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BDS2: Dental Material Science > Amalgam > Flashcards

Amalgam Flashcards

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0
Q

What is the viscosity of amalgam?

A

Packed/condensed

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1
Q

How has amalgam been adapted to reduce the risk of corrosion?

A

Copper enriched
Polishing margins
Avoidance of galvanic cells

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2
Q

What is the thermal expansion of amalgam?

A

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

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3
Q

How is amalgam retained in a cavity?

A

Mechanical retention of an undercut cavity preparation

*Does NOT bond to tooth

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4
Q

What is the thermal conductivity of amalgam?

A

High

May need to use liner/varnish in deep cavities to protect the pulp

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5
Q

What are the two different types of copper enriched alloys?

A

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%

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6
Q

How long should amalgam be mixed for?

A

7 or 8 seconds

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7
Q

What are the disadvantages of amalgam?

A 
Aesthetics
Doesn't bond to tooth
Corrosion
Leakage
Mercury - perceived toxicity and environmental impact
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8
Q

What cavity classes can amalgam be used with?

A

Class I, II, V and VI

So only posterior restorations

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9
Q

What happens to the mercury levels in amalgam when you condense it?

A

The mercury levels are reduced

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10
Q

What are the benefits of polishing an amalgam restoration?

A

More resistant to corrosion

More resistant to plaque accumulation

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11
Q

Do spherical amalgams contain zinc?

A

No, but admixed amalgams contain 1% or less

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12
Q

What problems can arise if amalgam contains more than 2% zinc?

A

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.

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13
Q

When carving amalgam, should you go from amalgam to enamel or enamel to amalgam?

A

The carving instrument should be pulled from enamel to amalgam to avoid overcarving.

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14
Q

What are the advantages of spherical high copper alloy?

A

High strength (especially early)
10% less mercury
Smaller particles therefore smoother surface and improved corrosion resistance

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15
Q

What are the advantages of the lathe-cut shape of amalgam particles?

A

Resists condensation more than spherical
Adapts well to cavity walls
Easier to obtain good interproximal contacts

16
Q

What are the advantages of spherical shaped amalgam particles?

A

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

17
Q

What percentage is each phase in the amalgam mixture?

A

Gamma - 25 to 40%
Gamma 1 - 50 to 60%
Gamma 2 - 10 to 15%

18
Q

What is the setting reaction for amalgam?

A

gamma + mercury -> gamma + gamma 1 + gamma 2

Ag3Sn + Hg -> Ag3Sn + Ag2Hg3 + Sn7 + Hg9

19
Q

What problems arise when zinc is added to amalgam?

A

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.

20
Q

What is the abrasion resistance of amalgam?

A

High - suitable for posterior teeth

Sometimes too high for deciduous teeth

21
Q

What factors decrease the strength of amalgam?

A

Undermixing
Hg content too high after condensation
Slow rate of packing
Corrosion

22
Q

What is creep?

A

Permanent deformation of a material caused by repeat low level stress for long periods of time.
Occurs in amalgam, alloys, waxes and plastics.

23
Q

Is amalgam biocompatible?

A

Yes. Concern about mercury toxicity but it is present in very low levels and released in very small quantities so is considered safe.

24
Q

How do you reduce corrosion in amalgam?

A

Copper enriched amalgam
Polishing margins
Avoiding galvanic cells (redox reactions)

25
Q

What is the setting reaction of copper enriched amalgam?

A 
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
26
Q

What are the properties of gamma?

A

Good strength and corrosion resistance

Highest tensile strength

27
Q

What are the properties of gamma 1?

A

Good corrosion resistance

28
Q

What are the properties of gamma 2?

A

Weak and poor corrosion resistance

Lowest tensile strength

29
Q

What is the tensile strength of each phase in the amalgam mixture?

A

gamma - 170 MPa
gamma 1 - 30 MPa
gamma 2 - 20 MPa
Overall tensile strength - 60 MPa

BDS2: Dental Material Science (9 decks)

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