DENTAL MATERIALS

Mercury and dentistryThe controversy continues.

Ben Balevi, BEng, dds

Until such a time as low levels of mercury are definitively linked toill-health, we must assume that the use of mercury amalgam is safe.

However we should be diligent in protecting our patients and ourselvesfrom mercury in our working environment, especially in light of thepercentage ofoffices where mercury levels exceed the recommended

levels.

The introduction of dental amalgam restorativetechniques to North America by the Crawcour

brothers in 1833revolutionized dentistry1. Prior to this,conservative restorative dentistry consisted entirely ofgold-foil restorations. Because of the expense of gold-foil, access to restorative dental treatment was limited tothe wealthy. Amalgam restorations simplified restorative dentistry and reduced its cost making dentaltreatment available to a large sector of society. Despitethis, the use of amalgam restorative techniques wassurrounded by controversy.

Initially, the Crawcour brothers (1833) recommendedamalgam placement without cavity preparation therebypromoting amalgam restorations as dental treatmentthat was both inexpensive and painless1,2. Manytreatment failures occurred as a result of this unsoundphilosophy. These failures as well as amalgam'spotential neurotoxicity resulted in much discord in thedental profession at the time. At the height of the"amalgam war" of 1843, the American Society of DentalSurgeons (A.S.D.S) condemned the use of amalgam byits members1. This did little to deter its use since theA.S.D.S. represented less than one-third of practisingdentists.

At the turn of the century, Dr. G.V. Black appliedsound scientific principles to the study of mechanicalproperties of amalgam and its application to restorativedentistry3.This led to the eventual standardization of themanufacture of dental amalgam and design of amalgam cavity preparations4. Except for the introductionof high copper amalgam alloys in the 1960's5 the basicconstituents and placement of amalgams has seen littlechange.

Mercury in amalgam was a minor issue during theamalgam wars due to the lack of understanding ofmercury toxicity by the general public and the absenceof regulatory agencies.

June 1988/Vol. 78/No. 6

The potential toxic effects of mercury in amalgamwere first reported in the mid-twenties by Stock (1926)6who postulated that mercury absorbed from dentalamalgams could lead to serious health problems. Afterfurther consideration Stock recanted his earlier claims2.Throughout this century and especially in the 1980's,isolated cases of neurotoxicity and apparent systemicdisorders have been supposedly linked to dentalamalgam.

In a recent article in the lay press (Toronto Star:January 13, 1988), a 31-year-old man attributed a15-yearhistory of fever,breathing difficulty,legcramps,lethargy, and reduced academic performance to thepresence of amalgam restorations placed 20 yearspreviously. The patient claimed that it was not until hisamalgam restorations were replaced with compositeresins that the quality of his life improved.

These unscientific reports create public hysteria aboutsimple dental restorative treatment. Although as recently as 15years ago public concern over the health hazardsof dental amalgam was unheard of, today it is notunusual for a dentist to have to answer patientsquestions concerning the safety of dental amalgams.This being the case, it is important that the dentist havean appreciation of how recent research in this subject hasdemonstrated that aside from those few who exhibit atrue allergy to mercury, amalgam poses no health hazardto the general population. Mercury allergy is rare and ismediated via a type IV hypersensitivity reaction7.It maymanifest clinically as dermatitis or stomatitis2 and lichenplanus and lichenoid lesions of the mouth may be

Dr. Balevi is with the Dept. of Dentistry at St. Michael's Hospital inToronto, Ont.

We welcome this original article submitted exclusively to OralHealth.

21

DENTAL MATERIALS.

Fast trackIt appears that any risk of mercury inamalgam is acceptably low.. . furthermore,for approximately 150 years amalgam hasbeen used as a dental restorative materialwith no proven deleterious effects.

associated with true mercury sensitivity2. In addition,it is important that the practitioner be familiar withthe potential occupational hazards of using mercury-amalgam. This dissertation will address these twoproblems.

