Сraig. Dental Materials

compounds. Liquid mercury reacts with silver to form an inorganic silver-mercury compound via a metallic bond. Reports of people and animals being poisoned by eating food high in mercury are traced to the contamination of these foods by methyl mercury.

Mercury vapor is released, in minute quantities, during all procedures involving amalgam, including mixing, setting, polishing, and removal. Mercury vapor has also been reported to be released during mastication and drinking of hot beverages. The amount of mercury on amalgam surfaces has correlated with the quantity of mercury used during trituration. However, measuring the flow and flow rate is difficult and not precise, especially when working with a small area such as the mouth. Furthermore, ambient mercury must be considered, especially if such readings are taken in a dentist's office. With good ventilation, mercury levels return to background levels 10 to 20 minutes after placing an amalgam, and a charcoal filter system decreases levels 25% during the operative procedure. Fresh amalgams release more mercury than 2-year- old amalgams even with a Streptococcus mutans biofilm, and it has been shown that most oral organisms can grow in dental plaque containing 2 pg of mercury. Under normal conditions amalgam is covered by saliva, tending to reduce vapor pressure. Amalgams can also be constrained with a sealant resin for the first several days after insertion. Adding indium (8% to 14%) also decreases the vapor pressure.

CONCENTRATIONS OF MERCURY

OSHA has set a Threshold Limit Value (TLV) of 0.05 mg/m3 as the maximum amount of mercury vapor allowed in the workplace. Nearly all dental offices worldwide comply with this standard. As an example of the factor of safety in this boundary, the fetuses of pregnant rats exposed to atmospheres with mercury concentrations of 2 mg/m3 showed no ill effects. Fetuses exposed to mercury concentrations of 5 mg/m3, or 40 times the allowable concentration, were stillborn. The lowest dose of mercury that illicits a toxic reaction is 3 to 7 pg/kg body weight.

Paresthesia (tingling of extremities) occurs at about 500 pg/kg of body weight, followed by ataxia at 1000 pg/kg of body weight, joint pain at 2000 pg/kg of body weight, and hearing loss and death at 4000 pg/kg of body weight. Therefore these values are much greater in magnitude than the exposure to mercury from amalgam or from a normal diet.

Mercury in Urin e The body cannot retain metallic mercury, but passes it through the urine. By using radioactive mercury in amalgams, it is possible to monitor the mercury levels in urine caused only by dental amalgams. One study showed that urine mercury levels peak at 2.54 pg/L 4 days after placing amalgams and, after 7 days return to zero. On removal of amalgam, urine mercury levels reach a maximum value of 4 pg/L and return to zero after a week. Although mercury is readily cleared in both cases, peak urine levels of mercury are nearly twice as great when amalgam is removed rather than inserted. The same is true for mercury vapor, with higher levels recorded on removal of an amalgam than on insertion. Other studies, using more sensitive techniques such as atomic absorption spectroscopy, show conflicting findings. There are reports demonstrating no increase in urine mercury levels and reports showing higher levels. Even in those cases in which urine mercury is elevated, the concentrations are still less than 1 pg/L.

As a comparison, consider the WHO estimate that eating seafood once a week will raise urine mercury to 5 to 20 pg/L, or two to eight times the level of exposure from amalgam determined in the study just cited. Neurological changes are not detected until urine mercury levels exceed 500 pg/L, nearly 170 times the peak levels found on insertion of an amalgam.

Mercury in Blood The maximum allowable level of mercury in the blood is 3 pg/L. Several studies have shown that freshly placed amalgam restorations elevate blood mercury levels to 1 to 2 pg/L. Removal of amalgam decreases blood mercury levels, with a half time of approximately 1 to 2 months for elimination of mercury.

However, as with urine mercury levels, there is first an increase of around 1.5 yg/L, which decreases in about 3 days. One study monitored blood mercury levels for a year and showed that patients with amalgams had a lower than average blood mercury level (0.6 pg/L) than patients without amalgams (0.8 pg/L). Presumably the blood mercury level is easily influenced by other factors and therefore cannot be explicitly related to amalgam. Evidently, a relationship exists between plasma and urine mercury levels.

