ADEX Dental Biomaterials 4 — Questions and Answers
Question 1: Which impression material is considered the most dimensionally accurate and is preferred for fixed prosthodontic impressions?
- Alginate (irreversible hydrocolloid)
- Polyvinyl siloxane (addition silicone) (Correct answer)
- Polysulfide (rubber base)
- Zinc oxide eugenol impression paste
Correct answer: Polyvinyl siloxane (addition silicone)
Polyvinyl siloxane (addition silicone) is considered the gold standard for fixed prosthodontic impressions due to its exceptional dimensional accuracy and stability.
Question 2: In ceramics used for dental restorations, leucite crystals are added primarily to:
- Reduce the firing temperature of the porcelain
- Increase fracture toughness and adjust thermal expansion coefficient (Correct answer)
- Improve translucency and esthetic appearance
- Enhance chemical bonding to metal substructures
Correct answer: Increase fracture toughness and adjust thermal expansion coefficient
Leucite reinforcement increases the fracture toughness of porcelain and raises its coefficient of thermal expansion to match metal substructures in PFM restorations.
Question 3: When mixing alginate impression material, using cold water will:
- Accelerate the setting reaction
- Decrease the working and setting time
- Prolong the working and setting time (Correct answer)
- Have no effect on setting time
Correct answer: Prolong the working and setting time
Cold water slows the chemical reaction in alginate, extending both working and setting times, which is useful when more manipulation time is needed.
Question 4: What is the primary cause of polymerization shrinkage in composite resins?
- Evaporation of solvent from the resin matrix during curing
- Van der Waals distances converting to shorter covalent bonds during crosslinking (Correct answer)
- Loss of water from the hydrophilic resin component
- Thermal contraction as the material cools after light activation
Correct answer: Van der Waals distances converting to shorter covalent bonds during crosslinking
During polymerization, monomers linked by van der Waals forces form shorter covalent bonds, reducing intermolecular distance and causing volumetric shrinkage.
Question 5: Agar (reversible hydrocolloid) impression material converts from gel to sol by:
- Chemical reaction with a hardening agent
- Heating to approximately 100°C (Correct answer)
- Absorption of moisture from surrounding tissue
- Addition of a sulfate accelerator
Correct answer: Heating to approximately 100°C
Agar is a thermoreversible material that liquefies (sol) upon heating to ~100°C and gels upon cooling, allowing it to be reused multiple times.
Question 6: Which property is most critical when selecting a luting cement for cementing a porcelain-fused-to-metal crown?
- High solubility to allow easy removal if needed
- Low film thickness to ensure accurate seating (Correct answer)
- High thermal conductivity to protect the pulp
- High radioopacity to detect overhangs on radiographs
Correct answer: Low film thickness to ensure accurate seating
Film thickness of a luting cement must be minimal (ideally ≤25 µm per ADA specification) to allow complete seating of the casting without distorting the occlusion.
Question 7: Which statement about zirconia (zirconium oxide) dental ceramic is correct?
- It is highly translucent and used primarily for anterior esthetic veneers
- It is opaque and relies on transformation toughening for high strength (Correct answer)
- It bonds easily to resin cements without surface treatment
- It is a glass-ceramic that can be etched with hydrofluoric acid
Correct answer: It is opaque and relies on transformation toughening for high strength
Zirconia is a polycrystalline ceramic that undergoes transformation toughening (tetragonal to monoclinic phase transformation) under stress, giving it exceptional fracture resistance.
ADEX Dental Biomaterials 4
Which impression material is considered the most dimensionally accurate and is preferred for fixed prosthodontic impressions?