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 Table of Contents  
REVIEW ARTICLE
Year : 2019  |  Volume : 10  |  Issue : 4  |  Page : 220-226

Solving dental amalgam dilemma: An integrated toxicology and its management strategies – A systematic review


1 Department of Orthodontics and Dentofacial Orthopedics, Coorg Institute of Dental Sciences, Kodagu, Karnataka, India
2 Department of Public Health Dentistry, D.A.P.M.R.V Dental College, Bengaluru, Karnataka, India
3 Private Practice of Orthodontics, Alappuzha, Kerala, India
4 Private Practice of Oral Surgery, Mandya, Karnataka, India

Date of Submission09-Aug-2019
Date of Acceptance24-Oct-2019
Date of Web Publication22-Jan-2020

Correspondence Address:
Dr. Mysuru Vasudevamurthy Akshatha
Room No. 06, Dept. of Orthodontics Coorg Institute of Dental Sciences, Kanjithanda Kuhalappa Campus Virajpet- 571218, Kodagu
India
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DOI: 10.4103/srmjrds.srmjrds_57_19

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  Abstract 

Mercury (Hg) has been well known as one of the most toxic nonradioactive materials. Anthropogenic mercury is now a major worldwide concern and is an international priority toxic pollutant. Mercury vapour release from amalgam is the most important form of mercury that determines human exposure. Autopsy studies have shown higher levels of mercury in the tissues of human with amalgam fillings as opposed to those who were not similarly exposed. study objective is to sensitize the dentist and dental staff about mercury toxicity and strategies for proper handling of amalgam in order to reduce the health hazards caused by mercury for both dentist, patient and environment. Data search included PubMed, PubMed Central, Cochrane and Google Scholar until January 2019, out of 59 publications, 43 full text articles were deemed important were analysis. Discussion focus on an integrated toxicological based on release, uptake. toxicology and management protocol to understand interrelationship of these substantial contents. Added insight of Indian scenario in this review educate dental fraternity of correspondence between consumerism and bionomic loop, where safety consensuses are questionable. Growing advances in technology and raising aesthetic demands have led way aesthetic dentistry. Hence promoting the use of cost-effective and clinically effectual mercury free alternatives for dental restoration is inevitable in years to come.

Keywords: Management of mercury toxicity from amalgam, mercury release from amalgam, mercury toxicity


How to cite this article:
Akshatha MV, Krishna V, Thampan A, Mallesh N. Solving dental amalgam dilemma: An integrated toxicology and its management strategies – A systematic review. SRM J Res Dent Sci 2019;10:220-6

How to cite this URL:
Akshatha MV, Krishna V, Thampan A, Mallesh N. Solving dental amalgam dilemma: An integrated toxicology and its management strategies – A systematic review. SRM J Res Dent Sci [serial online] 2019 [cited 2020 Feb 25];10:220-6. Available from: http://www.srmjrds.in/text.asp?2019/10/4/220/276373


  Introduction Top


Mercury (Hg) is popular as one of the most toxic nonradioactive materials. It is one of the naturally occurring elements, anthropologically now posing a major global public health concern and has become an international priority toxic pollutant. Dental amalgam is widely used posterior restorative material worldwide, which comprises elemental mercury as a paramount component. From the middle of the 19th century, use of mercury in dentistry has been topic of debate which has only gained momentum over the past 25 years, since sensitive analytical chemical techniques showed continuous release of mercury from dental amalgam.

The recommended daily mercury exposure from dental amalgam during conventional procedures is estimated to be 1–29 μs (US PHS, 1993).[1] Autopsy studies have shown higher levels of mercury in the tissues of human with amalgam fillings as opposed to those who were not similarly exposed. Increased mercury concentration in the exhaled air, saliva, blood, feces, urine, amniotic fluid, colostrum, breast milk, and various tissues, including the brain, pituitary gland, liver, and kidney, is associated with increasing amalgam load.[1],[2],[3],[4],[5],[6],[7],[8],[9] Studies have even shown risk of miscarriage in these groups.[5],[10],[11],[12]

Evidence show that if not practiced properly, handling mercury causes more threat to the dentist than to patient. Continuous exposure of dentist with amalgam on daily basis place them at higher risk and levels of mercury than the general population.[13],[14] Although the controversies still exist in many countries, amalgam is still the most commonly used posterior restorative material because of its cost, durability, long-term performance, and ease of manipulation.

