amalgam Flashcards
what is dental amalgam?
an alloy formed by the reaction of mercury(l) and silver/tin/copper/other metal powder
what are the 2 classifications of amalgam?
traditional
copper enriched
what are the functions of silver/tin
gamma phase
reacts with Hg liquid to form amalgam
what is the function of copper?
increases strength and hardness
what is the function of zinc
scavenger during production
preferentially oxidises and slag formed/removed
-some zinc free
what is the function of Hg in powder?
pre-amalgamated alloys react faster
describe lathe cut particles
coarse, medium, fine
formed by filing ingots
describe spherical particles?
range of particle sizes
formed by spraying molten metal into inert atmosphere
what is the simplified setting reaction?
powder + liquid -> unreacted particles + amalgam matrix
describe gamma particles
good strength & corrosion resistance
describe gamma 1
good corrosion resistance
describe gamma 2
weak poor corrosion resistance
which gamma has the highest tensile strength?
gamma
what are the setting dimensional changes in traditional amalgam?
initial contraction -solution of alloy particles in Hg
expansion - gamma 1 crystalisation
what are the setting dimensional changes in modern amalgam?
small contraction
solid solution of Hg in Ag3Sn
why does zinc cause expansion?
bubbles of H2 formed within amalgam
pressure build up causes expansion
downward pressure can cause pulpal pain
upward- rest sits proud
what affects amalgam properties?
handling factors -proportioning & trituration -condensation -carving & polishing dependent on cavity design affected by corrosion
what are the advantages of spherical particles?
less Hg required higher tensile strength higher early compressive strength less sensitive to condensation easier to carve
describe the strength of amalgam
early poor
late (>24hrs) ok
what factors decrease strength?
undermixing too high Hg content after condensation too low condensation pressure slow rate of packing -increments dont bond corrosion
describe creep in amalgam
visco-elastic
-results in deformation
high in traditional
affects marginal integrity
what is creep?
when a material experiences low-level stress levels which are applied repeatedly
over prolonged time it may flow-permanent deformation
what materials does creep affect?
amalgam
alloys
waxes
plastics
what happens when amalgam experiences creep?
sits proud of tooth causes ditched margins vulnerable to fracture at margins causes microleakage leads to secondary caries
what is the biocompatibility concern in amalgam?
mercury toxicity
what is the thermal expansion?
x3 of tooth
what is the thermal conductivity?
high
may need to use liner/varnish in deep cavities
what is the thermal diffusivity?
high
when transient thermal stimuli applied amalgams temp rises quickly
describe amalgam bonding
doesnt bond to tooth
needs mechanical retention
describe the handling/viscosity
mixing, working, setting times ok
viscosity
-packing into cavity= user friendly
describe corrosion
gamma 2 most electronegative weakens material at margins may contribute to sealing margins reduced by: -copper enriched -polishing margins -avoiding galvanic cells
aesthetics?
poor
radiopaque?
yes
anticariogenic?
no
smooth surface?
yes, if well polished, may deteriorate
setting shrinkage?
net overall shrinkage
what are the 2 types of copper enriched amalgam?
dispersion modified
single composition types
-introduced to increase uptake by profession
-more common
what is dispersion modified amalgam?
Ag-Cu spheres + conventional lathe cut alloy
what is the benefit of copper?
modification to setting reaction
what are the benefits of copper enriched?
higher early strength
less creep
higher corrosion resistance
increased durability of margins
what is the thermal expansion rate the same as?
enamel and dentine
what is the failure rate lower than?
all composites
what are the advantages of amalgam?
strong hard durable radiopaque user friendly
what are the disadvantages of amalgam?
corrosion leakage poor aesthetics mercury -perceived toxicity -environmental impact
which type of amalgam has the least microleakage?
permite
