Dental Amalgam Dentistry Notes

Dental Amalgam

An amalgam is defined as a special type of alloy in which mercury is one of the components. Mercury is able to react with certain alloys to form a plastic mass, which is conveniently packed into a prepared cavity in a tooth. This plastic mass hardens and is stronger than any dental cement or anterior filling material. Dental amalgam is the most widely used filling material for posterior teeth.

The alloys before combining with mercury are known as dental amalgam alloys. Strictly speaking, however, this is a misnomer as they are not dental amalgam alloys but alloys from which dental amalgam is prepared.

In dentistry, amalgam has been successfully used for more than a century as a restoration material for tooth decay. Over the years its quality has greatly improved, thanks to a lower amount of mercury and the addition of new components which can reduce its corrosion in the oral cavity.

Read And Learn More: Basic Dental Materials Notes

Indications

• As a permanent filling material for
  • Class 1 and class 2 cavities
  • Class 5 cavities where esthetics is not required
• In combination with retentive pins to restore a crown.
• For making dies.
• In retrograde root canal fillings.
• As a core material in abutment teeth.

Contraindications

• Amalgam should not be placed in patients with impaired kidney function.
• Individuals with allergic hypersensitivity to mercury or components of the alloy
• New amalgam fillings should not be placed in contact with nonamalgam restoration like gold and metal devices, such as orthodontic braces.

Classification Of Amalgam Alloys

Based on Copper Content

• Low copper alloys Contain less than 6% copper (conventional alloys)
• High copper alloys Contain between 13–30% copper

The high copper alloys are further classified as

• Admixed or dispersion or blended alloys.
• Single composition or uni-compositional alloys

Based on Zinc Content

• Zinc-containing alloys Contain more than 0.01% zinc
• Zinc-free alloys Contain less than 0.01% zinc

Based on Shape of the Alloy Particle

• Lathe-cut alloys (irregular shape)
• Spherical alloys
• Spheroidal alloys

Based on the Number of Alloyed Metals

• Binary alloys, e.g. silver-tin
• Ternary alloys, e.g. silver-tin-copper
• Quaternary alloys, e.g. silver-tin-copper-indium

Based on the Size of the Alloy Powder Particle

• Microcut
• Macrocut

Manufacture Of Alloy Powder

The various components of the amalgam alloy are combined together by melting to form ingots. The ingots have to be heat-treated in an oven for a set period of time. This process is called annealing. Annealing improves the homogeneity and grain structure of the alloy.

Lathe-Cut Alloy Powder

An annealed ingot of silver-tin alloy is placed in a lathe and fed into a cutting tool. The resulting chips obtained are often needlelike and some manufacturers reduce the chip size by ball-milling.

Aging, Acid Treatment, And Annealing Of Particles

• A freshly cut alloy reacts too rapidly with mercury. If the alloy filings are stored at room temperature for a few months, the reactivity gradually decreases. Such alloys are said to have been aged. The filings can be aged faster by boiling in water for 30 minutes. Aging also improves the shelf life of the product.
• Some manufacturers treat the filings with acid to improve reactivity.
• The stresses induced during the cutting and grinding process must be relieved by an annealing process (100 °C for several hours). Failure to anneal results in a slow release of stress over time (during storage) which can adversely affect the properties of the amalgam.

Spherical Alloy Powder

The spherical alloy is prepared by an atomization process. The liquid alloy is sprayed under high pressure of an inert gas through a fine crack into a large chamber. If the droplets solidify before hitting a surface, the spherical shape is preserved. Like the lathe-cut powders, spherical powders are aged. A comparison of two types of powders.

Supplied As

• Bulk powder and mercury in separate containers.
• Alloy and mercury in disposable capsules.
• Preweighed alloy as tablet form in tubes and mercury in sachets.

Composition

Function Of Constituents

Silver

• A major element in the reaction.
• Whitens the alloy.
• Decreases the creep.
• Increases the strength.
• Increases the expansion on the setting.
• Increases tarnish resistance in the resulting amalgam.

Tin

• Tin controls the reaction between silver and mercury. Without tin, the reaction would be too fast and the setting expansion would be unacceptable.
• Reduces strength and hardness.
• Reduces the resistance to tarnish and corrosion, hence the tin content should be controlled.

Copper

• Increases hardness and strength.
• Increases setting expansion.

Zinc

• In small amounts, it does not influence the setting reaction or properties of amalgam. Zinc acts as a scavenger or deoxidizer during manufacture, thus preventing the oxidation of Alloy/mercury in preproportioned capsule form. Alloy in tablet form.important elements like silver, copper, or tin. Oxidation of these elements would seriously affect the properties of the alloy and amalgam. Alloys without zinc are more brittle, and the amalgam formed by them is less plastic.
• Zinc causes delayed expansion if the amalgam mix is contaminated with moisture during manipulation.

Mercury

In some brands, a small amount of mercury (up to 3%) is added to the alloy. They are known as pre-amalgamated alloys. Pre-amalgamation produces a more rapid reaction.

Platinum

Hardens the alloy and increases resistance to corrosion.

Palladium

Hardens and whitens the alloy.