The American Dental Association (ADA) has established the recommendation for appropriate mercury hygiene within the dental offices, but they are not practiced strictly. Moreover, the effectiveness of regulatory authority, especially in countries like India to govern mercury hygiene practice, is still primal. Therefore, the purpose of this review is to sensitize the dentist and dental staff about mercury toxicity and strategies for proper handling of amalgam as per the recommendations to reduce the health hazards caused by mercury for both dentist and patient and environment.


  Materials and Methods Top


Source of data

PubMed, PubMed Central, Cochrane, and Google Scholar databases were searched until January 2019 to identify the relevant studies using MeSH terms mentioned below. Additional studies were found by handsearching and reference list. A total of 59 articles were obtained using the above-mentioned database and appropriate Boolean operators, of which 44 articles were of interest from 1983 to 2019.

Finalized MeSH terms for building the Search Queries

  1. Amalgam restorations
  2. Mercury release
  3. Mercury toxicity
  4. Occupational risk to dentist
  5. Mercury hygiene
  6. Dental practice in India.


Search strategy

A search with the terms shown in [Table 1] gave the following results: PubMed yielded 45 publications, PubMed Central yielded 8 publications, Cochrane Central Registry 4 publications, and Google Scholar, 2 publications. Limits activated were from 1989 to January 2019 and full-text articles.
Table 1: Building search queries by combining various MeSH terms

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  Results Top


The search query was categorized based on the MeSH terms [Table 1] shows literature search, Data synthesis included articles with Hg toxicity in dentistry, impact of Hg toxicity on humans, amalgam removal, article regarding amalgam restoration in pregnant women and children and occupational hazards related to dentist. The comprehensive search yielded 59 potentially relevant articles. Out of 59 articles, screening excluded 12 publication based on titles and abstract. The full text analysis of 43 studies were conducted. There were 19 articles which discussed Hg toxicity in dentistry, while 24 articles reviewed dental amalgam and its impact of Hg toxicity on humans. 3 articles discussed concerns regarding amalgam removal and clinical protocols. These articles also covered risks related to amalgam restoration in pregnant women. Only 2 articles discussed amalgam restoration in children while 5 articles highlighted the occupational risk related to dentist. Hygiene practices and guidelines were discussed in 6 articles.{Table 1}


  Discussion Top


The mercury kinetics and effect on human health can be discussed under the following headings:

  1. Mercury release from dental amalgam
  2. Mercury uptake and kinetics
  3. Mercury toxicity
  4. Mercury hygiene practice
  5. Indian scenario.


Mercury release from dental amalgam

Mercury is ubiquitous in environment, and humans are routinely exposed through air, water, and food. Mercury exists in organic, inorganic, and elemental forms; mercury oxidative states, namely the mercurous (Hg+1) and mercuric (Hg+2) ions, readily react with cysteine and glutathione to form sulfides. The resulting compounds are methylated by bacteria into methylmercury (MeHg) and dimethylmercury (Me2Hg) and organic compounds, which, due to their greater absorption rate, are even more toxic.[15]

Mercury vapor released from amalgam is the most important form of mercury that determines human exposure.[16],[17] Once the amalgamation is complete, amalgam fillings release mercury vapor continuously at low levels. The amount of Hg release from amalgam restorations ranged from <1.0 to 11.4 mg/day. The release rate is dependent on filling size, tooth and surface placement, texture of food, tooth grinding and brushing as well as surface area, composition, and age of the amalgam.[15]

Mercury release while handling amalgam is the highest during dry polishing: 44 μg followed by the removal of amalgam restoration under water spray and high-volume suction: 15–20 μg; placement of amalgam restoration releases: 6–8 μg; and trituration: 1–2 μg being minimal.[18]

