0008-3194/96/169-178/$2.000/©JCCA 1996 The dental amalgam controversy: a review George Feuer, PhD, CMSc, FRIC* H Stephen Injeyan, MSc, PhD, DC* In spite ofthe long history ofmercury amalgam as a En depit de 1'historique e'tendu de l'amalgame au dental restorative material, its use continues to be mercure a titre de materiel de restauration dentaire, son controversial. utilisation demeure controverse'e. L'amalgame dentaire Mercury vapour is continuously releasedfrom dental rejette continuellement des vapeurs de mercure qui, en amalgam and is ultimately absorbed into a variety of bout de ligne, sont absorbe'es par une varie'te de tissus. tissues. Experimental data have demonstrated that the Les donne'es expe'rimentales ont de'montre' que uptake, tissue retention and excretion ofmercuryfrom 1'absorption, la retention par les tissus et I'excrition du dental amalgam is significant. Evidence has mercure a partir des amalgames dentaires etaient accumulated indicating a relationship between tissue significatives. Des preuves ont ete' accumule'es indiquant mercury levels and a multitude ofclinical une relation entre le niveau de mercure des tissus et une manifestations. However, the clinical significance of multitude de manifestations cliniques. Cependant, la mercury toxicityfrom dental amalgams is a matterfor signification clinique de la toxicite du mercure a' partir debate. The literature is devoid ofrandomized clinical des amalgames dentaires reste un sujet de debat. La trials that are rigorously designed to address this issue. documentation a' ce sujet est de'pourvue de'tudes Thus, although research data renders the notion of cliniques re'parties au hasard qui sont rigoureusement amalgam safety questionable, the dental community conCues pour evaluer ce probleme. Donc, bien que les appears determined to continue its use as long as donne'es des chercheurs remettent en question la se'curite unequivocal evidence correlating amalgam mercury des amalgames, la communaute' dentaire semble toxicity to specific clinical conditions is lacking. de'termine'e a' continuer d'utiliser ces derniers tant (JCCA 1996; 40(3):169-178) qu'aucune preuve categorique mettant en correlation la toxicite de 1'amalgame au mercure et certaines conditions cliniques ne serafournie. (JCCA 1996; 40(3):169-178) KEY WORDS: dental amalgam, mercury toxicity. MOTS C SLES: amalgame dentaire, toxicite du mercure. Introduction since its introduction in the early 1800's in France.' Many The safety of dental amalgam as the primary material side effects have been associated with the uses of mercury applied in dental restoration treatments has been debated amalgam in dentistry and the controversy was sparked again in the 1990's.2-8 The renewal of the century old debate was mainly due to three factors: (a) A great number * Professor of Clinical Biochemistry and Pharmacology of studies have been published describing the adverse (Toxicology), University of Toronto, Banting Institute, effects of dental amalgam on physiological processes, (b) 100 College Street, Toronto, Ontario M5G 1L5; and Professor of Toxicology, Division of Biological Sciences, the rise of public awareness to any type of environmental Canadian Memorial Chiropractic College, 1900 Bayview Avenue, contaminants representing health hazards was expanded Toronto, Ontario M4G 3E6. to mercury poisoning, and (c) with the great advances of ** Director, Division of Biological Sciences, Canadian analytical chemistry extremely low amounts of Memorial Chiropractic College, 1900 Bayview Avenue, mercury Toronto, Ontario M4G 3E6, (416) 482-2340. could be detected in blood, urine or other tissue extracts. C JCCA 1996. As primary health care practitioners, chiropractors are J Can Chiropr Assoc 1996; 40(3) 169 Dental amalgam often asked for advice on health issues which may not be results in a stronger, less corrosive amalgam with im- directly within their scope of practice. However, with the proved clinical performance.'4 The amalgamation reac- patient's overall well-being in mind, it is important that tion does not proceed stochiometrically and unreacted their knowledge is well-rounded and, in particular, they silver-alloy particles and free liquid mercury are present are up to date with issues that are of general public health after the settling reactions.13 interest. One such issue is the question ofdental amalgams Various types of amalgams can deteriorate by electro- and whether or not they are contributing factors to the chemical corrosion.13 Chewing and abrasion, brushing development of disease. and increased temperature strongly stimulate amalgam In order to review this subject comprehensively, a corrosion and evaporation of mercury. Cracking due to search was conducted utilizing the Medline and Chirolars stress and mechanical wear can also contribute to deterio- databases for the period 1985-1995. Articles were se- ration. Every amalgam restoration is attacked by corrosion lected for review if they contained information relevant to to some extent resulting in a surface layer of corrosion the topic ofdental amalgam composition, mercury