Indium

Indium when added to the mercury reduces mercury vapor and improves wetting. Indium can also added to the powder. Though it reduces early strength it increases the final strength. It reduces creep.

Comparison Of Lathe Cut And Spherical Alloys

A comparison of the two types of powders.

Low Copper Alloys

Historically amalgam alloys were low copper alloys. The composition recommended by GV. Black in the late 18th century remained virtually unchanged until the late 1960s when the high copper amalgams were introduced.

Composition

Available As

• Lathe-cut alloys are further available as coarse or fine grain (fine grain type is preferred, because of the ease of carving).
• Spherical alloys.
• A blend of lathe-cut and spherical particles.

Setting Reaction

• When alloy powder and mercury are triturated, the silver and tin in the outer portion of the particles dissolve into the mercury. Simultaneously, the mercury diffuses into the alloy particles and starts reacting with the silver and tin forming crystals of silver-mercury (Ag₂Hg₃) and tin-mercury compounds (Sn₈Hg).
• Silver-tin compound (unreacted alloy powder) is known as the gamma (γ) phase. The silver mercury compound is known as the gamma 1 (γ₁) phase and the tin-mercury as the gamma 2 (γ₂) phase.
• A simplified reaction is outlined below
  Ag₃Sn + Hg → Ag₂Hg₃ + Sn₈Hg + (unreacted) Ag₃Sn
• The alloy particles do not react completely with mercury. About 27% of the original Ag₃Sn remains as unreacted particles, which as previously mentioned is known as the gamma (γ) phase.

Microstructure

Set amalgam consists of unreacted particles (γ) surrounded by a matrix of the reaction products (γ₁ and γ₂).

Note The properties of the hardened amalgam depend upon the proportion of each of the reaction phases. If more unconsumed Ag₃Sn (γ phase) is present, the stronger the amalgam. The γ₂ phase is the weakest component and is least stable to the corrosion process. Also present are Cu₃Sn phase (ε or epsilon) formed from the small amounts of copper present in the composition.

High Copper Alloys

High copper alloys contain between 13 to 30% wt. copper. The majority of amalgam restorations placed currently are high copper. They are preferred because of their improved mechanical properties, resistance to corrosion, and better marginal integrity.

Types

• Admixed alloy.
• Single-composition alloy.

Admixed Alloy Powder

• The admixed alloy was introduced in 1963 and was originally made by mixing 1 part silver-copper eutectic alloy (high copper spherical particles) with 2 parts silvertin alloy (low-copper lathe-cut particles).
• (An eutectic alloy is one in which the components exhibit complete liquid solubility but limited solid solubility. The silver-copper phase exhibits a eutectic structure at the composition of silver 71.9% and copper 28.1%).

• Amalgam made from admixed powders is stronger than amalgam made from lathe-cut low-copper powder, because of three reasons
  • A change in the nature of the filler particles. The silver-copper particles are present in greater amounts, in addition to the silver-tin particles.
  • A greater residual filler content thereby changing the filler-to-matrix ratio.
  • A reduction in the weaker γ₂ phase.
    Synonyms Dispersed phase alloy.

Types Admixed alloys are of two types

• Regular or conventional admixed alloy—contains irregular and spherical alloy particles having different compositions (low copper and high copper).
• Unicomposition admixed alloys—contain irregular and spherical particles of uniform composition.

Composition

Admixed alloy powders usually contain between 30 to 55 weight percent spherical high copper alloy powder. The total copper content ranges from 9 to 20 weight percent. A sample composition is presented below.

Setting Reaction

• When the components are mixed the mercury begins to dissolve the outer portion of the particles. Silver from the silver-copper eutectic alloy particles and both silver and tin from the silver-tin alloy particles enter the mercury. The tin dissolved in the mercury reacts with the copper of the silver-copper particles and forms the Cu₆Sn₅ (η or Eta). The η crystals form around the unreacted silver-copper particle. At the same time, the γ1 phase is also formed. As in the low copper alloys, γ1 surrounds everything forming the matrix. γ2 is also formed at the same time but is later replaced by η. Thus in admixed alloy the undesirable γ2 phase is greatly reduced.
• The reaction may be simplified as follows
  Ag₃Sn + Ag-Cu + Hg → Ag₂Hg₃ + Cu₆Sn₅ + Ag₃Sn unreacted + Ag-Cu unreacted

Note In this reaction, γ2 has been eliminated and is replaced by η phase. To accomplish this, it is necessary to have a net copper content of at least 12 percent in the alloy powder.

Microstructure of Set high copper admixed Amalgam The Cu₆Sn₅ is present as a ‘halo’ surrounding the Ag-Cu particles.

The final set material consists of

Core particles of

• Unreacted Ag₃Sn, (γ phase) and
• Unreacted Ag-Cu surrounded by a halo of Cu₆Sn₅ (η).

Embedded in a matrix made up of

• γ₁ (Ag₂Hg₃).

Schematic representation of the setting reaction and microstructure is shown in

Single Composition Alloys

High copper amalgam was developed by a Canadian metallurgist, Dr. William Youdelis in 1963. Single composition alloys are high copper amalgam alloys. Unlike admixed alloy powders, each particle of the alloy powder has the same composition. Therefore they are called single composition or ‘unicompositional alloys’. The spherical alloy particles are 5 to 40 µm in size.

