All dental amalgam alloys have Ag3Sn as their pri- mary component, which reacts with mercury to form Ag2Hg3, the major matrix phase of the set amalgam.
The amalgam alloy is intimately mixed with liquid TABLE 10.1 Approximate Composition of Low- and High-Copper Amalgam Alloys
Alloy
Particle Shape
Element (wt%)
Ag Sn Cu Zn In Pd
Admixed
regular Irregular
Spherical 40–70
40–65 26–30
0–30 2–30
20–40 0–2
0–1 0
0 0
0–1 Admixed
unicomposition Irregular
Spherical 52–53
52–53 17–18
17–18 29–30
29–30 0
0 0
0 0.3
0.3
Unicompositional Spherical 40–60 22–30 13–30 0 0–5 0–1
FIG. 10.1 Amalgam restorations from a low-copper lathe cut alloy (left) and a high-copper admix alloy (right) after 3 years of clinical service. (Courtesy David B. Mahler, OHSU School of Dentistry, Portland, OR.)
mercury in a process called trituration to wet the surface of the particles and facilitate their reaction with mercury. During this process, mercury diffuses into the alloy particles and reacts with the silver and tin portions of the particles to form predominantly a silver-mercury compound, Ag2Hg3, known as the gamma one (γ1) phase. This phase forms act as a matrix to hold the unreacted amalgam alloy together.
While crystals of the γ1 phase are being formed, the amalgam is relatively soft and easily condensable and carvable. As time progresses, more crystals of γ1
are formed; the amalgam becomes harder and stron- ger and is no longer condensable or carvable. The lapse of time between the end of the trituration and when the amalgam hardens and is no longer work- able is called working time. Completion of the reac- tion may take several days to several weeks, which is reflected by the change in mechanical proper- ties over this time. Patients are typically advised to avoid chewing directly on amalgam restorations for the first 24 hours after placement to be safe, though
considerable strength has been achieved within the first few hours.
The amount of liquid mercury used to amalgam- ate the alloy particles is not sufficient to react with the particles completely. Therefore the set mass of amal- gam contains about 27% unreacted particles, which actually enhance the strength of the final material.
In high-copper admix alloys, additional copper has typically been supplied by adding spherical particles of the silver-copper eutectic alloy to the silver-tin alloy. The solubility of silver, tin, and cop- per in mercury differs considerably. Approximately 1 mg of copper, 10 mg of silver, and 170 mg of tin can dissolve in mercury, all at the same tempera- ture. Therefore while mercury is dissolving mainly the silver and tin in Ag3Sn, as described earlier, very little of the silver-copper eutectic particles are dis- solved. However, some of the tin and copper that are dissolved by the mercury react to form a copper-tin compound, Cu6Sn5, referred to as the eta prime (η′) phase. It is the presence of this tin-copper compound, A
B C
FIG. 10.2 Scanning electron micrographs. (A) Lathe-cut; (B) spherical; and (C) admixed amalgam alloys.
rather than the formation of a weak, corrosion-prone tin-mercury compound, that gives the high-copper amalgams their superior performance as compared with the older low-copper amalgams.
The amalgamation reaction may be simplified as follows:
γ (Ag3Sn) + Ag-Cu (eutectic) + Hg →γ1 (Ag2Hg3) + η (Cu6Sn5) + unreacted γ (Ag3Sn) + unreacted
Ag-Cu (eutectic)
Reaction of Mercury in a Unicompositional Alloy
In high-copper unicompositional alloys, the alloy particles, typically spherical, contain both Ag3Sn (γ) and Cu3Sn (ε). When liquid mercury is mixed with these alloys, it diffuses into the surface of these par- ticles and Ag2Hg3 and Cu6Sn5 are formed, as in the admixed alloy. The difference is that because all of the copper is present within the single particles, the reaction of copper and tin occurs in a ring around the spherical particles, which become surrounded by the silver-mercury matrix.
Microstructure of Amalgam
Color-coded images of the microstructures of the set amalgams of the high-copper admix and the high- copper unicompositional types are shown in Fig.
10.3. The blue/black matrix phase is A2Hg3 (γ1). The tangerine color is for the Cu6Sn5 (η′) phase, which is very evident around spherical particles.
Physical and Mechanical Properties
ANSI/ADA Specification for Amalgam Alloy ANSI/ADA specification No. 1 (ISO 24234) for amal- gam alloy contains requirements that help control the qualities of commercially available dental amalgam. The specification lists three physical properties as a measure of amalgam quality: compressive strength, creep, and dimensional change. The minimum allowable com- pressive strength is 80 MPa for 1 hour after setting and 300 MPa for 24 hours after setting, the maximum allow- able creep is 1%, and the dimensional change between 5 minutes and 24 hours must fall within the range of
−15 to +20 μm/cm. The physical properties for several amalgams are shown in Table 10.2.
