The profound knowledge of the root canal anatomical complexity generated by accessory canals, apical deltas and isthmuses, increases the likelihood of success in endodontic treatment, given that such sites may be a reservoir of microorganisms and organic matter that may limit the action of instruments, irrigant substances and intracanal medications. The objective of this study was to determine the incidence and type of isthmus in the various segments of mesio-buccal roots of first upper molars and mesial root of first lower molars. Seventy-two extracted first molars, thirty-six upper and thirty-six lower were radiographed and different anatomic variants such as number of canals, curvature and root configuration were studied. Later, mesio-buccal and mesial roots were cut and embedded in resin and sectioned with a diamond disc, five serial horizontal sections were made perpendicular to the root long axis. These were stained with crystal violet and examined under stereomicroscope in order to determine the presence of isthmuses and their classification. Results showed a high incidence of isthmus associated with roots of flattened configuration. The closer to the apex, the lower the incidence of isthmus observed in both in the upper and lower roots; the isthmus were most frequently found at 7 mm from the apex, the type of isthmus found more frequently was type II in both types of root.

KEY WORDS: Isthmus; Anatomy; Molar; Endodontics.