Synonyms Single composition, unicompositional, non-gamma 2.

Composition

Setting Reaction

• Though each particle has the same composition, the silver, tin, and copper present exist in various phases within the particle. Thus each particle contains Ag₃Sn (γ), AgSn (β) and Cu3Sn (ε ). When triturated, silver and tin from the particle dissolve in mercury forming the γ₁ (Ag₂-Hg₃) crystal matrix that binds together the partially dissolved alloy particles. At this stage very little copper dissolves. Later, a layer of η (Cu₆Sn₅) crystals is formed at the surface of alloy particles. Some η (Cu₆Sn₅) crystals also form in the matrix.
• The overall simplified reaction is
  Ag-Sn-Cu + Hg → Cu₆Sn₅ + Ag₂Hg₃ + Ag-Sn-Cu

Note The undesirable γ₂ does not usually form in most single composition alloys. The η(Cu₆Sn₅) crystals are much larger and rod-shaped than those in the admixed amalgam.

Microstructure of set single-composition amalgam

Final set material consists of

Particles of

• Unreacted Ag₃Sn (γ phase) and surrounded by a mesh of rod shaped η (Cu₆Sn₅).

Embedded in a matrix made up of

• γ₁ (Ag₂Hg₃)

Advantages/disadvantages of Spherical high-copper amalgam

Advantages

• Faster set.
• Lower residual mercury.
• Lower creep during condensation.
• Faster finishing.
• Higher early strength.
• Low condensation pressure.

Disadvantages

• Less working time.
• Condensation pressure is not sufficient to displace the matrix during condensation (while restoring proximal cavities). Contouring of matrix band required.

Properties Of Set Amalgam

Microleakage

• Penetration of fluids and debris around the margins may cause secondary caries. Dental amalgam has an exceptionally fine record of clinical performance because of its tendency to minimize marginal leakage (see tarnish and corrosion).
• Self-sealing The small amount of leakage under amalgam restorations is unique. If the restoration is properly inserted, leakage decreases as the restoration ages in the mouth. This may be due to the formation of corrosion products in the tooth-restoration interface. Over a period of time they seal the interface and reduce leakage. Thus amalgam is a self sealingrestoration. Both low and high-copper amalgams are capable of sealing against microleakage but the accumulation of corrosion products is slower with the high-copper alloys. Initial leakage can be reduced through the application of varnish on the cavity walls. The use of dentin bonding agents (bonded amalgam technique) also shows promise.

Dimensional Change

• The earliest amalgams exhibited expansion while setting. This was because of the greater mercury/alloy ratio used. Amalgams may expand or contract, depending on its manipulation. Ideally, dimensional change should be small. Excessive contraction can lead to microleakage, sensitivity, and secondary caries. Excessive expansion can produce pressure on the pulp and postoperative sensitivity. Protrusion of the restoration can also occur.
• ISO Sp. 24234:2015 requires that amalgam should not expand more than 0.15% or contract less than –0.1% at 37 °C, during hardening. Mechanically, triturated modern amalgams, both low and high copper, prepared from low mercury/alloy ratios show a slight contraction.
• Theory of dimensional change
  • Contraction When the alloy and mercury are mixed contraction results initially as the particles dissolve and the γ₁ grows. The final volume of γ₁ is less than the initial volumes of silver and mercury that go into making the γ₁. Therefore, contraction will continue as long as the growth of γ₁ continues.
  • Expansion The γ₁ crystals as they grow, impinge against one another, and produce an outward pressure tending to oppose contraction. If there is sufficient mercury present to provide a plastic matrix, an expansion will occur when γ₁ crystals impinge on each other. After a rigid γ₁ matrix has formed, the growth of γ₁ crystals cannot force the matrix to expand. Instead, γ₁ crystals will grow into interstices containing mercury, consuming mercury, and producing continued reactions. Therefore, reducing mercury in the mix will favor contraction.
  • Thus, factors favoring contraction are
    • Low mercury/alloy ratio
    • Higher condensation pressure (squeezes out mercury)
    • Smaller particles (consume more mercury because of increased surface area)
    • More trituration (accelerates setting)
  • Modern amalgams show a net contraction, whereas older amalgams always show expansion. Two reasons for this difference are
    • Older amalgams contained larger alloy particles and were mixed at higher mercury/alloy ratios.
    • Hand trituration was used before. Modern amalgams are mixed with high-speed amalgamators (equivalent to an increase in trituration time).

Effect of Moisture Contamination (Delayed Expansion)

• If a zinc-containing-low-copper or high-copper amalgam is contaminated by moisture during trituration or condensation, a large expansion can take place. It usually starts after 3-5 days and may continue for months, reaching values greater than 400 µm (4%). This is known as delayed expansion or secondary expansion. The expansion is caused by the release of hydrogen gas from the reaction of zinc with water.

