Variations in Root and Canal Morphology of Mandibular First Molars: A Retrospective Cone-Beam Computed Tomography Study in a Saudi Subpopulation

<b>Background/Objectives</b>: Successful endodontic treatment relies on a precise understanding of root canal morphology and effective disinfection of the entire canal system. Anatomical variations, such as additional canals, canal merging, and intercanal communications, may affect clinical outcomes. This retrospective CBCT study evaluated the root and canal morphology of mandibular first molars in a Saudi population, with specific focus on the prevalence and configuration patterns of middle mesial canals (MMCs) and isthmuses in the mesial root. <b>Methods</b>: A total of 552 CBCT scans taken between 2017 and 2021 were screened. After applying strict eligibility criteria, 167 scans from Saudi patients containing fully erupted mandibular first molars with completely formed apices were included, yielding 322 teeth. Teeth with prior root canal treatment or distorted scans were excluded. Root canal configurations were classified according to Weine's system. The presence of MMCs, isthmuses, intercanal distance, and bilateral symmetry was recorded. Descriptive and comparative analyses were performed using Fisher's exact test, chi-square test, Kruskal-Wallis test, and McNemar's test. <b>Results</b>: Most mandibular first molars (75.5%) had two roots, whereas 24.5% exhibited two distinct distal roots. Mesial roots predominantly demonstrated Weine type II (50.9%) and type III (48.1%) configurations, while distal roots were mainly type I (69.3%). MMCs were rare (0.98%). Isthmuses were highly prevalent (82%) and occurred significantly more often in type III mesial roots (72.3%) compared with type II (1.2%) (<i>p</i> < 0.0001). Bilateral symmetry was substantial in both mesial and distal roots (86.8% and 88.4%, respectively). <b>Conclusions</b>: Middle mesial canals were uncommon, whereas isthmuses were frequently observed in the mesial roots of mandibular first molars among Saudi individuals. The high prevalence of isthmuses underscores the need to anticipate intercanal communications during endodontic treatment. CBCT evaluation enhances detection of such complex anatomical features and supports improved diagnostic accuracy and treatment planning.