INTRODUCTION

The in-office technique of external bleaching is a popular method used to whiten discolored teeth. The development of high-concentrations of hydrogen peroxide-based bleaching gels has facilitated these in-office bleaching procedures. The significant advantage to using this technique is obtaining immediate results without requiring patient compliance.

Although previous studies have reported the efficacy of bleaching agents that use high-concentrations of hydrogen peroxide, there is still controversy as to whether these agents could adversely affect dental hard tissues.1–5

Studies that investigated the effect of bleaching gels on enamel microhardness using the in-office technique are scarce and the results have been controversial. Oltu and Gürgan6 observed a reduction in enamel microhardness when 35% carbamide peroxide bleach was used; nevertheless, phosphoric acid was applied onto the enamel before the bleaching agent, which could affect the results. Lewinstein and others7 and Attin and others8 showed a reduction in surface enamel microhardness after the bleaching treatment. Al-Salehi and others9 reported that the reduction in bleached enamel microhardness was inversely proportional to the concentration of hydrogen peroxide. However, Park and others10 observed no alterations in 30% hydrogen peroxide bleached enamel microhardness.

Sulieman and others3 have reported that, following treatment with 35% hydrogen peroxide, there was no evidence of deleterious effects on enamel or dentin. They believe that studies reporting adverse effects on bleached enamel and or dentin do not reflect the bleach itself; instead, they reflect on the pH of the formulation used.

The loss of demineralization and calcium are alterations that can occur in the organic composition of bleached hydroxiapatite.5911–14

Attempts to enhance the remineralization of bleached enamel have been tried; however, the results are controversial. The addition of fluoride and calcium in the bleaching agent did not significantly enhance the enamel microhardness.2 In a study by Burgmaier and others,15 they also did not observe any improvement in fluoride uptake in bleached enamel. On the other hand, previous studies showed that post-bleaching fluoridation prevented mineral loss and restored the softened dental tissues.716 It was also found that, by adding dicalcium phosphate dehydrate to a whitening agent, it reduced caries susceptibility when compared with a gel without fluoride17 and that enamel treated with a fluoridated bleaching agent exhibited higher caries resistance than non-bleached enamel.18

Current knowledge of the effects of in-office bleaching agents on the surface and subsurface microhardness of enamel is still limited and controversial, as well as the addition of remineralizing agents in bleaching agents. Therefore, this study investigated the effect on the enamel surface and subsurface microhardness of adding calcium and fluoride to a 35% hydrogen peroxide gel. It was hypothesized that the addition of remineralizing agents in the bleaching agent would improve the microhardness of enamel.

METHODS AND MATERIALS

Microhardness Measurements

Vickers microhardness, at a load of 50g, with an indentation time of 10 seconds, was determined using a microhardness tester (FM-700, Future-Tech, Tokyo, Japan). Three indentations were performed on the surface of each specimen, with a distance of 100 μm between them, and averaged.

To determine the subsurface enamel microhardness, the specimens were sectioned in the central portion, using the water-cooled diamond disc, and the halves were fixed in an acrylic resin cylinder using sticky wax.

The inner portion of the specimens was exposed to allow for polishing, as described previously. After polishing, indentations were performed in the specimens from the surface to a depth of 125 μm, with intervals of 25 μm in-between. Three indentations were performed at each depth, with a distance of 100 μm between them and the total was then averaged (Figure 1).

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RESULTS

Surface Microhardness

The application of one-way ANOVA revealed significant differences for the different groups (p=0.00). The Tukey's test was then applied and showed that the bleached group exhibited the lowest microhardness means and the group bleached with the calcium-added agent showed the highest values (Table 1).

Table 1 Means and Standard Deviation (SD) Data for the Tested Groups and Results of Tukey's Test for the Surface and Subsurface Vickers Microhardness (5%)

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DISCUSSION

The adverse effects of the in-office bleaching treatment on enamel microhardness were previously studied, and the results were controversial. Some authors reported significant alterations in bleached enamel microhardness,7–919–20 but, in other studies, the alterations were not significant.310

In the current study, the 35% hydrogen peroxide bleaching treatment significantly reduced the enamel surface and subsurface microhardness. Although this effect was previously attributed to the pH of the bleaching gel,3 the bleaching agent used was not acidic enough to cause demineralization of the enamel.

The exact mechanism by which hydrogen peroxide affects dental tissue has not yet been fully understood. The microhardness alterations are probably attributed to the reaction ability of the bleaching agent in relation to the organic phase of enamel, consequently affecting its mineral content. The strong oxidizing effect of hydrogen peroxide on the organic matrix plays a predominant role in the post-bleaching alterations observed in bleached teeth; this can be enhanced by the low pH of the bleaching agent, leading to a decrease in enamel and dentin microhardness.1921–22

The results of the current study showed no significant alteration according to the evaluated depth, that is, the effects of the bleaching treatment on the enamel surface were similar to the effects on the enamel subsurface. Therefore, the authors of the current study can suggest that the potential deleterious effects that occur on the enamel surface extend to the subsurface, as was also observed by Attin and others.8 In fact, peroxide can penetrate the dental structure, due to its low molecular weigh, exerting its oxidative effect also on the subsurface of enamel where the organic content is higher.21

The use of fluorides following bleaching has been shown to restore the surface hardness of softened, bleached enamel.7 In addition, the attempt to reduce the demineralization of bleached enamel has been successfully accomplished by means of adding fluorides to bleaching agents.23–24 A fluoridated bleaching gel can also reduce the time needed for bleached enamel hardness to recover, compared with unfluoridated gel20 by means of the fluoridated hydroxyapatite and calcium fluoride formation, which are favorable to the remineralization process of the tooth surface.25 In the current study, the addition of fluoride to the bleaching gel significantly enhanced the microhardness of the enamel compared to the bleaching group.

The bleaching treatment with high-concentrations of hydrogen peroxide can reduce the Ca²⁺ concentration of the enamel surface,913–14 thus decreasing its microhardness.26 Although these alterations are reversible and may not have clinical significance,111323 knowledge of their adverse effects to bleached enamel is important, so that bleaching procedures are carried out correctly, respecting the time and concentration of agents, according to the manufacturers' instructions and as substantiated by scientific knowledge, thereby ensuring the safety of the treatment.9

Additionally, previous attempts to increase the remineralization potential of the dental structure were made by adding calcium compounds to chewing gum27 and toothpastes,28–29 thus obtaining satisfactory results. It was also observed that the addition of small amounts of calcium to acid solutions can reduce enamel loss.30

In the current study, adding calcium chloride to bleaching gel increased enamel microhardness, as opposed to the results obtained by de Oliveira and others,2 who found no remineralizing effect in bleached enamel using 10% carbamide peroxide with calcium.

In addition to inhibition of the deleterious effects of bleaching agents on enamel mineral content, the benefits of using remineralizing agents in bleaching agents could include the reduction of enamel solubility and reduced sensitivity due to mineral deposition in enamel crystallites.31

This hypothesis was confirmed, as there was a significant increase in both surface and subsurface enamel microhardness for the groups bleached with fluoride and calcium-containing bleaching gels. However, the current in vitro study did not take into account the remineralizing action of saliva and, therefore, future clinical studies should be performed to confirm the beneficial action of remineralizing agents in bleaching gels, as well as the optimal concentrations of these compounds.

CONCLUSIONS

According to the limitations of the current study, it can be concluded that the 35% hydrogen peroxide bleaching agent resulted in a reduction in both surface and subsurface enamel microhardness and that the addition of fluoride and calcium in the bleaching agent significantly increased the microhardness of bleached enamel.