Clinical Anatomy 11:396–400 (1998)
Masseteric Nerve: A Possible Donor for Facial Nerve Anastomosis? ERICH BRENNER1* 2Clinics
AND
THOMAS SCHOELLER2
1Institute of Anatomy, University of Innsbruck, Innsbruck, Austria for Plastic and Reconstructive Surgery, University of Innsbruck, Innsbruck, Austria
In the medical treatment of facial nerve paralysis a large number of different techniques have been developed to restore the function of the facial nerve. These include (a) the ipsilateral nerve grafting (e.g., partial hypoglossal-facial, spinal accessory-facial, partial glossopharyngealfacial), (b) crossfacial nerve grafting and (c) temporal muscle flaps or even free muscle transfers. None of these techniques uses the masseteric nerve as a graft for reconstruction of the facial nerve. This preliminary report deals with the anatomical basis, which could lead to a new technique. The masseteric nerve leaves the infratemporal fossa through the mandibular notch, accompanied by the masseteric artery. At this level the nerve consists in nine of 36 cases studied of only one branch (25.0%), in 17 cases of two branches (47.0%), in nine cases of three (25.0%), and in the remaining case of four branches (2.8%). There are three main reasons for considering the masseteric nerve as a possible donor for at least the orbicular branch of the facial nerve: (1) The approach to the mandibular notch is quite simple; (2) since the nerve consists of two or more branches in 75.0% of the cases, severe dysfunction of the masseter muscle should not occur; (3) if there is complete denervation of the masseter muscle, its function may be taken over by the temporalis muscle. Clin. Anat. 11:396–400, 1998. r 1998 Wiley-Liss, Inc. Key words: masseteric nerve; facial nerve paralysis; nerve anastomosis; nerve babysitting
INTRODUCTION Function of the facial nerve is a crucial part not only of communication but also for principal movements as closure of the eye and the mouth. There is a wide range of disturbances in this system leading to paralysis and loss of these functions. The number of attempts to restore facial movement is quite large. The classical cross-face procedure was introduced in the early 70’s, e.g., Smith (1971), Anderl (1973), or Scaramelia and Tobias (1973). It still provides the optimal therapy for irreversible facial paralysis. The contralateral facial nerve as motor donor is the only one which can restore voluntary mimetic expressions in an adequate and highly differentiated way. The time frame from the onset of the paralysis up to the point when motor function is restored is a very critical period. Since we know that within approximately one year after denervation of the mimetic muscles irreversible atrophy will occur, the reconstructive surgeon endeavors to start this two stage cross-face operation as early as possible. To overcome this time related problem, the concept of ‘‘facial nerve baby-sitting’’ has
r 1998 Wiley-Liss, Inc.
been introduced by Terzis (1994). This descriptive term means a motor donor in the region of the affected facial nerve is temporarily coapted to this nerve to provide the mimetic muscles with an early neuronal input in order to increase the period during which axonal regeneration through the cross-face nerve grafts will take place (expected in approximately six months). Different suitable potential donor nerves such as the hypoglossal nerve, the accessory nerve or the masseteric nerve among others have been described in the literature (Sunder-Plassmann et al., 1970; Spira, 1978; Conley and Backer, 1979). The same nerves have been first used for definitive reconstruction in facial palsy as early as 1925 (e.g., Escat and Viela, 1925, cited by Nicolai, 1983; masseteric nerve), and are still used in cases of bilateral paralysis (e.g., Mo¨bius-Syndrome) where these nerves are the only available motor donors. Although these different nerves have already *Correspondence to: Dr. Erich Brenner, Institute of Anatomy, University of Innsbruck, Mu¨llerstraße 59, A-6010 Innsbruck, Austria. E-mail:
[email protected] Received 24 July 1997; Revised 9 March 1998
The Masseteric Nerve
been investigated together with their potential anatomical variations in detail by Norre (1975), no anatomical study describes the different characteristics of each possible donor nerve for such a cross-over nerve graft. The masseteric nerve seems to be an interesting potential donor nerve for facial paralysis baby-sitting. The masseteric nerve leaves the mandibular nerve shortly after its entrance to the infratemporal fossa. The nerve passes over the lateral pterygoid muscle and leaves the infratemporal fossa through the mandibular notch, also called sigmoid notch, accompanied by the masseteric artery. Escat and Viela (1925) measured the distance between the mandibular notch and the entrance to the muscle with an average of 32 mm. At this site the nerve can easily be exposed. As early as 1977, Spira (1978) reported in a presentation at the annual meeting of the American Association of Plastic Surgeons an anastomosis of the masseteric nerve to the lower division of the facial nerve in three cases. Following our literature research, this preliminary report was the only report given on this topic. Rakhawy et al. (1976) cited Reid (1920), who observed one principal nerve entry to the masseter muscle. In their own observations, Rakhawy et al. (1976) found in 12 of 15 cases one and in the remaining three cases two branches of the masseteric nerve entering the muscle. Furthermore, they found no plexus formation within the masseter muscle. One of the major disadvantages of anastomosing the masseteric nerve to the facial nerve is the loss of masseter muscle function reported by Spira (1978). Our study attempts to reevaluate the ramification of the masseteric nerve as it approaches the muscle in order to find out if a residual function can be retained.
