Hydrocolloid impression materials and impression disinfection Flashcards
what are the 2 types of impression tray?
stock trays
special trays
what are stock trays?
made to fit the average patients mouth
perforated
what are special trays
unique in dimension to the patient
not perforated
what are the 2 classifications of impression material?
non elastic
elastic
what are the 2 types of elastic impression materials?
hydrocolloids
synthetic elastomers
what are the 2 types of hydrocolloids with examples?
reversible - agar
irreversible - alginate
3 examples of synthetic elastomers
polysulphides
silicone rubbers
polyethers
how are silicone rubbers set? 2
condensation curing
addition curing
what are hydrocolloids?
colloidal suspensions of polysaccharides in water
what is agar derived from?
seaweed
steps to use agar 3
supplied in gel
water bath conditioning
placed in water cooled trays
advantages of agar 2
takes up moisture - accurate impressions
can be re-used in a lab setting
disadvantages agar 4
special equipment
cost
low tear strength
dimensionally unstable due to water content
what is syneresis in agar?
loss of water from squeezed chains
what is imbibition in agar?
uptake of water - swelling due to chain separation
clinical use of agar 3
partial dentures, crowns and bridges
lab use of agar
duplicating material for pre-existing models
how are alginates supplied and used?
supplied as powder
mixed with water
composition of alginate 4
Na or K salt of alginic acid
calcium sulphate dihydrate
sodium phosphate
inert filler
what is the function of Na or K salt of alginic acid in alginate?
forms a sol with water and in set becomes a cross-linked gel
what is the function of calcium sulphate dihydrate in alginate?
source of calcium to cross link the chains of alginic acid
what is the function of sodium phosphate in alginate?
retardant
controls the working time by limiting the availability of calcium ions
what is the function of inert filler in alginate?
provide body for manipulation
what is in some alginates to signify set?
reaction indicator
what are modified alginates
alginates incorporated with silicone polymers
advantages of modified alginates 2
better detail reproduction and tear resistance
disadvantage of modified alginates
poor dimensional stability
how does DDH disinfect impressions?
ACTICHLOR
name an alternative to impressions
CAD CAM - intra-oral scanners