H₂O + Zn → ZnO + H₂ (gas)

• This hydrogen gas does not combine with the amalgam, but collects within the restoration, creating extreme internal pressure and expansion of the mass. This causes protrusion of the restoration out of the cavity, increased creep, increased microleakage, pitted surfaces, and corrosion. Dental pain, recurrence of caries, and fracture of the restoration are seen as a result of these poorly inserted restorations.

Note Moisture contamination after the cavity has been filled does not cause delayed expansion. Nonzinc alloys do not show this type of expansion when contaminated with water. However, moisture contamination of the mix of any alloy results in inferior physical properties.

Indications for zinc-free alloys

Amalgam without zinc tends to be less plastic and less workable. These alloys are used only for cases where it is difficult to control moisture, e.g. patients having excessive salivation, retrograde root canal filling, subgingival lesions, etc.

Strength

Well-designed amalgam restorations have sufficient compressive strength to withstand normal intraoral masticatory forces.

Tensile Strength

Amalgam cannot withstand high tensile or bending stresses and can fracture easily in improperly designed restorations. Therefore, the cavity should be designed so that the restoration will receive minimal tension or shear forces in service.

Factors affecting strength

• Effect of rate of hardening Amalgams do not gain strength as rapidly as might be desired. After 20 minutes, compressive strength may be only 6% of the one-week strength. ISO specifications stipulate a minimum of 100 MPa at one hour and 350 MPa after 24 hours. Since the initial strength of amalgam is low, patients should be cautioned not to bite too hard for a least 8 hours after placement, the time at which at least 70% of its strength is gained. The one-hour compressive strength of high-copper single-composition amalgams is exceptionally high (262 MPa), so the chances of accidental fracture are less.
• Even after six months, some amalgams may still be increasing in strength, suggesting that the reactions between the matrix phases and the alloy particles may continue indefinitely.
• Clinical significance The rate of hardening should be considered during the placement of amalgam. In class II restorations where a supporting matrix has been placed, removal of the matrix should be done at the appropriate time. Early removal can result in fracture. Excessive pressure on the restoration by the patient prematurely to test the occlusion can also result in fracture.
• Effect of trituration Either under-trituration or over-trituration will decrease the strength of both low-copper and high-copper amalgams.

• Effect of mercury content Sufficient mercury should be mixed with the alloy to wet each particle of the alloy. Insufficient mercury produces a dry, granular mix which can result in a rough and pitted restoration which is prone to corrosion.
• Excess mercury in the mix can produce a marked reduction in strength because of the higher γ₂ content (which is the weakest phase—see setting reaction).
• Effect of condensation Higher condensation pressure results in higher compressive strength (only for lathe-cut alloys).
  Reason A good condensation technique will minimize porosity and remove excess mercury from lathe-cut amalgams. If heavy pressures are used in spherical amalgams, the condenser will punch through. However, spherical amalgams condensed with lighter pressures produce adequate strength.
• Effect of porosity Voids and porosities reduce strength.
  Porosity is caused by
  • Decreased plasticity of the mix (caused by too low Hg/ alloy ratio, under trituration, and over trituration).
  • Inadequate condensation pressure.
  • Irregularly shaped particles of alloy powder.
  • Insertion of too large increments.
• Increased condensation pressure improves adaptation and decreases voids. Fortunately, voids are not a problem with spherical alloys.
• Effect of Cavity Design
  • The cavity should be designed to reduce tensile stresses.
  • Amalgam has strength in bulk, therefore, the cavity should have adequate depth and width.

Creep

It is defined as a time-dependent plastic deformation. A creep of dental amalgam is a slow progressive permanent deformation of a set amalgam, which occurs under constant stress (static creep) or intermittent stress (dynamic creep).

Significance of creep

Creep is related to the marginal breakdown of low-copper amalgams. The higher the creep, the greater the degree of marginal deterioration.

Creep values

In general lathe-cut low-copper alloys show the highest creep values, often exceeding ADA limits. The lowest creep values are shown by the high copper amalgams.

Factors affecting creep

Microstructure The γ₁ (Ag-Hg) phase has a big effect on low-copper amalgam creep rates. Increased creep rate is shown by larger γ₁ volume fractions. The decreased creep rate is shown by larger γ₁ grain sizes. The γ₂ phase is associated with higher creep rates.

Single-composition high-copper amalgams have very low creep rates, due to the absence of the γ₂ phase and due to the presence of η (Cu₆Sn₅) rods, which act as a barrier to the deformation of the γ₁ phase. Increased zinc content reduces creep.

Effect of manipulative variables For increased strength and low creep values

• Mercury/alloy ratio should be minimal.
• Condensation pressure should be maximum for lathe-cut or admixed alloys.
• Careful attention should be paid to the timing of trituration and condensation. Either under or over-trituration or delayed condensation tends to increase the creep rate.

Retention of Amalgam

Amalgam does not adhere to tooth structure. Retention of the amalgam filling is obtained through mechanical locking. This is achieved by proper cavity design (see cavity design in technical considerations). Additional retention if needed can be obtained by placing pins within the cavity. Amalgam can also be bonded using special bonding agents.

