| Abstract|| |
Objective: To evaluate the bond strength of a composite resin to the bovine enamel bleached with 35% hydrogen peroxide. It was used an etching-and-rinse adhesive system employed immediately, 7 and 14 days after the bleaching.
Materials and Methods: Twenty bovine teeth were randomly distributed into 4 groups (n = 5), 3 experimental and 1 control. G1: Unbleached + restoration 14 days after storage in artificial saliva (control); G2: Bleached + restoration immediately after bleaching; G3: Bleached + restoration 7 days after bleaching; G4: Bleached + restoration 14 days after bleaching. Their buccal enamel surfaces were flattened, and a 25 mm² (5 × 5 mm) area from each one of these regions was outlined so as to standardize the experimental region. Universal hybrid composite resin Filtek™Z350 was inserted into four layers of 1 mm each and photo-activated. The bond strength was quantitatively evaluated by a microtensile test (1.0 mm/min) 24 h after the restorative procedures. The failure mode was assessed through scanning electron microscopy (SEM).
Results: There was a significant reduction in the bond strength of the restored teeth immediately after the bleaching (G2). There were no significant differences in enamel bond strength between groups G1, G3, and G4. There was a predominance of adhesive and mixed (cohesive + adhesive) failure in all groups.
Conclusion: The 7-day-period after the end of the bleaching with 35% hydrogen peroxide was enough to achieve the appropriate values of bond strength to the enamel.
Keywords: Dental enamel, tensile strength, tooth bleaching
|How to cite this article:|
Lago AD, Garone-Netto N. Microtensile bond strength of enamel after bleaching. Indian J Dent Res 2013;24:104-9
Dental bleaching currently represents a feasible and conservative treatment alternative to most teeth with color alteration. Patients who have their teeth bleached must be aware that, once this treatment is over, restorations of composite resin or anterior teeth prosthesis will have a different color compared to the bleached teeth. Thus, they may need to be changed.
The first reports of tooth bleaching date back to the mid-19 th century, with the employment of different chemical agents, such as the chloride ion, sulfur vapors, oxalic acid, calcium, and sodium chloride. 
Bleaching is a procedure performed with hydrogen and/or carbamide peroxide in different concentrations both in vital and devitalized teeth. The hydrogen peroxide is a powerful oxidizing agent with the ability to penetrate the enamel and dentin, due to its low molecular weight, and remove the stains. ,,,
Hydrogen peroxide is an unstable molecule and tends to dissociate giving rise to free radicals and other subproducts. Free radicals generated have great ability to spread through the dental tissues and strong attraction to carbon compounds as those found in the pigments of dental structures. The reaction between peroxide and organic materials on the surface or in the subsurface enamel can result in morphological alterations. ,
Due to oxygen liberation during the bleaching, bleaching agents used in dentistry may cause morphological alterations to enamel. ,,, These alterations may affect the bond strength, jeopardizing the polymerization of adhesive materials. ,
The enamel is a highly mineralized structure, thus, it is subject to suffering acid etching. The bonding to the enamel is a procedure, which is already accepted in literature, and its result is very uniform. However, the bleaching effect on the enamel surface in relation to the bond strength is controversial in literature. Some studies found a reduction in bond strength when restorative procedures were performed immediately after the bleaching; ,,, others concluded that this reduction occurred due to the type of adhesive employed and not due to the bleaching. ,
The reduction in bond strength after bleaching is time-depending  and much debated in literature. This study aimed at assessing the bond strength to bovine enamel using an etching-and-rinse adhesive system (Adper Single Bond 2™, 3M/Espe, St. Paul, MN, USA) employed immediately, 7 and 14 days after the end of the bleaching with 35% hydrogen peroxide (Whiteness HP, FGM, Joinville, SC, Brazil). The null hypothesis tested was bond strength is not influenced by the 7 and 14 days after the end of the bleaching.
| Materials and Methods|| |
This research was approved by the Committee of Ethics in Research - Subcommittee of Bio-ethics of Animals of the School of Dentistry of the University of São Paulo, under protocol number 06/08.
