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DENTAL AMALGAM

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INDEX

Definitions
History
Classification
Composition
Effects of components

Manufacture of amalgam alloys
Setting reaction
Microstructure
Properties
Technical considerations

Clinical considerations
Uses of amalgam
Advantages
Disadvantages
References

Definitions
Amalgam: Alloy of mercury with one or more metals.
Amalgam alloy: Alloy which combines with mercury to form amalgam.
Dental amalgam alloy: Alloy that is combined with mercury to form amalgam used for dental purposes.
Amalgamation: Setting reaction of amalgam alloy with mercury.
Trituration: The act of mixing amalgam alloy with mercury.
Condensation: The process of placing the plastic amalgam mass into the tooth cavity and applying forces on it to adapt amalgam to the cavity walls.
Burnishing: The act of smoothening the surface and margins of amalgam after condensation.

Back to top History
Amalgam was first used by the Chinese. There is a mention of Silver-Mercury paste by Su Kung (A.D. 659) in the Chinese materia medica.

Back to top Classification of Amalgam Alloys

According to content

  1. Silver Amalgam: Silver more than 65%.
  2. Copper Amalgam: 70% Hg and 30% Cu.
  3. Preamalgamated alloys: Contain less than 3% of Hg.
  4. Noble metal amalgam alloys: Contain Au and/or Pd.

According to presence or absence of Zinc

  1. Zinc-containing alloys: More than 0.01% Zn.
  2. Zinc-free alloys: Less than 0.01% Zn.

According to Copper content

According to number of metals in the alloy

  1. Binary alloy: Ag; Sn
  2. Ternary alloy: Ag; Sn; Cu
  3. Quarternary alloys: Ag; Sn; Cu; In.

According to the shape of alloy particles

  1. Spherical (Smooth shaped spheres)
  2. Spheroidal (Irregular shaped spheres)
  3. Lathe-cut (Irregular shavings or filings)

According to development of Amalgam alloys

  1. 1st generation amalgam alloys
  2. 2nd generation amalgam alloys
  3. 3rd generation amalgam alloys
  4. 4th generation amalgam alloys
  5. 5th generation amalgam alloys
  6. 6th generation amalgam alloys

Back to top Composition of Amalgam Alloys

Low Copper Alloys

Silver 68-72%
Tin 26-28%
Copper 02-04%
Zinc 00-02%

Admixed Alloys
1/3rd lathe cut eutectic alloys, either

Silver 71.9%
Copper 28.1%
or
Silver 62%
Copper 28%
Palladium 10%

with 2/3rd lathe-cut or spherical low Copper alloy.

Ternary Alloys

  Sybraloy Tytin
Silver 40% 60%
Tin 30% 27%
Copper 30% 13%

Quarternary Alloys

Silver 40-60%
Tin 22-35%
Copper 13-30%
Indium 5%

Spherical particles of a ternary Ag-Sn-Cu alloy with lathe-cut particles containing Ag3Sn or Ag-Sn-Cu has been called as Hybrid System. E.g., Arjalloy, Contour, Oralloy, Permitec, Vivalloy HR.

Back to top Effects of Various Components of Amalgam Alloys

SILVER COPPER TIN
Increases Strength Increases Strength Decreases Strength
Increases Expansion Increases Expansion Decreases Expansion
Decreases Flow Decreases Flow Increases Flow
Decreases Setting time Decreases Setting time Decreases Setting time
Increases Corrosion resistance Increases Corrosion resistance Decreases Corrosion resistance
  Decreases Plasticity Increases Plasticity
  Increases Hardness  
  Increases Brittleness  

ZINC INDIUM GOLD
Increases Strength Increases Strength Increases Strength
Increases Expansion Increases Expansion Increases Corrosion resistance
Increases Flow Increases Flow  
Increases Setting time Increases Setting time MERCURY
Decreases Corrosion resistance Amalgamation more difficult Decreases setting time
Increases Plasticity Deoxidiser Decreases delayed expansion
Decreases Hardness  
Decreases Brittleness    
Scavenger    

Mercury is used in less than 3% in pre-amalgamated alloys. Mercury is incorporated by washing alloy particles with Mercuric Chloride. To remove excess chloride on the surface of the particles, the alloy particles are washed in acid waters. These acid waters remove zinc on the surface of the alloy particles along with chloride. Since zinc is responsible for delayed expansion, its removal results in reduced delayed expansion in pre-amalgamated alloys.

