EFDA Impression Materials and Techniques 3 — Questions and Answers
Question 1: Which impression material is classified as a reversible hydrocolloid?
- Alginate
- Agar (Correct answer)
- Polyether
- Polysulfide
Correct answer: Agar
Agar (colloid) is reversible because it can be repeatedly liquefied by heating and re-gelled by cooling, unlike alginate which sets irreversibly.
Question 2: When taking a preliminary alginate impression, the dentist notices voids in the palate area. What is the most likely cause?
- Too much water used in the mix
- Air entrapment during spatulation or tray seating (Correct answer)
- The alginate was over-spatulated
- The tray was seated too slowly
Correct answer: Air entrapment during spatulation or tray seating
Voids in the impression are commonly caused by air bubbles trapped during mixing or when the material fails to flow into all areas during tray placement.
Question 3: The water-to-powder ratio for alginate primarily affects which property?
- Color stability
- Tear strength and flow (Correct answer)
- Setting time only
- Dimensional accuracy only
Correct answer: Tear strength and flow
Altering the water-to-powder ratio changes both the consistency (flow) and the tear strength of the set alginate.
Question 4: A polysulfide impression has a distinctive smell. What causes this?
- Eugenol in the accelerator paste
- Sulfur-containing compounds in the base paste reacting with lead dioxide (Correct answer)
- Acrylic monomers released during setting
- Zinc oxide filler particles
Correct answer: Sulfur-containing compounds in the base paste reacting with lead dioxide
Polysulfide rubber contains mercaptan (thiol) sulfur groups that react with lead dioxide accelerator, producing a strong sulfur odor.
Question 5: What is the recommended action if an alginate impression cannot be poured immediately?
- Store in an airtight bag with a moist paper towel for up to 1 hour (Correct answer)
- Submerge fully in water for up to 24 hours
- Spray with disinfectant and leave uncovered on the counter
- Freeze the impression to halt dimensional change
Correct answer: Store in an airtight bag with a moist paper towel for up to 1 hour
Alginate should be wrapped in a damp towel and sealed in a bag to prevent syneresis and imbibition; ideally poured within 1 hour.
Question 6: Which type of impression tray is custom-made from the patient's diagnostic model?
- Stock metal tray
- Perforated plastic tray
- Individual custom tray (Correct answer)
- Quadrant bite tray
Correct answer: Individual custom tray
Custom trays are fabricated directly over a patient's cast, providing a uniform spacer distance from tissues and improving final impression accuracy.
Question 7: During impression removal, the tray should be removed with:
- A slow rocking motion from side to side
- A quick, decisive snap in the direction of insertion (Correct answer)
- Gradual pulling straight down on the lower arch
- Rotating the tray 90 degrees first
Correct answer: A quick, decisive snap in the direction of insertion
A swift, single-direction removal minimizes distortion by reducing the time elastic materials are stressed during withdrawal.
EFDA Impression Materials and Techniques 3
Which impression material is classified as a reversible hydrocolloid?