How Do Different Physical Stressors' Affect the Mercury Release from Dental Amalgam Fillings and Microleakage? A Systematic Review.

Background: Approximately 50% of dental amalgam is elemental mercury by weight. Accumulating body of evidence now shows that not only static magnetic fields (SMF) but both ionizing and non-ionizing electromagnetic radiations can increase the rate of mercury release from dental amalgam fillings. Iranian scientists firstly addressed this issue in 2008 but more than 10 years later, it became viral worldwide. Objective: This review was aimed at evaluating available data on the magnitude of the effects of different physical stressors (excluding chewing and brushing) on the release of toxic mercury from dental amalgam fillings and microleakage. Material and
Methods: The papers reviewed in this study were searched from PubMed, Google Scholar, and Scopus (up to 1 December 2019). The keywords were identified from our initial research matching them with those existing on the database of Medical Subject Headings (MeSH). The non-English papers and other types of articles were not included in this review.
Results: Our review shows that exposure to static magnetic fields (SMF) such as those generated by MRI, electromagnetic fields (EMF) such as those produced by mobile phones; ionizing electromagnetic radiations such as X-rays and non- Ionizing electromagnetic radiation such as lasers and light cure devices can significantly increase the release of mercury from dental amalgam restorations and/or cause microleakage.
Conclusion: The results of this review show that a wide variety of physical stressors ranging from non-ionizing electromagnetic fields to ionizing radiations can significantly accelerate the release of mercury from amalgam and cause microleakage. Copyright: © Journal of Biomedical Physics and Engineering.

Introduction

Despite the wide application of mercury in industry and medicine, it has known toxic effects. Methylmercury (MeHg), mercury vapor (Hg0), and ethylmercury (EtHg) are the three main forms of mercury which are the origin of concerns to human populations non-occupationally exposed to this toxic element [ 1 ]. Scientists’ effort for increasing human knowledge about the risks linked to mercury exposure is complicated due to its variable environmental fate as well as the key role of environmental, biological, and socioeconomic factors [ 2 ]. Substantial data suggests that mercury causes a wide variety of physiological and adverse health effects ranging from bioaccumulation in the central nervous system (CNS), liver and kidney as its major targets to promoting carcinogenesis, immunotoxicity, kidney damage that leads to nephrotoxicity, declined neurological capacity, and neurobehavioral function, changed the functioning of 3 main endocrine axes, and impaired reproduction quality and altered offspring quality [ 3 - 24 ]. Due to the ability of inhaled mercury vapor which can cross the blood-brain barrier, it can cause serious damage to the CNS [ 11 ].

Today, exposure of humans to mercury is a major public health concern. These exposures can be due to a wide variety of sources ranging from industrial processes, occupational and household uses of mercury, mercury-containing vaccines, dental amalgams, and consumption of fish [ 25 ]. The problems can be appeared differently over 250 symptoms in the clinical picture, involving many systems other than the immune system (e.g. neurological, renal, respiratory, gastrointestinal, cardiovascular, hepatic, and reproductive), along with fetotoxicity and genotoxicity [ 25 , 26 ].

Among the humans, children are believed to have a greater risk of developing detrimental neurological effects of mercury. The case of a 3-month-old infant reported in a study demonstrated that Hg causes poisoning and finally, it developed pneumothorax and respiratory failure. These kinds of cases highlight that Hg exposure should be considered as a crucial issue [ 27 ].

Hg vapor inhalation is the major route of contamination and dental amalgam fillings (~ 50% Hg), are also a significant source of mercury in general population [ 25 ].

Several studies have reported that magnetic resonance imaging (MRI) [ 28 ], chewing and brushing in an artificial mouth [ 29 ], Nd:YAG laser pulse energy [ 30 ], radiofrequency radiation sources including Wi-Fi routers, smartphones, light-curing tools [ 31 ] and also X-rays [ 32 ] might change the rate of evaporation of mercury from amalgam fillings. Iranian scientists firstly addressed this issue in 2008 [ 33 ] but more than 10 years later, it became viral worldwide [ 34 ].