Tarnish and Corrosion

• Amalgam restorations often tarnish and corrode in the mouth. Black silver sulfide can form on the surface of an amalgam restoration in some patients. Both high and low-copper amalgams show corrosion. However, corrosion in high-copper amalgams is limited because η phase is less susceptible.
• Factors related to excess tarnish and corrosion
  • High residual mercury.
  • Surface texture—small scratches and exposed voids.
  • Contact of dissimilar metals, e.g. gold, and amalgam.
  • Patients on a high sulfur diet.
  • Moisture contamination during condensation.
  • Type of alloy—low copper amalgam is more susceptible to corrosion (due to higher content) than high copper. Also, η (Cu₆Sn₅) phase of high copper is less susceptible to corrosion.
  • A high-copper amalgam is cathodic in respect to a low-copper amalgam. Therefore, mixed high-copper and low-copper restorations should be avoided.
• Corrosion of amalgam can be reduced by
  • Smoothing and polishing the restoration.
  • Correct Hg/alloy ratio and proper manipulation.
  • Avoid dissimilar metals including mixing of high and low copper amalgams.

Biological considerations Two types of potential biological effects can occur.

• Adverse systemic effects of the mercury component.
• Contact reaction of the mucosa with amalgam or amalgam corrosion products. (Oral lichenoid reaction).

Mercury Mercury is toxic to the human body and to the environment. Fortunately risk to the patient of mercury exposure from dental restorations is very low even with multiple restorations. However, mercury vapors pose a greater risk to dental personnel as they are more easily inhaled and absorbed through the lungs.

Mercury vapors may be produced during trituration and condensation of amalgam and removal of old restorations. Further information is under the section Mercury Toxicity at the end of the chapter.

Contact reactions Reactions occurring from proximity or contact with amalgam are rare. The symptoms are normally classified as delayed hypersensitivity reactions (type 4), and they were called oral lichenoid reactions (OLRs) by Finne et al.

These are lesions that clinically and histologically resemble lichen planus but have an identifiable etiology. These reactions are presumably due to allergic or toxic reactions to compounds released or generated from the restoration.

It is recommended that patch tests should be performed in patients with OLR if the lesions are in close contact with amalgam fillings. Replacement of such restorations is recommended if there is a positive patch test reaction to mercury or components of amalgam and if there are no signs of concomitant generalized lichen planus.

Technical considerations

Manipulation of Amalgam

The clinical success of amalgam restorations is highly dependent on the correct cavity design and selection and manipulation of the alloy. If a restoration is defective, it is usually the fault of the operator and not the material.

Cavity Design

Providing retention Since amalgam does not adhere to tooth structure, proper design of the cavity is very important. The amalgam cavity is designed to provide maximum mechanical locking of the amalgam.

This is achieved by creating a cavity with walls that diverge towards the floor of the cavity (or converge towards the mouth of the cavity). This results in a cavity mouth that is narrower, effectively locking the amalgam within the cavity. Additional retention if needed can be obtained by placing pins within the cavity.

• Four-wall support For effective condensation, the cavity should have four walls and a floor. If one or more of the walls of the cavity is absent, a stainless steel matrix can compensate for the missing walls. Failure to have a four-wall support can result in inadequate condensation which can weaken the amalgam. Additional retention can be obtained with amalgam pins or screws.
• Preventing tensile fracture Since amalgam has poor tensile strength, the cavity should have sufficient depth and width in order to provide sufficient bulk to the amalgam, especially those in high-stress areas.
• Cavosurface angle The junction of the cavity with the external surface should be as close to a right angle as possible. Beveling is not indicated for amalgam as it can cause fracture of the amalgam at the margins.

Selection Of Materials

Alloy The alloy is selected based on clinical need.

• For restorations subjected to occlusal forces, an amalgam with high resistance to marginal fracture is desirable.
• If strength is needed quickly the best choice is spherical or high copper alloys, but they require a fast operator.
• A nonzinc alloy is selected in cases where it is difficult to control moisture.
• Indium-containing alloys Indium performs the same functions as zinc and in addition, it decreases the γ2 phase.

Mercury

There is only one requisite for dental mercury and that is its purity. Common contaminating elements such as arsenic, can lead to pulpal damage. A lack of purity may also adversely affect physical properties. High-purity mercury is labeled as ‘triple distilled’.

• Freezing point –38.87 °C
• Boiling point 356.9 °C

ADA Sp. No. 6 for dental mercury requires that the mercury should possess no surface contamination and less than 0.02% nonvolatile residue.

Dispensers

Because proportioning is important, manufacturers have developed some simple dispensers for alloy and mercury. Dispensing by volume is unreliable because it is affected by particle size and the degree of packing (trapped air and voids) in the dispenser.

Tablets

This is the most accurate method of dispensing. Manufacturers compress alloy powder into tablets of controlled weight which is used with measured amounts of mercury.

Preproportioned Capsules

Preproportioned capsules containing alloy powder and mercury in compartments separated by a membrane are available. They usually contain 400, 600, 800, or in rare cases 1200 mg of alloy powder with the corresponding proportion of mercury. Before use, the membrane is ruptured by compressing the capsule, and the capsule is then placed in a mechanical amalgamator.

Advantages

• Consistent proportioning.
• Low mercury/alloy ratio.
• Physical handling is not required thus reducing health hazards.

