Qualitative and quantitative analysis of Mercury ions on the surface of amalgam restorations after home bleaching

Análise qualitativa e quantitativa de íons Mercúrio na superfície de restaurações de amálgama após o clareamento caseiro

ABSTRACT

Objective

The bleaching agents may interact with restorative materials, such as dental amalgam, providing an increase mercury ions (Hg) release, whose toxic effect is known. Whereas many patients have amalgam restorations (AR) and seek bleaching treatments, the present study aimed to evaluate in vitro the amount of Hg released from AR made with spherical alloy, after being subjected to different concentrations of carbamide peroxide (CP) for home bleaching.

Methods

40 class I AR were prepared in bovine teeth. Afterthe restoration, the samples were randomly allocated into 4 groups (n = 10): C (control group), CP10 (CP 10%) CP15 (CP 15%) CP20 (CP 20%) and its treatments were simulated for 14 days for 6 hours daily. The Hg ions released was measured by energy dispersive X-ray spectroscopy (EDS) and surface changes were assessed in the Scanning Electron Microscope (SEM). Hg levels (percent mass) were recorded and the differences were statistically analyzed using the Kruskal-Wallis test and Dunn’s “Post hoc” test.

Results

Statisticalanalysis showed the bleaching treatment resulted in a higher Hg release (p <0.05), although no visible changes were identified in micrographs when comparing the control group with the other groups (CP10, CP15, CP20).

Conclusion

Increased Hg release may be observed during simulated home bleaching.

Indexing terms
Dental amalgam; Mercury; Tooth bleaching

RESUMO

Objetivo

Os agentes clareadores podem interagir com materiais restauradores, como o amálgama, proporcionando um aumento na liberação de íons de mercúrio (Hg), cujo efeito tóxico é conhecido. Considerando que muitos pacientes possuem restaurações de amálgama (AR) e buscam tratamentos clareadores, o presente estudo teve como objetivo avaliar in vitro a quantidade de Hg liberado a partir da AR confeccionada com liga tipo esférica, após serem submetidas a diferentes concentrações de peróxido de carbamida (PC) para clareamento caseiro.

Métodos

40 AR foram confeccionadas em dentes bovinos classe I. Após a restauração, as amostras foram distribuídas aleatoriamente em 4 grupos (n = 10): C (grupo controle), CP10 (PC 10%) CP15 (PC 15%) CP20 (PC 20%) e seus tratamentos foram simulados por 14 dias, por 6 horas diárias. Os íons de mercúrio liberados foram medidos por espectroscopia de energia dispersiva de raios-X (EDS) e as mudanças de superfície foram avaliadas pelo Microscópio Eletrônico de Varredura (MEV).

Resultados

A análise estatística demonstrou que o tratamento clareador resultou em uma liberação de íons mercúrio (p <0,05), embora nenhuma alteração visível tenha sido identificada nas micrografias quando compara-se o grupo C com os outros grupos (CP10, CP15, CP20).

Conclusão

Pode-se observar aumento da liberação de mercúrio durante o clareamento caseiro simulado.

Termos de indexação
Amálgama dentário; Mercúrio; Clareamento dental

INTRODUCTION

Dental bleaching has become very popular in dentistry [¹] and is considered as an effective and non-invasive treatment [²,³]. Besides, this technique is relatively safe in terms of potential risk for an alteration of hard dental tissues [⁴]. When done under professional supervision, it is considered an efficient and reasonable method that requires reduced office time and still preserves the dental structure [⁵].

Among the techniques for vital teeth bleaching, the home bleaching stands out [⁶]. In general, the bleaching agent is applied on the tooth, using a flexible tray, for a few hours [³,⁷,⁸]. During this procedure, hydrogen peroxide (HP) or peroxide-releasing agents, such as carbamide peroxide (CP), are used, being commercially available in different concentrations [⁹,¹⁰].

Home bleaching has been extensively researched since 1989. However, not all questions about its use have been clarified. Some of them are related to their effects on restorative materials. Therefore, the influence of several bleaching agents on the physical properties and surface morphology of different restorative materials has been investigated in some studies [¹⁰,¹¹].

Considering dental amalgam restorations, some studies have shown that the exposure of amalgam to bleaching agents causes an increase in Mercury (Hg) levels on the amalgam surface [¹²,¹³], as well as an additional Hg release [¹⁴-¹⁷]. Besides, a variation in the Hg release was found among the different types of amalgam exposed to the bleaching agent [¹⁵,¹⁶]. The factors that may increase the Hg release are related to the bleaching treatment duration, the bleaching gel concentration, the amalgam age, the unpolished surfaces of the restorations and the acidic pH of the bleaching material, promoted by the oxidation from the bleaching agents [¹⁴,¹⁸]. In a study that used different dental alloys, the authors observed that all groups with spherical alloy exposed to bleaching agents showed an increase in the Hg release over time [¹⁸].

