Dental Impressions DA 122 Dental Materials

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

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

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

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

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

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

Tray Compound Set-up

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)

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

Aqueous Elastomeric

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

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

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

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

Non-Aqueous Elastomeric