Incidence of Middle Mesial Canals in Mandibular Molars After Guided Troughing Under High Magnifaction: An In Vivo Investigation

Disinfection of the root canal system is the primary goal of endodontic treatment. Schilder coined the term root canal system in the 1960’s to acknowledge the complexity of endodontic treatment. During tooth development, Hetrwig’s epithelial root sheath directs the formation of the root. Dentinogenesis results in the axial thickening of the root encroaching on the primordial dental papillae. As the root dentin increases the space for the dental papillae is reduced until full root development has occurred and an adult root form and dental pulp has been achieved.

But what determines how much root development occurs and its subsequent shape? Genetics, of course, determines whether a tooth would develop as an incisor or molar. But within the same tooth class what determines whether a molar will have three canals or four or possibly five? The ultimate shape of the root canal system appears to be directed by the vascular system. Endothelial cells of the pulpal vasculature send chemical messengers to the odontoblasts shutting down root development at certain point, likely a distance relative to the ability of the messenger molecule to diffuse from the endothelial cell. The resultant root canal system shape then is determined to a large degree by the complexity or simplicity of the vascular system itself.

This complexity of the root canal system with its lateral canals, ramifications, fins, cul de sacs, and apical branching is the greatest challenge to the Dentist reaching his endodontic goal. Every Dentist who has placed an instrument in a canal knows the frustration of negotiating a difficult passage to the apical canal terminus in a seemingly simple, straight canal. Furthermore, root canal systems often contain extra, unexpected canals. Everyone should be familiar with the MB2 canals of maxillary first and second molars which occur 70-80% of the time but fewer are aware of the mid-mesial canal of the mandibular molar.

Azim et al, JOE 41:2, February 2015, set out to determine the prevalence of the mid-mesial canal in mandibular molars (the mid-mesial canal occurs between the MB and ML canal in the mesial root) in this in vivo study. The authors examined 91 mandibular molars under the dental operating microscope. Following access canal orifices were identified and recorded. If no mid-mesial canal was located the MB and ML canals were cleaned and shaped, ready to fill, and then the isthmus between the MB and ML canals was troughed with a #2 Munce Discovery Bur (essentially a #2 round bur on the end of a Gates-Glidden shaft) to a depth of no more than 2 mm. If a mid-mesial canal was found, it was negotiated with hand files and then cleaned and shaped ready to fill.

The authors found that in the 91 molars evaluated, 6 (6.6%) teeth had mid-mesial canals that could be located without troughing. Following troughing, a further 36 (39.6%) mid-mesial canals were found with 60% occurring in the second molars. Of the 42 mid-mesial canals found, 4 (9.5%) had a separate coronal and apical orifice, i.e. were a completely separate canal. In the remaining teeth the mid-mesial canals joined either the MB or ML canal prior to the apical terminus. The authors also found that as the patient age increased the prevalence of mid-mesial canals decreased, likely due to canal calcification.

From a clinical perspective one might argue that if a large percentage of mid-mesial canal join there nearby MB or ML canal how important could they be? As an Endodontist we are charged with revision or retreatment of endodontically treated teeth that are failing. Often these teeth have previous endodontic treatment which fill the parameters of the “the look” of a good endodontic result. Many times during retreatment, with the help of 3D imaging and microscopic evaluation, extra, unfilled canal anatomy is discovered and treated resulting in a high proportion (85%) going on to heal. From a practical perspective the mesial root of mandibular molars is hourglass shaped with the narrow neck the home of the mid-mesial canal so troughing should be attempted with due respect to root shape.


Dr. Bittner was born and raised in the Surrey/Langley area. Following his pre-dental training at Simon Fraser University, he received his Doctor of Dental Medicine from the University of British Columbia in 1982 and his Certificate in Advanced Graduate Studies in Endodontics from Boston University’s Goldman School of Dental Medicine in 1995.

Dr. Bittner was in private practice in general dentistry for 11 years in Langley prior to his Endodontic specialty training. He has been practicing the Dental Specialty of Endodontics since 1995.

In his free time, Dr. Bittner loves to participate in a variety of sports including most recent, golfing! He also enjoys being a new grandfather, which if you ask him is just the best!

On March 30, 2015, posted in: News for Doctors by