A B
C
FIG. 10.3 Microstructures of the set amalgams of the (A) low-copper, lathe-cut; (B) the high-copper, admix; and (C) the high-copper, unicompositional types. These photographs were made by superimposing microprobe x-ray scans of the ele- ments through colored filters: blue for silver, red for copper, and green for tin. The blue/black matrix phase is A2Hg3 (γ1) for all amalgams. The green-colored Sn7–8Hg (γ2) phase is only present in the low-copper alloy (A). The tangerine-colored Cu6Sn5 (η′) phase is minimal in alloy (A) but substantial in alloys (B) and (C). In alloy (B), the source of the increased copper is in the spherical silver-copper eutectic phase where the η′ surrounds this spherical particle. In alloy (C), the source of the increased copper is in the additional Cu3Sn added to the spherical Ag3Sn particle and Cu6Sn5 (ηʹ) phases are present around this spherical particle. (Courtesy David B. Mahler, OHSU School of Dentistry, Portland, OR.)
TABLE 10.2 Mercury in Mix, Compressive strength, Tensile strength, Creep, and Dimensional Change
Mercury in Mix (%)
Compressive
Strength (MPa) Tensile Strength (MPa)
Creep (%)
Dimensional Change (mm/cm)
1 h 7 days 15 min 7 days
LOW COPPER Alloys Lathe-cut
Caulk 20th century 53.7 45 302 3.2 51 6.3 −19.7
Spherical
Caulk spherical 46.2 141 366 4.7 55 1.5 −10.6
HIGH COPPER Alloys Admixed
Dispersalloy 50.0 118 387 3.8 43 0.45 −1.9
Unicompositional
Sybraloy 46.0 252 455 8.5 49 0.05 −8.8
Tytin 43.0 292 516 8.1 56 0.09 −8.1
Modified from Malhotra ML, Asgar K. Physical properties of dental silver-tin amalgams with high and low copper contents. J Am Dent Assoc.
1978;96:444–450.
Mercury in Mix
In general, irregular particles have higher surface areas than spherical particles, and therefore require more mercury to wet their surfaces. In turn, higher percentages of mercury in the mix will result in higher mercury contents and lower strengths of the hardened amalgams. This effect is clearly shown in Table 10.2.
Compressive Strength
Resistance to compression forces is an important strength characteristic of amalgam. Because amal- gam is strongest in compression and much weaker in tension and shear, the prepared cavity design should maximize compressive stresses in service and minimize tensile or shear stresses. When sub- jected to a rapid application of force either in ten- sion or in compression, a dental amalgam does not exhibit significant deformation or elongation and, as a result, functions as a brittle material.
Therefore a sudden application of excessive force to amalgam may lead to fracture of the amalgam restoration.
The early compressive strengths after 1 hour of setting for several low- and high-copper alloys are listed in Table 10.2. The high-copper unicomposi- tional materials have the highest early compressive strengths of more than 250 MPa. High values for
early compressive strength are an advantage for an amalgam, because they reduce the possibility of frac- ture by the application of prematurely high occlu- sal forces by the patient before the final strength is reached. The compressive strengths at 7 days are again highest for the high-copper unicompositional alloys, with only modest differences in the other alloys.
Tensile Strength
The tensile strengths of various amalgams after 15 minutes and 7 days are also listed in Table 10.2. The tensile strengths at 7 days for low- and high-copper amalgams are about the same. The tensile strengths are only a fraction of their compressive strengths;
therefore cavity designs should be constructed to reduce tensile stresses resulting from biting forces.
The tensile strengths at 15 minutes for the high- copper unicompositional alloys are significantly higher than for the other alloys.
Elastic Modulus
The elastic modulus, or the stiffness, for dental amal- gam is in the range of 40 to 60 GPa. As a comparison, the elastic modulus of resin composites is only 5 to 15 GPa, which can be of significance when consid- ering amalgam versus composites in certain clinical applications.
Creep
Creep is the time-dependent inelastic deformation of materials that are used at temperatures that are close to their melting points. Expressed in absolute temperatures, the melting point of the major matrix phase (γ1) in dental amalgam is 400 K, whereas it is used at the mouth temperature of 310 K for a ratio of 0.8. In metals, ratios that exceed 0.5 are considered to be a forerunner for examining creep behavior.
Therefore dental amalgam is an appropriate candi- date for this examination.
The creep test in the specifications is conducted by applying a compressive stress of 36 MPa on a 7-day-old cylindrical specimen in a 37°C environ- ment. Creep is measured by the shortening of the test specimen between 1 and 4 hours of testing, and the specification sets the acceptable limit for creep of 1.0%.
However, amalgams whose creep values vary within the range of less than 1.0% do not show differ- ences in clinical performance. For example, referring to Table 10.2, unicompositional alloys with creep val- ues of 0.05 and 0.09 do not show superior clinical per- formance compared to the admix alloy with a creep value of 0.45. Therefore physical properties are help- ful in predicting clinical performance but care should be exercised in the limits of their interpretation.
The ability of creep to demonstrate the permanent deformation of dental amalgam in the clinical envi- ronment is shown to be of significance in Fig. 10.1.
