ADAT - Advanced Dental Admission Test Dental Materials & Biomechanics 1

Question 1

Which type of dental ceramic exhibits the highest flexural strength due to its tetragonal zirconia polycrystal (TZP) microstructure?

Accepted Answer

Yttria-stabilized zirconia (Y-TZP)

Answer

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:

Accepted Answer

18 GPa

Answer

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?

Accepted Answer

Ionic exchange between carboxylate groups and calcium in hydroxyapatite

Answer

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?

Accepted Answer

Tensile stress on the superior aspect of the connector

Answer

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?

Accepted Answer

Acid–base reaction forming zinc phosphate salt crystals within an unreacted zinc oxide core

Answer

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.

Advanced Dental Admission Test (ADAT) Practice Test

Advanced Dental Admission Test (ADAT) Practice Test

ADAT - Advanced Dental Admission Test Dental Materials & Biomechanics 1