Сraig. Dental Materials

-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.

SELECTION O F 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 y, 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 unicompositional (spherical) and admixed types, with the admixed being favored slightly. A high-copper alloy is selected because the result is a restoration with no y, 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 areaholume 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 surfxes with a lower areaholume 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.

Liquid

-Powder

Fis. 11-9 Cross-sectional sketch of a disposable capsule containing amalgam alloy and mercury.

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:l; 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.

Fig. 11-11 Types of capsules with and without pestles.

Size ofMi x 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.

MIXING OF AMALGAM

Trituration of amalgam alloy and mercury is done with a mechanical mixing device called an amalgamator or triturato~"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 auton~aticallysets 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 undermixed 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 (t,), 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 5t,. The value of t, 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 undertritu-

ration 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. Highor low-copper alloys respond alike. Overtrituration results in slightly higher contraction for all types of alloys. Highand 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 highcopper 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 overtriturating 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 overtriturated amalgam is better than a slightly undertriturated 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.

Smallto 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

308 Chapter 11 AMALGAM

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 pai-tial 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~considerabletime 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 Ag2Hg, and Sn,-,Hg, the y, and y, phases, thereby leaving less unreacted Ag3Sn, the y phase. As discussed earlier, both y, 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 mercurylalloy 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 mois-

ture 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 zinccontaining 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.

FACTORS RELATED TO FINISHING

AMALGAM RESTORATIONS

When an amalgam restoration has been properly placed, with adequate condensation, and the excess mercury has been removed from the final surface layer of the restoration, it will be sufficiently hardened within a few minutes to permit careful carving. If the restoration is not well condensed, it will not harden promptly, and the carving operation must be delayed. Usually the amalgam is sufficiently well set and hardened that carving with sharp instruments can be started almost immediately after condensation.

Burnishing, or rubbing the newly condensed amalgam with a metal instrument having a broad surface contact, can be employed to smooth the

surface, thereby making the amalgam more susceptible to finishing and polishing. Burnishing can produce a tenfold reduction in surface roughness.

If final finishing and polishing are to be done at a second appointment, the restoration should be left undisturbed for a period of at least 24 hours. The patient should be cautioned that the freshly inserted restoration is relatively weak and that heavy biting forces should be avoided for a few hours after the time of insertion. Occlusal contacts must be carefully established. However, current all-spherical high-copper alloys have a much higher early strength than other types and can withstand biting forces sooner than earlier amalgams. One-hour compressive strengths of spherical high-copper alloys are about twice as high as high-copper admixed types and are comparable with those of low-copper alloys at 6 to 7 hours.

High-copper unicompositional amalgams with high early strengths can be finished at the first appointment. After condensation the surface is burnished and carved for clear definition of the margins, and all excess amalgam is removed. A creamy paste of triple-x silex and water is applied gently with an unwebbed rubber cup and a slow-speed handpiece. Light pressure should be applied for no more than 30 seconds per surface, and polishing should be directed from the center toward the margins of the restoration.

This early finishing begins 8 to 10 minutes after the start of trituration, depending on the particular alloy. Results of a 3-year clinical study have shown that restorations polished 8 minutes after trituration and those polished after 24 hours had no difference in longevity. Also, as time in the mouth increased, it became difficult to determine which method had been used to finish the restoration. The 24-hour polishing procedure used in the study was that normally used for polishing amalgam restorations. The procedure used for the 8-minute polish was different; no polishing bur was used, and the amalgam was carved carefully. Because the 24-hour polishing technique requires a second appointment, many

310 Chapter II AMALGAM

restorations go without polishing. The main advantage of the 8-minute polishing technique is the elimination of the second appointment. This technique is limited to those amalgams that have high early-compressive strengths.

A well-finished and well-polished restoration will retain its surface appearance and be easier to keep clean than one that is poorly finished, because a rough surface on the restoration contains microscopic pits in which acids and small food particles from the mouth accumulate. These pits tend to encourage galvanic action on the surface of the restoration, leading to tarnish and perhaps even a corroded appearance.

The final polish at a second appointment is developed through a series of final finishing and polishing steps after a careful carving operation. This final polish is accomplished through a sequence of operations that includes the use of fine stones and abrasive disks or strips. To develop the final polish, a rotating soft brush is used to apply a suitable polishing agent, such as extrafine silex, followed by a thin slurry of tin oxide.

