Prosthodontics MCQ Answer 52

0 comments

Share

E is the correct answer. It is false statement

1. Is a true statement. Over tapering of the opposing axial walls can be corrected if a band of several millimeters of tooth structure can be prepared circumferentially with a restricted taper of approximately 6 degrees. It is probably unnecessary to further modify the preparation to compensate for the areas of excessive reduction in the occlusal third. If this is not the case, one can used an approach slightly less conservative of tooth structure such as uprighting overtapered axial walls to obtain the mechanical advantage of increased retention or using grooves, boxes, or pinholes as needed.

2. Is a true statement. Theoretically, maximum retention is obtained if a tooth preparation has parallel walls, but, a slight convergence, or taper, is necessary in the completed preparation. As long as this taper is small, the movement of the cemented restoration will be effectively retained by the preparation and will have what is known as a limited path of withdrawal. As taper increases, the free movement of the restoration will do so likewise, and reduce the retention.

3. Is a true statement. Crowns with long axial walls are more retentive than those with short axial walls. Molar crowns are more retentive than premolar crowns of similar taper.

Additional information: The factors influencing the resistance of cemented restorations include luting agents of the following in order of decreasing resistance: adhesive resin, glass ionomer, zinc phosphate, polycarboxylate, zinc oxide-eugenol

4. Is a true statement. In a short or excessively tapered complete crown, resistance form is minimal because most of the buccal wall is missing. A mesiodistal groove should be placed to increase resistance form.

A 7/8 crown with grooves has less retention than a complete crown with no grooves. According to a study by Potts RG et al: J Pros Dent 43:303, 1980. The removal force for a complete crown with no grooves was 1080 N versus the 7/8 crown with grooves which required only 507 N of removal force.

References: Rosenthiel, Land and Fujimoto: Contemporary Fixed Prosthodontics, Third Ed., Mosby, 2001.

Potts RG et al: J Pros Dent 43:303, 1980.