Impression Materials and Techniques Flashcards

What is the function of a tray adhesive when used with elastomeric impressions?

Answer: To bond the impression material to the tray and prevent separation

Tray adhesive creates a chemical and mechanical bond between the impression material and the tray walls, preventing distortion on removal.

Which dimension error occurs when an alginate impression is left in the mouth too long after setting?

Answer: Syneresis causing shrinkage

Prolonged oral contact allows syneresis — expulsion of liquid from the gel — causing the alginate to shrink slightly.

A bite registration material's primary requirement is:

Answer: Dimensional rigidity after setting to maintain occlusal relationship

Bite registrations must be rigid after setting so that casts can be reproducibly articulated without movement.

Polyvinyl siloxane impression materials release which byproduct during setting?

Answer: Hydrogen gas

Early-generation addition silicones released hydrogen gas, which could cause porosity in poured stone if models were made too soon after the impression.

What is the purpose of applying a retraction cord before taking a crown impression?

Answer: To displace gingival tissue and expose the margin subgingivally

Retraction cord mechanically (and sometimes chemically) retracts and displaces gingival tissue to expose the prepared margin for complete capture.

Which impression material is categorized as a non-aqueous elastomer?

Answer: Polysulfide rubber

Polysulfide, polyether, condensation silicone, and addition silicone are all non-aqueous elastomers; alginate and agar are hydrocolloids.

When disinfecting an alginate impression before sending to the lab, which method is recommended?

Answer: Spraying with an EPA-registered disinfectant and sealing in a bag for 10 minutes

Alginate impressions should be sprayed with an approved disinfectant, sealed for the contact time, rinsed, and then poured; immersion causes dimensional change.