MATERIALS AND METHODS Twenty-five heads were fixed with formaldehyde and phenol at low percentages. On 23 right sides and 13 left sides the masseteric nerve was exposed at the mandibular notch and followed to its entrance into the masseter muscle. The dorsal part of the superior attachment of the muscle to the zygomatic arch was detached in order to observe the nerve’s entry site. Dissecting into the muscle was carefully avoided. After detailed preparation of this region, the number of branches of the masseteric nerve at the level of the mandibular notch as well as the number of branches at the entrance to the muscle were recorded.
RESULTS In Figure 1 a photograph of the lateral region of the face shows the branches of the facial nerve arising from
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Fig. 1. Survey of the lateral region of the face. P, parotid gland; M, masseter muscle.
the parotid gland. In a closer view (Fig. 2), the most superior fibers of the masseter muscle have been detached from the zygomatic arch, thus presenting the masseteric nerve approaching under the zygomatic arch in the mandibular notch. In this case the nerve consists of two main branches with the major branch running more inferiorly. Both branches divide into five branches in total when entering the muscle. At the level of the mandibular notch the masseteric nerve consists mainly of two branches (17 cases; 47.2%). In nine cases (25.0%) we observed only one branch. Another nine cases (25.0%) showed three branches and in the remaining specimen (2.8%) we observed four branches (Table 1, Fig. 3). Whereas the right masseteric nerves showed an average number of 1.8 branches, the left ones had 2.5 branches. If there is more than one branch at this level, one of the branches is larger than the others. This major branch runs diagonally to the masseter muscle whereas the other ones take another course. The second branch runs more parallel to the muscle and enters inferior to the
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Brenner and Schoeller TABLE 1. Branches of The Masseteric Nerve at The Mandibular Notch N Row % Col. % Total % 1 branch
2 branches
3 branches
Right
Left
9 100.0% 39.1% 25.0%
0
7 41.2% 53.8% 19.4%
4 44.4% 17.4% 11.1%
5 55.6% 38.5% 13.9%
0
1 100.0% 7.7% 2.8%
17 47.2% 9 25.0% 1 2.8%
N
23 63.9%
13 36.1%
36
⌺ of branches
41
33
74
average
Fig. 2. Detail of the region of interest. f, facial nerve; m, masseter muscle; n, masseteric nerve; p, parotid gland; z, zygomatic arch.
9 25.0%
10 58.8% 43.5% 27.8%
4 branches
Total
1.8
2.5
2.1
where the right side showed more branches than the left side, once at the mandibular notch (3 branches vs. 2 branches), the other time at the muscular entrance (4 branches vs. 3 branches).
DISCUSSION main branch. A third branch, if found, runs along the inferior border of the zygomatic arch, sometimes also below the arch, and enters the muscle superior to the main branch. A fourth branch, which was found in one case on the left side, accompanied the main branch. At the entrance to the masseter muscle we found one to five branches. More than half of our cases (55.6%) showed three branches entering the muscle. In nine cases (25.0%) we found two branches, four cases (11.1%) had four branches. In two cases (5.6%) there were five branches and only in one right case (2.8%) we could expose only one branch entering the muscle (Table 2, Fig. 4). At this level the average number of branches of the masseteric nerve is significantly smaller on the right side (2.6 vs. 3.5 at the left side). The course of the branches is similar to the above-mentioned situation. If there are more than three branches, the additional branches run together with the main branch. In average the left side shows more branches than the right side. Nevertheless, there are two specimens,
Our data reveal that multiple branches of the masseteric nerve innervate the masseter muscle in 97.2% at the level of the muscular entrance. The ramification of the nerve takes place at least between the mandibular notch and the entrance into the
Fig. 3.
Number of branches at the mandibular notch.
