35. Amalgam

Question 1

L35: What toxic element does amalgam contain?

Answer 1

Mercury (no proof amalgam fillings pose a health risk)

Question 2

L35: What type of metal is amalgam?

Answer 2

An alloy (metal containing 2 or more types of metals)

Question 3

L35: What is the composition of amalgam?

Answer 3

• Mercury (Hg) (liquid);

- Silver, tin, copper and other metals (powder).

Question 4

L35: How are different amalgams classified?

Answer 4

• Composition: traditional or copper enriched;

- Particle shape and size.

Question 5

L35: What is the function of silver and tin in amalgam?

Answer 5

• Intermetallic compounds;

- Gamma (g) phase, interacts with Hg liquid to form amalgam.

Question 6

L35: What is the function of copper in amalgam?

Answer 6

Increases strength and hardness

Question 7

L35: What is the function of zinc in amalgam?

Answer 7

• Scavenger during production;
• Preferentially oxidises and then slag formed/ removed.

[some zinc free]

Question 8

L35: What is the function of mercury (powder), in amalgam?

Answer 8

‘Pre-amalgamated’ alloys, react faster

Question 9

L35: What is the function of mercury (liquid), in amalgam?

Answer 9

• Triple distilled (very pure);

- Reacts with other metals.

Question 10

L35: What are the two particle types of amalgam?

Answer 10

• Lathe cut (coarse/ medium or fine, formed by filing ingots);
• Shperical/ spheroidal (range of particle sizes, formed by spraying molten metal into inert atmosphere).

Question 11

L35: What is the overall setting reaction equation for amalgam?

Answer 11

Ag3Sn + Hg > Ag3Sn (unreacted particles, g) + Ag2Hg3 (g1) + Sn7Hg9 (g2) (amalgam matrix)

Question 12

L35: How do gamma phases differ with regards to strength and corrosion resistance?

Answer 12

g: good strength and corrosion resistance;
g1: good corrosion resistance;
g2: weak and poor corrosion resistance.

Question 13

L35: Which gamma phase has the greatest tensile strength?

Answer 13

g

Question 14

L35: What is the average tensile strength of amalgam?

Answer 14

60 MPa

Question 15

L35: When modern amalgam sets, are there any dimensional changes and if so how do these differ to traditional amalgams?

Answer 15

• Small contraction;

- Traditional: initial contraction followed by expansion due to g1 crystallisation.

Question 16

L35: Why are zinc-free amalgams sometimes favoured?

Answer 16

• Interaction with saliva/ blood;
• Zn + H2O > ZnO + H2;
• Bubbles of hydrogen formed within amalgam;
• Pressure build up causes expansion;
• Downward pressure causes pulpal pain;
• Upward, restoration sitting proud of surface.

Question 17

L35: What are amalgam properties dependent on?

Answer 17

• Handling factors: proportioning and trituration, condensation, carving and polishing;
• Cavity design;
• Product variation;
• Corrosion.

Question 18

L35: What mechanical properties do amalgams have?

Answer 18

• Strength: compressive;

- High abrasion resistance (good for posterior teeth, too high for deciduous teeth).

Question 19

L35: What factors can decrease the strength of amalgam restorations?

Answer 19

• Undermixing;
• Too high Hg content for condensation;
• Too low condensation pressure;
• Slow rate of packing (increments not bound);
• Corrosion.

Question 20

L35: What is material creep?

Answer 20

When a material is repeatedly stressed for long periods at low stress levels (below elastic limit), it may flow and result in permanent deformation

Question 21

L35: How does creep effect amalgam restorations?

Answer 21

• Affects marginal integrity (also depends on cavity design and corrosion);
• Change of shape at restoration boundaries and vulnerable to fracture.

Question 22

L35: Describe the thermal expansion and conductivity of amalgam.

Answer 22

• Both high;
• Expansion 3x as much as tooth;
• Conductive so may require use of cavity liner/ varnish in deep cavities.

Question 23

L35: How do amalgam restorations ‘bond’ to teeth?

Answer 23

• Do not bond;

- Require mechanical retention (undercuts).

Question 24

L35: Describe the handling/ viscosity of amalgam.

Answer 24

• Mixing, working and setting times ok (varies between types);
• Viscosity, packed and condensed into cavity.

Question 25

L35: Describe the aesthetics, radiopacity, anticariogenicity, smooth surface and setting shrinkage properties of amalgam.

Answer 25

- Aesthetics: poor; - Radiopaque: yes; - Anticariogenic: no; - Smooth surface: yes (if polished well); - Setting shrinkage: small (modern only).

Question 26

L35: How is corrosion of amalgam material reduced?

Answer 26

- Addition of copper (copper enrichment); | - Polishing margins.

Question 27

L35: What are the advantages of spherical particles?

Answer 27

- Less Hg required; - Higher tensile strength; - Higher early compressive strength; - Less sensitive to condensation; - Easier to carve.

Question 28

L35: How does copper effect the setting reaction of amalgam alloys?

Answer 28

Beneficial modifications to the setting reaction 1. As conventional material (g + Hg > g + g1 + g2) 2. g2 +Ag-Cu > Cu6Sn5 + g1 (takes several days) Ag-Cu particles, surrounded by Cu6Sn5 'halo' - stronger

Question 29

L35: What are the benefits of copper enriched amalgams?

Answer 29

- Higher early strength; - Less creep; - Higher corrosion resistance; - Increased durability of margins.

Question 30

L35: How do the compressive strengths of different amalgam compositions differ?

Answer 30

- Traditional lathe (45, 302); - Traditional spherical (120, 370); - Cu dispersion (118, 387); - Cu single (272, 485). [first number: after 1 day, second number: after 2 days]

Question 31

L35: Compare the thermal expansion coefficients for dentine, enamel, ceramic, GIC, gold alloy, amalgam, composite.

Answer 31

``` Enamel: 11 Dentine: 8 Ceramic: 8-14 GIC: 10-11 Gold alloy: 12-15 Amalgam: 22-28 Composite: 25-68 ``` ppm/ degrees C

Question 32

L35: What is permite amalgam?

Answer 32

Non-gamma 2, spherical and lathe cut

Question 33

L35: What is the typical tensile strength of permite amalgam and how does this change over 1hr-24hr-7 days?

Answer 33

1 hour: 28 MPa 24 hours: 54 MPa 7 days: NA

Question 34

L35: Compare the mechanical properties (compressive strength, tensile strength, elastic modulus and hardness) of different restorative materials.

Answer 34

See screenshot on desktop!

Question 35

L35: How does the rate of microleakage differ between permite and other amalgams?

Answer 35

Significantly less! 0.04 ml/min, some up to 1.97ml/min

Question 36

L35: How does the failure rate of amalgam restorations compare to other restorative materials?

Answer 36

- Lowest failure rate; - 5.8% over 8 years; - Composites: 13.7% on average.