Oral Health Group

Expecting the Unexpected. The Beauty of Endodontics: Part I Middle Mesial Canals of Mandibular First Molars

December 1, 2005
by Glen Partnoy, DDS, MS, FRCD(C)

Bacteria are the primary etiologic agents in pulpal and periapical infection.1-3 Therefore, the aim of successful endodontic therapy should be to thoroughly debride the canal system of organic substrate, infected pulp tissue and microorganisms and to seal the canal space to prevent reinfection of the pulp cavity.4 A missed canal is neither debrided nor thoroughly sealed, and thus may result in the development or persistence of periapical inflammation. With the use of magnification and fiber-optic illumination (in the form of either loupes and headlamp, or microscope) becoming mainstream among dental practitioners, accepting to expect the unexpected allows us to more thoroughly address the complex root canal system that was so beautifully described by Hess5 over 80 years ago.

A peek into the literature reveals the middle mesial canal of mandibular first molars was first described by Vertucci and Williams in 1974,6 and since that time its prevalence has been reported by various authors at an incidence of 1-15%.7-10 In virtually all cases this canal will join either the mesiolingual or mesiobuccal canal in the apical third, having an independent apical foramen in only 5% of cases.9 Alternatively, it may be argued that the middle mesial canal is not an extra canal but rather the sequelae of instrumenting the isthmus between the mesiobuccal and mesiolingual canals.11



A 48-year-old South African Caucasian male presented to my office in March, 2005 complaining of a history of intense pain to cold, and spontaneous pain related to tooth #36 for approximately five days (Fig. 1A). Clinical exam revealed the tooth had an extensive, deep composite resin restoration, had a dull, lingering response to cold testing and was percussion positive.

A diagnosis of irreversible pulpitis with acute apical periodontitis was made and consent for root canal therapy was obtained. Access was achieved while wearing 4.5X magnification loupes and a fiber-optic surgical headlight. The pulp was hemorrhagic upon access. Initially, only the three main canals (MB, ML and D) were located. These were subsequently cleaned and shaped until hemorrhage arrested (inflamed pulpal tissue was removed). Next, a #2 round bur in a slow-speed handpiece was used to gently trough and fully expose the dentino-pulpal groove between the MB and ML canals (this is done routinely for all molars) and explored with a DG-16 endodontic explorer.

Under 8X magnification using the surgical operating microscope a middle mesial canal was located, absence of perforation was confirmed with an electronic apex locator, and then all canals were chemomechanically cleaned, shaped and obturated using a warm gutta-percha technique (Fig. 1B). The middle mesial canal converged with the mesiobuccal canal in the apical third (Fig. 1C).


A 43-year-old South African (coincidentally) Caucasian female presented to my office in November, 2005 complaining of a history of pain to cold and biting on tooth #46 that had been present for a couple of months but just recently became spontaneous in origin, preventing her from sleeping the night before (Fig. 2A).

Clinical exam on the tooth revealed the presence of a broad MOD amalgam restoration that elicited a severe lingering cold response (she had to sit up and cover her mouth from the discomfort). The tooth was also sensitive to percussion. A diagnosis of irreversible pulipitis with acute apical periodontitis was made and consent for treatment was obtained.

Access was achieved while wearing 4.5X magnification loupes and a fiber-optic surgical headlight. The four main canals (MB, ML, DB and DL) were immediately identified, and then a #2 round bur was used to gently trough the dentino-pulpal groove between the MB and ML canals unveiling the middle mesial canal. An electronic apex locator was utilized to ensure a perforation of the pulpal floor had not occurred, and all five canals were cleaned shaped and obturated (Figs. 2B & 2C).


A 31-year-old Caucasian female presented to my office in November 2005 complaining of throbbing pain from tooth #36 for four days (Fig. 3A). Clinical exam on the tooth revealed it was extremely percussion sensitive and had a lingering response to cold.

A diagnosis of irreversible pulpitits with acute apical periodontitis was made and consent for treatment was obtained. Treatment was performed in the same manner as case #2. In this case, the middle mesial canal merged into the mesiolingual canal in the apical third (Figs. 3B & 3C).


Traditional teaching of root canal morphology has for years misled dental practitioners into believing there are a predetermined number of root canals in any given tooth. The absence of magnification and inadequate lighting has resulted in dentists squinting into the dark and scary access cavity of molars hoping they will identify three (and if they are really lucky), maybe four canals.

I propose that we should be laying aside conventional teachings of root canal anatomy and carefully identifying and exploring the dentino-pulpal groove that maps every multi-rooted tooth. This absolutely requires magnification and fiber-optic illumination! Only then will the practitioner be enabled a greater opportunity to more adequately clean, shape, and subsequently seal the root canal system, thereby increasing the predictability of the final treatment.

Dr. Partnoy received his DDS in 1998 from the University of Toronto, and a Masters of Science and Certificate in Endodontics from the University of Detroit Mercy in 2002. He maintains a private practice limited to endodontics, Endodontic Specialists, in Toronto.

Oral Health welcomes this original article.


1.Kakehashi S, Stanley HR, Fitzgerald RJ. The effects of surgical exposure of dental pulps in germ-free and conventional rats, Oral Surg 1965;20:340-9.

2.Moller AJR, Fabricius L, Dahlen G, Ohman AE, Heyden G. Influence of periapical tissues of indigenous oral bacteria and necrotic pulp tissue. An experimental study in monkeys. Scand J Dent Res 1981; 89:475-84.

3.Dahlen G, Bergenholtz G. Endodontic activity in teeth with necrotic pulps. J Dent Res 1980;59:1033-40.

4.Lin LM, Scribner JE, Gaengler P. Factors associated with endodontic treatment failures. JOE 1992; 18:625-7.

5.Hess, W. The Anatomy of the Root Canals of the Teeth of the Permanent Dentition. William Wood & Co. New York, 1925:45.

6.Vertucci F, Williams R. Root canal anatomy of the mandibular first molar. JNJ Dent Assoc. 1974; 48 27-8.

7.Vertucci F. Root canal anatomy of the human permanent teeth. Oral Surg 1984;58:589-99.

8.Pomeranz H, Eidelman D, Goldberg M. Treatment considerations of the middle mesial canal of mandibular first and second molars. J Endodon 1981;7:565-8.

9.Fabra-Campos H. Three canals in the mesial root of mandibular permanent first molars; a clinical study. Int Endod J. 1989;22:39-43.

10.Goel NK, Gill KS, Taneja JR. Study of root canal configurations in mandibular first permanent molar. J Indian Soc Pedod Prev Dent 1991;8:12-4

11.Mortman RE, Ahn S. Mandibular first molars with three mesial canals. Gen Dent. 2003;51:549-51.

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