NBDHE Dental Materials and Procedures 2 — Questions and Answers

Question 1: Which property of amalgam restorations is responsible for the initial poor marginal seal shortly after placement?

Dimensional change — initial contraction before expansion (Correct answer)
Immediate galvanic corrosion
Excessive plasticity
High coefficient of thermal expansion

Correct answer: Dimensional change — initial contraction before expansion

Fresh amalgam undergoes slight contraction immediately after setting, before its later characteristic expansion, creating initial marginal gaps that may allow microleakage.

Amalgam undergoes a dimensional change during and after setting. Modern high-copper amalgams show minimal overall dimensional change (near zero), but even a slight initial contraction creates temporary marginal gaps. Over time, corrosion byproducts (chloride compounds) accumulate at the margins and provide a self-sealing effect, which is one reason why amalgam restorations often show improved marginal integrity over years. High-copper amalgams have reduced gamma-2 phase (which was more prone to corrosion and creep), improving long-term performance.

Question 2: Which bonding mechanism primarily explains the adhesion of glass ionomer cement to tooth structure?

Chemical adhesion through ionic bonding with hydroxyapatite (Correct answer)
Micromechanical interlocking only
Van der Waals forces
Hydrophobic bonding

Correct answer: Chemical adhesion through ionic bonding with hydroxyapatite

Glass ionomer cements adhere to tooth structure primarily through ionic bonding between the carboxylate groups of the polyacrylic acid and the calcium in hydroxyapatite.

Glass ionomer cement (GIC) contains polyacrylic acid, which chemically bonds to tooth structure. The carboxylate (–COO⁻) groups in the polyacid chelate with calcium ions (Ca²⁺) in hydroxyapatite, forming a true chemical bond. This distinguishes GIC from composite resins, which rely primarily on micromechanical interlocking after acid etching. GIC also releases fluoride ions that promote remineralization and provide some degree of caries inhibition at restoration margins. Resin-modified GICs combine chemical adhesion with light-curing for improved handling.

Question 3: What is the purpose of the smear layer in restorative dentistry, and how does acid etching affect it?

It plugs dentinal tubules; acid etching removes it and opens tubules (Correct answer)
It strengthens dentin; acid etching hardens it further
It prevents bacterial invasion; acid etching reinforces it
It has no clinical significance; acid etching has no effect

Correct answer: It plugs dentinal tubules; acid etching removes it and opens tubules

The smear layer is a thin film of debris on prepared dentin that plugs tubule openings; acid etching dissolves it and opens dentinal tubules, enabling resin tag formation.

The smear layer is formed during cavity preparation and consists of amorphous debris, bacteria, and collagen fragments approximately 1–2 µm thick. It partially obliterates dentinal tubule openings (smear plugs), reducing dentinal fluid flow. Phosphoric acid etching (37%) removes the smear layer, opens tubule orifices, and demineralizes intertubular dentin, exposing a collagen fibril network for hybrid layer formation. Self-etch primers dissolve rather than fully remove the smear layer, incorporating it into the adhesive interface. Eighth-generation universal adhesives offer flexibility in handling the smear layer.

Question 4: Which type of impression material is classified as an irreversible hydrocolloid?

Alginate (Correct answer)
Agar
Polyvinyl siloxane
Zinc oxide eugenol

Correct answer: Alginate

Alginate is an irreversible hydrocolloid because its gelation is a chemical reaction that cannot be reversed; it cannot be re-liquefied and reused.

Impression materials are classified by their mechanism of setting. Irreversible hydrocolloids (alginate) undergo chemical gelation via a cross-linking reaction between sodium alginate and calcium sulfate; once set, they cannot be returned to a fluid state. Reversible hydrocolloids (agar) are thermoplastic — they gel on cooling and can be re-liquefied by heating. Elastomers such as polyvinyl siloxane (PVS/addition silicone) are irreversible elastomers. Zinc oxide eugenol impression paste is an irreversible rigid material. Alginate is the most widely used impression material due to ease of use and economy.

Question 5: Composite resin restorations undergo polymerization shrinkage. Which technique BEST reduces the clinical consequences of this shrinkage?

Incremental placement technique with each increment ≤2 mm (Correct answer)
Bulk placement in a single increment
Placing composite without bonding agent
Using a high-intensity curing light for the full bulk at once

Correct answer: Incremental placement technique with each increment ≤2 mm

Placing composite in increments of 2 mm or less and curing each layer independently reduces the volume contracting at any one time, minimizing stress at the tooth-restoration interface.

Composite resins polymerize with approximately 1.5–5% volumetric shrinkage, generating stress at the bonded interfaces that can cause cuspal deflection, marginal gap formation, postoperative sensitivity, and secondary caries. The incremental technique limits each layer to ≤2 mm, reducing the overall shrinkage per cure cycle. Placing the first increment obliquely on the pulpal floor (C-factor optimization) also helps. Bulk-fill composites are engineered to reduce shrinkage stress through photoinitiator modifications and stress-relief monomers, allowing 4–5 mm increments, but traditional composites should not be placed in bulk.

Question 6: Which component of local anesthetic solution is primarily responsible for preventing oxidation of the vasoconstrictor?

Sodium bisulfite (sodium metabisulfite) (Correct answer)
Sodium chloride
Methylparaben
Hydrochloric acid

Correct answer: Sodium bisulfite (sodium metabisulfite)

Sodium bisulfite (or sodium metabisulfite) is the antioxidant added to local anesthetic cartridges containing vasoconstrictors (epinephrine) to prevent their oxidation and degradation.

Epinephrine and other catecholamine vasoconstrictors are highly susceptible to oxidation, which renders them inactive. Sodium bisulfite or sodium metabisulfite acts as a reducing agent (antioxidant) to scavenge free radicals and prevent epinephrine breakdown. Patients allergic to sulfites may report reactions to local anesthetics with vasoconstrictors; plain (vasoconstrictor-free) anesthetics do not contain bisulfites. Methylparaben was formerly used as a preservative in multi-dose vials but is no longer used in dental cartridges due to allergy potential. Sodium chloride adjusts tonicity.

NBDHE Dental Materials and Procedures 2

Which property of amalgam restorations is responsible for the initial poor marginal seal shortly after placement?