NBDE Restorative Dentistry Questions and Answers — Questions and Answers
Question 1: In high-copper dental amalgams, what is the primary function of the increased copper content?
- To increase the setting expansion of the material.
- To reduce corrosion potential by eliminating the Gamma-2 (Sn-Hg) phase. (Correct answer)
- To improve the esthetics and polishability of the final restoration.
- To decrease the compressive strength of the set amalgam.
Correct answer: To reduce corrosion potential by eliminating the Gamma-2 (Sn-Hg) phase.
High-copper amalgams were developed to eliminate the weak and corrosion-prone Gamma-2 (Sn7-8Hg) phase. The copper reacts preferentially with tin, preventing the formation of the Gamma-2 phase, which significantly improves the longevity, strength, and marginal integrity of the restoration.
Question 2: Which component of dental composite resin is responsible for chemically bonding the inorganic filler particles to the organic resin matrix?
- Initiator (e.g., camphorquinone)
- Resin matrix (e.g., Bis-GMA)
- Inhibitor (e.g., BHT)
- Silane coupling agent (Correct answer)
Correct answer: Silane coupling agent
The silane coupling agent is a bifunctional molecule that creates a durable bond between the inorganic filler particles (like glass or quartz) and the organic resin matrix. This bond is crucial for transferring stress from the weaker resin to the stronger filler, improving the composite's mechanical properties and longevity.
Question 3: A clinician is preparing a Class II cavity for an amalgam restoration on a mandibular molar. To ensure adequate resistance form, which of the following design features is most important?
- Convergent buccal and lingual walls.
- A flat pulpal floor perpendicular to the long axis of the tooth. (Correct answer)
- Beveling the axiopulpal line angle.
- Ensuring the isthmus width is less than one-fourth the intercuspal distance.
Correct answer: A flat pulpal floor perpendicular to the long axis of the tooth.
Resistance form is the shape of the preparation that enables both the tooth and restoration to withstand masticatory forces without fracture. A flat pulpal floor, oriented perpendicular to the long axis of the tooth, is a key feature that helps distribute occlusal forces and prevents fracture. While isthmus width (D) is also related to resistance form, the flat floor is a more fundamental principle. Convergent walls (A) contribute primarily to retention form.
Question 4: When preparing a tooth for a composite restoration, what is the primary purpose of applying 30-40% phosphoric acid to the enamel surface?
- To disinfect the prepared cavity surface.
- To remove the smear layer and create microporosities for micromechanical retention. (Correct answer)
- To chemically activate the primer in the bonding agent.
- To expose collagen fibrils for hybridization in the enamel.
Correct answer: To remove the smear layer and create microporosities for micromechanical retention.
Acid etching the enamel with phosphoric acid selectively dissolves the ends of the enamel rods, creating microscopic surface irregularities or microporosities. This increases surface energy and allows the low-viscosity resin adhesive to penetrate these porosities, creating a strong micromechanical bond upon polymerization. While it also removes the smear layer, its primary function is creating this retentive pattern. Collagen fibrils (D) are a component of dentin, not enamel.
Question 5: A pediatric patient with high caries risk requires a restoration on a newly erupted first permanent molar where complete moisture control is difficult. Which of the following materials is most indicated due to its unique chemical properties?
- Packable composite resin
- Dental amalgam
- Resin-modified glass ionomer (RMGI) (Correct answer)
- Ceramic inlay
Correct answer: Resin-modified glass ionomer (RMGI)
Resin-modified glass ionomer (RMGI) is an ideal choice in this scenario. It has two key properties that make it suitable: 1) It chemically bonds to tooth structure and is more tolerant of moisture than composite resin. 2) It releases fluoride over time, which provides a therapeutic, caries-protective effect for a patient with high caries risk.
Question 6: For a tooth that has undergone endodontic treatment and requires a full coverage crown, what is the primary purpose of ensuring a 1.5 to 2.0 mm ferrule?
- To provide adequate retention for the temporary crown.
- To create a friction-locked seal at the crown margin.
- To improve the esthetics of the final restoration by hiding the margin.
- To resist functional lever forces and protect the tooth from vertical fracture. (Correct answer)
Correct answer: To resist functional lever forces and protect the tooth from vertical fracture.
The ferrule is a 360-degree collar of sound tooth structure at the gingival margin of a crown preparation. Its primary purpose is to encircle the tooth and protect the root from fracture by resisting lateral and functional lever forces. This "bracing" action significantly improves the fracture resistance and long-term prognosis of endodontically treated teeth.
NBDE Restorative Dentistry Questions and Answers
In high-copper dental amalgams, what is the primary function of the increased copper content?