The effect of hydrogen peroxide concentration on metal ion release from dental amalgam
Introduction
Tooth bleaching is an increasingly popular aesthetic procedure used in dentistry.1, 2, 3, 4 It is relatively simple highly effective, and can often preclude the need for operative intervention.1, 2 There are various agents available for bleaching vital teeth, although they invariably involve the application or generation of hydrogen peroxide (HP), a strong oxidising agent. Despite increased popularity, controversy surrounds the use of peroxide-based bleaching systems to whiten teeth. The situation has not been helped by conflicting reports in the scientific literature and media, further compounded by a lack of standardisation in methodology or presentation of data. Some studies suggested the relatively high concentrations of peroxide used for topical bleaching altered the chemical structure of tooth tissues.3, 4, 5, 6 While vital bleaching does not appear to cause macroscopic changes to the dental hard tissues, microscopic changes have been reported, particularly where peroxide was applied at high concentrations.7, 8
Reports in the dental literature have suggested that bleaching agents may have adverse effects on the physical properties of dental restorative materials.9, 10, 11, 12 They have also been reported to increase mercury release from dental amalgams.13, 14, 15, 16 Data from the latter studies is summarised below, with our own calculation of ion release provided in brackets to assist comparison. Robertello et al.13 tested three peroxide containing commercial tooth bleaching agents and saline control on a high copper amalgam. After 80 h of bleaching only 0.98 mg m−3 (0.98 μg/l) of mercury was detected for one of the products. Hummert et al.14 tested two tooth bleaching agents and saline on mercury release from four different amalgams. The level of mercury released was reported to be between 109 and 158 ng ml−1 (109 and 158 μg/l) for the bleaching agents and 5 ng ml−1 (5 μg/l) for saline after 8 h treatment. Mackert and Berglund15 reported similar levels of mercury release in their tests. Rotstein et al.16 tested the effects of 10% carbamide peroxide (CP) bleaching agent on four different amalgams and reported mercury release to be in the range 23–161 μg ml−1 (23,000–161,000 μg/l) after 48 h treatment. While there are also a few reports that suggested HP has no significant effect on restorative materials10, 17 the weight of evidence to the contrary means that this still remains a source of concern. Several methods have been used to model the effects of bleaching agents on dental amalgam. This lack of standardisation is reflected in the large variation in their reported data which in turn may also explain the very different conclusions reached by some authors.18, 19
Several bleaching methods exist, including in office bleaching with or without a light source,20, 21 mouth guard bleaching under supervision of a dentist, and bleaching kits that are sold over the counter (where individuals apply the bleaching agent without the supervision of the dentist). We reported previously on metal ion release from dental amalgam following exposure to CP at a single concentration.17 Active HP concentrations may, however, vary enormously and can be as high as 35% (v/v)9 even though current UK law limits the peroxide content to 0.1%. Despite widespread debate, there is currently a trend towards employing greater concentrations of HP as the active agent in tooth bleaching preparations.22, 23 The aim of the present study was therefore to investigate the effect of increasing HP concentrations (0 to 30% v/v) on ion release from amalgam. This research will provide data of value in informing current discussion and scientific debate regarding the safety and efficacy of tooth bleaching agents.
Section snippets
Materials
The amalgam selected for this study was Tytin (Kerr UK, Peterborough, UK). This is a typical restorative material based on a high copper, unicompositional spherical alloy. The Kerr data sheet gives the composition (%, w/w) as 59% Ag, 28% Sn, 13% Cu and final mercury concentration as 42.5% (w/w).
Preparation of amalgam discs (n = 25) for testing followed a strict protocol. Plastic moulds (10 mm diameter by 2 mm height) were designed and manufactured. Amalgam was packed into the moulds wearing
Results
The relationships between ion release and HP concentration is shown in Fig. 1. Values for the mean and standard deviation of ion release data for mercury, silver, copper and tin are shown in Table 1, at 1%, 3%, 10% and 30% HP concentrations.
Metal ion release increased with increasing hydrogen peroxide concentration for all elements. The highest ion releases were those for mercury followed closely by silver then tin and finally copper. The distribution of the recorded ion release data, not shown
Discussion
Metal ions (Hg, Ag, Cu and Sn) were detected in all samples following experimental treatments. For all the elements, ion release increased with increasing HP concentrations, with mercury release consistently being the highest (Fig. 1). The p-values of the transformed data (Table 2) show that there are significant differences between the control and each of the four HP concentrations for all elements (p < 0.05). The statistical tests for the surface roughness data showed no statistical
Conclusions
Metal ions (Hg, Ag, Cu and Sn) were released from amalgam following all treatments. The rate of ion release increased with increasing HP concentration and was statistically significant compared with control treatment (p < 0.05). Differences in surface roughness values before and after bleaching were not statistically significant (p > 0.05). We conclude that the trend towards using increasing concentrations of HP in tooth bleaching is undesirable without further research before these products are
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