The use of bovine teeth in researches that evaluate the bond strength to enamel surface is already established in literature, since the results are similar to those found in human teeth. ,
Two weeks after the extraction, 20 bovine incisors were selected and cleaned with periodontal curettes (Duflex, SS White, Rio de Janeiro, RJ, Brazil) to remove any remaining tissue and rubber cup (KG Sorensen, Barueri, SP, Brazil) with pumice (SS White, Rio de Janeiro, RJ, Brazil) and water. Following, they were examined in a binocular stereoscopic microscope (Olympus, Hongo, Tokyo, Japan) with a ×25 magnificence, whose goal was to verify the presence of cracks or grooves on the buccal surface. Those teeth which presented any of these alterations were discarded. The [Figure 1] illustrates the arrangement of the experiment.
|Figure 1: Schematic diagram illustrating the experimental procedures. (a) delimited area; (b) removal root; (c) bleaching agent; (d) removal bleaching agent; (e) groups; (f) etching with phosphoric acid; (g) bonding procedures; (h) preparations of samples for microtensile test; (i) sticks; (j) microtensile test; (k) the stick rupture.|
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The buccal surfaces of the bovine teeth were flattened with silicone carbide sandpaper with decreasing granulations (#120, #240, #400, #600) (Carbinet, Buehler, Lake Bluff, IL, USA) attached to the Politriz Ecomet 3 polishing machine (Buehler, Lake Bluff, IL, USA). The abrasion was made until it was possible to obtain a flat surface and maintain approximately 1.5 mm of enamel remnant, which was measured by a digital caliper.
These maneuvers were performed by the same operator and under abundant irrigation for 10 seconds with each sandpaper, and during 60 seconds with the sandpaper #600. The purpose was to standardize the smear layer and obtain flat enamel surfaces. The flattening and polishing procedure, whose intensity was controlled by a binocular stereoscopic microscope (Olympus, Hongo, Tokyo, Japan) with 40x magnificence, were carefully performed so as not to expose the dentin surface. In addition to that, it was also observed if the enamel surface was microscopically flat so that it would not interfere in the adhesion.
The flattened area of approximately 25 mm 2 (5 × 5 mm) on the buccal enamel surface was marked with a cosmetic nail polish (Risqué, São Paulo, SP, Brazil), in order to delimit the area where the bleaching and restorative procedures were performed [Figure 1]a.
By using a 15LC series diamond disk (Buehler, Lake Bluff, IL, USA), the roots were removed and the samples were stored in distilled water at 37°C for 24 h before the experiment was initiated. [Figure 1]b. After this period, they were randomly distributed into 4 groups (n = 5) according to the description in [Table 1].
The buccal surface delimited in groups G2, G3, and G4 were submitted to bleaching with 35% hydrogen peroxide (Whiteness HP, FGM, Joinvile, SC, Brazil) [Figure 1]c and d. This bleaching agent was employed by following the manufacturer's instructions, which determine two applications of approximately 1 mm layer for 15 minutes for each application, without photo-activating the bleaching agent. This technique is recommended when bleaching at the dentist's office.
The samples which were destined for restoration after 7 and 14 days were stored in artificial saliva (Saliform, Fórmula e Ação, São Paulo, SP, Brazil) with pH = 6.8 at 37 0 C, which was changed daily during these periods [Figure 1]e. The objective of this storage was to simulate the situation found in the oral cavity, where the re-mineralization of the dental structures occurs. , The composition of the bleaching agent and the artificial saliva are described in [Table 2].
The samples that were not bleached were stored in artificial saliva for 14 days before they were submitted to restorative procedures.
