MFDS Dental Materials 2 — Questions and Answers
Question 1: The polymerisation shrinkage of conventional composite resins is approximately:
- 0.1–0.5%
- 1–3% (Correct answer)
- 5–8%
- 10–15%
Correct answer: 1–3%
Conventional methacrylate-based composites shrink by approximately 1–3% by volume during polymerisation as monomers convert to a polymer network.
Question 2: Which component of amalgam is primarily responsible for its corrosion resistance?
- Tin
- Copper (Correct answer)
- Silver
- Zinc
Correct answer: Copper
High-copper amalgam alloys suppress the corrosion-prone tin-mercury (gamma-2) phase by converting it to copper-tin (eta) phase, greatly improving corrosion resistance.
Question 3: What does the term 'thixotropic' mean when applied to impression materials?
- Sets faster when heated
- Becomes less viscous under shear stress (Correct answer)
- Releases hydrogen gas on setting
- Has permanent deformation on removal
Correct answer: Becomes less viscous under shear stress
Thixotropic materials decrease in viscosity when subjected to shear stress (e.g. mixing or syringing) and return to a gel-like state at rest, aiding injection and flow.
Question 4: Galvanic corrosion in the mouth most commonly occurs between:
- Enamel and dentine
- Two dissimilar metals in contact (Correct answer)
- Porcelain and metal
- Composite and GIC
Correct answer: Two dissimilar metals in contact
Galvanic corrosion occurs when two dissimilar metals are in contact in the presence of saliva as electrolyte, causing the less noble metal to oxidise.
Question 5: Which property is most important when selecting a luting cement for a post with a long, parallel preparation?
- High film thickness
- Low film thickness (Correct answer)
- High setting expansion
- High solubility
Correct answer: Low film thickness
Low film thickness is critical for long parallel-sided posts to allow full seating; excess thickness prevents the post from reaching full depth.
Question 6: The process of 'stress relaxation' in impression materials refers to:
- Dimensional change on storage
- Decrease in internal stress over time under constant strain (Correct answer)
- Increased stiffness after setting
- Water sorption during storage
Correct answer: Decrease in internal stress over time under constant strain
Stress relaxation describes how a viscoelastic material that is held in a deformed state gradually reduces its internal stress without further change in strain.
MFDS Dental Materials 2
The polymerisation shrinkage of conventional composite resins is approximately: