Amalgam Flashcards
(14 cards)
good properties of amalgam
high compressive strength
wear resistance
durability
abrasion resistance
long lasting
cheap
not technique sensitive
ease of use
bad properties of amalgam
poor aesthetics
more destructive tooth prep
mercury content toxic
high thermal expansion, conductivity, diffusivity
creep *
corrosive
what factors affect amalgam strength
undermixing
high Hg after condensation
slow rate of packing
corrosion via oral fluid
components of amalgam
powder =
Ag, T
Cu - incr strength + hardness
Zn - scavenger, oxidises, slag
liquid =
Hg
triple distilled
what is slag
by-product of industrial process where metal is extracted from ore
what is the setting reaction of amalgam
silver/tin powder reacts with liquid mercury
some unreacted silver/tin remains
silvery mercury (y1) and tin mercury (y2) form amalgam matrix
what is non-y2 amalgam
copper enriched amalgam
increase copper content >=12%
tin preferentially reacts with copper instead of mercury
forms Cu5Sn6 instead of y2
lathe cut
properties/purpose of non-y2 amalgam
increased corrosion resistance, durability, compressive strength
reduced creep + post-op sensitivity
longer lasting rest
reduced breakdown/ditching
what is zinc free amalgam
added as a scavenger/deoxidiser to remove 02 during manufacturing process
prevent oxidisation of other metals
to improve handling + longevity
problems with zinc free amalgam
moisture contamination
produces hydrogen gas when mixed with water
gradual expansion via gas
causes marginal discrepancies, micro leakage, fracture
causes pulpal pain due to pressure
what is creep
material experiencing low level stresses [less than elastic limit] applied repeatedly over time
may flow
results in permanent deformation
what is retention form cavity design
features preventing loss of rest
= undercuts
what is resistance form cavity design
features preventing loss of material due to distortion or fracture by masticatory forces
cavity floor parallel to occlusal surface
sufficient depth 1.5-2mm
IP box 90* to axial wall
how to avoid corrosion
copper enriched amalgam
polishing margins
avoid galvanic cells