Chemistry and biology of mercuryMercury is a peculiar element in that it is the only metalthat is liquid at room temperature. Its vapour pressure istemperature sensitive and it is therefore a potentialhealth hazard in enclosed areas. Mercury is found inthree major forms: elemental or metallic mercury (whichincludes mercury vapour), inorganic mercury (includingmercury salts such as mercury chloride) and organicmercury (such as methyl mercury).

Methyl mercury is the most toxic form of mercury andis usually found in poisoned foods such as fish frompolluted waters or insecticide-sprayed grain prod-ucts2.7.s.9.io jt js ijqujd soluble and is easily absorbedthrough the gastro-intestinal tract into the bloodstreamwhere it readily crosses the blood-brain barrier8-9. Oncein the brain, methyl mercury is oxidized to ionic mercurywhich remains in the tissues of the central nervous

system. A toxic dose of methyl mercury will classicallymanifest itself clinically as a neurologic disorder. Inaddition, methyl mercury tends to accumulate in thegastro-intestinal tract mucosa and in the acinar cells ofthe salivary glands. As a result, disturbances of thegastro-intestinal tract, gingivitis, loss of alveolar boneand a peculiar metallic taste may be associated withmercury toxicity. A complete list of the potential sideeffects of mercury are listed in Table 1.

Fortunately, mercury from amalgam makes itselfavailable to the body as either elemental mercury or as amercury salt210. Recent reports that certain oral bacteriacan methylate mercury in vitro have not been supportedby in vivo investigations2,10.

Eighty percent of inhaled mercury vapour is believedto be absorbed by the lungs2-910. Another avenue for theabsorption of elemental mercury as well as inorganicmercury is via the gastro-intestinal tract210. Fortunately,only 10 percent of mercury in these forms is absorbed bythe lining of the G.I. tract.

Once in the blood, elemental mercury is quickly oxidized by the red blood cells into the ionic form (Hg 2 + ).Since ionic mercury does not pass the blood-tissuebarrier well it collects in the kidneys. An insignificantquantity of mercury is not oxidized making it availableto the brain. Toxic levels of elemental mercury and inorganic mercury are associated with neurotoxicity and

22

disturbances of the gastro-intestinal tract, gingiva andsaliva".

Risk of mercury toxicity from amalgam restorationsCombustion products of fossil fuel, ingested foods,applied soaps and cosmetics and dental amalgam are afewof the more common sources of mercury in the body.The dental community has always been concerned withthe hazards of mercury from dental restorations210.

The patientElemental and inorganic mercury are the only potentialtoxic forms of mercury from dental amalgam restorations. Accidental introduction of amalgam material intothe tissue of the mouth (the so-called amalgam tattoo)are thought to be trivial in regards to toxic mercurialeffects210. Ingestion of free mercury in the form ofcorrosion products or newly-set amalgam particlesduring insertion have minimal toxic effects since such asmall percentage of mercury is absorbed through thelining of the digestive tract210.The threshold-limit-value(TLV), set at 50 ug/ M 3 is the mercury concentration inthe air of a working environment which is consideredsafe210 and is based on an average working week of 40hours. Others consider half the TLV to be acceptable forthe normal non-mercury working person10. In addition,short-term exposure below 500 ug/M 3 is consideredsafe10. These values are derived from data on thetoxicokinetics and toxicodynamics of ingested mercury.

Table 1: Signs and symptoms of acute highdoses or chronic low exposure tomercury.

Central Nervous convulsionsSystem anorexia

irritabilitydepressionfatiguepersonality disordersinsomnia

Neuromuscular loss of fine motor

control

tremors

Gastro-intestinal nausea

diarrhea

gastritis

Genito-urinary nephritis

Oral manifestations gingival mercurydeposits

loosening of teethulceration of oral

mucosa

enlarged tongueexcess salivationmetallic taste

ORAL HEALTH

Enwonwu (1987) argues that this derivation is basedon "many scientific uncertainties and assumptions"9.Nevertheless these values serve as guidelines so as tomake comparative analysis with data that is actuallyavailable and not as undisputed absolutes.

