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Flashcards in DML4: Amalgams Deck (46)
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Define an amalgam

When mercury is mixed with another metal (commonly mercury and a silver tin alloy)


What is conventional/traditional amalgam

Low copper amalgam ~ 0 - 6%


What type of amalgam is commonly used today

High copper amalgam ~ 12 - 30%


Describe the variable components of high copper amalgam

1. Mixture of two alloys (admix or dispersed phase)
2. A single ternary alloy (not containing zinc)
3. Quaternary alloy


What is the function of silver in amalgam

- increase strength
- increase tarnish and corrosion resistance
- increases expansion
- decreases flow and creep
- decreases setting time


What is the function of tin in amalgam

- increases setting time
- increases flow and creep
- decreases strength
- decreases expansion
- decreases corrosion resistance


What is the function of copper in amalgam

- increases strength and hardness
- increases tarnish and corrosion resistance
- increases expansion
- decreases flow and creep
- decreases setting time


What are the main advantages of copper in amalgam

increased strength and decreased creep


What is the function of zinc in amalgam

- scavenger
- decreases oxidation of other metals


What is the function of mercury in amalgam

Activated the reaction; without this there will only be an alloy powder


What is the function of palladium in amalgam

- increases tarnish and corrosion resistance
- increases strength


What is the function of indium in amalgam

- increases strength
- increases expansion
- increases setting time
- decreases flow and creep
- decreases surface tension and so the amount of mercury needed is less


Describe for lathe-cut particles are achieved

1. Lathe-cut particles are aged by annealing at 100 degrees C to relieve internal strains
2. The surface is treated with acid to increase reactivity by removing the oxides formed on the surface

Lathe-cut particles have a rectangular shape and can be
- micro-cut
- fine-cut
- coarse-cut


How are spherical particles achieved

1. Atomising molten alloy in an inert atmosphere
2. Acid washing to increase reactivity


Why are spherical particles more advantageous in amalgams than lathe-cut

1. Require less mercury as there is a smaller surface area
2. Require less mixing time
3. Require lower condensation pressures (rounded particles move over more easily)
4. Harden more rapidly
5. Smooth surface for overall restoration


Outline the setting reaction for dental amalgams

1. Mercury initially dissolves the surface of the alloy particles
2. New mercury containing alloys formed
3. Need to condense prior to crystallisation of new alloys
4. Reaction is never complete
5. Set amalgam contains unreacted Ag3Sn in a matriculates of the newly formed alloy giving a cored structure


What happens in conventional low copper amalgams upon setting

Mercury dissolves Ag and Sn from alloy and a new intermetallic compound is formed


What is the strongest phase in amalgam

Gamma Ag3Sn
- this corrodes the second least
- forms 30% of the set amalgam volume


What is the second strongest phase in amalgam

Gamma 1 = Ag2Hg3
- matrix for unreacted alloy
- this corrodes the least
- 60% of volume


What is the weakest and softest phase in amalgam

Gamma 2 = Sn7-8Hg
- corrodes fastest, forming voids within amalgam
- 10% of volume (this decreases over time due to erosion)


Outline the setting reaction for admix high-copper amalgams

1. Hg dissolves Ag from Ag-Cu spherical eutectic particles
2. Hg dissolves Ag and Sn from Ag3Sn particles
3. Gamma 2 reacts with Ag-Cu to form Cu6Sn5
4. This happens around unconsumed Ag-Cu particles


Outline the setting reaction in single phase high-copper amalgams

- Ag and Sn dissolve in Hg
- Gamma 1 crystals grow to bind gamma particles together
- Cu6Sn5 forms within gamma 1 phase

This is a more direct setting reaction compared to admix reactions (there is no gamma 2 phase so the set amalgam is stronger and corrodes less)


Outline the dimensional changes occurring in amalgams

Net shrinkage
1. Initial shrinkage from dissolution of the alloy
2. Expansion from the growth of crystalline phases and formation of intermediate compounds
3. The higher the mercury content, the higher the expansion
4. Remains stable after 6-8 hours


When is the dimensional shrinkage greater

1. Smaller particle size as more dissolution
2. Higher Hg/alloy ratio as more dissolution
3. Trituration time longer and more rapid as more dissolution
4. Condensation pressure higher as lower mercury and less alloy formation


What is trituration and how is this carried out

The mixing of dental amalgams; done by a mechanical mixer


Describe the appearance of an under-triturated amalgam

- dry
- dull
- crumbly


Describe the appearance of a properly triturated amalgam

- shiny mix
- separated from capsule in a single mass


Describe the appearance of an over triturated amalgam

- shiny
- hot
- wet
- sticks to capsule


Why is an amalgam condensed and what does this mean

This is the process of incrementally packing the amalgam into the cavity
- this removes excess mercury
- it prevents voids
- gives optimum marginal adaption


What is the difference in condensation for lathe-cut and spherical particle containing amalgams

Lathe-cut = smaller condenser, higher pressure
Spherical = larger condenser, lower pressure