J. Anat. (1966), 100, 3, pp. 639-650 639With 13 figures

Printed in Great Britain

The peripheral course of the inferior dental nerve

By MARJORIE WEDGWOODDepartment of Dental Medicine, Dental School, University of Bristol.

INTRODUCTION

Varied effects of block anaesthesia of the inferior dental nerve have been ob-served clinically. These variations have been attributed to contralateral innerva-tion of the incisor teeth and mucous membrane, but this has not been proved.Accessory innervation of mandibular teeth by nerves other than the inferior dentalnerve has been described.

Seldin (1942) observed in 2-3 % of human mandibles, at the medial side of thepremolars, a foramen through which the cutaneous colli nerve may pass to supplythese teeth. According to Sicher (1952), a small branch of the mylohyoid nervesometimes enters the mandible to supply the lower incisors.

Olivier (1927) investigated the incisor nerves in fifty human mandibles withoutfinding any communication between the two sides. Starkie & Stewart (1931) tracedan anastamosis across the symphysis in man, but found no communicating fibresin the cat, rabbit or sheep.

Deductions about the nerve supply to the mandibular incisor teeth have beenmade following local anaesthesia in man. Stewart & Wilson (1928) found that inmany patients there is no mid-line overlap, and when it occurs it rarely extendspast the central incisors. Similarly, Rubin (1952) found diminished sensitivity ofthe incisor teeth in the side opposite to a blocked inferior dental nerve.The results of Simon (1952) do not agree. He anaesthetized the inferior dental

and infraorbital nerves and investigated the cutaneous sensation of temperature,touch and pain. There was a clear demarcation between right and left nerves. Ina later review (1956) he suggested that sympathetic pain conducting fibres supplythe alveolar periosteum. No experimental evidence was offered to support thisview. In support, Kocsis (1958) pointed out anastomoses between left and rightinferior dental arteries with an accompanying sympathetic plexus.Olund (1954) concluded on clinical evidence that contralateral innervation ini the

mandible is rare enough to be regarded as an individual variation. Sprinz (1955)mapped out areas on the skin supplied by the mental nerve in 159 patients. Thedistribution was constant and no mid-line overlap was found.Simpson (1958) surveyed fifty patients with injured inferior dental nerves follow-

ing extraction of lower third molar teeth or fracture of the mandible. On the skinof the lower lip, paraesthesia was restricted to one side. He stated afterwards (1961)that it was less easy to find a similar limitation on the mucous membrane insidethe lip and overlap may occur.

These reports are conflicting. It was thought that more accurate informationmight be obtained by tracing the course of the incisor nerves microscopically,using unilateral Wallerian degeneration to distinguish between the nerves of rightand left sides.

640 MARJORIE WEDGWOOD

MATERIALS AND -METHODS

Inspection and radiographic examination of the dried skulls of the commonlaboratory animals, cat, dog, rabbit, rat, Rhesus monkey and pig, showed that allexcept the adult pig and the monkey have a persistent suture in the mid-line ofthe mandible.The significance of this feature in the question of overlapping between nerve

fibres of opposite sides was unknown, so the present experiment was restricted toan animal where full ossification occurs in the adult mandible. Young pigs weresuitable, aged 7-9 weeks. The mid-line suture was still apparent at this age, althoughit was in the process of closing at the anterior end.

Five animals were used, all from the same litter. They were anaesthetized bythe injection of Nembutal into an ear vein. An incision was made just above theangle of the jaw, then the anterior edge of the masseter muscle was freed, andretracted to expose the bone of the body of the mandible. Several bur-holes weremade in the bone, to form a circle 7-8 mm in diameter. The outer plate of themandible was removed, so that the inferior dental canal was exposed. The siteselected was just below the crypt of the third molar tooth. The inferior dental nervewas identified, and a portion of it 3-4 mm long was removed completely, taking careto preserve intact the accompanying blood vessels.

