Amalgam and Metallic Materials

Question 1

What is amalgam in dentistry

Answer 1

Material formed when a silver based alloy is mixed with liquid mercury to form a paste which sets hard

Question 2

Composition of amalgam

Answer 2

• Silver
• Tin
• Copper
• Mercury
• Also sometimes zinc, palladium, indium

Question 3

What are the benefits and limitations of copper in amalgam

Answer 3

• High copper has better strength due to reduciton of gamma 2 phase
• Greater than 6% copper = increased expansion = pain/fracture

Question 4

Amalgam working time

Answer 4

Approx 4 mins

Question 5

How long does amalgam to reach 90% strength

Answer 5

12 hours

Question 6

Notes on physical properties of amalgam

Answer 6

• Lower hardness and elastic modulus than enamel
• Higher thermal conductivity and electrical conductivity
• High radiopacity
• Colour - lustrous, shiny
• Significant creep particularly in low copper type

Question 7

What is creep?

Answer 7

the tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses. It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material.

Question 8

What is chemical corrosion in amalgams and what is electrochemical erosion

Answer 8

• Chemical corrosion is the reaction of silver on the surface with sulphur in saliva
• Electrochemical corrosion is much more destructive, where amalgams act as anodes which are susceuptible to high corrosion rates

Question 9

Electrochemical corrosion includes what 3 types:

And contributes to:

Answer 9

• Galvanic corosion (amalgam coupled with gold casting alloys)
• Crevice corrosion (concentration cell)
• Stress corrosion (occlusal and proximal contact areas)

Contributes to marginal leakage and failure of restorations

Question 11

Toxic effects of amalgam:

Answer 11

• Acute exposure high concentration can cause severe respiratory damage
• Chronic exposure lower levels is primarily associated with CNS damage and behavioural changes

Question 12

Dimensional changes (creep) within amalgam are due to:

Answer 12

• Particle size and shape
• Mercury to alloy ratio
• Trituration time
• Condensation pressure used
• Long term continuation of chemical reaction

Question 13

Why is it a good idea to condense amalgam with high pressure

Answer 13

Less dimensional changes

Question 14

4 methods of increasing amalgam retention

Answer 14

• Retention grooves, locks, slots
• Dovetail
• Pins
• Amalgam bonding