Amalgam

Question 1

Compare the compressive strength of different morphology of alloy

Answer 1

Spherical particle alloys & Copper enriched alloys > Conventional fine grain lathe-cut alloys > Conventional coarse grain lathe-cut alloys

Question 2

Time taken to reach final strength

Answer 2

24h

Question 3

Variations in alloy morphology?

Answer 3

Lathe-cut: Coarse-grain or Fine-grain Spherical

Question 4

Composition of conventional amalgam

Answer 4

Ag: 65% (min) Cu: 6% (max) Sn: 29% (max) Zn: 2% (max) Hg: 3% (max)

Question 5

ISO limit of expansion/contraction

Answer 5

+/- 0.1%

Question 6

Main cause of creep

Answer 6

y2 phase (SnxHg) note: it is not exclusively responsible

Question 7

Which product undergoes the most corrosion in copper-enriched amalgam?

Answer 7

Cu6Sn5 (anode)

Question 8

Difference in reaction between conventional amalgam and high copper amalgam?

Answer 8

Production of y2 phase (SnxHg), which is prone to creep and erosion

Question 9

List 6 things dental clinics utilizing dental amalgam should note

Answer 9

.1) Floor should not be lined with carpet/liners to allow easy and thorough clean-up in case of spillage 2) Amalgam should be stored away from heat sources 3) Place amalgam on a tray lined with aluminium foil and do not handle it with bare hands 4) All waste amalgam should be stored in a sealed container approved by amalgam contractor 5) Use mercury spillage kit in case of spillage 6) Always have aspirator in the patient’s mouth to suck out the mercury vapour

Question 10

What is the mixing of amalgam called?

Answer 10

Trituration

Question 11

Placement to function time

Answer 11

15 to 20 mins

Question 12

Consequence of creep

Answer 12

Creep causes the amalgam to flow, such that unsupported amalgam protrudes from the margin of the cavity. These unsupported edges are weak and may be further weakened by corrosion

Question 13

Composition of copper-enriched amalgam

Answer 13

Ag: 40% (min) Cu: 30% (max) Sn: 32% (max) Zn: 2% (max) Hg: 3% (max)

Question 14

Harmful biological properties of amalgam? (3 effects of mercury)

Answer 14

• Mercury compounds can have a harmful effect on the CNS –> Multiple sclerosis / Psychiatric disturbances / Behavioural problems - Mercury can be concentrated in the placenta and passed from mother to child, potentially causing spontaneous abortion or birth abnormalities - Environmental pollution: i. Release of Hg gas during trituration ii. Contamination of water by waste amalgam products iii. Mercury vapourizes during crematorial, releasing Hg into the atmosphere

Question 15

Thermal properties of amalgam?

Answer 15

1. Relatively high value of thermal diffusivity Thus, it is necessary to line the base of the cavity with an insulating, cavity lining material prior to condensing the amalgam –> Reduces the harmful effects of thermal stimuli on the pulp 2. Coefficient of thermal expansion value is 3x greater than dentine. Coupled with greater diffusivity, it results in more expansion and contraction in the restoration than the surrounding tooth

Question 16

Compare the different forms of strength - compressive, tensile, transverse

Answer 16

Compressive strength is much higher than either Tensile or Transverse strength

Question 17

Determinants of strength

Answer 17

Hg content (optimally at 44-48% Hg)

Question 18

Difference between conventional and high copper amalgam?

Answer 18

Increase in copper composition from 6% to 30%

Question 19

What is creep in relation to dental amalgam?

Answer 19

Plastic deformation under intra-oral stresses/static loads

Question 20

Why does corrosion occur?

Answer 20

Amalgam has a heterogeneous, multiphase structure and saliva within the mouth can act as the electrolyte

Question 21

Which is the most electrochemically reactive compound?

Answer 21

y2 phase (SnxHg) hence, it readily forms the anode in an electrolytic cell it breaks down to give tin-containing corrosion products and mercury which may be able to combine with unreacted alloy (y), but not all can combine rapidly and hence, small amounts get ingested.

Question 22

High copper amalgam setting reaction

Answer 22

Ag3Sn + Cu + Hg –> Ag2Hg3 + Cu6Sn5 + Ag3Sn (y + Cu + Hg –> y1 + Cu6Sn5 + y)

Question 23

What is dental amalgam?

Answer 23

Mercury combined with a powdered silver-tin alloy, reacting to form a silvery hard grey mass

Question 24

What happens if a zinc-containing amalgam is contaminated with moisture during condensation?

Answer 24

Result in a greater than expected expansion due to the liberation of hydrogen gas

Question 25

Conventional amalgam setting reaction

Answer 25

Ag3Sn + Hg –> Ag2Hg3 + SnxHg + Ag3Sn (y + Hg –> y1 + y2 + y)

Question 26

What is the benefit of corrosion?

Answer 26

Corrosion products gather at the restoration tooth interface and eventually forms a seal which prevents microleakage

Question 27

Dimensional changes

Answer 27

Small contraction in the first half hour, as mercury diffuses into alloy Followed by expansion in the next half hour, as crystallisation takes place. (Expansion is caused by an outward thrust of growing crystals) Overall effect can be a net expansion or contraction

Question 28

What is the role of zinc in amalgam?

Answer 28

Zinc acts as a scavenger in alloy production - At an elevated temperature where the alloy is formed by melting all the constituent metals, oxidation can occur. Zinc will react with the oxygen produced to form zinc oxide, which is easily removed. Note: Zinc can be eliminated if the product is manufactured in an inert atmosphere

Question 29

Role of copper

Answer 29

Strengthening effect on amalgam

Question 30

What are the advantages of encapsulation of amalgam? (x8)

Answer 30

1. Constituents are in optimum conditions to react
2. Precisely metered compositions
3. More consistent and controlled mixing
4. Easier to mix
5. Easier to dispense
6. Less mercury vapour released into the atmosphere
7. Less risk of mercury spillage
8. Less risk to mercury contamination to the work environment