Dental Materials Science Flashcards
6 cards from real MJDF practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Dental Materials Science flashcards as text
What is the difference between thermoplastic and thermoset polymers in dental materials?
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).
What makes zirconia (Y-TZP) a suitable material for posterior dental crowns?
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.
What is the setting contraction of zinc phosphate cement and its clinical implication?
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.
What is the clinical significance of the coefficient of thermal expansion (CTE) in restorative materials?
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.
What is the primary failure mode of all-ceramic crowns?
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.
What does 'thixotropy' mean in the context of impression or cement materials?
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.