## 1 Introduction  
Endodontic treatment is one of the most challenging procedures in dentistry, and knowledge of the common anatomy of the endodontic space and its variations is critical to the success of endodontic treatment [1]. According to the American Association of Endodontists, the difficulty of endodontic treatment depends on anatomical variations in the tooth and its position in the jaw, among other factors [2]. The morphology of the tooth is formed during the late bell stage, and root development represents the interaction between the dental sac, dental papilla and [PERSON]'s sheath, where, after complex and unique interactions, the full length of the root is achieved with an appropriate system of root canals [3]. During tooth formation, deviations from the usual morphology, and numerous anatomical variations can occur in each tooth type, including the number and shape of roots and root canals [4]. The lower second molar, although it usually has two roots, the mesial, which usually has two canals, and the distal, which usually has one canal, is often subject to anatomic variations. One of the most common anatomical variations affecting the roots and canals of lower second molars is the occurrence of a \"C\"-shaped canal [5, 6]. In addition to mandibular molars, it can also be found in maxillary molars [7, 8]. The \"C\"-shaped root is thought to be caused by a defect in the fusion of the Hertwig's sheath or by the deposition of cementum over time [9]. A \"C\"-shaped root always contains a \"C\"-shaped canal due to the fusion of either the buccal or lingual aspect of the mesial and distal roots while the two roots stay connected by an interradicular ribbon [10]. The peculiarity of this anatomical anomaly is that the pulp chamber is positioned below the cementoenamel junction. Although it is named a canal, it is rare to find a classic cylindrical or conical space containing pulp tissue. \"C\"-shaped canal actually refers either to an irregular space resembling the letter \"C\", or to multiple canals connected by a network of accessory canals [10]. Furthermore, \"C\"-shaped root canals have thin lingual walls, which presents a further challenge for root canal instrumentation [11]. According to [PERSON] et al. [12], for a lower second molar to be diagnosed with having a \"C\"-shaped root canal it must exhibit three main characteristics: fused roots, a longitudinal groove on the lingual or buccal surface of the root and at least one cross section of the root canal having the continuous C or semicolon shape. Clinically, a \"C\"-shaped root canal can be recognized according to the appearance of the floor of the pulp chamber and a large pulp chamber with low bifurcation. One of the first clinical indications of this anatomical variation may be localized periodontal disease due to narrow grooves on the root surface [13]. Among other morphological anomalies and variations in the lower molars, the presence of an additional root is also possible. An additional root on the lingual side is radix entomolaris while radix paramolaris is an additional root located bucucally [6]. Before root canal treatment, in the majority of cases, a preoperative radiograph is available, and it can provide certain clues regarding root canal morphology. However, it is debatable if X-rays are helpful in diagnosing a \"C\"-shaped root canal. According to some data from the literature, a few radiographic characteristics can help predict \"C\"-shaped root canals: radicular fusion, radicular proximity, a large distal canal or a blurred image of a third canal in between [14; 15]. In the case of a lower second molar, signs of a \"C\"-shaped canal on X-ray include the appearance of a single root or as two roots with communication or a large and deep pulp chamber which is usually present in \"C\"-shaped roots [13].