EFDA Impression Materials and Techniques 4 — Questions and Answers
Question 1: What is the function of a tray adhesive when used with elastomeric impressions?
- To accelerate the setting reaction
- To bond the impression material to the tray and prevent separation (Correct answer)
- To improve the flow of material into sulcus areas
- To extend working time
Correct 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.
Question 2: Which dimension error occurs when an alginate impression is left in the mouth too long after setting?
- Imbibition causing expansion
- Syneresis causing shrinkage (Correct answer)
- No dimensional change occurs after setting
- Polymerization continues causing contraction
Correct answer: Syneresis causing shrinkage
Prolonged oral contact allows syneresis — expulsion of liquid from the gel — causing the alginate to shrink slightly.
Question 3: A bite registration material's primary requirement is:
- High flow to capture soft tissue detail
- Dimensional rigidity after setting to maintain occlusal relationship (Correct answer)
- Extended working time of 10+ minutes
- High tear strength for multiple uses
Correct 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.
Question 4: Polyvinyl siloxane impression materials release which byproduct during setting?
- Hydrogen gas (Correct answer)
- Carbon dioxide
- Water vapor
- No byproduct — it is a true addition reaction
Correct 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.
Question 5: What is the purpose of applying a retraction cord before taking a crown impression?
- To displace gingival tissue and expose the margin subgingivally (Correct answer)
- To anesthetize the sulcus before impression material is placed
- To hold the impression tray in position during setting
- To absorb saliva and improve hydrophilicity of the material
Correct 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.
Question 6: Which impression material is categorized as a non-aqueous elastomer?
- Alginate
- Agar hydrocolloid
- Polysulfide rubber (Correct answer)
- Plaster impression material
Correct answer: Polysulfide rubber
Polysulfide, polyether, condensation silicone, and addition silicone are all non-aqueous elastomers; alginate and agar are hydrocolloids.
Question 7: When disinfecting an alginate impression before sending to the lab, which method is recommended?
- Immersing in 2% glutaraldehyde for 60 minutes
- Spraying with an EPA-registered disinfectant and sealing in a bag for 10 minutes (Correct answer)
- Autoclaving at 121°C
- Ultraviolet light exposure for 30 minutes
Correct 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.
EFDA Impression Materials and Techniques 4
What is the function of a tray adhesive when used with elastomeric impressions?