The majority of Caucasian first molars are two-rooted with two mesial and one distal canal. The tooth usually has two roots with two or three canals in the mesial root and one, two, or three canals in the distal root. In 1% to 15 % of mandibular first molars a middle mesial canal (MMC) is present in the developmental groove between the MB and ML canals. The presence of two separate distal roots is rare, but this does occur.
When only one distal canal is present, the orifice is oval buccolingually and the opening is in the middle. If the orifice is narrow and round, a second distal canal may be present. It is said if a #25 k-file could not reach the apex, there should be an extra canal in distal root. If so, the file tip takes a sharp turn in a distobuccal or distolingual direction and the file handle moves in an opposite direction.
A variation in root morphology is the presence of an extra distolingual root. The radix entomolaris (RE) is a supernumerary root located distolingually in mandibular molars whereas the radix paramolaris (RP) is an extra root located mesiobuccally. They are common in Chinese population.
Variations in the canals and roots of mandibular first molars are listed below:
- Mesial root/canal
- Middle mesial canal (Figs.1 and 2)
- Mesial root + Paramolaris (a very rare supernumerary root at mesiobuccal)
- Distal root/canal
- Middle distal canal (Figs. 3 and 4)
- One distal root with one canal
- One distal root with two canals (Missed DB canals, Figs. 5 to 8 and Figs. 9 to 12, the latter case was not VRF!)
- Two separate distal roots [ distobuccal root + radix entomolaris (a supernumerary root at distolingual)] (Figs. 13 and 14)
Conclusion: Dentists who perform root canal treatment should have a profound knowledge of the root canal anatomy. Root canals which are left untreated because the dentist fails to recognize their presence, will lead to the endodontic treatment failure (Figs. 5 to 12).