Dental Amalgam – One of dentistry’s oldest and most widely used restorative material is dental amalgam. Amalgam is without a doubt a restorative material with an excellent clinical record and has served hundreds of millions of patients successfully for more than 100 years. Amalgam’s reputation as a relatively inexpensive, forgiving and durable restorative material is well deserved and well documented.
Advantages of dental amalgam
Though, the use of silver dental amalgam has declined considerably in recent time, still it is continuing to be one of the most important restorative material, because of its certain unmatchable properties. Dental amalgam continues to be used as a restorative material because of the following reasons:
• It adapts readily to cavity walls and is easy to insert.
• Little dimensional change occurs during the hardening process.
• The working time is sufficient for placement, condensation, and carving.
• It reaches compressive strengths as high as 80,000 psi, yet it does not wear the opposing tooth structure.
Marginal microleakage is minimal.
It endures the harsh oral environment very well.
Limitations of dental amalgam as restorative material
However, silver dental amalgam, as a restorative material, has certain intrinsic limitations, which caused the downfall of this material from the position of undisputed king of restorative materials, which it retained for more than 50 years by virtue of remaining the material of first choice in most of the restorative cases.
However, in last few decades, with the invention of certain new materials, its position has been challenged up to great extent, with the silver dental amalgam loosing its luster as a restorative material. Some of the reasons of its decline are as follows:
• The silver/gray/black colour is in sharp contrast to tooth structure and is esthetically undesirable.
• The presence of mercury in the material.
• Silver dental amalgam may be subject to discolouration or tarnish over time.
• Silver dental amalgam is non-adhesive and requires sufficient tooth structure support to be retained.
• Silver dental amalgam high thermal conductivity requires placement of sufficient base or liner to protect the pulp.
COMPOSITION of dental amalgam
The mixing of a metal alloy powder and mercury forms dental amalgam. Mercury is a metal that is liquid at room temperature. The alloy component contains three primary metals—silver, tin, and copper. Silver provides the strength, durability, and luster while increasing expansion. Tin decreases the expansion but will also lower the strength while improving the flow.
Workability is aided by the addition of tin, but tin tends to weaken the dental amalgam. Copper increases the hardness and expansion while decreasing the flow and setting time. Copper also contributes to corrosion resistance. In some dental amalgam alloys, zinc may be added. Zinc acts as a scavenger molecule to entrap impurities. It also reacts with oxygen and prevents it from combining with the silver, tin, or copper.
It therefore minimizes the oxidation of other metals. In addition zinc reacts with water if moisture contamination occurs in dental amalgam containing zinc, expansion may be excessive. Other metals such as palladium or indium may be added in trace amounts to improve the alloys physical properties. Traditional dental amalgam, created under ADA specification #1, are to have a minimum silver content of 65%, a maximum tin content of 29%, a maximum copper content of 6%, and maximum zinc content of 2%.
However, modern metallurgical advances have altered these specifications significantly. When a non-metallic substance reacts chemically with a metal, corrosion occurs. Alloys with high copper content (some as high as 30%) have been developed that eliminate the most corrosive phase in the amalgamation process (the Gamma II Phase, during which tin reacts with mercury). With copper present in high concentrations, the tin will react with the copper instead of with the mercury. High-copper alloys demonstrate superior physical properties. Most modem dental amalgam are zinc-free and have greater than 12% copper (high copper content).
High-copper alloys can be placed into two categories according to shape: spherical (round, ball-shaped) and non-spherical, or cut particle, alloys. From this, there are two distinct classes of dental amalgam: spherical—only with spherical particles, and admix—both spherical and cut particles.
Although these designations are directly related to the manufacturing process by which the ahoy particles are created, the resultant handling properties of the two particle shapes are also quite different. Mixing the appropriate amounts of molten metals—pure silver, tin, copper, and any additional trace metals creates all alloys. The molten alloy is then either cast into ingots (metal bars) or atomized (reduced to tiny particles).
The atomization process sprays the molten metal into a cool, inert environment. This process results in droplets that solidify into spherical particles. These particles are then sold as the spherical alloy. The metal ingots are placed into a cutting/milling machine that creates filings (irregular fragments), which may be further reduced by other miffing procedures to create the fmal powder form of the cut-particle alloy. Dental alloys are then sold in this powder form or pressed in a pill machine to create tablets. A dispenser is needed for both the alloy and mercury. The proper proportion of the two components can then be measured.
The admix alloys will require more pressure to pack into the tooth or cavity preparation. They exhibit a condensation sound or “crunch.” The spherical particles will pack much easier. An operator accustomed to an admix alloy will find the spherical alloy mushy. On the other hand, an operator accustomed to spherical alloys may insufficiently pack the admix alloy and leave a void.
