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Impression Materials Lecture

Impression Materials

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

  • Replicas are used to fabricate restorations or other dental appliances.

    • Impression: Negative reproduction of oral tissues.

    • Replica: Positive reproduction of oral structures.

Types of Replicas

  • Study Model: For diagnosis and treatment planning.

  • Casts: Used in the construction of restorations or appliances.

  • Die: Replica of a single tooth.

Impression Materials Systems

  • 3 Different Systems:

    • Powder-water

    • Paste-paste

    • Melted

  • Cost Variability: Costs vary depending on the system.

  • Importance of Adequate Impressions: Ensures less retakes and better restorations.

Impression Trays

  • Function: Serve as a carrier for impression materials.

  • Variety: Available in various sizes and shapes.

    • Materials Used: Plastic vs. metal trays.

Types of Impression Trays

  • Custom Trays: Tailored to individual anatomy.

  • Special Use Trays: Designed for specific functions.

  • Bite Registration Trays: Record the occlusal relationship between dental arches.

  • Quadrant Trays: For a section of dental arch.

  • “J” Trays: Uniquely shaped for specific applications.

  • Sideless Trays: Improve access and visibility during impression taking.

  • Full Arch Trays: Capture the entire dental arch.

  • Anterior and Posterior Trays: Shaped specifically for the front or back teeth, respectively.

Classification of Impression Materials

  • Type of Change:

    • Chemical Change: Thermoset.

    • Physical Change: Thermoplastic.

  • Applications:

    • Elastic: Capable of returning to original state after deformation.

    • Inelastic: Remains deformed after removal.

    • Accuracy and Detail Reproduction: Both crucial for effective impressions.

Types of Impression Materials

  • Inelastic Materials:

    • Types:

    • Plaster: Rarely used today.

    • Wax: First impression material used; cheap, clean, and easy to use.

    • Zinc Oxide-Eugenol (ZOE): Used less frequently; sets to a hard, brittle mass.

  • Aqueous (Hydrocolloid) Elastomeric Materials:

    • Types:

    • Alginate

    • Agar

  • Nonaqueous Elastomeric Materials:

    • Types:

    • Polysulfides

    • Condensation silicones

    • Polyethers

    • Addition silicones (rubbers)

Inelastic Impression Material

  • Plaster: Rarely used today for impressions.

  • Wax and Impression Compound:

    • First impression materials used; advantages include being cost-effective, clean, and easy to use.

    • Thermoplastic Nature: Becomes plastic on heating and hardens on cooling.

Zinc Oxide-Eugenol (ZOE)

  • Nature: Paste-paste system with a clove scent.

  • Usage: Less commonly used, hardens to a brittle mass after setting.

  • Preparing: Mix to achieve one homogeneous color.

Aqueous Elastomeric Impression Materials

  • Hydrocolloid: Main component is water (hydro = water, colloid = gelatin).

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

  • Classifications:

    • Irreversible: Alginate (sets via chemical reaction).

    • Reversible: Agar (sets through a physical change).

Irreversible Hydrocolloids

  • Commonly known as alginate.

  • Setting Mechanism: Through a chemical reaction.

Alginate Material

  • Forms: Comes in bulk canisters with a scoop or in pre-measured packets.

  • Water Measurement: Supplied plastic cylinder for mixing; 1 scoop of powder requires 1 measure of water.

Setting Times for Alginate

  • Normal Set:

    • Working Time: 2 minutes.

    • Setting Time: Up to 4.5 minutes.

  • Fast Set:

    • Working Time: 1.25 minutes.

    • Setting Time: 1-2 minutes.

Alginate Color Changes

  • Common colors: Purple, pink, and white are indicators of different formulations.

Alginate Usage Guidelines

  • Mixing: Achieve a creamy consistency (~1 min of mixing).

  • Removal: Use quick motion to detach from the mouth.

  • Post-Processing: Disinfect and immediately pour to create a model.

Applications of Alginate

  • Uses: Suitable for study models, orthodontic appliances, mouthguards, and trays for bleaching & fluoride treatments.

  • Advantages: - Can minimize air bubbles if teeth are wet. - Easily poured into models using gypsum materials.

  • Disadvantages: Must be poured immediately after taking the impression due to syneresis and imbibition effects.

Syneresis and Imbibition

  • Syneresis: Hydrocolloid materials contract slightly after setting and extrude water; occurs very slowly; reasoning for quick pouring of hydrocolloid impressions.

  • Imbibition: Occurs when the hydrocolloid absorbs water, swells, and distorts the shape; it is essential to limit the time the impression is exposed to an aqueous disinfecting solution.

Final Notes

  • Both syneresis and imbibition can compromise the quality of impression materials and should be overseen during practice to obtain optimal results.