Properties of amalgam Flashcards
Discuss the composition of dental amalgam
Amalgam is produced by mixing liquid mercury with an alloy made of silver, tin, and copper solid particles
How is amalgam classified?
- 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.
What are the main components in amalgam?
- Basic metals: Mercury, Silver, Copper, Tin
* Other metals: Zinc, Indium, Palladium
Describe the properties of sliver
- increases strength
* increases expansion
Describe the properties of tin
- decreases expansion
- decreased strength
- increases setting time
Describe the properties of copper
- increases strength
- No Gamma 2 formation
- reduces tarnish and corrosion
- reduces creep
Describe the properties of zinc
- delayed expansion if contaminated by moisture during placement
- increases clinical performance
- reduces marginal breakdown
Describe the properties of indium
- decreases surface tension
- reduces amount of mercury necessary
- reduces emitted mercury vapor
- reduces creep and marginal breakdown
Describe the properties of palladium
- reduces corrosion
* greater luster
What are the 2 main amalgam components?
- Low copper: 70% silver, 26% tin, 3-4% copper
* High copper: 41-61% silver, 28-31% tin, 12-27% copper
List the properties of amalgam
Compressive strength
Dimensional change
Strength
Tarnishing
Crevice corrosion
Corrosion fatigue
Galvanic corrosion
Creep
Rigidity
Describe amalgams compressive strength
- 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
Describe amalgams dimensional change
- 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
Describe amalgams strength
- 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
Describe amalgams tarnishing
- 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
Describe amalgams cervice corrosion
- 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
Describe amalgams corrosion fatigue
• Fine branch like penetrations around the margins of restorations: both the gamma phase and the eta phase can corrode – a very slow process
Describe amalgams galvanic corrosion
• contact between dissimilar metals within an electrolyte, e.g. saliva acts as an electrolyte between amalgam and cast gold, may lead to galvanic corrosion
Describe amalgams creep
- 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
Describe amalgams rigidity
- 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
List the advantages of amalgam as a dental restorative material
- 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
List the disadvantages of amalgam as a dental restorative material
- Need for mechanical retention
- Susceptibility to corrosion
- Marginal breakdown - possible sensitivity
- Concerns about mercury content
- Colour (aesthetics)
