Which of the following statements applies to globular dentin?

 # Which of the following statements applies to globular dentin? 

a. Deposited first along the dentinoenamel junction. 

b. Mineralized by matrix vesicles. 

c. Separates mantle dentin from circumpulpal dentin. 

d. Secreted by immature odontoblasts. 

The correct answer is C. Separates mantle dentin from circumpulpal dentin.

Sometimes mineralization of dentin begins in small globular areas that fail to coalesce into a homogeneous mass. This results in zones of hypomineralization between the globules. These zones are known as globular dentin or interglobular spaces. This dentin forms in the crowns of teeth in the circumpulpal dentin just below the mantle dentin, and it follows the incremental pattern. The dentinal tubules pass uninterruptedly through interglobular dentin, thus demonstrating defect of mineralization and not of matrix formation. 

Which is NOT a paired bone of the skull?

 # Which is NOT a paired bone of the skull?

a. Nasal.

b. Maxilla.

c. Lacrimal.

d. Sphenoid.

The correct answer is D. Sphenoid. 

Out of the 8 cranial bones, Parietal and Temporal are only paired. Occipital, Sphenoid, Frontal and Ethmoid are unpaired bones of the skull. Nasal, Maxilla and Lacrimal bones are paired though they don't constitute the cranium, they are facial bones.

Admixed high copper alloy powder contains:

  # Admixed high copper alloy powder contains:

A. 9-20% copper

B. 13-20% copper

C. 9-30% copper

D. 13-30% copper

The correct answer is A. 9-20% copper.

In low copper alloys, copper is less than 6% by weight. In high copper single composition alloys, 13-30% copper by weight is present whereas in high copper admixed alloys, 9-20% copper is present.



Back pressure porosity can be avoided by:

  # Back pressure porosity can be avoided by:

A. Using asbestos liners

B. Placing the sprue at least one quarter inch away from the end of casting ring

C. Preventing rapid heating of investment

D. Using a short and wide sprue

The correct answer is B. Placing the sprue at least one quarter inch away from the end of casting ring

Entrapped air bubbles on the inner surface of the casting, sometimes referred to as back-pressure porosity, can produce large concave depressions. This is caused by the inability of the air in the mold to escape through the pores in the investment or by the pressure gradient that displaces the air pocket toward the end of the investment via the molten sprue and button. The incidence of entrapped air can be increased by the use of the dense modern investments.

All castings probably contain a certain amount of porosity, as exemplified by the photomicrographs. This should be minimized because it may adversely affect the physical properties of the casting. Proper burnout, an adequate mold and casting temperature, a sufficiently high casting pressure, and a proper investment L/P ratio can minimize or eliminate entrapped air porosity. It is good practice to make sure that the thickness of investment between the tip of the pattern and the end of the ring is not greater than 6 mm.