Dental amalgam is still the most common material for premolars and molars restoration [¹⁹]. Many patients undergoing home bleaching have dental amalgam restorations, becoming the possible increase of Hg release a concern leading to the deleterious effects of this metal, including the development of chronic diseases [²⁰]. As far as this problem is concerned, the studies mostly use amalgam samples, not simulated restorations, which can give erroneous results when it is considered clinically. The simulation of restoration may reduce the number of surfaces in contact with the bleaching agent, reducing the Hg release. Therefore, the present study aimed to evaluate the amount of Hg released from amalgam restorations made with spherical alloy, after being subjected to different concentrations of CP for home bleaching.

METHODS

For this study, 40 freshly extracted bovine incisors were selected, obtained by donating a local slaughterhouse, and were stored at -4°C until the beginning of the research procedures. The teeth were sectioned perpendicularly to the long axis, at the height of the cement-enamel junction boundary, and was cut using an Isomet 1000 cutter (Buehler, Lake Bluff, IL, USA) and diamond disc (series 15LC Diamond no. 11-4254, Buehler, Lake Bluff, IL, USA) at a 275 rpm low speed under water cooling. The root portions were appropriately discarded as biological waste and the root canal was restored with composite resin (Filtek™ Z350 XT, 3M ESPE, Brazil).

Class I preparations were performed on the vestibular face using the diamond tip (#3131, KG Sorensen, Cotia, Sao Paulo, Brazil), with the depth corresponding to the height of the diamond point active part (4 mm) with an extension of 5 mm by 2.5 mm. The preparations were cleaned with cavity detergent (Tergentol, Formula & Action Pharmacy, Sao Paulo, Brazil) and restored with spherical amalgam alloy (DFL Alloy, DFL, Rio de Janeiro, Brazil).

The pre-dosed amalgam capsules were subjected to the amalgamation process (Duomat 2, Degussa, Germany) according to the manufacturer’s instructions, and the material was condensed into the cavity following the condensers sequence. After the condensation, the pre-sculpture honing, the sculpture and the post-sculpture honing were performed, always following the tooth vestibular face contour. After the initial settling time/crystallization indicated by the manufacturer (8 to 12 minutes), the restored samples were kept in distilled water for 48 hours until the complete material cure. Afterward, all restorations were polished, since the release of Hg is higher when this procedure is not performed [²¹]. Abrasive rubbers (Amalgam Kit, Microdont, São Paulo, Brazil) and diamond paste (Diamond Excel, FGM, Joinville, Brazil) were used for polishing. All preparation and restoration steps were performed by the same researcher.

Bleaching stage

The 40 samples were randomly distributed into four groups (n = 10) (table 1) [¹⁰], according to the different concentrations of CP (Opalescence™ PF, Ultradent, South Jordan, UT, USA). The bleaching gel was kept on the surface of the restoration for 6 hours daily. After this time, the samples were washed and stored in distilled water. The total bleaching time was 14 days, which corresponds to the mean time required to obtain a satisfactory treatment [⁶,²²].

Table 1
Groups according to bleaching gel concentration.

Groups	Surface treatment
C	No bleaching treatment
CP10	Bleaching with 10% carbamide peroxide
CP15	Bleaching with 15% carbamide peroxide
CP20	Bleaching with 20% carbamide peroxide

Scanning Electron Microscopy

The surface morphology evaluation was performed by a qualitative analysis with Scanning Electron microscopy (SEM) (Shimadzu Corporation, model SSX-550, Japan). In the SEM analysis, the humidity of the samples was removed and positioned on a rotating base with the carbon tape, in order to the sample face was perpendicular to the incident electron beam, during the analysis. Micrographs were performed at 1000X with 1kV of acceleration voltage.

Energy dispersive X-ray spectrometer (EDS)

For quantitative analysis of the chemical elements on the sample surface, an analysis was performed by Energy Dispersive X-ray Spectrometer (EDS) (Shimadzu Corporation, model SSX-550, Japan). Three measurements of each sample with 100X amplification, 25kV voltage were performed. All specimens were evaluated using the average of 3 measurements for each sample to quantify the Hg present in each sample.

Statistical analysis

Hg levels (percent mass) were recorded and the differences were statistically analyzed using the Kruskal-Wallis test and Dunn’s “Post hoc” test. Statistical software (SPSS version 20.0 – Statistical Package for the Social Sciences, IBM Corporation, Armonk, NY, USA) was used, and all tests were conducted at a significance level of 5%.

RESULTS

In the EDS analysis, several chemical elements were observed (figure 1). However, only the quantitative analysis of Hg was performed.

Figure 1
EDS Analysis showing peaks of various chemical elements.

The bleaching treatment resulted in an increase in Hg release when compared to the control group (table 2). However, there were no differences between the experimental groups (CP10, CP15 and CP20).

Table 2
The Comparison of Hg levels between groups.

Chemical element		Groups	Median	Mean	Standard deviation	p-valor
Hg**	Control		44,52A	44,72	2,40	0,044
	CP10		46,46B	47,00	3,16
	CP15		45,97B	47,01	3,68
	CP20		47,10B	47,35	1,67

• Note:
• ^**
  Kruskal-Wallis test.
• ***Different letters denote statistically differences through the Dunn’s Test.

