1. Dental Impressions
DA 122 Dental Materials

2. What is a dental impression?
A negative copy of an oral structure
Can involve a
Single tooth
An entire arch
A quadrant
With or without teeth
Soft or hard tissue

3. Positive Copy
Gypsum or stone is poured into the impression to create a
MODEL of the oral structure
This MODEL is considered to be a
POSITIVE COPY of the original oral structure
Models are sometimes referred to as
CASTS
A DIE

4. Review: Original Oral Structure = Patient Negative Copy = Impression
Positive Copy = Model

5. Properties of Impression Materials
Must be able to flow and adapt itself around structures of interest, in a semi-liquid state
Must harden into a solid that is rigid enough to be removed from mouth without being distorted: elasticity and strength
Must be accurate
Must have dimensional stability (after removal from mouth)
Must have adequate working time between start of mix and placement in mouth
Must have reasonable setting time (required to harden)
Must have biocompatibility and aesthetics (taste, odor, appearance): non-toxic
Must be chemically compatible with material that is used to create the model

6. Categories of Impression Materials
Inelastic (rigid)
Aqueous (water- based)
Non-Aqueous (rubber- based)

7. INELASTIC (Rigid) Original impression material
Not used much or at all today
Rigid
Can be brittle and break during removal
Best for edentulous mouths when making dentures
EXAMPLES:
Plaster (Type I gypsum)
Impression and Tray Compound
Zinc Oxide Eugenol impression paste

8. Tray Compound Set-up

9. Elastomeric Impression Material
Can stretch over tooth structures during removal and return to original form
Can be water-based (aqueous) or rubber- based (non-aqueous)

10. Aqueous Elastomeric Examples: Water-based
Irreversible hydrocolloid (alginate)
Reversible hydrocolloid (agar)

11. Aqueous Elastomeric

12. Hydrocolloid Another word for aqueous elastomeric
Colloid = material in which particles remain uniformly in suspension (gelatin + water = Jello)
Sol = liquid solution of colloid
Gel = hardened colloid

13. Hydrocolloid (continued)
Gelation = rubbery semi-solid state
(Think J-E-L-L-O)
Reversible Hydrocolloid Impression Material:
Material is capable of changing from sol to gel and back to sol
Temperature changes from sol to gel
Warm to cool to warm (liquid)
Irreversible:
Chemical changes from sol to gel
Unable to return to liquid state
Temperature does not change it
Gel Strength is important; material can tear; needs to be removed from mouth quickly to avoid tears from stress

14. Distortion of Impressions from:
Syneresis = too little water leads to shrinkage from evaporation
Imbibition: too much water leads to expansion from swelling

15. Non-aqueous Elastomeric
Rubber-based
Commonly used for Final Impressions of crowns, bridges, veneers, etc.—produce fine detail
Examples:
Polysulfide rubber
Polyether rubber
Polyvinyl siloxane

16. Non-Aqueous Elastomeric