J. Anat. (1989), 167, pp. 195-198 195With 2 figures

Printed in Great Britain

The mandibular incisive foramen

NEILL J. SERMAN

School of Dental and Oral Surgery, Columbia University, 630 West 168th Street,New York, N.Y. 10032, USA

(Accepted 2 May 1989)

INTRODUCTION

Although no specific reference could be found in the literature, it is generallyaccepted that the inferior alveolar nerve runs an entirely intra-osseous course from itsentry into the mandible at the mandibular foramen. The nerve is accompanied in themandibular canal by the inferior alveolar artery, a branch of the maxillary artery.While in the canal, the nerve and artery supply the teeth on that side of the dental arch(Weisse, 1886). Opposite the mental foramen, the inferior alveolar artery and nervegive off, respectively, the mental artery and nerve which emerge onto the superficialregion of the face through the mental foramen (Weisse, 1886). This foramen is locatedin the region inferior to, or between, the first and second premolar teeth (Sicher &Du Brul, 1975) or, in about 50% of cases, below the apex of the second premolar tooth(Dixon, 1986). In man, the mental foramen is usually single but may be multiple, theusual number of foramina being two (de Villiers, 1968). Starkie & Stewart (1930)noted that, immediately after entering the mandibular canal, the nerve divides intoseveral branches which run forward in a plexiform manner and then re-unite. Thisprocedure is then repeated and results in an increased number of branches, so that, atthe level of the mental foramen, there are fifteen or more of these. Le Double (1903)discussed multiple mental foramina in some detail but did not mention the foramencomplex under discussion in this paper. Double mental foramina are well described inthe literature and are presumed to be the result of branching of the mental nerve priorto its exit from the mental foramen. There is a slight variation according to tribal andracial groups with regard to the number of mental foramina. The number of foraminaranges from 1 02 % on the left side in the Egyptian and Polynesian groups to 111 %on the right side in the Melanesian group (Riesenfeld, 1956). Whether the mentalforamen on one side of the mandible is single or multiple, it is accepted in the literaturethat in all cases, the nerve emerges from the mental foramen. Serman (1987), workingon dry human mandibles, put forward the hypothesis that, in a small percentage ofcases, there exists an anatomical variation where there is a (mandibular) incisiveforamen complex. In these instances the nerve and blood vessels re-enter the mandiblethrough the anterior foramen to supply the anterior part of the body of the mandibleand to form its main nerve and blood supply. The results of dissection are nowpresented in support of that hypothesis. In these cases, the inferior alveolar nerve hasa short extra-osseous course where the division of the nerve into its two terminalbranches occurs outside the bone (Fig. 1) and the incisive nerve then re-enters thebone.

N. J. SERMAN

Fig. 1. The extra-osseous course of the inferior alveolar nerve between the arrows No. I and 2.Arrow No. 3 indicates the mental nerve and vessels.

MATERIALS AND METHODS

Some 408 dry human mandibles from the Department of Anatomy and Histology,Faculty of Medicine, University of Stellenbosch, South Africa were examined in theregion of the mental foramina; the mental foramen region of 88 cadavers from theDepartment ofAnatomy, Faculty of Medicine, Medical University of Southern Africaas well as the mental foramen region of 12 cadavers from the Department ofAnatomy,College of Physicians and Surgeons, Columbia University, New York were dissected.

RESULTS

A mental-incisive foramen complex that is distinct from double mental foramina, asdescribed in the literature, was found in seven mandibles, with two cases beingbilateral, making a total of nine cases (0-88 %). Three cases (two unilateral and onebilateral) were found in dry human mandibles as previously described (Serman, 1987)with four cases (one bilateral) found on dissection. The races of the dry specimens werenot known. The examples found at the Medical University of Southern Africa are notsignificant with regard to race as that hospital serves only black people. Both cases

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Mandibular incisive foramen

Fig. 2. Mental-incisive foramina complex showing larger posterior foramen with an anteriorlydirected lumen. The smaller anterior, mandibular incisive foramen has its lumen directed in aposterior direction. Both foramina are on the same horizontal plane.