Note: Gallium may be used instead of Hg to form Cu-Ga-Sn intermetallic compound. These Silver-Gallium alloys are still in experimental stage. More about these later!

Back to top Manufacture of Amalgam Alloys

Filings: Filings are irregularly shaped particles. Ingredients are melted and poured into a mould of 3.8 cm diam and 20-25 cm length. An ingot is obtained by cooling which is then heated to 400C for 6 to 8 hrs for homogenization. Ingot is then lathe cut or ball milled. Cut particles are then passed through a fine sieve of 100 mesh. Ageing is performed by heating to 60-100 C for 1 to 6 hrs. To make the particles more reactive, surface treatment with acid waters may be done. Size of the particles vary from 28-35µ.
Spherical particles: These are produced by atomizing the molten alloy in a closed chamber filled with inert gas. Droplets of alloy solidify into spheres as they fall through the gas to the floor of the chamber. The solidified spheres are then heat-treated and acid washed. Size of spheres may vary from 2-4µ to 25-35µ.

Back to top Setting Reaction

This is a process by which liquid Hg reacts with dental amalgam alloy particles to produce a matrix of intermetallic compounds of Hg with metals of the alloy.

Low Copper alloys

Ag3Sn Hg Ag2Hg3 Sn7Hg8 Ag3Sn
Excess γ phase       γ1 Phase   γ2 Phase   Unreacted γ Phase
                 

Admixed alloys

Reaction 1
Ag3Sn Ag3Cu2 Hg Ag2Hg3 Sn7Hg8 Ag3Sn Ag3Cu2
Excess γ phase   Silver-Copper Eutectic       γ1 Phase   γ2 Phase   Unreacted γ Phase   unreacted Eutectic phase
Reaction 2
Ag3Cu2 Sn7Hg8     Ag2Hg3 Cu6Sn5        
                η (eta) phase        

The second reaction occurs at mouth temperature for 1-2 weeks and γ2 phase is thus eliminated. The matrix is formed by γ1, Cu3Sn ("ε" epsilon)and η phases.

Single composition alloys

Ag3Sn Cu3Sn Hg Ag2Hg3 Cu6Sn5 Ag3Sn Cu3Sn
Excess γ phase   Excess ε phase       γ1 Phase   η (eta) phase   Unreacted γ phase   Unreacted ε phases

Here no γ2 phase is formed.

Theory behind the setting reaction of unicompositional alloys
Solubility of Hg in Cu is 1 mg, in Ag 10 mg, and in Sn 170 mg. Since the solubility of Hg is more in Sn, the Sn on the surface of the alloy particles will be depleted by the formation of γ2 phase, while the percentage of Cu will relatively increase as a result of limited reaction with Hg. Therefore alloy particles are surrounded by γ1 and γ2 phases, whereas the periphery of the alloy particle becomes an eutectic alloy of Ag and Cu. As with admixed alloys, this Ag-Cu phase reacts with γ2 phase to form η phase and more γ1 phase, eliminating γ2 phase. So here the alloy particles function like Ag-Sn alloy initially providing sufficient working time and ease in manipulation.

Sixth generation amalgam alloys

Back to top Microstructure of Amalgam

The set mass of amalgam consists of unreacted particles of gamma (γ) and epsilon (ε) phases surrounded by a matrix of reaction products (γ1, γ2 and η phases). The reaction between the two constituent materials is a rapid amalgamation of the outer layers of alloy particles. When once these layers have formed, further amalgamation proceeds at a slower rate. This surface alloying is a solution of amalgam alloy in mercury, and is accompanied by reduction in total volume of metals. Crystallization of the new phases from amalgamated alloy occurs, while at the same time solution continues inwards towards the centre of the alloy particles. These two conflicting processes of solution and crystallization continue until the formation of new phases stifles the solution process.

Back to top Properties Of Dental Amalgam

The properties that would be discussed are:

A. Dimensional Change

Expansion that occurs due to reaction of Hg with alloy components is termed primary expansion or mercuroscopic expansion. Expansion that occurs after 1 to 7 days due to moisture contamination during trituration or condensation before the amalgam mass is set, is termed secondary expansion or delayed expansion.

There is an initial volumetric contraction due to reduction in total volume of alloying elements. But as crystallization of various phases occurs, the impinging of crystals against each other results in expansion. Release of mercury from γ2 phase during corrosion results in additional crystallization of phases on reaction with unreacted γ phase, causing further expansion. This is also termed mercuroscopic expansion.

  1. Components: Increased γ phase or β phase increased expansion; Increased traces of Tin, decreased expansion
  2. Particle size: Decreased size, there is contraction initially (due to increased surface area/ unit volume and increased dissolution of Hg) but later expansion (due to outward thrust of forming crystals).
  3. Particle shape: Smoother shape (as in spherical type) there is better wetting with Hg causing in faster amalgamation resulting in contraction.
  4. Hg/Alloy ratio: Increased Hg/Alloy ratio Increased expansion (mercuroscopic)
  5. Trituration: Rapid trituration and longer trituration within limits results in contraction because of
  6. Condensation: Increased condensation pressure causes closer contact of Hg with alloy particles and squeezing of excess Hg from the mix resulting in contraction.
  7. Moisture contamination: Alloys containing Zn, if contaminated with moisture before amalgam is set, may evince delayed (or) secondary expansion. This is due to release of H2 gas within the restoration creating an internal pressure of nearly 2,000 psi. Since the gas cannot escape out, it causes expansion of the restoration.The gas is formed as follows:

    Zn + H2O ZnO + H2

Effects of dimensional change

Expansion >> 4%

Contraction >> than 50/cm

Back to properties B. Strength

The approximate values of compressive and tensile strengths of different types of amalgam are as follows:

Compressive Strength (psi) Tensile strength (psi)
30 min 1 hour 1 day 7 days 7 days

Low Copper Alloys

  • Fine cut
  • Spherical
7,500
15,000
13,000
24,500
62,000
50,000
  8,700
7,250

High Copper Alloys

  • Admixed
  • Unicompositional

10,000
16,000

16,000
27,000
58,000
65,000
62,600
74,000
7,250
9,300

Factors affecting strength of Dental Amalgam

  1. Particle size: Decreased size results in increased strength (due to increased surface area / unit volume)
  2. Particle shape: Regular uniform shape result increased strength (due to more wettability, more coherent mass, less interrupted coherent interphases)
  3. Microstructure of amalgam:
  4. Porosities and voids in amalgam: Decreased strength. Formed due to:
  5. Hg/Alloy ratio: Increased Hg/Alloy ratio, decreased strength, because increased Hg results in
  6. Trituration
  7. Condensation pressure Increased pressure results in increased strength (due to removal of excess Hg within amalgam resulting in less residual Hg)
  8. Temperature Amalgam loses 15% of its strength when its temperature is increased from room temperature to mouth temperature. It loses 50% of its strength when temperature is elevated beyond 60C (as in overjealous polishing).
  9. Corrosion activity: Decreased corrosion activity results in increased adhesive integrity and therefore increased strength.

Back to properties C. Creep

The values of creep of various amalgam alloys are as follows:

Creep (%)

Low Copper Alloys

  • Fine cut
  • Spherical

02.6
0.8 -1.5

High Copper Alloys

  • Admixed
  • Unicompositional

0.44%
0.05-0.09

Creep occurs because of grain boundary sliding. η crystals on γ1 grains prevent grain boundary sliding and therefore are responsible for decreased creep values of high copper alloys. Higher creep is associated with flow of amalgam over cavity margins which is thin and easily fractures under occlusal stress ("ditched amalgam").

