NBDHE Test Dental Materials and Procedures 1 — Questions and Answers
Question 1: Which composite resin curing shrinkage direction is most clinically significant when bonding to cavity walls?
- Shrinkage toward the light source, pulling away from cavity walls
- Shrinkage toward the bonded tooth structure, creating stress at the restoration interface (Correct answer)
- Uniform isotropic shrinkage with no preferred direction
- Shrinkage away from the light source only
Correct answer: Shrinkage toward the bonded tooth structure, creating stress at the restoration interface
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?
- Type IV has a higher water-to-powder ratio, making it easier to mix
- Type IV contains calcium sulfate hemihydrate instead of dihydrate
- Type IV has lower expansion and greater compressive strength, making it suitable for dies (Correct answer)
- Type IV sets faster but with less dimensional accuracy
Correct answer: Type IV has lower expansion and greater compressive strength, making it suitable for dies
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?
- Amalgam expands at a rate 2–3 times greater than enamel under temperature changes, repeatedly stressing the margin (Correct answer)
- Amalgam contracts upon cooling, pulling away from tooth structure permanently
- Amalgam and enamel have identical coefficients of thermal expansion, so no mismatch occurs
- Amalgam undergoes only creep deformation, not thermal dimensional change
Correct answer: Amalgam expands at a rate 2–3 times greater than enamel under temperature changes, repeatedly stressing the margin
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?
- To prevent the acid from evaporating before the cement sets
- To slow the exothermic reaction and allow longer working time while achieving adequate strength (Correct answer)
- To increase the final fluoride release from the cement
- To ensure the zinc oxide dissolves completely before the phosphate reaction begins
Correct answer: To slow the exothermic reaction and allow longer working time while achieving adequate strength
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?
- Hardness
- Fatigue resistance (Correct answer)
- Compressive strength
- Modulus of elasticity
Correct answer: Fatigue resistance
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.
Question 6: What is the primary purpose of using a cavity varnish under a zinc phosphate cement base in a deep preparation?
- To chemically bond the cement to dentin
- To seal dentinal tubules and reduce pulpal irritation from cement acids (Correct answer)
- To increase the thickness of the cement base for thermal insulation
- To accelerate the setting reaction of the zinc phosphate cement
Correct answer: To seal dentinal tubules and reduce pulpal irritation from cement acids
Cavity varnish — a resin dissolved in a volatile solvent — is applied to seal exposed dentinal tubules before placing zinc phosphate cement. This reduces the penetration of free phosphoric acid from the unset cement into the pulp, minimizing postoperative sensitivity and pulpal inflammation.
