Posted by John Doe at Dental Assistant on May 14, 2013.
Categories: Dental Materials
MIXING OF AMALGAM
Trituration of amalgam alloy and mercury is done with a mechanical mixing device called an amalgamator or triturator. The two amalgamators shown in Fig. 11-12 have controls for the speed and duration of trituration. The amalgamator shown on the left has a slot on the lower right for the insertion of plastic cards. There is a separate card for each size mix; insertion of the card automatically sets the correct mixing time and speed. Each of the amalgamators has a housing that is placed over the capsule area during trituration to confine any mercury lost from the capsule during mixing.
The capsule holder is attached to a motor that rotates the holder and capsule eccentrically. The trituration may be accomplished simply by the agitation of the alloy particles and mercury, or the manufacturer may have included a plastic pestle to aid in the mixing.
Spherical or irregular low-copper alloys may be triturated at low speed (low energy), but most high-copper alloys require high speed (high energy). Effective trituration depends on a combination of the duration and speed of mixing. Duration of amalgamation is the easiest factor to vary; however, it should be emphasized that variations of 2 to 3 seconds of mixing time may be enough to produce an amalgam that is under-mixed or overmixed. Mechanical amalgamators allow some variation in speed to adjust to differing amounts of alloy and mercury in the capsules.
Low-, medium-, and high-speed amalgamators operate at about 3200 to 3400, 3700 to 3800, and 4000 to 4400 cycles per minute, respectively, at correct live voltage. However, an amalgamator set at la speed of 3300 cpm may actually be operating at 3000 cpm with a decrease in line voltage from 120 to 100 volts, and undermixed amalgams may result. This problem can be avoided by installing a voltage regulator between the line plug and the amalgamator. Using a parameter called the coherence time (tc), defined as the minimum mixing time required for an amalgam to form a single coherent pellet, it has been found that the compressive strength, dimensional change, and creep are optimized if mixing is carried out for a time of 5tc. The value of tc can be determined experimentally for a particular amalgam alloy, size of mix, and speed of the amalgamator. However, most packages of amalgam alloys will contain recommendations for times and speeds for a variety of amalgamators, and these guidelines should be followed.
With the introduction of disposable capsules containing premeasured amounts of amalgam alloy and mercury, mercury and alloy dispensers have become obsolete, as have reusable capsules; however, the discussion of their selection and use is described in the 9th and earlier editions of this textbook.
Mixing Variables Undermixing, normal mixing, or overmixing can result from variations in the condition of trituration of the alloy and mercury. The three mixes have a different appearance and respond differently to subsequent manipulation. The undermixed amalgam appears dull and is crumbly, the normal mix appears shiny and separates in a single mass from the capsule, and the overmixed amalgam appears soupy and tends to stick to the inside of the capsule. Examples of these mixes are shown in Fig. 11-13. The three types of mixes have characteristically different mechanical properties of dimensional change, strength, and creep. These three conditions can be developed from variations in the mixing variables described earlier. Therefore the type of mix contributes to the success or failure of the amalgam restoration.
Not all types of alloys respond in the same manner to overtrituration and undertrituration. Spherical and lathe-cut alloys respond differently. The effect of overtrituration and undertrituration of amalgam on working time, dimensional change, compressive and tensile strengths, and creep is summarized as follows.
Working Time and Dimensional Change
Working time of all types of amalgam, spherical or irregular, decreases with overtrituration. Higher low-copper alloys respond alike. Overtrituration results in slightly higher contraction for all types of alloys. High- and low-copper alloys show the same effect.
Compressive and Tensile Strengths Both compressive and tensile strengths of irregular shaped alloys increase by overtrituration. However, this is not true for spherical alloys. Compressive and tensile strengths of spherical alloys are greatest at normal trituration time. Both overtrituration and undertrituration reduce compressive and tensile strengths. The admixed high-copper alloys consist of both shapes of particles and behave like spherical alloys; normal trituration times produce the highest strength values,
whereas overtrituration results in significant decreases in strength.