Disadvantages

Mercury and alloy may leak. The dentist is forced to use one alloy/mercury ratio for all situations when using disposable capsules. Also, the disposable capsules are expensive.

Mercury: Alloy Ratio (Proportioning)

Prior to mechanical triturators, when amalgam was triturated manually excess mercury had to be used in order to achieve smooth and plastic amalgam mixes.

This excess mercury was removed from the amalgam by

• Use a squeeze cloth to squeeze out the excess mercury.
• Increasing dryness technique During condensation of each increment, a mercury rich soft layer comes to the surface. This is removed by condensing excess amalgam and carving off the excess.

Eames’ technique

• The better method of reducing mercury content is to reduce the original mercury/alloy ratio. In 1959 Dr. Wilmer Eames proposed a 1:1 ratio of mercury: alloy. This is came to be known as the minimal mercury or Eames’ technique (mercury/alloy 1:1). (Prior to this manufacturers usually recommended higher mercury/alloy ratios of 6.5:5,7:5 and even 8:5 to ensure adequate amalgamation.) However, it is still necessary to squeeze mercury out of the mix using the increasing dryness technique. Hence, with this technique, 50% or less mercury will be in the final restoration, with obvious advantages.
• Mercury alloy ratios ranges from 43 to 54%. In preproportioned capsules, the mercury/alloy ratio is determined by the manufacturer and is usually less than 50%.
• Low mercury/alloy ratios are not easy to triturate manually. In order to benefit from a low mercury/alloy ratio a high-speed mechanical triturator (amalgamator) is absolutely essential.

Trituration

The objective of trituration is to wet all the surfaces of the alloy particles with mercury. For proper wetting, the alloy surface should be clean. Rubbing of the particles mechanically removes the oxide film coating on alloy particles.

Trituration is achieved either by

• Manually by hand
• Mechanical mixing

Manual mixing

• A glass mortar and pestle is used. The mortar has its inner surface roughened to increase the friction between the amalgam and the glass surface. A rough surface can be maintained by occasional grinding with carborundum paste. A pestle is a glass rod with a round end.
• The three factors to obtain a well-mixed amalgam mass are
  • The number of rotations,
  • The speed of rotation and
  • The magnitude of pressure placed on the pestle. Typically a 25 to 45-second period is sufficient.

Mechanical trituration

Mechanical amalgamators are more commonly used to triturate amalgam alloys and mercury.

• The disposable capsule serves as a mortar. Some capsules have a cylindrical metal or plastic piece in the capsule which serves as the pestle. The capsule is inserted between the arms on top of the machines. When switched on, the arms holding the capsule oscillate at high speed thus triturating the amalgam. Most amalgamators have hoods that cover the arms holding the capsule in order to confine mercury spray and prevent accidents.
• Reusable capsules are available with friction fit or screw-type lids. This type uses alloy in tablet form and capsulated mercury. At one time not more than two pellets of alloy should be mixed in a capsule.

• With either type, the lid should fit the capsule tightly, otherwise, the mercury can spray out from the capsule, and the inhalation of a fine mist of mercury droplets is a health hazard.
• Amalgamators have automatic timers and speed control devices. The speed ranges from 3200 to 4400 cycles per minute. High copper alloys require higher mixing speeds.
• Mixing time The mixing time can vary depending on the speed, oscillating pattern, and capsule designs. Spherical alloys usually require less amalgamation time than do lathe-cut alloys. A large mix requires a slightly longer mixing time than a smaller one. Manufacturer’s recommendations should be followed when determining mixing speed and time.

Advantages of mechanical trituration

• Shorter mixing time.
• More standardized procedure.
• Requires less mercury when compared to the hand mixing technique.

Under-triturated mix

• It is rough and grainy and may crumble.
• It gives a rough surface after carving and tarnish and corrosion can occur.
• Strength is less.
• Mix hardens too rapidly and excess mercury will remain.

Normal mix

• It has a shiny surface and a smooth and soft consistency.
• It may be warm (not hot) when removed from the capsule.
• It has the best compressive and tensile strength.
• The carved surface retains its luster after polishing, hence increased resistance to tarnish and corrosion.

Over-triturated mix

• The mix is soupy, difficult to remove from the capsule, and too plastic to manipulate.
• Working time is decreased.
• Results in higher contraction of the amalgam.
• Strength increases for lathe-cut alloys, whereas it is reduced in high copper alloys.
• Creep is increased.

Mulling

Mulling is actually a continuation of trituration. It is done to improve the homogeneity of the mass and get a single consistent mix. It can be accomplished in two ways

• The mix is enveloped in a dry piece of rubber dam and vigorously rubbed between the first finger and thumb, or the thumb of one hand and palm of another hand for 2–5 seconds.
• After trituration, the pestle is removed and the mix is triturated in the pestle-free capsule for 2–3 seconds.

Mulling is not required for mechanical triturated amalgams.

Condensation

The amalgam is placed in the cavity after trituration, and packed (condensed) using suitable instruments.