toxicity products and slow and continuous release of mercury.'5 and clinical consequences of dental amalgam use. Review It has been demonstrated that the released mercury articles and textbooks were also used as a source to re- vapour from the amalgam filling is present in the air, and trieve much valuable information. absorbed by the lungs.2'1S'8 It has been estimated that the In this review we provide an overall picture of the daily average intake of mercury from dental amalgam documented associations between mercury applied in den- ranges between 2-20 pg.2'19-21 Furthermore, due to corro- tal amalgams and various possible neurological, immuno- sion, mercuric ions and microscopic amalgam particles logical and other disorders. In addition, in order to facili- are also released into the saliva.22 tate a better understanding of the effects of mercury, we give a brief account of the physical properties of dental 2. Mercury Toxicity amalgams and a synopsis of other sources of mercury and its half-life in humans. The literature is very wide2'9'10 and a) Environmental occurrence and dietary intake it is not possible to discuss every detail of this issue which On the basis of toxicological properties, there are three concerns dentists, toxicologists, neurologists, immunolo- forms of mercury: metallic (elemental), inorganic and gists and in a broad sense all health professionals. A recent organic compounds.9 Metallic mercury volatilizes at room article by Lorschider and coauthors1l provides a review of temperature, and exposure can occur by inhalation. It is the evidence for mercury toxicity and questions the tradi- lipid soluble and readily diffuses across alveolar mem- tional dental paradigm that continues to embrace the use of branes. The gastrointestinal tract does not effectively ab- mercury amalgam. The present article assesses the status sorb elemental mercury. Metallic mercury has an affinity of dental amalgam effects and provides the reader with for the central nervous system and erythrocytes. current educational information in the hopes of enhancing Inorganic mercury usually occurs as mercurous or the clinician's appreciation of the issues surrounding the mercuric salts. Gastrointestinal absorption of these com- dental amalgam controversy. (See addendum) pounds show large variations.23 Following exposure to inorganic mercury compounds, the kidney contains the 1. Physical-chemical properties of dental amalgams greatest concentration. Unlike elemental mercury, inor- Amalgams are alloys of mercury with other metals and ganic mercury compounds do not have a special affinity they have many advantages for use in dentistry.9" 0'12'13 for red blood cells or the central nervous system. The literature provides wide ranging data on the composi- Organic mercury, which occurs mainly in the alkylated tion of the amalgam. Accordingly, the conventional silver form (e.g. methyl mercury), is primarily produced by amalgam contains in approximate percentages by weight, microorganisms including plankton and the gastroin- 48-60 parts/g of Hg, 15-37 Ag, 12-13 Sn, 0-3 Cu and testinal flora. Thus, humans acquire alkyl mercury prima- 0-1 Zn.10'12 Newer brands used almost exclusively in rily from the consumption of fish and to some extent from North America and Europe utilize higher concentrations inorganic mercury compounds converted by the bacterial of Cu (up to 26 parts/g).9 The higher copper composition flora of the mouth and intestine.2 Alkyl mercury is lipid 170 J Can Chiropr Assoc 1996; 40(3) G Feuer, HS Injeyan soluble, can be easily absorbed and partially transformed b) Biological half-life to inorganic mercuric salts or oxides. The biological half-life ofmercury varies depending on its Excretion ofmercury from the body occurs primarily by chemical status and the tissue(s) in which it is stored. It is urine and feces. Unabsorbed elemental mercury is elimi- generally accepted that the average half-life of methyl nated in the feces. The fecal route is also predominant in mercury in human adults is 70 days.26 In oxidized form the the excretion of inorganic mercury compounds.2'21 In biological half life of mercury in human volunteers was time, however, urinary excretion of inorganic mercury is about 58 days in the whole body, 64 days in kidneys, 21 increased. Almost 90% of alkyl mercury is excreted in the days in the head region27 and 3.3 days in the blood.'9'28 feces. The biological half-life of ingested mercuric salts was Excluding occupational exposure, dietary intake is the 29-41 days in women and 32-60 days in men suggesting a most important source of mercury next to dental amalgam sex related difference. It is likely that the half life of restorations.6'9'24 Thus, when the contribution of amalgam mercury compounds is longer in the kidney than in the fillings to body mercury is discussed it is essential to con- whole body due to the presence of mercury binding pro- siderthe absorption ofmercury salts contained in food. Me- teins.23 thyl mercury found in fish is the most important dietary The biological half-life of methyl mercury was 71 days form of mercury.