Another study showed that patients with and without amalgams do not differ in the mean number or percentage of lymphocytes. Some studies have shown the blood mercury levels of dentists to be normal, whereas others report an increase. For those studies that indicate higher blood mercury levels in dentists, results have varied regarding any correlation between mercury concentration and number of amalgams placed. Elevated blood mercury levels may relate to mercury spills in the office, a factor that can easily be controlled. Both blood and serum mercury levels seem to correlate best with occupational exposure and not with the number of amalgams or length of time with amalgams in place.

Release of Corrosion Products Mercury release into various media, including water, saline, buffered citric and phosphoric acid, and synthetic saliva, has been measured by a number of techniques, such as atomic emission spectroscopy and atomic absorption spectroscopy. Ion release tends to be greatest in the first 1 to 24 hours after trituration. Once the amalgam is fully set, ionic dissolution is very low. This reduction in ion flux with time probably results from a combination of the chemical reaction progressing further and the formation of a passive surface film. In general, low-copper alloys release more ions than high-copper alloys, because of their inferior corrosion resistance. Greater amounts of mercury and silver are released from unpolished specimens than from polished specimens.

The effect of electrolytic concentration on corrosion has been compared for conventional and

high-copper admixed alloys following storage for 4 months. The main corrosion products were tin compounds at the surface of the amalgams. Low-copper amalgam showed surface corrosion only, whereas subsurface corrosion occurred with high-copper amalgam, especially following immersion in an NaCl solution without phosphate. For low-copper amalgam, the release of elements decreased with time, possibly indicating passivation. For high-copper amalgam, the release of elements increased with time, except for copper and tin in a solution with a high concentration of phosphate, indicating that phosphate inhibits corrosion of the copper-tin phases. Other studies have revealed a tendency for tin and copper to be preferentially released from amalgam. Presumably, tin release originates from surface corrosion, whereas copper release results from subsurface corrosion. Stronger galvanic influences enhance copper release and, to a lesser extent, zinc release. Tin tends to provide a passive layer and to suppress the dissolution of mercury. It is suspected that indium functions similarly. In zinc-free alloys, the tin oxide is mercury depleted.

Another recent study has shown that following 1 week of aging in 0.9% NaCl solution at 37" C, the amount of mercury released from y, was 14 to 60 times that released from amalgam and j times that released from PI. The y, phase released the least amount of mercury.

ALLERGIC REACTIONS AND DISEASE

Allergic reactions to mercury in amalgam restorations are rare, although there are case reports of allergic contact dermatitis, gingivitis, stomatitis, and remote cutaneous reactions. Such responses usually disappear on removal of the amalgam. Other local or systemic effects from mercury contained in dental amalgam have not been demonstrated. No well-conducted scientific study has conclusively shown that dental amalgam produces any ill effects.

Random reports of various diseases, such as multiple sclerosis, cannot unequivocally link the diseases to amalgams and therefore must be interpreted with caution. Reports of multiple

314 Chapter 11 AMALGAM

sclerosis patients being instantly cured when amalgam is removed cannot be upheld scientifically. Because a week must pass for all mercury to be cleared by the body, an instantaneous recovery after removing the potential source of the mercury is unlikely.

Local Reactions In patients with oral lesions near amalgam sites, positive patch tests have been reported. However, the appropriate patch test has still not been determined, and many of the materials used for patch testing contain excessive concentrations of mercury. There are also reports of inflammatory reactions of the dentin and pulp, similar to the reactions to many other restorative materials. Mercury has been found in the lysosomes of macrophages and fibroblasts in some patients with lesions. Inflammation can usually be alleviated with a cavity liner. With the increased use of more corrosion-resistant amalgams, the volume of corrosion products and subsequent reactions are reduced.

