Maxillary First Molars and Their Four Canals...The Benefits of Magnification

Ingle stated that a major cause of root canal therapy failure is the inability to recognize the presence of the canals of the root canal system and to adequately obturate all of them.1 In 2 prior articles appearing in Dentistry Today (July 1998 and June 1999), we demonstrated the importance of the dental operating microscope in locating all the canals in complex root canal systems. The microscope and knowledge of these anatomic characteristics assist the dentist in providing quality treatment.

One of the biggest mysteries in endodontics is the elusive “mesiolingual” or “mesiopalatal” canal. Prior to an article written by Weine et al in 1969,2 virtually all dentists thought the mesiobuccal root of the maxillary first molar had only one canal. In his textbook, Weine stated that “MB-2” is a poor and inappropriate name for this canal. Instead, “mesiolingual” (ML) is the term that best describes the canal and its location.3 A classic article published in 1990 by Kulild and Peters demonstrated a ML canal in up to 95.2% of the teeth studied.4 In fact, a maxillary first molar has 4 canals, a few have 3 canals, and some even have 5 canals. The mesiobuccal root of the maxillary first molar is often a ribbon of tissue that may end in 1, 2, or 3 separate foramina.

Figures 1a and 1b. The position of the ML canal.  

The location of the ML orifice is approximately 2 mm lingual to the mesiobuccal orifice.2-4 Sometimes, this orifice is along a direct line between the mesiobuccal and palatal canals. Most often, it is mesial to the line connecting these 2 canals, appearing to be under the mesial marginal ridge (Figures 1a and 1b). A careful dissection of this area is essential to successful root canal therapy for this tooth.

Figure 2. Type II maxillary left first molar. Figure 3. Type II maxillary left first molar. (Notice the lateral canal in the palatal root).
Figure 4. Maxillary right first molar demonstrating a type III configuration. Figure 5. Type III maxillary right first molar.
Figure 6. Type III maxillary right first molar.

There are 4 major types of canal systems in the mesiobuccal root of the maxillary first molar: type I, single canal from pulp chamber to apex; type II, 2 separate canals leaving the pulp chamber, but merging short of the apex to form a single canal (Figures 2 and 3); type III, 2 separate canals leaving the chamber and exiting in separate foramina (Figures 4 through 6); and type IV, one canal leaving the chamber and dividing into 2 separate canals with 2 separate foramina.5 Other variations, such as 3 separate canals, have also been reported in the literature. Type III canals have been demonstrated to occur in 45.8% of maxillary first molars.4

Approximately half of all maxillary first molars have 2 separate foramina exiting the mesiobuccal root. Frequent failure of endodontic treatment of these teeth is likely due to the failure to locate and obturate the ML canal. A clinical investigation of second mesiobuccal canals in endodontically treated and re-treated maxillary molars was initiated by Wolcott et al.6 The study concluded that the significant difference in the incidence of a MB-2 canal between initial treatments and re-treatments suggests that failure to find and treat existing MB-2 canals will decrease the long-term prognosis.

Five recent articles in the Journal of Endodontics suggest that the dental operating microscope is an indispensable tool for root canal therapy on maxillary first molars. With the added advantage of magnification and illumination provided by the use of the surgical operating microscope, location of the ML canal in the maxillary first and second molars is enhanced.7 The dental operating microscope facilitates the ability to negotiate the ML canal,8 and the use of the dental operating microscope increases the number of root canal orifices located.9 Yoshioka et al found that surgical loupes were relatively ineffective compared with the microscope for detecting orifices.10

The most convincing evidence necessitating use of the dental operating microscope for maxillary molars was demonstrated in a clinical investigation by Stropko.11 In this 8-year retrospective study, an ML canal was found in 73.2% of first molars. In the last 2 years of the study, a dramatic increase in ML detection was noted. In fact, in the last year of the study, an ML canal was found in 93.0% of maxillary first molars. Stropko attributed this 20% increase in locating ML canals in maxillary first molars to scheduling adequate chair time, utilizing new technology (specifically the dental operating microscope), and more consistent utilization of current clinical aids available to the operator.

The dental operating microscope has experienced slow acceptance.12 The first binocular-operating microscope was marketed in 1953 by the Carl Zeiss Company of West Germany. Forty-four years later, mandatory training became a new standard for advanced specialty education programs in endodontics. Since this recognition, many academic institutions are researching and documenting the benefits of this technology.

CONCLUSION

This paper demonstrates the advantages of the dental operating microscope’s ability to aid in the detection and instrumentation of the ML canal of the maxillary first molar to the terminus. The dental operating microscope can also assist the practitioner in broken file removal, the identification and instrumentation of calcified canals, and in endodontic surgery. New uses beyond endodontics in restorative and periodontal procedures are encouraging. As new instruments and techniques develop to aid the practitioner in microscopic dentistry, we may find the microscope to be a necessity rather than a luxury.


References

1. Ingle JI, Bakland LK. Endodontics. 4th ed. Baltimore, Md: Williams & Wilkins; 1994.

2. Weine FS, Healy HJ, Gerstein H, et al. Canal configuration in the mesiobuccal root of the maxillary first molar and its endodontic significance. Oral Surg. 1969;28:419-425.

3. Weine FS. Endodontic Therapy. 5th ed. St Louis, Mo: Mosby; 1996.

4. Kulild JC, Peters DD. Incidence and configuration of canal systems in the mesiobuccal root of maxillary first and second molars. J Endod. 1990;16:311-317.

5. Eskoz N, Weine FS. Canal configuration of the mesiobuccal root of the maxillary second molar. J Endod. 1995;21:38-42.

6. Wolcott J, Ishley D, Kennedy W, et al. Clinical investigation of second mesiobuccal canals in endodontically treated and retreated maxillary molars. J Endod. 2002;28:477-479.

7. Baldassari-Cruz LA, Lilly JP, Rivera EM. The influence of dental operating microscope in locating the mesiolingual canal orifice. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2002;93:190-194.

8. Gorduysus MO, Gorduysus M, Friedman S. Operating microscope improves negotiation of second mesiobuccal canals in maxillary molars. J Endod. 2001;27:683-686.

9. de Carvalho MC, Zuolo ML. Orifice locating with a microscope. J Endod. 2000;26:532-534.

10. Yoshioka T, Kobayashi C, Suda H. Detection rate of root canal orifices with a microscope. J Endod. 2002;28:452-453.

11. Stropko JJ. Canal morphology of maxillary molars: clinical observations of canal configurations. J Endod. 1999;25:446-450.

12. Selden HS. The dental-operating microscope and its slow acceptance. J Endod. 2002;28:206-207.



Dr. Mortman is chairperson of the Palm Beach County Dental Research Clinic and is in private practice in West Palm Beach, Fla. He can be reached at (561) 684-1312.

Dr. Ahn is co-chairperson of the Palm Beach County Dental Research Clinic and is inprivate practice in West Palm Beach, Fla. She can be reached at (561) 684-1312.



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