Root canal anatomical complexities, such as isthmus, may limit the action of the endodontic instruments, irrigant solutions and intracanal medications, leading to endodontic treatment failure. Objectives: This in vitro study assessed the type, position and incidence of isthmus in human permanent molars. Material and Methods: One hundred roots were embedded in transparent resin, and then split at different distances from the apex (1.0–2.5–4.0–5.5–7.0 mm). Following the sample examination in stereomicroscope, ! The highest isthmus incidence was at 7.0 mm from the root apex in all samples, except the distal root of lower molars (at 5.5 mm). In upper and lower molars, type V (complete isthmus with a continuous opening between the two main root canals) was the most " #$% % & ' " molars, type IV had the highest incidence (36% and 23.9%, respectively). Conclusion: It " + - close to the apex. In upper and lower molars, these structures were most frequently found at 7 mm from the apex
