Dental Amalgams Flashcards
Define amalgam
When mercury is mixed with another metal
What metal is mercury mixed with in dental amalgams?
Mercury + silver tin alloy
List the chemical symbols for the following;
- Silver
- Tin
- Copper
- Zinc
- Palladium
- Indium
- Mercury
- Ag
- Sn
- Cu
- Zn
- Pd
- In
- Hg
What is the composition of conventional amalgam? (5)
What intermettalic compound is found in the composition?
- Silver (67-74%)
- Tin (25-28%)
- Copper (0-6%)
- Zinc (0-2%)
- Mercury
(Alloy contains gamma phase Ag3Sn An intermetallic compound)
What are other names for conventional amalgam? (2)
- Traditional amalgam
- Low copper amalgam
What is the composition of high copper amalgam? (7)
- Silver (40-70%)
- Tin (21-30%)
- Copper (12-30%)
- Zinc (0-1%)
- Palladium (0-0.5%)
- Indium (0-4%)
- Mercury
Give an example of a single ternary alloy used in high copper amalgams?
What are its components?
- Tytin
- Ag-Sn-Cu
What are the components of a quaternary alloy used in high copper amalgams?
Ag-Sn-Cu-In
What does silver do in dental amalgams?
Increase (3)
Decreases (2)
- Increases strength
- Increases tarnish and corrosion resistance
- Increases expansion
- Decreases flow and creep
- Decreases setting time
What does tin do in dental amalgams?
Increase (2)
Decreases (3)
- Increases setting time
- Increases flow and creep
- Decreases strength
- Decreases tarnish and corrosion resistance
- Decreases expansion
What does copper do in dental amalgams?
Increase (3)
Decreases (2)
- Increases strength + hardness
- Increases tarnish and corrosion resistance
- Increases expansion
- Decreases flow and creep
- Decreases setting time
What is the function of zinc in copper amalgams?
Acts as a scavenger
decreases oxidation of other metals
What is the function of mercury in copper amalgams?
Activates reaction
What does palladium increase in copper amalgams? (2)
- Increases strength
- Increases tarnish and corrosion resistance
What does indium do in dental amalgams?
Increase (3)
Decreases (2)
- Increases strength
- Increases setting time
- Increases expansion
- Decreases flow and creep
- Decreases surface tension and so the amount of mercury required is less
When the alloys for the amalgams are ‘lathe-cut’ what process need to occur to form the gamma phase of the alloy?
What are the alloy particles cut from?
At what temperature do particles anneal?
What is surface treated with to increase reactivity?
- Needs homogenising to form gamma phase
(400°C for up to 8 hours) - Particles cut from ingot
- Annealed at 100°C to relieve internal strains
- Surface treated with acid to increase reactivity
How are spherical alloy particles produced?
What is surface treated with to increase reactivity?
- Spherical particles are produced by atomising molten alloy in an inert atmosphere
- Acid washed to increase reactivity
What are the advantages of using spherical particles over lathe-cut? (5)
- Require less mercury
- Require less mixing time
- Require lower condensation pressures
- Harden more rapidly
- Smoother surface for restoration
What are the new mercury intermetallic compounds formed after mixing of low copper amalgam? (2)
- Ag2Hg3
- Sn7Hg
What is 30% of set low copper amalgam formed of?
Unreacted Ag3Sn
strongest phase
What is 60% of set low copper amalgam formed of?
Ag2Hg3
2nd strongest phase
What is 10% of set low copper amalgam formed of?
Sn7Hg
weakest phase
Explain the series of dimensional changes of amalgams
- Initial shrinkage due to alloy dissolution
- Expansion occurs from growth of crystalline phases
- Stable after 6-8 hours
How does mercury content affect expansion of amalgams?
The higher the mercury content the higher the expansion
What factors of dental amalgams increase shrinkage? (4)
- Smaller particle size
- Higher Hg:Alloy
- Trituration time longer and more rapid
- Condensation pressure higher
What is a Trituration?
- Mechanical mixer
- 3000 rpm
- 5-20 seconds
What is the gamma phase?
Ag3Sn
What is epsilon phase?
Cu3Sn
What is gamma 2 phase?
Sn7-8Hg
What is eta phase?
Cu6Sn5
If amalgam is under-trituated what would you see?
Dull, dry and crumbly mix
If amalgam is over-trituated what would you see?
Shiny, hot, wet mix, sticks to capsule
Why are amalgams condensed? (3)
- Removes excess mercury
- Prevent voids
- Give optimum marginal adaptation
Why may delayed expansion of the amalgam occur?
- If a zinc containing alloy becomes contaminated with water
- Hydrogen is evolved.
- Which causes an expansion
What is tarnish of amalgams?
What compound causes this?
Surface discolouration caused by black silver sulphide
Put the phases in order of corrosion resistance
Ag2Hg3 (gamma 1) Ag3Sn (gamma) Ag3Cu2 Cu3Sn (epsilon) Cu6Sn5 (eta) Sn7-8Hg (gamma 2)
What is galvanic corrosion?
When there are 2 opposing dissimilar metals and one corrodes the other
What is crevice corrosion?
Plaque build-up increases electronegativity with respect to clean surface. Causing the plaque surface to corrode
What is stress corrosion?
Occurs under a sustained tensile force in corrosive environment
What is the longevity of amalgams?
12 years
What are some disadvantages of amalgams? (6)
- Non adhesive
- Conducts heat/electricity
- Corrosion/Tarnish
- Poor aesthetics
- Brittle
- Viscoelastic
How can the quality of the amalgam be controlled by the dentist? (5)
- Trituration procedures
- Cavity designs
- Condensation technique
- Marginal integrity
- Anatomy
What is an amalgam tattoo?
Accidental implantation of silver containing compounds into oral mucosal tissue.
(Grayish/black pigmentation)
Why aren’t gallium alloys used anymore? (4)
- Poor corrosion resistance
- Prone to delayed expansion
- Expensive
- Very wet mix and difficult to handle
What are fluoridated amalgams?
- Contains fluoride containing glass particles
- More susceptible to erosion
