Dental Amalgam Chapter 9

Dental Amalgam Dental amalgam has been used successfully for longer than 170 years for the restoration of teeth. Approximately 50 million amalgam restorations are placed each year. Lack of esthetics and patient concerns about amalgam safety have prompted an approximately 40% drop in its use over the past 15 years.

Alloy Versus Amalgam An alloy is a mixture of two or more metals. The alloy used in dental amalgam is composed predominantly of silver but also contains copper and tin. A variety of other metals, such as palladium, iridium, or zinc, may be added in smaller quantities. When the silver-based alloy is mixed with mercury, the reaction that occurs is called amalgamation.

Silver-Based Amalgam Alloy Particles Silver-based amalgam alloys are classified according to the shape of the particles in the powder as lathe-cut, admixed, or spherical. Each of these particle shapes contributes certain handling characteristics to the amalgam.

Silver-Based Amalgam Alloy Particles (cont’d) Lathe-cut particles are formed by shaving fine particles off an ingot of the alloy with the use of a lathe. The particles are sifted to separate them into fine and ultra-fine particles.

Silver-Based Amalgam Alloy Particles (cont’d) Spherical particles are produced by spraying molten alloy into an inert gas. Spherical particles are formed as the atomized droplets cool. Admixed particles consist of a mixture of lathe-cut and spherical particles.

Composition Modern dental alloys are considered to be high in copper content (13% to 30%) compared with their predecessors (4% to 6%). They generally contain 40% to 70% silver. Approximately 12% to 30% tin is included. They are mixed with mercury 43% to 50% by weight. Zinc may inhibit corrosion by reducing oxidation of other metals in the amalgam, however, it can cause expansion in low-copper amalgams

Setting Transformation When the alloy in powder form is mixed with the mercury, a chemical reaction occurs. The mixture has a putty-like consistency that can be packed into the cavity preparation. It gradually becomes firmer.

Setting Transformation (cont’d) During the firming phase, the amalgam can be carved to the anatomic shape of the tooth. Once it reaches its initial set, it no longer can be carved. It is firm but is not fully reacted. It is relatively brittle at this point. 24 hours is needed for a full set.

Setting Reactions The chemical reaction that occurs when the alloy and the mercury are mixed has three phases. The first phase, called the gamma phase, is the silver alloy phase. It is the strongest phase and involves the least corrosion.

Setting Reactions (cont’d) The second phase is the gamma-1 phase, which consists of mercury reacting with silver. It is strong and corrosion resistant. The third phase is the gamma-2 phase, which consists of the reaction of mercury with tin. Gamma-2 is weak and corrodes easily.

Tarnish The oxidation that attacks the surface of the amalgam and extends slightly below the surface Results from contact with oxygen, chlorides, and sulfides in the mouth Causes a dark, dull appearance that is not destructive to the amalgam

Corrosion Corrosion can be caused by a chemical reaction between amalgam and substances in saliva or food that results in oxidation. Corrosion can occur when two dissimilar metals interact in a solution that contains electrolytes. An electrical current is generated between the metals (galvanism), resulting in corrosion.

Creep Creep in dental amalgams refers to the gradual change in shape of the restoration resulting from compression by the opposing dentition. This phenomenon is associated with the gamma-2 phase seen with low copper alloys and results in deterioration of the margins.

Dimensional Change Ideally, the dimensions of a newly placed amalgam should not change. If it contracts excessively, it will open gaps at the margins and contribute to microleakage. If the material expands excessively, it puts pressure on the cusps that can cause fracture.

Strength Amalgam is among the strongest of the directly placed restorative materials. Amalgams are stronger in compression than are composites or glass ionomers. If they are too thin in areas, however, amalgams may be broken by the biting forces.

Manipulation of Amalgam The dentist selects the dental alloy on the basis of his or her personal preference for its handling characteristics. High-copper alloys are used almost universally because they have superior properties to low-copper alloys.

Dispensing of Alloy and Mercury The amalgam must be handled properly from the start through the entire placement process if the restoration is to be successful. Use of premeasured capsules is the most hygienic way to work with these materials. Chances for contamination of material or for spills are fewer.

Trituration Powder and mercury are mixed together in a mechanical device called a triturator, or amalgamator. Each manufacturer of amalgam materials provides specific time and speed specifications regarding the mixing of a particular material.

Trituration (cont’d) Under-triturated amalgam appears dry and crumbly and sets too quickly. Over-triturated amalgam appears wet and sets quickly, producing heat. Properly triturated amalgam has a satin-like appearance. See Fig. 9-4

Placement and Condensation After mixing, the amalgam is removed from the capsule and is placed into an amalgam well. The amalgam is picked up in small increments with the use of an amalgam carrier. Condensers are used to work the material into all areas of the preparation. The preparation must be overfilled to allow room for carving of the anatomy.

Burnishing and Carving Burnishing before carving produces a much smoother and denser surface. Burnishing also helps to seal margins and enhances the longevity of the restoration. Carving removes extra material from the cavosurface margins and allows the missing anatomy to be reformed.

Finishing and Polishing Finishing is best done 24 hours or longer after initial placement, to allow crystallization within the amalgam to go to completion. Because a second visit is involved for finishing and polishing amalgam restorations, many clinicians do not polish them. Refer to chart on pg. 116 Advantages vs. Disadvantages of Amalgam

Bonding of Amalgam Amalgam is retained in the cavity preparation by parallel walls or undercut walls and through its adaptation to irregularities in the tooth created during preparation. It is not bonded to the tooth surface but is retained mechanically. Recently, many dentists have begun bonding amalgam with resin bonding agents. Wet resin technique

Mercury Safety Mercury is toxic at high levels. The amount of mercury that is released from a set amalgam is minimal. No substantiated research has proved otherwise. When old restorations are removed, it is important to use a water spray to minimize vapor.

Mercury Safety (cont’d) When one is disposing of amalgam waste, it is important to do so properly. Commercial containers can be purchased for scrap amalgam disposal and recycling. Although some offices do so, scrap amalgam should never be thrown into the trash can.

Mercury-Free Amalgam A mercury-free metal compound was developed that has an appearance and handling characteristics similar to those of amalgam. This was not a good alternative because it was highly corrosive. It is no longer used in the United States.

Summary Dental amalgam plays a large role in restorative dentistry. It is an economical, durable, and versatile material. Patients should be educated about the pros and cons of a material so that they can make informed decisions.