Amalgam is the major source of inorganic mercury (Hg), Hg emitted from amalgam by this process may be in one of the two forms:

  1. Mercury vapor (HG0): It passes into the oral air and from here may be either inspired into the lungs or expired into the outside air[19]
  2. Mercury ions (Hg2+): It passes into the saliva and from there into the gastrointestinal (GI) tract. The spherical alloy has an increased surface area for immediate reaction with liquid mercury than the admixed alloy as they have more irregular surface. The factors, which disrupt the surface-pacifying layer, such as tooth brushing and chewing can increase Hg release. Thus, Hg release from amalgam is phasic and consists of a very low baseline release and increased stimulated release as a result of toothbrushing or chewing.[17],[20]


Mercury uptake and kinetics

It is important to understand uptake and kinetics of mercury to identify and abolish early sign and symptoms of mercury poisoning. Metallic mercury vapors outgassed from the amalgam are by large are elemental in nature. Eighty percent of these vapors are absorbed mainly through inhalation, remaining 7%–10% through ingestion, and about 1% through skin contact.[2],[17]

Majority of mercury vapor absorbed in the lungs, rapidly enters the bloodstream. On entering the circulation, metallic mercury rapidly oxidizes to mercuric mercury and remains in certain tissues for longer periods of time, since the half-life of excretion is prolonged. The central nervous system (CNS) and kidneys are the two primary target organs of concern.[3],[4]

Apart from the brain and kidneys, metallic mercury is also deposited in the lungs, myocardium, liver, adrenals, pancreas, thyroid, breast, muscles, skin, salivary glands, sweat glands, testes, prostate, and enterocytes and may cause dysfunction.

Mercury influences T-cell function, as it has affinity for binding sites on the surface of T-cells and for sulfhydryl groups. Mercury deposits readily in placenta and fetal tissues and is found in breast milk.[2],[5],[6]

Metallic mercury is largely excreted as mercuric mercury. Depending on the organ of deposition and redox state, the excretory half-lives of metallic and mercuric mercury vary widely; values can range from a few days to several months, and in some pools (e.g., CNS), half-lives exceed several years, although there is no evidence of hair mercury correlating to brain content of metallic mercury.[2],[8]

Although significant amounts are shed through the sweat, tears, breast milk, and saliva, mercuric mercury is largely excreted through urine and stool.[2],[8],[21] As with metallic mercury, half-lives of mercuric mercury appear to be multiphasic, with human studies suggesting an effective half-life of 42 days for 80% of an oral tracer dose; the other 20% did not appear to have a measurable rate of excretion,[7],[9] as explained in [Figure 1].
Figure 1: Flowchart showing mercury metabolism

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Mercury toxicity

In the periodic table, mercury (Hg) is one of the heavy metals known for toxicity, highlighted for most horrific public health disaster in Minamata Bay, Japan, and Iraq.[22] Mode of action of Hg is by altering the tertiary and quaternary structure of proteins by binding with sulfhydryl (−SH) and selenohydryl groups which unwires cellular function at all structural hierarchy of an organism.

Hg interaction with soluble protein bond − SH group results in the production of reactive oxygen species (ROS) which is capable to produce oxidative injury to the tissue.[4],[16]

Amalgam restorations are mercury banks in saliva, and it is significantly correlates with both the number of restorations and the amalgam surfaces known to generate ROS which results in significant decrease in salivary antioxidant capacity.[23] Depleted iron level induces the iron-transfer system and hence Hg2+ in plasma or bound on erythrocytes, metallothionein, or glutathione competes with Fe2+ in the iron transfer system in the GI tract causing anemia.[24]

Markers of mercury (Hg) exposure have shown both positive and negative associations when it comes to cardiovascular disease.[25] Prevailing research polls the CNS as the most susceptible target organ for mercury vapor exposure.[15],[26],[27],[28] Infants and children are the main domains with rapid developing nervous system to suffer from neurodevelopmental delays, cognitive deficits, and lower body weight. The cerebral cortex and the pituitary gland account for the highest mercury concentration.[29],[30] A published relationship exists between increased mercury levels among dentists and carpal tunnel syndrome and polyneuropathy.[31] The human brain and kidney have shown to have the highest affection to Hg and can store up to 2–3-fold and 9-fold higher, respectively, in people with dental amalgam restorations compared with those without.[31],[32] Evidence suggest that patients having amalgam filling exhibit significant concentration of the apolipoprotein E4-allele than healthy individuals. Apo-E-4 lacks − SH groups decreasing its detoxifying abilities and is a major risk factor for developing Alzheimer's disease.[15]