Intact amalgam restorations have been a concern tothe dentist since the time it was found that higher levelsof mercury vapour were found in the expired air ofpersons with amalgam restorations210. The level ofmercury in the expiredair isdependenton thenumberofsurfaces restored with amalgams210. In addition, anincrease of as much as tenfold has been measured soonafter brushing the teethor chewing gum although levelsreturned to pre-brushing levels quickly210. It is important to realize that reported values vary by a factor of 10probably because of the different methods of measuringmercury in expired air2-910. Expired air mercury levelsranged from 4.91 ug/M 3 (prior to chewing) to 29.10ug/M 3 during chewing gum in a report by Vimy andLorscheider (1985)". Otto and co-workers (1984)reported valuesthat weresignificantly lower (0.29 ug/M3 and 1.35 ug/M 3 prior to and during chewing

respectively12). Mackert (1987) demonstrated that Vimyand Lorsheider's values should be reduced by a factor of16because the rate of breathing and tidal volume used intheir experimental technique differed substantially fromnormal respiration13. Although higher levels of mercuryare found in the expired air of those with many amalgamrestorations, the values remain well below the recommended TLV13.

Mercury levels in blood and urine are relatively goodindicators of mercury absorption2-8-910. The relationshipbetween the number of amalgam restorations and thelevel of mercury in the blood or urine has not beenthoroughly investigated as in that of expired air.However, all reported values thus far2-810 are well belowthe upper limit of normal which is 20 ug/100 ml bloodand 15 ug/1 for urine2.

The relationship of amalgam restorations to theaccumulation of mercury in brain tissue was addressedby Eggleston and Nylander (1987)u. They measured theamount of mercury in brain tissue taken from cadaverswith or without amalgam dental restorations and founda positive correlation between the number of occlusalsurfaces of dental amalgam and mercury levels in thebrain. Eggleston and Nylander did not however, havedata to support any deleterious side effects associatedwith the presence of these increased mercury levelswhilethe subjects were alive.

Risks to dental health personnelAlthough as health-care workers we should always beconcerned with minimizing the mercury-relatedhealth-risks to our patients, we must also consider thedangers to dental personnel since it is these personswho are chronically exposed to mercury in their working environment. The sources ofmercury in the workingenvironment are summarized in Table 2.

From 10 to 17 percent of dental offices in NorthAmerica reported mercury levels in the working

June 1988/Vol. 78/No. 6

environment above the TLV215. This has been a concernto the dental profession and steps have been taken byprofessional societies to eliminate this threat (Table3).

Pre-packaged disposable amalgam capsules havedone much to reduce mercury spillage in the dentaloffice but they are still major sources of mercury vapourduring trituration2-815. Removal of old amalgam restorations using proper water-spray and high-volume

Table 2: Potential sources of mercurycontamination in the dental office.

• Accidental mercury spills.• Mechanical amalgamators.• Amalgam condensation.• Ultrasonic amalgam condensors.• Removal of old amalgam restorations.• Poor disposal of unused mercury amalgam.• Hot-air sterilization of amalgam-contaminated

instruments.

Table 3: Recommended mercury hygieneprocedures.18

• Prompt clean-up of all mercury spills.• Well ventilated operatories.• Monitoring for air-borne mercury once per year.• Use of disposable pre-capsulated amalgams.• Amalgam triturator arms should be enclosed

during trituration.• No contact of mercury with skin.• All amalgam scraps should be salvaged and

stored in a tightly closed container partially filledwith a sulfide solution such as dental x-ray fixersolution.

suction has been shown to eliminate the risks of

ingesting amalgam and inhaling mercury vapour2-8.Both the dentist and the dental assistant are at risk

from exposure to mercury vapour during the restorationof teeth with amalgam materials although perhaps thedental assistant is at greater risk due to handling ofamalgam and her proximity to the triturator. Despite thedefinite increase in ingested mercury in dental personnelthere is no evidence to suggest that dental assistants, orthe wives of dentists suffer from increased spontaneousabortions or abnormalities in their offspring2.

