Impression Materials DH 363 Dental Materials I

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

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.

Classification

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

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

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

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)

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

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

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

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

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.

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

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

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

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)

Alginates Packaged in canisters, pouches or individual pouches

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)

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??

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.

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.

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): 446-53, 1998 Flanagan, Denise A., Palenik, Charles John, Setcos, James C., Miller, Chris H., Antimicrobial activities of dental impression materials, Dental Materials, 14: 399-404, 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)

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)

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)

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

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

Impregum

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

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

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.

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

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?