The findings from two concurrently conducted clinical trials reports that amalgam restorations could be a source of nephrotoxicity because of increased N-acetyl-β-D-glucosaminidase, y-glutamyl-transferase, and albumin excretion in urine.[33] Correlation exists between the number of amalgam surfaces and increased creatinine levels in urine, manifesting subtle, preclinical effects in the most susceptible individuals.[4],[28]

Remarkably, mercury has effects on maternal and fetal health; Hg from maternal amalgam fillings lead to a significant increase of mercury concentration in the amniotic fluid, breast milk, tissues and the hair of fetuses, and newborn children.[29],[34] Mercury levels significantly correlate with the number of maternal dental amalgam fillings and the mercury concentration in the fetal blood, could shoot up to thirty times greater than the mother's blood.[5],[10] This alarming rise is because metallic mercury easily crosses the placental barrier and fetal blood–brain barrier depositing in the fetal brain.[2],[26] Studies document expecting female dental assistants preparing fifty mercury amalgams a week are at an increased risk of miscarriage, and infertility among females preparing thirty amalgams per week who employ poor mercury hygiene practice.[11],[12],[35]

A subtle relationship exit between oral lichenoid reactions in patients with amalgam restorations and most (97.1%) of patient's condition improved from the removal of these restorations.[8],[28],[32],[36] The case report documents severe dermatitis due to cross-reaction between the nickel and palladium. Further, interaction of this complex with mercury contributed to deterioration of the symptoms severe dermatitis in this case. The condition was improved following the replacement with palladium-free materials.[14]

Mercury hygiene practice

About 19.9% of commoners know about adverse reactions to dental amalgams and 7.5% of them have heard about people recovering from an illness after the removal of their filling,[37] hence, on initial examination, it is advised to record when the thorough medical and dental history, along with factors affecting overall fitness of the body which include its ability to eliminate toxins, wear/attrition rate of teeth, biting pressure, type of daily diet consumed, oral hygiene practice, and saturation of metals if present in the mouth should be obtained.[38]

Common use of carpets and porous floor materials, contaminated air-conditioner filters, removal of old amalgam fillings without suction, and inadequate ventilation are found to be major contributory factors for high mercury vapor levels in the dental office.[39],[40],[41],[42] Moreover, leaking capsules/cartridges, open amalgam mixers without exhaust, using amalgam carrying instruments in close proximity of heat sources add to increasing mercury vapor levels. Hand trituration and mulling of amalgam, use of squeeze cloths, and open transfers of mercury resulting in mercury spills contributing further to raise the mercury vapor level.[42] It has been documented that filling removal procedure, which lasts approximately 10 min, and Hg level in breathing zone of dentist accounts on an average of 0.1 mg/m3 which in some cases exceeds 2.5 mg/m3.[41] It is strongly recommended that while treating expecting or breastfeeding women, amalgam removal should not be done until postpartum or she completely stops breastfeeding. In these cases, Vitamin C supplements are helpful and are recommended as it helps to detoxify mercury that is stored in the body.[39],[43]

Since provocation is safe and inexpensive indication to safeguard absorption through inhalation, sublingually absorption through mucosal tissues or blood–brain barrier, these chairside procedures to be followed while removing amalgam fillings.[39]

  • First line of protection with a filtered mask, eye and hair protection, and a face shields for operator
  • Patient should be draped with a plastic apron under the dental bib to cover clothing
  • Use of customized rubber dam to fit the existing tooth/teeth to prevent vapor leakage from contacting the oral mucosa
  • Chelating agents such as activated charcoal or chlorella can be placed, along with a cotton roll and gauze underneath the dam. Accumulation of particles on the sublingual tissues and lateral border of the tongue must be prevented as this leads to fast absorption into the body
  • Liner must be used drape patient's face under the dam
  • Eyeshield and hair cap are worn for protection
  • Removal of amalgam with a new dental bur to ensure easy removal
  • Cutting in big chunks with continuous water spray as it produced less aerosols
  • Use of high volume suction
  • Controlling breathing space with the use of positive pressure respirators, for example, half-mask respirator with mercury-rated filtered cartridges available from the safety equipment suppliers.