Whiteand Brandt (1976) report a positive correlationbetween hypersensitivity to mercury in dental studentsand their academic year in dentistry16. Recentlv.however. Miller and co-workers (1987) contended thisconcept suggesting that White and Brandt's findingswere related to the increased use of disposable alloy

23

DENTAL MATERIALS.

capsules as well as improved mercury hygiene practisedin modern dental schools17.

It appears that any risk of mercuryin amalgamto thedental health team and the general public is acceptablylow. It seems that mercury toxicity related to dentalamalgam comes exclusively from elemental and inorganic forms. Furthermore, research demonstrates thatlevels of mercury in the expired air of patients, and theblood and urine of dental health personnel were withinrecognized acceptable limits. The concentration ofmercury in the ambient air of most dental offices wasalso within accepted values. Furthermore, for approximately 150 years, amalgam has been used as a dentalrestorative material with no proven deleteriouseffects. Q

References

1. Ring, M.E.; Dentistry: An Illustrated History. C.V. Mosby Company, St.Louis, 1985.

2. Langan,D.C., Fan,P.L.and Hous, A.A. The Use of Mercuryin Dentistry: Acritical review of the recent literature. J. Am. Dent. Assoc. 115: 867-880(1987).

3. D.C. Smith: Biocompatibility of Dental Materials.Editor Smith, D.C. andWilliams, D.F., CRC press, Baco Ranton Florida 1982.

4. Black, G.V.; Operative Dentistry, Medicodental Published, Chicago1914.

5. Innes, D.B. and Yaudelis, W.V. Dispersion strengthened amalgams:J. Can.

Dent. Assoc. 29: 587-593 (1963).6. Anusavice, K.J.: Consequences of Mercury Exposure in Dentistry: A review

of the literature. Florida Dent. J. 54: 17-21 (1983).7. Rose, L.F. and Kay, D. Internal Medicine for Dentistry. C.V. Mosby

Company, St. Louis (1983).8. Wyngaarden, J.B. and Smith, L.H. Cecil: Textbook of Medicine. W.B.

Saunders Co. p. 2220 (1982).9. Enwonwu, CO. Review: Potential Health Hazards of Use of Mercury in

Dentistry: Critical reviews of the literature. Environmental Research 42:257-274(1987).

10. Eley, B.M. and Cox, S.W. Mercury from dental amalgam fillings inpatients. Brt. Dent. J. 163: 221-225 (1987).

11.Vimy, M.J. and Lorscheider, F.L. Intra-oral mercury released from dentalamalgams. J. Dent. Res. 64: 1069-1071 (1985).

12.Ott, K.H.R. et al. Mercury burden due to amalgam fillings. DtschZahnarztlZ39: 199-205(1984).

13. Macket Jr., J.R. Factors affecting estimation of dental mercury exposurefrom measurements of mercury vapour levels in intra-oral and expired air.J. Dent. Res. 66: 1775-1780 (1987).

14.Eggleston,D.W. and Nylander, M. Correlationsof dental with mercury inbrain tissue. J. ofProstho. 58: 704-707(1987).

15. de Freitas, J.F. Mercury in the dental work-place. An assessment of thehealth hazards and safeguards.Aust. DentalJ. 26: 156-161 (1983).

16. White, R.R. and Brandt, R.L. Development of mercury hypersensitivityamong dental students. J. Am. Dent. Assoc. 92; 1204-1207 (1976).

17. Miller, E.G., Perry, W.L. and Wagner, M.J. Prevalence of Mercuryhypersensitivity in dental students. J. ofProstho. 58: 235-237 (1987).

18.Council on Dental Materials, Instruments and Equipment. Recommendations in dental mercury hygiene. J. Am. Dent.Assoc. 109: 617-619(1984).

Acknowledgement:The author would like to acknowledge Dr. R.E. Wood for hisassistance in the preparation of this manuscript.

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