NBDHE Test Dental Materials and Procedures 1

Question 1

Which composite resin curing shrinkage direction is most clinically significant when bonding to cavity walls?

Accepted Answer

Shrinkage toward the bonded tooth structure, creating stress at the restoration interface

Answer

Composite resins shrink toward bonded surfaces during polymerization. When the resin is bonded to cavity walls, the contraction stress pulls toward those walls, potentially breaking the adhesive bond or creating marginal gaps on unbonded surfaces — a phenomenon described by the C-factor (configuration factor).

Question 2

What distinguishes a Type IV gypsum product (die stone) from a Type III gypsum (dental stone) in laboratory use?

Accepted Answer

Type IV has lower expansion and greater compressive strength, making it suitable for dies

Answer

Type IV die stone has reduced setting expansion and higher compressive strength compared to Type III stone. These properties are critical for accurate dies used in fabricating cast restorations, where even minor dimensional changes would result in ill-fitting prostheses.

Question 3

Which mechanism accounts for the thermal expansion mismatch between amalgam restorations and tooth structure that contributes to marginal breakdown over time?

Accepted Answer

Amalgam expands at a rate 2–3 times greater than enamel under temperature changes, repeatedly stressing the margin

Answer

Amalgam's coefficient of thermal expansion is approximately 25 ppm/°C, roughly 2–3 times that of enamel (~11 ppm/°C). This mismatch causes cyclical stress at the restoration margin with each temperature fluctuation, gradually fatiguing and widening the marginal seal over years.

Question 4

In zinc phosphate cement, why must the powder be incorporated into the liquid in small, successive increments over a cool glass slab?

Accepted Answer

To slow the exothermic reaction and allow longer working time while achieving adequate strength

Answer

The reaction between zinc oxide powder and phosphoric acid is strongly exothermic. Spatulating small increments over a large, cool glass slab dissipates heat and slows the reaction rate, extending working time and allowing maximum powder incorporation, which increases the cement's compressive strength and reduces its solubility.

Question 5

Which property best describes the ability of a dental material to withstand repeated masticatory loads without fracturing over time?

Accepted Answer

Fatigue resistance

Answer

Fatigue resistance refers to a material's ability to withstand cyclic or repeated loading below its ultimate strength without failure. In the oral environment, restorations endure thousands of masticatory cycles daily, so fatigue resistance — not single-load compressive strength — determines long-term clinical durability.

NBDHE Test Practice Test

NBDHE Test Practice Test

NBDHE Test Dental Materials and Procedures 1