NBDHE NBDHE Dental Materials and Procedures 4 — Questions and Answers
Question 1: Which dental ceramic material is most commonly used for all-ceramic crowns requiring the highest esthetics in the anterior region?
- Zirconia
- Leucite-reinforced ceramic (IPS Empress) (Correct answer)
- Feldspathic porcelain (veneering only)
- Alumina core ceramic
Correct answer: Leucite-reinforced ceramic (IPS Empress)
Leucite-reinforced glass ceramics provide excellent optical properties and translucency that mimic natural tooth enamel, making them preferred for anterior esthetic restorations.
Question 2: What is the working time of a dental material defined as?
- Time from start of mix until the material can no longer be manipulated (Correct answer)
- Time from start of mix until final set
- Time the impression can remain in the mouth
- Time the material takes to reach body temperature
Correct answer: Time from start of mix until the material can no longer be manipulated
Working time is the period from the beginning of mixing during which the material can be manipulated and placed without compromising its final properties.
Question 3: What is the significance of the 'critical surface tension' concept when bonding to enamel?
- Higher surface tension of the adhesive increases bond strength
- The adhesive must have lower surface tension than the substrate to wet and spread properly (Correct answer)
- Critical surface tension determines fluoride release rate
- Higher substrate surface tension reduces penetration of resin tags
Correct answer: The adhesive must have lower surface tension than the substrate to wet and spread properly
For proper wetting and adhesion, the surface tension of the adhesive must be lower than the critical surface tension of the substrate so the adhesive can spread and flow into surface irregularities.
Question 4: A patient's porcelain-fused-to-metal crown shows repeated porcelain fracture at the incisal edge. What is the most likely cause?
- Insufficient metal support (coping) beneath the porcelain (Correct answer)
- Too thick a layer of opaque porcelain
- Overcontouring of the gingival margin
- Using feldspathic porcelain instead of leucite-reinforced
Correct answer: Insufficient metal support (coping) beneath the porcelain
Porcelain requires adequate metal support; without it, flexure of the metal coping during function creates tensile stresses in the brittle porcelain, causing fracture.
Question 5: Which property of dental amalgam is most responsible for its long clinical service life despite inferior marginal integrity compared to composites?
- High compressive strength and hardness
- Self-sealing behavior due to corrosion products (Correct answer)
- High coefficient of thermal expansion
- Ability to release fluoride
Correct answer: Self-sealing behavior due to corrosion products
Corrosion products (oxides and chlorides) at amalgam margins expand to seal the marginal gap over time, reducing microleakage despite the initially poor marginal fit.
Question 6: When fabricating a custom acrylic tray, why is a spacer wax placed over the diagnostic cast?
- To prevent the tray from bonding to the stone cast
- To create space for an adequate thickness of impression material (Correct answer)
- To prevent distortion of the tray during curing
- To increase the strength of the tray walls
Correct answer: To create space for an adequate thickness of impression material
The spacer wax creates a uniform space between the tray and the tissue, ensuring there is enough room for the impression material to flow and record fine detail.
Question 7: What is the role of eugenol in zinc oxide eugenol (ZOE) cement?
- It acts as the acid component that reacts with zinc oxide
- It acts as a chelating agent that reacts with zinc oxide to form zinc eugenolate (Correct answer)
- It lowers the pH to create an acidic environment for bonding
- It increases the setting expansion of the cement
Correct answer: It acts as a chelating agent that reacts with zinc oxide to form zinc eugenolate
Eugenol chelates with zinc oxide to form an amorphous zinc eugenolate matrix that is the primary reaction product giving ZOE its sedative and setting properties.
NBDHE NBDHE Dental Materials and Procedures 4
Which dental ceramic material is most commonly used for all-ceramic crowns requiring the highest esthetics in the anterior region?