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
