MJDF Dental Materials Science 3

Question 1

What is the difference between thermoplastic and thermoset polymers in dental materials?

Accepted Answer

Thermoplastics soften on reheating (e.g., PMMA tray acrylic); thermosets have irreversible cross-links and cannot be remoulded (e.g., vulcanised rubber)

Answer

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?

Accepted Answer

Transformation toughening (tetragonal-to-monoclinic phase transformation absorbs crack energy) — extremely high fracture toughness

Answer

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?

Accepted Answer

Sets with slight expansion (ZPC has minimal net dimensional change — slight expansion compensates for contraction); adequate film thickness controlled by mixing technique

Answer

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?

Accepted Answer

If CTE of restoration differs significantly from tooth structure, repeated thermal cycling causes microleakage at margins

Answer

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?

Accepted Answer

Brittle fracture (low fracture toughness compared to metals) — ceramics have high compressive strength but are weak in tension

Answer

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.

MJDF Part 1 Practice Test

MJDF Part 1 Practice Test

MJDF Dental Materials Science 3