The Masseteric Nerve TABLE 2. Branches of The Masseteric Nerve at The Entrance to The Masseter Musle N Row % Col. % Total % 1 branch
2 branches
3 branches
Right
Left
Total
1 100.0% 4.3% 2.8%
0
1 2.8%
8 88.9% 34.8% 22.2%
1 11.1% 7.7% 2.8%
14 70.0% 60.9% 38.9%
6 30.0% 46.2% 16.7%
20
4 100.0% 30.8% 11.1%
4
0 4 branches
0 5 branches
23 63.9%
13 36.1%
⌺ of branches
59
46
2.6
25.0%
55.6%
11.1%
2 100.0% 15.4% 5.6%
N
average
9
2 5.6% 36 105
3.5
2.9
muscle, sometimes even earlier in the course of the nerve. As a rule, the ramification takes place earlier on the left side. The site of ramification can normally be exposed easily. Only under certain conditions, if there is a late ramification, the most dorsal part of the superior attachment of the masseter muscle has to be detached from the zygomatic arch to expose the ramification. In no cases should it be necessary to detach the muscle completely. The length of the nerve, measured between the mandibular notch and
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the entrance to the muscle (Escat and Viela, 1925), seems to be sufficient to attach the facial nerve branches. The dissection of these branches should be performed after exposure of the masseteric nerve in order to gain the right length. The side differences of the number of the branches at both levels, which were not statistically analyzed due to the limited number of specimens, cannot be explained from the data of this study. The difference of the number of branches found in this study compared to other workers (e.g., Reid, 1920; Rakhawy et al., 1976) could be explained by the difference in the approach to the nerve’s entry and by the age of the specimens used. Reid stated that he found one principal branch. So he did not look for additional, minor branches. Furthermore, his investigation was performed on only one specimen. Rakhawy et al. (1976) performed their study on eight infants from full-term to 1 year of age. At this age the more detailed ramification as seen in our specimens has not yet taken place. Due to our data, the surgeon may preserve a residual innervation of the masseter muscle if just one of the branches is used for babysitting or even permanent anastomosis. Furthermore, some of the muscle’s function may be taken over by the temporalis muscle. In an animal study performed on Wistar-derived rats, Carter and Harkness (1995) found a decrease in mass of the masseter muscle and an increase in mass of the temporalis muscle. Furthermore, they could not reveal larger changes of the mandible after resection of the masseteric nerve. One major advantage of the use of the masseteric nerve deals with the function. Since the innervation of the masseter muscle results in the lifting of the lower jaw, a shutting action, the transferred masseteric nerve easily serves as ⬎⬎shutter⬍⬍ for the mimetic muscles as well as for the eye and the mouth.
REFERENCES
Fig. 4.
Number of branches at the entrance to the muscle.
Anderl, H. 1973 Reconstruction of the face through cross-facenerve transplantation in facial paralysis. Chir. Plast. 2:17–46. Carter, G. M. and E. M. Harkness 1995 Alterations to mandibular form following motor denervation of the masseter muscle. An experimental study in the rat. J. Anat. 186:541–548. Conley, J. and D. C. Baker 1979 Hypoglossal-facial nerve anastomosis for reinnervation of the paralyzed face. Plast. Reconstr. Surg. 63:63–72. Escat, E. and A. Vie´la 1925 Manuel operatoire de l’anastomose du nerf facial avec le nerf masserterin. Ann. Mal. Oreille Larynx 77:1149–1159. Nicolai, J.-P. A. 1983 Irreversible facial paralysis and its treatment, Wageningen: Ponsen & Looijcn, p. 71.
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Norre, M. E. 1975 Introductory study on the cranial nerves V, VII, IX, X, XI, XII. Acta Otorhinolaryngol. Belg. 29:932–951. Rakhawy, M. T., S. H. Shehata and Z. H. Badawy 1976 The points of nerve entry and the intramuscular nerve branchings in the human muscles of mastication. Acta Anat. 94:609–616. Reid, R. W. 1920 Motor points in relation to the surface of the body. J. Anat. 54:271–275. Scaramelia, L. F. and E. Tobias 1973 Facial nerve anastomosis. Laryngoscope 83:1834–1840. Smith, J. W. 1971 A new technique of facial animation. In
Transactions 5th International Congr. of Plastic and Reconstructive Surgery. Australia: Butterworth’s, pp. 83–84. Spira, M. 1978 Anastomosis of masseteric nerve to lower division of facial nerve for correction of lower facial paralysis. Preliminary report. Plast Reconstr Surg. 61:330–334. Sunder-Plassmann, M., V. Grunert and J. A. Ganglberger 1970 Results of accessory and hypoglossal nerve in surgical treatment of facial paralysis. Wien. Med. Wochenschr. 120:880–882. Terzis, J. K. 1994 The ‘‘Babysitter’’ principle: Experience and results in 25 cases. Eur Arch Otorhinolaryngol Suppl 393.
