amalgam Flashcards
what is amalgam
an alloy formed by the reaction of mercury (liquid) and a powder consisting of metals (commonly silver tin and copper)
function of silver and tin in amalgam (gamma phase)
reacts with mercury in liquid to form amalgam
function of copper in amalgams
increases strength and hardness
zinc and amalgams
not contained in modern amalgams
interactions between zinc and water from blood or saliva could cause hydrogen bubbles to form in amalgam that would cause expansion
this could add downward pressure and therefore pain or push upwards and make the restoration proud
why might mercury be added to the powder component of amalgam as well as the liquid
to accelerate the setting time
how are lathe cut particles of amalgam achieved
by filing solid ingots (blocks) of amalgam
particles vary in size and are irregular in shape
how are spherical amalgam particles achieved
by spraying molten amalgam in an inert atmosphere
particles vary in size but are spherical in shape
gamma phase
tin and silver powder
good strength and corrosion resistance
present on both sides of equation
powder of tin and silver on left and also unreacted powder left over on right
gamma 1 phase
silver and mercury
holds gamma and gamma 2 phase together by wrapping around them
good corrosion resistance
gamma 2 phase
tin and mercury
weakest phase - low strength and lacks corrosion resistance
what phase contributes most to amalgams strength
gamma phase
what are the advantages of using spherical particles
less mercury required
easier to carve
less sensitive to condensation
higher tensile and early compressive strength
amalgam condensation
compacting/compressing each increment into the cavity
name 3 factors that decrease amalgam strength
undermixing
too little condensation pressure
slow rate of packing
corrosion (over time)
discuss amalgams thermal properties
amalgam has a high conductivity and diffusivity which isnt good
it also has a thermal expansion co efficient which is much greater than that of a tooth
for this reason deep cavities may need a lining to protect the pulp
how much copper must be in amalgams for them to be classed as cooper enriched
6%
what are copper enriched amalgams also referred to as
non gamma 2 phase amalgams
what are the two types of copper enriched amalgams
dispersion modified
single composition
(setting reactions are different)
what is the powder compromised of in single composition copper enriched amalgams
copper tin and silver
describe the setting reaction of single composition copper enriched amalgams
powder (Sn, Ag, Cu) and liquid (mercury) to powder (Sn, Ag, Cu) and gamma 1 ( Ag, Hg) and CuSn
gamma 1 phase holds the other two components together
how does the dispersion modified copper enriched amalgam setting reaction differ from that of traditional amalgams
the inital step is the same - production of gamma, gamma 1 and gamma 2 phases
after several days however the gamma 2 phase interacts with AgCu to form gamma 1 phase and CuSn
benefits of cu enriched amalgams compared to conventional
greater strength
less creep
higher corrosion resistance
increased durability of margins
amalgam tattoo
traumatic implantation of amalgam into soft tissues leaving behind a grey discolouration
why is amalgam still used
long lasting and good long term performance
durable
radiopaque
cheap (time = money, quick to place)
what are 3 disadvantages to amalgam
poor aesthetics
poor thermal qualities
may require extra destruction of sound tooth to create a retentive cavity
possibility of mercury toxicity
lichenoid lesions
whiteish markings on ssoft tissues (resembles lichen planus)
possible cause = hypersensitivity to amalgam
galvanic response
‘battery effect’
electric current between 2 disimilar metals. e.g amalgam and gold
retention form
features of a cavity that prevent loss of restoration in any direction
resistance form
features of a cavity that prevent the loss of material due to distortion or fracture by masticatory forces
what are the 2 options when preparing an inter proximal cavity for an amalgam restoration
self retentive box - undercut areas along axial walls
proximo occlusal - take out fissures as well as proximal caries to achieve retention
why does dentine need to be sealed with a lining etc before amalgam is placed
due to amalgams ability to transfer heat readily
trituration
mixing to form amalgam
minimata convention
global treaty to protect human health and environment from the adverse effects of mercury
