01 - Amalgam Flashcards
Define: amalgam & amalgamation & trituration.
- amalgam = alloy metal containing mercury
- amalgamation = chemical reaction of mixing liquid mercury with 1 or more metals/ alloys to form amalgam
- trituration = the mechanical process of mixing mercury and alloy powder.
What occurs during amalgamation?
- when metal particles mix with mercury, the outer portion of particles dissolve into mercury.
- at the same time, mercury diffuses into metal particles.
- when the solubility of metal in mercury is exceeded, crystals precipitate within the mercury.
- as the liquid mercury decreases, due to formation of crystals -> mixture hardens.
What are the main types of dental amalgam?
1) copper amalgam (not used)
2) conventional low-copper amalgam
- dispersalloy
- 12% copper
3) high copper amalgam
- tytin
- 28% copper
note: in both, the main components are silver and tin.
What are the components of amalgam?
1) Basic
- silver
- tin
- copper
- mercury
2) Other
- zinc -> acts as deoxiders, which minimises the formation of oxides, during melting. Making amalgam less brittle.
- indium
- palladium
What properties can we use to measure the quality of amalgam?
- dimensional change
- corrosion
- compressive strength
- creep -> the amount of deformation under compressive strength
What causes the contraction and expansion of amalgam?
1) Contraction
- as particles dissolve & y1 crystals grow, they impinge against on another
- Low Hg: alloy ratio & high condensation pressure causes less Hg in the mix -> contraction
- long trituration time & use of smaller particle size alloys causes accelerate setting -> contraction
2) Expansion
- when there is sufficient Hg and the y1 crystals impinge against each other.
- zinc-containing amalgam contaminated by moisture during trituration or condensation -> delays expansion -> can cause creeping
What can contraction of amalgam lead to?
- microleakage
- plaque accumulation
- secondary caries
What is tarnish on amalgam?
- an oxidation that attacks the surface of the amalgam and extends slightly below the surface -> resulting silver sulfide forming on surface
What are the most common corrosion products from amalgam?
- oxides & chlorides of tin
What is the order of corrosion resistance?
- Ag2Hg3 (y1) > Ag3Sn (y) > Ag3Cu2 > Cu3Sn (ε) > Cu6Sn5 (η) > Sn7-8Hg (y2)
Which phase is the most corrodible in low Cu amalgam?
- Sn7-8Hg (y2)
- corrosion -> liberated Hg + Tin oxide or Tin Chloride
What alters the compressive strength of amalgam?
1) Trituration:
- Either undertrituration or overtrituration decreases strength
2) Mercury content:
- Inadequate Hg -> dry granular mix -> rough pitted surface -> corrosion
- Higher Hg content -> formation y2 phase -> incidence & severity of fracture increases as amalgam ages
3) Condensation
a) Lathe-cut alloys
- greater condensation pressure needed to minimise porosite
- higher compressive strength
b) spherical amalgams
- lighter condensation pressure for adequate strength
- heavy pressure -> condenser may punch through the amalgam
4) Porosity
- Voids & porosity may reduce compressive strength of set amalgams
- Delayed condensation or undertriturition -> porosity in set amalgams
- Insufficient condensation pressure on lathe-cut alloys -> more porosity
- Increasing condensation pressure -> improved adaptation & decreases number of voids
5) Amalgam hardening rate
- High-Cu single composition amalgams may be strong enough shortly after placement to permit amalgam build-ups to be prepared for crowns
What is creeping
- definition: time-dependent strain or deformation produced by stress
- Can cause amalgam to extend out of the restoration site
- Increase susceptibility to marginal breakdown (gradual fracture around perimeter of filling)
- Occurs when solid material slowly deforms plastically under influence of stress
- Correlates with marginal breakdown of conventional low-copper amalgams
What are variables that dentists can control when using amalgam?
- Trituration
- Condensation
- Burnishing
- Polishing
Why would you polish amalgam?
- increase smoothness
- decrease plaque retention
- decrease corrosion
Why burnish amalgam?
1) Pre-carve:
- Removes excess mercury
- Improves margin adaptation
2) Post-carve:
- Improves smoothness
3) Combined:
- Less leakage
What happens when you over or under triturate?
1) Overtrituration:
- Hot mix
- Sticks to capsule
- Decreases working/setting time
- Slight increase in setting contraction
2) Undertrituration:
- Grainy, crumbly mix
What are the advantages and disadvantages of amalgam?
1) Advantages
- Easy to insert
- Not overly technique sensitive
- Maintain anatomical form
- Adequate resistance to fracture
- Prevent marginal leakage after period of time in mouth
- Can be used in stress bearing areas
- Relatively long life
2) Disadvantages
- Colour match
- Brittle
- Subject to corrosion & galvanic action
- May demonstrate degree marginal breakdown
- Don’t help retain weakened tooth structure
- Regulatory concerns regarding disposal
What are the major components of amalgam?
- MAJOR components in amalgams = Silver & Tin
o Copper added to harden & strengthen silver-tin alloy
o Zinc also added
What are the phases of amalgam?
- Each phase given Greek symbol based on how much Tin (Sn) they contain – from 0-100%
- Amalgam alloys have narrow range of compositions:
o B + y phase
o y phase
What are the different types of alloy powders?
1) Lathe-cut
- more irregular
- resist condensation better
2) Spherical
- freshly triturated spherical amalgam very plastic
- require less mercury than lathe-cut
- lower mercury content = better properties
What is condensation of amalgam?
- compact they alloy into the prepared cavity so greatest possible density & sufficient mercury to ensure continuity of matrix phase
- More Hg left in the mass after condensation = weaker alloy
