SELECTION OF THE ALLOY

Several criteria are involved in the selection of an amalgam alloy. The first criterion is that the alloy should meet the requirements of the ADA Specification No. 1 or the corresponding ISO specification for dental amalgam alloys.

The manipulative characteristics of dental amalgam are extremely important and a matter of subjective preference. Rate of hardening, smoothness of the mix, and ease of condensation and finishing vary with the alloy. For example, the resistance felt with lathe-cut amalgams during condensation is entirely different from that with spherical amalgams. The alloy selected must be one with which the dentist feels comfortable, because the operator variable is a major factor influencing the clinical lifetime of the restoration. Use of alloys and techniques that encourage standardization in the manipulation and placement of the amalgam enhances the quality of the service rendered. Coincident with this is the delivery system provided by the manufacturer—its convenience, expediency, and ability to reduce human variables.

Obviously the physical properties should be reviewed in the light of claims made for the superiority of one alloy over competing products. Ideally such a list of properties should be accompanied by documented clinical performance in the form of well-controlled clinical studies. Although the cost of the alloy is a factor, this criterion should not be overemphasized when balanced against the alloy’s ability to render maximum clinical service. The dentist should always consider the fractional costs of any material to the overall total charges for a dental procedure when making price comparisons between brands, particularly when comparing a brand with documented clinical performance against a generic brand of material.

Dental amalgam alloys generally are available as either small filings called lathe-cut alloys or spherical particles called spherical alloys. Spherical alloys tend to amalgamate readily. Therefore amalgamation can be accomplished with smaller amounts of mercury than required for lathe-cut alloys, and the material gains strength more rapidly. Also, the condensation pressure and technique employed by the dentist in placing the restoration are somewhat less critical in achieving the same properties of the amalgam. This is an advantage in difficult clinical situations in which optimal access for condensation is limited. Spherical amalgam alloys have a somewhat different feel during condensation and require less condensation pressure than lathe-cut alloys. The dentist and auxiliary should familiarize themselves with the handling characteristics of a new alloy before clinical restorations are placed.

HIGH-COPPER ALLOYS

The original dental amalgam alloys were alloys of silver and tin with a maximum of 6% copper. When significantly more copper is available, improved laboratory properties and clinical performance have been demonstrated. This improvement has been attributed to the displacement of the tin-mercury reaction product with a copper-tin phase during the amalgamation reaction. Alloys that contain enough copper to eliminate the formation of the tin-mercury phase (11% to 30%) are called high-copper amalgam alloys. The first such alloy of this type was an admixed system. Small spherical particles of a silver-copper alloy were added to filings of a conventional silver-tin alloy. High-copper alloys also can be made using single composition particles. Each of these alloy particles has the same chemical composition, usually silver, copper, and tin. Amalgams made from high-copper alloys have low creep. Creep is the tendency of a material to deform continuously under a constant applied stress. This property has been associated with the marginal breakdown (ditching) commonly noted with amalgam restoration. Although the ADA specification for dental amalgam permits a maximum of 3% creep, creep of a modern high-copper amalgam alloy should not exceed 1%.

Choice of amalgam alloy today should be limited to high-copper alloy systems.

Regardless of the alloy used, manipulation plays a vital role in controlling the properties and the clinical performance of the restoration.

MERCURY/ALLOY RATIO

Most of the properties of amalgam restorations have been shown to depend on the relative amount of mercury contained in the finished restoration (the residual mercury). One of the variables that control the final mercury content is the amount of mercury used to mix the amalgam.

Although dental amalgam alloy still may be available in the form of powder or preweighed compressed pellets and bulk mercury can be dispensed by volume, most of the amalgam alloy sold today is in the form of prefilled, disposable mixing capsules containing the proper amounts of alloy and mercury. This delivery system should be used for several reasons. The alloy/mercury ratio is accurately preproportioned. The need for disinfection procedures is minimized because the capsule system is discarded after use. Most importantly, exposure of dental personnel and environmental contamination by mercury vapor is minimized. These prefilled capsules are usually available for different size mixes, often called single- or double-spill capsules.

TRITURATION

The second manipulative variable that controls the residual mercury content is trituration. Trituration time can significantly influence both consistency and working time of the mixed amalgam. These in turn relate to the ability to bring excess mercury to the surface during condensation. The correct trituration time varies depending on the composition of the alloy, the mercury/alloy ratio, the size of mix, and other factors. The best practice is to acquire an appreciation for the appearance of a proper mix and then to adjust the trituration time accordingly. The most serious error in amalgamation generally is undertrituration. An undertriturated mix appears dry and sandy and does not cohere into a single mass. Such an amalgam will set too rapidly, which results in a high residual mercury content, reduced strength, and the increased likelihood of fracture or marginal breakdown. Properly mixed amalgam is a shiny, coherent mass that can be readily removed from the capsule.

