Amalgam Flashcards
What Amalgam contain?
- Silver
- Tin
- Copper
- Zinc
- Palladium
- Indium
- Mercury
(STiCZPIM)
Definition of Dental Amalgam
When Mercury is mixed with silver tin alloy
Low copper Amalgam composition
- 67 -74% Silver
- 25 -28% Tin
- 0 - 6% Copper
- 0 - 2% Zinc
+ Mercury
High Copper Amalgam composition
- 40 - 70% Silver
- 21 - 30% Tin
- 12 - 30% Copper
- 0 - 1% Zinc
- 0 - 0.5% Palladium
- 0 - 4% Indium
+ Mercury
Function of Silver in Amalgam
Increases
- Strength
- Tarnish and Corrosion Resistance
- Expansion
Decreases
- Flow and Creep
- Setting Time
Definition of Amalgam
When mercury is mixed with another metal
Function of Tin in Amalgam
Increases
- setting time
- flow and creep
Decreases
- strength
- expansion
- corrosion resistance
Function of Copper in Amalgam
Increases
- Strength and hardness
- Tarnish and corrosion resistance
- expansion
Decreases
- Flow and Creep
- Setting Time
Function of Zinc in Amalgam
- Acts as Scavenger
- Decreases oxidation of other metals
Function of Mercury in Amalgam
Activates the reaction
Function of Palladium in Amalgam
Increases
- Tarnish and Corrosion Resistance
- Strength
Function of Indium in Amalgam
Increases
- Strength
- Expansion
- Setting time
Decreases
- Flow and Creep
- Surface Tension and thus reduces amount of Hg needed
How do you form Lathe-cut particles?
- Heat ingot to homogenise (400C for 8hrs)
- Cut particles from ingot
- Anneal particles at 100C
- Treat surface with acid to increase reactivity
How do you for Spherical Particles?
- Atomise molten alloy
- Quench
- Acid Wash to increase reactivity
Lathe-cut VS Spherical
Spherical
- Less mercury 40-45%
- Less mixing time
- Lower condensation pressures
- Harden quicker
- Smoother surface
What categories can you classify Dental Amalgams based on? 4
- Cu Content
- Zn Content
- Alloy
- Shape
Conventional Amalgams 2
- Lathe cut Ag3Sn - Spherical Ag3Sn
High Cu Amalgams (Admix) 2
- Lathe cut Ag3Sn + spherical Ag-Cu
- Lathe cut Ag3Sn + spherical Ag-Sn-Cu (or Ag-Cu-Pd)
High Cu Amalgams (Single shape) 2
- All spherical Ag-Sn-Cu
- Lathe cut Ag-Sn-Cu
Quaternary Alloy Amalgam 1
Ag-Sn-Cu-In
Setting Reaction Process
- Mercury initially dissolves the surface of the alloy particles
- New mercury with alloys is formed
- Need to condense before crystallisation of new alloys
- Reaction is never complete
- Set amalgam contains unreacted Ag3Sn in a matrix of the newly formed alloys
- A ‘cored structure’
Conventional Low Cu Alloys Setting
- Hg dissolves Ag and Sn from alloy
- New inter-metallic compounds formed
- Ag2Hg3 – γ1
- Sn7-8Hg – γ2
Gamma Phase (γ) Ag3Sn Properties 3
- Unreacted Alloy
- Strongest phase and corrodes 2nd least
- Forms roughly 30% of volume of set amalgam
Gamma 1 Phase (γ1) Ag2Hg3 Properties 4
- Matrix for unreacted alloy
- 2nd Strongest phase and corrodes least
- Forms roughly 60% of volume
- Fills gaps between Ag3Sn
Gamma 2 Phase (γ2) Sn7-8Hg Properties 4
- Weakest and softest phase
- Corrodes fast, voids form
- Forms roughly 10% of volume
- Volume decreases with time due to corrosion
Setting reaction for Admixed high copper alloy
- Hg dissolves Ag from Ag-Cu spherical eutectic particles
- Hg dissolves Ag and Sn from Ag3SN particles
Reaction
- γ + Hg -> γ + γ1 + γ2
- AgCu remains unreacted
Final Reaction for Admixed High Copper Alloys
Sn7-8Hg+ Ag-Cu → Cu6Sn5 + Ag2Hg3
Single Phase High Copper Setting Reaction
(γ + Cu3Sn) + Hg -> γ + Cu3Sn + γ1 + Cu6Sn5
Why does Single phase higher copper alloy corrode the least?
No γ2 in final set product
Structure of Single Phase High Copper Alloy
γ with Cu3Sn coating
Modern Amalgam Shrinkage process
- Initial shrinkage for roughly 20mins
- Expansion from growth of cyrstalline phases
- More Mercury = More expansion
- Stable after 6-8 hours
OVERALL NET SHRINKAGE
Reasons for higher shrinkage
- Particle Size Smaller
- Higher Hg/Alloy Ratio
- Trituration time longer and more rapid
- Condensation pressure higher
Visual properties of Properly triturated Amalgam
- Shiny
- Separates from capsule in a single mass
What is condensation?
Packing incrementally into the cavity
3 types of finishing
- Carving
- Burnishing
- Polishing
What Element causes Delayed Expansion?
- Zn
Zn + H2O -> ZnO + H2
What compound is Tarnish?
Black Silver Sulphide
No long term Problems
Order of corrosion in pure phases?
- γ1
- γ
- Ag3Cu2
- Cu3Sn
- Cu6Sn5
- γ2
Types of corrosion in Amalgam 3
- Galvanic Corrosion
- Crevice Corrosion
- Stress Corrosion
Mechanical Properties of Amalgam
- High Compressive Strength
- Good Wear Resistance
- Mercury content must be < 50% for optimum results
Disadvantages of Dental Amalgams
- Non Adhesive (relies on mechanical retention)
- Conducts Heat
- Corrosion and Tarnish
- Poor Aesthetics
- Brittle
- Viscoelastic (creep)
What is marginal breakdown?
Thin layers of amalgam fracture easily as it is brittle
Known as ‘Ditching’
How does the manufacturer affect the quality of Amalgam?
- Alloy Composition
- Heat Treatment
- Size, Shape and production method of particles
- Particle surface treatment
- Where it is supplied from
- Mercury:Alloy Ratio
How does the dentist affect the quality of Amalgam?
- Trituration Procedures
- Cavity Design
- Condensation Technique
- Marginal Integrity
- Anatomy
