|Year : 2021 | Volume
| Issue : 3 | Page : 135-139
Branching pattern of terminal branches of the facial nerve: A cadaveric study
Assistant Professor, Department of Anatomy, Government Medical College, Thrissur, Kerala, India
|Date of Submission||15-Nov-2020|
|Date of Decision||17-Dec-2020|
|Date of Acceptance||15-Mar-2021|
|Date of Web Publication||30-Jul-2021|
A K Sapna
Thannissery House, Minalur PO, Thrissur - 680 581, Kerala
Source of Support: None, Conflict of Interest: None
Background: For a successful facial surgery, it is vital to have a good knowledge of the distribution of the facial nerve in the face. The aim of this study was to highlight the pattern in the branching of the end part of the facial nerve in adults and fetuses and to look for any anastomoses between them. The branching pattern in the two groups is compared for any significant association. Methodology: Cross sectional study done in cadavers and stillborn fetuses. Subjects and Methods: Done in 100 facial halves. Statistical Analysis Used: Chi square test is used to look for any significant association between the branching pattern and gender & side of the face. Results: The branching pattern was classified into six types as per Davis et al. classification in the two study groups – adults and fetuses. The frequencies of occurrence in the first group were Type I – 1%, Type II – 7%, Type III – 18%, Type IV – 17%, Type V – 4%, and Type VI – 3% and in the second group were Type I – 1%, Type II – 8%, Type III – 20%, Type IV – 15%, Type V – 5%, and Type VI – 1%. No significant association was found in the branching pattern with respect to gender and side of the face in the two groups of study. Conclusions: There are multiple anastomotic communications among the end branches of the facial nerve. More the anatomical knowledge of these communications, lesser will be any unwanted facial nerve injury.
Keywords: Anatomic variation, cadaver, facial nerve, facial nerve injuries
|How to cite this article:|
Sapna A K. Branching pattern of terminal branches of the facial nerve: A cadaveric study. Natl J Clin Anat 2021;10:135-9
| Introduction|| |
The human face is complex and portrays myriads of expressions and emotions aided by its muscles and the facial nerve. It is the surgeons, especially for those specialized in the head and neck to have a profound knowledge of the distribution of facial nerve in the face. The facial nerve soon after exiting through the stylomastoid foramen of the skull, courses through the parotid gland, divides into upper and lower trunks which, in turn, gives temporal, zygomatic, buccal, marginal mandibular, and cervical branches. They are joined in between from an early fetal life to form a plexus of nerves which shows lots of variations. If the various anatomical variations in the pattern of branching are known to the surgeons, it can decrease the chance for any facial nerve weakness postsurgery., These anastomoses are important surgically and can be the reason why an accidental incision of a minor branch of the facial nerve will not lead to the expected facial nerve paresis. In younger age groups, the superficial location of the facial nerve could put the nerve more in danger compared to adults. Although the depth of the facial nerve trunk and its branching pattern are studied in adults in the past, not many studies are done in fetuses or in children. Such a study could help to compare the differences in the facial nerve course across the age groups. The aim of the present study is to highlight the pattern of branching of the terminal facial nerve in adult as well as in fetuses and also to look for any difference in the branching pattern in the two study groups. The depth of the nerve trunk at its branching is also studied in the two groups. It can help the surgeons to be careful with the necessary precautions while doing facial surgeries as the nerve can be easily damaged in younger ages due to the superficial location of the nerve.
| Subjects and Methods|| |
The study is a cross-sectional study.
Sample size – The maximum prevalence of variations in the terminal branching pattern is 73%.
Sample size = 4pq/d2
p = Prevalence of variations = 73%
q = 100-p = 27%
d = Precision = 20% of P = 14.6
Sample size = 4 × 73 × 27/14.6 × 14.6 = 40. A minimum of 40 samples from adult and fetus samples were required for the study.
Specimens with severe facial injuries and gross pathological lesions were excluded. The study was done in 100 specimens. There were 50 facial halves from 25 cadavers (50–90 years) which include 22 males and 3 females used for undergraduate dissection and 50 from 25 fetuses (20–40 weeks gestation) which include 14 males and 11 females preserved in the Department of Anatomy over a course of 3 years (2012–2014). The study was started after ethical clearance from the institutional ethics committee.