Macrophages play an important role in the removal of foreign particulate matter from tissue. A number of cell culture studies have assessed the potential cytotoxicity of amalgam and its constituents. Unreacted mercury or copper leaching out from high-copper alloys has usually been the constituent leading to adverse responses. An in vitro study of the effects of particulate amalgams and their individual phases on macrophages showed that all particles except the y, are effectively phagocytized by macrophages. Cell damage was seen in treated cultures exposed to particulate y,.

muscle implantation model, biological reactions to amalgams were found to depend on the time of implantation. All amalgams were strongly toxic 1 hour after setting. After 7 days, only high-copper amalgam showed any reaction.

In another series of studies, lowand highcopper amalgam powders and various phases of

amalgam were implanted subcutaneously in guinea pigs. The result was a mild, early inflammatory response, in which particles were taken up by macrophages and giant cells. After 1.5 to 3 months, chronic granulomas developed. With low-copper amalgam, early changes occurred in the intracellular material, associated with the rapid degradation of the y, phase. Later, intracellular particles from both lowand high-copper amalgam underwent progressive degradation, producing fine secondary particles containing silver and tin, which were distributed throughout the lesions and gave rise to macroscopic tattooing of the skin. Secondary material and small, degrading, primary particles from both types of amalgam were detected in the submandibular lymph nodes.

Elevated mercury levels were detected in the blood, bile, kidneys, liver, spleen, and lungs, with the highest concentrations found in the renal cortex. Mercury was excreted in the urine and feces. Mercury levels in the blood, liver, renal cortex, and feces were lower with the highcopper amalgam.

Black, refractile particulate deposits approximately 1 to 3 pm in diameter were found in the cytoplasm and nuclei of kidney cells. The ratio of nuclear to cytoplasmic deposits was higher in animals receiving high-copper amalgam. The cytoplasmic deposits consisted of collections of fine particles within lysosomes. Both lysosomal and nuclear deposits contained mercury and selenium, which were present in the animals' diet at low levels. Neither this study nor others have demonstrated any changes in biochemical function of any of the laden organs.

Subcutaneous implantation of only the powdered y, phase led to a limited initial release of mercury from extracellular material. Thereafter, chronic granulomas developed around the implants, and particles degraded slowly in macrophages and giant cells. Fine secondary particles containing tin were produced. Subcutaneous implantation of only the powdered y, phase induced a severe initial tissue response, and the majority of the material was extruded from the healing wounds. This process was accompanied

by the release of significant amounts of mercury, which appeared in the body organs and excreta. The small numbers of particles remaining in the tissues underwent a slow degradation in macrophages and giant cells in chronic granulomas. Minute secondary particles containing silver and sulfur were deposited in the tissues and gave rise to macroscopic tattooing of the skin above the implants.

In another study, primates received occlusal amalgam fillings or maxillary bone implants of amalgam for 1 year. Amalgam fillings caused deposition of mercury in the spinal ganglia, anterior pituitary, adrenal, medulla, liver, kidneys, lungs, and intestinal lymph glands. Maxillary amalgam implants released mercury into the same organs, except for the liver, lungs, and intestinal lymph glands. Organs from control animals were devoid of precipitate.

Note that studies on powders probably overestimate the amount of breakdown products, and therefore biological response, because the surface area of powders can be 5 to 10 times the surface area of a solid component. It must also be emphasized that any reaction to amalgam, whether in cell culture, local tissue response, or systemic response, does not necessarily imply a reaction to mercury. Such reactions could be in response to some other constituent of the amalgam or corrosion product. For example, in vitro cell culture testing that measured fibroblasts affected by various elements and phases of amalgams has shown that pure copper and zinc show greater cytotoxicity than pure silver and mercury. Pure tin has not been shown to be cytotoxic (Fig. 11-14). The y, phase is moderately cytotoxic. Cytotoxicity is decreased by the addition of 1.5% and 5% tin (Fig. 11-15). However, the addition of 1.5% zinc to y, containing 1.5% tin increases cytotoxicity to the same level as that of pure zinc. Whenever zinc is present, higher cytotoxicity is revealed. High-copper amalgams show the same cytotoxicity as a zinc-free, lowcopper amalgam. The addition of selenium does not reduce amalgam cytotoxicity, and excessive additions of selenium increase cytotoxicity. The cytotoxicity of amalgams decreases after 24

Element

Fig. 11-14 Quantitative representation of the affected areas of fibroblasts, which reveals the magnitude of cytotoxicity of amalgam elements. Standard deviations are represented by vertical bars.

