ORE Part 1 Dental Materials Science 2 — Questions and Answers
Question 1: Which ADA/ISO type of gypsum product has the highest compressive strength?
- Type II — dental plaster
- Type III — dental stone
- Type IV — die stone (improved stone)
- Type V — high-strength, high-expansion stone (Correct answer)
Correct answer: Type V — high-strength, high-expansion stone
Type V gypsum (high-strength, high-expansion die stone) has the highest compressive strength of all gypsum types, exceeding 35 MPa, making it suitable for precision die work.
Question 2: Which monomer is the primary liquid component in heat-cured PMMA denture base resins?
- Methyl methacrylate (MMA) (Correct answer)
- Ethyl methacrylate (EMA)
- Bisphenol A-glycidyl methacrylate (Bis-GMA)
- Urethane dimethacrylate (UDMA)
Correct answer: Methyl methacrylate (MMA)
Methyl methacrylate (MMA) is the primary liquid monomer in conventional PMMA denture base resins; when polymerised it forms polymethyl methacrylate (PMMA).
Question 3: Which elastomeric impression material has the best dimensional stability after removal from the mouth?
- Polysulfide
- Polyether
- Condensation silicone
- Addition silicone (polyvinyl siloxane, PVS) (Correct answer)
Correct answer: Addition silicone (polyvinyl siloxane, PVS)
Addition silicone (PVS) has the best dimensional stability because its hydrosilylation setting reaction produces no volatile by-products, resulting in negligible post-set dimensional change.
Question 4: The primary mechanism by which 4th-generation dentine bonding agents achieve adhesion is:
- Direct chemical bonding to calcium in hydroxyapatite
- Micromechanical interlocking through hybrid layer formation in demineralised dentine (Correct answer)
- Ionic exchange with calcium ions at the dentine surface
- Covalent bonding directly to exposed collagen fibres
Correct answer: Micromechanical interlocking through hybrid layer formation in demineralised dentine
4th-generation bonding agents use a separate etch-prime-bond sequence to form a 'hybrid layer' — resin monomers infiltrate the acid-demineralised collagen network and polymerise, achieving micromechanical retention.
Question 5: What stabilises the tetragonal crystal phase of zirconia in yttria-stabilised tetragonal zirconia polycrystal (Y-TZP) ceramics?
- Aluminium oxide additions
- Yttrium oxide (yttria) doping (Correct answer)
- Rapid sintering at high temperature
- Lithium disilicate glass matrix
Correct answer: Yttrium oxide (yttria) doping
Yttrium oxide (yttria, Y2O3) is added in small amounts (3 mol%) to stabilise the tetragonal phase at room temperature, enabling transformation toughening when cracks propagate through the material.
Question 6: Which component in ZOE-based root canal sealers is primarily responsible for periapical tissue cytotoxicity?
- Zinc oxide powder
- Eugenol (Correct answer)
- Rosin
- Titanium dioxide
Correct answer: Eugenol
Eugenol is the cytotoxic component in ZOE-based sealers; it causes tissue irritation, inhibits pulp cell proliferation, and is toxic to periapical tissues when in direct contact.
Question 7: What is the primary setting reaction of addition silicone (PVS) impression material?
- Radical chain-growth polymerisation initiated by benzoyl peroxide
- Polycondensation releasing ethanol as a volatile by-product
- Platinum-catalysed hydrosilylation of Si-H groups across vinyl groups (Correct answer)
- Acid-base neutralisation between base and catalyst pastes
Correct answer: Platinum-catalysed hydrosilylation of Si-H groups across vinyl groups
Addition silicones set via a platinum-catalysed hydrosilylation reaction in which Si-H groups add across vinyl (Si-CH=CH2) groups with no by-product released, giving excellent accuracy and dimensional stability.
ORE Part 1 Dental Materials Science 2
Which ADA/ISO type of gypsum product has the highest compressive strength?