Amalgam Flashcards
What is the viscosity of amalgam?
Packed/condensed
How has amalgam been adapted to reduce the risk of corrosion?
Copper enriched
Polishing margins
Avoidance of galvanic cells
What is the thermal expansion of amalgam?
3 times that of tooth - so with intake of hot/cold foods/drink the restoration with expand/contract more than the tooth resulting in microleakage over time
How is amalgam retained in a cavity?
Mechanical retention of an undercut cavity preparation
*Does NOT bond to tooth
What is the thermal conductivity of amalgam?
High
May need to use liner/varnish in deep cavities to protect the pulp
What are the two different types of copper enriched alloys?
Dispersion modified:
Ag-Cu spheres & conventional lathe cut alloy
Single composition formulations:
powder is Ag-Sn-Cu, both spherical and lathe cut particles, copper 12-30%
How long should amalgam be mixed for?
7 or 8 seconds
What are the disadvantages of amalgam?
Aesthetics Doesn't bond to tooth Corrosion Leakage Mercury - perceived toxicity and environmental impact
What cavity classes can amalgam be used with?
Class I, II, V and VI
So only posterior restorations
What happens to the mercury levels in amalgam when you condense it?
The mercury levels are reduced
What are the benefits of polishing an amalgam restoration?
More resistant to corrosion
More resistant to plaque accumulation
Do spherical amalgams contain zinc?
No, but admixed amalgams contain 1% or less
What problems can arise if amalgam contains more than 2% zinc?
If the amalgam is contaminated with moisture during placement the zinc will react with H2O to form ZnOH and hydrogen gas which may cause the amalgam to expand about a week after placement. Expansion will put downwards pressure on the pulp and upwards pressure will result in the restoration sitting proud of the tooth surface leaving it viable to fracture.
When carving amalgam, should you go from amalgam to enamel or enamel to amalgam?
The carving instrument should be pulled from enamel to amalgam to avoid overcarving.
What are the advantages of spherical high copper alloy?
High strength (especially early)
10% less mercury
Smaller particles therefore smoother surface and improved corrosion resistance
What are the advantages of the lathe-cut shape of amalgam particles?
Resists condensation more than spherical
Adapts well to cavity walls
Easier to obtain good interproximal contacts
What are the advantages of spherical shaped amalgam particles?
Requires less mercury to wet the alloy surfaces
Sets rapidly and has greater early strength
Smoother surface and probably less corrosion
Less sensitive to condensation
Easier to carve
What percentage is each phase in the amalgam mixture?
Gamma - 25 to 40%
Gamma 1 - 50 to 60%
Gamma 2 - 10 to 15%
What is the setting reaction for amalgam?
gamma + mercury -> gamma + gamma 1 + gamma 2
Ag3Sn + Hg -> Ag3Sn + Ag2Hg3 + Sn7 + Hg9
What problems arise when zinc is added to amalgam?
Zinc reacts with water to form zinc oxide and hydrogen. The bubbles of hydrogen formed within amalgam then cause a pressure build up resulting in expansion, pulpal pain and the restoration sitting proud of the tooth’s surface.
What is the abrasion resistance of amalgam?
High - suitable for posterior teeth
Sometimes too high for deciduous teeth
What factors decrease the strength of amalgam?
Undermixing
Hg content too high after condensation
Slow rate of packing
Corrosion
What is creep?
Permanent deformation of a material caused by repeat low level stress for long periods of time.
Occurs in amalgam, alloys, waxes and plastics.
Is amalgam biocompatible?
Yes. Concern about mercury toxicity but it is present in very low levels and released in very small quantities so is considered safe.
How do you reduce corrosion in amalgam?
Copper enriched amalgam
Polishing margins
Avoiding galvanic cells (redox reactions)
What is the setting reaction of copper enriched amalgam?
Dispersion modified: First same as conventional material. Second reaction takes several days. gamma 2 + Ag-Cu -> Cu6Sn5 + gamma 1 Single composition formulation: Ag-Sn-Cu + Hg -> gamma 1 + Cu6Sn5
What are the properties of gamma?
Good strength and corrosion resistance
Highest tensile strength
What are the properties of gamma 1?
Good corrosion resistance
What are the properties of gamma 2?
Weak and poor corrosion resistance
Lowest tensile strength
What is the tensile strength of each phase in the amalgam mixture?
gamma - 170 MPa
gamma 1 - 30 MPa
gamma 2 - 20 MPa
Overall tensile strength - 60 MPa