It was possible to observe that the SEM micrographs presented similar surface topography for all groups. Figure 2 shows the surface texture in the different samples is similar, there being no cracked regions or characterizing the structural loss in the restorative material.

Figure 2
Absence of differences in surface topography. Photomicrograph of (a) control group, (b) CP10 group, (c) CP15 group and (d) CP20. The presence of Hg is represented by the darker regions.

DISCUSSION

The evaluation of the SEM micrographs did not present significant differences between all the groups studied. In the study of Gurgan et al. [²³], subtle differences in micrographs were observed only when 30% CP was used, and the concentration of 16% showed no significant changes. As the 30% CP is not indicated for the home bleaching technique, this concentration was not tested in the present study.

All CP concentrations studied (10%, 15% and 20%) showed an increase in the Hg ions release. These results are similar to Rotstein et al. [¹²], who reported a significant increase in Hg levels after 14 days of 10% CP bleaching. These authors state that prolonged treatment (more than 14 days) with bleaching agents can promote microstructural changes in the amalgam surfaces, possibly resulting in increased patient exposure to toxic byproducts. Similar results were observed by Oskoee et al. [¹³] who evaluated the effect of CP at 15% applied for 6h/day and observed that Hg levels were significantly higher than control group. At 16% and 30% CP concentrations, Gurgan et al. [²³] also founded an increase in the Hg release in the samples tested. On the other hand, the study by Al-Salehi et al. [⁹] did not find an increase Hg release in amalgam samples after 10% CP treatment. A systematic review reported that increasing CP concentrations led to an increase in Hg release [¹⁰].

Increased Hg levels have also been studied in sample stock solutions and some studies have shown an increase in solutions Hg levels [¹⁴-¹⁷,²⁴]. In our study the stock solution analysis of the samples was not carried out, which could bring additional information of Hg in the saliva, considering that Hg present in saliva can be absorbed by oral tissues and cause adverse effects [²⁰]. These effects are related to the active oxidation, that was assumed to be responsible for the increased release of amalgam components during extended 10% CP bleaching [¹⁰].

As for the amalgam restorations polishing, several studies [¹⁰,¹⁴,²⁵] reported higher Hg concentrations in the solution from non-polished amalgam samples compared to the polished one and that the bleaching agent caused minor corrosion potential for the polished amalgam when compared to non-polished. The non-polished surfaces lead to active oxidation, and it was reported that the use of 10% CP solution on non-polished amalgam samples and nickel-chromium specimens may cause corrosion of these materials, but not of noble alloys [¹⁰]. In the present study all samples were polished, which possibly provided a quantitative reduction in the Hg release.

Hydrogen peroxide at 3%, 5% and 6% concentrations are also indicated for home bleaching, however, additional experimental groups of these bleaching agents were not included as there is an equivalence between the agent’s concentrations with the CP. It is important to emphasize that the CP can be subdivided into 6.4% urea, ammonia, carbon dioxide, and approximately 3.6% HP [²⁴]. Subsequently, the HP decomposes and yields free radicals of OH- and O-, that are strong oxidizing agents, promoting changes in the surface levels of the amalgam restorations, being a potential source for mercury release [¹³]. The typical bleaching method uses 10 to 15% CP, that decomposes into 3 to 5% HP and 7 to 10% of urea once the bleaching interacts with the oral environment [²⁴]. According to Ahn et al. (2006) [²⁴], 10% CP corresponds to about 3.5% HP, leading to observe a close proximity in the concentrations between the different bleaching gels (CP 10% = HP 3%, CP 15% = HP 5.25%, CP 20% = HP 7%), therefore, so we chose not to include the groups with HP in this study.

In this study, all teeth were restored with spherical type alloy, however, differences in the Hg release have been reported depending on the type of alloy used26. When comparing a spherical alloy to mixed alloy, Certosimo et al. (2003)26 showed the Hg release from spherical alloy exceeded than mixed alloys. In addition, it was observed that all the groups with spherical alloy amalgam exposed to bleaching agents showed an increase Hg release over time18. These results are similar to observed in the present study.

CONCLUSION

All CP concentrations indicated for home bleaching were able to increase the Hg release in amalgam restorations when compared to the control. Although it is not possible to identify visible changes in micrographs when comparing all groups. In addition, prudence is required to indicate home bleaching for patients with amalgam restorations.

How to cite this article

• Bastos NA, Bitencourt SB, Bombonatti JFS, Nascimento APC, Mattos CMA, Freitas VP, et al. Qualitative and quantitative analysis of Mercury ions on the surface of amalgam restorations after home bleaching. RGO, Rev Gaúch Odontol. 2020;68:e20200037. http://dx.doi.org/10.1590/1981-863720200003720190113

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Publication Dates

• Publication in this collection
  21 Sept 2020
• Date of issue
  2020

History

• Received
  11 July 2019
• Reviewed
  18 Oct 2019
• Accepted
  22 Nov 2019