Recommendations adopted for safe handling of mercury by the ADA council of scientific affairs in October 1998

  • Educating personnel of the potential sources of mercury vapor in the dental laboratory, about spills, leaky capsules, leaky bulk mercury dispensers, open storage of amalgam scrap as well as disposal of used capsules, remains of amalgam, placement, polishing and removal, heating of amalgam-contaminated instruments
  • Work in well-ventilated spaces, with fresh air exchanges and outside exhaust
  • Periodically check the dental operatory atmosphere for mercury vapor
  • Monitoring the mercury spill or when there is a reasonable concern about the concentration of mercury vapor in the operatory. For rapid analysis, mercury vapor analyzers are appropriate, particularly after spill or cleanup. The established limit for mercury vapor currently by the OSHA is 50 μg/cu m
  • Designing a proper work area to facilitate spill contamination and cleanup. Lining floor with nonabsorbent and facilitate easy clean
  • Use of precapsulated alloys
  • Amalgamator should be completely closed arm
  • Properly dispose used capsules or amalgam according to waste disposal laws
  • Use high-volume evacuation when finishing or removing amalgam. Evacuation systems should have traps or filters. Check and clean or replace traps and filters periodically to remove waste amalgam from the waste stream
  • Salvage and store all scrap amalgams in a tightly closed container, under radiographic fixer solution. Amalgam scrap should not store dry because mercury vapor can escape into room air when the container is opened
  • Where feasible, recycle amalgam scrap and amalgam waste
  • Clean up spilled mercury properly using trap bottles tapes, or freshly mixed amalgam to pick up droplets, or use commercial cleanup kits
  • Remove professional clothing before leaving the workplace.


Indian Scenario

Despite extensive literature available on mercury toxicity, amalgam restorations still find place in dental practice in the Indian subcontinent, with the reasons including its long-term performance and inexpensive option. Estimates of mercury usage in dentistry suggest that the average dentist use 2–3 pounds (1–1.5 kg) annually.[43] The reason for the use of amalgam among dental students is attributed to the undergraduate curriculum set by the Dental Council of India; however, equal importance should be given for waste management. Another aspect of mercury usage in India is dental waste management.

The questionnaire survey conducted to estimate mercury management policy implementation among dental practitioners in India suggests that 40.6% of dentists dispose excess amalgam to common bin. Only 5.6% of dentists do not use amalgam in their clinical practice. Only 1.2% store it in a fixer solution as recommended by ADA. These vital data evoke necessity of sensitizing dental fraternity regarding toxicity and management of amalgam because only a minor percentage of population have the knowledge about health hazards of mercury and even if it exists expensive diagnostic procedure makes it a barrier. Dental fraternity insists on segregation, labeling, and record-keeping of sources of mercury waste.[28] Recycling the mercury waste and as far as possible limiting the use of mercury in literature administration and monitoring of waste disposal in India seem to be inefficient and inadequate owing to the lack of initial education and implementation of correct practice by personnel involved at all levels of waste management.[43]


  Conclusion Top


Amalgam restorations have served the professional well and will continue to do so in the years to come. Growing advances in technology and social behavior have led to a steady increase in esthetic restoration. Replacement of amalgam with material of its own kind is constructive for sure. On contrary to previously accepted concepts, now numerous data indicate that even very low doses of mercury cause toxicity. Hence, it is the responsibility of a dentist to educate patients about the adverse effect of mercury as it is a right of every individual toward good health. On the other hand, major task is with regulatory bodies in sharing equal efforts to educate and train the people involved in waste management.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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