(From: Kaga M, Seale NS, Hanawa T et al: Cytotoxicity of amalgams, alloys, and their elements and phases, Dent Mater 7:68, 1991.)

Amalgam phase

Fig. 11-15 Quantitative representation of the affected areas of fibroblasts, which reveals the magnitude of cytotoxicity of amalgam phases. Standard deviations are represented by vertical bars.

(From Kaga M, Seale NS, Hanawa T et al: Cytotoxicity of amalgams, alloys, and their elements and phases, Dent Mater 7:68, 1991.)

hours, possibly from the combined effects of surface oxidation and further amalgamation. The results of this study suggest that the major contributor to the cytotoxicity of amalgam alloy powders is probably copper, whereas that for amalgam is zinc.

316 Chapter II AMALGAM

RISKS T O DENTISTS A N D OFFICE

PERSONNEL

Of the two groups of people (i.e., patients and dental office personnel) potentially at risk to mercury exposure with dental amalgam, the dental office personnel are at greater risk because of frequent handling of the freshly mixed material. The concern with the potential for mercury toxicity therefore centers primarily on dental office personnel. The blood mercury levels of dentists have been shown to be normal in several studies, especially when the following recommendations in mercury hygiene are followed:

1.Store mercury in unbreakable, tightly sealed containers.

2 . To confine and facilitate the recovery of spilled mercury or amalgam, perform all operations involving mercury over areas that have impervious and suitably lipped surfaces.

3.Clean up any spilled mercury immediately. Droplets may be picked up with narrow-bore tubing connected (via a wash-bottle trap) to the low-volume aspirator of the dental unit.

4. Use tightly closed capsules during amalgamation.

I SELECTED PROBLEMS

Problem 1

An amalgam mix is difficult to remove from the capsule and appears excessively wet. What can be done to obtain a better mix?

Solution a

The most common cause is related to overtrituration. Trituration time should be de-

Use a no-touch technique for handling the amalgam.

Salvage all amalgam scrap and store it under water that contains sodium thiosulfate (photographic fixer is convenient).

Work in well-ventilated spaces. Avoid carpeting dental operatories; decontamination of carpeting is very difficult.

Eliminate the use of mercury-containing solutions.

Avoid heating mercury or amalgam.

Use water spray and suction when grinding dental amalgam.

Use conventional dental amalgam condensing procedures, manual and mechanical, but do not use ultrasonic amalgam condensers.

Perform yearly mercury determinations on all personnel regularly employed in dental offices.

Determine mercury vapor levels in operatories periodically.

Alert all personnel who handle mercury, especially during training or indoctrination periods, of the potential hazard of mercury vapor and the necessity for observing good mercury and amalgam hygiene practices.

creased by 1or 2 seconds, and the mix should then be tested for plasticity.

Solution b

The speed of trituration may have been too fast; a slower speed should be selected for the next mix. Remember that the work of trituration is important in obtaining a normal mix

and that the work is a function of the speed and the duration of trituration; increasing either increases the work of trituration.

Problem 2

Mixes of amalgam are consistently on the dry side and lack plasticity during condensation. What can cause a dry mix to occur, and how can it be corrected?

Solution a

In contrast to the previous case, a dry mix is often caused by undertrituration. Several mixes should be made at increased trituration times (an additional 1 to 2 seconds), and each one should be tested for plasticity. Listen for significant sound changes during trituration; the pestle can become wedged for the first few seconds, and the work-energy during that time is lost. Another choice would be to use a higher speed on the amalgamator.

