Amalgam
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name the classifications of amalgam
composition - traditional, copper enriched particle type - lathe cute, spherical
what is the composition of powder in traditional amalgam
silver tin, copper, zinc - 50% powder
what is the powder mixed with
mercury liquid
what is the setting reaction in traditional amalgam
AgSn + Hg - gamma, gamma 1, gamma 2
what makes up gamma and what is its properties
AgSn - in product, it is the unreacted particles, this improves the mechanical properties and is corrosion resistant
what makes up gamma 1 and what is its properties
AgHg - corrosion resistant
what makes up gamma 2 and what is its properties
SnHg - weak and corrosive
what reduces the mechanical properties
gamma 2 and the fact it takes a while to set, not good early strength with traditional amalgam insufficient mixing and condensing
what is creep
when low level stresses cause a change over time, can result in amalgam protruding above cavity, these are then vulnerable to fracture and can result in microleakage
what are the thermal properties of amalgam
high thermal conductivity - deep cavities require a liner thermal expansion is greater than that of the tooth but not as bad as composite
what is corrosion
when the material disperses and breaks down, reduces marginal integrity
what is the advantage of using spherical particles over lathe cut
easier to condense and pack into cavity, improves tensile strength and early compressive strength
what are copper enriched amalgams
improving the traditional amalgam by introducing copper to reduce the gamma 2, forms Ag-Sn-Cu which is stronger and corrosion resistant compared to HgSn
how can copper enriched amalgams be made
either dispersed or single composition formulas
what is dispersion modified copper enriched amalgam
when the traditional amalgam is produced, however, gamma 2 is then mixed with silver copper. copper binds to the Hg, removing the SnHg, gamma 2
what is single composition amalgam
when copper is added initially and no gamma 2 is produced
what is the advantages of copper enriched amalgam
greater earlier tensile strength, less creep, better corrosion resistant, better marginal integrity
what are the indications for amalgam
large posterior restoration required
when would amalgam not be used
small occlusal cavities or where aesthetics are important
what are some advantages of using amalgam over composite
good mechanical properties, cheaper (mostly as can be placed quicker so less operator time), durable
what are some disadvantages of amalgam
high thermal conductivity, requires a liner placed, poor aesthetics, may cause toxicity, corrosion, does not bond to tooth (marginal integrity), creep is an issue
name the two factors to consider when designing a cavity for amalgam
retention and resistance
what is retention is cavity design
as amalgam does not bond to tooth surface, it must be mechanically held in place. create undercuts - larger at the bottom than it is at the top. normally done when removing caries as the disease spreads further in dentine than it does in enamel
what is resistance in cavity design
preventing deformation of the material due to masticatory forces. cavity floor should be parallel to occlusal surface and the axial walls should be perpendicular to cavity floor. this reduces the chances of amalgam moving over time and ensures it is packed down to the gingival floor