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This article addresses the dental amalgam debate from two aspects. The first is a review of the current status regarding the appropriate treatment planning of direct restorations. The second is a discussion of the safety of amalgam and the mercury toxicity concerns. Dental research continues to support the use of amalgam while the search continues for the "ideal" restorative material.
The interaction between tooth and amalgam during in vitro corrosion of dental amalgams has been studied in this investigation. Extracted teeth have been restored with five commercial amalgams, one of which was gamma 2 -free and the others contained the gamma 2-phase. The restored teeth were immersed in a 1% NaC1 solution for 9 months. Post corrosion restorations have been examined by optical microscopy, scanning electron microscopy, and X-ray microanalysis. The results are: (1) gamma 2-containing amalgam surfaces were covered with Ca-Sn-P-rich corrosion products of various morphology which occasionally contained relatively low concentrations of C1 and/or Zn; (2) the corrosion products on the gamma 2-free amalgam surface indicated relatively high concentrations of Hg in addition to Ca, p, Sn, Cu, and Zn. These results agree with the past observations that corrosion of amalgam restorations is not an isolated process. Rather it may involve reactions of the restoration and the surrounding oral environment including tooth and oral fluids in which interactions of Sn, Zn, Hg, Ca and P take place.
Opponents of the use of dental amalgam fillings often hold specific views on health and illness. Their doubts about the safety of amalgam are based upon unproven theories and results obtained with invalidated diagnostic methods and case-histories. It is contended here that case-histories cannot be accepted as scientific evidence and do not warrant over-simplified and idle, yet threatening statements. Almost superfluously, the improbability (not identical to impossibility!) of toxic symptoms and the rare manifestation of allergic reactions are summarized, prior to the description of some measures to diminish the release of mercury from amalgam restorations. Guidelines are presented how to deal with patients with health complaints attributed by them to amalgam.
The creep and recovery of dental amalgam were studied with a torsional creep apparatus. The purpose was to investigate viscoelastic behavior in a low stress range that might result from normal chewing forces. The creep of specimens up to six months old was also studied. Four types of alloys for amalgam were used: two single-composition sphericals (SCS), an admixture-lathe-cut eutectic (ALE), and a conventional (CON). Constant torque was applied to dumbbell-shaped specimens for three hr and, after the stress was released, recovery was followed for 50 hr. All measurements were made in water at 37 degrees C after storage of the specimens for one week in 37 degrees C water. Creep was also measured at nine intervals on specimens aged from three hr to six months. The SCS amalgams exhibited the least amount of creep, followed by the ALE amalgam. The CON amalgam had the highest creep. The SCS amalgams approached linear viscoelastic behavior in the shear stress range of 0.78-4.10 MPa at 37 degrees C. They did not exhibit steady-state (viscous) creep at stresses less than 4.10 MPa. Recovery was complete but proceeded more slowly than creep. The ALE and CON amalgams exhibited viscous creep at 1.23 MPa and higher. The steady-state creep rate depended on stress, and the stress exponent ranged from 2.3-3.5. All except the ALE amalgam showed essentially constant creep compliance when aged up to six months, once the amalgamation reaction was completed after 24 to 48 hr. The compliance of the ALE amalgam gradually decreased over the six-month period.
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Questions about the materials which are in current use are posed and answered in this review, which speculates about the sort of dental materials we will be using, or would like to use in the next century. Consideration is given to the merits and failings of dental amalgam, composites and glass-ionomer cements prior to laying down the requirements of the ultimate direct filling material. The case for the continued use of dental alloys for large restorations is discussed and new ways of forming these are described. The use of many cunning techniques to improve the resistance of dental ceramics to catastrophic failure is reported and a general improvement in both the denture bases and orthodontic appliances of the future is predicted.
This critical review of the published literature on dental materials for the year 1995 has been compiled by the Dental Materials Panel of the United Kingdom. It continues the series of annual reviews started in 1973 and published in the Journal of Dentistry. Emphasis has been placed upon publications which report upon the materials science or clinical performance of the materials. The review has been divided by accepted materials classifications (fissure sealants, glass polyalkenoate cements, resin composites, dentine bonding, dental amalgam, endodontic materials, casting alloys, investment materials, resin-bonded bridges and ceramo-metallic restorations, all ceramic restorations, denture base and soft lining materials, impression materials, dental implants, orthodontic materials and biomechanics). Three hundred and thirty articles published in 68 titles have been reviewed.
Several manipulative variables have been shown to influence the physical properties of dental amalgam. Variation in trituration time has been reported to affect creep, compressive strength, and diametral tensile strength in various amalgams. Recent classification of amalgam alloys by particle microstructure reveals four categories: single-composition lathe-cut; single-composition spherical; lathe-cut and eutectic admixture; and lathe-cut and single-composition spherical admixture. The effect of variation of trituration times has not been studied for these newer high-copper alloys. A representative from each classification was selected and physical properties tested, with varied times for trituration utilized. Results showed little compromise in laboratory values for the properties tested. Additional laboratory and clinical trials are warranted, but this study supports the premise that clinicians may vary trituration time to improve handling characteristics without significantly affecting mechanical properties.
