1. Impression Materials
DH 363
Dental Materials I

2. Objectives List the oral structures for which impressions are made
Differentiate between a model, cast and die
Describe the various types of impression trays
Describe the composition and setting mechanisms of various impression materials
Differentiate between elastic and non-elastic impression materials, and between reversible and non-reversible impression materials
Describe the effect of water temperature on the setting rate of alginate
Describe the effect of water and heat on the setting rate of polysulfides
Describe the use of impression materials with indirect restorative procedures

3. Impression Materials
Used to make replicas of teeth and other oral tissues
Replicas used to construct restorations and other appliances
Impression must be an accurate duplication
Stable enough to allow disinfection and production of a model/cast
Not all impression materials are compatible with all model materials
Wide variety of products available to make impressions of oral tissues.

4. Classification

5. Impression Trays Disposable – plastic Reusable – metal
Stock trays – off the shelf
Custom trays – most accurate
Preliminary model taken with stock tray
Made on a model of the patient’s arch
Acrylic or other resin
Custom tray used to take final impression

6. More Trays
Bite registration tray – used to record occlusal surfaces of both arches and relate upper and lower casts in the lab just as they relate in the mouth
Triple tray – used to take impression, bite registration & opposing impression all at the same time

7. Classification Chemical reaction Physical change Thermoset
Chain lengthening, cross-linking or both
Physical change
Thermoplastic
Cooling
Solidification (hot wax sets up)
Gelation (Jell-O)
Thermoplastic not as stable as thermoset materials

8. Classifications Elastic – for impressions with teeth present
Inelastic – for edentulous impressions
Accuracy
Procedure determines which one to use
Flow and detail reproduction
Variety of viscosities available
Light body (runny) to putty (thickest)

9. Inelastic Impression Materials
Plaster
Same setting reaction and properties as gypsum products used for models/casts
Major use is for edentulous ridges
Wax
First material used in dentistry
Cheap, clean, easy to use
Thermoplastic material
May be used for impressions for complete dentures
Mainly used in fabrication of crowns, bridges, and other restorations

10. Compound Impression compound
Wax with a filler added to improve handling and stability
Stronger and more brittle than wax
Stiff thermoplastic material used for preliminary impressions for full dentures
Denture with green stick compound

11. ZOE Zinc oxide eugenol (ZOE)
Two paste system: Eugenol + fillers and ZO powder mixed with vegetable oil
Inexpensive and easy to use -- dentures

12. Aqueous Elastomeric Materials
Agar or reversible hydrocolloid
Requires special equipment
Rubbery material (gel) is boiled to become viscous liquid (sol)
Material then kept at 150º water bath, then just before use is ‘tempered’ at 110º
Impression trays use water circulating through tubes to cool material in the mouth more quickly
Inexpensive, accurate impressions

13. Hysteresis
Unlike common phase changes of water that freezes, melts, boils at the same temperature
Reversible hydrocolloid does not melt at the same temperature at which it gels.
Characteristic of having different properties that depend on the direction of the change is called hysteresis.

14. Hydrophilic Properties
Reversible and irreversible hydrocolloids are mostly water
Advantages
Will wet tooth surfaces and even absorb some oral fluids
Pouring models is easier than with elastomeric materials
Disadvantages
Evaporation – shrinkage
Syneresis – contraction and exude water
Imbibition – absorb, swell and distort

15. Alginate Impression Materials
Alginate impression materials are commonly used in general dentistry

16. Advantages of Alginates
Common method for obtaining diagnostic casts
Easy to manipulate
Comfortable for patients
Relatively inexpensive
Do not require special equipment
Alginate materials are supplied as powders that are mixed with water
Disinfectants can be added

17. Impression Ingredients
Diatomaceous Earth provides body for the impression
Key components: K+ alginate derived from algae (carbohydrate polymer) & CaSO4 (reactor)
Carboxylate groups react with calcium ions and cross-link the material to form a gel -- irreversible hydrocolloid
Reactor is same chemical component as in gypsum (used in making cast from the alginate impression)

18. Alginates
Packaged in canisters, pouches or individual pouches

19. Clinical Problems? Alginates:
1. Tear easily if the walls of the impression
become too thin.
2. Lack the detail of other impression materials.
3. Can distort easily if removed too soon after
placement or if removed too slowly.
4. Can have varying setting time based on
many factors. It’s a good idea to follow the
manufacturer’s instructions to achieve optimal
consistency. (Inoue et al., 1999)

20. Alginate Impression 9 month set??
Temperature of the water controls the setting reaction
Cooler water slows the reaction
Warmer water hastens the reaction
Fast set – 1-2 minutes
Regular set – 3-4 minutes
9 month set??

