NBDE Endodontics 4 — Questions and Answers
Question 1: Which microorganism is most commonly isolated from teeth with failed root canal treatment (persistent infection)?
- Streptococcus mutans
- Porphyromonas gingivalis
- Enterococcus faecalis (Correct answer)
- Fusobacterium nucleatum
Correct answer: Enterococcus faecalis
Enterococcus faecalis dominates persistent endodontic infections due to its ability to survive in nutrient-poor, alkaline environments and resist calcium hydroxide treatment.
Question 2: A patient presents with a draining sinus tract on the attached gingiva near tooth #30. The tract is traced with a gutta-percha cone. Where does the cone most likely terminate?
- Gingival sulcus of the adjacent tooth
- Furcation area of the same tooth
- Periapical region of the offending tooth (Correct answer)
- Palatal root of the maxillary molar
Correct answer: Periapical region of the offending tooth
Sinus tracts from endodontic origin typically track through bone and soft tissue to terminate at the periapical area of the causative tooth.
Question 3: Which sealer type is considered the current gold standard for biocompatibility and bond strength in root canal obturation?
- Zinc oxide eugenol-based sealers
- Epoxy resin-based sealers (AH Plus) (Correct answer)
- Calcium hydroxide-based sealers
- Silicone-based sealers
Correct answer: Epoxy resin-based sealers (AH Plus)
Epoxy resin-based sealers like AH Plus demonstrate superior bond strength to dentin, low solubility, and excellent long-term clinical performance.
Question 4: During access preparation of a mandibular second premolar, the clinician should anticipate which unusual canal configuration?
- Always one canal with one foramen
- High incidence of three separate canals
- Frequent occurrence of a C-shaped canal
- Possible two canals that may or may not merge (Correct answer)
Correct answer: Possible two canals that may or may not merge
Mandibular premolars have notoriously variable canal anatomy, with two canals present in a significant percentage that may merge or remain separate — a major source of treatment failure.
Question 5: When calcium hydroxide is used as an intracanal medicament, its primary antimicrobial mechanism is:
- Direct enzyme inhibition by calcium ions
- Disruption of bacterial membranes by hydroxyl ions raising pH above 11 (Correct answer)
- Formation of calcium carbonate precipitate that blocks tubules
- Chelation of essential metal cofactors
Correct answer: Disruption of bacterial membranes by hydroxyl ions raising pH above 11
The high pH (>11) generated by hydroxyl ion release denatures bacterial proteins and enzymes, disrupts membrane integrity, and damages bacterial DNA.
Question 6: A 9-year-old patient has an immature permanent incisor with a necrotic pulp. Which treatment option is most appropriate?
- Conventional root canal obturation with gutta-percha
- Extraction and space maintainer
- Apexification with MTA or regenerative endodontic procedure (Correct answer)
- Pulpotomy with formocresol
Correct answer: Apexification with MTA or regenerative endodontic procedure
Immature necrotic teeth require apexification (MTA barrier) or regenerative endodontic procedures to promote apical closure before or instead of conventional obturation.
Question 7: Which CBCT finding is most useful in identifying a missed MB2 canal in a maxillary first molar before retreatment?
- Increased root length measurement
- Separate radiolucent channel in the mesiobuccal root distinct from MB1 (Correct answer)
- Hyperdense area at the furcation
- Widened PDL space along the distal root
Correct answer: Separate radiolucent channel in the mesiobuccal root distinct from MB1
CBCT axial slices through the mesiobuccal root clearly reveal a separate MB2 canal as a distinct radiolucent channel, guiding retreatment access.
NBDE Endodontics 4
Which microorganism is most commonly isolated from teeth with failed root canal treatment (persistent infection)?