Impression materials Flashcards
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
