NDAEB Dental Materials and Equipment 2 — Questions and Answers
Question 1: Alginate impression material is classified as which type of impression material?
- Reversible hydrocolloid
- Irreversible hydrocolloid (Correct answer)
- Elastomeric material
- Non-elastic material
Correct answer: Irreversible hydrocolloid
Alginate is an irreversible hydrocolloid because once it undergoes its chemical setting reaction, it cannot be returned to its original state by heating.
Question 2: What is the correct storage method for an alginate impression before it can be poured with stone?
- In open air at room temperature
- Submerged completely in water
- Wrapped in a damp paper towel sealed in a plastic bag (Correct answer)
- Refrigerated without any moisture
Correct answer: Wrapped in a damp paper towel sealed in a plastic bag
Alginate impressions must be wrapped in a damp paper towel and sealed in a bag to prevent syneresis (shrinkage from drying) or imbibition (swelling from absorbing water).
Question 3: Polyvinyl siloxane (PVS) is classified under which category of impression materials?
- Irreversible hydrocolloid
- Reversible hydrocolloid
- Elastomeric impression material (Correct answer)
- Non-elastic zinc oxide eugenol
Correct answer: Elastomeric impression material
PVS is an addition silicone and belongs to the elastomeric category, meaning it sets to a flexible, rubber-like consistency that allows accurate reproduction of undercuts.
Question 4: Which type of gypsum product is MOST appropriate for fabricating dies used in crown and bridge construction?
- Type I dental plaster
- Type II model plaster
- Type III dental stone
- Type IV die stone (Correct answer)
Correct answer: Type IV die stone
Type IV die stone has the highest compressive strength and lowest setting expansion, providing the accuracy and hardness required for crown and bridge die fabrication.
Question 5: How does increasing the water-to-powder ratio of alginate primarily affect the mix?
- Shortens the setting time and increases strength
- Lengthens the setting time and produces a thinner mix (Correct answer)
- Has no effect on setting time
- Improves the dimensional stability of the final impression
Correct answer: Lengthens the setting time and produces a thinner mix
Adding more water to alginate dilutes the reaction, lengthening the setting time and producing a thinner, more fluid consistency that is weaker when set.
Question 6: Which type of impression tray provides the MOST accurate result when using polyvinyl siloxane material?
- Perforated plastic stock tray without adhesive
- Rim-lock tray with water
- Custom acrylic tray with PVS adhesive (Correct answer)
- Solid metal stock tray used dry
Correct answer: Custom acrylic tray with PVS adhesive
A custom acrylic tray creates uniform material thickness of 2–3 mm throughout, minimizes distortion, and with PVS adhesive ensures the material stays seated for maximum accuracy.
Question 7: Which material is most commonly used today to fabricate a provisional (temporary) crown after tooth preparation?
- Zinc oxide eugenol paste
- Bis-acrylic composite (Correct answer)
- Polycarboxylate cement
- Acrylic resin powder only
Correct answer: Bis-acrylic composite
Bis-acrylic composite is the standard material for provisional crowns because it offers good esthetics, adequate strength, easy trimming, and can be made chair-side using a matrix of the pre-op tooth.
NDAEB Dental Materials and Equipment 2
Alginate impression material is classified as which type of impression material?