Impression Materials

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 • 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. 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?? Alginate Impression

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. 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)

24. 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)

25. 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

26. 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

27. Impregum

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

29. 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

30. 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.

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

32. 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?