MECHANICAL AMALGAMATORS

When first introduced, mechanical amalgamators for dental amalgam operated at a single speed that was usually below 3000 cpm. High-copper alloys in prefilled, self-activating capsules are designed for shorter trituration times at higher trituration speeds. Failure to activate these capsules reliably results in undertrituration and is a common problem with the use of older single-speed amalgamators. Because amalgamators also deteriorate with time, replacement of an older unit with a new high-speed amalgamator is desirable. A unit that allows multiple speeds of operation should be selected, because numerous other products such as dental cements are now marketed in capsules to be mixed in a dental amalgamator. The trituration times suggested by the amalgam alloy supplier are starting points. Amalgamators may vary in operating speed even within the same brand, and a unit’s performance may vary with line voltage or the number of times it is used in rapid succession. Trituration speed, as well as time, significantly influences the rate at which some amalgams harden(Fig. 16-1).

Figure 16-1 The influence of amalgamator speed (lowmedium-high) on the hardening rate of a high-copper amalgam alloy as measured by the Brinell Hardness test (BHN). BHN = 1.0 indicates the working time, and BHN = 4.5 indicates the carving time.

(Redrawn from Brackett W. Master’s thesis. Indianapolis, Indiana University School of Dentistry, 1986.)

CONDENSATION

The purpose of condensation is to adapt the amalgam to the walls of the cavity preparation as closely as possible, to minimize the formation of internal voids, and to express excess mercury from the amalgam. Within reasonable limits, the greater the condensation pressure, the lower the amount of residual mercury left in the restoration and the greater the strength of the restoration. The selection of the condenser and the technique of “building” the amalgam should be designed to achieve those objectives, as described in detail in textbooks of operative dentistry, and should be tailored to the handling characteristics of the type of amalgam alloy chosen.

MOISTURE

Moisture contamination of an amalgam restoration can promote failure. If zinc is present in the alloy, it will react with water, and hydrogen gas will be formed. As this gas builds up within the amalgam, a significant delayed expansion can occur and may cause protrusion of the amalgam from the cavity preparation, which enhances the possibility of fracture at the margins.

Such moisture contamination can result from failure to maintain a dry field during the placement of the restoration. Exposure to saliva after the amalgam has been completely condensed is not harmful. It is only moisture incorporated within the amalgam as it is being prepared or inserted that must be avoided.

Zinc-free alloys are available, and their physical properties are generally comparable to those of their counterparts that contain zinc. A zinc-free, high-copper alloy should be used when the dentist operates in a field where moisture control is difficult.

MARGINAL BREAKDOWN AND BULK FRACTURE

Because dental amalgam is a brittle material, a commonly observed type of amalgam failure is the restoration in which the marginal areas have become severely chipped. The exact mechanisms that produce this breakdown of the amalgam or the adjoining tooth structure are not established, but it is likely that the deterioration is precipitated by manipulation and technique of finishing rather than by dimensional changes during setting.

If the restoration is improperly finished by the dentist, a thin ledge of amalgam may be left that extends slightly over the enamel at the margins. These thin edges of such a brittle material cannot support the forces of mastication. In time they fracture, leaving an opening at the margins.

Bulk fracture of amalgam is much less common with high-copper amalgam alloys. Those cases that do occur likely have one of two causes. Poor cavity design resulting in an insufficient bulk of material across the isthmus can lead to failure of even a high-strength alloy, as illustrated in Fig. 16-2. The other reason for bulk fracture is premature loading of the restoration. Unlike a resin matrix composite, amalgam gains strength slowly over the first 24 hours. Premature loading can result in minute fractures that are not apparent for weeks or even months. The use of a rapid-setting amalgam with a high 1-hour compressive strength should be considered when treating a pediatric patient in whom compliance with instructions to refrain from biting down hard on the freshly placed amalgam is in question.

Figure 16-2 Bulk fracture of an amalgam restoration. Such failure may occur from improper cavity design or premature occlusal loading.

(Photo courtesy of Dr. Jeffrey Platt)

BONDED AMALGAM RESTORATIONS

Because dental amalgam does not adhere to tooth structure, it must be retained mechanically by the design of the cavity preparation and/or mechanical devices such as pins. The placement of an amalgam does not strengthen the compromised remaining tooth structure and subsequent fracture may occur, particularly in molar teeth with relatively large mesiodistocclusal amalgam restorations. The use of dental adhesive systems, as described in detail in the section related to resin composites, as lining materials for amalgam to create a “bonded amalgam restoration” has been suggested. Several products are marketed specifically for this purpose. In general, they are chemically activated dentin-bonding systems over which the amalgam is condensed before the resin adhesive has hardened. This results in an intermixing of the unset resin and the plastic amalgam at the interface and forms a mechanical bond as both materials harden. It is important to distinguish this application from the use of a dental adhesive to seal the dentin surface and reduce early microleakage as previously discussed. When dental adhesives are used to seal the dentin surface, the adhesive should be polymerized before the amalgam is placed. Bond strengths reported in laboratory studies between amalgam and dentin are lower than the maximum reported for resin composite bonded to dentin. In vitro studies also show that teeth restored with bonded amalgams are more resistant to fracture than those in which amalgam is placed without a bonding adhesive.

These are relatively short-term laboratory studies. Even though longer-term clinical data are available, little is known about the potential influence of embedding the resin into the bulk of the amalgam on the long-term properties of the restoration. At the present time, amalgam bonding should />