| Abstract|| |
Objectives: As enamel bond strength of the resin-modified materials is reduced after bleaching, a delay bonding of at least 1 week is recommended. This study investigated the efficacy of 10% sodium ascorbate hydrogel on the bond strength of bleached enamel as an antioxidant agent.
Materials and Methods: Ninety-six human, sound molars divided into eight groups. Buccal enamel surfaces of specimens in two negative control groups after preparation were bonded by the Z100 CR using the single-bond adhesive and Vitremer RmGI. Specimens in other groups after enamel preparation were bleached with 9.5% hydrogen peroxide. In two positive control groups, bonding of CR and RmGI was performed immediately after bleaching. Specimens in two other groups were immersed in distilled water after bleaching, and then CR and RmGI were bonded after 1 week. In last two groups, 10% sodium ascorbate hydrogel was applied after bleaching, and then they were bonded by CR and RmGI. The specimens were placed under shear force.
Statistics: ANOVA and Duncan's multiple range tests were used for data analysis.
Results: Application of 10% sodium ascorbate hydrogel immediately before bonding and delay bonding for 1 week eliminated the negative effects of bleaching on CR and RmGI enamel bond strength.
Conclusion: Application of 10% sodium ascorbate hydrogel or 1 week period elapsed after bleaching could significantly increase the enamel bond strength to a normal value.
Keywords: Antioxidant agent, bleaching, bond strength, hydrogen peroxide, sodium ascorbate hydrogel
|How to cite this article:|
Mazaheri H, Khoroushi M, Shafiei E, Ghorbanipour R, Majdzade F. Bond strength of composite-resin and resin-modified glass ionomer to bleached enamel: Delay bonding versus an antioxidant agent. Indian J Dent Res 2011;22:432-5
A combination of esthetic restorative treatment and vital bleaching is an important treatment plan in esthetic dentistry.  In many instances, replacement of previous restorations after bleaching leads to better esthetic results  , because bleaching agents can not whiten the color of esthetic materials such as composite resin.  Vital teeth bleaching using a mouthguard is a common technique  and has been used in many researches which support its effectiveness according to scientific evidence.  The bleaching process adversely but transitionally reduces enamel and dentin bond strength when immediate bonding is performed after bleaching. ,,,,,,, This reduction can likely be related to a delayed release of oxygen from the bleaching agent that could either interfere with resin infiltration into etched enamel or inhibit polymerization of adhesive resin. ,,,, Up to now, the most acceptable recommendation is to delay any restorative bonding after bleaching to solve this transient adhesion problem.  It seems that the delay bonding at least 1 week after the last bleaching treatment is adequate. ,,,,, In addition, as slight tooth shade regression almost always is seen during the first few days after bleaching, delay bonding allows the tooth shade to stabilize. ,
|How to cite this URL:|
Mazaheri H, Khoroushi M, Shafiei E, Ghorbanipour R, Majdzade F. Bond strength of composite-resin and resin-modified glass ionomer to bleached enamel: Delay bonding versus an antioxidant agent. Indian J Dent Res [serial online] 2011 [cited 2019 Jul 23];22:432-5. Available from: http://www.ijdr.in/text.asp?2011/22/3/432/87066
Recent studies have shown that the reduction in bond strength can be reversed as a result of the application of sodium ascorbate as a biocompatible antioxidant agent. ,,,,, In some situations, delay restorative treatment may tire patients and decrease their motivation. Therefore, if the application of sodium ascorbate can be effective, it will be possible to perform the bonding procedure without any waiting period. On the other hand, tooth shade regression seems not to be very significant to affect the shade of esthetic materials selected especially by an experienced dentist. It could be minimized, when the patient is motivated to optimize his oral hygiene and temporarily does not consume pigment containing foodstuff. In most studies, the solution forms of sodium ascorbate have been used, ,,, which may not be clinically acceptable.  If the hydrogel form has similar effects on the solution forms, with 10% concentration and application time of at least 1 h which have been reported in four studies, ,,, it can be easily used by the patients under dentist supervision with minimal outpatient waiting. For cervical lesions, even in non-carious cases, RmGI restorations are a reasonable selection that can be applied using a final increment of composite resin for an improved esthetic result. It was reported that the overall clinical performance of the RmGI restorations is superior to the combination of one-bottle adhesive and resin composite restorations.  Therefore, this study investigated the effects of delay bonding for 1 week after bleaching and the application of sodium ascorbate hydrogel immediately before bonding, on the bond strength of CR and RmGI to bleached enamel by approaches similar to clinical conditions.
| Materials and Methods|| |
This was an in vitro study performed in Torabinejad Dental Research Center at Isfahan school of dentistry in autumn 2008. Ninety-six sound human molars; extracted in the course of 3 months and stored in 0.2% thymol solution, were randomly divided into eight groups (n=12). Each tooth was embedded in self-cured acrylic resin (Triplex, Ivoclar Vivadent AG, Schaan, Liechtenstein) using a cylindrical mold.