21. Storage
Consistency of the alginate material prior to taking the impression significantly effects accuracy. (Inoue et al., 1999)
Storage of impression in 100% humidity recommended for greatest accuracy
Some research indicates that disinfection of impressions by immersion in certain disinfectants can be just as accurate.

22. Summary
Alginates are the most common method for obtaining diagnostic casts.
The two main components K+ alginate (carbohydrate polymer) & CaSO4 (reactor) react to form a gel--->irreversible hydrocolloid.
Be aware of clinical problems you may encounter using alginates and take the necessary precautions to avoid them.
Accuracy of your impressions can be affected greatly by the alginate gaining or losing water. Initial consistency of the material is also important to the accuracy of the impression.

23. References Cited
Anusavice, Kenneth J. Phillips' Science of Dental Materials Tenth ed.(Chapter 6)
Eriksson, Anders, Öckert-Eriksson, Gudrun, Lockowandt, Paul, Lindén, Lars-Åke, Irreversible hydrocolloids for crown and bridge impressions: Effect of different treatments on compatibility of irreversible hydrocolloid impression material with type IV gypsums, Dental Impressions, 12: 74-82, 1996
Felton, David A., Lang, Brien R., Dimensional stability and detail reproduction of irreversible hydrocolloid and elastomeric impressions disinfected by immersion, The Journal of Prosthetic Dentistry, 79(4): , 1998
Flanagan, Denise A., Palenik, Charles John, Setcos, James C., Miller, Chris H., Antimicrobial activities of dental impression materials, Dental Materials, 14: , 1998
Inoue, K., Song, X., Fujii, K., Kadokawa, A., and Kanie, T. Consistency of alginate impression materials and their evaluation, Journal of Oral Rehabilitation, 26: 203-7, 1999
O’Brien, William J. Dental Materials and Their Selection Second ed. (Chapter 9)

24. Polysulfides
First non-aqueous elastomeric material developed for dentistry
Often called ‘rubber’ or ‘rubber base’
Uses specific adhesive in the tray
Set via condensation polymerization – chemical reaction
Water most common by-product (‘condensation’ polymerization)

25. Mixing Polysulfides
Two paste system (white base and brown accelerator)
Equal lengths dispensed on mixing pad
Pastes are swirled and stropped together
Mixed material is place in tray and the impression taken
Much more accurate than alginate (advantage)
Long working time (advantage)
Icky smell and taste (disadvantage)
Long setting time (disadvantage)

26. Condensation Silicones
Based on silicone rubber
Hydrophobic
Condensation reaction
Alcohol by-product
Cleaner than rubber, but hard to pour good models
Not used much anymore

27. Polyethers Developed in the late ’60’s
Cationic polymerization – similar to addition polymerization except a positive ion is the reactive molecule
No reaction by-products
Shorter working and setting times
Very accurate and easy to pour with gypsum
Impregum – most popular product

28. Impregum

29. Addition Silicones Most popular, especially for C&B Clean, no smell
Accurate, stable, and expensive
Also called:
Vinyl polysiloxanes
Polyvinylsiloxanes

30. Polyvinylsiloxanes Reactive group is C=C (vinyl group)
Free radicals, addition polymerization and cross-linking to form stable rubber material
No evaporation, no by-product formed
Viscosities include:
Light body – usually placed directly on prep
Medium body
Heavy body
Monophase
Putty

31. Mixing Each viscosity comes as two pastes Mixed like rubber base
Also can be supplied in automix gun
Putty – used in the tray as ‘body’ of the impression
Mixed by kneading two colors together with fingers – no latex gloves! Vinyl gloves OK.

32. Timing
Standard procedure is critical for mixing, working, and setting times!

33. Questions Why do we take alginate impressions?
What are ingredients of typical alginate impression materials and what is their source?
What clinical problems can occur when we take an alginate or other type of impression?
What operator variable most affects the accuracy of alginate impression materials?