Aims

• To compact the mass to increase the density of the restoration.
• To reduce voids.
• To remove excess mercury.
• To adapt the amalgam to the preparation walls and margins.
  Proper condensation increases the strength and decreases the creep of the amalgam. Condensation must always be done within the four walls and floor. If one or more walls of the cavity are missing, a steel matrix may be used to compensate for it. Failure to use a matrix can result in a poorly condensed and weak restoration. Amalgam can also escape into the interdental space resulting in inflammation, bleeding, and pain.

Condensers

• Condensers are instruments with serrated tips of different shapes and sizes. The shapes are oval, crescent, trapezoidal, triangular, circular, or square. The condenser type is selected as per the area and shape of the cavity.
• The smaller the condenser, the greater is the pressure exerted on the amalgam. Condensation can be done manually or mechanically. For spherical amalgams, a large condenser tip should be selected to reduce punching through and improve condensation.

Manual condensation

The mixed amalgam is held in an amalgam well. The mixed material is packed in increments. Each increment is carried to the prepared cavity by means of small forceps or an amalgam carrier.

• Once inserted, it should be condensed immediately with sufficient pressure (approximately 3 to 4 pounds). Condensation is started at the center, and the condenser point is stepped sequentially towards the cavity walls.
• The smaller the condenser the greater the force. Serrated condensers are preferable. The shape of the condenser should conform to the area under condensation. A large circular condenser may be ineffective for cavity corners. For corners, a smaller point or a triangular or rectangular condenser is more effective.

• As the mix is condensed some mercury-rich material rises to the surface. Some of this can be removed, to reduce the final mercury content and improve the mechanical properties. The remainder will assist in bonding with the next increment.
• Modern amalgams are fast setting and so working time is short. Therefore, condensation should be as rapid as possible. A fresh mix of amalgam should be ready if condensation takes more than 3 or 4 minutes. Long delays between mixing and condensation, result in weaker amalgam.
• Spherical alloys have little ‘body’ and thus offer only mild resistance to condensation. When condensing these alloys, a larger condenser is recommended.

The cavity is overfilled so that the excess mercury-rich layer can be subsequently trimmed away during the carving process.

Mechanical condensation

Mechanical condensers provide a vibration or impact type of force to pack the amalgam mix. Less effort is needed than for hand condensation.

Shaping And Finishing

Precarve Burnishing

Some operators perform a preserve burnishing. This condenses and smooths the surface amalgam and reduces the voids and irregularities caused by the serrated condenser. It also removes some of the overfilled mercury-rich surface layer from the surface.

Carving

The restoration is carved to reproduce the tooth anatomy. Carving also removes the weaker mercury-rich surface layer. The carving should not be started until the amalgam is hard enough to offer resistance to the carving instrument.

A scraping or ringing sound should be heard when it is carved. If the carving is started too soon, the amalgam may be so plastic that it may pull away from the margins. Sharp carvers are used with strokes proceeding from the tooth surface to the amalgam surface. Various carving instruments.

Burnishing

After the carving, the restoration is smoothened, by burnishing the surface and margins of the restoration. Burnishing is the plastic deformation of a surface caused by sliding contact

• with another object. Fast-setting alloys gain sufficient strength by this time to resist rubbing pressure. Burnishing slow-setting alloys can damage the margins of the restoration.
• Burnishing is done with various types of burnishers using light strokes proceeding from the amalgam surface to the tooth surface. Final smoothing can be done by rubbing the surface with a moist cotton pellet.

Polishing

• Polishing minimizes corrosion and prevents the adherence of plaque. The polishing should be delayed for at least 24 hours after condensation, or preferably longer. Wet polishing is advised, so a wet abrasive powder in a paste form is used. Dry polishing powders can raise the temperature above 60°C.
• If the temperature rises above 60°C, mercury is released which may cause corrosion and fracture at the margins. High copper unicompositional alloys with high early strength may be polished at the same sitting after the materials have hardened sufficiently.
• However, polishing should be carried out delicately using soft abrasives and gentle pressure. A completed amalgam restoration.

Amalgam Bonding

• Amalgam restorations do not reinforce the teeth. Teeth with MOD cavities are susceptible to cuspal fractures. Bonding of the amalgam with a suitable adhesive (4-META) has been shown to improve the fracture resistance of the tooth (twice as much as nonbonded restorations).
• Amalgam bonding also reduces marginal leakage and postoperative sensitivity. The bonding mechanism is similar to that of resin bonding agents. The bonding agent penetrates the dentinal tubules and forms a hybrid zone.
• The bond with the amalgam is micromechanical through the formation of resin tags when amalgam is condensed into the uncured bonding agent. However, amalgam to amalgam bond is not so effective and therefore repair with bonding agents is not recommended. A variety of amalgam bonding agents are available commercially.

Mercury Toxicity

• Mercury is toxic to living creatures. Free mercury should not be sprayed or exposed to the atmosphere. This hazard can arise during trituration, condensation, and finishing of the restoration, and also during the removal of old restorations at high speed. Mercury vapors can be inhaled. Skin contact with mercury should be avoided as it can be absorbed.
• Any excess mercury should not be allowed to get into the sink, as it reacts with some of the alloys used in plumbing. It also reacts with gold ornaments.
• Mercury has a cumulative toxic effect. Dentists and dental assistants are at high risk. Though it can be absorbed by the skin or by ingestion, the primary risk is from inhalation.

