Impression Materials Flashcards
What is the function of impression materials (IM)?
produce an accurate replica of the surface
and shape of hard and soft oral tissues
What is an impression and what is dental stone used for?
a negative reproduction of tissues
used to fill the impression and produce a positive replica (stone cast)
What are clinical classfications of IM?
- mucostatic
- mucocompressive
What is mucostatic and what are examples?
- fluid materials that displace the soft tissues slightly
- ie give an impression of the undisplaced mucosa.
eg. zinc oxide eugenol, low viscosity alginates
What is mucocompressive and what are examples?
- viscous materials that record an impression of the mucosa under load
- ie give impression of displaced soft tissue.
eg. impression compound, high viscosity alginates/elastomers
What decreases permanent strain in viscoelastic IM?
If the LOAD time is LESS…ie the impression tray is removed with a sharp pull, there is LESS overall PERMANENT STRAIN (ie permanent deformation)
wait for a time after removing the tray before you pour the cast – so as to minimise permanent strain (deformation)
What are examples of elastic materials?
Hydrocolloids
* Agar - reversible (lab duplicating)…..no longer used!
* Alginate – irreversible
Elastomers
– Polysulphides
– Polyethers
– Silicones (conventional & addition cured)
What happens to non-elastic IM when removing?
either deformed or fractured
What are the ideal properties that ensure accuracy?
flow
- surface detail recorded
setting changes
* NO dimensional change
removal
* NO effect on oral tissues
* NO change dimensionally (ie no thermal contraction)
* COMPLETE ELASTIC RECOVERY
storage
* does not change dimensionally over time (moisture)
decontamination
* dimensions and surfaces unaltered
high tear strength (some elascity required)
What is the ideal expansion coefficient for IM?
ideally low (to cope with mouth to room temperature transition)
What are the 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 for operator convenience?
- quick, simple technique
- convenient working/ setting times
- must be able to be decontaminated
- cost (inexpensive) is a consideration BUT not a property)
What are examples of non-elastic IM?
Impression compound
Impression paste
What is alginate made of?
what do the components do
sodium alginate (reacts with ca ions)
calcium sulfate (provides ca ions)
trisodium phosphate (delays gel formation)
filler, and modifiers (improve surface, ph, taste)
How does trisodium phosphate work and why?
trisodium phosphate acts as a retarder for the gel formation of alginate by reacting with and depleting the calcium ions in calcium sulfate first, before allowing the alginate crosslinking reaction to occur.
this is to increase mixing and loading time before setting and allow working time
