NBDE Restorative Dentistry 5 — Questions and Answers
Question 1: Which type of taper is recommended for full-crown preparations to balance retention and resistance form?
- 0–2 degrees total convergence angle
- 6–10 degrees total convergence angle (Correct answer)
- 15–20 degrees total convergence angle
- 25–30 degrees total convergence angle
Correct answer: 6–10 degrees total convergence angle
A total occlusal convergence of 6–10 degrees provides adequate retention while allowing seating of the crown and appropriate resistance form.
Question 2: A patient has a defective marginal ridge on an existing MOD amalgam. The MOST likely clinical consequence if left untreated is:
- Pulpitis from microleakage
- Fracture of the marginal ridge and proximal wall
- Loss of contact and food impaction leading to periodontal disease (Correct answer)
- Occlusal wear of opposing teeth
Correct answer: Loss of contact and food impaction leading to periodontal disease
A defective marginal ridge can collapse, eliminating the proximal contact and causing food impaction that leads to periodontal bone loss and caries.
Question 3: The ADA Specification No. 1 for dental amalgam specifies maximum allowable dimensional change after setting. Which behavior is ACCEPTABLE?
- Contraction of 10 μm/cm
- Contraction of 20 μm/cm
- Expansion of 20 μm/cm or less (Correct answer)
- Expansion of any amount
Correct answer: Expansion of 20 μm/cm or less
ADA Specification No. 1 allows a maximum expansion of 20 μm/cm and a maximum contraction of 0 μm/cm (no net contraction permitted) for amalgam.
Question 4: Bonding to enamel using phosphoric acid etching creates micro-tags by:
- Dissolving the enamel surface entirely
- Selectively removing enamel rod cores or peripheries to create micro-porosities (Correct answer)
- Depositing a mineral layer on the enamel surface
- Dehydrating the enamel surface to improve wettability
Correct answer: Selectively removing enamel rod cores or peripheries to create micro-porosities
Phosphoric acid selectively dissolves enamel rod cores or peripheries depending on the etching pattern, creating micro-porosities into which resin flows and polymerizes.
Question 5: A full gold crown is being cemented with zinc phosphate. To minimize postoperative sensitivity, the dentist should:
- Mix the cement on a warm glass slab
- Use a thick mix with a short spatulation time
- Mix the cement on a chilled glass slab with prolonged spatulation to slow setting (Correct answer)
- Apply the cement only to the crown, not to the tooth
Correct answer: Mix the cement on a chilled glass slab with prolonged spatulation to slow setting
Mixing zinc phosphate on a chilled slab and spatulating over a larger area slows the exothermic reaction, lowers the initial pH, and reduces pulpal irritation.
Question 6: Which of the following is a contraindication for placing a porcelain-fused-to-metal crown on a maxillary first molar?
- Deep overbite with limited interocclusal clearance (Correct answer)
- Endodontically treated tooth
- Heavily restored tooth with multiple previous restorations
- Presence of a large existing amalgam
Correct answer: Deep overbite with limited interocclusal clearance
Insufficient occlusal clearance (deep overbite) prevents adequate reduction for the metal coping and overlying ceramic, risking restoration fracture.
Question 7: Smear layer removal during dentin bonding is important primarily because it:
- Reduces postoperative sensitivity by sealing dentinal tubules
- Allows resin monomers to infiltrate the underlying dentin for bonding (Correct answer)
- Increases the thickness of the hybrid layer
- Prevents bacterial contamination of the preparation
Correct answer: Allows resin monomers to infiltrate the underlying dentin for bonding
The smear layer is a physical barrier; removing or modifying it exposes collagen fibers and open tubules so resin can infiltrate and form a hybrid layer for adhesion.