Restorative procedures and microtensile test
It was used an one-bottle total-etch adhesive system, Adper Single Bond 2™ (3M Espe, St Paul, MN, USA), and it was employed according to manufacturer's clinical protocol. After its photo-activation, the Filtek™ Z350 composite resin (3M Espe, St Paul, MN, USA) was inserted in the outlined buccal surface of the enamel [Figure 1]f and g.
Four composite resin layers of approximately 1 mm each were employed through incremental technique, up to approximately 4 mm high. The restorative procedures were built on the top of the enamel. No cavity preparation was performed, because the purpose of this study was to evaluate the bond strength of adhesive systems to enamel only. Each layer was photo-activated for 20 seconds with the halogen lamp photo-activator equipment Jetlite 4000 (J Morita, Irvine, CA, USA) with intensity of 1000 mW/cm 2 . After the restorative procedures, the samples were stored in artificial saliva at 37°C for 24 h.
An acrylic resin cone was joined to the palatine portion of each sample (crown + adhesive + composite resin) with the purpose of facilitating its embedding in the PVC tube with wax, and then, attaching them to the serial cutting machine. Following, these samples were cut to obtain sticks shape [Figure 1]h and i.
In order to assess the bond strength to the microtensile, each stick was individually fixed to a Geraldeli's microtensile device with special setting cyanoacrylate glue (Super Bonder; Loctite, São Paulo, Brazil). After that, it was adapted to the Mini Instron universal test machine, model 4442 (Instron Corporation, Canton, MA, USA). This fixation occurred by positioning the adhesive interface perpendicularly to the direction of power employment. 
The microtensile test was run at a speed of 1.0 mm/min until the rupture of the sticks. Next, the rupture charge value, expressed in Newtons (N), was recorded [Figure 1]j and k. Each of the obtained value (N) was divided by the adhesion area, therefore, the values were converted to Mega Pascal (MPa)  The adhesion area was measured with a digital caliper (Mitutoyo, Tokyo, Japan).
After the microtensile test, each fragment of stick was carefully separated from the universal test machine devices and examined in a binocular stereoscopic microscope, with 40x magnificence for an initial assessment of the failure mode. The fragments of a stick from each group were chosen randomly and submitted to scanning electron microscopy.
The evaluation of the data on enamel was conducted by applying the analysis of variance (ANOVA) so as to compare the experimental groups. For the individual checks, the Tukey test was employed by using a multiple-comparison test. The level of a significance was 5% (P ≤ 0.05).
| Results|| |
The data obtained in this study resulted from the mean bond strength of the sticks obtained in each tooth.
The results of this analysis (ANOVA) showed that there are significant differences among the groups. The Tukey test was employed for the comparisons among the mean values, two by two.
The statistical significance was regarded for P ≤ 0.05 values. All analyzes were carried out by using the BioEstat statistical software, version 5.0.
The mean values used for these comparisons are described and illustrated in [Graph 1].
By analyzing these mean values, it may be observed that there was a significant difference in the microtensile strength of enamel between the control group (G1) and the group in which the restoration was done immediately after the bleaching (G2). Regarding groups G3 and G4, the values of bond strength were not different from that obtained in the control group.
In group G2, all the observed failures were from the adhesive type. On the other hand, in groups G1, G3, and G4, there was little occurrence of cohesive failure mode (failure in the tooth substrate); however, the predominance was of mixed (adhesive + cohesive) and adhesive failures mode.
[Figure 2] and [Figure 3] show the photo-micrographs, obtained with a ×350 magnificence, exemplifying the types of failure found.
|Figure 2: Photo-micrograph of the mixed failure type on enamel surface with a ×350 magnificence. A - adhesive; B - enamel|
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|Figure 3: Photo-micrographs of the adhesive failure type on enamel surface with a ×350 magnificence|
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| Discussion|| |
The bond strength in teeth that were bleached with high concentrations of hydrogen peroxide may vary according to the bleaching agent used and the waiting time after the end of bleaching for the accomplishment of restorative procedures. ,
In this study, there was a decrease in the bond strength to enamel when the restorative procedures were performed immediately after the bleaching, and these findings are in agreement with the studies of Dishman et al.;  Ben-Amar et al.;  Cavalli et al.;  Lai et al.;  Barbosa et al.  In the studies of Perdigao et al.;  Arcari et al.,  and Amaral et al.,  no decrease was found in the bond strength to the bleached enamel; it is believed that it is due to the different methodology employed.
