In Vitro Evaluation of Mercury Release from Dental Amalgam After Magnetic Resonance Imaging

Introduction

The alloy that mercury forms with other metals is known as amalgam. Mercury, being in liquid state at room temperature, dissolves into metal particles and results in the formation of crystals of mercury-containing compounds. This mixture has a plastic consistency and is used in restoring cavities.¹

Dental amalgam is prepared by triturating liquid mercury with a mixture of other metals such as silver, tin, copper and zinc, palladium and indium in traces.² It has been used as a preferred restorative material due to the low cost, ease of use, durability, strength and bacteriostatic effect.³

Despite its popularity, there exists some concern over its safety due to the release of mercury. Mercury is released during all phases of restoring a tooth with amalgam. Mercury vapour is released during trituration, condensation of the restorative material, polishing the surface and removal of the restoration.⁴

Mercury vapour is also released during normal activities such as chewing, brushing, etc. Mercury vapour is one of the most toxic non-radioactive elements and may cause toxicity even at low doses. Ortendahl et al. studied the cause for metallic taste among deep sea divers performing underwater electric welding and cutting, and showed that dental amalgams exposed to magnetic fields underwent changes in their composition.⁵ It has been shown that persons with amalgam restorations have a higher concentration of mercury in their blood, saliva and urine. Inorganic mercury has an effect on various organs of body, including the nervous system, renal system, immunity, respiratory and other systems.⁶

Intraoral radiographs, panoramic radiographs, computed tomography scans, magnetic resonance imaging (MRI), and ultrasound scans play a major role in diagnosis of oral and maxillofacial disorders and MRI scans are often requested due to the non-ionizing radiation.³ A few studies have reported that there is increased microleakage from amalgam restorations on exposure to MRI.^7,8

Hence, this study was conducted to assess mercury release from amalgam when exposed to MRI.

Materials and Methods

The study was approved by the Institution Review Board (Ref/IRB/CODE-KLE/JAN-2022/02). In this in vitro study, zinc free non-gamma 2 alloy [DPI ALLOY] and mercury were mixed using amalgamator. Seven blocks of amalgam, each measuring about 5×5×5 mm were prepared (Figure 1). The amalgam blocks were placed in 20 mL artificial saliva for 48 hours (Figure 2).

Artificial saliva was prepared according to Macknight Hane and Whitford (1992) formula.⁹ The composition of artificial saliva was-Methyl-p-hydroxybenzoate (2.00 g/L), Sodium carboxymethyl cellulose (10.00 g/L), KCl (0.625 g/L), MgCl₂.6H₂O (0.059 g/L), CaCl₂.2H₂O (0.166 g/L), K₂HPO₄ (0.804 g/L), K₂HPO₄ (0.326 g/L). The pH of artificial saliva was adjusted to 6.75 with KOH.

After 48 hours, mercury concentration in artificial saliva was estimated using inductive coupled plasma mass spectrometry (ICP-MS). The amalgam blocks were then placed into fresh artificial saliva (20 mL) and exposed to MRI (Figure 3). MRI procedures were performed with T2 flare, T2 coronal, T2 axial, T2 due, SWAN sequence using 1.5T magnetic field strength for 10 minutes. Mercury levels in artificial saliva was estimated 48 hours after MR imaging by ICP-MS.

Results

The mean concentration of mercury from all amalgam blocks before exposure to MRI was 4.74 ppb. The mean level after exposure to MRI was 8.68 ppb. There was statistically significant difference in the mean mercury content after exposure to MRI. Mercury levels before and after exposure to MRI are depicted in Table 1 and Figure 4.

Discussion

Dental amalgam is one of the most used restorative materials. Amalgam accounts for almost 75% of all restorations prepared by dentists. It has been in clinical use since over 165 years. Amalgam is popular as it is easy to prepare and manipulate, economical, and long-lasting.¹⁰

The first mention of dental amalgam was in Materia Medica in 659 AD. It was used in the 16th century in Europe. The paste consisted of 100 parts mercury mixed with 900 parts of tin and 45 parts silver. It was introduced in America in 1833 as ‘Royal mineral succedaneum’. This however, ran into disrepute and resulted in the ‘amalgam war’. The tide was turned in 1877, when clinical research by Flagg proved the safety of amalgam. Amalgam gained universal acceptance after the investigations of GV Black in 1895. Black developed an alloy of composition of 68.5% silver, 25.5% tin, 5% gold, 1% zinc and designed the principles of cavity preparation to overcome the shortcomings of amalgam. Further improvement took place when a 1:1 ratio of mercury to alloy was recommended. Atomization of the alloy particles further improved the ease of handling amalgam.¹⁰

