Creep—The time-dependent strain or deformation that is produced by a stress. The creep process can cause an amalgam restoration to extend out of the restoration site, thereby increasing its susceptibility to marginal breakdown.
Delayed expansion—The gradual expansion of a zinc-containing amalgam over a period of weeks to months. This expansion is associated with the development of hydrogen gas, which is caused by the incorporation of moisture in the plastic mass during its manipulation in a cavity preparation.
Dental amalgam—An alloy that is formed by reacting mercury with silver, copper, and tin, and which may also contain palladium, zinc, and other elements to improve handling characteristics and clinical performance.
By definition, an amalgam is an alloy that contains mercury. Mercury, a liquid at room temperature, can dissolve and react to form an alloy with numerous metals. When metal particles are mixed with mercury, the outer portion of the particles dissolves into mercury. At the same time, mercury diffuses into the metal particles. When the solubility of the metal in mercury is exceeded, crystals of mercury-containing compounds start to precipitate within the mercury. During this period of reaction the metal particles coexist with the liquid mercury, giving the mix a plastic consistency. This means that the mixture can be adapted to any shape with a light pressure. As the content of liquid mercury in the mixture decreases by the formation of precipitates, the mixture hardens. This process is called amalgamation and the material has been used for restoring tooth structure. The first use of amalgam for tooth filling was recorded in the Chinese medical literature in 659 A.D. In this chapter the term restoration refers to the amalgam filling and the adjacent tooth structure.
The amalgams used today are largely based on the formulation published by G.V. Black in 1895, although the amalgam concept was introduced in the United States in 1833 by the Crawcours brothers, who were from France. Since then, major modifications of G.V. Black’s formulation included the incorporation of a higher copper content and spherical particles, both of which were introduced in the early 1960s. Despite its long history of success as a restorative material, there have been periodic concerns regarding the potential adverse health effects arising from exposure to mercury released from dental amalgam. Because of advances in resin-based composites and adhesive technology in dentistry, the use of amalgam has declined substantially. Its use may be limited in the near future and it may eventually disappear from the clinician’s armamentarium.
As is true for other materials discussed in this book, practical skill and sound scientific familiarity with the material are needed for making high-quality restorations. Even when no new amalgams are being placed in patients, there will still be billions of amalgam restorations remaining in patients’ mouths. These restorations require periodic examinations to decide whether defective fillings should be replaced, repaired, or refinished. However, these clinical decisions require an understanding of several key terms commonly used by the dental profession. In this chapter, amalgam structure, properties, and manipulation characteristics are discussed.
Before these alloys are reacted with mercury, they are known as dental amalgam alloys or alloys for dental amalgam. They are usually provided as (1) irregularly shaped particles (Figure 15-1) produced by milling or lathe-cutting a cast ingot of the amalgam alloy, (2) as spherical particles (Figure 15-2) produced by atomizing the liquid alloy in a chamber filled with inert gas, or (3) as a mixture of both lathe-cut and spherical particles (Figure 15-3).
Originally, the dentist dispensed alloy powder and mercury in a mortar and mixed them together with a pestle by hand. The process of mixing is called trituration. Later, the powder was compacted in the form of pellets, which were dispensed along with mercury in a reusable capsule with a pestle (Figure 15-4, A) and triturated by a power-driven triturator or amalgamator (Figure 15-5). Today, disposable amalgam capsules are widely available (Figure 15-4, B & C). Each capsule contains a predetermined amount of alloy powder and mercury in a sealed pouch, and the capsule is sealed to prevent evaporation of the mercury.