Solution b

A dry mix can result from loss of mercury from the capsule during trituration. The two portions of most premeasured capsules are held together by a friction fit and occasionally some mercury can leak out during mixing. Check the inside of the housing covering the capsule area for small droplets of mercury, which also may appear like dust. If mercury is found follow the clean up procedures listed in the text at the end of this chapter. Changing to a hermetically sealed capsule is suggested if the problem persists with the product being used.

Problem 3

When larger restorative procedures are performed with amalgam, the amalgam is difficult to carve and seems to set before an adequate carving can be completed.What can cause this problem?

Solution a

The working time of an alloy can be influenced by the specific composition or particle

size of an alloy and by the aging treatment during manufacturing. If the working characteristics of a particular alloy appear to change from those previously experienced, the cause may be an alteration made by the manufacturer. On the other hand, if the faster reaction rate occurs at initial trials with a new or unfamiliar alloy, this may simply indicate that the alloy has a short setting time and is unsuitable for use by certain operators or with specific techniques.

Solution b

The two most common manipulative variables that can accelerate the reaction and make carving difficult are overtrituration and decreased mercury/alloy ratios. An overtriturated mix is recognizable by its shiny and very wet appearance and by its high initial plasticity. A low mercury/alloy ratio appears quite dry during condensation and presents difficulty in handling.

Solution c

The increased rate of the reaction may be compensated for by making several smaller mixes as material is used. Do not try to complete large restorations from a single mix or continue to use a mix after it has exceeded the usable range for plasticity. Also, the technique should be evaluated; most carving problems can be remedied by obtaining assistance and improving operator speed.

Problem 4

When trying a new alloy, some amalgams may appear dry and brittle at the carving stage and tend to break away in large increments rather than to carve smoothly. What can cause this problem?

Solution a

A check should be made with a stopwatch to determine the point after initiating the mix at which this brittleness or loss of plasticity is first noticed. Prolonged condensation involves

working the material beyond its limit of plasticity, and the loss of cohesiveness between increments complicates carving. Delayed condensation, in which there is a short, unavoidable interruption during the procedure, can also result in working the material after significant matrix has formed, causing the structure to break down. The result is a weak, friable surface that will not carve smoothly. It is very important to condense and carve an alloy in one continuous operation and within the time framework of the setting reaction for the alloy being used.

Solution b

The setting or working time of the alloy could be too short for the particular procedure being performed. Newer alloys appear to be faster setting and somewhat less consistent with respect to working time. The causes for a shortened setting time or an increased reaction rate are reviewed in the preceding problem.

Solution c

In certain instances a lack of condensation force can result in a restoration with a large number of air voids or poor cohesion between increments. This often occurs when

It can also occur when moisture contamination interferes with cohesion between increments.

Problem 5

Solution b

Patients on a high-sulfur diet or dietary supplement show increased tarnish on amalgam restorations. A well-polished surface is the best preventive measure to minimize tarnishing.

Solution c

Surface texture is also important in preventing corrosion. Small scratches and exposed voids develop concentration cells, with saliva as the electrolyte. Thus corrosion weakens the amalgam in critical areas such as the margin interface and begins the breakdown process. One of the major advantages of polishing amalgam surfaces to a smooth texture is that polishing minimizes the effects of corrosion and thus enhances clinical performance.

Solution d

Galvanic action can also develop in the mouth when two dissimilar metals come into contact. The most common cause of galvanic action is gold and amalgam placed in adjacent teeth. The effects can be seen in the darkened corrosion products appearing on the surface of the amalgam. This does not occur in every mouth, and the severity may relate to salivary composition and its function as an electrolyte.

Solution e

Moisture contamination during condensation causes air voids to develop throughout the mass of the restoration and corrosion to progress at a faster rate.

What factors are related to excessive tarnish and corrosion that appear several years after placement?

Solution a

A high residual mercury level in the final restoration can lead to increased corrosion as a result of an increase in the tin-mercury (y2) phase. This mercury can result from a mercury/alloy ratio that is too high in the initial mix or from inadequate condensation to remove excessive mercury.

