L11 Dental Silver Amalgams 1

Question 1

What is amalgam?

Answer 1

A mixture of 2 or more metals one of which is mercury

Question 2

What is a dental silver amalgam made of?

Answer 2

Mercury and silver/tin alloy

Question 3

What does amalgamation reaction result in?

Answer 3

Hard shiny silver-grey restoration

Question 4

Do amalgams last longer than composites and why?

Answer 4

Much easier to set than resin composites thus last much longer.

Question 5

Is amalgam toxic and how is this prevented?

Answer 5

Presented in a capsule to prevent from exposure to liquid mercury (as the vapour is toxic). Once set and in place there is no evidence to suggest that it releases any toxic vapours.

Question 6

What does pressing the capsule do?

Answer 6

Breaks the seal between the powder and liquid mercury

Question 7

The amalgamated oscillates at a high frequency for a set number of seconds, what does this do?

Answer 7

Every particle of inter metallic alloy is coated by mercury.

Question 8

What should the amalgam in the capsule look like?

Answer 8

Should not be crumbly or dull - this should not be used. Should be shiny and liquid.

Question 9

Why is amalgam still around in the UK?

Answer 9

It is a cost-effective material. Mercury is liquid at room temperature so can create a plastic mass at room temperature and mould into a cavity chairside.

Question 10

Why is dental silver amalgam being phased down?

Answer 10

Less conservative
Environmental concern
Cosmetic reasons

Question 11

What is the minimata convention?

Answer 11

People getting very ill from mercury contamination

Question 12

What are the EU regulations on mercury (heavily regulated and restricted)?

Answer 12

Cannot be used in:
- pregnant or breastfeeding women
- adolescents - children under 15
- deciduous teeth
(With exceptions if it is deemed strictly necessary)

Question 13

What is the intermetallic compound (alloy)?

Answer 13

Ag3Sn

Question 14

What is the intermetallic phase referred to as?

Answer 14

Intermetallic phase (silver/tin alloy) is referred to as the gamma phase which readily amalgamates with Hg

Question 15

What is the expansion of tin and what does it do?

Answer 15

<25% expansion
Expansion means it fills the gaps, so less likely of infection occurring and failure of the material

Question 16

What is the contraction of tin?

Answer 16

> 29%

Question 17

What does tin do for the amalgam?

Answer 17

Tin controls the physical change of the material when it is in the cavity and when it is setting. Do not want too much shrinkage to cause stress at the interface and cause gaps (not much of an issue in amalgam because of the rate of set, stress is dissipated over time)

Question 18

What does stress create?

Answer 18

Stress might cause micro cracks and pressure on cusps and enamel, or gaps might cause secondary infection.

Question 19

What does copper do in amalgam?

Answer 19

Dispersion strengthening agent
Increases strength
Reduces tarnish, corrosion
Decreases creep, marginal deterioration (improving mechanical properties)

Question 20

Zinc is not in all amalgams, but when it is what does it do?

Answer 20

Lowers rate of marginal fracture (delays corrosion)
Scavenges oxygen, keeping the alloy clean during production

Question 21

What might moisture contaminate Zn-containing alloys cause?

Answer 21

Delayed expansion
Surface blistering
Internal corrosion

Question 22

What is the general setting reaction for dental amalgam?

Answer 22

y + Hg —> y + y1 + y2
where gamma y is the intermetallic compound
gamma1 y1 is the silver mercury phase
gamma2 y2 is the tin mercury phase

Question 23

What is an intermetallic compound?

Answer 23

2 or more metals combine with a discrete composition

Question 24

How does the setting reaction work?

Answer 24

Vigorous mixing - Hg diffuses into outer layer of Ag3Sn particles
Particle size decreases
Solubility of Ag in Hg is much less than Sn in Hg, so Ag precipitates out first as gamma1 occupying a significant proportion of the microstructure followed by Sn as gamma2

Question 25

Properties of the phases

Answer 25

y - hard and strong
y1 - brittle, midrange strength
y2 - soft, low strength, high flow and corrosion

Question 26

Why are the compounds with mercury weaker?

Answer 26

Due to the electro negativity and softness of mercury

Question 27

What is the limitation of the conventional amalgam?

Answer 27

Y2 phase is soft, prone to creep and prone to corrosion
Target is to reduce or eliminate y2

Question 28

What is the target to bettering conventional amalgam?

Answer 28

Reduce or eliminate the y2 phase

Question 29

What happens in the development of modern amalgams?

Answer 29

Dispersed phase: admix of Ag-Cu and Ag-Sn; Cu more uniformly distributed; physical properties improved
Spherical particles introduced, affecting setting and handling properties

Question 30

What happens in the high copper dispersed phase amalgam?

Answer 30

Copper content increased modifies the setting reaction
The weak y2 phase is then removed. Sn has a greater affinity for Cu than Hg, Cu combines with Sn to form ‘eta’ phase

Question 31

What is the setting reaction for high copper dispersed phase?

Answer 31

1: y + Hg —> y + y1 + y2
2: y2 + AgCu2 —> ‘eta’ phase + y1 (+y)
eta phase = Cu6Sn5

Question 32

Compare the microstructure of conventional amalgam alloy with dispersed phase amalgam alloy

Answer 32

Pg62 microstructure image

Question 33

What is the high Cu single phase amalgam?

Answer 33

Instead of having 2 binary alloys, combine them all together to create a terniary alloy. No formation and removal of y2, just formation of the ‘eta’ phase

Question 34

What is the setting reaction for the high Cu single phase amalgam?

Answer 34

Ag-Sn-Cu + Hg —> y + y1 + eta phase
(Terniary spherical alloy)
Microstructure image on pg 62

Question 35

What does the copper content need to be in modern amalgams?

Answer 35

At least 12%

Question 36

What are the advantages of copper in amalgam?

Answer 36

Reach full strength more quickly
Easier to condense
Easier to carve and polish
Lower mercury content
Higher strength
More resistant to corrosion