Agent used as a die hardener

# Which of the following agents is used as a die hardener?

A. Cyanoacrylate

B. Nail polish

C. Volatile relief agents

D. Composite resin

The correct answer is A. Cyanoacrylate.

Gypsum dies are sometimes modified to (1) make them more abrasion-resistant, (2) change the dimensions of the dies, (3) increase the refractoriness of the dies, or (4) produce a combination

of these effects. Several means are used to increase the abrasion resistance, including silver plating, coating the surface with cyanoacrylate adhesive, and adding a die hardener to the gypsum.

Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, Anusavice, Shen, Rawls, 12th Edition, Page No: 211

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Factors associated with external root resorption

 # Following are the factors associated with external root resorption except:

A. Grafting of alveolar clefts

B. Cysts

C. Orthodontic therapy

D. Endodontic therapy

The correct answer is A. Graft of alveolar clefts.

Risk factors associated with severe resorption are:

- Shorter than average roots.

- Previously traumatized teeth.

- Nonvital teeth.

- Teeth subjected to excessive forces.

- Combination of orthodontic and orthognathic procedures.

Reference: Shafer’s Textbook of ORAL PATHOLOGY, 7th Edition, Page No: 930

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Test for measuring hardness

 # Which of the following is a test for measuring hardness?

A. Knoop

B. Toughness

C. Yield strength

D. Resilience

The correct answer is A. Knoop.

The tests most frequently used in determining the hardness of dental materials are known by the names

Barcol, Brinell, Rockwell, Shore, Vickers, and Knoop. Selection of the test should be determined on the basis of the material being measured.

The Knoop hardness test employs a diamond-tipped tool. The impression is rhombic in outline, and the length of the largest diagonal is measured. The load is divided by the projected area to give the Knoop hardness number (usually abbreviated as HK or KHN).

Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, Anusavice, Shen, Rawls, 12th Edition, Page No: 64

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Mechanism of adhesion of glass ionomer cement restoration with tooth surface:

 # The mechanism of adhesion of glass ionomer cement restoration with tooth surface is by means of:

A. Carboxyl group of polymer chains

B. Chelates with metal ion

C. C=C double bond

D. Hydroxyl group

The correct answer is A. Carboxyl group of polymer chains.

Glass ionomers bond to tooth structure by chelation of the carboxyl groups of the polyacrylic acids with the calcium in the apatite of the enamel and dentin, in a manner similar to polycarboxylate cement.

Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, Anusavice, Shen, Rawls, 12th Edition, Page No: 321

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During the setting of alginate impression materials:

 # During the setting of alginate impression materials:

A. Trisodium phosphate reacts preferentially with calcium sulfate

B. Trisodium phosphate reacts preferentially with sodium alginate

C. The colloidal gel state changes to a sol

D. The material in contact with the soft tissues set last

The correct answer is. A. Trisodium phosphate reacts preferentially with calcium sulfate

A water-soluble phosphate salt (e.g., trisodium phosphate) is added to the composition as a retarder to extend the working time. The strategy is that the calcium ions will react preferentially with the phosphate ions in the solution. Thus, the rapid reaction between calcium ions and the soluble alginate is

deferred until phosphate ions from trisodium phosphate are exhausted. The amount of retarder is adjusted to provide the proper setting time. A similar gelation process is expected with the modified alginate product.

Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, Anusavice, Shen, Rawls, 12th Edition, Page No: 172

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Ease with which impression can be poured without bubble formation is related to:

# Ease with which impression can be poured without bubble formation is related to:

A. Hardness of impression material

B. Resiliency of impression material

C. Contact angle of the die material with the impression material

D. Surface contamination

The correct answer is C. Contact angle of the die material with the impression material.

When gypsum products are mixed with water to pour dental models in various types of impressions, wetting must occur between gypsum and the impression to ensure good surface quality of the gypsum model. The basic ingredient of the most popular silicone-based elastomeric impression material today is considered to be hydrophobic, because a contact angle of greater than 90° is observed when an aqueous medium is poured into this elastomeric mold. To improve the wettability of the set impression material by a gypsum-water mixture, the operator usually sprays a surfactant (also called debubblizer). The most direct approach by the manufacturer is to incorporate a wetting agent (also known as a surfactant)

in the impression material during the manufacturing process. 

Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, Anusavice, Shen, Rawls, 12th Edition, Page No: 27

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Which portion of the flame is used while soldering:

# Which portion of the flame is used while soldering?

A. Cold mixing zone

B. Partial combustion zone

C. Tip of the reducing zone

D. Oxidizing zone

The correct answer is C. Tip of reducing zone.

The next zone, dimly blue and located just beyond the tip of the green combustion zone, is the reducing zone. This is the hottest part of the flame, and it should be kept constantly on the alloy during melting. The outer cone (oxidizing zone) is the area in which combustion occurs with the oxygen in the air. Under no circumstances should this portion of the flame be used to melt the alloy. Not only is its temperature lower than that of the reducing zone but it also oxidizes the alloy.

Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, Anusavice, Shen, Rawls, 12th Edition, Page No: 221

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