Table 1. Age of pigs at death and duration of Wallerian degeneration

Pig Sex Age at Age at death Duration ofNo. operation Wallerian

Weeks Days degenerationWeeks Days (days)

1 7 3 10 4 222 F 8 5 12 2 253 F 7 2 10 1 204 F 7 5 10 5 215 AI 9 2 11 5 17

The animals were killed by an overdose of Nembutal 17-25 days later. The ageat death is shown in Table 1. Bony deposition had taken place in the operated areawith apparently complete healing. At the time of death, all deciduous teeth haderupted.The mandibles were removed from the pigs immediately after death, and fixed

in 10 0 formol-saline for 1 week. Decalcification was carried out in 5 0 nitric acid.The mandible of each animal was divided into one anterior and two posteriorpieces. The anterior segment, including all the lower incisor teeth and canines, andthe posterior segment of the operated side, extending approximately from 1 cmbehind the operated area to 1 cm in front, were treated as follows:

(1) Cut into slices in the bucco-lingual plane not more than 1 cm thick.(2) Immersed in potassium dichromate for 14 days (potassium dichromate,

3 %0 solution in distilled water).(3) Changed to Marchi's solution for 3 weeks, changing every 4-5 days or six

Inferior dental nervechanges in all (Marchi's solution: 1 %0 osmium tetroxide solution to 2 parts of 3 %0potassium dichromate solution).

(4) After embedding in paraffin wax at 56° C, sections were cut serially, in thebucco-lingual plane 7-8jim thick.

(5) The sections were counterstained with haematoxylin and eosin to simplifyinterpretation.

In order to test the possibility that immersion in 5 %0 nitric acid in the process ofdecalcification could affect the subsequent staining by the AMarchi technique, apiece of degenerated nerve was dissected from a rabbit used in a control experi-ment. This section of nerve was cut into two pieces, only one of which was treatedwith nitric acid, simulating decalcification. The two pieces of degenerated nervewere then stained by the method outlined above. No significant differences weredetected between the staining of the degenerated fibres in the two pieces of nervoustissue. It is assumed from this that the decalcification necessary in the pig speci-mens is unlikely to interfere with the interpretation of the results.

RESULTS

There was a marked similarity in anatomical and histological appearance in allanimals.

First, in order to establish that the section of the inferior dental nerve had beencomplete, the area of operation was examined, including the proximal and distalsevered ends of the nerve.Normal nerve was seen proximally, including the beginning of the lingual nerve

which was undamaged. In the distal part of the inferior dental nerve, below thefirst molar tooth, there is no doubt that section of the nerve was always complete,and all fibres degenerated, as judged by the disintegration of the myelin sheath. Atthe peripheral endings where myelin sheaths were extremely thin and difficult todistinguish, it was possible to recognize degenerated nerves by the shrunkenappearance of their axons.

In the inferior dental canal, the artery and all of its five or six accompanyingveins were intact, and it is believed that the blood supply to the mandible re-mained unaffected for the duration of the experiment.The premolar region was not examined, but just posterior to the deciduous

canine tooth, the inferior dental nerve divides into two. The larger proportion ofthe fibres, approximately 60-700%, emerges through the mental foramen to thebuccal alveolar surface, constituting the mental nerve. The remainder continuesin the bone as the incisor nerve (Fig. 1).The course of this incisor nerve then follows a particular pattern unlike any seen

in descriptions of the human jaw. The main incisor nerve splits up into four smallerincisor nerves, one for each 'dental unit' (i.e. an erupting deciduous tooth and itscorresponding developing permanent tooth). Each of these subsidiary incisor nervessplits into four branches:

(1) A large superficial 'muco-cutaneous branch', consisting of about one-fifth ofthe available fibres.

(2) A small 'pulpal branch' to the deciduous tooth, with about one-tenth of thetotal fibres.

641

MARJORIE WEDGWOOD(3) A small branch going directly to the periodontal membrane of the deciduous

tooth, 'the short periodontal branch', with about one tenth of the total fibres.(4) A medium-sized terminal branch, which will be referred to as the 'long

periodontal branch', with about three-fifths of the total fibres. This supplies theperiodontal membrane of the deciduous tooth, and part of the developing permanenttooth.

These are shown diagrammatically in Fig 2.

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Fig. 1. A, Mental nerve; B, main incisor nerve before division intobranches to separate 'dental units'. x 10.

(1) Course of the muco-cutaneous branchThe muco-cutaneous branch of each incisive nerve is given off first, passing

anteriorly and laterally to emerge through a small foramen on the buccal aspectof the mandible, anterior to its respective deciduous tooth, about 6-7 mm belowthe level of the alveolar crest.The muco-cutaneous branch adjacent to the deciduous canine tooth mingles with

the fibres of the mental nerve after emerging on to the external surface of the bone(Fig. 3).The corresponding branches of the incisive nerves for the second and third de-

ciduous incisor teeth supply the buccal mucous membrane of the related area, anddo not extend to the lingual aspect.

