Dental products students are taught that amalgams are composed of a mixture of metals with varying composition of mercury, silver, zinc, tin and copper. They are told that these individual metals are all toxic, but when amalgamated their toxic effects are eliminated. …The facts, however, are at variance with these views.
Dental amalgam is still the most widely used restorative in dentistry. Dental students are taught that amalgams are composed of a mixture of metals with varying composition of mercury, silver, zinc, tin and copper. They are told that these individual metals are all toxic, but when amalgamated their toxic effects are eliminated. They are reassured that the most widely used and toxic of the five metals mercury, is bound in such a way that it is not released from the restoration and presents no hazards to patients. There are still many dentists who believe that these teachings still hold true and that the use of mercury amalgam is safe.
The facts, however, are at variance with these views. Research has shown that mercury and the other metals are released from amalgam restorations during their lifetime and can be absorbed from the lungs and the gastrointestinal tract; be distributed throughout the body and concentrated at varying levels in organs and tissues.
The issue currently being debated often with more heat than light is “are the amounts of mercury and other metals from amalgam restorations that reach the target organs and tissues below the levels at which toxic effects are detectable.”
The conventional view is that if amalgam presented a toxic hazard to the average patient any effects would have become evident by now as restorations containing mercury have been in use for over a hundred years. However, modern methods of analysis and diagnosis are many times more sensitive than they were only twenty years ago. Sensitive mercury detectors can pick up the mercury released from one filling after only two minutes chewing or brushing and neurophysiological techniques can detect significant deficits in central and peripheral neuronal function in dental personnel that can be correlated with their body burden of mercury.
The amounts of mercury found in the bodies of both dental workers and patients reflects the degree of exposure. Dentists who have worked extensively with amalgam restorations over long periods and patients with many fillings tend to have higher levels of mercury in their bodies and be at greater risk from its toxic effects. In some countries the authorities have decided that the health hazards from amalgam restorations are unacceptable and outweigh the benefits from them.
Sweden has banned the use of mercury amalgams in pregnant women as the mercury readily passes into the foetus which is believed to be particularly sensitive to the toxic effects of mercury. It is expected that this ban will be extended to cover the whole population by 1997.
Other countries such as Austria are planning to phase out the use of mercury amalgam by the end of the century and in Germany the drug regulatory body has signalled its intention to ban the production of amalgam.
Toxicity of Mercury
The distinguished Swedish toxicologist, Prof. Lars Friberg, who is an adviser to WHO on heavy metal toxicity, has stated that in his view mercury should be regarded like chemical carcinogens and that there is “no safe level of mercury”.
Modern techniques of amalgam preparation and taking effective precautions during removal of amalgam fillings can reduce the exposure to mercury for both the dental staff and patients. However, even under the best operating conditions the levels of mercury vapour present in dental surgeries is higher than considered healthy and in some cases has been shown to exceed the TLV (Threshold Limiting Value) for industrial environments. Although such cases are fortunately rare there are well documented cases of dentists dying as a direct result of mercury toxicity acquired as a result of exposure during their work.
One of the earliest signs of mercury poisoning is an adverse effect on fine muscular co-ordination resulting from a neurotoxic rather than a musculo-toxic effect. This effect is thought to be on the central rather than the peripheral nervous system. Recently mercury has been shown to inhibit the formation of the micro-tubules in cells of the central nervous system. This can affect their function and can lead to impaired brain function and eventual neuro-degeneration. Some patients can show a hypersensitivity response to mercury and this may be a factor in the development of oral lichen planus. Removal of amalgam fillings in these patients can result in elimination of the condition.
A recent collaborative study among three U.S. universities has found that oral and intestinal bacteria can show a degree of cross resistance to both antibiotics and mercury. The mercury resistant bacteria were also resistant to many of the widely used antibiotics including ampicillin, tetracyclines, erythromycin and kanamycin. This may be the first example of a non-antibiotic factor inducing antibiotic resistance in potentially pathogenic bacteria. The full implications of this finding have still to be fully explored.
