NBDHE Dental Materials and Procedures Flashcards

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?

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.

What is the primary function of a cavity varnish when used under amalgam restorations?

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.

In an ideal casting procedure, what is the purpose of the sprue former attached to the wax pattern?

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.

Which characteristic of zirconia (Y-TZP) makes it the strongest all-ceramic material available for posterior crowns?

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.

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?

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.

When selecting a shade for a ceramic restoration, at what time of day and under what light should shade matching ideally be performed?

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.

What is the main disadvantage of resin-modified glass ionomer cement (RMGI) compared to conventional glass ionomer when used as a base?

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.