Dental Materials For Dental Assistants And Hygienists 3rd Edition By Stephan – Test Bank

Chapter 11: Casting Metals, Solders, and Wrought Metal Alloys

MULTIPLE CHOICE

1. Which of the following statements is true of noble dental casting alloys?
a. They require gold and must contain at least 25% noble metals by weight.
b. They do not require gold and require less than 25% by weight of noble metals.
c. They do not require any gold but must contain at least 25% noble metals by weight.
d. They must contain at least 60% by weight of noble alloys, of which gold must account for at least 40% by weight.

ANS: A
Noble dental casting alloys do not require any gold but must contain at least 25% noble metals by weight. High-noble alloys must contain at least 60% by weight of noble alloys, of which gold must account for at least 40% by weight. Base-metal alloys do not require gold and require less than 25% by weight of noble metals.

REF: p. 171 TOP: Casting Alloys (All-Metal Castings)

2. Which description is correct regarding palladium?
a. Is more expensive than gold
b. Has poor corrosion resistance
c. Increases the hardness of the alloy
d. Is a precious metal but it is not considered noble

ANS: C
Palladium increases the hardness of the alloy. It is widely used because it has good corrosion resistance and is less expensive than gold. Platinum and palladium are noble metals that are sometimes referred to as precious metals because of their high monetary value. Although silver is considered to be a precious metal, it is not considered noble because of its corrosion in the oral cavity.

REF: p. 171 TOP: Casting Alloys (Other Noble Metals for Casting Alloys)

3. Which of the following is a noble metal used for dental casting alloys?
a. Silver (Ag)
b. Platinum (Pt)
c. Copper (Cu)
d. Titanium (Ti)

ANS: B
Platinum, palladium, and gold are noble metals used for dental casting alloys. The primary base metals used in casting alloys are copper, nickel, silver, zinc, tin, and titanium. Copper and silver are often added to gold alloys to increase their hardness. Zinc is added to reduce oxidation when the alloy is cast. Because of their low cost, base metals have also been called nonprecious metals. Although they are not considered to be as good as the noble metals, the base metals are essential for many applications in dentistry.

REF: p. 171 TOP: Casting Alloys (Other Noble Metals for Casting Alloys)

4. Which of the following is true concerning grain formation in dental casting alloys?
a. Reheating gold-based alloys will degrade the physical properties.
b. Reheating base-metal alloys will not degrade the physical properties.
c. Large crystals produce more desirable properties in the metal alloy than small crystals do.
d. Iridium or ruthenium is added to gold-base alloys to keep the crystals from growing too large.

ANS: D
Iridium or ruthenium is added to gold-base alloys to keep the crystals from growing too large. Small crystals produce more desirable properties in the metal alloy than large crystals do. Reheating gold-based alloys (called annealing) can improve some of the properties. However, with base-metal alloys, reheating will degrade the physical properties.

REF: p. 172 TOP: Casting Alloys (Crystal Formation [Grains])

5. The metals in porcelain bonding alloys are selected and blended so that they have which of the following properties?
a. Small amounts of metals such as indium, iron, tin, or gallium are added to form oxides on the metal surface to which the porcelain will bond.
b. They have the ability to withstand the low temperatures at which porcelain is fired without distorting or melting.
c. They have a higher thermal expansion than gold alloys used for all-metal crowns.
d. Titanium and titanium alloys are added for easy handling.

ANS: A
With metals in porcelain bonding alloys, small amounts of metals such as indium, iron, tin, or gallium are added to form oxides on the metal surface to which porcelain will bond. They have the ability to withstand the high temperatures at which porcelain is fired without distorting or melting. They have a lower thermal expansion than gold alloys used for all-metal crowns. Titanium and titanium alloys have such high melting temperatures and difficulty in handling that they have limited use for ceramic-metal crowns.

REF: pp. 172-173 TOP: Casting Alloys (Porcelain Bonding Alloys)

6. Which of the following is true of the preparation of the metal substructure for porcelain-bonded-to-metal crowns?
a. The metal substructure should have sharp angles.
b. The metal substructure must be concave in shape.
c. The metal substructure must be at least 1.5 mm thick.
d. To make room for the metal substructure and porcelain, these crowns require greater reduction of the tooth than all-metal crowns.

ANS: D
To make room for the metal substructure and porcelain, these crowns require greater reduction of the tooth than all-metal crowns. Porcelain-bonded-to-metal crowns have a metal substructure that is covered with layers of porcelain. The metal substructure must be at least 0.3 mm thick to prevent distortion at high temperatures and to be convex in shape with no sharp angles.

REF: p. 173 TOP: Casting Alloys (Preparing the Metal)

7. After the substructure for a porcelain-bonded-to-metal crown is cast in metal, why is it heated at high temperature?
a. Anneal the metal
b. Reduce the grain size
c. Form oxides on the surface of the metal
d. Cause the metal to slump and adapt to the die

ANS: C
After the substructure for a porcelain-bonded-to-metal crown is cast in metal, it is heated at high temperature to form oxides on the surface of the metal. This process is called degassing, although oxidizing would be a more accurate term.

REF: p. 173 TOP: Casting Alloys (Preparing the Metal)

8. Which of the following represents the correct order for application of the porcelain layers during the creation of a porcelain-bonded-to-metal crown?
1. Incisal porcelain
2. Opaque porcelain
3. Body porcelain
a. 1, 2, 3
b. 1, 3, 2
c. 2, 3, 1
d. 2, 1, 3

ANS: C
The correct order is opaque porcelain, which is the first layer of porcelain applied to the metal, and then body and incisal porcelains, which are built up or stacked to simulate dentin and enamel colors and translucency. After final contouring of the crown, another firing while maintaining the temperature at the fusing temperature for a while will produce a surface glaze.

REF: p. 173 TOP: Casting Alloys (Fusing Porcelain to Metal)

9. Which of the following is the most esthetic margin for a ceramic-metal crown?
a. Disappearing metal margin
b. Porcelain facial margin
c. All-metal margin
d. All-metal facing

ANS: B
The porcelain facial margin is the most esthetic margin. This is achieved by not extending the metal substructure all the way to the margin and leaving room to place porcelain to complete the margin. The metal at the margin of a disappearing metal margin crown has been ground to a thin knife-edge where it extends to the edge of the preparation margin. The all-metal margin is used only when it is not in an esthetic zone.

REF: p. 173 TOP: Casting Alloys (Crown Design)

10. Which of the following base metals has the highest incidence of allergic response?
a. Nickel
b. Chromium
c. Beryllium
d. Gallium

ANS: A
Nickel has the highest incidence of allergic response. The allergic response is sometimes seen around the free gingival tissues, especially at the margins of base-metal crowns. It is less common for removable partial dentures because the metal often is not in direct contact with the tissues, and they are not worn constantly as with fixed partial dentures or single crowns. Some responses to nickel cause a skin reaction rather than a response in the mouth, even though the oral cavity is the source of the nickel.

REF: p. 175 TOP: Casting Alloys (Biocompatibility)