Problem 6

As amalgam restorations wear, the marginal integrity is usually the first area to show signs of failure. Small increments of amalgam or unsupported enamel fracture and crevices develop, thus leading to increased leakage and eventual secondary caries. What factors contribute to marginal deterioration of this type?

Solution a

Initially, every margin of a preparation should be examined for potential areas of enamel

failure. Unsupported enamel rods and undercut walls are potential sites for fracture when subjected to occlusal forces. All cavosurface margins should be smooth-flowing curves and be free of unsupported enamel. Cavity walls should meet the external surface of the tooth at a 90-degree angle to provide optimum support for the tooth and sufficient bulk in the amalgam to resist fracture along the margin.

Solution b

Carving of the amalgam should be continuous with existing tooth form and should provide an accurate adaptation to the exposed cavity margin. Thin overextensions of amalgam beyond the margins and onto enamel can fracture readily into the bulk of amalgam and leave a crevice.

Solution c

Inadequate condensation of the amalgam in areas adjacent to the margins, especially in the areas of occlusal overpacking, causes a high residual mercury level to remain at the margin interface. The excessive y, phase in that area leads to an increase in flow and corrosion and a decrease in strength that predisposes the restoration to fracture.

Solution d

Use of an alloy with a higher creep value, such as a microcut, results in evidence of early marginal fractures when subjected to occlusal function. The high-copper content alloys have less creep and demonstrate more durable marginal adaptation.

Problem 7

Small interproximal restorations often fail by fracturing across the occlusal isthmus. How can this type of failure be avoided?

Solution a

The major cause of gross fracture of amalgam restorations is usually found in the design of the cavity preparation. Sufficient bulk of material must be provided to support occlusal forces. This can best be accomplished by

keeping the isthmus narrow and providing adequate cavity depth; however, on occasion the reverse might be necessary to avoid pulpal involvement. The axiopulpal line angle should be rounded to reduce stress concentration in that area.

Solution b

Occlusal contacts should be adjusted to avoid excessive contact on the marginal ridge. A torquing action places the isthmus under tension and results in fracture sooner.

Solution c

A smaller condenser must be used in the isthmus area so that adequate condensation can be accomplished. Inadequately condensed amalgam results in a weakening of the area and a predisposition to fracture.

Solution d

If enough dentin is removed during cavity preparation to require the placement of a cement base, a sufficiently rigid material must be selected for use. Zinc phosphate cement is the best material having a high enough modulus to minimize deflection of the amalgam. Other dental cements, particularly zinc oxideeugenol types, have low moduli; under occlusal function they allow too much deflection and brittle failure is likely to occur in the amalgam. The axiopulpal line angle is a critical area and should be reconstructed in a supporting base of zinc phosphate cement.

Problem 8

A high-copper, fast-setting amalgam could not be finished by the early polishing procedure until 20 minutes after amalgamation and a satisfactory finish could not be obtained. What was the cause, and what would be the proper clinical procedure at this point?

Solution

For amalgams of this type to be polished at the first appointment, finishing should be started 8 to 10 minutes after trituration, depending on the alloy. If the finishing is delayed until

20 minutes after trituration, the setting of the amalgam has proceeded too far, and strength of the alloy is too great to complete the finishing with triple-x silex and water. If the finishing is delayed too long, attempts at early finishing should be stopped, and final finishing should be done in the standard manner at a second appointment.

Problem 9

A spherical amalgam mix was condensed with a 2-mm diameter condenser to obtain a high condensation pressure and a well-condensed restoration. However, a low-strength amalgam restoration and failure resulted. Why?

Solution

Except for the rare allergic reaction there is no scientific documentation of local or systemic effects of dental amalgams. Patients with nine occlusal surfaces of amalgam will inhale less than 1Yo of the mercury that a person would inhale in a workplace having a level allowed by OSHA. Ingested mercury is eliminated through feces and urine. The daily intake of rnerculy from air, water, and food exceeds that from dental amalgams.

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