The dissection was carried out in the supine position of the cadaver. The temporal branches of the facial nerve can be identified crossing the zygomatic arch anterior to the ear. The fascia covering the parotid gland was cut and dissected and looked for the presence of any vessels, nerves and for parotid duct which emerged at the margins of the gland. The zygomatic branches of the facial nerve were traced superior to the parotid duct. The buccal branches were traced till the buccinator. The marginal mandibular branch and the cervical branches were identified. The facial nerve trunk was located by tracing back any one of the facial nerve branches. The nerve was traced to its exit from the skull and the exact site of branching of the trunk of the facial nerve into the upper and lower trunk was found. The angle of the mandible (a point where the lower border of the mandible meets its posterior border) is taken as a landmark to measure the distance from it to the bifurcation of the facial nerve. This distance was measured by a Vernier caliper and approximated to the nearest millimeter. The parotid gland was removed as small pieces during dissection. The pattern of branching and type of anastomoses of the peripheral facial nerve branches as per Davis et al. classification is observed and recorded as:
- Type I – absence of anastomoses between upper and lower divisions
- Type II – anastomosis only between the branches of upper division
- Type III – single anastomosis between the branches of the upper and lower divisions
- Type IV – combination pattern of types II and III
- Type V – double anastomoses among the two divisions
- Type VI – complex anastomoses among the two divisions. Here, the buccal branch receives fibers from the lower division as well as from the marginal mandibular branch.
The retromandibular vein was exposed and its relation to the nerve was studied. Photographs were taken to document the important findings. The results obtained were then tabulated against their group. The findings were compared with that of earlier workers.
The statistical test includes the Chi-square test to find whether the observed series of frequency differ between the study groups based on sex and side of the face. This statistical significance of probability was expressed as P value. Statistical analysis was done under the guidance of a statistician by using the SPSS software, version 26, IBM Corporation (SPSS Inc., USA).
| Results|| |
The facial nerve was traced in the face until it divides into temporal, zygomatic, buccal, marginal mandibular, and cervical branches. In all the 100 specimens, the nerve had only a single trunk. After its exit, the nerve was divided into two trunks-temporo-facial trunk (upper division) and lower cervico-facial trunk (lower division). The upper division gave temporal and zygomatic branches while the lower division gave marginal mandibular and cervical branches. Buccal branches had its origin from either one or from both the trunks.
Distance between the points of bifurcation of the facial nerve trunk to the angle of the mandible
Distance between the points of bifurcation of the facial nerve trunk to the angle of the mandible was measured in the two study groups (adult and fetus).In adult cadavers, the distance ranged from 22 to 38 mm with an average of 30.4 mm. In the second group, the value was 11–15 mm in 54% and 5–10 mm in 46% with an average of 10.2 mm distance.
Branching pattern of the facial nerve in adults and fetuses
Different patterns of branching and anastomoses were exhibited by the end branches of the facial nerve in adults and fetuses. Of the 100 specimens, 18 adults and 20 fetuses showed anastomoses among the branches of the upper and lower trunks of facial nerve classified as Type III [Figure 1] and [Figure 2]. Anastomoses among the branches of the upper division classified as Type II [Figure 3] were seen in 15 cases (15%). Thirty-two specimens (32%) showed Type IV pattern, which is a combination of Type II and Type III patterns. Double anastomoses between the upper and lower divisions-Type V pattern was seen in nine cases (9%) and complex anastomotic pattern-Type VI was seen in four cases (4%). In Type I [Figure 4], no anastomosis was seen between the terminal branches. It was seen only in two specimens (2%). As shown in [Table 1], Type III pattern was seen in majority of specimens, followed by Type IV and Type II. In 44 cadavers, the branching pattern on both sides of the face was different (88%) and bilaterally similar in six cadavers (12%). There is no significant association in the pattern of branching between the sides of the face as shown in [Table 1]. No significant association was found between the branching pattern of the facial nerve between males and females or between the adult and fetuses as the P value was 0.92 and 0.59, respectively.
|Figure 1: The right side of the face showing Type III branching pattern of the facial nerve in adult cadaver (single anastomosis between upper and lower divisions) T: Temporal branch, Z: Zygomatic branch, B: Buccal branch, M: Marginal mandibular branch|
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|Figure 2: The left side of the face showing Type III branching pattern of the facial nerve in the fetus (single anastomosis between upper and lower divisions), T: Temporal branch, Z: Zygomatic branch, B: Buccal branch, M: Marginal mandibular branch|
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|Figure 3: The right side of the face showing Type II branching pattern of the facial nerve (anastomosis only among the branches of upper division). UD: Upper division, LD: Lower division, T: Temporal branch, Z: Zygomatic branch, B: Buccal branch, M: Marginal mandibular branch, C: Cervical branch)|
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|Figure 4: The left side of the face showing Type I branching pattern of the facial nerve (no anastomosis between branches of the facial nerve), T: Temporal branch, Z: Zygomatic branch, BU: Upper Buccal branch, BL: Lower buccal branch, M: Marginal mandibular branch, C: Cervical branch|
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The retromandibular vein was in close contact with the facial nerve trunk and divisions. The vein was observed below the divisions of the facial nerve in adults as seen in [Figure 2] in 47 cases (94%). On observing the relation to the trunks, in three cases (6%), the retromandibular vein was superficial to the upper one and deep to the lower trunk.
| Discussion|| |
Knowledge about the anatomy of the facial nerve and its branching is vital to head and neck surgeons.
