Amalgam - Dental amalgam
Posted by John Doe at Dental Assistant on February 2, 2012.
Categories: Dental Materials
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Values for creep are determined in an instrument similar to that shown in Fig. 11-6. A cylindrical sample is placed in the position indicated by the arrow 7 days after preparation. A static stress of 36 MPa is applied by the spring. The change in length of the sample is determined at 37 ± 0.3° C by a calibrated differential trans-
former, the output of which is recorded on a chart. The change in length between 1 hour and 4 hours after placing the static stress is used to calculate the percentage creep.
Creep values for various amalgams are listed in Table 11-2. The highest value of 6.3% was found for the low-copper cut alloy, and the lowest values (0.05% to 0.09%) were determined for the high-copper unicompositional spherical alloys. The high-copper admixed alloy and one of the low-copper alloys had slightly higher creep values of 0.45% to 0.50%, and the remaining two low-copper spherical alloys had values of 1.3% to 1.5%.
Multiple regression analyses of creep data have shown that the most influential variables are volume percentage of the r\' phase, grain size of the Yi phase, volume percentage of the Y and ? phases, number of very small T|' crystals (less than 1.5 (xm)/mm, and weight percentage of mercury. All of these values except weight percentage of mercury correlate negatively with creep. When y1 has a concentration of tin greater than 1%, creep is controlled more by the distribution of tin and tin-mercury intergranular precipitates than by grain size. After aging at oral temperature for 6 months, amalgam exhibits a decrease in creep. This decrease in creep is related to p\ formation and not to changes in either Yi grain size or composition.
A direct relationship exists between y2 content and a high incidence of marginal fracture of amalgam restorations. In addition, there is a general relationship between low static creep values and low marginal fracture in clinical service, which may be explained by the fact that the time to rupture under a constant load is inversely proportional to creep rate. Amalgams having higher compressive strengths at 7 days, determined at slow rates of loading, have demonstrated better marginal integrity. In general, amalgams having low values of creep and high 7-day compressive strength at slow rates of loading have better clinical performance.
Fig. 11-6 An instrument for measuring creep of amalgam. Arrow points to specimen.
Note that the low creep values of high-copper amalgams increase the brittleness of the amalgam and decrease the relief of stresses at contact areas under load. As a result, a high-modulus base under a high-copper amalgam is essential to minimize deformation and the development of tensile stresses at the amalgam-cement base interface.
Dimensional Change The dimensional change during the setting of amalgam is one of its most characteristic properties. Modern amalgams mixed with mechanical amalgamators usually have negative dimensional changes. The initial contraction after a short time (the first 20 minutes) is believed to be associated with the solution of mercury in the alloy particles. After this period an expansion occurs, although the total change remains negative, which is believed to be a result of the reaction of mercury with silver and tin and the formation of the intermetallic compounds. 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 resulting from contamination of a zinc-containing alloy with water during tritura-tion or condensation.
The dimensional change may be determined with an instrument such as the one shown in Fig. 11-7. The amalgam specimens identified by the arrows are placed in position 5 minutes after setting, and the probe is placed on top of them. The probe is mechanically attached to a differential transformer, and the electrical output is used to determine expansion or contraction. 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 centimeter for the various alloys are listed in Table 11-3. The largest dimensional change of -19.7 urn/cm occurred with the low-copper, lathe-cut alloy, and the lowest change of -1.9 |j,m/cm was for the high-copper admixed alloy. The remainder of the alloys had values ranging from -8.8 to -14.8 u.m/cm. All the amalgams meet the requirements of ANSI/ADA Specification No. 1 of ±20 um/cm. Notice that the ranking of the dimensional change does not correlate with any of the other mechanical properties. The dimensional change is susceptible to influence from various manipulative factors, especially final mercury content. Higher mercury content results in less shrinkage but also in lower mechanical strength.
Some question remains concerning the significance of dimensional change with respect to clinical success. The belief was that if amalgam expanded during hardening, leakage around the margins of restorations would be eliminated. With current alloys and proper techniques of trituration, however, most alloys show some shrinkage. Evidently the detrimental effect of shrinkage occurs when the amalgam mass shrinks more than 50 um. ANSI/ADA Specification No. 1 for dental amalgam allows up to
20 (im/crn shrinkage, and no correlation of clinical success with the magnitude of the shrinkage determined in the laboratory has been shown. Furthermore, the expansion of an amalgam mass may seem to have a beneficial effect for one-surface restorations such as Class 1 and 5, but offers hardly any advantage when Class 2 and 6 restorations are considered. The expanded amalgam around the cervical areas of Class 2 and 6 restorations would have to pull away from the preparation, and this may have as undesirable an effect as the shrinking of amalgams for one-surface restorations.
