MJDF Dental Materials Science 3 — Questions and Answers
Question 1: What is the difference between thermoplastic and thermoset polymers in dental materials?
- Thermoplastics soften on reheating (e.g., PMMA tray acrylic); thermosets have irreversible cross-links and cannot be remoulded (e.g., vulcanised rubber) (Correct answer)
- They are identical
- Thermosets are always stronger
- Thermoplastics cannot be used in the mouth
Correct answer: Thermoplastics soften on reheating (e.g., PMMA tray acrylic); thermosets have irreversible cross-links and cannot be remoulded (e.g., vulcanised rubber)
Thermoplastics can be repeatedly softened and reshaped on heating (PMMA, polycarbonate); thermosets form irreversible cross-linked networks during curing and cannot be remoulded (dental composites, rubbers).
Question 2: What makes zirconia (Y-TZP) a suitable material for posterior dental crowns?
- Transformation toughening (tetragonal-to-monoclinic phase transformation absorbs crack energy) — extremely high fracture toughness (Correct answer)
- Its translucency (it is the most translucent ceramic)
- Low modulus of elasticity similar to enamel
- Bonds with glass ionomer without surface treatment
Correct answer: Transformation toughening (tetragonal-to-monoclinic phase transformation absorbs crack energy) — extremely high fracture toughness
Y-TZP zirconia undergoes stress-induced tetragonal-to-monoclinic phase transformation at crack tips, dissipating energy and arresting crack propagation — giving it exceptional fracture toughness for posterior restorations.
Question 3: What is the setting contraction of zinc phosphate cement and its clinical implication?
- Sets with slight expansion (ZPC has minimal net dimensional change — slight expansion compensates for contraction); adequate film thickness controlled by mixing technique (Correct answer)
- Zero dimensional change
- Large shrinkage requiring compensation
- Large expansion causing post-cementation pain always
Correct answer: Sets with slight expansion (ZPC has minimal net dimensional change — slight expansion compensates for contraction); adequate film thickness controlled by mixing technique
Zinc phosphate cement has a complex dimensional change; its relatively low film thickness (25–40 µm when mixed properly on a cold slab) is its critical property for precise crown seating.
Question 4: What is the clinical significance of the coefficient of thermal expansion (CTE) in restorative materials?
- If CTE of restoration differs significantly from tooth structure, repeated thermal cycling causes microleakage at margins (Correct answer)
- Thermal expansion only affects aesthetics
- CTE determines setting time
- CTE is only relevant to all-ceramic restorations
Correct answer: If CTE of restoration differs significantly from tooth structure, repeated thermal cycling causes microleakage at margins
When a restoration and tooth expand/contract differently with temperature change, cyclic thermal stress occurs at the margin; materials with CTE close to tooth structure (enamel ~11 ppm/°C, dentine ~8 ppm/°C) have better marginal integrity over time.
Question 5: What is the primary failure mode of all-ceramic crowns?
- Brittle fracture (low fracture toughness compared to metals) — ceramics have high compressive strength but are weak in tension (Correct answer)
- Corrosion
- Creep deformation
- Dissolution in oral fluids
Correct answer: Brittle fracture (low fracture toughness compared to metals) — ceramics have high compressive strength but are weak in tension
Ceramics are strong in compression but weak in tension (low fracture toughness); failures typically initiate from tensile stress concentrations at surface defects or in the cementation surface.
Question 6: What does 'thixotropy' mean in the context of impression or cement materials?
- The material becomes less viscous under shear (mixing or pressure) and returns to higher viscosity at rest (Correct answer)
- Permanent change in viscosity after mixing
- A measure of the material's hardness
- The rate of chemical reaction
Correct answer: The material becomes less viscous under shear (mixing or pressure) and returns to higher viscosity at rest
Thixotropy is shear-thinning behaviour: the material flows easily when manipulated or injected but becomes viscous at rest once placed, aiding flow during placement and stability thereafter.
MJDF Dental Materials Science 3
What is the difference between thermoplastic and thermoset polymers in dental materials?