ADAT - Advanced Dental Admission Test Dental Materials & Biomechanics 1 — Questions and Answers
Question 1: Which type of dental ceramic exhibits the highest flexural strength due to its tetragonal zirconia polycrystal (TZP) microstructure?
- Feldspathic porcelain
- Leucite-reinforced glass ceramic
- Yttria-stabilized zirconia (Y-TZP) (Correct answer)
- Lithium disilicate glass ceramic
Correct answer: Yttria-stabilized zirconia (Y-TZP)
Y-TZP derives its exceptional flexural strength (900–1200 MPa) from transformation toughening: stress at a crack tip converts tetragonal ZrO₂ grains to the monoclinic phase, creating compressive stresses that resist crack propagation. Feldspathic and leucite ceramics are far weaker, and lithium disilicate, while strong, does not exhibit this mechanism.
Question 2: The modulus of elasticity (Young's modulus) of dentin is approximately:
- 2 GPa
- 18 GPa (Correct answer)
- 70 GPa
- 200 GPa
Correct answer: 18 GPa
Dentin has a modulus of elasticity of roughly 14–21 GPa (commonly cited as ~18 GPa), which is intermediate between enamel (~84 GPa) and soft tissue. This viscoelastic stiffness allows dentin to absorb occlusal loads and flex slightly without fracture, serving as a resilient support for the stiffer enamel crown.
Question 3: Which bonding mechanism is primarily responsible for the adhesion of glass ionomer cement to tooth structure?
- Micromechanical interlocking with etched enamel prisms
- Ionic exchange between carboxylate groups and calcium in hydroxyapatite (Correct answer)
- Covalent bonding between MDP monomer and zirconia oxide
- Hydrophobic resin infiltration of the hybrid layer
Correct answer: Ionic exchange between carboxylate groups and calcium in hydroxyapatite
Glass ionomer cements bond chemically via chelation: the polyacrylic acid (polycarboxylate) chains displace phosphate ions and form ionic crosslinks with calcium ions in hydroxyapatite. This provides a true chemical adhesion that is unique among dental cements and does not require acid etching or a separate bonding agent.
Question 4: A cantilever fixed partial denture (FPD) places the pontic abutment junction under which primary stress?
- Compressive stress on the occlusal surface of the abutment
- Tensile stress on the superior aspect of the connector (Correct answer)
- Shear stress uniformly distributed along the span
- Torsional stress only at the gingival embrasure
Correct answer: Tensile stress on the superior aspect of the connector
In a cantilever FPD, the unsupported pontic creates a bending moment about the connector. The connector's superior (occlusal) surface is placed in tension while the inferior surface is in compression. Because ceramics and porcelain are weak in tension, this geometry makes cantilever designs especially prone to connector fracture at the occlusal aspect of the joint.
Question 5: Which setting reaction best describes the hardening of zinc phosphate cement?
- Free-radical polymerization of methacrylate monomers
- Acid–base reaction forming zinc phosphate salt crystals within an unreacted zinc oxide core (Correct answer)
- Chelation of zinc ions by polyacrylic acid chains
- Hydration of calcium silicate phases producing calcium silicate hydrate gel
Correct answer: Acid–base reaction forming zinc phosphate salt crystals within an unreacted zinc oxide core
Zinc phosphate cement sets by an exothermic acid–base reaction: phosphoric acid attacks zinc oxide powder, forming an amorphous zinc phosphate gel matrix that crystallizes around unreacted ZnO particles. The resulting crystalline microstructure, with its ZnO core–shell architecture, provides the compressive strength and rigidity characteristic of this traditional luting cement.
Question 6: Creep in dental amalgam restorations is clinically significant because it leads to:
- Increased marginal adaptation and reduced microleakage over time
- Ditching and marginal breakdown as the alloy flows under sustained occlusal load (Correct answer)
- Spontaneous conversion of gamma-2 phase to gamma-1 phase
- Enhanced corrosion resistance through work hardening at the margins
Correct answer: Ditching and marginal breakdown as the alloy flows under sustained occlusal load
Creep is the time-dependent plastic deformation of amalgam under sustained compressive stress. At the restoration margin, the metal flows beyond the cavosurface angle, creating an unsupported overhang that fractures under cyclic loads — a phenomenon called 'ditching.' High-copper amalgams largely eliminated the weak gamma-2 phase and exhibit significantly lower creep values, improving marginal integrity.
ADAT - Advanced Dental Admission Test Dental Materials & Biomechanics 1
Which type of dental ceramic exhibits the highest flexural strength due to its tetragonal zirconia polycrystal (TZP) microstructure?