Mahdi Tabrizizadeh
Yousef Khalil Sefat
Mahdi Hakimian

Background: Accessibility to all sites of root canal and its mechanical and chemical cleaning is mandatory for successful root canal therapy. The presence of isthmus is a major hindrance to complete root canal accessibility. Objectives: The purpose of the present study was to determine the relative frequency and type of isthmuses in the apical region of mesial root of the first mandibular molar extracted in Yazd. Materials and Methods: In this descriptive-laboratory study, ۱۰۰ mandibular first molar teeth were collected. The mesial roots were excised at the cervical region and three horizontal sections perpendicular to the longitudinal axis of the root were secured with ۱-, ۳-, and ۵-mm distances upwards from apex region of the roots. The incised surfaces were stained using India ink and viewed under stereomicroscope with a magnifying power of ×۶۰ and photographed. The obtained images were studied regarding the presence or absence of isthmuses and the various anatomical forms of isthmuses were recorded based on Hsu and Kim taxonomy. Results: Isthmus was present in ۵۴% of teeth. The greatest frequency of isthmuses was observed in the ۵ mm from the apex. The type V isthmus was the most prevalent isthmuses between all levels of roots. Conclusions: The frequency of isthmuses in the mesial root of mandibular first molars was high. The results of clinical and surgical endodontic procedures may be affected by this aspect of root canal anatomy.