Creep Overtrituration increases creep, and undertrituration lowers it. As mentioned earlier in this chapter, two properties that are closely related to the clinical behavior of alloys are low creep and high compressive strength. By over-triturating irregular amalgams, a higher compressive strength can be obtained, which is beneficial. However, the amalgam has a higher creep, a property that is not desirable. If there is doubt about the correct trituration time, a slightly over-triturated amalgam is better than a slightly un-dertriturated one. This suggestion is particularly true for high-copper alloys.
Some manufacturers recommend altering the trituration time to obtain a longer or shorter working time. Altering the trituration time does change the working time of amalgam, but it also affects other properties. When amalgam is triturated for shorter than normal times, mercury does not completely wet the outer surface of
amalgam particles. As a result, mercury does not react with the amalgam alloy over the entire surface of the particle. The mass remains soft for a longer period of time, producing an amalgam with a longer working time. Such an amalgam mass contains excessive amounts of porosity, has lower strength, and possesses poorer corrosion resistance.
Overtrituration reduces working time, causing the reaction rate to increase because the amalgamated mass becomes hot. When amalgams with longer or shorter working times are desired, one should use amalgam alloys that are designed to react faster or slower and not attempt to achieve the change by altering the trituration time.
CONDENSATION OF AMALGAM
During condensation, adaptation of the amalgam mass to the cavity walls is accomplished and the operator controls the amount of mercury that will remain in the finished restoration, which in turn influences the dimensional change, creep, and compressive strength. In general, the more mercury left in the mass after condensation, the weaker the alloy. With irregularly shaped alloys, in which a higher percentage of mercury is used initially, the operator should remove as much mercury as possible during condensation by using as great a force as possible on the condenser. With spherical alloys, the amount of mercury supplied in the capsules is lower, and it is not necessary to remove as much mercury as for the irregularly shaped alloys; however, increasing the condensation pressure from 3 to 7 MPa results in a significant increase in compressive strength. Further increase in condensation pressure to 14 MPa does not result in additional compressive strength.
Hand or Mechanical Condensation
A large number of instruments designed for hand condensation of amalgam have been available to the dental profession for many years. The instruments and the techniques for their use have been described in textbooks of operative dentistry. In general, a suitable instrument for hand condensation of amalgam would be shaped so that the operator could readily grasp it and exert a force of condensation by appropriately placing one finger on a finger rest of the instrument. Hand instruments that do not permit a convenient grasping may inhibit proper condensation practices and mercury removal. In many instances, circular condenser tips may prove adequate, whereas in other cavity areas and designs, the triangular, oval, crescent, or other shape of tip may be effective. In general, a condenser tip that is too small in cross section tends to be ineffective in condensing a reasonable quantity of amalgam. The size of the condenser tip and the direction and magnitude of the force placed on the condenser also depend on the type of amalgam alloy selected.
With irregularly shaped alloys, one should use condensers with a relatively small tip, 1 to 2 mm, and apply high condensation forces in a vertical direction. During condensation, as much mercury-rich mass as possible should be removed from the restoration.
When condensers with small tips are used with high condensation forces on spherical amalgams, the particles tend to roll over one another, the tip penetrates the amalgam, and the mass does not adapt well to the cavity walls. With spherical alloys one should use condensers with larger tips, almost as large as the cavity permits. For example, at the cervical margin of a Class 2 preparation with a small opening, a condenser with a very small tip should be used. As the cavity is filled and the opening toward the occlusal surface becomes larger, condensers with larger tips should be used. Because of the spherical shape of the particles, a lateral direction of condensation provides better adaptation of amalgam to cavity walls than of condensation toward the pulpal floor. With high-copper spherical amalgams, a vertical and lateral direction of condensation with vibration is recommended.
Small- to medium-diameter condensers are advocated with admixed high-copper alloys with a medium-to-high force and vertical and lateral directions of condensation.
Many mechanical devices are available for condensing amalgam. These devices are more popular and more useful for condensing irregularly shaped alloys when high condensation forces are required. With the development of spherical alloys, the need for mechanical condensers was eliminated. Ultrasonic condensers are not recommended because during condensation they increase the mercury vapor level to values above the safety standards for mercury in the dental office.
