35. Amalgam Flashcards
L35: What toxic element does amalgam contain?
Mercury (no proof amalgam fillings pose a health risk)
L35: What type of metal is amalgam?
An alloy (metal containing 2 or more types of metals)
L35: What is the composition of amalgam?
- Mercury (Hg) (liquid);
- Silver, tin, copper and other metals (powder).
L35: How are different amalgams classified?
- Composition: traditional or copper enriched;
- Particle shape and size.
L35: What is the function of silver and tin in amalgam?
- Intermetallic compounds;
- Gamma (g) phase, interacts with Hg liquid to form amalgam.
L35: What is the function of copper in amalgam?
Increases strength and hardness
L35: What is the function of zinc in amalgam?
- Scavenger during production;
- Preferentially oxidises and then slag formed/ removed.
[some zinc free]
L35: What is the function of mercury (powder), in amalgam?
‘Pre-amalgamated’ alloys, react faster
L35: What is the function of mercury (liquid), in amalgam?
- Triple distilled (very pure);
- Reacts with other metals.
L35: What are the two particle types of amalgam?
- Lathe cut (coarse/ medium or fine, formed by filing ingots);
- Shperical/ spheroidal (range of particle sizes, formed by spraying molten metal into inert atmosphere).
L35: What is the overall setting reaction equation for amalgam?
Ag3Sn + Hg > Ag3Sn (unreacted particles, g) + Ag2Hg3 (g1) + Sn7Hg9 (g2) (amalgam matrix)
L35: How do gamma phases differ with regards to strength and corrosion resistance?
g: good strength and corrosion resistance;
g1: good corrosion resistance;
g2: weak and poor corrosion resistance.
L35: Which gamma phase has the greatest tensile strength?
g
L35: What is the average tensile strength of amalgam?
60 MPa
L35: When modern amalgam sets, are there any dimensional changes and if so how do these differ to traditional amalgams?
- Small contraction;
- Traditional: initial contraction followed by expansion due to g1 crystallisation.
L35: Why are zinc-free amalgams sometimes favoured?
- 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.
L35: What are amalgam properties dependent on?
- Handling factors: proportioning and trituration, condensation, carving and polishing;
- Cavity design;
- Product variation;
- Corrosion.
L35: What mechanical properties do amalgams have?
- Strength: compressive;
- High abrasion resistance (good for posterior teeth, too high for deciduous teeth).
L35: What factors can decrease the strength of amalgam restorations?
- Undermixing;
- Too high Hg content for condensation;
- Too low condensation pressure;
- Slow rate of packing (increments not bound);
- Corrosion.
L35: What is material creep?
When a material is repeatedly stressed for long periods at low stress levels (below elastic limit), it may flow and result in permanent deformation
L35: How does creep effect amalgam restorations?
- Affects marginal integrity (also depends on cavity design and corrosion);
- Change of shape at restoration boundaries and vulnerable to fracture.
L35: Describe the thermal expansion and conductivity of amalgam.
- Both high;
- Expansion 3x as much as tooth;
- Conductive so may require use of cavity liner/ varnish in deep cavities.
L35: How do amalgam restorations ‘bond’ to teeth?
- Do not bond;
- Require mechanical retention (undercuts).
L35: Describe the handling/ viscosity of amalgam.
- Mixing, working and setting times ok (varies between types);
- Viscosity, packed and condensed into cavity.
L35: Describe the aesthetics, radiopacity, anticariogenicity, smooth surface and setting shrinkage properties of amalgam.
- Aesthetics: poor;
- Radiopaque: yes;
- Anticariogenic: no;
- Smooth surface: yes (if polished well);
- Setting shrinkage: small (modern only).
L35: How is corrosion of amalgam material reduced?
- Addition of copper (copper enrichment);
- Polishing margins.
L35: What are the advantages of spherical particles?
- Less Hg required;
- Higher tensile strength;
- Higher early compressive strength;
- Less sensitive to condensation;
- Easier to carve.
L35: How does copper effect the setting reaction of amalgam alloys?
Beneficial modifications to the setting reaction
- As conventional material (g + Hg > g + g1 + g2)
- g2 +Ag-Cu > Cu6Sn5 + g1 (takes several days)
Ag-Cu particles, surrounded by Cu6Sn5 ‘halo’ - stronger
L35: What are the benefits of copper enriched amalgams?
- Higher early strength;
- Less creep;
- Higher corrosion resistance;
- Increased durability of margins.
L35: How do the compressive strengths of different amalgam compositions differ?
- 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]
L35: Compare the thermal expansion coefficients for dentine, enamel, ceramic, GIC, gold alloy, amalgam, composite.
Enamel: 11 Dentine: 8 Ceramic: 8-14 GIC: 10-11 Gold alloy: 12-15 Amalgam: 22-28 Composite: 25-68
ppm/ degrees C
L35: What is permite amalgam?
Non-gamma 2, spherical and lathe cut
L35: What is the typical tensile strength of permite amalgam and how does this change over 1hr-24hr-7 days?
1 hour: 28 MPa
24 hours: 54 MPa
7 days: NA
L35: Compare the mechanical properties (compressive strength, tensile strength, elastic modulus and hardness) of different restorative materials.
See screenshot on desktop!
L35: How does the rate of microleakage differ between permite and other amalgams?
Significantly less! 0.04 ml/min, some up to 1.97ml/min
L35: How does the failure rate of amalgam restorations compare to other restorative materials?
- Lowest failure rate;
- 5.8% over 8 years;
- Composites: 13.7% on average.