In a study, the authors investigated the urinary mercury from dental amalgam fillings in MRI-exposed and control groups and reported a significant difference between these groups [ 28 ]. Moreover, in another study, electromagnetic radiation from Wi-Fi routers and mobile phones could increase the concentration of mercury released from amalgam restorations [ 31 ]. Furthermore, the level of mercury vapor release was significantly linked to the pulse energy of Nd:YAG laser [ 30 ]. Recently, it has been suggested that exposure of women with dental amalgam fillings to electromagnetic fields may increase the risk of autism due to higher release of mercury from dental amalgam [ 35 ]. A literature review shows that over the past years many publications have been reported to be adversely affected by the key shortcoming of ignoring the role of static magnetic fields or electromagnetic radiation in accelerating the release of mercury from amalgam and microleakage [ 36 - 38 ]. In the current study, the main purpose is to review physical stressor impact on mercury release and amalgam microleakage.

Material and Methods

This study was performed as a systematic review. Figure 1 briefly illustrates the process of data collection and analysis.

Figure 1

Data collection and analysis process used in this study

Search strategy

The papers reviewed in this study were searched from PubMed, Google Scholar, and Scopus (up to 1 December 2019). The keywords were identified from our initial research matching them with those existing on the database of Medical Subject Headings (MeSH) managing by the US National Library of Medicine. Furthermore, to improve the quality of searches, we got helped by an expert who worked in the Medical Branch libraries. The search strings were selected MeSH, title, abstract, and keywords based on “amalgam” “or “microleakage” or “mercury release” or “electromagnetic field”.

Inclusion and exclusion criteria

The original articles were chosen based on inclusion criteria, by contrast, the non-English papers, and any types of articles related to the review papers, letters, etc. were considered as exclusion criteria.

Selection process

At the first sage, the papers were screened by a reviewer in the point of titles and abstracts views and then they were classified into three sets completely; the first step was included the papers with certain inclusion criteria, the second one has contained the papers without the clear inclusion criteria from reviewers, and the third one, set referred to the papers with no inclusion criteria which were not appropriate and then excluded from the screening.

Data extraction

Four variables for responding to the research questions were extracted in this phase contained mercury release, amalgam microleakage, electromagnetic fields, and radiofrequency wave.

Results

The data extracted from the 13 articles reviewed are shown in Table 1. The fourth variables of the selected papers are represented in the Table 1. As presented in the fourth column of the Table 1, a wide variety of physical stressors were studied. Our review shows that exposure to:

Table 1

Summary of the data extracted from 13 articles finally included in our review

	Study	End point	Physical stressor	Method	Outcome summery
1	Unal Erzurumlu 2019 [ 39 ]	Micro-leakage of amalgam restorations	MRI	Using extracted molar teeth, various groups exposed to 1.5 or 3-T MRI.	Microleakage was higher in the gingival region compared to occlusal region in all groups. The strength of the magnetic field directly determined the level of microleakage.
2	Yilmaz et al. 2018 [ 40 ]	Mercury release from dental amalgam	MRI	Extracted caries-free molar or premolar teeth were exposed to 1.5 or 0.7 T MRI.	While mercury was released from amalgam fillings after exposure to 0.7 T, 1.5-T MRI did not change the release.
3	Hosseini et al. 2018 [ 41 ]	Mercury release from dental amalgam	Wi-Fi and X-Ray radiation	Extracted premolars were divided into five groups; control, CT, “CT+Wi-Fi”, “Wi-Fi+CT”, and “Wi-Fi only”. Mercury level was measured 24 and 48 hours after exposure.	The mercury released from teeth exposed to Wi-Fi and CT scan (ionizing radiation) was higher.
4	Paknahad et al. 2016 [ 42 ]	Mercury release from amalgam restorations	Radiofrequency radiation from Wi-Fi devices	Non-carious extracted human premolars were exposed to Wi-Fi..	Conventional Wi-Fi routers could increase the release of mercury from amalgam fillings.
5	Mortazavi et al. 2016 [ 43 ]	Dental amalgam microleakage	Radiofrequency electromagnetic radiation	Investigation of the mechanisms behind the accelerated microleakage of amalgam after exposure to electromagnetic fields.	Multiple reflections of the radiofrequency radiation on the inner walls of the tiny spaces between amalgam and teeth and their interferences produces some “hot spots”, then rapid expansion of the gas bubbles lead to increase in the micro-leakage of amalgam (the so-called Triple M” effect).
6	Mortazavi et al. 2016 [ 44 ]	Microleakage of amalgam restorations	Radiofrequency electromagnetic fields of dental light cure devices and mobile phones	Identical class V cavities were prepared on the buccal surfaces of 60 non-carious extracted human teeth, exposed to dental light cure devices and smartphones.	Both light cure devices and mobile phones can increase the microleakage of amalgam restorations meaningfully.
7	Kursun et al. 2014 [ 32 ]	Mercury release from dental amalgam	X-rays and MRI	Amalgam capsules were molded into discs. The samples were exposed to X-rays or MRI in a soft tissue-equivalent material.	Increased release of mercury was observed in X-ray group; while no change was seen in MRI group.
8	Marti Akgun et al. 2014 [ 45 ]	Microleakage of amalgam restorations	MRI	Permanent molar teeth (class II cavities with gingival margins ending 1 mm below the cementoenamel junction were exposed to MRI.	No difference found in surface microleakages in MRI and control samples.
9	Savadi Oskoee et al. 2013 [ 30 ]	Mercury vapor release from the dental amalgam	Nd:YAG laser pulse energy	Aamalgam samples in sealed containers were exposed to Nd:YAG laser (pulse energies of 50, 150, and 250 mJ at a distance of 1mm from the surface of amalgam for 4 seconds).	A significant increase was observed in the of release of mercury vapor.
10	Shahidi et al. 2009 [ 46 ]	Microleakage of amalgam restorations	MRI	Eextracted premolar teeth were divided into 3 groups. Three high-copper amalgams were used to restore standard class V. MRI was randomly applied.	Increased micro-leakage was reported in MRI-exposed teeth. The authors believed that thermo-electromagnetic convection caused the enhancement of the diffusion process, grain boundary migration, and vacancy formation that finally resulted in microleakage.
11	Mortazavi et al. 2008 [ 33 ]	Mercury release from dental amalgam	MRI and following mobile phone	Stimulated saliva samples from 30 patients were collected just before and after 0.23-T MRI. Thirty patients were investigated. In the second phase of the study, fourteen female healthy university students who had not used mobile phones before the study and did not have any previous amalgam restorations, were studied. In this phase urine sample was studied.	Both MRI and mobile phone radiation could significantly increase the mercury release from amalgam.
12	Berglund A et al. 1998 [ 47 ]	Intra-oral release of mercury vapor from amalgam restorations.	Low frequency magnetic fields	Subjects with amalgam fillings were exposed to magnetic fields (flux densities of 20 µT at 30 kHz and 500 µT at 50 Hz).	No increase was found in the mercury release from dental amalgam fillings.
13	Muller-Min et al. 1996 [ 48 ]	Mercury dissolution from dental amalgam fillings	MRI	Dental cavities were filled with amalgam and the mercury release was measured for 2 weeks in a nonmagnetic condition. Then, the samples were divided into two groups; i.e. exposed to a static magnetic field and exposed to a repetitive gradient-echo sequence.	There was no difference in mercury release between the groups.

MRI: Magnetic resonance imaging, CT: Computed tomography

1. Static magnetic fields (SMF) such as MRI

2. Electromagnetic fields (EMF) such as mobile phones

3. Ionizing Electromagnetic Radiation such as X-rays

4. Non- Ionizing Electromagnetic Radiation such as lasers, and light cure devices can significantly increase the mercury release from amalgam fillings and/or cause microleakage.

Mercury release or amalgam microleakage applies in physical stressors study

As shown in the fifth column of Table 1, mercury release and amalgam microleakage have been applied in physical stressors. The fifth and sixth columns of the Table 1 demonstrate the method of various studies and the effects of different physical stressors on mercury release and amalgam microleakage, respectively.

