Dental Impressions - PowerPoint PPT Presentation

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  1. Dental Impressions DA 122 Dental Materials

  2. What is a dental impression? • A negative copy of an oral structure • Can involve a • Single tooth • An entire arch • A quadrant • With or without teeth • Soft or hard tissue

  3. Positive Copy • Gypsum or stone is poured into the impression to create a MODEL of the oral structure • This MODEL is considered to be a • POSITIVE COPY of the original oral structure • Models are sometimes referred to as • CASTS • A DIE

  4. Review: • Original Oral Structure = Patient • Negative Copy = Impression • Positive Copy = Model

  5. Properties of Impression Materials • Must be able to flow and adapt itself around structures of interest, in a semi-liquid state • Must harden into a solid that is rigid enough to be removed from mouth without being distorted: elasticity and strength • Must be accurate • Must have dimensional stability (after removal from mouth) • Must have adequate working time between start of mix and placement in mouth • Must have reasonable setting time (required to harden) • Must have biocompatibility and aesthetics (taste, odor, appearance): non-toxic • Must be chemically compatible with material that is used to create the model

  6. Categories of Impression Materials • Inelastic (rigid) • Aqueous (water-based) • Non-Aqueous (rubber-based)

  7. INELASTIC (Rigid) • Original impression material • Not used much or at all today • Rigid • Can be brittle and break during removal • Best for edentulous mouths when making dentures • EXAMPLES: • Plaster (Type I gypsum) • Impression and Tray Compound • Zinc Oxide Eugenol impression paste

  8. Tray Compound Set-up

  9. Elastomeric Impression Material • Can stretch over tooth structures during removal and return to original form • Can be water-based (aqueous) or rubber-based (non-aqueous)

  10. Aqueous Elastomeric • Water-based • Examples: • Irreversible hydrocolloid (alginate) • Reversible hydrocolloid (agar)

  11. Aqueous Elastomeric

  12. Hydrocolloid • Another word for aqueous elastomeric • Colloid = material in which particles remain uniformly in suspension (gelatin + water = Jello) • Sol = liquid solution of colloid • Gel = hardened colloid

  13. Hydrocolloid (continued) • Gelation = rubbery semi-solid state • (Think J-E-L-L-O) • Reversible Hydrocolloid Impression Material: • Material is capable of changing from sol to gel and back to sol • Temperature changes from sol to gel • Warm to cool to warm (liquid) • Irreversible: • Chemical changes from sol to gel • Unable to return to liquid state • Temperature does not change it • Gel Strength is important; material can tear; needs to be removed from mouth quickly to avoid tears from stress

  14. Distortion of Impressions from: • Syneresis = too little water leads to shrinkage from evaporation • Imbibition: too much water leads to expansion from swelling

  15. Non-aqueous Elastomeric • Rubber-based • Commonly used for Final Impressions of crowns, bridges, veneers, etc.—produce fine detail • Examples: • Polysulfide rubber • Polyether rubber • Polyvinyl siloxane

  16. Non-Aqueous Elastomeric