Abstract: Objective: To evaluate the clinical effect of using dental operating microscope to detect and negotiate the middle mesial canals in mandibular molars. Methods: One hundred and thirty-nine mandibular molars from 126 patients were recruited. The detection and negotiation of middle mesial canals in mandibular molars were performed by three senior endodontists by naked eye or with the aid of dental operating microscope. The location of middle mesial canal orifices was recorded and the pathway of the canals was characterized by periapical angulated projection of X ray. Results: With the aid of microscope, the total number of detected and negotiated middle mesial canals increased by 11.5% and 10.1% respectively compared with naked eye. The number of negotiated middle mesial canals increased from 5.0% to15.0% in the first molars, and from 3.4% to 13.6% in the second molars. The number of detected middle mesial canals in the first and second molars increased by 12.5% and 10.2% under microscope. All of the detected middle mesial canal orifices were located in the mesial subpulpal groove, closer to the mesiolingual canal (46.2%), in the middle of the groove (30.8%), or closer to the mesiobuccal canal (23.1%). There was 45% of the negotiated confluenting with the mesiolingual canal and there was no middle mesial canals with an independent apical foramen. Conclusion: With the aid of dental operating microscope, the percentages of detected and negotiated middle mesial canals in mandibular molars were increased significantly.

Key words: Middle mesial canal, Mandibular molars, Dental operating microscope