Background and Aim: A considerable percentage of failure in Endodontic treatments in maxillary molars is attributed to undiscovered second mesiobuccal canal (MB₂).There are different methods for discovering and accessing to this canal. The purpose of this ex vivo study was to compare the detection ability of three methods (direct look, fiberoptic loup and surgical microscope) to find MB₂ after troughing with ultrasonic.

Materials and Methods: In this experimental study, we selected 90 extracted maxillary molars (45 first and 45 second molars) in which after access cavity preparation MB₂ canal was not discovered by direct vision and endodontic explorer. They were divided into 3 groups (n=30). The dentinal shelf between mesiobuccal and palatal canals was eliminated by an endodontic ultrasonic tip (troughing). After that, first group was searched by direct vision, second group by a loup and fiberoptic light and third group by dental operating microscope. Data were analyzed, specificity and sensitivity were calculated.

Results: The results showed that 21%, 61%, and 92% of MB₂ canals after troughing was found by direct vision, fiberoptic loup, and surgical microscope, respectively.

Conclusion: Based on the results of this study, surgical microscope and loup with fiberoptic are preferred methods for discovering MB₂ canal. Troughing with ultrasonic can help find MB₂ canal in all methods.