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Inferior dental nerveThe muco-cutaneous branch of the deciduous central incisor is, like the other

muco-cutaneous nerves, inferior to the tooth, and tending to move medially as itpasses forward. This brings the nerve on each side very close to the median suture,where endochondral ossification is taking place (Fig. 4). However, as long as this

Fig. 2. Diagram of the nerve supply to the deciduous central incisor and the dentalorgan of the developing permanent central incisor in the pig. A, Long periodontalbranch; B, short periodontal branch; C, pulpal branch; D, muco-cutaneous branch.

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Inferior dental nerve 645nerve is contained within the bone, it does not pass into the fibrous tissue, althoughat times the separation between nerve and suture consists of only one trabecula ofbone. These two nerves, one from each side, have been traced to their emergence onthe surface (Fig. 5), close to the mid-line without any lateral branching being seenwithin the bone.When the nerves reach the corium underlying the epidermis, they divide into

two or three smaller bundles, and in all five pigs, some of these bundles are seencrossing the mid-line (Fig. 6).

Degenerated and normal fibres lie side by side supplying the dermis (Fig. 7)opposite the deciduous central incisor. The terminal branches can be traced fartherlaterally but were not seen extending beyond the contralateral first incisor.

(2) Course of the pulpal branchThe pulpal branch is the smallest branch of all. The individual pulpal branch for

each dental unit can be traced through the apices of the first and third deciduousincisors. The second incisor is not fully formed at this stage of development, but anerve can be traced to the region of its developing pulp.The appearance of the fibres entering the apex is always consistent, i.e. on the

operated side they are degenerated, and on the contralateral side they are normal.There is no indication of any crossing over of innervation, within the bone. Fartheraway from the apex, within the pulp, the nerves split into small branches and losetheir mvelin sheaths, but degenerated axons are identifiable by their shrunkenappearance.

(3) Short periodontal branchThe small separate branch to the periodontal membrane of the deciduous tooth

runs at first close to the fibres of the pulpal branch. It contains approximately thesame number of fibres as those traced to the pulp, and supplies the periodontalmembranes near the apex of the tooth.

(4) Course of the long periodontal branchThe long periodontal branch is given off last, and may be regarded as the con-

tinuation of an individual incisive nerve to a 'dental unit' after giving off its otherthree branches.A few fibres are given off to supply the periodontal membrane of the deciduous

tooth immediately, but most of the fibres constitute a bundle that is placed

Fig. 3. A, Deciduous canine tooth; B, muco-cutaneous nerve; C, mental nerve. x 20.Fig. 4. A, MIuco-cutaneous nerve on operated side; B, muco-cutaneous nerve on un-operated side; C, median suture; D, labial surface of bone. x 29.

Fig. 5. A, -Muco-cutaneous nerve on operated side; B, muco-cutaneous nerve on un-operated side; C, median suture. x 29.Fig. 6. A, Degenerated nerve fibres; B, normal nerve fibres from opposite side; C, normalnerve fibres crossing mid-line laterally; D, median suture. x 22.Fig. 7. Area outlined in Fig. 6 at higher magnification. A, Degenerated nerve fibrebundle; B, normal nerve fibre bundle. x 135.Fig. 8. A, Apex of forming root of deciduous second incisor, operated side; B, muco-cutaneous branch; C, long periodontal branch. x 18.41 Anat. I100

646 MARJORIE WEDGWOOD

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Inferior dental nerve 647medially and lingually with respect to the apex of the deciduous tooth, while themuco-cutaneous nerve is placed buccally (Fig. 8).As the central deciduous incisor is fully erupted at 2 months, and the central

permanent incisor is in the late cap stage of development, the subsequent course ofthe long periodontal branch is most easily traced in relation to this area, althoughthe same course is taken in relation to the other anterior teeth, by their respectiveincisor nerves.The long periodontal nerve continues to move medially and superiorly (lingu-

ally) as it passes forwards. This brings it nearer to the central suture, but like themuco-cutaneous branch, it does not give off lateral branches within the bone, nordoes it pass medially into the fibrous or cartilaginous tissue.

This course brings the long periodontal branch nearer to the crypt of the de-veloping permanent tooth, into which it enters on the medial side from below(Fig. 9). The nerve fibres remain near the wall of the crypt and cannot be tracedinto the dentinal papilla of the developing tooth. Within the crypt, several smallerbundles split off, and come to lie in the follicle. The bundles of nerve fibres in thissituation are so small that it is difficult to distinguish any myelin sheath. They havebeen traced continuously from the parent nerve, and no nervous communication isseen across any bony area, small though it is, between the two crypts of right andleft sides (Figs. 10, 11).The remaining fibres of the periodontal branch emerge from the crypt as a

bundle, running at first on the medial aspect of the crypt and then moving slightlyaway from the mid-line as it passes forward.The bundle finally enters the periodontal membrane of the erupted deciduous

central incisor, and terminates by dividing into individual fibres along the medialaspect of the tooth between the fibres of the periodontal membrane (Figs. 12, 13).