The introduction of a higher copper content in amalgam alloys marked a significant change in the properties of amalgams. It is now customary to classify amalgam alloys as either low-copper (conventional) or high-copper alloys (Table 15-1). In both types, the major components of the alloys are silver and tin.
|COMPOSITION (MASS %)|
|Low copper (lathe-cut)||70.3||25.9||2.8||0.9|
|Low copper (spherical)||72.0||25.0||3.0||–|
|High copper (admix)||69.5||17.7||11.8||1.0|
|High copper (spherical)||61.0||26.0||13.0||–|
Figure 15-6 is an equilibrium-phase diagram of the silver-tin alloy system. Conventionally, starting from the left of the phase diagram, each phase is designated with a Greek letter in alphabetical order from 0% Sn to 100% Sn. The ratio of silver to tin in Table 15-1 shows that amalgam alloys have a narrow range of compositions, falling within the β + γ and the γ phases of the diagram shown in Figure 15-6.
Silver-tin alloys are quite brittle and difficult to blend uniformly unless a small amount of copper is substituted for silver. Within the limited range of copper solubility, an increased copper content hardens and strengthens the silver-tin alloy. The chief function of zinc in an amalgam alloy is to act as a deoxidizer, which is an oxygen scavenger that minimizes the formation of oxides of other elements in the amalgam alloys during melting. Alloys without zinc are more brittle, and their amalgams tend to be less plastic during condensation and carving. The American Dental Association (ADA) Specification No. 1 for amalgam alloys allows some mercury in the alloy powder.
The ranges of conventional alloy composition by weight in the early 1980s were 66.7% to 71.5% silver, 24.3% to 27.6% tin, 1.2% to 5.5% copper, 0% to 1.5% zinc, and 0% to 4.7% mercury. The structure of these conventional alloys was dominated by Ag3Sn (γ phase) with some Cu3Sn (ε phase).
The first high-copper alloy was formulated by mixing one part of silver-copper, spherical eutectic (Ag-Cu; 71.9% silver and 28.1% copper by weight) particles to two parts of Ag3Sn, provided as lathe-cut particles. This modification raises the copper content to 11.8% by weight. This is often called “dispersed-phase alloy” or “admixed high-copper alloy.”
A second type of high-copper alloy was made by melting all components of the dispersed phase alloy. This process yields a single composition system. The presence of the higher copper content makes mechanical cutting of ingots into particles difficult. Thus, they are often provided in a spherical form that is produced by an atomization process. The copper content of this group of alloys can be as high as 30% by weight. Various amounts of indium or palladium have been included in some commercial systems.
In an attempt to eliminate mercury from direct metallic restorative materials, gallium, which is also a liquid when alloyed with indium and tin at room temperature, has been considered as a substitute. Like mercury, this metal element can be triturated with alloys for high-copper amalgam.
Dental amalgams are made by mixing alloy powders with mercury. The reaction begins at the particle/mercury interface. Therefore, the physical configuration and condition of the particles will have a significant influence on the setting process.
To produce this powder, an as-cast ingot is first annealed to retain a uniform phase and then it is placed in a milling machine or in a lathe to be fragmented by a cutting tool or bit. The powders obtained from cutting are acid-washed to produce a more reactive surface. Since the stresses induced into the particle during cutting are not uniform and can slowly be self-relieved over time, they will cause the performance of the amalgam to be inconsistent. However, a stress-relief process can be performed by annealing the powder particles at a moderate temperature.
The liquid metal is atomized into fine spherical droplets of metal in a chamber of inert gas. If the droplets solidify before hitting a surface, the spherical shape is preserved as spherical powders. Like the lathe-cut powders, spherical powders are given an annealing heat treatment and surface washing.
The average particle sizes of modern powders range between 15 µm and 35 µm. Smaller particles greatly increase the surface area per unit volume of the powder. A powder containing tiny particles requires a greater amount of mercury to form an acceptable amalgam. It is critical to maintain an optimal particle size and size distribution.
The particle size distribution can affect the character of the finished surface. When the amalgam has partially hardened, the tooth anatomy is carved in the amalgam with a sharp instrument. During carving, the larger particles may be pulled out of the matrix, producing a rough surface. Such a surface is probably more susceptible to corrosion than a smooth surface. A smaller average particle size tends to produce a more rapid hardening of the amalgam with greater early strength.
