Impression Materials Flashcards
What does good dimensional stability mean?
Material is likely to retain their size and shape well
If a material does not have good dimensional stability, what is likely to happen to it?
The material can shrink, expand, warp, depending on the storage conditions and time
When are rigid impression materials used?
Enters the mouth as a viscous liquid and leaves as a solid
Edentulous patients without bony undercuts
What are the properties of plaster of paris?
Based on gypsum - calcium sulphate hemihydrate
Low viscosity so mucostatic - does not compress the mucosa
Can be used with a wash material with compo
Good dimensional stability
How is plaster of paris formed?
Powder mixed with water which initiates the setting reaction
What are the properties of impression compound?
- Thermoplastic resin or wax based material
- High viscosity so mucocompressive - compresses the mucosa
- Poor dimensional stability
How does compound work?
- Material heated above its glass transition temperature above which it melts and is a liquid
- Sets on cooling
- Used to make special tray, can use ZnoEu afterwards
What are the general properties of ZnOEu?
- 2 paste system comprising ZnO with various oils and eugenol with various fillers
- Low viscosity so mucostatic
- Good dimensional stability
- Can cause a burning sensation and persistent taste in the mouth
- Does not need adhesive in tray
- Special tray is required - fitted to patient first
- v v good detail
What are the advantages of PoP?
- No unpleasant flavour
- Good dimensional stability
What are the disadvantages of PoP?
- Must take care to avoid air bubbles which create an inaccurate impression
- Special tray required
What are the disadvantages of compo?
- Poor dimensional stability, model must be poured quickly
- Poor detail reproduction owing to high viscosity
- Water bath and substantial time required for heating
What are the advantages of ZnOEu?
- Good dimensional stability
What are the disadvantages of ZnOEu?
- Some patients find taste/sensation unpleasant
- Special tray required
What are the properties of agar?
- A thermoplastic reversible impression material
- Sets when cooled and melts when warmed
What is agar made of?
- Seaweed
- A long chain of polysaccharides derivatives from marine algae
- Warmed in a water bath, agar becomes a liquid. Cools and sets to a flexible solid.
- Hydrophilic - adapts well to moist surfaces so has good detail reproduction
What are the properties of alginate?
- Chemically setting impression material
- Sets when a chemical reaction occurs - not reversible
- Widely used on clinic when only moderate accuracy and stability is required.
- Can be used on broken skin - biocompatible
When is alginate used?
Mouthguards
Dentures
Whitening trays
What is alginate made of?
- Polymer - sodium alginate and calcium salts
- Fillers and other materials to give appropriate physical properties and setting times
- When powder is mixed with water, the setting reaction starts
How does alginate set?
Water is mixed with the powder causing the calcium salts to dissolve releasing Calcium ions which replace the sodium ions in the alginate
Divalent calcium forms bridges between adjacent polymer chains
Liquid becomes a flexible solid which can be removed from undercuts
Fillers provide strength
Must be kept moist during transit or else will shrink and become brittle
What are the advantages of agar?
Good detail reproduction
Cheap
What are the disadvantages of agar?
Poor dimensional stability
Required a specialist water bath
Special water cooled tray required for clinical use
Main used in lab
What are the advantages of alginate?
Cheap
Moderately good detail reproduction
Can hasten setting by using warm water
What are the disadvantages of alginate?
Takes practice to mix properly
Poor dimensional stability and can tear easily on removal
Difficult to disinfect - short term spray causes little distortion but anything more causes impression to swell/distort
What are the properties of silicones? (addition cured polyvinyl siloxanes)
- Polymer based on an Si-O-Si-O-Si backbone with vinyl groups
- 2 paste system supplied in a cartridge that mixes whilst it dispenses
- Various viscosities and wash versions
- Expensive
What are the general properties of Polyethers?
- Polymer based on a C-C-C backbone with imine groups
- A two-paste system mixed in a cartridge or by hand
- Some viscosities available but fewer available than silicones
- Cheaper but more time consuming
What are the properties of condensation cured polyvinyl siloxanes?
- An alternative silicone that released a by-product (a light alcohol) on setting
- Has poor dimensional stability compared to addition cured silicones hence are less popular
What are the properties of polysulfides?
Polymer that sets by cross linking and lengthening of the chains via mercaptan -SH groups
Foul smell and taste means that this material has been superseded by other alternatives
Smell like rotten eggs