Negative control groups: 1 (NC1) and 5 (NC2)
Specimens stored in distilled water at 37 °C for 1 week. After enamel preparation, bonding was performed by using CR and RmGI respectively for groups 1 (NC1) and 5 (NC2).
Positive control groups: 2 (PC1) and 6 (PC2)
After fabrication of the custom trays for each specimen, the enamel preparation was performed and the specimens were bleached. Then, they were rinsed and immersed in distilled water for 10 min. The bonding was carried out by using CR and RmGI respectively for groups 2 (PC1) and 6 (PC2).
Delay bonding groups: 3 (DB1) and 7 (DB2)
After the enamel preparation and bleaching similar to positive control groups, specimens were rinsed and immersed in distilled water at 37°C for 1 week.
Then bonding was done with CR and RmGI respectively for group 3 (DB1) and 7 (DB2).
Sodium ascorbate groups: 4 (SA1) and 8 (SA2)
After enamel preparation and bleaching, 10% sodium ascorbate hydrogel was applied in the same way as that of bleaching agent for 10 h at 37°C and 100% relative humidity. Specimens were then rinsed and immersed in distilled water for 10 min to dissolve the sodium ascorbate crystals deposited on the bonding surfaces. , Then, the bonding was carried out by using CR and RmGI respectively for groups 4 (SA1) and 8 (SA2).
Buccal enamel surface of the specimens were polished with wet 600-grit silicon carbide abrasive paper using polishing wheels for 60 s to create a flat enamel surface and eliminate surface aprismatic enamel. ,
The custom trays fabricated for each specimen with a reservoir space in buccal surface using a heat vacuum forming unit (Er Kodent, Erko form, RVE, Germany) and bleaching gel (Day white ACP, Discuss Dental Inc., Culver City, CA, USA) containing 9.5 hydrogen peroxide was applied for 6 hours a day for five consecutive days at 37 °C and 100% relative humidity. This daily regime was consistent with one of the recommendations provided by the manufacturer. After completion of the daily bleaching procedure, the specimens were rinsed and immersed in distilled water at 37 °C.
For specimens in groups 4(SA1) and 8 (SA2), 10% sodium ascorbate hydrogel was applied in the same way as bleaching agent for 10 hours-one third of total bleaching time-according to the method described by Lai et al. 
For each specimen in groups 1 to 4 a hole of 4 mm in diameter was made in acetate strips (Hawe Neos Dental, Bioggio, Switzerland) that was fixed to buccal surfaces prepared for bleaching and bonding. After etching of the open surfaces using 35% phosphoric acid (Scotch Bond Etchant, 3M ESPE, Dental products, St Paul, MN, USA) and the application of adhesive (Single Bond; 3M ESPE, Dental products, St Paul, MN, USA) according to the manufacturer's instructions [Table 1], translucent plastic cylindrical molds with a circular hole of 3 mm in inner diameter and 4mm in length were placed on the open surfaces. Composite resin (Z100; 3M ESPE, Dental products, St Paul, MN, USA) was placed into each cylinder in two layers, each layer was light cured with a conventional quartz halogen LC unit (Coltolux 50, Coltene, Whaledent Inc, Mahwal, NJ, USA) for 40 s in the opposite sides with moving the light. Additional light curing was carried out for 20 s from the end of the cylinder .After removing the molds and strips, the specimens were stored in distilled water at 37 °C for 24 h. For specimens in groups 5 to 8, resin modified glass ionomer (Vitremer; 3M ESPE, Dental products, St Paul, MN, USA) was applied according to the manufacturer's instructions, [Table 1], in the same way as composite bonding approach. Specimens were stored in distilled water at 37 °C for 24 h.
|Table 1: Materials used in the study and mode of their applications according to the manufacturer's instructions (3M ESPE)|
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Shear bond strength testing
Shear bond strength of each cylinder was determined using a universal testing machine (Dartec, Series HC10, England) at a crosshead speed of 1 mm/min. The data were expressed in MPa.
