Amalgam Flashcards
Compare the compressive strength of different morphology of alloy
Spherical particle alloys & Copper enriched alloys > Conventional fine grain lathe-cut alloys > Conventional coarse grain lathe-cut alloys
Time taken to reach final strength
24h
Variations in alloy morphology?
Lathe-cut: Coarse-grain or Fine-grain Spherical
Composition of conventional amalgam
Ag: 65% (min) Cu: 6% (max) Sn: 29% (max) Zn: 2% (max) Hg: 3% (max)
ISO limit of expansion/contraction
+/- 0.1%
Main cause of creep
y2 phase (SnxHg) note: it is not exclusively responsible
Which product undergoes the most corrosion in copper-enriched amalgam?
Cu6Sn5 (anode)
Difference in reaction between conventional amalgam and high copper amalgam?
Production of y2 phase (SnxHg), which is prone to creep and erosion
List 6 things dental clinics utilizing dental amalgam should note
.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
What is the mixing of amalgam called?
Trituration
Placement to function time
15 to 20 mins
Consequence of creep
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
Composition of copper-enriched amalgam
Ag: 40% (min) Cu: 30% (max) Sn: 32% (max) Zn: 2% (max) Hg: 3% (max)
Harmful biological properties of amalgam? (3 effects of mercury)
- 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
Thermal properties of amalgam?
- 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
Compare the different forms of strength - compressive, tensile, transverse
Compressive strength is much higher than either Tensile or Transverse strength
Determinants of strength
Hg content (optimally at 44-48% Hg)
Difference between conventional and high copper amalgam?
Increase in copper composition from 6% to 30%
What is creep in relation to dental amalgam?
Plastic deformation under intra-oral stresses/static loads
Why does corrosion occur?
Amalgam has a heterogeneous, multiphase structure and saliva within the mouth can act as the electrolyte
Which is the most electrochemically reactive compound?
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.
High copper amalgam setting reaction
Ag3Sn + Cu + Hg –> Ag2Hg3 + Cu6Sn5 + Ag3Sn (y + Cu + Hg –> y1 + Cu6Sn5 + y)
What is dental amalgam?
Mercury combined with a powdered silver-tin alloy, reacting to form a silvery hard grey mass
What happens if a zinc-containing amalgam is contaminated with moisture during condensation?
Result in a greater than expected expansion due to the liberation of hydrogen gas
Conventional amalgam setting reaction
Ag3Sn + Hg –> Ag2Hg3 + SnxHg + Ag3Sn (y + Hg –> y1 + y2 + y)
What is the benefit of corrosion?
Corrosion products gather at the restoration tooth interface and eventually forms a seal which prevents microleakage
Dimensional changes
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
What is the role of zinc in amalgam?
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
Role of copper
Strengthening effect on amalgam
What are the advantages of encapsulation of amalgam? (x8)
- Constituents are in optimum conditions to react
- Precisely metered compositions
- More consistent and controlled mixing
- Easier to mix
- Easier to dispense
- Less mercury vapour released into the atmosphere
- Less risk of mercury spillage
- Less risk to mercury contamination to the work environment