found at Columbia University were black people. Thus the sample is too small tomake any assumption regarding racial predilection in these cases.At first glance, the cases under discussion appear similar to other double mental

foramina described in the literature. On closer examination it can be seen that in allthese cases the larger foramen is always situated more posteriorly, with an anteriorlydirected lumen. The smaller foramen is situated anteriorly but with a posteriorlydirected lumen. Both foramina are always situated on the same horizontal plane witha groove between them in most cases (Fig. 2). No intra-osseous canal or alternatenerve supply were found between these two foramina. This mental foramina complexcould not be distinguished radiologically from other double mental foramina and inmany cases it could not be recognised radiologically at all. The increased film-objectdistance and the intervening bone of the mandible may, in part, be the reason for this.

DISCUSSION

The foramen complex under discussion can be explained in the following manner.The undivided inferior alveolar nerve (Fig. 1) leaves the canal through the mentalforamen. Once it has reached the surface, lying in the groove between the foramina,the nerve divides into the terminal mental and incisive branches. The incisive nervethus has an extra-osseous commencement and runs a short extra-osseous coursebefore re-entering the mandible through the more anteriorly situated foramen. Theposteriorly situated foramen therefore has a larger lumen to accommodate theundivided inferior alveolar nerve and has an anteriorly directed lumen (Fig. 2). Themore anteriorly situated foramen is smaller as it only has to accommodate the incisivenerve and is directed posteriorly as the incisive nerve is coursing in an antero-mesialdirection as it re-enters the mandible. It must be stressed that in these cases the incisive

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foramen represents the re-entry of the incisive nerve into the mandible. In these casestoo, the blood vessels follow the course of the nerve. An important feature of thesecases is the lack of a mandibular canal between the two foramina. The groove betweenthe foramina may be the remnant of the mandibular canal which has lost its covering.As stated previously (Serman, 1987) it is important for surgeons operating in this areato be aware of this anatomical variant. It is suggested that the anterior foramen in themandible be named the mandibular incisive foramen.

SUMMARY

An anatomical variant in the region of the mental foramen is discussed. In thesecases the inferior alveolar nerve divides into its two terminal branches only after it hasexited through the mental foramen. The incisive nerve thus commences outside themandible, and has a short extra-osseous course before it enters the mandible througha separate foramen on the same horizontal plane. For the distance between these twoforamina there is no nerve supply within the mandible. The groove between the twoforamina may be the remnants of the mandibular canal. The foramen through whichthe nerve enters the bone is a separate anatomical entity from the mental foramen andshould be recognised as such. It is proposed that this foramen be named themandibular incisive foramen.

REFERENCES

DE VILLIERS, H. (1968). The Skull of the South African Negro, Ch. 10, pp. 148-154. Johannesburg:Witwatersrand University Press.

DIXON A. D. (1986). Anatomy for Students of Dentistry, 5th ed., Ch. 4, p. 132. Edinburgh: ChurchillLivingstone.

LE DOUBLE, A. F. (1903). Traite des Variations des Os de la Face de l'Homme. [Characteristics of the variationsof the facial bones of man]: maxillaire inferieure, pp. 328-334. Paris: Vigot Freres.

RIESENFELD, A. (1956). Multiple infra-orbital, ethmoidal and mental foramina in the races of man. AmericanJournal of Physical Anthropology 14, 85-99.

SERMAN, N. J. (1987). Differentiation of double mental foramina from extra bony coursing of the incisivebranch of the mandibular nerve. An anatomic study. Journal of Dental Medicine 5(3), 20-22.

SICHER, H. & Du BRUL, E. L. (1975). Oral Anatomy, 6th ed., Ch. 2, pp. 115-121. St Louis: C. V. Mosby Co.STARKIE, C. & STEWART, D. (1930). The intramandibular course of the inferior dental nerve. Journal ofAnatomy 65, 319-323.

WEISSE, F. D. (1886). Practical Human Anatomy, p. 352. New York: William Wood & Co.

198 N. J. SERMAN