Definition: Impression materials are used to produce a detailed negative replica of the teeth and oral tissues. This impression is then poured to make a positive model/cast used for constructing complete dentures, partial dentures, crowns, bridges, and inlays.
Clinical Case
A 62-year-old edentulous patient requires new complete dentures. They have a moderately resorbed ridge with a small area of mobile anterior tissue. You take a preliminary impression, then use a special tray to record a more accurate master impression, choosing an impression material and technique that records detail while minimising distortion of the mobile tissues.
Uses
Study models / diagnostic casts
Complete dentures (primary and master impressions)
Crowns, bridges, inlays (indirect restorations)
Record of soft-tissue contours and sulcus detail (where required)
Classification
Elastic | Non-elastic | |
|---|---|---|
Plaster | ||
Alginate (irreversible | Zinc oxide eugenol | |
Elastomers | Silicones (addition or condensation-cured) | Impression compound |
Polyether | Impression wax | |
Polysulphides | ||
Elastic impression materials:
Can engage undercuts and are used in dentate, partially dentate and ednetulous mouths
Non-elastic (rigid) impression materials:
Cannot be withdrawn from undercuts without distortion of fracture → genearlly restricted to edentulous patients without bony undercuts.
Tray selection and why it matters
Impression trays support the material during insertion, setting, removal, and pouring
Stock trays: convenient, pre-formed; best with higher-viscosity materials that can be controlled in the tray.
Special trays: close-fitting custom trays that allow more controlled thickness and accuracy; often required for very fluid or technique-sensitive materials.
Practical rule: If the material is very fluid, it is harder to contain reliably in a stock tray → a close-fitting special tray is typically needed.
Examples often requiring a special tray:
ZOE, polyether, and polysulphide (not typically available as high-viscosity “stock tray friendly” versions).
Requirements of an impression material
1. Accurate reproduction of surface detail:
Low enough viscosity to flow and adapt to tissues and capture fine detail
But not so low that it becomes difficult to contain and control in the tra
Moisture compatibility matters:
Some materials are hydrophobic and are repelled by moisture → trapped air may cause blow holes and loss of detail
If a dry field is not feasible, choose a material/technique more tolerant of saliva/moisture
2. Tissue displacement behaviour (mucostatic vs mucocompressive)
Mucocompressive materials may displace mobile tissues (e.g., flabby ridges) → impression distortion → discomfort and instability in the final prosthesis
Ideally, the material is sufficiently fluid on placement to be more mucostatic, recording tissues with minimal displacement where appropriate
3. Dimensional accuracy and stability
Dimensional change can occur from:
Setting shrinkage/expansion (chemical reaction or physical change)
Thermal contraction as the impression cools from mouth temperature to room temperature
Tray distortion if the tray is flexible
Poor tray–material adhesion (material pulls away and distorts)