Introduction
The facial nerve (FN) gains its
importance clinically in facial palsy which is of a great concern
for the clinicians and for the patients. Iatrogenic injury to the
facial nerve most often is seen after cervicofacial
rhytidectomies, surgery of the parotid gland, acoustic neuroma
resection, or tumor resection at any point along the passage of
the facial nerve. If there is facial paralysis following surgery,
post-operative investigation should be done (1).
Hypoglossal-facial neurorrhaphy has been extensively used for
reanimation of paralyzed facial muscles after irreparable injury
of the facial nerve.(2)
Facial nerve emerges from the stylomastoid foramen (SMF) and goes into the parotid gland through its postero-medial surface. It further furcates into an upper division (temporofacial) and a lower division (cervicofacial), which further gives the five terminal branches. The segment of the FN between its point of appearance from the skull through the SMF and its furcation into upper and lower branches at the parotid area is known as the main trunk of the FN.(3) In infants, the FN trunk is in a more superficial plane and is liable to injury during surgical incision and trauma to the retromandibular area.(4) In the adults, this region is surrounded by soft tissue which poses difficulty in dissecting the FN trunk. Therefore, a thorough knowledge is necessary to know the exact location of the path of the FN, which would help the clinicians and surgeons in clinical diagnosis and in performing surgery.
Exploration of the various branches of the FN
using anatomic landmarks is difficult because of high variability of the
branching pattern. Soft tissue benchmarks are frequently used but they are
usually mobile and are of inconstant morphology. Bony benchmarks are more
consistent but only few have been described in literature.(5)
Hence this study was made to note the morphometric dimensions of facial nerve and its relation with transverse process of atlas and tip of mastoid process which was found lacking in the literature.
Materials and Methods
Twenty hemi-faces of unknown age and sex were dissected. The FN was identified at the SMF and traced along its course through the parotid gland. The distance between the FN trunk from its exit to the tip of the mastoid process (FM) and the distance between the exit of the FN trunk to the transverse process of the atlas (FA) was measured using the Vernier calliper. The length of the FN trunk was measured from its exit to the point of its bifurcation into two divisions within the parotid gland (LOT). The length of the two divisions, upper division (LOUD) and lower division (LOLD), up to the emergence of the branches was measured. Gross variations in the branches and the two divisions were also noted and photographed.
Results
The mean of
1. The length of the facial nerve trunk (LOT) was 1.380±0.082
2. The length of the upper division (LOUD) was 0.820±0.108*
3. The length of the lower division (LOLD) was 0.890±0.090*
*- significant with Length of facial nerve trunk
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Fig 1. A: Facial nerve trunk of facial nerve immediately bifurcates into two divisions, the upper trunk gives temporal, zygomatic and buccal. There is communication between temporal, zygomatic and upper buccal (Type I). Tr Trunk of facial nerve, U- Upper division. L lower division, T temporal branch, Z zygomatic branch, UB upper buccal branch, LB lower buccal branch, M marginal mandibular branch, C cervical branch, PG parotid gland, PD parotid duct.
Fig 1. B: Trunk bifurcates into upper and lower division, the lower division gives the lower buccal branch, marginal mandibular and cervical. Upper division gives zygomatic and temporal and upper buccal (Type II).
Fig 1. C: There are two major trunks emerging from the parotid gland. Upper trunk divides into temporal, zygomatic and upper buccal. The lower trunk divides into lower buccal, marginal mandibular and cervical. The stylomastoid artery (A) emerges between the two trunks. There is communication marginal mandibular and lower buccal (Type III).
