Impression materials Flashcards
what is the function of impression material
- accurate replica of the surface and shape of hard and soft oral tissues
4 classification types of impression types
elastix
non elastix
mucocompressive
mucostatic
what is the diff between mucostatic and mucocompressive
mucocompressive
high viscosity alginates
impression of displaced soft tissues
mucostatic
low viscosity alginates
does not displace soft tissues
is elastic ideal
yes but no material is 100% elastix, there will be a slight mismatch
why is non elastic not ideal
fractures
deforms
not good for cuspal areas
break after undercuts
describe viscoelastic behaviour of impression material
load -> elastix strain -> load removed -> elastix recovery -> permanent strain
how to minimize permanent strain?
wait for a time after removing the tray before pouring cast
less load time and sharp pull
what does shorter load time mean?
remove the impression tray with a SHARP PULL
Impression process
- preparation
- insertion
- setting
- removal
- storage
- casting
Ideal properties of impression materials
- accuracy
- flow/viscosity and record details
- no dimensional change after setting and removal
- no effect on oral tissues upon remove
- ideally have complete elastic recovery
- no dimensional change or warping during storage
- decontaminated without changing dimensions
- compatible with gypsum
- LOW thermal expansion, so that there is no change in dimensions when going from mouth temp 37deg to room temp of 22deg
- withstand tearing when removal from undercuts
- non toxic
- taste
- short setting time
- removable without damage to oral tissues
is hydrocolloid elastix or non elastix
it is elastic
can be reversible hydrocolloid or irreversible
what is alginate classification
irreversible hydrocolloid
wat are examples of elastomers
polysulphides
polyethers
silicones
wat is agar
reversible hydrocolloid
what is a colloid and a hydrocolloid?
A colloid is a two phase system
1. fine particles
2. dispersing medium
Hydrocolloid
1. fine particles
2. medium is water
- SOL
o viscous liquid state of hydrocolloid
o can flow around the impression tray
- GEL
o jelly like consistency state of hydrocolloid
o after setting
o agglomeration of particles
o entangled framework of solid particles enclosing liquid by capillary forces
Composition of alginate
- salt of alginic acid
- calcium sulphate
- trisodium phosphate
- filler
- modifiers
- flavourings
purpose of trisodium phosphate
delay gel formation
- trisodium phosphate reacts with calcium sulphate PREFERENTIALLY => calcium phosphate
- sodium alginate reacts with calcium in the calcium phosphate
Setting reaction of alginate
sodium alginate + calcium sulphate -> calcium alginate
increasing temp does what to alginate setting time
speeds up
why is it beneficial to wait after removing the tray from the mouth?
*CROSSLINKING CONTINUES AFTER SET SO GREATER ELASTIC PROPERTIES IF YOU WAIT BEFORE REMOVING FROM MOUTH
temp of water to use to mix alginate
18-24
Alginate properties
- permanent distortion and tearing reduced by bulk
- poor tear strength
- non toxic
- taste ok
- setting time ok
- easy to use
- storage must avoid moisture -> cast asap to avoid dimensional changes
- flow details ok
- little dimensional changes
- nearly fully elastic
syneresis
imbibition
syneresis = release of water
imbibition = uptake of water
*** in relation to storage of alginate impression, can distort and change dimensions when in contact with moisture
what are the 3 types of elastomers?
- polyether
- polysulphides
- silicones
what are the ideal properties of impression materials?
- appropriate working and setting time
- flow readily
- wettability
- flexible
- 100% elastic recovery ideal
- high tear strength
- detail reproduction
- limited syneresis and imbibition
what is the elastic recovery of impregum and virutal
virutal = 99.5%
impregum = 98%
what is the ISO standard for detail reproduction
20um / 50um
what is the tear strength of virtual and impregum
Virtual = 9 MPa
Impregum = 1.8 MPa
what is the def of tear strength
stress a material will withstand before fracturing
what is the def of rigidity
How much a material changes shale depending on the force applied to it
why should an impression material be flexible
flexible means low amounts of stress required to produce a large change in shape
flexibility allows the impression to be removed from undercuts and interdental regions without fracturing
what is the working time of polyether and silicones
2 min polyether
4 min silicone
what is the setting time of poly ether and silicone
polyether 5min
silicones 6 min
what is the elastic recovery % of virutal?
> 99.5%
what is the linear dimensional change of virutal?
less than 0.20%
what is the fine detail reproduction of virtual?
20um