The assessment of different waiting times after the end of the bleaching made in this study was based on the literature that recommends the waiting period of 7 days ,, or 14 days , so that residual oxygen is eliminated.
According to Dishman et al.,  the bleaching effect on bond strength of composite resin to tooth substrate is time-depending.
In this study, similar values of bond strength between bleached and unbleached teeth as well as the restoration after 7 and 14 days were found without statistically significant differences among them. Thus, the null hypothesis was accepted.
The replacement of restorations after the bleaching may become necessary since the bleaching agent does not change the color of existing restorations, which affects the dental harmony. Thus, it is inconvenient to replace them immediately after the bleaching, because the adhesion of composite resin to enamel and to dentin would be jeopardized, and, instead of waiting 14 days or more, this study found that a 7-day period would be adequate to safely perform the restorative procedures.
It is convenient that more studies are carried out in order to assess the bond strength in bleached teeth with a lower waiting time to perform the restorative procedures. Dentistry have advanced a great deal in the last decades, and the patients who have been following this evolution want that, after bleaching, any esthetic restorative procedures may be performed.
Bulut et al.,  Kaya et al.,  and Lai et al.  evaluated the use of sodium ascorbate after bleaching to reduce the waiting time after bleaching. Lewinstein et al.  used a solution of fluoride to recompose changes in enamel after bleaching so that you can perform the accession safely. Shortly after, Moule et al.  used the CPP-ACP with the same goal. This study did not use any of these substances, because the purpose was to determine which waiting time after bleaching is the most suitable to achieve high levels of bond strength in the most simple and the next one found in the clinical, with fewer steps operative.
The evaluation of bond strength in bleached teeth should be performed not only quantitatively but also qualitatively, through microscopic analysis of the type of fracture. In this study, there was a predominance of adhesive and mixed failures. The group of which teeth was restored immediately after the bleaching, there was a great incidence of adhesive failures, which suggests the bleaching agent interference in the adhesion, causing lower values of bond strength.
In the groups restored 7 and 14 days after the bleaching, there were predominantly adhesive and mixed failures, which suggests that the microtensile test was coherent since there was not enough stress on the adhesive interface to jeopardize the test. Moreover, these results are consonant with the high values of bond strength found in these groups, and it was probably due to the elimination of residual oxygen and the perfect penetration of the adhesive into the enamel porosities. 
More research studies are necessary in order to assess the bond strength to the bleached enamel relating the values found with failures mode. In addition, they need to verify the adhesive interface through scanning electron microscopy before the failure with the aim of observing the adhesive quality, and, thus, being able to explain this relation better.
| Conclusion|| |
Based on the conditions of this in vitro study, we conclude that:
- Restorative procedures carried out immediately after the enamel bleaching presented the lowest bond strength values
- Bond strength values in 7 and 14-day-periods were not different from those in the control group
- In the control group and in the groups bleached and restored after 7 and 14 days, there were mainly adhesive and mixed failures, presenting the highest incidence of cohesive fractures among all groups as well
- For groups restored immediately after the bleaching, there were only adhesive failures.
| Acknowledgement|| |
The authors would like to thank the financial support given by FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) grant # 2007/57131-9. The authors also wish to thank Mrs. Carla Almeida for her critical review on the English language. The authors would like to thank the companies 3M Espe and FGM for the supply of materials in order to conduct the experimental part of this study.
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Andréa Dias Neves Lago
Department of Restorative Dentistry, School of Dentistry of São Paulo, University of São Paulo, São Paulo, SP, 05508-900
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2]