Amalgam Restorations Flashcards
What is an amalgam?
Any mixture, or blending, or mercury with another metal or alloy.
Why is mercury used?
Ability to dissolve any metal as its liquid at room temp.
What are the three metals used in amalgam?
What other metals can be added and why?
Mercury
Silver
Tin
Copper - to increase the final strength
Zinc - to reduce oxidation
How is the amalgam delivered (what system?)
A powder-liquid system.
Mercury liquid, silver+tin solid.
What is the maximum % that amalgam can be added in weight?
3%
When producing the gamma phase (silver+tin), what is the main problem?
Melting components at high temperatures with silver+tin oxidising easily.
Zinc needed to remove the oxygen as zinc oxide.
How is the amalgam powder produced by the liquid metals?
Two methods
1) Lathe cut = cooled down and mechanically grinded
2) Spherical = atomisation in an inert atmosphere
What particles are produced by a lathe-cut and a spherical method?
Lathe cut = irregular sized
Spherical = spherically sized
Why does dental amalgam use both spherical and irregular shaped particles?
To increase packing efficiency, reduce the need for mercury and increases performance of restoration.
Name some differences between lathe-cut and spherical alloys
Lathe cut:
- More mercury needed
- More condensation force
- Less ease in carving and burnishing
- Less overhangs and strong proximal contacts
Spherical:
- Requires less mercury
- Less condensation force
- Easier to carve and burnish
- Overhangs and weak proximal contacts
Setting reaction of amalgam:
1) Initial dissolution
2) Formation of gamma 1
3) Formation of gamma 2
4) Set amalgam
1) Outer surface of silver/tin dissolves in liquid mercury.
2) Reaction of mercury + silver. Forms grains which can stick along alloy particles.
3) Reaction of mercury + tin.
4) Set amalgam - all free mercury has reacted with silver and tin.
What phase of amalgam is the weakest?
Gamma 2 (mercury + tin)
How long does it take for amalgam to fully set in the mouth?
A week - amalgam wont be strong at this time so good to recall patient after a week to refine edges, polish ect.
Why does amalgam have such good mechanical properties?
They are similar to that of a tooth.
How does the dimension of amalgam change on setting?
Why is this a problem?
Initially, there is contraction due to dissolution into mercury.
Then, expansion due to crystallisation of gamma 1 and 2 phases.
Contraction of the material can lead to marginal gaps to allow infiltration and failure of the restoration and secondary caries or tooth cracks.
Expansion can result in protrusions or even tooth cracks.
What are the main thermal properties of amalgam?
- High thermal diffusivity
- Thermal expansion can occur leading to micro leakage
Are amalgams corrosive?
Yes.
Corrosion can lead to the release of free mercury and can be reduced by polishing the restoration to a smooth surface.
Is amalgam an elastic or plastic material?
Plastic - subjective to deformation from mechanical load in chewing.
Why do we use high copper amalgams?
They eliminate gamma 2 (mercury+tin) to give an increased strength and reduced corrosion and creep.
What is the dispersed phase in high copper amalgams?
A combination of 2 parts of conventional lathe cut (copper) and 1 part eutectic (silver and copper).
The phase contains two copper-tin molecules with different numbers of each atom.
How is an amalgam placed?
1) Prepare the cavity, pulp protection if required
2) Isolating the tooth (rubber dam, matrix bands, wedges)
3) Amalgam trituration = Powder and liquid mix, into the amalgamator machine to regulate timing and energy of the mixing procedure
4) Application of a small amount of amalgam to fill all the cavity corners
5) Remove excess mercury
6) Repeat until filled whole cavity
7) Need to carve to the shape we want to keep, burnishing of the metal is important to keep marginal adaptation and to remove any remaining reactive mercury from the mix.
8) Remove matrix
9) Access to all edges of the restoration to give more carving
10) Finish and polish final restoration - controlling contact point to avoid periodontal disease, polishing is important but can be postponed to a second appointment (1 week later)
How do we prepare a cavity for amalgam?
- Avoid unsupported enamel as this can lead to chipping
- Corners of cavity need to be shaped as a tooth curp othweise chewing forces will lead to cracks
- Undercuts needed for macro-mechanical retention
What does under and over trituration lead to?
Under = no adequate formation of gamma 1 and 2 phases resulting in crumbly mix
Over = excessive contraction once placed
Why do we condense amalgam once its in a cavity?
Remove the excess mercury
To ensure there are no voids in the restoration
Ensure margins integrity
