NBDHE NBDHE Dental Materials and Procedures 5 — Questions and Answers
Question 1: Which type of elastomeric impression material requires the use of a custom or stock tray with adhesive AND has the longest setting time among elastomers?
- Polyether
- Polysulfide (mercaptan) (Correct answer)
- Condensation silicone
- Addition silicone (PVS)
Correct answer: Polysulfide (mercaptan)
Polysulfide (mercaptan) impression materials have the longest setting time of all elastomers, require an adhesive for tray retention, and must be poured promptly due to poor dimensional stability.
Question 2: What is the primary function of a cavity varnish when used under amalgam restorations?
- To stimulate reparative dentin formation
- To reduce microleakage and protect the pulp from chemical irritants (Correct answer)
- To increase retention of the amalgam
- To release fluoride into the surrounding dentin
Correct answer: To reduce microleakage and protect the pulp from chemical irritants
Cavity varnish seals the dentinal tubules to reduce penetration of corrosion products and chemical irritants from amalgam into the pulp until self-sealing occurs.
Question 3: In an ideal casting procedure, what is the purpose of the sprue former attached to the wax pattern?
- To vent gases during casting
- To provide a channel for molten metal to enter the mold cavity (Correct answer)
- To support the wax pattern during investing
- To control the setting expansion of the investment
Correct answer: To provide a channel for molten metal to enter the mold cavity
The sprue former, when removed, creates a channel (sprue) through which molten alloy is forced under centrifugal or pressure casting into the mold space.
Question 4: Which characteristic of zirconia (Y-TZP) makes it the strongest all-ceramic material available for posterior crowns?
- High silica content that increases translucency
- Transformation toughening mechanism that arrests crack propagation (Correct answer)
- Ability to be fired to a high gloss without glazing
- Chemical bond formation with resin cements
Correct answer: Transformation toughening mechanism that arrests crack propagation
Yttria-stabilized zirconia undergoes stress-induced phase transformation from tetragonal to monoclinic at crack tips, which increases volume and closes the crack — a toughening mechanism called transformation toughening.
Question 5: A patient reports sensitivity after placement of a resin composite in a Class II preparation. Which procedural step most likely prevented adequate hybrid layer formation?
- Applying too many coats of the bonding agent
- Over-drying the etched dentin, causing dentin collapse (Correct answer)
- Extending the acid etch time on enamel beyond 30 seconds
- Using a self-cure activator with a light-cure adhesive
Correct answer: Over-drying the etched dentin, causing dentin collapse
Over-drying etched dentin collapses the collagen fibril network, preventing resin monomer penetration and resulting in an incomplete hybrid layer with residual sensitivity.
Question 6: When selecting a shade for a ceramic restoration, at what time of day and under what light should shade matching ideally be performed?
- Late afternoon under fluorescent operatory lights
- Early in the appointment under natural daylight or balanced daylight bulbs (Correct answer)
- After rubber dam placement under the dental curing light
- During try-in under the patient's home lighting conditions
Correct answer: Early in the appointment under natural daylight or balanced daylight bulbs
Shade matching should be done early before the patient's teeth dehydrate and lighten, using natural daylight or a balanced color-corrected light source to avoid metamerism.
Question 7: What is the main disadvantage of resin-modified glass ionomer cement (RMGI) compared to conventional glass ionomer when used as a base?
- Inferior fluoride release over time
- Hygroscopic expansion that can fracture all-ceramic restorations (Correct answer)
- Inability to bond chemically to tooth structure
- Lower biocompatibility near the pulp
Correct answer: Hygroscopic expansion that can fracture all-ceramic restorations
RMGI absorbs water and expands hygroscopically after setting, which can generate stresses sufficient to crack brittle all-ceramic restorations placed over them.
NBDHE NBDHE Dental Materials and Procedures 5
Which type of elastomeric impression material requires the use of a custom or stock tray with adhesive AND has the longest setting time among elastomers?