Amalgam Flashcards
ILO 1.6c: be familiar with the chemical and physical properties as well as the clinical uses of a range of dental materials
what is the composition of traditional amalgam?
6
powder - silver, tin, copper, zinc, mercury
liquid - mercury
what is the function of the constituents of traditional amalgam?
6
silver/tin - gamma phase, reacts with liquid Hg to form amalgam
copper - increases strength and hardness
zinc - reacts with oxygen
mercury powder - accelerate the tting reaction
mercury liquid - very pure and reacts with metals
what are the different particle types? how are they formed? and describe them
2
lathe cut - coarse, medium, fine, formed by filing ingots
spherical - range of sizes, formed by spraying molten metal into inert atmosphere
what is the setting reaction of amalgam in simple terms?
Ag3Sn(γ) + Hg -> Ag3Sn(γ) + Ag2Hg3(γ1) + Sn7Hg9(γ2)
Ag2Hg3(γ1) + Sn7Hg9(γ2) is the amalgam matrix
Ag2Hg3(γ1) + Sn7Hg9(γ2) is the amalgam matrix
describe each gamma phase in amalgam’s structure
3
γ - good strength and corrosion resistance
γ1 - good corrosion resistance
γ2 - weak and poor corrosion resistance
γ1 holds γ and γ2 together
what is the difference in setting dimensional changes with traditional and modern mixed amalgams?
traditional amalgam initially contracts then expands
modern amalgam contracts a small amount then maintains it
traditional = hand mixed, modern = mechnically mixed
why is zinc used less in amalgam?
interacts with saliva/blood
* Zn + H2O -> ZnO + H2
bubbles of H2 formed within amalgam
* pressure build up causes expansion
* downward pressure causes pulpal pain
what are the advantages of spherical particles?
5
less Hg required
higher tensile strength
higher early compressive strength
less sensitive to condensation
easier to carve
what factors decrease the strength of amalgam?
5
undermixing
too high Hg content after condensation
too low condensation pressure
slow rate of packing - increments don’t bond
corrosion
what is creep?
when a material experiences low-level stresses that are applied repeatedly over a period of time
is amalgam biocompatible?
yes because there is no evidence that it harms humans
what are the thermal properties of amalgam?
thermal expansion - 3x greater than tooth tissue
thermal conductivity - high so may need a liner in deep cavities
thermal diffusivity - high so when a thermal stimuli is applied, amalgam’s temperature rises quickly and can damage the pulp
what is used to mix amalgam? describe it
amalgamator/capsule mixer
* capsule is loaded with amalgam powder and liquid mercury separated by a membrane
* once started, membrane is punctured and ultrasonic vibrations ensure even mixing
what can provide the conditions for corrosion of amalgam?
3
saliva, pH changes, drinks consumption
how do you decrease amalgam’s corrosivity?
copper enriched, polished margins
avoiding galvanic cells
what is the composition of dispersion modified copper enriched amalgam?
- Ag-Cu spheres added to conventional lathe cut alloy
- γ and γ1 particles present
- Cu6Sn5 forms a halo around Ag/Cu spheres
- all γ2 is consumed so less corrosive
