MB2 Canal Upper Molar — ORE Part 1 MCQ
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Correct answer: B — Second mesiobuccal canal (MB2)
The second mesiobuccal canal (MB2) is the most commonly missed canal in endodontic treatment of upper first molars. Multiple CBCT studies confirm that approximately 45–46% of clinically treated upper molars have missed MB2 canals, and when missed, about 70% develop apical periodontitis. In molar retreatment cases, 93% of missed canals in upper first molars are located in the mesiobuccal root, predominantly the MB2. The distobuccal canal is the second most commonly missed but remains significantly less common than MB2. The palatal and MB1 canals are typically easier to locate due to their larger size and distinct locations; the mesiopalatal (fourth canal) is a rare anatomical variant. The high prevalence of MB2 (approximately 70% overall, up to 92% with direct access) combined with its challenging anatomy (close proximity to MB1, small orifice, variable curvature) makes this canal a frequent source of endodontic treatment failure.
Reference: Prevalence of Missed Canals and Their Association with Apical Periodontitis in Posterior Endodontically Treated Teeth: A CBCT Study. PubMed PMC8257386 (2022). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8257386/