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The mean of
1. The distance between facial nerve trunk and tip of the mastoid process (FM) was 1.300±0.089
2. The distance between the facial nerve trunk and transverse process of atlas (FA) was 1.645±0.108
Table 1: Types of branching patterns |
| |
No. of Specimens |
Percentage |
Type 1 (Fig. 1. A) |
2 |
10% |
Type II (Fig. 1. B) |
15 |
75% |
Type III (Fig. 1. C) |
1 |
5% |
Type IV (Fig. 2. A) |
1 |
5% |
Type V (Fig. 2. B) |
1 |
5% |
 |
Fig 2. A: Single trunk gives all the branches. There is communication between auriculotemporal nerve and the trunk of facial nerve. There is communication between MM and C, MM and LB, UB and Z (Type IV). Tr Trunk of facial nerve, U- Upper division. L lower division, T temporal branch, Z zygomatic branch, UB upper buccal branch, LB lower buccal branch, M marginal mandibular branch, C cervical branch, ATN- Auriculotemporal nerve.
Fig 2. B: Loop formation between two trunks. Absence of a single trunk, FN bifurcates before the gland. There is communication between LB and MM (Type V). Tr1 First trunk, Tr2- Second trunk
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Discussion
Conservative parotidectomy is the treatment for parotid gland tumors. A report on retrospective studies on 351 parotid gland surgeries suggests that post-operative facial nerve dysfunction occurs mainly in radical parotidectomy.(6)
There is 1-4 percent likelihood of injuring the facial nerve during arthroscopic puncture during temporomandibular joint (TMJ) surgeries.(7) The styloid process is used as a bony benchmark for recognition of FN for classical parotid gland surgeries, any variation in the styloid process prevents it from being used as reliable landmark. Hence Dimitrov et al. used the insertion of posterior belly of digastric and tympanomastoid fissure as the landmarks for parotid surgery in 37 patients with good results.(8) In a study, which included 40 human cadaveric heads were dissected by a surgeon and anatomist using substantial landmarks to locate the FN trunk easily and swiftly. An incision was made in the middle of a triangle formed by temporomandibular joint, angle of mandible and mastoid process. Dissection was progressed till the main trunk of facial nerve was reached. The average distance between the expected and the real position of the nerve was 1.42 mm.(9)
In a previous study conducted by Pather et al on 40 cadaveric heads, the morphological association on the FN trunk to the surrounding structures was assessed. The distance of facial nerve in mm from the styloid process was found to be 9.8, transverse process of the axis was 16.9 and mandible angle was 38.1. The length of the facial nerve trunk from its point of exit from the SMF to its bifurcation is 14.0. This study supports use of transverse process of axis as it is effortlessly felt as a consistent landmark and warrants least threat of damage to FN trunk.(3) Similar reference points were used in our study. We used the transverse process of atlas and tip of mastoid process as bony reference points, both the bony landmarks are easily palpable and appreciated. The mean distance in centimetres between facial nerve trunk and tip of the mastoid process (FM) was found to be 1.300±0.089 and the mean length between the facial nerve and atlas (FA) was found to be 1.645±0.108.
In a study conducted by Shin et.al. on 29 specimens, the direct distance from the mastoid process tip (MPT) to the SMF was found to be 14.1±2.9 mm. The distance from the MPT to the FN origin was 8.6±2.8 mm in the front and 5.9±2.8 mm above.(10) In a study of 45 parotid glands, the mean distance from the angle of mandible to FN was 28.8 mm. Communication between FN and auriculotemporal nerve was seen in 93.3% cases.(11) Similarly, in the present study the communication between auriculotemporal nerve and facial nerve is seen one case (Fig. 2. A).
Dissection of 30 heads showed that mean depth of
SMF from the exterior of the skin was 21.0 mm and distance of foramen from the
bifurcation of FN is 13.0 mm. In the same study, buccal branches showed lot of
patterns and accordingly were classified into different types. In type I (13.8%
of cases), two main divisions of the trunk gave rise to the buccal branch. In
type II (44.8% of cases), the two main divisions gave rise to buccal branch were
interconnected with the zygomatic branch. In type III (17.3% of cases), buccal
branch, which originated from the upper and lower divisions was interconnected
to marginal mandibular branch. In type IV (17.3% of cases), buccal branch was
interconnected to the branches from zygomatic and marginal mandibular.(12) In our study, in one case there is communication between temporal, zygomatic and upper buccal branches (Fig. 1. A). Also, a communication between marginal mandibular and lower buccal branches was observed (Fig. 1. C). In yet another case, a single trunk giving all the branches and establishment of communication between all the branches was observed (Fig. 2. A).
