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Learning Objectives:

After participating in the activity, the individual will learn:
  • variations in root canal anatomy found in mandibular first molars, and
  • the clinical technique for endodontically treating mandibular first molars with 4 canals.

About the Authors

Dr. Polesel graduated in dentistry at the University of Genoa (Italy) in 1995. He has been an active member of the Italian Endodontic Society (SIE) since 2001, active member of the European Society of Endodontology (ESE) since 2003, and associate member of the American Association of Endodontists since 2007. He maintains a private practice in endodontics and restorative dentistry in Arenzano and Genoa, Italy. He can be reached via e-mail at
Dr. Castelluccigraduated in medicine at the University of Florence (Italy) in 1973 and specialized in dentistry at the same university in 1977. As well as maintaining a practice limited to endodontics in Florence, Dr. Castellucci is past president of the Italian Endodontic Society, past president of the International Federation of Endodontic Associations, an active member of the European Society of Endodontology, an active member of the American Association of Endodontists, and a visiting professor of endodontics at the University of Florence Dental School. He is editor of The Italian Journal of Endodontics and of The Endodontic Informer. An international lecturer, he is the author of the text Endodontics, which is now available in English. He can be reached via e-mail at


Figure 1 Image
Figure 1. The access cavity must have the shape of the projection of the pulp chamber floor onto the occlusal surface. The point of entry of the bur is identified by the central fossa.

Figure 2 Image
Figure 2. Mandibular first molar with ramifications (subdivisions) and lateral canals. Hess Anatomical Plates4 is considered one of the foundations of modern endodontics. Approximately 5,000 teeth were analyzed to code morphological features of the endodontium. (Reprinted with permission from Nicola Perrini, Le Tavole Anatomiche di W. Hess. Keller O, Ed. Edizioni Scientifiche Oral B. 1988: Plate 7, Mandibular First Molar.)

Figure 3 Image
Figure 3. Access cavity of a mandibular first molar with 3 canals. Magnification and ultrasonic tips allow conservative preparations.

Figure 4a Image
Figure 4a. Postoperative radiograph of the mandibular first molar.

Figure 4b Image
Figure 4b. The mesio-distal view of the same tooth demonstrates that in addition to the 2 canals of the distal root, the mesial canals have independent apical foramina that communicate at the level of the middle one third.

Figure 5a Image
Figure 5a. Mandibular first molar with 2 apical foramina in the distal root. The mesial root demonstrates 2 canals and 2 foramina.

Figure 5b Image
Figure 5b. The access cavity preparation, after isolation and pretreatment, shows 3 orifices and the traditional triangular outline. Straight-line access and smooth axial walls are essential for endodontic success.