The cytotoxicities of oxides, phosphates and sulphides, which possibly form on dental alloys, were tested using the 'agar method'. The relative magnitudes of the cytotoxicities of these compounds exhibited tendencies similar to those of the metal components in the compounds. In correlation with dental amalgams, zinc oxide was the most cytotoxic, just as with metallic zinc, while tin oxides and mercury oxides exhibited no cytotoxicity. Zinc was the most influential element in the cytotoxicity of dental amalgams. On the other hand, in correlation with Ni-Cr alloys, Co-Cr alloys, and/or stainless steels a chromium oxide and a cobalt oxide showed serious cytotoxicity, whereas the iron oxides were non-cytotoxic. Sulphides, which possibly form on silver alloys, showed cytotoxicity. Oxides and phosphates of titanium and aluminium, and hydroxyapatite, were not cytotoxic, supporting the use of these materials for implants. Not only the valence of an element but also the chemical species combining with the element affect the cytotoxicity of the alloy containing it.
Extension for prevention has been an integral part of dentistry for over 100 years. Because this concept advocated the removal of sound tooth structure, it was not totally accepted at the turn of the century. The advent of the gold casting catapulted extension for prevention into general acceptance. In 1883, Webb presented a concept of "prevention of extension of decay". This concept advocated a proximal cavity preparation extending toward the buccal and lingual aspects of the tooth so that contact with adjacent teeth would not be at the margins. The separation of the margins, along with proper restoration contours, was thought to promote natural cleansing of the embrasures with saliva and fluids in the diet. GV Black's 1891 idea of "extension for prevention" was to provide extension of the preparation to the facial and lingual line angles in order to bring about "self-cleansing" margins via food excursion. Black's concept also included extending preparations through fissures to allow cavosurface margins to be on non-fissured enamel. Black integrated the extension of the proximal margins with his concept of an occlusal isthmus for a Class II amalgam preparation one-third the faciolingual width of the occlusal surface. Challenges to this concept of extension for prevention were immediate; and, by the 1950's, narrower, more conservative preparations were seen by a few as being more effective in preserving teeth. Not only occlusal width was reassessed, but the need to routinely extend proximal margins to the buccal and lingual line angles was also questioned. By the mid-1960's and early 1970's a more conservative approach to amalgam preparation was advocated and was being taught in some dental schools. Today, a standardized outline form should not be used or taught as a principle of cavity preparation. In areas where fissure caries has necessitated a preparation extending into dentin, a composite resin or dental amalgam restoration should be placed, and a fissure sealant should be used to protect remaining susceptible fissures from carious attack. This current form of the concept of extension for prevention, which is supported by clinical research, preserves sound tooth structure that, using outdated concepts, would have been cut away. Placing proximal margins in sound tooth structure that just clears an adjacent tooth is also strongly advocated. Sound enamel margins in certain areas may occasionally be left in contact with adjacent teeth for amalgam preparations. For Class II preparations for composite resin, facial or lingual proximal bevels will usually suffice to separate the margins from the adjacent tooth to allow finishing and polishing at the margins. Preventing unnecessary extension and allowing sounder tooth structure to remain is one important aspect of helping patients to maintain their teeth for their lifetimes.
Urinary mercury levels and excretion rates were measured to determine the effect of dental amalgam restoration on the mercury body burden. No consistent increase in urinary mercury concentrations was found among subjects who had a single restoration, but a continuously increasing statistically significant (P < 0.05) trend, that was 33% above background levels, was detected between 9 and 12 days after restoration, in the subject with four restorations in a single day. The current findings suggested that even though amalgam restorations can cause an increase in mercury body burden, the elevation above background levels is small and thus the risks associated with the use of this material are considered minimal for the general population.
Based upon their answers to a questionnaire, 1147 respondents could be divided into four groups. One group has a critical attitude to amalgam, the second is 'uncritical', the third 'uninformed' and the fourth, among which many with health complaints attributed to amalgam fillings, is named 'imaginary group'. The division is based upon nine out of 15 variables: 1. potential harmfulness of amalgam; 2. worry about the general health because of amalgam; 3. attribution of existing health complaints to amalgam; 4. reading/hearing about harmfulness of amalgam; 5. visits to alternative healers and quacks; 6. knowing that fish contains mercury; 7. knowing that amalgam contains mercury; 8. opinions on homeopathic medicines; 9. presence of dental amalgam. Among others preventive health and environmental actions were not of importance. The respondents, distinguished by sex, age and education, are not evenly distributed. An analysis of a second sample is considered in general to corroborate the findings. Although the analysis cannot reveal causal relationships, the results suggest the possibility that the population is kidded into distrust towards amalgam by the mass media and alternative healers.