The mechanism by which mercury exerts its toxic effects is still not fully understood but recent evidence suggests the involvement of toxic free radicals. These are short lived but highly reactive molecules containing oxygen produced within cells, examples are superoxide, hydrogen peroxide and hydroxyl radicals. These may be a by-product of metabolism, particularly lipid metabolism or produced specifically by certain white blood cells to destroy engulfed viruses and bacteria. Free radicals can also be produced by other dental procedures, X-rays, OV light used in resin curing and local anaesthetics may all produce free radicals and although the individual amounts may be small their additive effects may be of toxicological significance.
The first and most important protective procedure is to minimise the exposure of mercury to the atmosphere during amalgam preparation. Mercury has a low vapour pressure and is highly volatile at the temperatures and atmospheric pressures found in the dental surgery. Modern containment procedures are designed to minimise the release of mercury vapour into the atmosphere and hence its subsequent inhalation and absorption. However, unless the dental team are prepared to dress in space suit type protective clothing while filling and drilling to remove amalgam, mercury vapour can transfer into the atmosphere of the surgery and this can then be inhaled. As mercury is only slowly excreted from the body the burden of mercury is likely to increase with exposure, the longer the exposure the more mercury is found in the body.
Intervention to minimise the toxic effects of mercury and other free radical inducers is feasible for both the dental team and patients. Probably the most effective protective agent against the hazards of mercury poisoning is the element selenium. This has the advantage in that it can combine with mercury to reduce the bioavailability of the mercury, possibly by competing with it for critical low molecular weight proteins used in metal transport. Selenium is a vital component of the metallo-protein enzyme glutathione peroxidase. This is a major component in the body’s free radical defence system. The availability of selenium is the limiting factor in the production of glutathione peroxidase, as modern day diets tend to be deficient in selenium its addition in the form of a nutritional supplement is the surest way of ensuring that adequate amounts are available for detoxification purposes.
Other protective agents that protect against free radical activity are the various anti-oxidant vitamins. The major antioxidant vitamins are Vitamins A, C and E and the vitamin A precursor, ß-carotene; these all are capable of quenching excess free radical activity and preventing their toxic effects. As vitamin A can be toxic if taken in excess the preferred form of this vitamin is ß-carotene.
Some dental teams already take nutritional supplements of selenium and anti-oxidant vitamins and have noticed significant improvement in their health, particularly in their increased ability to undertake procedures involving repeated fine movements. As the attitude of the health authorities in the UK towards the hazards from mercury toxicity appears to be lagging behind that of more progressive countries it is good policy to supplement the diet of the dental team with selenium and an antioxidant supplement. There is no evidence that the amounts of selenium and the other vitamins present in good quality nutritional supplements are harmful and it can be considered to be good insurance for the health of the team. Some dentists advocate the removal of all amalgam fillings as in many cases this has been found to benefit the health of patients and particularly in those suffering from disorders of the immune system such as M.E. Others believe that the removal of amalgam fillings increases the risk of exposure to a greater degree than that of leaving them intact. However, the evidence suggests that it is good practice to advise patients to ensure suitable nutritional cover with selenium and antioxidant supplements prior to removal of amalgam restorations and for some time after in order to minimise any potential mercury toxicity.
Selenium in its inorganic form is poorly absorbed by the body and most of the body’s selenium comes from organic sources, where it exists, combined with sulphur-containing amino acids, the commonest is L-selenomethionine. Some nutritional supplements contain the poorly absorbed inorganic selenium but for maximum absorption of selenium a formulation containing L-selenomethionine is the one of choice.
An organic combination of selenium with other anti-oxidants provides the best insurance against possible mercury toxicity and a preparation that combines Selenomethionine with the anti-oxidant Vitamins A, B6, C, and E together with Zinc in a balanced combination is Bio-Selenium+Zinc. This preparation originates from Denmark where many dentists who are exposed to mercury take this preparation as a preventive measure against mercury toxicity.