In a study with hundred cadaveric heads the MMB
ran below the lower border of the angle of mandible with an average vertical
distance of one centimeter. The divisions of the FN were in the area between the
apex of the parotid gland and the Lateral palpebral line. The dividing
configurations of the FN were categorized into six varieties. The least common
(1%) was type I (absence of anastomosis), while type V, the complex pattern of
branching (29%). Symmetrical branching pattern occurred in 30% of cases.(13)
(Fig. 2. B)
FN branching as seen in 20 cases of parotidectomy in 75% patients, the mean length of main trunk of facial nerve between 16 20 mm. In 60% patients, the distance of facial nerve from tympanomastoid suture was between 2.5-3mm.(14) In the present study length of the facial nerve trunk was 1.380±0.082 mm.
In a study conducted on facial nerve pattern in 94 parotid gland tumour surgeries, bifurcation was seen in 76% and trifurcation was seen in 24% of cases, also the branching pattern was classified into 3 groups per the rebranching of the cervical and facial branches.(15) FN studied in foetal cadavers showed the maximum common branching type of the facial nerve was bifurcation (53.33%) shadowed by trifurcation (33.33%). Several divisions of the FN were also perceived in 13.34% of the specimens. Remarkably, a communication between the cervical branch of facial and the great auricular nerves was also detected in the study.(16) In our study bifurcation (Figs. 1. A & B) was seen in 90% of cases and single trunk was seen in 10% of cases (Fig. 2. A).
Conclusion
Precise understanding of the anatomy of the facial nerve trunk and its branching pattern along with its variation is thus vital for performing several surgical techniques on the mastoid process, parotid gland, temporomandibular joint and surgical procedure directed at facial nerve restoration.
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- Farrior JB, Santini H. Facial nerve identification in children. Otolaryngol Head Neck Surg. 1985;93(2):1736.
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- Natsuki S.Clinico-anatomical study on damage to facial nerve and artery by arthroabspic puncture. J Oral Maxillofac Surg. 1991;49(8):108.
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- Pereira JA, Merν A, Potau J, et al. A simple method for safe identification of the facial nerve using palpable landmarks. Arch Surg. 2004;139(7):745-7.
- Shin DS, Bae HG, Shim JJ, et al. Morphometric Study of Hypoglossal Nerve and Facial Nerve on the Submandibular Region in Korean. J Korean Neurosurg Soc. 2012;51(5):253-61.
- Park IY, Lee ME. A Morphological Study of the Parotid Gland and the Peripheral Branches of the Facial Nerve in Koreans. Yonsei Med J. 1977;18(1):45-51.
- Kwak HH, Park HD, Youn KH, et al. Branching patterns of the facial nerve and its communication with the auriculotemporal nerve. Surg Radiol Anat. 2004;26:494500.
- Weerapanta E, Bunaprasertb T, Chokrungvaranontc P, et al. Anatomy of the facial nerve branching patterns, the marginal mandibular branch and its extraparotid ramification in relation to the lateral palpebral line. Asian Biomed. 2010;4:603-8.
- Malik NUID, Verma D, Varshney S, et al. Facial nerve branching pattern as seen in parotidectomy in Indian population A single centre experience. EJPMR. 2016;3(3):359-61.
- Saki N, Nikakhlagh S, Mashali L, et al. Pattern of Distribution in Facial Nerve in the Benign Parotid Gland Tumors. Biomed Pharmacol J. 2015;8:47-50.
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