ADAT Dental Materials & Biomechanics 2 — Questions and Answers
Question 1: What is the significance of the critical stress intensity factor (KIC) in dental ceramics?
- It measures resistance to thermal shock
- It quantifies the material's resistance to crack propagation (Correct answer)
- It determines the material's biocompatibility
- It measures flexural strength under loading
Correct answer: It quantifies the material's resistance to crack propagation
KIC (fracture toughness) quantifies a ceramic's resistance to crack propagation; higher KIC means the material can tolerate larger flaws before catastrophic fracture.
Question 2: Which cement is most appropriate for luting a porcelain-fused-to-metal (PFM) crown on a vital tooth with adequate retention form?
- Resin cement (adhesive cementation)
- Zinc phosphate cement
- Resin-modified glass ionomer cement (RMGIC) (Correct answer)
- Zinc oxide eugenol (ZOE) cement
Correct answer: Resin-modified glass ionomer cement (RMGIC)
RMGIC combines the fluoride release of GIC with improved retention and reduced pulpal irritation, making it ideal for PFM crowns on vital teeth with good preparation.
Question 3: What is the ideal water-to-powder ratio for dental stone (Type III gypsum) compared to plaster (Type II)?
- Stone uses more water than plaster for better flow
- Stone uses less water than plaster, producing a denser, stronger product (Correct answer)
- Both use the same water-to-powder ratio
- Stone requires twice the water of plaster for proper setting
Correct answer: Stone uses less water than plaster, producing a denser, stronger product
Dental stone requires approximately 0.28–0.30 mL/g water versus 0.45–0.50 mL/g for plaster, resulting in a denser, harder material with lower porosity.
Question 4: Which property of wax makes it suitable for occlusal registration but problematic if not handled correctly?
- High melting point prevents distortion
- Thermal contraction during cooling can cause dimensional inaccuracy (Correct answer)
- Chemical bonding to tooth structure provides stability
- Radiopacity allows verification on radiographs
Correct answer: Thermal contraction during cooling can cause dimensional inaccuracy
Wax contracts significantly during cooling from mouth temperature, causing distortion of bite registrations if not supported or poured immediately.
Question 5: Which type of indirect restoration is most appropriate for a large MOD cavity in a mandibular molar with a cracked marginal ridge?
- Direct composite resin
- Amalgam restoration with pins
- Full-coverage ceramic crown
- Indirect composite or ceramic onlay (Correct answer)
Correct answer: Indirect composite or ceramic onlay
An onlay is the most conservative option that provides cusp coverage to protect cracked marginal ridges without unnecessary removal of sound tooth structure.
Question 6: What causes 'low-temperature degradation' (aging) in zirconia dental restorations?
- UV light causes surface cracking over time
- Hydrothermal transformation from tetragonal to monoclinic phase in the presence of water (Correct answer)
- Thermal cycling causes microcrack propagation at grain boundaries
- Salivary enzymes chemically attack the zirconia surface
Correct answer: Hydrothermal transformation from tetragonal to monoclinic phase in the presence of water
Water molecules trigger spontaneous tetragonal-to-monoclinic phase transformation in zirconia at low temperatures, causing surface roughening and strength reduction over time.
ADAT Dental Materials & Biomechanics 2
What is the significance of the critical stress intensity factor (KIC) in dental ceramics?