MJDF Dental Materials Science 1 — Questions and Answers
Question 1: What is the Young's modulus (modulus of elasticity) and why is it important in dental materials?
- Measure of stiffness (stress/strain in elastic region); higher modulus means stiffer material — important for predicting deflection under load (Correct answer)
- Measure of strength before fracture
- A measure of hardness
- The thermal expansion coefficient
Correct answer: Measure of stiffness (stress/strain in elastic region); higher modulus means stiffer material — important for predicting deflection under load
Young's modulus quantifies material stiffness; a high modulus (e.g., ceramics, metals) means less deformation under load, while low modulus (e.g., PMMA, PDMS) means more flexibility — critical for prosthesis and restoration design.
Question 2: What is the main chemical composition of dental amalgam?
- Silver, tin, copper, and zinc alloy powder mixed with mercury (forming gamma, gamma-1, gamma-2 phases) (Correct answer)
- Gold, silver, and platinum
- Resin and silica filler
- Calcium silicate and water
Correct answer: Silver, tin, copper, and zinc alloy powder mixed with mercury (forming gamma, gamma-1, gamma-2 phases)
Dental amalgam is formed by mixing a silver-tin-copper-zinc alloy (the alloy powder) with liquid mercury; the resulting microstructure contains gamma (Ag₃Sn), gamma-1 (Ag₂Hg₃), and gamma-2 (Sn₇Hg) phases.
Question 3: What is the 'critical surface tension' concept in adhesive dentistry?
- The surface tension of the substrate must be higher than the wetting liquid for adequate wetting and bonding (Correct answer)
- The maximum stress at the bonding surface
- The contact angle of water on dentine
- The viscosity of the adhesive
Correct answer: The surface tension of the substrate must be higher than the wetting liquid for adequate wetting and bonding
For a liquid adhesive to wet and spread on a substrate, the surface energy of the substrate must exceed the surface tension of the liquid; low-energy surfaces (e.g., contaminated enamel) impair bonding.
Question 4: What is the setting reaction of zinc phosphate cement?
- Acid-base reaction between zinc oxide powder and phosphoric acid liquid producing zinc phosphate matrix (Correct answer)
- Polymerisation of resin monomers
- Chelation reaction
- Hydration of calcium silicate
Correct answer: Acid-base reaction between zinc oxide powder and phosphoric acid liquid producing zinc phosphate matrix
Zinc phosphate sets via an acid-base reaction: phosphoric acid dissolves zinc oxide, forming zinc phosphate and zinc aluminophosphate, which crystallise into an amorphous zinc phosphate matrix.
Question 5: Which impression material records the finest surface detail?
- Addition silicone (polyvinylsiloxane, PVS) (Correct answer)
- Alginate
- Zinc oxide eugenol paste
- Impression compound
Correct answer: Addition silicone (polyvinylsiloxane, PVS)
Addition silicone (PVS/A-silicone) has the best dimensional stability, lowest polymerisation shrinkage, and finest surface detail reproduction among impression materials.
Question 6: What is the significance of the 'gel point' in alginate impressions?
- The point at which the irreversible sol-to-gel transformation occurs and the impression material becomes rigid (Correct answer)
- The time at which impression material is mixed
- The temperature at which alginate dissolves
- The maximum loading time before shrinkage
Correct answer: The point at which the irreversible sol-to-gel transformation occurs and the impression material becomes rigid
At the gel point, the irreversible hydrocolloid network forms; removing the tray before this point distorts the impression, while excessive delay causes permanent distortion from continued reaction.