Amalgams made from lathe-cut powders or admixed powders tend to resist condensation better than amalgams made entirely from spherical powders. Since freshly triturated amalgams from spherical powders are very plastic, one cannot rely on the pressure of condensation to establish a proximal contour for a class II restoration. Spherical alloys require less mercury than typical lathe-cut alloys because spherical alloy powder has a smaller surface area per volume ratio than does the lathe-cut powder. Amalgams with low mercury content generally have better properties.
As discussed earlier, modern dental amalgam alloys are based on the intermetallic compound Ag3Sn; therefore, the main reaction after trituration occurs between Ag3Sn and mercury. Other elements in the alloy, especially copper, also play a significant role in the final microstructures of set amalgams.
The main reactions of low-copper alloy produce alloy phases including the body-centered cubic Ag2Hg3 (γ1) phase and the hexagonal Sn7-8Hg (γ2) phase (Figure 15-7). Both phases are designated as the γ phase because they happen to be the third phase of the respective Ag-Hg and Sn-Hg systems. Since the solubility of silver in mercury is much lower than that of tin, the Ag2Hg3 phase precipitates first and is designated as γ1, whereas the Sn7-8Hg phase precipitates later and is designated as γ2. There is evidence from x-ray diffraction and thermal analyses of set amalgam that a δ phase, which is richer in mercury than γ2, is present in low-copper alloy instead of γ2. The existence of γ2 or δ phases in low-copper amalgams remains an unresolved question. For the purpose of this discussion the Sn-Hg phase is designated as γ2 in this chapter.
The low-copper alloy is usually mixed with mercury in a ratio of about 1 : 1. This is an insufficient amount of mercury to consume the original alloy particles completely; consequently, unconsumed particles are present in the set amalgam. Thus, a typical low-copper amalgam is a composite in which the unconsumed particles are embedded within γ1 and γ2 phases. The sequence of amalgamation of the silver-tin alloy is shown schematically in Figure 15-8.
The physical properties of the hardened amalgam depend on the relative percentages of each of the microstructural phases. The greater the number of unconsumed Ag-Sn particles retained in the final structure, the stronger the amalgam will be. The γ2 phase is the weakest and least stable in a corrosive environment and may suffer corrosion attack especially in crevices of the restorations.
A reaction between Cu3Sn (ε phase) and γ2 also occurs and yields Cu6Sn5 (η′ phase). Because of the low copper content, a majority of γ2 remains. Figure 15-9 illustrates the features found in a typical microstructure of amalgam made from a lathe-cut, low-copper alloy. As described later, the addition of more than 6% of copper by weight can reduce or eliminate the γ2 phase by formation of the Cu-Sn phase.
According to the phase diagram of Cu-Sn, the Cu6Sn5 phase exhibits a solid state transformation at 186 °C to 189 °C. The new phase is different structurally from the previous phase with no change in composition. The high-temperature phase is designated as η and the lower temperature phase is η′. Since the formation of the Cu6Sn5 phase in amalgam occurs at room temperature, η′ is used throughout the following discussion.
Two different types of high-copper alloy powders are available. The first is a two-phase admixed powder and the second is a single-composition single-phase powder. Both types contain more than 6% of copper by weight.
When mercury reacts with an admixed powder, silver in Ag-Cu spheres and silver and tin from Ag-Sn particles dissolve into the mercury. Whereas both γ1 and γ2 crystals form, as in lathe-cut alloys, the tin in mercury diffuses to the surfaces of the Ag-Cu alloy particles and reacts with the copper to form a layer of η′ phase crystals on the surface. The η′ layer on the surface of Ag-Cu alloy particles also contains γ1 crystals, since γ1 and η′ phases form simultaneously. As in the low-copper amalgams, γ1 is the matrix phase (i.e., the phase that binds the unconsumed alloy particles together). In this reaction, the γ2 phase does form along with the η′ phase but later reacts with copper from Ag-Cu particles, yielding additional η′ phase (the secondary solid-state reaction in Figure 15-7). The γ2 phase can be eliminated with at least 11.8% of copper by weight in the alloy powder.
