ORE Part 1 Dental Materials Science 1 — Questions and Answers
Question 1: Which phase in conventional dental amalgam is most susceptible to corrosion?
- Gamma phase (Ag3Sn)
- Gamma-1 phase (Ag2Hg3)
- Gamma-2 phase (Sn8Hg) (Correct answer)
- Beta phase (Ag2Hg)
Correct answer: Gamma-2 phase (Sn8Hg)
The gamma-2 phase (Sn8Hg) is the weakest and most corrosion-susceptible phase in conventional amalgam, which is why high-copper amalgams were developed to eliminate it.
Question 2: What is the approximate working time of zinc phosphate cement when mixed at room temperature (23°C)?
- 1–2 minutes
- 3–5 minutes (Correct answer)
- 7–9 minutes
- 10–12 minutes
Correct answer: 3–5 minutes
Zinc phosphate cement has a working time of approximately 3–5 minutes at 23°C, which can be extended by cooling the mixing slab to slow the exothermic reaction.
Question 3: Which impression material is classified as an irreversible hydrocolloid?
- Agar
- Alginate (Correct answer)
- Polysulfide
- Condensation silicone
Correct answer: Alginate
Alginate is an irreversible hydrocolloid because its gel formation proceeds via a chemical reaction that cannot be reversed by temperature change, unlike agar which is reversible.
Question 4: What is the primary clinical advantage of resin-modified glass ionomer cement (RMGIC) over conventional GIC?
- Significantly higher fluoride release
- Greater compressive strength
- Better translucency and aesthetics
- Reduced sensitivity to moisture during initial setting (Correct answer)
Correct answer: Reduced sensitivity to moisture during initial setting
RMGIC incorporates a resin component that provides an immediate light-cure set, greatly reducing the moisture sensitivity that is the chief clinical weakness of conventional GIC during early setting.
Question 5: Which type of composite resin contains the smallest filler particle size?
- Macrofilled composite (10–50 μm)
- Hybrid composite (0.4–5 μm)
- Microfilled composite (0.04–0.4 μm) (Correct answer)
- Packable composite (0.7–3.5 μm)
Correct answer: Microfilled composite (0.04–0.4 μm)
Microfilled composites contain colloidal silica particles of 0.04–0.4 μm, giving them outstanding polish retention and smooth surface finish, though at the cost of lower mechanical strength.
Question 6: What is the role of the silane coupling agent in composite resin?
- Initiates the polymerisation reaction when light-activated
- Forms a chemical bond between the organic resin matrix and inorganic filler particles (Correct answer)
- Prevents discolouration of the restoration over time
- Reduces volumetric polymerisation shrinkage
Correct answer: Forms a chemical bond between the organic resin matrix and inorganic filler particles
Silane coupling agents create a chemical (covalent) bond between the organic resin matrix and the inorganic filler particle surface, which is critical for load transfer and durability.
Question 7: The Vickers Hardness (VHN) test measures which material property?
- Tensile strength
- Resistance to permanent surface indentation (Correct answer)
- Elastic (Young's) modulus
- Fracture toughness
Correct answer: Resistance to permanent surface indentation
The Vickers Hardness test measures a material's resistance to permanent surface deformation by pressing a diamond pyramid indenter into the surface under a defined load and measuring the indentation size.
ORE Part 1 Dental Materials Science 1
Which phase in conventional dental amalgam is most susceptible to corrosion?