Factors affecting Creep

  1. Microstructure of amalgam
  2. Hg/Alloy ratio Increased Hg/Alloy ratio, increased creep (due to more residual Hg)
  3. Trituration
  4. Condensation pressure Increased pressure, decreased creep (due to less residual Hg)
  5. Delay between trituration and condensation Increased creep

Back to propertiesD. Modulus of elasticity

At low rates of loading, the elastic modulus of amalgam is 11-20 X 103 MN/m2. At high rates of loading, the elastic modulus is 62 X 103 MN/m2

E. Resistance to corrosion

Passive layer of chlorides, sulphides, and /or oxides seen on amalgam surface in unhygienic mouths. Electrolytic corrosion of dissimilar portions of the filling. Corrosion products that form at the margins over a period serve to seal the marginal gaps. Therefore marginal integrity of amalgam restorations improves with time. Corrosion resistance of various phases in descending order are as follows:

Back to top Technical Considerations

A. Selection of the Alloy: Various factors to be considered are:

  1. Particle size: Smaller size rapid hardening, high early strength, smoother surface.
  2. Particle shape: Whether lathe cut or spherical .
  3. Presence or absence of zinc: Responsible for delayed expansion.
  4. High Cu or Low Cu: High Cu alloys have little or no γ2 phase, which is the weakest phase.
  5. Rate of hardening: Affects working time.
  6. Smoothness of the mix.
  7. Ease of condensation.
  8. Economical factor: Cheaper brand if all the other factors equal.

Differences between Lathe cut and Spherical alloys

LATHE CUT
SPHERICAL
  • Require more mercury (50%)
  • Require more condensation force
  • Overtrituration increases strength
  • Undertrituration decreases creep
  • Require smaller condenser points
  • Less ease in carving and burnishing
  • Require less mercury (42%)
  • Require less condensation force
  • Overtrituration decreases strength
  • Undertrituration increases creep
  • Require broader condenser points
  • Smooth surface during carving & burnishing

B. Selection of Mercury
Should have less than 0.02% of non-volatile residue.

C. Mercury Alloy ratio

  1. Lathe cut alloys 1:1 or Eames ratio (50% Hg)
  2. Spherical alloys 40.0% Hg
  3. High copper alloys 43.0% Hg
  4. Low copper alloys 53.7% Hg

Methods of Dispensing Alloy and Hg

Methods to reduce excess residual Hg

D. Size of the mix
Depends on the cavity. Usually 400mg to 800 mg alloy with adequate amount of Hg.

E. Trituration

Objectives:

  1. To dissolve Hg in alloy particles so as to obtain a plastic mass of amalgam which can be condensed into the cavity. This also achieves a workable mass with sufficient working time.
  2. To remove oxide film on the surface of the alloy particles.
  3. To pulverize the alloy particles for proper wetting by Hg (by decreasing the particle size and increasing the surface area and thus increasing the wettability).

Methods:

Factors affecting trituration

Mulling: After the amalgam alloy and Hg are mixed for a specific time in the mechanical amalgamator, the pestle is removed and the mix left in place within the capsule and the amalgamator is turned on for a period of 2 to 3 seconds. This result in a homogenous mix of the amalgam mass. The improved coherence of the mix makes it possible to remove the mass as a whole from the capsule. This process is called mulling. Mulling can be also performed by kneading the plastic amalgam mix in a piece of rubber dam.

F. Condensation

Objectives:

  1. To condense unattacked gamma particles closely together (to increase strength and decrease creep).
  2. To adapt amalgam to the cavity walls.
  3. To remove excess Hg.
  4. To bring Hg on the top of each increment so as to bind the increments to one another (increasing dryness technique).
  5. To increase the density of the restoration by development of an uniform compact mass with minimal voids.
  6. To increase the rate of hardening so that carving operation need not be unduly delayed.