During the final polishing operation, the restoration should remain moist to avoid overheating from the use of dry polishing surfaces. Because the amalgam is weak in tension and shear resistance, it should not be drawn over the margin by burnishing or drawing operations that tend to produce extensions that subsequently will be fractured from the amalgam mass. To avoid such overextensions, all recommended operative practices should be followed faithfully.

BONDING OF AMALGAM

Although amalgam has been a highly successful restorative material when used as an intracoronal restoration, it does not restore the strength of the clinical crown to its original strength. Additional features, such as pins, slots, holes and grooves to increase retention of the restoration, must be supplied with the preparations for large amalgam restorations, but they do not reinforce the amalgam or increase its strength.

With the development of adhesive systems for

dental composites came the opportunity to attempt to bond amalgams to tooth structure. Ad- hesive plastics containing 4-META, an acronym for 4-methacryloxyethyl trimellitic anhydride (see Chapter lo), have been the most successf~~l products. Shear bond strengths of amalgam to dentin as high as 10 MPa have been reported using these adhesives. Comparable values for the shear bond strength of microfilled composites to dentin using these same adhesives have been 20 to 22 MPa. The fracture resistance of teeth restored with amalgam-bonded MOD restorations was more than twice that of restorations containing unbonded amalgams. Also, in spite of the lower shear strength of amalgam-bonded-to- dentin test samples compared with composites, the fracture strength of MODS in teeth restored with bonded amalgams was as high as for composites, although neither were as high (45% to 80%) as values for the intact tooth. As expected, amalgam bonded MODS with narrow preparations had higher strengths than those with wide preparations. Other studies showed the retention

of amalgam-bonded MODSwith proximal boxes was as great as pin-retained amalgams. In addition, amalgam-bonded restorations decreased marginal leakage in Class 5 restorations compared with unbonded amalgams. Finally, the plastic bonding agents for amalgam have not been successful in increasing the ainalgam-to- amalgam bond strength in the repair of amalgam restorations. Thus at this stage of development, adhesive bonding of amalgam restorations to tooth structures is an improvement over nonbonded amalgams.

Amalgams have been used for 150 years; about 200 million amalgams are inserted each year in the United States and Europe. In spite of its substantial history, however, periodically concern arises about the biocompatibility of amalgam. Allergic reactions to mercury in amalgam restorations do occur, albeit infrequently. This is

not surprising, because there is no material that 100% of the population is immune to 100% of the time. However, such allergic responses usually disappear in a few days or, if not, on removal of the amalgam. Aside from varying reports of mercury accumulation, no other local or systemic effects from mercury contained in dental amalgam have been demonstrated. If amalgam is used correctly, biocompatibility should not be a problem.

Even in their passive condition, metals are not inert. In vitro and in vivo experiments have established that there is a passive dissolution from all metals. The following eight questions are linked to the issues of dissolution, corrosion, and potential allergic response and toxicity:

Is any material released into the mouth? What material is released?

What is the form of the released material? How much material is released?

In what subsequent reactions do the released products get involved?

What percentage of the released products is excreted and what percentage is retained?

Where does the retained percentage accumulate?

What biological responses will result from the retained fraction?

Therefore any analysis of the literature and discussion of mercury toxicity in amalgams must continually refer to these eight questions, particularly questions 3 and 4, relating to the dosage and form of the mercury to which the body is exposed.

SOURCES OF MERCURY

In addressing these eight questions, the sources of the potential toxins must be evaluated. Exposure to mercury can occur from many different sources, including diet, water, air, and occupational exposure (Table 11-4). The World Health Organization (WHO) has estimated that eating seafood once a week raises urine mercury levels to 5 to 20 pg/L, two to eight times the level of exposure from amalgam (1 pg/L = 1 mg/m3 = 1 part per billion [ppb]). Thus the amount of mercury vapor released from amalgam is less than that received from eating many common fish. It has been estimated that a patient with 9 amalgam occlusal surfaces will inhale daily only about 1% of the amount the Occupational Safety and Health Administration (OSHA) allows to be inhaled in the workplace. Blood and urine mercury levels are easily influenced by other factors and cannot often be directly linked to amalgam. In general, elemental mercury from amalgam seems to make only a small contribution to the total body burden of mercury. On the basis of epidemiological studies, blood and serum mercury levels correlate highly with occupational exposure and diet, whereas urine mercury relates to amalgam burden. Urine mercury levels relate to methods of condensation and ventilation more than to the amalgam per se.

FORMS OF MERCURY

Mercury has many forms, including organic and inorganic compounds. The most toxic organic compounds are methyl and ethyl mercury, and the next most toxic form is mercury vapor. The least toxic forms of mercury are the inorganic