The principal ingredient of modeling plastic is:

 # The principal ingredient of modeling plastic is:

A. Waxes

B. Zinc oxide

C. Aluminium oxide

D. Silicon oxide

The correct answer is A. Waxes.

In general, impression compounds (modelling plastic) are composed of a mixture of waxes, thermoplastic resins, filler, and a coloring agent. Shellac, stearic acid, and gutta-percha are added to improve plasticity and workability. The waxes or resins in the impression compound are the principal ingredients of the matrix. Filler is added to increase the viscosity at temperatures above that of the mouth and to increase the rigidity of the compound at room temperature.

Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, Anusavice, Shen, Rawls, 12th Edition, Page No: 178

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Chemically gypsum is:

# Chemically gypsum is:

A. Calcium sulphate dihydrate

B. Calcium aluminofluorosilicate

C. Calcium carbonate

D. Magnesium carbonate

The correct answer is A. Calcium sulphate dihydrate.

Gypsum (CaSO4•2H2O; calcium sulfate dihydrate) is a mineral mined in various parts of the world, but it is also produced as a by-product of flue gas desulfurization in some coal-fired electric power plants. Various crystalline forms of gypsum, such as selenite and alabaster, exist in nature. Gypsum products are supplied as fine hemihydrate powders that are produced by heating ground gypsum particles. After mixing with water, the mixture reverts back to gypsum. This unique property of gypsum products has led to numerous applications through history.

Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, Anusavice, Shen, Rawls, 12th Edition

Page no: 182

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Serial extraction is indicated primarily in:

 # Serial extraction is indicated primarily in:

A. Class I malocclusion

B. Class II malocclusion

C. Class III malocclusion

D. Lower anterior crowding only

The correct answer is D. Lower anterior crowding only.

Serial extraction is directed toward severe dental crowding. For this reason, it is best used when no skeletal problem exists and the space discrepancy is large-greater than l0mm per arch. If the crowding is severe, little space will remain after the teeth are aligned, which means there will be little tipping and uncontrolled movement of the adjacent teeth into the extraction sites. If the initial discrepancy is smaller, more residual space must be anticipated. It is unwise to start serial extraction in a child who has a skeletal problem, because the closure of extraction spaces would be affected by how the skeletal problem was being addressed.

Serial extraction treatment begins in the early mixed dentition with extraction of primary incisors if necessary, followed by extraction of the primary canines to allow eruption and alignment of the permanent incisors. 

Ref: Comtemporary Orthodontics, 4th Edition, Proffit, Page 490

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Eruption of permanent teeth in cases of extremely early loss of deciduous teeth:

# The eruption of permanent teeth in cases of extremely early loss of deciduous teeth  will result in:

A. No change in the time of eruption

B. Early eruption of the permanent teeth

C. Delayed eruption of the permanent teeth

D. A class II division 1 malocclusion

The correct answer is C. Delayed eruption of the permanent teeth.

Sometimes incisors fail to erupt even when there is no retained or overlying primary tooth or supernumerary  teeth present. Changes in the overlying keratinized tissue occur in long-standing edentulous regions, and this contributes to slow eruption of a permanent incisor when its predecessor was lost prematurely. 

Ref: Comtemporary Orthodontics, Proffit 4th Edition, Page 450

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