However, exposure to high levels of mercury vapor can cause neurobehavioral, cognitive changes and renal injury. Mercury vapor is released during placement of the restoration, normal chewing activities, and during removal. FDA has noted that exposure to mercury vapor in persons with amalgam restorations is not expected to exceed health exposure reference values. Those with a high number of amalgam restorations may have greater exposure, but even this is well below the levels required to cause adverse health effects.¹¹

MRI investigations are becoming popular as they do not expose individuals to ionizing radiation while providing good soft tissue resolution. Current MRI scanners use magnets of strength 1.5 to 7 Tesla, with most systems being 1.5 or 3.0 T.12 MRI is contraindicated in the instances of patients wearing pacemakers, metal implants or prostheses; however, MRI scan can safely be carried out in the presence of amalgam restorations. On combining with mercury, silver becomes paramagnetic, and hence is not likely to be dislodged during MRI.⁷

Although a satisfactory explanation for release of mercury from amalgam on exposure to MRI has not been reached, the following are well documented. Mercury is released from amalgam restorations during preparation, placement and removal of restorations. It can also occur continually during wear.¹³

The magnetic fields and the radiofrequency pulses used during MR imaging induce eddy currents in body tissues. This leads to undesired heating, which in turn leads to release of mercury. Mercury is thought to be released from amalgam restorations, as amalgam has a higher electric conductivity compared to soft tissues, and hence gets heated rapidly.¹⁴

The effect of MRI on amalgam restorations was proposed by Mehdizadeh AR et al. as the ‘Triple M’ effect.⁸ This effect has been described as the formation of ‘hot spots’ in small amount of saliva that is trapped between the amalgam restoration and the tooth, on exposure to MRI. This accelerates microleakage from amalgam.

The joint FAO/WHO expert committee on food additives recommended a tolerable weekly intake of inorganic mercury of 4000 ng kg-1 body weight.¹⁵ The total amount of mercury released during setting and maturation of amalgam is about 72.83 ng mL^-1. Allison et al. exposed amalgam blocks embedded in hydroxyapatite to 7T MRI and found the mean total mercury release to be 15.43 ng mL^-1. They concluded that the maximum amount of mercury liberated in their study on exposure to 7T MRI was 4.7 times lower than the amount of mercury released during amalgam maturation. It was also 20.3 times lower for a 3T MRI for the duration of the 20 min scan session.¹⁴

In our study, the mercury released after exposure to 1.5 T MRI was 8.68 ng mL^-1 (ppb). This is higher than the levels detected by Allison et al., probably since exposure to MRI was carried out 48 hours after preparation of the blocks. However, it is much lower than the tolerable weekly limit proposed by WHO.

Gul et al. estimated the amount of mercury released over time from amalgam after treatment in healthy subjects. The resultant values ranged from 0.4 to 0.9 ng mL^-1 over 35 days. They found no statistically significant difference among the values of mercury released over time.¹³ It has been reported that the tolerable maximum level of mercury in blood is 3 ng/mL.¹⁶ Skoner et al. reported toxic mercury doses in the blood as 200 ng/mL and lethal mercury doses as 600 ng/mL.¹⁷

Our study supports the increased leaching of mercury from amalgam blocks when exposed to MRI. However, the quantity of mercury released appears to be far below permissible limits. This combined with the triple M effect may increase mercury release from amalgam restorations. However, the threat from this release is not great and does not imply that amalgam restorations are unsafe.

Several limitations were noted in this study. Clinically amalgam is placed as a restorative material in prepared cavities, while we used only amalgam blocks. Further, these blocks were placed in stagnant artificial saliva which did not simulate the exact oral environment. Amalgam restorative material from different manufacturers and of different types could have been evaluated. Further investigations of amalgam restorations with larger samples using 3.0 Tesla MRI scans and other forms of electromagnetic radiation will throw light on this phenomenon.

Conflict of interest

The authors declare that they do not have any conflict of interest.

Acknowledgement

This study was conducted under RGUHS research grant number UG21DEN124.