Figure 15-10 illustrates the microstructure of an admixed amalgam. Included in this structure are the γ phase particles, Ag-Cu particles, ε particles, the γ1 matrix, and η′ reaction layers. In some admixed amalgams, a small number of the η′ crystals are also found amid the γ1 matrix.
The major components of single-composition particles are usually silver, copper, and tin. The copper content of various single-composition alloys ranges from 13% to 30% by weight. In addition, small amounts of indium or palladium are included in some of the single-composition alloys. A number of phases are found in each single-composition alloy particle, including the β phase (Ag-Sn), γ phase (Ag3Sn), and ε phase (Cu3Sn). Some of the alloys may also contain some η′ phase.
When triturated with mercury, silver and tin from the Ag-Sn phases dissolve in mercury. Very little copper dissolves in mercury. The γ1 crystals grow, forming a matrix that binds together the partially dissolved alloy particles. The η′ crystals are found as meshes of rodlike crystals at the surfaces of alloy particles, dispersed in the matrix. In most single-composition amalgams, little or no γ2 forms (Figure 15-7). Figure 15-11 shows the microstructure of a typical single-composition amalgam. This structure includes unconsumed alloy particles (P), γ1 grains (G1), and η′ crystals (H).
Figure 15-12, A, shows a scanning electron micrograph of a high-copper single-composition amalgam fractured a few minutes after condensation when the amalgamation reaction was still taking place. Two kinds of crystals are seen on the surface: polyhedral crystals (γ1), shown by arrow A, between the unconsumed alloy particles, and meshes of η′ rod crystals, shown by arrow B, for the unconsumed alloy particles.
Figure 15-12, B, shows details of the marked areas in Figure 15-11, A. In addition to a mesh of η′ crystals (arrow B), which formed on an unconsumed particle, η′ rods (arrow C) are seen embedded in a γ1 crystal (A). Meshed η′ crystals on unconsumed alloy particles may strengthen bonding between the alloy particles and γ1 grains, and η′ crystals dispersed between γ1 grains may interlock γ1 grains. This interlocking is believed to improve the amalgam’s resistance to deformation.
A good modern dental amalgam alloy can be manipulated so that the restoration lasts, on average, 15 years. The cavity preparation must be designed correctly and the amalgam must be manipulated properly so that the set amalgam restoration is not placed under excessive tensile stress.
Obviously the selection of one type of amalgam over others should be based on clinical performance; lacking such information, it should be based on the physical and mechanical properties. However, the initial analysis of properties should be compared with clinical performance as such data become available. Another criterion is that the alloy should meet the requirements of the ADA’s Specification No. 1 or ISO 1559. However, it is the operator who controls the performance of a restoration. Thus, it is essential that the alloy selected be one with which the dentist and the assistant feel comfortable. Use of alloys and techniques that are technique insensitive relative to the manipulation and placement of the amalgam will enhance the quality and durability of the restoration.
The amount of alloy and mercury to be used can be described as the mercury/alloy ratio, which signifies the number of parts by weight of mercury divided by the number of parts of alloy to be used for the particular technique. Sufficient mercury must be present in the original mix to provide a coherent and plastic mass after trituration, but it must be low enough that the mercury content of the restoration is at an acceptable level without the need to remove an appreciable amount of mercury during condensation. The mercury content of the lathe-cut alloy is about 50% by weight and that for spherical alloys is 42% by weight.
When mortar and pestle were used for mixing amalgam, it was necessary to use an excess amount of mercury to achieve a smooth and plastic amalgam. Removal of excess mercury was accomplished by squeezing or wringing the mixed amalgam in a squeeze cloth prior to insertion of the increments into the prepared cavity. However, the amount of mercury removed by the squeeze cloth process and during condensation varied. Thus, there was a considerable chance for error.
