AGD Dental Materials and Application 2 — Questions and Answers
Question 1: Which polymerization shrinkage characteristic of composite resins is most clinically significant?
- Shrinkage occurs toward the light source (Correct answer)
- Shrinkage pulls away from the tooth walls creating gaps
- Shrinkage is greatest in bulk-fill composites
- Shrinkage is eliminated by incremental layering
Correct answer: Shrinkage occurs toward the light source
Composite resins shrink toward the curing light, which is why proper light placement and incremental layering techniques are used to minimize marginal gaps.
Question 2: What is the primary advantage of using resin-modified glass ionomer cement (RMGIC) over conventional glass ionomer?
- Higher compressive strength than composite
- Improved physical properties while retaining fluoride release and adhesion (Correct answer)
- Complete elimination of sensitivity
- Better esthetic match than ceramic
Correct answer: Improved physical properties while retaining fluoride release and adhesion
RMGIC incorporates resin monomers to improve strength and working time while retaining the fluoride release and chemical adhesion to tooth structure of conventional GIC.
Question 3: Which amalgam alloy phase is associated with the greatest dimensional change and corrosion susceptibility?
- Gamma-1 phase (Ag2Hg3)
- Gamma phase (Ag3Sn)
- Gamma-2 phase (Sn7-8Hg) (Correct answer)
- Delta phase (Cu3Sn)
Correct answer: Gamma-2 phase (Sn7-8Hg)
The gamma-2 phase (Sn7-8Hg) is the weakest, most corrosion-prone phase in amalgam, which is why high-copper alloys were developed to eliminate it.
Question 4: A patient presents with a cracked cusp on a maxillary molar. Which indirect restorative material best resists crack propagation?
- Full-coverage porcelain-fused-to-metal crown
- Feldspathic porcelain veneer
- Direct composite onlay
- Lithium disilicate ceramic onlay (Correct answer)
Correct answer: Lithium disilicate ceramic onlay
Lithium disilicate (e.max) has high flexural strength (~400 MPa) and fracture toughness, making it ideal for resisting propagation of cracks in posterior teeth.
Question 5: What effect does the addition of palladium have in metal-ceramic alloys?
- Increases ductility for easier casting
- Raises the melting range and improves oxidation resistance (Correct answer)
- Reduces the modulus of elasticity
- Increases radiopacity for radiographic detection
Correct answer: Raises the melting range and improves oxidation resistance
Palladium raises the melting temperature and improves corrosion/oxidation resistance in metal-ceramic alloys, making them suitable for high-firing porcelains.
Question 6: Which property of zinc phosphate cement makes it suitable as a base under metallic restorations?
- Its acidic pH provides antibacterial action
- High film thickness seals dentinal tubules
- High thermal insulating capacity protects the pulp (Correct answer)
- It chemically bonds to both tooth and metal
Correct answer: High thermal insulating capacity protects the pulp
Zinc phosphate cement serves as a thermal insulator under metallic restorations, protecting the pulp from temperature changes conducted through metal.
Question 7: During impression making, which material requires the use of a custom tray to achieve optimal accuracy?
- Irreversible hydrocolloid (alginate)
- Zinc oxide eugenol impression paste (Correct answer)
- Addition silicone (vinyl polysiloxane)
- Polysulfide rubber
Correct answer: Zinc oxide eugenol impression paste
Zinc oxide eugenol paste requires a custom tray due to its minimal working and setting characteristics that demand close adaptation to the tissues.
AGD Dental Materials and Application 2
Which polymerization shrinkage characteristic of composite resins is most clinically significant?