Dental Amalgam

Question 1

How is amalgam formed

Answer 1

It is the product between a metal alloy and mercury

Question 2

What are the two types of compositions

Answer 2

Conventional- 65% silver, 6% copper
High copper- 30% copper and 40% silver

Other components are- Ag, Sn, Cu, Hg, Zn

Question 3

Dental amalgam can be cut two ways

Answer 3

Lathe cut- irregular cut

spherical

Question 4

What is the setting reaction of dental amalgam with convention

Answer 4

Ag3Sn + Hg—-> Ag2Hg3 + SnHg + Ag3Sn

Gamma———-> Gamma1 +gamma2+ gamma

Question 5

What is the setting reaction for high copper amalgam

Answer 5

Ag3Sn + Cu+ Hg—-> Ag2Hg3+ CuSn+Ag3Sn

Gamma—–> Gamma 1+ Gamma

Question 6

Why has high copper become more popular

Answer 6

This is because the gamma 2 phase of the conventional method is more prone to corrosion

Therefore the high copper- provides a strengthening effect on the amalgam

Question 7

What is the mixing of amalgam termed

Answer 7

Tirtuitration

Question 8

What occurs in tiruitration

Answer 8

1) 30 mins- it undergoes small shrinkage- where the mercury diffuses into the alloy
2) 30-60 min- expansion occurs due to crystallisation

Question 9

What is the purpose of the other metals

Answer 9

They are needed because if they aren’t present- only expansion would take place e.g. Sn reduces the expansion of amalgam. Although if we only use Sn, the strength decreases and corrosion occurs.

Question 10

What happens if there is poor moisture control and a zinc alloy?

Answer 10

Zinc would react with the water to release hydrogen- it would act as a deoxidiser- scavenger

Question 11

The compressive strength is greater than the tensile strength which means

Answer 11

This means that it is weak in thin sections

Question 12

How long does the reaction take

Answer 12

To reach function- 15/20 min

full strength- a day

Question 13

What are the properties of amalgam

Answer 13

1) CREEP-plastic deformation under load- clinically flows manifesting as protrusions at the restoration margined can fracture off producing a ditch that can predispose caries around the restoration.
2) corrosion- electrochemical interactions of different alloy phases with saliva as the electrolyte
3) thermal- high thermal diffusivity- need to place a thermal lining beneath the amalgam filling so pulp is no t affected.
4) Biological- Hg compounds known to be toxic- may be able to cross the placenta
5) Environmental pollution- Disposal
6) particle size of alloy- smaller achieves higher strength. If extremely small- too rapid a set cannot remove excess Hg