Clinical aspects of Amalgam

Question 1

amalgam is an ALLOY of metals?

Answer 1

Hg (mercury) —> the liquid

Ag (silver) —> power
Sn (tin) —> powder
the powders exist as Ag+ Sn –> Ag3Sn= gamma phase

Cu (copper)

Zn (zinc)

Question 2

what is the gamma phase?

Answer 2

Ag + Sn –> Ag3Sn.

the most common phase found in amalgam

Question 3

what is the gamma 1 phase?

Answer 3

Ag + Hg –> Ag2Hg3

desired product

Question 4

what is the gamma 2 phase?

Answer 4

Hg + Sn –> Sn7Hg

Question 5

why do we want to reduce the amount of gamma 2 phase/ Sn7Hg3?

Answer 5

1. this product is susceptible to CORROSION (which happens when the amalgam oxidises over time) –> changes the restoration, especially at the margins= causes MARGINAL LEAKAGE/ FAILURE = secondary caries
2. causes discolouration/ STAINING of surrounding tissue
3. causes CREEP
4. reduces the overall STRENGTH of amalgam

Question 6

how do we remove the phase 2 amalgam/ Sn7Hg3?

Answer 6

1. increase the amount of COPPER (Cu3Sn)
2. causes a reaction where the copper will react with phase 2 amalgam and the gamma phase amalgam to get even more desirable gamma 1 phase amalgam plus some copper crystals:

Cu3Sn + Sn7Hg3 (gamma 2) + Ag3Sn (gamma) –> Cu6Sn5 + Ag2Hg3 (gamma 1)

Question 7

why is it important to overfill an amalgam restoration?

Answer 7

so that when you pack amalgam with condenser, you allow the MERCURY RICH LAYER to enter the top, then you can carve it off to reduce the amount of mercury in the final restoration.

Question 8

spherical cut amalgam is…

Answer 8

softer and more flowable

Question 9

lathe cut (irregular shaped) amalgam is…

Answer 9

stronger but less flowable

Question 10

what is admixed amalgam

Answer 10

combination of spherical and lathe cut amalgam plus some copper

Question 11

amalgam does NOT bond directly to teeth. how do we achieve retention and what are the requirements of the cavity prep?

Answer 11

MACRO- mechanical retention via undercut (90° or BIGGER cavo-surface angle, 70° or BIGGER amalgam -margin angle)

has to be 2mm deep minimum because amalgam has low tensile strength

grooves/ slots

dentine pins

Question 12

-ves and +ves of amalgam

Answer 12

BAD:

not aesthetic
doesnt bond directly to teeth
mercury content (especially when handling, but low in the set material)
requires 24hrs to reach reasonably high strength value
low tensile strength/ in thin sections
lichenoid reactions
forms corrosion layer on the surface

GOOD:

good compressive strength 
moisture control not essential 
longevity (12 years life span compared to 5yr life span for composite)
cheaper 
easier to use/ quick
better wear resistance