Methods:

Condensation pressure: 3 to 4 lbs. For this pressure, force at the tip of the condenser point of 2 mm diameter is around 600 -800 psi. For 10 lbs of pressure, the force at the tip of the condenser point of 2 mm diameter is 2,000 psi. However this 10 lbs pressure is manually not possible.

G. Burnishing

Objectives:

  1. To further decrease the size and number of voids.
  2. To express excess Hg on the surface of the amalgam restoration.
  3. To adapt amalgam to the cavosurface anatomy.
  4. To condition surface of the amalgam for carving.

Method: Performed using Beaver tail burnisher or Sprately burnisher.

H. Carving

Objectives:To produce a restoration with

  1. No underhangs (no shouldering or shelving).
  2. Proper physiological contours.
  3. Minimal flash (no overhangs).
  4. Functional, non-interfering occlusal anatomy.
  5. Adequate, compatible marginal ridges.
  6. Proper size, location, extent and inter-relationship of contact areas.
  7. Physiologically compatible embrasures.
  8. No interference with integrity of periodontium.

Method: Performed by using various varieties of amalgam carvers available ( like Hollenback's carver). Carving is always from the tooth surface to the restoration surface. This is done to avoid removal of amalgam at the margins.

I. Finishing and Polishing

Objectives:

  1. To remove amalgam flash that has been left behind during carving.
  2. To remove overhangs, major sometimes.
  3. To correct minor enamel underhangs.
  4. To convert superficial amalgam into a relatively inert layer galvanically, to decrease electrolytic corrosion.
  5. To remove superficial scratches and irregularities: decreases fatigue failure, decreases concentration cell corrosion and decreases accumulation or adherence of plaque.
  6. To make the restoration aesthetically more appealing.

Method: Performed with extrafine silex, slurry of tin oxide, or pumice-wet mix in a paste form to avoid heat generation (If temperature is >> 60C, release of Hg occurs).

Back to top Clinical Considerations

  1. Ditched Amalgam: Fracture of amalgam at the margins.

    Causes are:

2. Marginal leakage: Gap formed between the wall of the cavity and the restored amalgam because of the contraction of the filling material. The gap formed is a potential area for food impaction and plaque accumulation, which in turn result in secondary (recurrent) caries. As the restoration ages, deposition of corrosion products in the gap aid in sealing the margins.

3. Corrosion: If the amalgam surface is not well polished, the rough surface not only attracts plaque but may also undergo crevicular corrosion.

Back to top Uses of Amalgam

  1. As a filling material for Class I and Class II cavities.
  2. Can be used for Class V cavities of posterior teeth.
  3. Sometimes can be used for cuspal restorations (with pins usually).
  4. As a core build-up material prior to cast restoration.
  5. As a retrograde filling material.
  6. In combination with Composite resin for cavities in posterior teeth. Resin veneer over amalgam.
  7. As a die material.

Back to top Advantages of Amalgam

  1. Relatively inexpensive.
  2. Easy to manipulate.
  3. Restoration is completed within one sitting without requiring much chair time.
  4. Well-condensed and triturated amalgams have good compressive strengths.
  5. Sealing ability improves with age by formation of corrosion products at tooth-amalgam interface.
  6. Relatively not technique sensitive.

Back to top Disadvantages and Failure of Amalgam Restorations

  1. Marginal breakdown and fracture.
  2. Tarnish and corrosion.
  3. Unnatural appearance (not aesthetic).
  4. Metallic taste and Galvanic shock.
  5. Marginal leakage.
  6. Discolouration of the tooth structure.
  7. Lack of chemical or mechanical adhesion to the tooth structure.
  8. Mercury toxicity.
  9. High rate of secondary caries.
  10. Thermal conductivity.
  11. Promotes plaque adhesion.
  12. Delayed expansion.

Back to top REFERENCES

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These web pages created and maintained by
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Dr R. V. Subramanyam
Professor, Dept. of Oral Pathology
College of Dental Sciences, Davangere 577 004
Karnataka, India.

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