RDA Restorative and Impression Materials 4 — Questions and Answers
Question 1: Which impression material is most affected by the presence of moisture or blood during the impression-taking procedure?
- Polyvinyl siloxane (PVS) (Correct answer)
- Zinc oxide eugenol (ZOE)
- Alginate
- Polyether
Correct answer: Polyvinyl siloxane (PVS)
Polyvinyl siloxane (PVS) is hydrophobic and is significantly affected by moisture, blood, or saliva contamination, leading to voids and inaccurate impressions.
Question 2: When mixing glass ionomer cement for a base, what is the ideal powder-to-liquid ratio compared to a luting consistency?
- Lower powder ratio than luting
- Same powder ratio as luting
- Higher powder ratio than luting (Correct answer)
- No liquid is used for a base
Correct answer: Higher powder ratio than luting
A base consistency of glass ionomer requires a higher powder-to-liquid ratio to achieve a stiffer, more viscous mix that can withstand condensation forces.
Question 3: What is the purpose of applying a cavity varnish before placing an amalgam restoration?
- To bond the amalgam to the tooth structure
- To reduce microleakage and protect the pulp from irritants (Correct answer)
- To increase the strength of the amalgam
- To improve the color match of the restoration
Correct answer: To reduce microleakage and protect the pulp from irritants
Cavity varnish seals dentinal tubules to reduce microleakage and protects the pulp from chemical irritants until the amalgam has fully set.
Question 4: A custom tray for a final impression should be constructed to leave how much space for impression material between the tray and the tissues?
- 0.5 mm
- 2–3 mm (Correct answer)
- 5–6 mm
- No space — the tray should contact the tissues directly
Correct answer: 2–3 mm
A uniform 2–3 mm space between the custom tray and tissues allows for an adequate thickness of impression material for dimensional accuracy.
Question 5: Which restorative material undergoes the LEAST thermal expansion and contraction, making it most dimensionally stable in the oral cavity?
- Composite resin
- Amalgam
- Gold alloy
- Ceramic (porcelain) (Correct answer)
Correct answer: Ceramic (porcelain)
Ceramic/porcelain has a coefficient of thermal expansion very close to that of natural tooth structure, making it the most dimensionally stable under temperature changes.
Question 6: During the setting of zinc phosphate cement, why is the powder incorporated into the liquid in small increments over a wide, cool glass slab?
- To increase the final strength of the cement
- To speed up the setting reaction
- To dissipate heat and extend working time (Correct answer)
- To improve the powder-liquid homogeneity only
Correct answer: To dissipate heat and extend working time
The setting reaction of zinc phosphate is exothermic; spatulating on a cool slab and adding powder incrementally dissipates heat and prolongs working time.
Question 7: Which type of gypsum product is used to construct dies for crown and bridge fabrication due to its high compressive strength and minimal setting expansion?
- Plaster of Paris (Type I)
- Dental stone (Type III)
- Die stone (Type IV) (Correct answer)
- Impression plaster (Type II)
Correct answer: Die stone (Type IV)
Type IV die stone (high-strength stone) has the greatest compressive strength and lowest setting expansion, making it ideal for accurate die fabrication.
RDA Restorative and Impression Materials 4
Which impression material is most affected by the presence of moisture or blood during the impression-taking procedure?