DISCUSSION

There appears to be a tendency in the pig for the establishment of an overallpattern of distribution of incisive nerves into cutaneous and deeper branches, com-parable to the behaviour of the inferior dental nerve, and following the pattern ofdistribution of a typical segmental or spinal nerve. On the whole, only a very smallproportion of fibres actually reaches the pulps of the teeth, the greater part going tothe dermis of the lower lip. In the young pig the richness of the innervation of the

Fig. 9. A, Median suture, islands of cartilage; B, follicle surrounding developingpermanent central incisor; C, neuro-vascular bundle containing degenerated longperiodontal nerve just entering crypt from below. x 36.Fig. 10. A, Enamel organs of permanent central incisors; B, bundle of fibres of longperiodontal nerve (normal side); C, bundle of fibres of long periodontal nerve (degene-rated side); D, Suture. x 36.Fig. 11. Higher magnification of area outlined in Fig. 10. A, Bundles of fibres of longperiodontal nerve; B, dental follicle of developing permanent central incisor; C, enamelorgan. x 100.Fig. 12. A, Fully erupted deciduous central incisor; B, bundle of fibres from long perio-dontal nerve. x 32.

Fig. 13. Higher magnification of area outlined in Fig. 12. A, Cementum of deciduouscentral incisor; B, bundle of nerve fibres, just before dividing into terminal fibres. x 200.

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648 MARJORIE WEDGWOODperiodontal membrane surrounding the teeth that had already erupted at birth(deciduous third incisor and canine) was surprisingly great.Thomas (1946) made an analysis of the inferior dental and mental nerves in the

cat, and gave the following proportions for the distribution of the fibres: molarteeth, 12%0; periodontal membrane, 13440; canine and incisor teeth, 26-7 0;mental nerve, 47-9 %0. He did not try to subdivide 'canine and incisor 26 7 % ' anyfurther into nerves to pulp or elsewhere, and it is not improbable that some of the26-7 %0 fibres did not actually terminate in tooth pulp.The only completely acceptable argument for contralateral innervation of the

pulps of the incisor teeth would be the tracing of a nerve fibre from one side to theother within the bone. This is not seen in the pig, nor is any nerve fibre observedto enter the fibrous or cartilaginous tissue dividing one side of the mandible fromthe other. The proximity of nerve fibres to the central line is so striking at times inthe pigs that it is difficult to understand the mechanism limiting the nerve sostrictly to its own side of origin until it emerges from the bone. Even in the mucosathe overlap is so slight that it gives the appearance of an entirely fortuitous growthof the terminal fibres, perhaps due to a slight stretching of the skin as a result ofasymmetry between the developing connective tissue masses of either side.The ossification adjacent to the symphysis in the pig is taking place in cartilage.

It is usual to see islands of cartilage cells arranged in rows down the centre orien-tated at right angles to the plane of the suture. Since most of the rest of the mandibleis developing in membrane, and the actual pattern of its earlier development wasrelated to the course of the immature inferior dental nerve, it is possible that thepresence of cartilage in itself presented an insuperable barrier to the further medialor forward passage of the nerve. There is, however, no ultimate essential differencebetween ossification in membrane and ossification in cartilage, and one cannoteasily accept the mere presence of active ossification as an adequate barrier.

AIohiuddin (1950) studied the fate of the nerves of the deciduous teeth in cats.He considered that spontaneous degeneration in the nerves preceded root resorp-tion. He saw these changes only in the axons in the pulp. They did not spreadcentrally to involve the axons in the dental nerve trunk. He thought it likely thatboth sets of teeth are supplied by branches from the same stem fibres. It is notknown whether or not such degeneration preceding shedding of the deciduousteeth occurs in the pigs being studied. The possibility that this could affect theinterpretation of the appearance of Wallerian degeneration in the nerves enteringthe apices of the pigs' teeth was dismissed as the fibres examined were quite largeenough (7-l10tm) to enable degenerated myelin to be seen, whereas Mohiuddin wasreferring to smaller axons within the pulp. Furthermore, the pigs' deciduous teethare not fully formed, and it is unlikely that any changes connected with exfoliationare taking place prior to full development.