A wide variety of mercury and alloy dispensers are available. The most common is the dispenser based on volumetric proportioning. Preweighed pellets or tablets are first placed in a capsule. In mechanical mixing, the capsule serves as a mortar. As a liquid, mercury can be measured by volume without appreciable loss of accuracy. The dispenser should be held vertically to ensure consistent dispensing of mercury. The dispenser should be at least half full when it is used. If it is not, the weight of mercury dispensed may be erratic. Probably the most common cause of inaccurate delivery of mercury is the entrapment of contaminants in the reservoir and orifice of the device. This variable could decrease the amount of mercury by 3% or 4% and this may lead to an unusable mix. Regardless of the alloy or triturator used, no more than two pellets of alloy should be mixed in a capsule at one time.
Disposable capsules containing preproportioned aliquots of mercury and alloy are now widely used. To prevent any amalgamation from occurring during storage, the mercury and alloy are physically separated from each other. The older types of preproportioned capsules require activation before trituration to allow the mercury to enter the compartment with the alloy. Some alloys are now available in self-activating capsules, which bring the alloy and mercury together automatically during the first few oscillations of the triturator. Although the preproportioned material is more expensive, it is more convenient, it eliminates the chance of mercury spills during proportioning, and it should result in a reliable mercury/alloy ratio. However, these capsules do not provide an opportunity to make minor adjustments in the mercury/alloy ratio to accommodate personal preferences.
The objective of trituration is to provide proper amalgamation of the mercury and alloy. There is always an oxide layer of the alloy surface that hinders diffusion of mercury into the alloy. This film must be disrupted so that a clean surface of alloy can make intimate contact with the mercury. The oxide layer is removed by abrasion when the alloy particles and mercury are triturated.
The main mixing mechanism of a mechanical triturator is a reciprocating arm that holds the capsule under a protective hood. The purpose of the hood is to confine mercury that might escape into the room or to prevent a capsule from being accidentally ejected from the triturator during trituration. A commonly used older model is a single-speed device with an automatic timer for controlling the length of the mixing time. Later models have multiple speed settings. A modern triturator is often microprocessor controlled (Figure 15-5) and contains preset trituration programs for a number of materials. It can also be programmed by the operator to include other materials. A cylindrical metal or plastic piston of smaller diameter than the capsule is inserted into the capsule, and this serves as the pestle (Figure 15-4, A & B). Spherical alloys often do not need a pestle (Figure 15-4, C).
A triturator should be used at the speed recommended by the alloy manufacturer. Older triturators do not operate at a sufficient rate of speed to amalgamate high-copper alloys properly with minimal mercury. Self-activating capsules are usually very sensitive to trituration speed.
Manufacturers often provide a list of recommended time schedules and speed settings for their alloys and various types of triturators. Because of the speed variations in triturators, the schedule should serve only as a rough guide. Dentists and assistants can adjust the amalgamation time required to attain a mix of correct consistency. For a given alloy and mercury/alloy ratio, increased trituration time and/or speed shorten the working and setting times. In addition, alloys differ in their sensitivity to trituration time (Figure 15-13).
A reusable capsule should be clean and free of previously mixed, hardened alloy. At the end of each trituration procedure, one should quickly remove the pestle from the capsule, replace the lid, reinsert the capsule in the triturator, turn it on for a second or two, and then remove the amalgam. This mulling process generally causes the mix to cohere so that it can be readily removed from the capsule with minimal residue in the capsule. It minimizes the need of scraping out partially hardened alloy, which usually produces scratches in the capsule.
The proper time of mixing can be determined by observing the consistency of the mix. For example, the very grainy mix (Figure 15-14, A) indicates undertrituration. Not only will the amalgam restoration made from this mix be weak, but also the rough surface left after carving of the granular amalgam will increase its susceptibility to tarnish. If the trituration has produced an amalgam of the general appearance shown in Figure 15-14, B, the strength will be optimal and the smooth carved surface will retain its luster long after polishing. Because of the friction between particles during trituration, such an amalgam mix should be warm (not hot) when it is removed from the capsule. This will have no effect on the physical properti/>