MJDF Dental Materials Science 4 — Questions and Answers
Question 1: Which ceramic system is most appropriate for a single-unit anterior crown requiring high aesthetics?
- Lithium disilicate (e.g., IPS e.max) — excellent translucency and adequate strength for anterior teeth (Correct answer)
- High-strength zirconia (most opaque type)
- Feldspathic porcelain unsupported
- Metalloceramic — gold-porcelain bonded
Correct answer: Lithium disilicate (e.g., IPS e.max) — excellent translucency and adequate strength for anterior teeth
Lithium disilicate has superior translucency and aesthetics compared to zirconia, with adequate flexural strength (400 MPa) for single-unit anterior and premolar crowns, making it the preferred system for aesthetic anterior work.
Question 2: What is the working and setting time of addition silicone impression material?
- Working time: 2–3 minutes; total setting time: 5–6 minutes (varies by viscosity and manufacturer) (Correct answer)
- Setting time: 30 minutes
- Working time: 10 minutes; setting: 15 minutes
- Sets immediately on contact with moisture
Correct answer: Working time: 2–3 minutes; total setting time: 5–6 minutes (varies by viscosity and manufacturer)
Addition silicone provides a working time of approximately 2–3 minutes and complete setting in 5–6 minutes, allowing adequate time for tray loading, positioning, and seating while ensuring dimensional accuracy.
Question 3: What is the key advantage of ether-based polyether impression material compared to addition silicone?
- Hydrophilic nature — better wetting in a moist oral environment; high stiffness in set state (Correct answer)
- More flexible in set state
- Lower cost
- Indefinite storage life
Correct answer: Hydrophilic nature — better wetting in a moist oral environment; high stiffness in set state
Polyethers are hydrophilic, tolerating moisture better during setting than addition silicones (which are hydrophobic without special surfactants), giving better adaptation to wet gingival sulcus tissue.
Question 4: What is the purpose of a cavity varnish in amalgam restorations?
- Seal dentinal tubules to reduce microleakage and post-operative sensitivity before placement of amalgam (mostly historic — now replaced by bonded amalgam or liners) (Correct answer)
- Bond the amalgam to dentine
- Act as a thermal insulator
- Provide fluoride release
Correct answer: Seal dentinal tubules to reduce microleakage and post-operative sensitivity before placement of amalgam (mostly historic — now replaced by bonded amalgam or liners)
Cavity varnish was traditionally applied to seal dentinal tubules, reducing early microleakage (before amalgam corrodes to seal margins) and post-operative sensitivity; its use has largely been superseded by adhesive systems.
Question 5: What is the difference between resilience and toughness in dental materials science?
- Resilience is energy absorbed in elastic deformation (before permanent deformation); toughness is total energy to fracture (elastic + plastic) (Correct answer)
- They are identical
- Toughness measures only elastic deformation
- Resilience is relevant only to metals
Correct answer: Resilience is energy absorbed in elastic deformation (before permanent deformation); toughness is total energy to fracture (elastic + plastic)
Resilience is the area under the elastic portion of the stress-strain curve (energy returned on unloading); toughness is the total area under the entire stress-strain curve to fracture — including plastic deformation.
Question 6: What is the curing mechanism of dual-cure composite resin cements?
- Primary light activation of camphorquinone photoinitiator + secondary chemical cure from benzoyl peroxide/amine redox system in areas of inadequate light access (Correct answer)
- Light cure only
- Chemical cure only (no photoinitiator)
- Thermal activation at body temperature
Correct answer: Primary light activation of camphorquinone photoinitiator + secondary chemical cure from benzoyl peroxide/amine redox system in areas of inadequate light access
Dual-cure cements combine light activation (camphorquinone absorbs blue light, generates free radicals) with a chemical redox cure system (tertiary amine + peroxide), ensuring cure in light-inaccessible regions.
MJDF Dental Materials Science 4
Which ceramic system is most appropriate for a single-unit anterior crown requiring high aesthetics?