Properties of amalgam

Question 1

Discuss the composition of dental amalgam

Answer 1

Amalgam is produced by mixing liquid mercury with an alloy made of silver, tin, and copper solid particles

Question 2

How is amalgam classified?

Answer 2

• Lathe cut - irregularly shaped filings, and Spherical - round particles
• Lathe-cuts have a 1:1 alloy to mercury ration, where as spherical alloys have a higher alloy ratio to mercury
• Mercury: activates reaction. Is liquid at room temp. Their role is to wet the alloy components as they are rather dry
• Spherical alloys require less mercury. It is easier for mercury to wet due to smaller SA
• Admixed alloys require more mercury. Lathe-cut particles are harder to wet.

Question 3

What are the main components in amalgam?

Answer 3

• Basic metals: Mercury, Silver, Copper, Tin

* Other metals: Zinc, Indium, Palladium

Question 4

Describe the properties of sliver

Answer 4

• increases strength

* increases expansion

Question 5

Describe the properties of tin

Answer 5

• decreases expansion
• decreased strength
• increases setting time

Question 6

Describe the properties of copper

Answer 6

• increases strength
• No Gamma 2 formation
• reduces tarnish and corrosion
• reduces creep

Question 7

Describe the properties of zinc

Answer 7

• delayed expansion if contaminated by moisture during placement
• increases clinical performance
• reduces marginal breakdown

Question 8

Describe the properties of indium

Answer 8

• decreases surface tension
• reduces amount of mercury necessary
• reduces emitted mercury vapor
• reduces creep and marginal breakdown

Question 9

Describe the properties of palladium

Answer 9

• reduces corrosion

* greater luster

Question 10

What are the 2 main amalgam components?

Answer 10

• Low copper: 70% silver, 26% tin, 3-4% copper

* High copper: 41-61% silver, 28-31% tin, 12-27% copper

Question 11

List the properties of amalgam

Answer 11

Compressive strength

Dimensional change

Strength

Tarnishing

Crevice corrosion

Corrosion fatigue

Galvanic corrosion

Creep

Rigidity

Question 12

Describe amalgams compressive strength

Answer 12

• High copper single compositions have a compressive strength of 262 MPa, therefore chances of fracture are less
• Amalgams must be able to achieve a certain minimum compressive strength one hour after mixing to avoid the danger of early fracture through masticatory stresses

Question 13

Describe amalgams dimensional change

Answer 13

• Most high-copper amalgams undergo a net contraction
• Contraction leaves a marginal gap, leads to initial leakage and post-operative sensitivity, and it is reduced with corrosion over time
• Spherical alloys have more contraction
• Greater condensation = higher contraction
• Over-trituration causes higher contraction
• Conversely water during placement causes expansion

Question 14

Describe amalgams strength

Answer 14

• Develops slowly; 24 hrs= 90% strength
• Spherical alloys strengthen faster
• Weak in thin sections. Its stronger in bulk amounts
• High copper spherical amalgams have the highest physical properties of all amalgam

Question 15

Describe amalgams tarnishing

Answer 15

• Formulation of a film of oxides, sulphides and hydroxides results in a dull or discoloured restoration
• Doesn’t cause resto to fail
• Corrosion results from chemical reactions that penetrate into the body of the amalgam
• Tarnish and corrosion occur more on amalgams with rough surfaces

Question 16

Describe amalgams cervice corrosion

Answer 16

• A differential oxygen concentration cell may develop at the tooth/restoration interface
• This results in the formation of corrosion products containing tin and copper that actually seal the margin between the amalgam and the cavity wall
• Thus it’s not necessarily a bad thing

Question 17

Describe amalgams corrosion fatigue

Answer 17

• Fine branch like penetrations around the margins of restorations: both the gamma phase and the eta phase can corrode – a very slow process

Question 18

Describe amalgams galvanic corrosion

Answer 18

• contact between dissimilar metals within an electrolyte, e.g. saliva acts as an electrolyte between amalgam and cast gold, may lead to galvanic corrosion

Question 19

Describe amalgams creep

Answer 19

• Progressive permanent deformation of a set amalgam under load
• The presence of the Gamma 2 is the principal factor influencing creep
• High-copper amalgams have creep resistance prevention of gamma 2

Question 20

Describe amalgams rigidity

Answer 20

• The modulus of elasticity of high copper amalgams far exceeds that of glass ionomer and composite resin
• High copper amalgams are more rigid than low-copper amalgams – close to the modulus of elasticity of enamel

Question 21

List the advantages of amalgam as a dental restorative material

Answer 21

• Technically very ‘forgiving’ – least technique sensitive of all restorative materials
• Ease of manipulation by clinician
• Strong and rigid
• Good longevity, high clinical success rate (compared with CR and GI)
• Cost effective
• Suitable for large restoration, if necessary

Question 22

List the disadvantages of amalgam as a dental restorative material

Answer 22

• Need for mechanical retention
• Susceptibility to corrosion
• Marginal breakdown - possible sensitivity
• Concerns about mercury content
• Colour (aesthetics)