During the development of the facial nerve, primordia of the nerve appear in 3rd week, while the peripheral branches can be identified during 7–9 weeks. By 16th week, all major communications of the facial nerve are established. The facial nerve approximates the anatomy of the adult at birth itself, except for the fact that it lies more superficial. Adult anatomy is acquired as the mastoid tip develops after birth.
The approximate location of divisions and the beginning of branching of the facial nerve can be estimated on measuring the distance between the points of bifurcation of the nerve from a bony landmark. In the present study, an average distance of 30.4 mm was found from the angle of the mandible to the point of bifurcation of nerve in adults and 10.2 mm in fetuses. This is comparable to the study by Davis et al. who found an average distance of 32 mm in adults. Myint et al. found the average distance as 28.06 mm. Knowledge about this aspect could help to avoid injury to facial nerve branches, especially while dealing with infants and younger ages by keeping in mind the superficial location of the facial nerve.,
The branching pattern of the facial nerve was studied by many workers previously which is summarized in [Table 2]. Knowledge about various types of branching pattern and its incidence will help to predict the extent of injury any accidental damage to the branches can lead to. Type I pattern branches can be easily injured due to lack of anastomosis in between them while other types have an anastomosis to overcome the functions of an injured nerve depending on the location of the anastomosis. This can affect the muscles of facial expression which could cause psychological trauma to the patient. Type VI has a much complex pattern which is protective to the nerve. It is found to receive multiple anastomoses from most branches which could save the nerve from getting affected during minor surgical injuries. Gataa and Faris have summarized that while considering the safety of the individual branches, it is advisable to preserve the marginal mandibular branch and cervical branch during parotid surgeries as they are found to have fewer anastomoses compared to other branches. Most harm occurs if injury to the facial nerve was found in branches with fewer anastomosis like at the marginal mandibular branch, followed by the buccal branch and the cervicofacial division due to their lack of extensive anastomoses. It has been observed that sacrificing the buccal branch of the facial nerve need not always cause impairment of facial nerve function for a longer period when the temporozygomatic and cervicomandibular branches were undisturbed. Even in the case of marginal mandibular nerve injury, a paresis of the lower lip was reported without any severe damage which was attributed to a communication that existed between the buccal and marginal mandibular branch. Hence, if a surgeon needs to sacrifice a branch for a better access, it is better to go for zygomatic or upper buccal considering the possibilities like the presence of extensive anastomoses in the middle of the face between the buccal and zygomatic branches and fewer anastamoses between the marginal mandibular and cervical branches. While dealing with Type I pattern, which is the least common pattern one has to be extra careful, since there is an absence of anastomoses between the end branches.
|Table 2: Comparison of types of branching pattern of the facial nerve from various studies|
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Farooq et al. found that the branching pattern is bilaterally different in the face. They found bilaterally different patterns of branching three times more common. In studies pertaining to fetuses, with respect to the length of the facial nerve trunk, no statistically significant difference was found among female and male fetuses or between right and left sides.[ 13]
The retromandibular vein can act as a guide to identify the location of the parotid gland tumor with respect to the facial nerve. Usually, the vein lies deep to the two trunks of the facial nerve. The trunks more commonly, upper trunk may pass under the retromandibular vein. Touré and Vacher found that in 86%, the retromandibular vein was deep to the nerve trunk and its divisions. The vein can lie anterior to the facial nerve and can even get forked between the nerve and its branches. Future studies in this field could explore more morphometric parameters, especially those parameters which could help to locate and identify individual branches of the facial nerve in order to locate their safe zones during surgery.
| Conclusions|| |
Anastomotic communications of different patterns develop among the end branches of the facial nerve before birth itself. The various patterns of branching are uniformly seen across the different age groups. There is no significance in the variation in the branching of the facial nerve with respect to age, sex, or the side of the face. Surgeons have to be extra careful while dealing with patterns with less intercommunication like Type I pattern. The branches that have to be preserved are the marginal mandibular, cervical branches due to lack of any major communications. During facial surgeries in any age group, more the anatomical knowledge of these anastomoses, lesser will be any chances of unwanted facial nerve injury.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2]