Dental Amalgam Manufacturing Process

Amalgam

• Amalgam is any alloy containing mercury.
• Dental amalgam alloy is an alloy made from silver, tin, and copper. Zinc is sometimes added.
• Dental amalgam is produced by mixing dental amalgam alloy with mercury at room temperature. This gives a plastic mix that's placed into a cavity.

Composition of dental amalgam alloy

• Dental amalgam alloy's composition includes silver, tin, copper, and sometimes zinc.
• Silver constitutes 69% of the alloy.
• Tin makes up 18% of the alloy.
• Copper comprises 12% of the alloy.
• Zinc accounts for 1% of the alloy.

Composition of dental amalgam

• The main role of silver is to control the reaction between silver and mercury.
• Tin controls the reaction between silver and mercury.
• Copper plays a role in the mix of the restoration.
• Mercury is the liquid that makes up the plastic mix of the restoration.

Role of zinc

• Zinc acts as a scavenger (deoxidizer).
• Zinc improves plasticity during condensation.

Classification of dental amalgam

• Classification is based on particle shape.
  • Lathe cut (irregular)
  • Spherical
  • Spheroidal
• Classification is based on copper content.
  • Low copper
  • High copper
  • Admixed
  • Unicompositional
• Classification is based on zinc content.
  • Zinc containing
  • Zinc free

Percentage composition of dental amalgam alloy

• The percentage composition of silver, tin, copper, and zinc in low copper and high copper amalgam is presented.

Manufacturing of dental amalgam

• Ingot production: The constituent elements are melted and cast into a cylindrical ingot (3.8 x 25 cm). Then it is slowly cooled under 480°C.
  • An intermetallic compound Ag3Sn(γ phase) is produced.
• Homogenous heat treatment: The cored ingot is subjected to homogenization heat treatment to improve mechanical properties and corrosion resistance. This is done by heating to 400°C for 24 hours.
• Powder production:
  • Lathe cut: The ingot is cut or milled into irregular particles.
  • Spherical: The constituent elements are melted and atomized (sprayed) into inert gas.
  • Spheroidal: The constituent elements are melted and atomized (sprayed) into water.
• Aging or annealing heat treatment:
  • Aging: Store the powder at room temperature for several months.
  • Annealing: Heat the powder at 100°C for 1-6 hours.
  • Spherical & spheroidal require homogenization heat treatment only; they don't need aging/annealing.

Advantages of spherical particles over lathe cut

• Lower surface area, requiring less mercury to obtain a homogenous mix.
• Produces amalgam with superior properties and increases corrosion resistance.
• Produces a smooth surface during carving and finishing.
• Requires less condensation pressure.

Amalgamation reaction

• Amalgamation reaction involves a series of phases (γ, γ1, γ2, η, ε) with their compositions.
• Low copper amalgam: The powder is mainly Ag3Sn (γ Phase).
  • Wetting: liquid mercury is added and triturated.
  • Diffusion: mercury diffuses into silver and tin.
  • Surface reaction: mercury reacts with silver and tin leading to new phases.
• The new phases increase over time, hardening the mix.
• The new phases surround the unreacted parts of the mix.

Setting reaction

• The setting reaction occurs in two steps:
  • Amalgamation reaction
  • Solid-state reaction
• In low-copper amalgam, Ag3Sn (γ) + Hg yields Ag2Hg3 (γ1)+ Sn8Hg2 (γ2).
• Unreacted Ag3Sn (γ) and voids are left behind.
• In solid state reaction, Sn8Hg(γ2) elimination between Ag-Cu (eutectic) and Sn8Hg (γ2) occurs.
• Ag2Hg3 (γ1) and a new phase Cu6Sn5 (η) are created.

Microstructure

• Cored structure: Formed from unreacted Ag3Sn (r) and Sn8Hg (82) surrounded by a matrix of Ag2Hg3 (81), with voids.

Properties of dental amalgam phases

• Ag3Sn (γ) phase: Strongest & most resistant to corrosion.
• Ag2Hg3 (81) phase: Less strong and less resistant to corrosion.
• Sn8Hg (82) phase: Weakest and least resistant to corrosion.
• Excess mercury reduces strength and corrosion resistance.
• Elimination of Sn8Hg (r2) phase improves amalgam properties.

High copper amalgam

• Increasing copper content eliminates Sn8Hg (82) phase and improves properties.
• Copper content varies from 13-30%.

Admixed high copper amalgam

• The copper content increases by mixing silver-copper (Ag-Cu) eutectic alloy and silver-tin Ag3Sn (r) alloy particles.

Setting reaction (steps)

• Amalgamation reaction (like low copper).
• Solid-state reaction.