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.
The following compounds have been identified on dental amalgams in patients: SnO, Sn02, Sn4(OH)sCl2, Cu20, CuCl2 3Cu(OH)2, CuCl, CuSCN, and AgSCN.
Because of their different chemical compositions, the different phases of an amalgam have different corrosion potentials. Electrochemical measurements on pure phases have shown that the Ag2Hg3 (Yj) phase has the highest corrosion resistance, followed by Ag3Sn (y), Ag3Cu2, Cu3Sn (e), CusSn5 On'), and Sn7_8Hg (y2). However, the order of corrosion resistance assigned is true only if these phases are pure and they are not in the pure state in dental amalgam.
The presence of small amounts of tin, silver, and copper that may dissolve in various amalgam phases has a great influence on their corrosion resistance. The Yi phase has a composition close to Ag2Hg3 with 1% to 3% of dissolved tin. The higher the tin concentration of Ag2Hg3 (Yi), the lower its corrosion resistance. In general, the tin content of the Yi phase is higher for low-copper alloys than for high-copper alloys. The presence of a relatively high percentage of tin in low-copper alloys reduces the corrosion resistance of their Yi phase so it is lower than their Y phase. This is not true for high-copper alloys. The average depth of corrosion for most amalgam alloys is 100 to 500 Jim.
In the low-copper amalgam system, the most corrodible phase is the Sn7_8Hg or y2 phase. Although a relatively small portion (11% to 13%) of the amalgam mass consists of the y2 phase, in time and in an oral environment the structure of such an amalgam will contain a higher percentage of corroded phase. On the other hand, neither the y nor the Yj phase is corroded as easily. Studies have shown that corrosion of the y2 phase occurs throughout the restoration, because it is a network structure. Corrosion results in the formation of tin oxychloride from the tin in the y2, and also liberates mercury, as shown in the following equation:
Sn7_8Hg + V202 + H20 + Cl" > Sn4(OH)6Cl2 + Hg
The reaction of the liberated mercury with unre-acted y can produce additional Yi and y2. It is proposed that the dissolution of the tin oxide or tin chloride and the production of additional y1 and y2 result in porosity and lower strength.
The high-copper admixed and unicompo-sitional alloys do not have any y2 phase in the final set mass. The Cu6Sn5 or T|' phase formed with high-copper alloys is not an interconnected phase such as the y2 phase, and it has better corrosion resistance. However, t)' is the least corrosion-resistant phase in high-copper amalgams; and a corrosion product, CuCl2 • 3Cu(OH)2, has been associated with storage of amalgams in synthetic saliva, as shown below.
Cu6 Sn5 + V202 + H20 + Cl > Cu Cl2 3 Cu (OH)2 + Sn O
Phosphate buffer solutions inhibit the corrosion process; 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 compositions 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 Y particles. The distribution of phases in the clinically aged amalgams, however, differed from that of new amalgams. The low-copper amalgams had decreased amounts of y, ylt and y2 and increased px and tin-chloride. High-copper admixed amalgams had decreased y1, increased (3^ and enlarged reaction rings of y1 and r\'. There was also evidence of a conversion of y1 to (^ and T2 to if.
Note that the processes of corrosion and wear are frequently coupled and that wear can lower the corrosion potential and increase the corrosion rate by an order of magnitude.
Fig. 11-8 compares an amalgam restoration on the distal portion of a tooth prepared from a low-copper spherical alloy with one on the mesial portion prepared from high-copper admixed alloy. The restorations have been in service for 3 years, and the higher marginal fracture, presumably resulting from the corrosion of the y2 phase of the low-copper amalgam, is readily apparent.
Surface tarnish of low-copper amalgams is more associated with y than yv whereas in high-copper amalgams surface tarnish is related to the copper-rich phases, r\' and silver-copper eutectic.
PROPERTIES OF MERCURY
ANSI/AD A Specification No. 6 for dental mercury requires that mercury have a clean reflecting surface that is free from surface film when agitated in air. It should have no visible evidence of surface contamination and contain less than 0.02% nonvolatile residue. Mercury that complies with the requirements of the United States Pharmacopoeia also meets requirements for purity in ANSI/ AD A Specification No. 6. Mercury amalgamates with small amounts of many metals and is contaminated by sulfur gases in the atmosphere, which combine with mercury to form sulfides. Small quantities of these foreign materials in mercury destroy its bright, mirror-like surface and can be readily detected by visual inspection.
