AGD Dental Materials and Biomaterials 3 — Questions and Answers
Question 1: Which optical property of ceramics is most critical for matching the appearance of natural tooth enamel?
- Fluorescence only
- Translucency (Correct answer)
- Opacity
- Surface gloss alone
Correct answer: Translucency
Natural enamel is semi-translucent, so ceramic translucency is essential for an esthetic match, allowing light to pass through similarly.
Question 2: The coefficient of thermal expansion (CTE) mismatch between a ceramic and its underlying metal substructure can cause:
- Increased bond strength
- Ceramic fracture or delamination (Correct answer)
- Improved marginal fit
- Reduced porosity
Correct answer: Ceramic fracture or delamination
CTE mismatch creates thermal stress during firing and intraoral temperature changes, leading to ceramic fracture or delamination from the metal.
Question 3: Which mechanism primarily accounts for the superior strength of zirconia compared to other dental ceramics?
- Higher filler loading
- Transformation toughening (Correct answer)
- Leucite crystal reinforcement
- Increased glass phase content
Correct answer: Transformation toughening
Zirconia undergoes stress-induced tetragonal-to-monoclinic phase transformation, absorbing crack energy and dramatically increasing fracture toughness.
Question 4: Galvanic corrosion in the oral cavity is most likely to occur when:
- Two identical alloys contact each other
- Dissimilar metals are in contact with saliva (Correct answer)
- Gold alloys are used alone
- Ceramic restorations contact metal restorations
Correct answer: Dissimilar metals are in contact with saliva
Galvanic corrosion occurs when dissimilar metals with different electrode potentials are in electrical contact through an electrolyte like saliva.
Question 5: Which component of dental porcelain gives it its characteristic glass-like quality?
- Alumina (Al₂O₃)
- Leucite crystals
- Silica-based glass matrix (Correct answer)
- Zirconia particles
Correct answer: Silica-based glass matrix
The silica-based glass matrix provides translucency and the glass-like optical and physical properties characteristic of dental porcelain.
Question 6: A patient presents with a failed composite bond after only 6 months. Which material failure mode is most likely if the fracture is cohesive within the adhesive layer?
- Enamel fracture
- Adhesive failure at the tooth-material interface
- Cohesive failure within the adhesive (Correct answer)
- Marginal discoloration from microleakage
Correct answer: Cohesive failure within the adhesive
Cohesive failure within the adhesive layer indicates the adhesive material itself was the weakest link, often due to inadequate thickness or incomplete cure.
Question 7: Which setting characteristic distinguishes type IV gypsum (die stone) from type II plaster?
- Type IV has higher water-to-powder ratio and lower strength
- Type IV has lower expansion and higher compressive strength (Correct answer)
- Type II has lower water absorption and higher hardness
- Type II requires less water and sets faster
Correct answer: Type IV has lower expansion and higher compressive strength
Type IV die stone requires less water (0.22–0.24 w/p ratio), exhibits minimal setting expansion (<0.10%), and achieves compressive strength >34 MPa.
AGD Dental Materials and Biomaterials 3
Which optical property of ceramics is most critical for matching the appearance of natural tooth enamel?