Unicompositional high copper amalgam

• The amalgam alloy powder is one particle type with a copper content of 13-30%.
• Each particle contains Ag3Sn (r) and Cu3Sn (ɛ).

Microstructure (Unicompositional)

• Matrix of Ag2Hg3 (γ1) surrounds unreacted particles of [Ag3Sn (γ) + Cu3Sn (ε)].
• Cu6Sn5 (η) phase is in a rod-shaped structure, surrounding unreacted particles.
• Cu6Sn5 (η) crystals are embedded in the Ag2Hg3 (r1) phase.

Manipulation

• The presentation modes of amalgam include tablets & mercury, preproportioned capsules, and powder & mercury.

• Amalgam alloy selection depends on factors like particle shape, particle size, zinc content, and copper content.

• Proportioning techniques include wet and dry methods (Eam's technique).

• Using preproportioned capsules is now common.

• Excess mercury results in decreased strength and increased setting expansion/creep.

  • Less mercury results in a flawed/friable mix and reduced strength.
  • Proper trituration leads to a shiny surface/soft mix with best strength and resistance.
  • Over/under trituration negatively affects working time and properties.

• Mulling: Rubbing triturated mix into rubber finger-stall.

• Condensation: Packing triturated amalgam mix into cavity with increment-by-increment condensation.

• Objectives of condensation: Increase restoration adaptation to the cavity wall, reduce mercury content (transfer it to the surface), and create a strong, compact mass.

  • Not performed after 3 minutes to avoid reducing strength and adaptation.

• Delayed condensation reduces strength, worsens adaptation, and diminishes bonding between increments.

• Carving: The cavity is filled; then the top mercury-rich layer is carved,

  • Carving objectives: Reproducing tooth anatomy, removing the top mercury-rich layer.

• Finishing & polishing: Using finishing burs to finish and polishing with rubber cups and pumice to enhance the surface and increase patient acceptance.

  • This is best performed 24 hours after conventional low-copper amalgam and shortly after high-copper amalgam insertion.

Properties

• Dimensional changes (immediate and delayed).

  • Contraction: Leads to leakage, recurrent caries, hypersensitivity.
  • Expansion: Causes tooth fracture or protrusion.

• Flow and Creep:

  • Flow: Time-dependent plastic deformation during setting (before restoration fully sets).
  • Creep: Time-dependent plastic deformation after setting (from below-yield strength stress application).
  • Causes: Amalgam's viscoelastic nature, stresses in the mouth, and oral temperature being close to amalgam's softening temperature.
  • Significance: Marginal breakdown, gingival irritation, contact area issues.
  • Methods for reduction: Using high-copper alloy composition, decreasing mercury content, and proper condensation,
  • Strength: Amalgam builds strength slowly, reaching 80% of its final strength within an hour. The immediate compressive strength is more crucial than ultimate strength to prevent fracture due to patient biting. Amalgam is brittle (strong in compression, weak in tension). It's viscoelastic, sensitive to loading rate.

• Maximizing strength in an amalgam restoration through:

  • Proper cavity preparation (acceptable depth and width to maintain bulk).
  • Using a 90-degree cavo-surface angle to prevent thin restoration.
  • Rounding sharp line angles to reduce stress concentration.
  • Insulating bases to prevent heat transfer.
  • Selecting high-copper amalgam, decreasing mercury ratio, and proper trituration.
  • Performing appropriate finishing and polishing.

• Bond to tooth structure: Dental amalgam is bonded to tooth structure through macro-mechanical retention, with some amalgam bonding systems now in use.

• Biological properties: Important aspects include corrosion, thermal properties, and mercury toxicity.

  • Corrosion: Results in reduced strength and metallic product release.
    • Reducing corrosion is done by selecting copper-enriched alloy composition, controlling mercury-alloy ratio, ensuring appropriate condensation, and proper surface finishing and polishing.
  • Thermal properties: Amalgam conducts heat; an insulating base is essential for deep cavities to prevent thermal sensitivity.
  • Mercury toxicity: Pure mercury has a high vapor pressure at room temperature; amalgam is considered highly toxic at 4000 mm/kg of body weight.

Mercury hygiene

• Using no-touch technique (e.g., for spills)
• Cleaning mercury spills immediately.
• Storing amalgam scrap under sodium thiosulfate.
• Using water coolant and suction when removing old amalgam restorations.

Test your knowledge on the intricacies of dental amalgam, including its composition, manufacturing process, and properties. This quiz covers critical aspects such as zinc's role, the types of amalgam, and the effects of heat treatment. Perfect for dental students and professionals seeking to reinforce their understanding.