Precautions

In placing and removing amalgam fillings, dentists should use techniques and equipment to minimize the exposure of the patient and the dentist to mercury vapor and to prevent amalgam waste from being flushed into municipal sewage systems

• The clinic should be well-ventilated.
• Use preproportioned capsules to control the mercury alloy ratio.
• The alloy mercury capsules should have a tightly fitting cap to avoid leakage.
• Whenever possible use protective barriers such as rubber dam isolation.
• High-volume evacuation should be used when condensing and carving amalgam.
• While removing old fillings, a water spray, mouth mask, and high-volume suction should be used.
• The use of an ultrasonic amalgam condenser is not recommended as a spray of small mercury droplets is observed surrounding the condenser point during condensation.
• All excess mercury and amalgam waste should be stored in well-sealed containers under fixer solutions.
• Amalgam scrap and materials contaminated with mercury or amalgam should not be subjected to heat sterilization.
• Proper disposal systems should be followed, to avoid environmental pollution. Amalgam separators should be used to capture the amalgam from the spittoon and the suction devices and to prevent amalgam waste from being flushed into public sewage systems
• Amalgam scrap should be disposed of or recycled through the appropriate agencies.
• Extracted teeth containing silver restorations should not be disposed of in the infectious waste and incinerated. Rather they should treated as hazardous amalgam scrap and disposed of accordingly.
• Spilled mercury is cleaned as soon as possible. Avoid carpeted floors in the operatory as it is extremely difficult to clean it from carpets. Vacuum cleaners are not used because they disperse the mercury further through the exhaust. Mercury suppressant powders are helpful but these are temporary measures. A mercury spill kit is commercially available for the management of mercury spills.

• Skin contact with mercury should be washed with soap and water.
• Annually, a program for handling toxic materials is monitored for actual exposure levels.

Amalgam Disposal

• Residual amalgam and empty amalgam capsules must be disposed of separately in special containers. These containers contain fixing salts or other chemical substances to bind mercury vapors. These containers must be disposed of according to legal requirements, returning them to the respective manufacturer. Local/regional instructions for waste disposal must be observed.
• Amalgam separators (ISO 11143:2008)
  • Dentists should use dental amalgam separators to catch and hold the excess amalgam waste coming from office spittoons. The amalgam separators separate the heavier metal particles from the liquid. Removal of old fillings and placement of new amalgam restorations are sources of amalgam contamination. Without dental amalgam separators, the excess amalgam waste will be released to the sewers via drains in the dental offices.
  • Many companies market amalgam separators. The unit acts as a collection tank allowing settling and decantation, thus separating liquids from solids. The maintenance is done on a periodic basis to decant off the liquid in your canister that will build up faster than the sediment. When full the heavy metal sediment unit is emptied into a separate container. The collected sludge is either shipped directly to a recycler or collected by an appropriate hauler who will ship it to a recycler.

• Extracted teeth containing amalgam fillings
  • Extracted teeth (not containing amalgam) are considered potentially infectious material and should be disposed into medical waste containers subject to the containerization and labeling provisions of the Occupational Safety and Health Administration (OSHA) blood-borne Pathogen Standard. However, extracted teeth containing amalgam should not be placed in a medical waste container that uses an incinerator for final disposal. Incinerating teeth releases mercury directly into the atmosphere. Many metal recycling companies will accept extracted teeth with amalgam. A recycler may be contacted and asked about their policies and any specific handling instructions they may have.
• How does amalgam waste affect the environment?
  • If improperly managed by dental offices, dental amalgam waste can be released into the environment. If an amalgam separator is not used, the excess amalgam waste will be released to the sewers via drains in the dental offices. Not all public sewage management facilities are equipped to manage hazardous waste like amalgam. At the treatment plant, the amalgam waste settles out as a component of sewage sludge that is then disposed of through various means.
    • In landfills thereby contaminating the surrounding land and water sources.
    • Through incineration, thereby contaminating the air.
    • Sludge when used as fertilizer contaminates agricultural land and water bodies and enters the food source.

Advantages And Disadvantages Of Amalgam Restorations

Advantages

• Reasonably easy to insert.
• Not overly technique-sensitive.
• Maintains anatomic form well.
• Has adequate resistance to fracture.
• Self-sealing; minimal-to-no shrinkage and resists leakage.
• Durable and long-lasting.
• Wears well and causes minimal wear of natural teeth.
• More economical than other alternative posterior restorative materials like cast gold alloys and composite.

Disadvantages

• The color does not match the tooth structure.
• They are more brittle and can fracture if incorrectly placed.
• Requires removal of some healthy tooth structures for cavity designing.
• They are subject to corrosion and may darken as it corrode.
• Corrosion products may stain teeth over time
• Galvanic action. Contact with other metals may cause occasional, minute electrical flow.
• They eventually show marginal breakdown.
• Temporary sensitivity to hot and cold because it is a metal.
• They do not bond to the tooth structure.
• Environmental mercury concerns.