Impression material is defined as any substance or combination of substance used / for making and impression / negative reproduction. Impression is defined as a negative likeness or copy in reverse of the surface of an object, an imprint of the teeth and adjacent structures for use in dentistry. Master impression is defined as a negative likeness made for the purpose of fabricating prosthesis. Impression technique s a method / manner used in making a negative likeness.
Desirable properties of an impression material are
- A pleasant odour, taste and acceptable colour
- Absence of toxic or irritant constituents
- Adequate shelf life for requirements of storage and distribution
- Economically commensurate with the results obtained
- Easy to use with the minimum equipment available
- Setting characteristics that meet clinical requirements
- Satisfactory consistency and texture
- Readily wets oral tissues
- Elastic properties that allow easy removal of the set material from the mouth and good elastic recovery
- Adequate strength to avoid breaking or tearing upon removal from the mouth
- Dimensional stability over temperature and humidity ranges normally found in clinical and laboratory procedures for a period long enough to permit the production of a cast or die
- Compatibility with cast and die materials
- Accuracy in clinical use
- Readily disinfected without loss of accuracy
- No release of gas or other by-products during the setting of the impression or cast and die materials
Classification of impression materials
Based on mode of setting and elasticity
Rigid
- Reversible / thermoplastic – Eg. Impression compound
- Irreversible / thermosetting – Eg. ZOE impression paste, Impression plaster
- Reversible – Eg. Agar
- Irreversible – Eg. Alginate
Elastic
- Hydrocolloid
- Elastomeric impression material – Eg. Polysulphides, Polyether, Addition silicones, Condensation silicones
Based in type of impression and area of use
Dentulous –
Primary – Eg. Alginate
Secondary – Eg.. Elastomers, Agar
Edentulous
Primary – Eg. Impression compound, Impression plaster, Alginate
Secondary – Eg. ZOE impression paste, Elastomers for CD
Based on their use in dentistry
- Edentulous –
Eg. Impression compound, ZOE impression paste, Alginate, Elastomers
- Dentulous –
Eg. Agar, Alginate, Elastomers
Based on the amount of pressure applied
- Mucocompressive – Eg. Impresion compound
- Mucostatic – Eg. Impression plaster
Based on manipulation
- Hand mixing – Kneading, Circular motion, Vigorous mixing
- Mechanical mixing
Based on the tray for impression
- Stock tray
- Special tray
Article by Dr. Siri P. B.