Discussion

Currently it seems unlikely that dental amalgams can lead to health problems in majority large proportion of humans. However, specific groups such as pregnant women, children, elderly people and hypersensitive individuals may be in risk [ 33 ]. Figure 2, shows studies conducted to date that indicate exposure to different physical stressors can lead to accelerated mercury release from amalgam and microleakage. According to several articles were mentioned in this study, magnetic resonance field has an increasing effect on mercury release. However, the study conducted by Muller-Miny et al., failed to show significant increase in mercury release after MRI [ 48 ]. Kursun et al., reported that magnetic resonance field (1.5 T) did not change the level of mercury release from dental amalgam [ 32 ]. Akgun et al., reported no statistically significant differences in microleakage in groups with or without exposure to MRI [ 45 ]. Exposure to pulsed electromagnetic fields (PEMF) exposure generated by Helmholtz coil, as reported by Mortazavi et al., could not increase the microleakage of amalgam restorations. However, in their experiments, X-ray exposure significantly increased the microleakage of amalgam. In their study, intraoral radiography increased microleakage of amalgam fillings [ 49 ]. Amalgam microleakage was not significantly different in the light cure-exposed group or those exposed to mobile phone radiation with that of the control group [ 44 ]. Akgun et al., reported that MRI cannot change the microleakage of bonded or nonbonded amalgam fillings [ 45 ]. Moreover, laser beams (Nd:YAG) with the pulse energies of 50, 150, and 250 mJ increased the mercury vapor release, that was dependent on the Nd:YAG laser pulse energy [ 30 ]. However, Pioch et al., showed that CO2 laser, no signs of amalgam ablation or mercury vapor release were observed [ 50 ]. In Figure 3 a possible mechanism that can be involved in accelerated microleakage of amalgam after exposure to radiofrequency radiation is demonstrated.

Figure 2

Studies conducted so far show that a wide variety of physical stressors can induce the accelerated release of mercury from dental amalgam fillings and microleakage.

Figure 3

Demonstration of a possible mechanism that can be involved in accelerated microleakage of amalgam after exposure to radiofrequency radiation as a major physical stressor. (Originally from Mortazavi et al, [ 43 ] modified and reproduced with permission)

Mortazavi et al., have previously introduced “Triple M effect”. According to Triple M” effect, in hot spots, increased amalgam microleakage after exposure to radiofrequency electromagnetic fields increases the temperature in saliva-filled tiny spaces between amalgam fillings and teeth. Reflection of radiofrequency radiation on the inner walls of these tiny spaces, and their interferences produce specific “hot spots” in these areas. High temperature and fast expansion of the bubbles is responsible for accelerated microleakage of amalgam. Appropriate experiments are needed to verify the validity of this theory [ 43 ].

Hypersensitive People and Pregnant Women Issues

Although what we know about mechanisms of hypersensitivity to the damaging effects of mercury is very limited, genetic factors can be involved in this phenomenon [ 51 ]. Studies conducted so far reveals that a proportion of any population may show hypersensitivity to mercury. Besides hypersensitive people, children and pregnant women with dental amalgam fillings require special attention. Regarding the pregnant women, it’s worth noting that the strong link between maternal and cord blood mercury levels are reported previously. Thus, in order to decrease the toxic effects of mercury in their fetuses, pregnant women with amalgam restorations are requested to limit their exposures (both exposure time and exposure intensity) to electromagnetic radiation.

Conclusion

In this paper, we systematically reviewed the physical stressor impact on mercury release and amalgam microleakage. The results of this investigation show that physical stressors such as Electromagnetic fields, MRI and laser, just to name a few, have an effect on mercury release and amalgam microleakage. Furthermore, it shows a new view in providing protections against mercury release and amalgam microleakage.

Authors’ Contribution

G. Mortazavi and M. Keshavarz conceived the idea. The first draft was written by all authors. The final manuscript was revised by G. Mortazavi. All the authors read, and approved the final version of the manuscript.

Conflict of Interest

None