AMohiuddin (1951), writing about the maturation of the inferior dental nerve,mentioned some fine fibres, the presence of which 'might be correlated with theearly maturation of the milk dentition, and they might remain in the adult nerveto innervate the permanent teeth through adult branches'. If one adds to thisstatement that the number of axons in the inferior dental nerve remains the samein the adult cat as it is in the kitten, it is interesting to speculate on what happens

to the 'long periodontal branch' of the pig's incisive nerve that reaches the perio-dontal membrane of the deciduous teeth after passing through the crypt of thepermanent tooth. It is reasonable to suppose that the terminal periodontal partsimply degenerates during shedding of the deciduous tooth, as far centrally as thedeveloping permanent tooth, and no doubt it will regenerate and later contributeto the innervation of the periodontal membrane of the permanent tooth.

It is concluded that, in the pig, transmedian overlap of innervation in the mandi-ble does not occur within the bone, and that therefore the pulps of the teeth are notaffected, nor are their periodontal membranes involved. The lower lip is affectedslightly, but only in relation to the innervation of the area overlaying the centralincisors.

These results do not exclude a possible overlapping of sensory innervation viathe sympathetic plexus adjacent to the blood vessels, the branches of which wereplentiful in the sutures.

SU1MAR Y

1. The peripheral course of the inferior dental nerve in five young pigs wasobserved histologically. This was done by tracing the Wallerian degenerationfollowing unilateral surgical section of the nerve posterior to the last molar tooth.

2. Anterior to the mental nerve, the remaining fibres of the inferior dental nerveform a bundle of incisor nerves within the bone. This bundle supplies a separateincisor nerve to each of the deciduous canine and incisor teeth, and their relatedstructures.

3. Each separate incisor nerve gives off four branches, one to the mucous mem-brane and skin, one to the pulp of the erupted deciduous tooth, and two to theperiodontal membrane of the deciduous tooth. One of the latter branches passesthrough the crypt of the developing permanent tooth.

4. Nerve fibres reach almost as far as the ossifying suture, but never enter it.Only in a small muco-cutaneous area, overlying the roots of the deciduous centralincisors, does transmedian innervation occur.

5. No fibres of the inferior dental nerve were found supplying teeth on theopposite side of the mandible.

This work was supported by a grant from the Research Fund of SheffieldUniversity. I am deeply indebted to Dr Rudolph Sprinz for advice and encourage-ment, and to Dr Carrie Sprinz for translation of German literature.

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Kocsis, A. G. (1958). Erscheinungen in Zusammenhang mit den Gerfdssen bei der Betdubungdes Nervus alveolaris inferior. Schweiz. .M1schr. Zahnheilk. 68, 97-106.

M1oiuUDDIN, A. (1950). The fate of the nerves of the deciduous teeth. J. Anat. 84, 319-323.M1OHIUDDIN, A. (1951). Post-natal development of the inferior dental nerve in the cat. J. Anat.

85, 24-36.OLIVIER, E. (1927). Le canal dentaire inferieur et son nerf chez l'adulte. Annls Anat. path. med-

chir. 4, 975-986.OLIUND, B. (1954). Innervation av underkakens fronttander. Odont. Revy. 5, 188-202.RUBIN, L. R. (1952). K problem obeshbolivaniia zubov. Stomatologiya. Mosk. 31(2), 44-49.SELDIN, H. MI. (1942). Practical Anaesthesia for Dental and Oral Surgery, p. 178. London: HenryKimpton.

Inferior dental nerve, 649

650 MARJORIE WEDGWOODSICHER, H. (1952). Oral Anatomy, p. 366. London: Henry Kimpton.SIMON, B. (1952). A felfileti beidegzes a neuritek egym6shoz val6 viszomya. Fogorv. Szle, 45, 1-9.SIMON, B. (1956). Problems of local anaesthesia in dental practice. Int. dent. J. Lond. 6, 158-173.SIMPSON, H. E. (1958). Injuries to the inferior dental and mental nerves. J. oral Surg. 16, 300-305.SIMPSON, H. E. (1961). Private communication.SPIIINZ, R. (1955). A contribution to the applied anatomy of regional dental anaesthesia. B.D.S.

Thesis, University of Sheffield.STARKIE, C. & STEWART, D. (1931). The intra-mandibular course of the inferior dental nerve.

J. Anat. 65, 319-323.STEWART, D. & WILSON, S. L. (1928). Regional anaesthesia and innervation of the teeth. Lancet

2, 809-811.THOMAS, B. 0. A. (1946). An analysis of the inferior alveolar and mental nerves in the cat.

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