Impression Materials Flashcards
What is the function of impression materials?
-produce an accurate replica of the surface and shape of hard and soft oral tissues
What is an impression?
A negative reproduction of tissues (hard and soft)
How can impression materials be classified clinically?
- mucostatic
- mucocompressive
What does mucostatic mean?
-fluid materials that displace the soft tissues slightly i.e. give an impression of the undisplaced mucosa
What does mucocompressive mean?
-viscous materials that record an impression of the mucosa under load i.e. give impression of displaced soft tissue
What are some examples of mucostatic materials?
- zinc oxide eugenol
- low viscosity alginates
What are some examples of mucocompressives ?
- impression compound
- high viscosity alginates/elastomers
How can impression materials be classes based on their properties?
- elastic
- non-elastic
During removal what does the impression material undergo?
Elastic strain as it is coming over the most bulbous part of the tooth
After removal, what does the impression material have to undergo?
Elastic recovery
What are the 2 categories of ‘elastic’ impression materials? (not perfectly elastic)
- hydrocolloids
- elastomers
What are some examples of hydrocolloids?
- agar (no longer used)
- alginate
What are some examples of elastomers?
-polyethers -silicones (conventional and addition cured)
What would happen to materials that are non-elastic when taking an impression?
Either be deformed or become fractured
Summarise the setting reaction for IM materials?
- chemical reaction
- polymerisation
- cooling
- forms solid replica
After the impression material has set, what happens next?
- tray removed from mouth
- disinfected
- stored
- cast prepared
What needs to be considered when storing the impression?
- how long can it be stored before dimensional changes become significant
- should you wait to allow elastic recovery?
Why do you need flow of an impression material?
so the surface detail can be recorded
What are the ideal properties for setting changes in an impression material?
NO dimensional change (dont want it to shrink or expand when removed)
What are the ideal properties of an impression material on removal?
- NO effect on oral tissues (irritate them or pull off etc)
- NO change dimensionally (ie no thermal contraction)
- complete elastic recovery
What are the ideal properties for decontamination of an impression material?
Dimensions and surfaces unaltered by decontamination
What properties afect the accuracy of an impression material?
- viscosity
- setting mechanism (does it cause contraction)
- thermal expansion coefficient
- hydrophobi/hydrophilic
- elasticity
- tear strength
How does viscosity affect accuracy?
-determines the ability of the material to flow over surface and so level of detail recorded
How does whether a material is hydrophobic or hydrophilic affect accuracy?
Can affect the surface contact Might need to remove saliva for good impression
How does tear strength affect accuracy?
-if the material isnt strong enough to withstand tearing when removed will have to take another impression
What aspect in the mouth can increase the likelihood of tearing?
undercuts
What are the ideal properties of impression materials for patient comfort?
-non-toxic, non-irritant -acceptable taste and smell -short setting time -removable wihtout damage to oral tissues
What are some non-elastic impression materials?
Impression compound
What are some elastic impression materials?
- hydrocolloids
- elastomers
What two states do hydrocolloids exist in?
SOL state
GEL state
Describe the SOL state of hydrocolloids.
Viscous liquid state
Describe the GEL state of hydrocolloids
-jelly-like consistenct (when the material undergoes reaction becomes firmer) -agglomeration of particles -entangles framework of solid particles enclsoing liquid by capillary forces
What are the components of alginate?
- salt of alginic acid (e.g. Na alginate)
- calcium sulphate
- trisodium phosphate
- filler
- modifiers, flavourings and chemical indicators
Function of the salt of alginic acid?
react with Ca ions provided by calcium sulpate
What happens before the salt of alginic acid reacts with the calcium ions?
The calcium ions react with the trisodium phosphate
What is the function of the trisodium phosphate?
To delay the gel formation
What is the function of the filler?
- cohesion
- strength
Setting reaction for alginate?
2NanAlg + nCaSo4 —> nNa2SO4 + CanAlg
