Impression Materials
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What different situations would result in different ideal properties for an impression material?
Single tooth Whole dentition Edentulous mouth
What is the function of an impression?
To produce an accurate replica of the surface & shape of hard and soft oral tissues
What are the classifications of impression materials?
Clinical - mucostatic, mucocompressive Properties - elastic, non elastic
What is a mucostatic impression material?
Fluid material that displaces the soft tissue slightly - giving an impression of the undisplaced mucosa. e.g. zinc oxide eugenol, low viscosity alginates
What is a mucocompressive impression material?
Viscous materials that record an impression of the mucosa under load - giving an impression of displaced mucosa. e.g. impression compound, high viscosity alginates/elastomers
What are the elastic impression materials?
Hydrocolloids - alginate (irreversible) Elastomers - polyethers, silicones (conventional & addition cured)
What are the non-elastic impression materials?
Impression compound Impression paste
What is impression compound?
Combination of resins, wax, stearic acid and filler pigments
What are the properties of impression compound?
High thermal expansion Poor fine detail Plastic deformation during removal Stability OK Non toxic, non irritant Setting time OK Sterilisation difficult Long shelf life Compatible with die materials
What is impression paste?
Two pastes - ZnO powder + oil - eugenol + inert filler \+ accelerator \+ hydrogenised resin a.k.a. ZOE paste
What are the properties of impression paste?
Setting time OK (3-8 mins) Fine details OK Setting changes OK Not elastic Storage OK Compatibility with stone OK Non toxic, non irritant (eugenol may cause allergies) Adheres to tissues (must coat lips with petroleum jelly) Shelf life OK Expensive
What are hydrocolloids?
2 phase system of fine particles (1-200nm) of one phase dispersed in another phase States - SOL (viscous liquid state), GEL (jelly like consistency) e.g. alginate & agar (not used anymore)
What are the properties of alginate?
Flow fine detail ok Setting changes ok Nearly elastic (viscoelastic) Poor tear strength Non toxic, non irritant Acceptable taste and smell Setting time OK Relatively easy to use Must be prepared ASAP as it changes shape
What is an elastomer?
A polymer with viscoelasticity & very weak intermolecular forces
What properties should you assess in an impression material?
Flow/viscosity Surface detail reproduction Contact angle/wettability Elastic recovery (%) Stiffness Tear strength Mixing time (min) Working time (min)
What are examples of elastomers?
Condensation cured silicone Addition cured silicone Polysulphide Polyether
What is the ideal viscosity of an impression material?
Must be able to flow readily
What is the ideal surface wetting of an impression material?
Must make intimate contact with teeth/mucosa Small contact angle
When should elastomers be used as an impression material?
When accuracy is paramount e.g. crown and bridge work and implants
What are the two types of hydrocolloids?
Reversible e.g. agar Irreversible e.g. alginate
What is impression compound used for?
Recording edentulous ridges Useful in denture cases with severe bone resorption
How are impression materials disinfected?
Removed from mouth Rinsed to remove saliva and blood Submerged in perform (disinfectant) for 10 minutes Rinse to remove disinfectant Wrap with wet tissues or gauze Bag, label & write lab card