dental02.jpg (11342 bytes)Fundamentals of
Dental Materials

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Restorative Materials

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1-7. USAGE AND PREPARATION OF AMALGAM

a. General. The dental specialist has the direct responsibility for the correct preparation and use of amalgam. Incorrect use may produce a faulty restoration that can cause or contribute to the loss of a tooth. Therefore, the dental specialist must use extreme care in preparing a good mix of amalgam that will provide the best qualities obtainable from the alloy.

b. Proportioning Alloy and Mercury. To proportion and mix dental alloy and mercury, the size of the mix (the number of alloy pellets to be used) and the alloy-mercury ratio must be known. The dental officer determines the size of the mix used. Dental alloy is supplied in pellet form or in sealed capsules containing premeasured mercury. The pellets are composed of silver alloy filings compressed under pressure without a binding agent. They are supplied in weights ranging from 4.8 to 6 grains per pellet. A special dispenser is used to drop the pellets individually. (See figure 1-2.) Since the pellets are preformed in a set amount of silver alloy, only the amount of mercury used with each pellet needs to be measured. The mercury dispenser is furnished with four interchangeable plungers lettered A through D. (See figure 1-2.) The manufacturer's instructions accompanying the pellets should be followed in selecting the size of plunger to use in providing the desired ratio of alloy to mercury. The alloy-mercury capsules are preweighed and premeasured, needing only to be combined internally. This is done by penetrating a membrane separating the alloy and mercury prior to trituration.

c. Trituration of Amalgam. Trituration is the mechanical mixing of the alloy and mercury. Trituration is done by hand using a mortar and pestle or a mechanical amalgamator. (See figure 1-3.) Trituration is done by setting the timer according to the manufacturer's instructions for the alloy and for the type amalgamator used. Special capsules are furnished with the mechanical amalgamator to hold the alloy-mercury mixture during trituration. Each capsule is fitted with a cap and a small rod-like pestle. A small funnel is also furnished to help in pouring the mercury into the capsule. The

Figure 1-2. Amalgam dispensing systems.

amalgamator mixes the amalgam in the capsule by rapid shaking or vibration. This produces a consistently uniform mix. The amalgamator reduces trituration to a matter of seconds. When using the pellet method, first insert the pestle in the capsule, dispense the required mercury, and then dispense the pellets. Usually one pellet is used for a small mix and two pellets for a large mix. Most manufacturers recommend mixing times of approximately 10 seconds per pellet. When the time selected has elapsed, the automatic timer will stop the machine. The dental specialist must be careful not to overtriturate or undertriturate. Overtrituration results in shorter setting time and increased shrinkage. Undertrituration results in increased expansion, lengthened setting time, and weakened amalgam.

d. Filling Amalgam Carrier. Modern dental amalgams use precise proportioning methods for dispensing the mercury with the alloy. Since the mercury content in the original mix is less than the maximum level of 55 percent, it is not necessary to eliminate mercury from the amalgam before it is carried to the cavity preparation. The amalgam is taken from the capsule and placed in an amalgam cup. The amalgam carrier is loaded by forcing the open cylinder of the amalgam carrier into the balled amalgam. The amalgam is then carried to the mouth and deposited in the cavity preparation.

 

Figure 1-3. Mechanical amalgamator.

e. Condensation and Carving of Amalgam. Condensation is the process of packing an amalgam mix into a cavity preparation. Both time and pressure are important to achieve the best results. Condensation must be accomplished before crystals start to form. Delay will result in a breakdown of these crystals and a weakened amalgam. Sufficient condensation pressure is necessary to prevent voids in the restoration. The amount of pressure varies with each type of amalgam. Usually the amalgam restoration is well set and hardened so that carving can be started with sharp instruments immediately after condensation. The carving operation results in a completely restored tooth.

David L. Heiserman, Editor

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Revised: May 22, 2017