Effect of Delay in Condensation
It is important that an amalgam be condensed into the tooth cavity promptly after the mercury and alloy are suitably mixed. Delay of the condensation operation permits the amalgam to set partially before being transferred to the cavity. A delay in the condensation operation with a partial reaction of the mercury and alloy makes it impossible to remove the mercury effectively during condensation. As a result, an amalgam mass that has remained uncondensed for any period of time will contain more mercury than one that is condensed promptly. The resulting amalgam with the additional mercury content will show less strength in compression and higher creep. Delay in the condensation operation reduces the plasticity of the mix, and amalgams with reduced plasticity do not adapt well to the cavity walls. In a large restoration involving, considerable time to place the amalgam mass, condensation of the final portions of amalgam becomes a problem. In such cases, it is preferable to make two smaller mixes of amalgam rather than one excessively large mix and not to use the amalgam if more than 3 or 4 minutes have elapsed from the time of initial mixing.
Mercury Content of Amalgam Restorations
Amalgam restorations containing greater amounts of mercury in the set mass demonstrate less favorable clinical characteristics. Having more mercury in the set amalgam produces a greater amount of Ag2Hg3 and Sn7.8Hg, the Yi and y2 phases, thereby leaving less unre-acted Ag3Sn, the y phase. As discussed earlier, both Yj and y2 have lower strength than the y phase. Therefore, when amalgam specimens are subjected to compressive stress, those containing increasing quantities of mercury exhibit decreasing strength values. The compressive strength decreases 1% for each 1% increase in mercury above 60%.
The mercury content of an amalgam restoration is not uniform throughout. Higher concentrations of mercury are located around the margins of the restoration. As a result, cavities should be overfilled and then carved back to minimize this problem. When using alloys that require higher mercury/alloy ratios, as much mercury as possible should be removed from the amalgamated mass. Note that the maximum allowable amount of mercury remaining in a hardened amalgam mass depends on the original mercury/ alloy ratio. In other words, for alloys requiring high mercury/alloy ratios for trituration, 50% mercury in the hardened amalgam might be acceptable; however, for alloys needing low mercury/alloy ratios for trituration, 50% mercury in the set amalgam would be detrimental.
Although the lower mercury/alloy ratios currently being used are favorable regarding the total quantity of mercury in the set mass, remember that condensation forces alter mercury content within the restoration. Because condensation brings mercury to the surface of the amalgam mass, such "plashy" material should be periodically removed when filling the cavity to prevent trapping high mercury concentrations within the restoration. Overfilling of the cavity is carried out for the same reason, that is, to remove the amalgam that contains higher mercury content from the restoration contour.
When alloys that permit lower mercury/alloy ratios are used to obtain a plastic mass suitable for condensation, the operator should expect a lesser volume of excess mercury to be brought to the surface for removal than was observed with older materials.
Moisture Contamination During Insertion
Moisture contamination during the mixing and condensing operations is the factor that may produce excessive expansion. There is no evidence, however, that the presence of moisture on the surface will cause any serious damage once the condensation operation is completed and the restoration is finished, except for trimming and polishing.
Because moisture in the saliva is a potential source of contamination for the amalgam, the tooth cavity must remain dry and the amalgam must be free from saliva contamination. Techniques and procedures in operative dentistry provide for an isolated field of operation, and these techniques should be followed to gain the best properties of the set amalgam.
With zinc-containing amalgam, the presence of saliva on the amalgam during condensation probably was a principal source of excessive delayed expansion and other poor qualities in the restoration. Moisture contamination of a zinc-containing amalgam mass from any source results in an excessive delayed expansion of several hundred micrometers per centimeter after the restoration has been placed in the tooth for several hours or days. This excessive expansion results from the decomposition of moisture. The trapped hydrogen gas in the amalgam restoration continues to be developed until sufficient force is produced to cause the excessive expansion. This decomposition of moisture results from the presence of zinc in the amalgam alloy and can be overcome by the use of non-zinc alloys.
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