Mercury is only metal that is liquid at room temperature. Any metal mixed with mercury is an “amalgam.” It is the mixing of the powdered alloy and mercury that creates the dental amalgam that has been the primary posterior restorative for many years. Dental amalgam is specifically mercury, silver, tin, copper, and any other metals used. Mercury is poisonous and care must be taken in its handling.
Pure mercury should have a bright, shiny, silvery, mirror-like surface. If the surface is dull or appears to be contaminated, the mercury should be filtered through a clean cloth. Mercury may be carefully handled in its liquid form because it is not readily absorbed, However, if vapourized by heat, it can be absorbed through the pores of the skin or inhaled and absorbed through the lungs. Almost all dental amalgam today is sold in predose capsules to minimize any handling of mercury.
Use a no-touch technique by wearing gloves, a mask, and glasses if you should have to work with mercury or dental amalgam. Mercury spill kits are available to safely clean up any loose, free mercury. These kits contain a powder (such as sulfur) that combines with the liquid mercury to form a compound that can be safely and easily cleaned up. The household vacuum should not be used to clean up mercury spills because it will tend to vaporize the mercury. Mercury vapour has no odour, colour, or taste and can not be easily detected. Polyethylene bags should be used to dispose of any mercury-contaminated items. Proper ventilation in treatment rooms (operatories) is necessary. Air-filter systems need to be checked and/or changed regularly to reduce the possibility of inhaling mercury vapours. Mercury vapour monitors or monitoring equipment is available. However, the use of precapsulated dental amalgam has virtually eliminated the need for them.
Alloy-to-Mercury Ratios and Mixing
Less than 10% of the dental alloy being utilized in Dental Offices in U.S.A. is in its non-capsulated powder or pellet form, and OSHA (the Occupational Safety and Health Administration) of U.S.A. wants it to be stopped altogether, due to its associated health hazard. These powders and pellets require dispensers to measure out the amounts of alloy and mercury to create the proper mixture. Early dental amalgam required excess mercury ratios of 7:5 (7 parts of mercury to 5 parts of alloy) or greater. The excess mercury would be forced to the surface of the restoration as the dental amalgam was being packed. The excess mercury would then be removed as the dental amalgam was carved to its anatomical form. Optimal amalgam strength is achieved with a low (1:1) mercury-to-alloy ratio.
More popular today are the precapsulated dental amalgam, which is ideal since it presents no possibility of mercury spill or leakage, Most capsule systems are pressed together with a frictional fit. Each capsule contains a pre measure amount of dental alloy and the corresponding amount of mercury. They are generally available in the following three sizes (or spills):
One spill 400 mg alloy approx. 400 mg mercury Two spill 600 mg alloy approx. 600 mg mercury Three spill 800mg alloy approx. 800 mg mercury
A one-spill precapsulated dental amalgam is about the same size as mixing one pellet of alloy with its corresponding mercury. However, a three- spill precapsulated dental amalgam corresponds to a two-pellet mix. The two-spill precapsulated dental amalgam would compare to a one-and-one-half pellet mix and was created because studies had shown that 600 mg would fill the most common sizes of cavity preparations.
The alloy and mercury are dispensed into the bottom piece of a two-piece capsule. A small pestle (piston or ball) is placed inside to aid in mixing, and the capsule is sealed. The capsule is placed into an automatic triturator (mechanical mixer; also called an amalgamator) for the prescribed time (5—25 seconds). The cover of the amalgamator is always closed to prevent the escape of mercury vapours. For precapsulated alloys, simply select the proper spill and place into the triturator for the prescribed time.
The dental amalgam is transferred to the tooth by using an amalgam carrier and placed into the cavity preparation in increments. In a process called condensation, the operator carefully packs each increment with an instrument called a condenser to ensure that there are no voids. The cavity preparation is completely filled with dental amalgam. The restoration is built up in this fashion until the dental amalgam is above the cavo-enamel margins (also called cavosurface margins) so it is overfilled.
Carving and Setting Times
The operator will then use carvers to recreate the anatomical shape of the tooth in the dental amalgam.
Older alloy formulas required 24 to 48 hours of setting time, Final set occurred in about seven days. Modem dental alloys reach sufficient hardness in 3 to 4 hours to allow normal mastication and reach their maximum hardness in about 24 hours.
FACTORS RESPONSIBLE FOR DECLINE IN USE OF DENTAL AMALGAM IN RECENT TIME
During past 20 years there has been a continual decrease in the use of dental amalgam. The decrease is due to numerous factors. Some of the main factors are as follows:
(I) Fluoridated water supplies, better oral hygiene, use of sealants and other strategies have significantly reduced dental caries in younger individuals.
(II) Dental amalgam contains mercury that is a health hazard, Dental personals can be exposed to mercury either in the form of vapour or as particulate amalgam dust that can be inhaled from the air.
III) Hypersensitivity to amalgam: The production of delayed hypersensitivity, contact reactions on the skin and mucous membrane including lichenoid lesions are reported but are rare.
(IV) Increased use of esthetic dental materials such as resin composites.