MJDF Dental Materials Science

Question 1

What is the Young's modulus (modulus of elasticity) and why is it important in dental materials?

Accepted Answer

Measure of stiffness (stress/strain in elastic region); higher modulus means stiffer material — important for predicting deflection under load

Answer

Measure of strength before fracture

Answer

A measure of hardness

Answer

The thermal expansion coefficient

Question 2

What is the main chemical composition of dental amalgam?

Accepted Answer

Silver, tin, copper, and zinc alloy powder mixed with mercury (forming gamma, gamma-1, gamma-2 phases)

Answer

Gold, silver, and platinum

Answer

Resin and silica filler

Answer

Calcium silicate and water

Question 3

What is the 'critical surface tension' concept in adhesive dentistry?

Accepted Answer

The surface tension of the substrate must be higher than the wetting liquid for adequate wetting and bonding

Answer

The maximum stress at the bonding surface

Answer

The contact angle of water on dentine

Answer

The viscosity of the adhesive

Question 4

What is the setting reaction of zinc phosphate cement?

Accepted Answer

Acid-base reaction between zinc oxide powder and phosphoric acid liquid producing zinc phosphate matrix

Answer

Polymerisation of resin monomers

Answer

Chelation reaction

Answer

Hydration of calcium silicate

Question 5

Which impression material records the finest surface detail?

Accepted Answer

Addition silicone (polyvinylsiloxane, PVS)

Answer

Alginate

Answer

Zinc oxide eugenol paste

Answer

Impression compound

Question 6

What is the significance of the 'gel point' in alginate impressions?

Accepted Answer

The point at which the irreversible sol-to-gel transformation occurs and the impression material becomes rigid

Answer

The time at which impression material is mixed

Answer

The temperature at which alginate dissolves

Answer

The maximum loading time before shrinkage