Mercury, which has a freezing point of -38.87° C, is the only metal that remains in the liquid state at room temperatures. It combines readily to form an amalgam with several metals such as gold, silver, copper, tin, and zinc, but does not combine under ordinary conditions with such metals as nickel, chromium, molybdenum, cobalt, and iron.
Mercury boils at 356.9° C, and, if pure, has a significant vapor pressure at room temperature. Extended inhalation can result in mercury poisoning. Globules dropped on a surface roll about freely without leaving a tail and retain their globular form. This tendency to form globules is related to the high surface tension of liquid mercury, which is 465 dynes/cm at 20° C, as compared with 72.8 dynes/cm for water. Mercury with a very high degree of purity exhibits a slight tarnish after a short time because impurities contaminate the metal and produce a dull surface appearance. Impurities in mercury can reduce the rate at which it combines with the silver alloy.
MANIPULATION OF AMALGAM
SELECTION OF ALLOY
The selection of an alloy involves a number of factors, including setting time, particle size and shape, and composition, particularly as it relates to the elimination of the y2 phase and the presence or absence of zinc. It is estimated that more than 90% of the dental amalgams currently placed are high-copper alloys. The majority of the alloys selected are high-copper unicomposi-tional (spherical) and admixed types, with the admixed being favored slightly. A high-copper alloy is selected because the result is a restoration with no y2, high early strength, low creep, good corrosion resistance, and good resistance to marginal fracture.
Finer particle sizes are used for low-copper, irregular alloys because of improved properties and enhanced clinical convenience. Finer particles produce a smoother surface during carving and finishing. The clinical manipulation of dental amalgam alloys is influenced to a modest extent by the shape of the particles. Lathe-cut alloys exhibit rough, irregular surfaces having a large area/volume ratio to react with mercury, and generally require nearly 50% or more mercury to obtain adequate plasticity during trituration. Spherical alloys are smoother, consist of various sizes of spheres (2 to 43 pm), which is important in packing, have more-regular surfaces with a lower area/volume ratio, and generally require less mercury for trituration and suitable plasticity development. Mercury concentrations as low as 42% permit acceptable handling characteristics with certain products.
Lathe-cut and spherical alloys react differently to condensation forces. These differences result from frictional forces within the amalgam mass that offer higher resistance to the face of the condenser in lathe-cut alloys than in spherical alloys. Carving the excess amalgam from the overfilled cavity to restore morphological and functional anatomy presents further differences.
Because of improved manufacturing, few products contain zinc because the contamination of a zinc-containing alloy by moisture may result in excessive dimensional change. If an alloy contains more than 0.01% zinc, the package must carry a printed precaution that the amalgam made from the material will show excessive corrosion and expansion if moisture is introduced during mixing and condensation.
PROPORTIONS OF ALLOY TO MERCURY
Correct proportioning of alloy and mercury is essential for forming a suitable mass of amalgam for placement in a prepared cavity. Some alloys require mercury-alloy ratios in excess of 1:1, whereas others use ratios of less than 1:1; the percentage of mercury varies from 43% to 54%. Automatic mechanical dispensers for alloy and mercury have been used in the past and are described in previous editions of this textbook. With the recommendation for "no touch" procedures for handling mercury and amalgam, capsules with preproportioned amounts of alloy and mercury have been substituted for mercury and
alloy dispensers. The correct amounts of alloy and mercury are kept separated in the capsule by a membrane, as shown in the sketch in Fig. 11-9. Just before the mix is triturated the membrane is ruptured by compression of the capsule, or it is automatically activated during trituration. Various manufacturers' amalgam alloys with their corresponding capsules are shown in Fig. 11-10. Some capsules contain a plastic pestle in the shape of a disk or rod, as illustrated in the disassembled capsules in Fig. 11-11. To prevent any escape of mercury from the friction-fitted capsule during trituration, some capsules are hermetically sealed; the mercury is contained in a small plastic film packet which ruptures during mixing.
DISPERSALLOY
Size of Mix Manufacturers commonly supply capsules containing 400, 600, or 800 mg of alloy and the appropriate amount of Hg, color coded for ease of identification. Clinical consensus is that these amounts are sufficient for most restorations. It is usually suggested that if larger amounts are required that several smaller mixes be made at staggered times so the consistency of the mixed amalgam remains reasonably constant during the preparation of the restoration. However capsules containing 1200 mg of alloy are available if a large amount of amalgam is needed to produce an amalgam core on a severely broken down tooth.
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