The purpose of this study was to compare microleakage around two types of restorations lined with polyalkenoate (glass ionomer) cements after thermocycling. Preparations were made in 48 molars to a diameter and depth of 2.0 mm. Half of the preparations were lined with glass ionomer, and the remainder were not lined. Dental amalgam or glass ionomer restorative material was placed and the amalgams were left unburnished and unpolished. Selected restorations were thermocycled 625 times between 4 degrees C and 50 degrees C. Teeth were immersed in 0.5 per cent methylene blue solution, sectioned and visually scored for microleakage at X 100 magnification. Data analysis indicated significant differences in microleakage because of: thermocycling (chi 2 = 103.38, d.f. = 19,2P less than 0.0004); presence of glass ionomer liners (chi 2 = 53.28, d.f. = 19,2P less than 0.0001); and type of restorative material (chi 2 = 103.44,d.f. = 19,2P less than 0.0004). The use of a glass ionomer liner significantly reduced microleakage in both amalgam and glass ionomer restorations when subjected to thermocycling.
The rapidly evolving field of materials science is providing dentistry with new treatments and alternatives. Calcium phosphate materials are under development for bone repair and replacement, root surface desensitization, and for caries prevention, by greatly increasing the remineralizing efficiency of fluorides. Composites fabricated from calcium phosphates and polymers may have application as pulp capping and cavity-basing materials. An alternative to mercury dental amalgam is being investigated; it is composed of silver powders that are cold-welded in the presence of a dilute acid. These materials are only a few of the technologies that may profoundly affect the future delivery of dental care.
Short-circuited electrodes of dental amalgams and a gold alloy were used in an attempt to study a possible relationship between the quantity of copper and tin ions released into an electrolyte and the charge transferred between the electrodes. It was found that under the experimental conditions used the quantity of tin ions released was related to the charge transferred between the conventional amalgam and the gold alloy. The same tendency was found using a saliva sample which confined and collected the released tin ions into a small volume when the conventional amalgam specimen was short-circuited to the gold alloy specimen in vivo.
OBJECTIVES: Millions of restorative procedures are performed annually in the United States, yet very little is known about their distribution in the general population. With increasing concern about potential adverse health effects of some restorative materials, a better understanding of the extent of exposure to these materials in the population is important. The purpose of this study is to report the prevalence, patterns, and distribution of dental restorative materials in a population of male veterans. METHODS: This collaborative study with the US Air Force examined 1,166 male veterans to assess exposure to dental amalgam and other restorative materials. An inventory of dental materials in the study population was obtained through oral examinations. Dental materials were classified into five categories: (1) amalgam; (2) resin; (3) porcelain, cement, or temporary, including ionomer (PCT); (4) cast gold alloys/direct filling gold; and (5) other metals (OM). The mean age of the study participants was 52.9 years. Over 94 percent of the study participants were dentate. RESULTS: The study participants averaged 45.8 restored/replaced surfaces. Restored/replaced surfaces increased with age while the number of teeth decreased with age. The most frequently used restorative material was amalgam, averaging 19.89 surfaces per subject, followed by PCT (9.38), resins (8.99), OM (5.52), and gold (4.91). The distributions of restorative materials varied by age, arch type, and location in the mouth. CONCLUSION: The study population experienced substantial exposure to dental materials.
The basic mechanism of dental amalgam corrosion has been thoroughly studied during the last 50 years by various experimental techniques, most often carried out in vitro. Electrochemical methods were extensively employed, and it was recognized that a gradual dealloying of the more electroactive components, Zn, Sn, and to a lesser extent Cu, contributed to change the surface composition. It is also well known that, in all circumstances, galvanic coupling threatens the longevity of the restoration. Among the patients examined at the hospital attached to the Department of Dental Surgery of the Paris 5 University, the authors observed, in a few exceptional circumstances of defective restorations, a release of liquid metal droplets in the surrounding tissue. These particles were identified as elemental mercury. This unexpected phenomenon was the result of the use of a low-silver-content alloy, combined with an enhanced corrosion process due to galvanic coupling with a silver alloy crown and brass screw-posts. In the described clinical case, the tooth, which had to be extracted, was axially sliced for observation by scanning electron microscopy coupled with elemental analysis by X-ray fluorescence energy dispersive spectroscopy. The observations showed a dramatic corrosion of the amalgam matrix with the formation of porosity even in the bulk of the material. Superposition of the element mappings proved (1) the electrochemical coupling of the amalgam with the silver-based crown, (2) the decalcification of the dentin, and (3) the formation of tin oxychloride precipitates in the matrix porosities and the interfacial gaps. These experimental results corroborate the corrosion mechanisms described hereupon. They support the current national and international recommendations for the dental amalgam formulation and for a rigorous professional practice.