In a study of 760 mandibular molars, Fabra found that 20 (2.6%) had three canals in the mesial root. In 13 of these (65%) the third canal joined the mesiobuccal canal in the apical third of the root and in 6 (30%) they converged with the mesiolingual canal, also in the apical third; the third canal ended as an independent canal in only 1 case. Many authors agree on the presence of three foramena in the mesial root but few report three independent canals, which presents itself as a rare anatomical variant. Walker cites 3 canals in the mesial root of the mandibular first molars as an infrequent ocurrence. Goel notes that the mesial root of permanent mandibular first molars presented two foramena in 60% of the specimens they examined, 6,7% had three and 3.3% even had four. In early studies by Hess, only 1 out of 55 teeth presented 2 distal roots and 5 canals did not appear in any of the teeth. However, many other authors confirm the hypothesis of a third canal in the mesial root of the permanent mandibular first molars According to Mortman, the third mesial canal is not an extra canal but rather the sequelae of instrumenting the isthmus between the mesiobuccal and mesiolingual canals. The isthmus is a narrow connection between two root canals that contains pulp tissue. In clinical practice, this isthmus is important in the conventional treatment of canals and in periapical surgery. In both cases it can lead to failure because it makes the canals very difficult to shape. An isthmus is located in between 54% and 89% of cases, most frequently between 4 mm and 6 mm from the apical foramen. The studies that examine series of cross-sections taken at different distances from the apex observe that isthmuses mainly occur at 3-5 mm from the apex. The prevalence and types of isthmus that can be identified in first molars by endoscopic examination during periapical surgery are as follows for the mesial root of the mandibular first molar: in a sample of 52 mesial roots, a canal with no isthmus was found in 3 cases (6%), two canals with no isthmus in 6 (11%) and two canals with isthmus in 43 (83%). Read more here.
Source: Navarro et al. (2007)