Amalgam Restorations

Question 1

What is an amalgam?

Answer 1

Any mixture, or blending, or mercury with another metal or alloy.

Question 2

Why is mercury used?

Answer 2

Ability to dissolve any metal as its liquid at room temp.

Question 3

What are the three metals used in amalgam?

What other metals can be added and why?

Answer 3

Mercury
Silver
Tin

Copper - to increase the final strength
Zinc - to reduce oxidation

Question 4

How is the amalgam delivered (what system?)

Answer 4

A powder-liquid system.

Mercury liquid, silver+tin solid.

Question 5

What is the maximum % that amalgam can be added in weight?

Answer 5

3%

Question 6

When producing the gamma phase (silver+tin), what is the main problem?

Answer 6

Melting components at high temperatures with silver+tin oxidising easily.

Zinc needed to remove the oxygen as zinc oxide.

Question 7

How is the amalgam powder produced by the liquid metals?

Answer 7

Two methods

1) Lathe cut = cooled down and mechanically grinded
2) Spherical = atomisation in an inert atmosphere

Question 8

What particles are produced by a lathe-cut and a spherical method?

Answer 8

Lathe cut = irregular sized

Spherical = spherically sized

Question 9

Why does dental amalgam use both spherical and irregular shaped particles?

Answer 9

To increase packing efficiency, reduce the need for mercury and increases performance of restoration.

Question 10

Name some differences between lathe-cut and spherical alloys

Answer 10

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

Question 11

Setting reaction of amalgam:

1) Initial dissolution
2) Formation of gamma 1
3) Formation of gamma 2
4) Set amalgam

Answer 11

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.

Question 12

What phase of amalgam is the weakest?

Answer 12

Gamma 2 (mercury + tin)

Question 13

How long does it take for amalgam to fully set in the mouth?

Answer 13

A week - amalgam wont be strong at this time so good to recall patient after a week to refine edges, polish ect.

Question 14

Why does amalgam have such good mechanical properties?

Answer 14

They are similar to that of a tooth.

Question 15

How does the dimension of amalgam change on setting?

Why is this a problem?

Answer 15

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.

Question 16

What are the main thermal properties of amalgam?

Answer 16

• High thermal diffusivity

- Thermal expansion can occur leading to micro leakage

Question 17

Are amalgams corrosive?

Answer 17

Yes.

Corrosion can lead to the release of free mercury and can be reduced by polishing the restoration to a smooth surface.

Question 18

Is amalgam an elastic or plastic material?

Answer 18

Plastic - subjective to deformation from mechanical load in chewing.

Question 19

Why do we use high copper amalgams?

Answer 19

They eliminate gamma 2 (mercury+tin) to give an increased strength and reduced corrosion and creep.

Question 20

What is the dispersed phase in high copper amalgams?

Answer 20

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.

Question 21

How is an amalgam placed?

Answer 21

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)

Question 22

How do we prepare a cavity for amalgam?

Answer 22

• 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

Question 23

What does under and over trituration lead to?

Answer 23

Under = no adequate formation of gamma 1 and 2 phases resulting in crumbly mix

Over = excessive contraction once placed

Question 24

Why do we condense amalgam once its in a cavity?

Answer 24

Remove the excess mercury
To ensure there are no voids in the restoration
Ensure margins integrity

Question 25

What is the best method when condensing amalgam?

Answer 25

Condense in small increments

Question 26

What are the limitations of amalgam?

Answer 26

• No adhesion to tooth substance
• Need a retentive cavity design (removes more tooth)
• Poor aesthetics
• Galvanic effects (corrosion when two different metals are in contact, leads to a sharp electric shock in tooth)

Question 27

What are the concerns with using mercury?

Answer 27

• Requires adequate measures for disposal of excess amalgam material
• Mercury is a polluting agent
• Mercury vapours injected can affect brain and kidney

Question 28

How can we minimise the risk to the operator and patient in amalgam removal?

Answer 28

• Avoid removing good amalgam restorations
• Use rubber dam on patient to avoid ingestion and inhalation of mercury micro -particles
• Operator wears full PPE and controls aerosols

Question 29

In what patients should amalgam not be used?

Answer 29

• Deciduous teeth
• Childen under 15
• Pregnant/breast feeding women