Crown & Bridge Fabrication Flashcards

When articulating casts for crown and bridge fabrication, the primary purpose of a facebow transfer is to:

Answer: Relate the maxillary cast to the hinge axis of the articulator

A facebow transfer records the spatial relationship of the maxillary arch to the patient's transverse hinge axis so the maxillary cast is correctly oriented on the articulator.

In the lost-wax casting process, what is the purpose of the 'burnout' stage performed before metal injection?

Answer: To eliminate the wax pattern and preheat the mold to receive molten metal

Burnout eliminates the wax pattern by incineration and preheats the investment mold, ensuring the molten alloy flows fully into the cavity without premature solidification.

What type of finish line preparation is considered most favorable for a full-cast metal crown?

Answer: Knife-edge (feather edge)

A knife-edge (feather edge) finish line is traditionally used for full-cast metal crowns because thin metal can be burnished to adapt closely to the tooth, achieving an excellent marginal seal.

A technician notices that a porcelain crown has developed vertical 'check cracks' after the glaze firing. The most likely cause is:

Answer: Rapid cooling through the thermal shock range

Check cracks develop when the crown is cooled too rapidly after firing, causing differential thermal contraction stresses that exceed the tensile strength of the porcelain.

Which factor is most responsible for galvanic corrosion when dissimilar metals are present in the mouth?

Answer: Difference in the electrode potential of the two alloys in saliva

Galvanic corrosion occurs because dissimilar alloys have different electrode (reduction) potentials; in the electrolytic environment of saliva, an electric current flows and the less noble alloy corrodes.

When adjusting the occlusion of a PFM crown on a semi-adjustable articulator, grinding should primarily be done:

Answer: On the porcelain surface using diamond burs followed by repolishing

When occlusal adjustment is unavoidable on porcelain, fine diamond burs are used followed by progressive polishing or glazing to restore the surface and minimize wear to opposing teeth.

Which investment material property is critical to compensate for the thermal contraction of gold alloys during solidification after casting?

Answer: Adequate setting and thermal expansion to expand the mold

Investment materials are formulated to expand (through setting expansion and thermal expansion) to compensate for the shrinkage of the alloy as it cools from liquid to solid.