Elsevier

Dental Materials

Volume 22, Issue 10, October 2006, Pages 948-953
Dental Materials

The effect of carbamide peroxide treatment on metal ion release from dental amalgam

https://doi.org/10.1016/j.dental.2005.10.006Get rights and content

Abstract

Objectives

There is concern that hydrogen peroxide generated by tooth bleaching agents may cause enhanced metal ion release (including mercury) from dental amalgam following contact. The aim of this in vitro study was therefore to investigate the effect of a carbamide peroxide (CP) based tooth bleaching gel on metal ion release from dental amalgam.

Methods

Dental amalgam discs were prepared according to the manufacturers’ instructions. These were treated with either a 10% carbamide peroxide (CP) gel or a 0% CP gel for 24 h. Discs were carefully wiped with cotton wool before immersion in distilled water (20 ml) for 24 h at 37 °C. Following immersion, water samples were taken for metal ion release determination (Ag, Cu, Hg and Sn) using inductively coupled plasma mass spectrometry methods. The specimens were further evaluated for surface changes using scanning electron microscopy (SEM) and Talysurf surface roughness measurements.

Results

The differences in concentration of metal ions released after treatment with the 10% CP gel and a placebo gel treatment were not statistically significant (p > 0.05). For example, mercury release following treatment with the 10% CP gel and the 0% CP gel was found to be 1.17(0.5) and 0.57(0.1) μg cm−2, respectively. Roughness measurements for samples treated with the 10% CP gel and 0% CP gel were 2.23(0.47) and 1.74(0.16) μm, respectively, again showing no significant difference between groups (p > 0.05). SEM images of the amalgam surfaces showed no apparent differences between treatments.

Significance

Treatment with a 10% CP gel did not significantly enhance subsequent metal ion release from dental amalgams compared to a control gel, contradicting previously published studies.

Introduction

In recent years there has been an increased demand for tooth bleaching in order to improve the whiteness and perceived aesthetic appearance of tooth tissue [1], [2], [3], [4]. This process commonly uses hydrogen peroxide, either directly or via its generation in a carbamide peroxide (CP) gel. The effects of peroxide on enamel and dentin have been extensively studied and there are numerous studies that report peroxide-containing products do not adversely affect enamel and dentin [5], [6], [7], [8], [9], [10]. In contrast, there have been studies that provided evidence that high concentrations of peroxide could alter the chemical and morphological structures of tooth tissues [11], [12], [13], [14]. While work is ongoing to determine the optimum, safe and effective concentration of peroxide for tooth whitening procedures, it is important to also consider other potential interactions that may occur in the oral cavity.

One potential interaction is that between peroxide and dental materials. Various reports have described the effects of bleaching agents on dental materials including glass-ionomer cements, ceramics and gold [8], [15], [16], [17], [18]. These reports generally concluded that there was little evidence for the bleaching systems causing significant changes to the materials. In the case of dental amalgam, however, several in vitro studies have reported a significant increase in mercury release as a result of treatment with peroxides compared to control treatments [19], [20], [21]. Robertello et al. [19] compared the effects of three peroxide-containing commercial tooth whitening products and saline control on a zinc-free, palladium-enriched high copper amalgam. After 80 h of bleaching, 0.98 mg m−3 of mercury was detected for one of the products. Hummert et al. [20] studied the effects of two tooth whitening products and saline on mercury release from four different amalgams. After 8 h treatment, the level of mercury released was between 109 and 158 ng ml−1 for the tooth whitening products and 5 ng ml−1 for saline. Rotstein et al. [21] studied the effects of 10% carabamide peroxide and phosphate buffer (both at pH 6.5) on four different amalgams. After 48 h, the level of mercury released was reported to be in the range 23–161 μg ml−1.

Consideration of published studies shows that a wide variety of methods have been used to model the effects of bleaching agents on tooth tissue and dental materials. This lack of standardisation is reflected by the data generated, and this in turn is the most likely explanation for the very different conclusions reached by authors. In addition to this broad criticism, it was also noted that relatively little attention has been directed at understanding the mechanisms responsible for metal ion release from dental amalgam following exposure to peroxides. The aim of this study was therefore to investigate metal ion release from amalgam discs which were prepared and finished to simulate clinical preparation.

Section snippets

Materials

The amalgam selected for this study was Sybraloy® (Kerr UK Ltd., Peterborough, Lot 71062). This is a typical restorative material based on a high copper, unicompositional spherical alloy. The composition (% w/w) of this alloy is reported as 41.8 Ag, 29.3 Sn, 28.2 Cu and 0.03 Zn. It is mixed at an alloy to mercury ratio of 1–0.92 (w/w). The Kerr data sheet gives the final mercury concentration as 45%, while the US material safety data sheet gives 44.5% (w/w). Discs (10 mm diameter × 2 mm thickness)

Metal ion release

The ion release data for mercury, silver, tin and copper are shown in Fig. 1, Fig. 2, Fig. 3, Fig. 4, respectively. Treatment with Sprite Light did not result in significant release of mercury or silver, but gave the highest release of copper in this study. There were no significant differences in metal ion release between 10% CP and 0% CP gel (p > 0.05).

Scanning electron microscopy

The surface of a typical amalgam disc under SEM, before and after treatment with the 10% carbamide peroxide gel, is shown in Fig. 5(a + b

Discussion

Metal ions, including mercury, were detected in water samples following all of the treatments including controls (Fig. 1, Fig. 2, Fig. 3, Fig. 4). The greatest mercury release followed treatment with the 10% CP gel. However, this was not significantly greater than the 0% CP gel treated group. This observation suggested that the physical removal of gel from the surface of the amalgam disc itself contributed to subsequent metal ion release. Treatment with Sprite-Light® (pH 2.84) resulted in

Conclusions

Metal ions, including mercury, silver, copper and tin, were released from amalgam following all treatments. Mercury release from the amalgam samples postbleaching was approximately 1 μg cm−2 over a 24 h period. While metal ion release was found to be above that associated with control treatment (0% CP gel) this was not statistically significant and the levels of metal ion release were not considered sufficiently high to represent a health hazard. It was also concluded that the mechanism for ion

Acknowledgement

The authors are grateful to Unilever plc for some financial support and donation of materials for this study and to Alan Cox in the Department of Chemistry for technical assistance with ion determination by ICP-MS.

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