Impression materials
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What is dental stone
Dental stone used to fill the impression and produce a positive replica (stone cast)
What are mucostatic materials
(eg zinc oxide eugenol, low viscosity alginates) - fluid materials that displace the soft tissues slightly - ie give an impression of the undisplaced mucosa.
What is a mucocompressive material and some examples
eg impression compound, high viscosity alginates/elastomers) - viscous materials that record an impression of the mucosa under load ie give impression of displaced soft tissue.
What is elasticity needed in impression materials
So the material returns to shape after removal (elastic recovery)
Why shouldn’t the cast be poured straight after removing the impression tray from the mouth
As the material needs time to recover and the longer the time left, the less permanent strain
How can permanent strain be limited by removal methods
Removing the tray as quick as possible (sharp pull) as there is less time spent with load acting on the tray and less overal strain occurs
What happens to non-elastic materials used for impressions
They are left deformed or fractured
What are the ideal properties for accruate impression material
Good flow - detail No dimensional changes on setting No effect on oral tissues, or changes dimensionally on removal Full elastic recovery Can be decontaminated without alteration Compatible with the cast material Can be stored without changes over time
What impression properties will determine accuracy
viscosity - determines ability to flow over surface & so level of detail recorded Setting mechanism (dimensional changes?) thermal expansion coefficient (ideally low) hydrophobic / hydrophilic – affects surface contact High elasticity High tear strength (removal from undercuts)
What are ideal properties for patient comfort
non-toxic, non-irritant acceptable taste and smell short setting time removable without damage to oral tissues
What are the ideal properties of impression materials for the clinician
quick, simple technique convenient working/ setting times must be able to be decontaminated cost (inexpensive) is a consideration
What are non elastic impression materials
Impression paste impression compound
What are the elastic impression materials
Hydrocolloids Elastomers
What are the states of hydrocolloids and their discriptions
SOL - viscous liquid state GEL - jelly like consistency -agglomeration of particles -entangled framework of solid particles enclosing liquid by capillary forces
What is the purpose ofcalcium sulphate in alginate
Provides calciumions to react with the salt of alginic acid
What component in alginate delays gel formation
Trisodium phosphate
What gives alginate its cohesion and strength
Filler (makes up 70%)
What is the alginate setting reaction
2 Nan Alg + n CaSO4 ——> nNa2SO4 + Ca n Alg
How does trisodium phosphate delay the alginate setting reaction
It reacts with the calcium ions first and then the sodium alginate will react with the calcium ions
What degrees should the water be when mixing alginate
18-24
Why should you wait a few minutes after you think the alginate is set
As the crosslinking continues after aparent set and there will be greater elastic properties the longer you wait
How is alginate stored to maximise accuracy
avoid moisture prepare cast ASAP to avoid dimensional change syneresis - release of water imbibition - uptake of water
What is syneresis
Release of water
What is imbibition
Uptake of water