RmGI did not bond to specimens in group 6 (PC2). Therefore, all the data was zero in this group. Other data was analyzed by One-way analysis of variance followed by Duncan's multiple range test at the significant level of P<0.05 by use of SPSS software (SPSS Inc., Chicago IL, USA).
| Results|| |
The results of shear bond strength tests are shown in [Table 2]. In comparison to the groups bonded with CR shear bond strength, the values in group 2 (PC1) were significantly lower than other groups. There was no significant difference among other groups.
In comparison to the groups bonded with RmGI, bonding did not occurred in group 6 (PC2) and there was no significant difference among other groups.
| Discussion|| |
In most previous studies about bleaching, 10% carbamide peroxide has been used. But, the selected bleaching agent in this study contained 9.5% hydrogen peroxide, designed for at-home use. Application of this approach to investigate the methods proposed to reverse the compromised bond strength seems to be interesting. The results of this study demonstrated that immediate bonding to bleached enamel leads to a significant reduction in composite bond strength that is in accordance to some previous studies. ,,,,,,, In specimens where RmGI was used the bonding did not occur. However, Homewood et al. reported that bleaching with 10% carbamide peroxide did not affect the enamel bonding of RmGI cement used for brackets at 24 hours and 14 days after bleaching.  Inconsistent results may be due to higher concentration and longer application time of the bleaching agent in the present study. The reason for the reduction in bond strength is not understood clearly, but it seems to be the result of presence of residual oxygen in the teeth,  that inhibit resin polymerization and interfere with resin infiltration into the etched enamel. ,,,, Furthermore, some researchers have studied on the morphological and organic alternations in enamel structures, the loss of calcium, and decrease in microhardness as important factors resulting in reduced enamel bond strength. , SEM evaluation in the interfaces between resin and bleached enamel has shown that large areas of the enamel surface were free of resin; present tags were fragmented, poorly defined, and penetrated to a lesser depth than in the unbleached controls. ,,
According to the results of this study, it seems that the ionic bond of RmGI is also adversely affected by bleaching possibly due to a significant decrease of enamel calcium content. Additionally, etching process probably helps to neutralize some components of the bleaching agent, which can interfere with ionic bonds of RmGI. And this process has not been applied to RmGI, which needs more investigation. A delay bonding of 1 week could completely eliminate the adverse effects of bleaching on CR and RmGI bonding according to the researchers that support the delay of bonding for 1-2 weeks after bleaching. ,,,,, This may be due to removing of the residual oxygen from the bleaching material as the result of the immersion process. 
Recent studies have shown that compromised bonding of composite resin to acid etched-bleached enamel can be reversed both by 10% sodium ascorbate solution ,,,,, or if the hydrogel form is used 10% concentration is reliable ,, and for maximum effectiveness, it should be applied at least for 60 min.  The antioxidizing ability of sodium ascorbate helps to neutralize and reverses the oxidizing effects of bleaching agents. Therefore, the altered redox potential of the oxidizing bonding substrate will restore and polymerization of the adhesive will continue without permanent termination.  The results of this study support these findings, since there was no significant difference among delay bonding and sodium ascorbate groups in comparison with unbleached control groups.
According to the results of this study, application of 10% sodium ascorbate hydrogel for 10 h is effective. It can eliminate long period elapsed before esthetic restorative treatment and compared to solution form, it leads to a more comfortable application and minimal outpatient waiting chair time. However, further investigations are necessary to find out a certain protocol for application of sodium ascorbate.
| Conclusions|| |
- Bleaching reduces immediate bond strength of composite resin and resin modified glass ionomer cement does never once bond in this condition.
- Application of 10% sodium ascorbate hydrogel or a delay bonding of 1 week completely return the bond strength of bleached enamel to normal values of both the composite resin and the resin-modified glass ionomer.
- There is no difference between the mentioned methods in reversing the reduced bond strength.
| Acknowledgment|| |
This study (nb.386166) was supported financially by Research Center of Isfahan University of Medical Sciences and Health Services, Isfahan, Iran. Also the authors wish to express their sincere respect to the deans and staff members of Prof. Torabinejad Dental Research Center.
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Department of Orthodontics, Isfahan Dental School, Isfahan University of Medical Sciences, Isfahan
Source of Support: Research Center of Isfahan University of Medical Sciences and Health Services, Isfahan, Iran., Conflict of Interest: None
[Table 1], [Table 2]