The long-term success of endodontic treatment is the result of good clinical practice, including a tapered preparation, maintenance of the original anatomy, the removal of all organic soft tissue, a 3-dimensional root canal obturation,1 and a protective restoration to prevent microleakage into the root canal system.2 The following are essential for proper shaping: a well-designed access preparation (Figure 1), correct interpretation of the appropriate radiographs, and knowledge of the root canal system, as morphology can vary even in anterior teeth such as the maxillary central incisors.3
Clinicians should approach the tooth to be treated with the assumption that anatomical variation often occurs. It is important for practitioners to have essential knowledge prior to beginning endodontic therapy. This includes familiarity with the description of endodontic anatomy4 (Figure 2), recent reports in the clinical scientific literature, the application of the buccal-object rule in endodontic radiography to identify the spatial position of an object and to localize additional canals,5 and the use of current technologies (magnification, fiber-optic illumination, and ultrasonics) that improve vision, access, and control when performing clinical procedures such as access and orifice location (Figure 3). According to the buccal-object rule, when a radiograph is taken at a certain angle, the object closer to the radiographic source–the buccal-object–is displaced in the radiograph in the same direction as the x-ray beam.
Together with the maxillary first molar, the mandibular first molar most frequently requires endodontic treatment.6 General dentists usually look for the fourth canal in the mesiobuccal root of maxillary first molars, but they do not do this in mandibular first molars. Two orifices in the distal root of the mandibular first molar occur frequently. This tooth is often extensively restored. It is almost always under heavy occlusal stress; therefore, the coronal pulp chambers are frequently calcified.6 Iatrogenic and pathological problems make the location of the second orifice more difficult.
This 2-part report will help clinicians understand and apply a modern paradigm for caries management. Part 1 will focus on diagnosis and will introduce treatment concepts. Part 2 will focus on practical protocols for caries management.
The mandibular first molar usually has 2 roots; occasionally it has 3. Further, there are generally 2 canals in the mesial root and one or 2 in the distal root. The canals of the distal root are larger than those of the mesial root. A canal orifice that is wide buccolingually indicates the possibility of a second canal, or a ribbonlike canal with complex webbing that can complicate cleaning and shaping.6 The most common configuration of the distal root of the mandibular first molar is one single canal and one apical foramen (66.1%).7 The occurrence of 2 canals and 2 foramina in the distal root is relatively rare (4.8%) (Figures 4a and 4b).7 Most commonly, the mesial root has 2 canals converging to form a common apical foramen (61.3%).7 Two canals with distinct apical foramina in the distal root of the mandibular first molar were found in 25% of cases by Goel, et al,8 11.1% of cases by Skidmore and Bjorndal,9 7% of cases by Vertucci and Gainesuille,10 5.7% of cases by Pineda and Kuttler,11 and 21% of cases by Sert, et al12 (Table). In a small percentage of cases the distal root subdivides to form 2 distinct canals13 (Figures 5a and 5b).
Root canal anatomy of the mandibular first molar is frequently more complex than what is obvious on the radiograph. Skidmore and Bjorndal9 stated that approximately one third of mandibular first molars studied had 4 root canals. Therefore, in these teeth the traditional triangular access form should be changed into a more rectangular form in order to permit better visualization and exploration for a possible fourth canal in the distal root.9 In mandibular first molar teeth, distal accessory foramina are sometimes located in a small distolingual root.10 The distobuccal root can also demonstrate 2 separated canals. A mandibular first molar with 3 roots and 5 canals has been described; 2 canals were found in the mesial root, 2 canals in the distobuccal root, and one canal in the distolingual root.14 A case of a patient with an additional distal root in each permanent mandibular first molar has been reported; inability to recognize these aberrations could lead to failure of endodontic treatment.15
The importance of radiographic examination and interpretation to identify distolingual canals and unusual canal morphology associated with mandibular first molar teeth is an important consideration.16 For example, Weine reported a mandibular first molar with 3 mesial canals (one accessory canal in the middle of the 2 mesial canals),17 and Quack-enbush18 reported a case of 5 distinct canals (2 canals in the mesial root and 3 canals in the distal root). Another case of a mandibular first molar with 5 canals has been described: 2 canals were found in the mesial root and 3 canals were found in 3 distal roots.19 Ricucci described endodontic therapy in a mandibular first molar with 3 canals in the mesial root.20 The mesiobuccal and the mesiolingual canals were found in their normal locations; the third canal was located between the other 2 canals. Each canal ended in its own foramen. Castellucci described a similar case, where the mesial root had 3 canals (mesiobuccal, mesiolingual, and the so-called "middle mesial canal," each one with an independent foramen) and the distal root had 2 separate canals (again, each one with an independent foramen).21
The morphological pattern of separate apical terminations of 3 mesial canals is a rare finding but was reported by Holtzmann.22 Beatty and Interian emphasized the need for the practitioner to perform a complete examination of the pulpal floor of the tooth even after the anticipated number of canal orifices have been identified.23
Mandibular first molars with 5 canals were described by Beatty and Krell.24
Mandibular first molars with 6 root canals were reported by Martinez-Berna and Badanelli.25 Reeh reported a mandibular first molar retreatment in which 7 canals were cleaned, shaped, and filled/refilled, leading to clinical resolution.26
Further, according to the endodontic literature, C-shaped root configuration is most frequently seen in the mandibular second molar; however, a case of a mandibular first molar with a C-shaped root configuration was reported.27 Although the most common anatomical variations in mandibular first molars are the number of canals, variations in canal shape do occur. This can influence the outline of the access cavity, which must always follow the shape of the pulp chamber floor.
To illustrate the proper protocol when endodontically treating mandibular first molar teeth, 2 cases will be presented: the retreatment of a mandibular first molar with 4 root canals and a large periapical lesion, and the treatment of a mandibular first molar with 4 canals.