Dimensional Change
The dimensional change during the setting of amal- gam is one of its most significant properties. Modern amalgams mixed with mechanical amalgamators usu- ally have negative dimensional changes. The initial contraction after a short time (the first 20 minutes) is believed to be associated with the solution of the alloy particles in mercury. After this period, an expansion occurs that is believed to be a result of the reaction of mercury with silver mainly and the formation of the matrix phase. The dimensions become nearly constant after 6 to 8 hours, and thus the values after 24 hours are final values. The only exception to this statement is the excessive delayed dimensional change seen clinically when some older zinc-containing alloys were contaminated with water-based fluids during trituration or condensation.
Dimensional change is measured by the change in length of an 8-mm cylindrical specimen between 5 minutes and 24 hours after trituration. The change in length can be determined continuously, although ANSI/ADA specification No. 1 requires only the value at 24 hours.
The dimensional changes in micrometers per cen- timeter for various alloys are listed in Table 10.2. The lowest dimensional change of −1.9 μm/cm was for
the high-copper admixed alloy. All the amalgams meet the requirements of ANSI/ADA specification No. 1 of −15 to +20 μm/cm but are susceptible to influence from various manipulative factors.
An additional clinical significance of dimen- sional change is related to the occasional occur- rence of postoperative sensitivity associated with newly placed amalgam restorations. Amalgam does not adhere to tooth structure; therefore a negative dimensional change would result in the presence of an interfacial gap between the amalgam restoration and tooth structure. When a cavity is prepared that cuts through dentin in a tooth requiring restoration, pulpal fluid in the tubules can flow outward into the interfacial gap. Changes in pressure of this fluid are considered to be one of the major causes of postoper- ative sensitivity. Apparently, the size of the interfacial gap is a key factor in determining whether sensitivity will occur, with teeth with restorations having larger gaps being more prone to being sensitive.
Although most alloys that pass ANSI/ADA speci- fication No. 1 for negative dimensional changes of
−15 μm/cm or less have not been shown to have an uncommon amount of postoperative sensitiv- ity, some high-copper amalgams consisting of only spherical particles have been reported to show a propensity for this sensitivity. The reason for this anomaly was found by in vitro microleakage stud- ies that showed that spherical particle alloys leaked more than lathe-cut particle alloys, even though their respective dimensional changes were not sig- nificantly different. Examination showed that the surfaces of these amalgams next to the cavity walls exhibited a relatively uneven texture for the spheri- cal particle alloys compared to a smoother texture for the lathe-cut alloys. Thus the interfacial space filled by pulpal fluid was greater for the spherical particle alloys. In Fig. 10.4, the microleakage of a number of dental amalgams is shown where spherical particle alloys are marked with a capital S. It is clear that the presence of higher microleakage values is associated with the spherical alloys. Bars that are shaded refer to alloys in which data were available to indicate an unusual propensity for postoperative sensitivity.
The use of film-forming agents such as dentin bonding agents to seal the dentinal tubules before placement of an amalgam restoration has proven to be an effective solution to the problem of postop- erative sensitivity of spherical particle amalgams.
However, this practice has not been widely adopted by the profession.
Corrosion
In general, corrosion is the progressive destruction of a metal by chemical or electrochemical reaction with its environment. Excessive corrosion can lead to increased porosity, reduced marginal integrity, loss
of strength, and the release of metallic products into the oral environment.
Corrosion products identified in dental amalgams include tin oxides, tin hydroxychlorides, copper oxides, copper chlorides, and other more complex compounds. The formation of oxides and chlorides is not surprising considering that the amalgams operate in an aerated environment containing salt solutions.
Because of their different chemical compositions, the different phases of an amalgam have different cor- rosion potentials. Electrochemical measurements on pure phases have shown that the Ag2Hg3 (γ1) phase has the highest corrosion resistance, followed by Ag3Sn (γ), Ag3Cu2, Cu3Sn (ε), Cu6Sn5 (η′), and Sn7–8Hg (γ2). However, the presence of small amounts of tin, silver, and copper that may dissolve in various amalgam phases has a great influence on their cor- rosion resistance. For example, in the γ1 phase, the silver-mercury phase always has some tin dissolved within it, and the higher the tin concentration, the lower is the corrosion resistance. The average depth of corrosion for most amalgam alloys is 100 to 500 μm, measured from the amalgam/tooth margin.
Phosphate buffer solutions inhibit the corrosion process, as do the formation of protein pellicles on the amalgam surface; thus saliva may provide some protection of dental amalgams from corrosion.
A study of amalgams that had been in service for 2 to 25 years revealed that the bulk elemental com- positions were similar to newly prepared amalgams, except for the presence of a small amount of chloride
and other contaminants. The compositions of the phases were also similar to new amalgams, except for internal amalgamation of the γ particles. The dis- tribution of phases in the clinically aged amalgams, however, differed from that of new amalgams, veri- fying that dental amalgam is a dynamic material that changes with time.
Surface tarnish in high-copper amalgams is related to the copper-rich phases.