Amalgam - Dental amalgam
Posted by John Doe at Dental Assistant on February 2, 2012.
Categories: Dental Materials
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RISKS TO DENTISTS AND 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:
- Store mercury in unbreakable, tightly sealed containers.
- 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.
- 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.
- Use tightly closed capsules during amalgamation.
- 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.
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 over-trituration. Trituration time should be decreased by 1 or 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 overtritu-rated 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 the cavity preparation is not confining and an unstable matrix technique is used. It can also occur when moisture contamination interferes with cohesion between increments.
Problem 5
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.
Solution b
Patients on a high-sulfur diet or dietaiy 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.
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 y2 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 pul-pal 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 oxide-eugenol 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
Spherical alloys triturated with mercury do not resist small condenser tips well, but allow them to penetrate the mass and thus reduce the condensation pressure. Larger diameter-tip condensers should be used that do not penetrate the mass as readily, thus allowing higher pressures and better condensation. These amalgams appear to condense easily and there is a tendency to use less than the desired condensation force. For optimum strength, a condensation pressure of 7 MPa should be used.
Problem 10
Why should scrap amalgam be stored in a sodium thiosulfate solution such as photographic fixer rather than just water?
Solution
Any mercury vapor released will react with the thiosulfate ions and lower the vapor pressure of mercury to levels below the instrument detection level of 0.01mg/m3. If scrap amalgam is stored under water only, the amount of mercury in the air above the water increases with the log of time.
Problem 11
What assurances can you give a patient to dispel fears of mercury toxicity from dental amalgams?
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 1% 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 mercury from air, water, and food exceeds that from dental amalgams.
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