Definition: Elastic impression materials can be removed from undercuts without permanent distortion (if handled correctly), so they are suitable for dentate, partially dentate, and edentulous patients. They fall into hydrocolloids (agar = reversible, alginate = irreversible) and synthetic elastomers (polysulphide, polyether, silicones), with elastomers generally offering the best surface detail and dimensional stability for fixed prosthodontics, provided moisture control and technique are appropriate.
Hydrocolloids vs Synthetic Elastomers
Feature | ||
|---|---|---|
Main types | Polysulphide, Polyether, Condensation silicone, Addition silicone | |
“Sol–gel” mechanism | Agar: physical gel ⇄ sol ⇄ gel via heating/cooling (reversible) \nAlginate:chemical cross-linking sol → gel(irreversible) | Chemical cross-linking of polymer chains into a 3-D network (liquid → rubber) |
Hydrophilicity / wetting | Generally hydrophilic (adapt readily to moist tissues) | Generally hydrophobic → moisture contamination can cause loss of detail (polyether wets best; silicones most problematic unless “hydrophilic” versions with surfactant) |
Surface detail | Agar: very good detail \nAlginate: lower detail (not recommended for crown/bridge) | Excellent detail when low-viscosity (wash) materials are used; often exceeds what gypsum can reproduce |
Dimensional stability on storage | Poorer: prone to syneresis (water loss → shrinkage) and imbibition (water uptake → distortion); typically pour ASAP / within ~1 hour | Better overall:
|
Tray requirements (typical) | Often stock trays; agar uses special water-cooled trays + water bath | Often special trays (esp. polysulphide/polyether); silicones support many techniques incl. putty/wash in stock trays |
Tear strength | Alginate has low tear strength (tears easily in undercuts) | Highest in polysulphide, then polyether, then silicones(adequate, but technique matters) |
Permanent deformation (viscoelasticity) | Viscoelastic → snap removal needed; alginate permanent deformation can be clinically relevant | Also viscoelastic → snap removal still important; silicones show least permanent set, polysulphides more viscous flow |
Handling/setting consistency | Agar requires equipment and temperature control; alginate is simple, cheap, time-controlled | Polysulphide and condensation silicone can be more variable with environment/mixing; polyether and addition silicone are more consistent; addition silicone may be inhibited by latex gloves |
Typical indications | Alginate: study models, prelim impressions, many denture impressions \nAgar: now uncommon (equipment/cross-infection issues) | Fixed prosthodontics: crowns/bridges/inlays; situations needing high accuracy and stability; duplicate dies (addition silicone) |