IMPRESSION MATERIALS

INSTRUCTOR INFORMATION

• Rhonda Soberdash, MA, RDH, CDA, EFDA

OBJECTIVES

• Describe the use of impression materials during indirect restorative procedures.

• List the oral structures from which impressions are made.

• Differentiate between a model, cast, and die.

• Describe the various types of impression trays.

• List the desirable qualities of an impression material.

• Differentiate between:

  • Elastic and inelastic impression materials

  • Reversible and irreversible impression materials

• Describe the composition and setting mechanism of:

  • Wax and impression compound

  • Zinc oxide - eugenol (ZOE)

  • Agar or reversible hydrocolloid

  • Alginate

• Compare the properties, use, and cost of the above impression materials.

• Describe the effect of water temperature on the setting rate of alginate.

IMPRESSION MATERIALS

• Used to make replicas of teeth and other oral tissues.

• Replicas used to fabricate restorations or other appliances.

  • Impression: Negative reproduction.

  • Replica: Positive reproduction.

CLARIFICATIONS OF TERMINOLOGY

• Study Model: Used for diagnosis and treatment planning.

• Casts: Used for construction of a restoration or appliance.

• Die: Replica of a single tooth.

IMPRESSION MATERIAL SYSTEMS

• 3 Different Systems:

  • Powder-water

  • Paste-paste

  • Melted

• Cost: Varies depending on the system used.

• Note: Adequate impressions lead to fewer retakes and better restorations.

IMPRESSION TRAYS

• Function: Serve as carriers for impression material.

• Various sizes and shapes available.

• Types of Impression Trays:

  • Plastic vs. Metal trays

  • Custom Trays: Tailored to specific patient needs.

  • Special Use Trays:

  • Bite registration trays

  • Quadrant trays

  • "J" trays

  • Sideless trays

  • Full arch trays

  • Anterior trays

  • Posterior trays

BITE REGISTRATION

• Records the occlusal relationship between dental arches.

• Examples:

  • Before and after bite registration images (22).

CLASSIFICATION OF IMPRESSION MATERIAL

• Based on type of reaction or physical changes:

  • Chemical change: Thermoset

  • Physical change: Thermoplastic

• Specifications for use:

  • Elastic vs. inelastic

  • Accuracy and detail reproduction

TYPES OF IMPRESSION MATERIALS

• Inelastic Materials:

  • Plaster

  • Wax

  • Zinc oxide-eugenol (ZOE)

• Aqueous Elastomeric Materials:

  • Alginate

  • Agar

• Nonaqueous Elastomeric Materials:

  • Polysulfides

  • Condensation silicones

  • Polyethers

  • Addition silicones (rubbers)

INELASTIC IMPRESSION MATERIAL

• Plaster: Rarely used today for impressions.

• Wax and Impression Compound:

  • First impression material used.

  • Characteristics:

  • Cheap

  • Clean

  • Easy to use

  • Thermoplastic characteristics: Becomes plastic on heating, hardens on cooling.

  • Used for wax patterns.

ZINC OXIDE-EUGENOL (ZOE)

• Type: Paste-paste system

• Characteristics:

  • Clove scented

  • Less commonly used

  • Sets to a hard, brittle mass

  • Mix until one homogeneous color is obtained

AQUEOUS ELASTOMERIC IMPRESSION MATERIALS

• Hydrocolloid: Main component is water.

  • Hydro (water) + colloid (gelatin)

• Gelation process: Change from a sol (viscous liquid state) to a gel (semisolid, rubbery state).

• Classifications:

  • Irreversible Hydrocolloid:

  • Alginates (set via chemical reaction)

  • Reversible Hydrocolloid:

  • Agar (set via physical change)

IRREVERSIBLE HYDROCOLLOIDS

• Definition: Materials that set via a chemical reaction.

• Alginates: Most common type of irreversible hydrocolloid.

ALGINATE MATERIAL

• Forms:

  • Powder in bulk canisters with provided scoop

  • Pre-measured packets

  • A water measurer (supplied plastic cylinder)

• Ratios:

  • 1 scoop of powder requires 1 measure of water

ALGINATE SETTING TIMES

• Normal Set:

  • Working Time: 2 min

  • Setting Time: Up to 4.5 min

• Fast Set:

  • Working Time: 1.25 min

  • Setting Time: 1-2 min

• Color Changes:

  • Alginates may be purple, pink, or white.

ALGINATE IMPRESSION PROCESS

• Mixing:

  • Mix powder and water to a creamy consistency (~1 min).

• Removal:

  • Remove with a quick motion.

• Disinfection:

  • Must disinfect and pour the impression as soon as possible.

ADVANTAGES AND DISADVANTAGES OF ALGINATE

• Advantages:

  • Easy to use

  • Inexpensive

  • Wet tooth technique reduces air bubbles

• Disadvantages:

  • Must be poured immediately to avoid degradation

  • Susceptible to syneresis and imbibition

SYNERESIS AND IMBIBITION IN HYDROCOLLOID MATERIAL

• Syneresis:

  • Hydrocolloid material contracts slightly after setting and exudes water.

  • Occurs slowly; hence the need to pour hydrocolloid impressions immediately.

• Imbibition:

  • Hydrocolloid absorbs water, swells, and distorts.

  • Important to limit time impression is in contact with aqueous disinfecting solutions.