includes ACP; however, ACP concentration has not been mentioned by the manufacturer. Increased levels of potassium nitrate up to 5% in relief ACP may maintain the depolarized state of the sensory nerves, decreasing pain perception capacity. [34],[35] Fluoride compounds, including sodium fluoride, are able to reduce tooth sensitivity through precipitation of fluoroapatite from calcium and phosphate ions of saliva and forming an insoluble precipitate. [34] One study reported that the incorporation of sodium fluoride into bleaching compounds increases remineralization by increasing mineral crystals containing fluoride within demineralized layer of tooth structure, decreasing tooth sensitivity. [25] Moreover, the incorporation of ACP into desensitizing agents has attracted a great deal of attention recently. On the basis of the generally accepted molecular formula for ACP [Ca3 (PO4)2 -nH2O], ACP also may be considered a tricalcium phosphate. There is no conclusive evidence that ACP is an integral mineral component in hard tissues. It likely plays a specific role as a precursor to bioapatite and as a transient phase in biomineralization. In solutions, ACP is readily converted to stable crystalline phases such as octacalcium phosphate or apatitic products. [36] According to some studies ACP can seal dentinal tubules and create a layer of hydroxyapatite on the surface of enamel. [23] This agent might also have rehardening effect on tooth structures.

The effects of the application of fluoride compounds after bleaching on the bonding of composite resin to enamel have been previously investigated. Among the studies which are in agreement with the present study Damon et al., [37] Kelclik et al., [38] Xiaojun et al., [39] and Chuang et al., [27] can be mentioned in all of which the compounds of relief gel were examined separately and did not show a reducing effect on composite bond strength to enamel. As previously mentioned, the compound used in the present study contained three desensitizers. In contrary, in the studies by Keilbassa et al., [40] and Metz et al., [21] with higher concentrations of fluoride (%2.26) and longer application periods the bonding of composite to enamel was reduced. However 24-h use of fluoride which was mentioned in the study conducted by Keilbassa et al., [40] is not clinically feasible. Moreover, Moule et al. investigated the effect of GC Tooth mousse (containing CPP-ACP compound) on the bonding of composite to enamel following bleaching and stated that after bleaching this substance reduces the bonding of both self-etch and total-etch bonding systems to enamel. [23]

The results of the present study did not show any statistically significant differences between the control values and treatment provided by desensitizer (Groups 1 and 5). However, all the bleached groups with immediate bonding, including Groups 2 and 6, had significantly lower values than the negative control values. The results of many previous studies are consistent with this results. [21],[28],[29],[30],[31],[41] Consistent with the outcomes of this study, qualitative comparisons of resin tags present in the bleached and unbleached specimens using scanning electron microscopy (SEM) have revealed few, thin and fragmented resin tags when hydrogen peroxide and carbamide peroxide are used. [32] Until now, no microstructural study has been performed on the qualification of bonding on the bleached/desensitized dental tissues.

The results of this study showed the superiority of two-week-delayed bonding over one-week-delayed bonding after bleaching, in two conditions of with and without the application of desensitizing gel. In addition, considering the results obtained for Groups 3 and 7, and also Groups 4 and 8, the application of desensitizer showed an insignificant decrease in bond strength. Consistent with this study, there have been several in vitro studies which have recommended waiting at least for two weeks before adhesive bonding. [10],[21],[26],[27],[28],[29],[30],[42],[43],[44] It seems discrepancies in the results of some previous studies regarding suitable time of bonding subsequent to bleaching might be attributed to differences in bleaching regimen/agent.

Bleaching agents have been reported to decrease enamel microhardness. [4],[6],[21],[44] In the present study, bleached enamel specimens showed an increase in adhesive failures compared with the negative control group. Consistent with the results of this in vitro study, some previous studies have shown a decrease in cohesive enamel fracture subsequent to bleaching. [21],[45] The use of remineralizing/rehardening or desensitizing agents fail to restore failure mode after bleaching (Group 6) but subsequent treatment of unbleached teeth with desensitizer (Group 5) resulted in an increase in cohesive enamel failures, and acceptable bond strengths were also recorded. However, it seems treatment of bleached dental tissues with these different agents as desensitizing and/or rehardening regimens and the real effect of each agent need to be further investigated.


   Conclusion Top


Within the limitations of the present study, it can be concluded that:

  • Bleaching and bleaching/desensitizer application has a significant influence on bond strengths of composite resin to enamel.
  • Subsequent to the completion of at-home bleaching or bleaching/desensitizing procedures, delaying in adhesive restorations for two weeks is preferred.



   Acknowlegement Top


The authors gratefully acknowledge that this report is based on a thesis submitted to the School of Dentistry, Isfahan University of Medical Sciences, in partial fulfillment of the requirement for a DDS degree (Grant No. 318180).

 
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Correspondence Address:
Maryam Khoroushi
Dental Materials Research Center and Department of Operative Dentistry, Isfahan University of Medical Sciences, Isfahan
Iran
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Source of Support: The research project (#318180) was supported financially by the vice chancellor for research at Isfahan University of Medical Sciences., Conflict of Interest: The authors declare that they have no individual, financial, or personal interests of any nature or kind in any commercial product and/or company that is presented in this article.


DOI: 10.4103/0970-9290.114961

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2 Effect of different surface treatment on the shear bond strength of metal brackets to bleached and desensitized enamel
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3 Frequently asked questions about vital tooth whitening
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4 Frequently asked questions about vital tooth whitening
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5 Casein Phosphopeptide-Amorphous Calcium Phosphate and Shear Bond Strength of Adhesives to Primary Teeth Enamel
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