INTRODUCTION
“He who devotes himself to surgery must be versed in the science of anatomy. Those who lack a good grasp of anatomy are prone to serious and even fatal mistakes.”Al-Zahrawi (936-1013 A. D.).
Facial nerve paralysis is a formidable complication of parotid surgery and thus is a great concern for the operating surgeon. The facial nerve is said to divide the parotid gland into a superficial and a deep lobe. However, this division is not anatomically based since, embryologically, the parotid gland entraps mesenchymal structures that later develop into the facial nerve which along with the accompanying vessels creates a potential space between the superficial and deep lobes. Dissection in this plane is possible only if the surgeon identifies the facial nerve and proceeds along its branches, and thus, a sound anatomical knowledge is a prerequisite for parotid surgery since damage to the nerve could result in facial paralysis which can be devastating for the patient with possible medicolegal implications for the surgeon. Postoperative paralysis may be temporary or permanent. Temporary weakness is much more common and is seen in 8.2%–65% of parotidectomies, whereas the incidence of permanent paralysis is generally reported to be 3%–5%.[1] Preserving the integrity of the facial nerve during parotid surgery depends on its proper identification.
There are two approaches to identify the facial nerve during parotidectomy – the conventional anterograde dissection and the retrograde dissection. In the former, the facial nerve trunk (FNT) is identified first followed by its bifurcation and its branches. In the retrograde approach, any of the peripheral branches can be isolated and followed to the main trunk. This latter approach is time-consuming, especially when the tumor is large, when there has been distortion of the intraparotid facial nerve plexus by bulky infiltrating tumors, or when scarring has occurred due to previous procedures.[2] This technique has largely been abandoned because of these drawbacks.[3] However, when difficulty is encountered in locating the nerve proximally due to fibrosis or a large or infiltrating tumor, etc., this technique can be helpful. In retrograde dissection, it is recommended that the buccal branch and not the mandibular branch should be traced since such dissection places the branch followed at risk and the paralysis of the mandibular branch leads to an ugly cosmetic deformity.[4] It can also be used for the small tail of the parotid tumors in which it has the advantage of allowing the patient to receive only a cervical mastoid incision and thus avoid unnecessary exposure of the main trunk and the upper branches.[5]
Surgeons today use the most consistent portion of the nerve – the FNT for identification of the nerve. The FNT is defined as that part of the nerve which emerges from the stylomastoid foramen to its bifurcation into upper and lower branches.[6] Occasionally, the nerve divides within the mastoid and exits as separate branches.[4] The first surgeon to describe the exposure of the main trunk at its origin was Janes[7] in 1940. Since then, various authors have described several surgical landmarks to identify the facial nerve. These include the stylomastoid foramen, stylomastoid artery, tympanomastoid suture (TMS), posterior belly of the digastric muscle (PBDM), tragal pointer (TP), retromandibular vein, transverse process of the Atlas, styloid process, mastoid process, and external auditory canal (EAC). The use of so many landmarks to identify the FNT points to the fact that there is a lack of consensus regarding the safety and reliability of each of these landmarks.[8]
The purpose of this study was to review the four commonly used landmarks for identification of the main trunk of the facial nerve, namely the TP, TMS, PBDM, and junction of the bony and cartilaginous EAC.
MATERIALS AND METHODS
The study was conducted in the Department of ENT and Head Neck Surgery and the Department of Anatomy, Government Medical College Jammu to evaluate the relationship of the FNT to the above-mentioned four anatomical landmarks. Prior approval of the Institutional Ethical Committee of Government Medical College Jammu was obtained. The study included 15 patients who underwent parotidectomy for various etiologies and 15 cadavers from the laboratory of human anatomy. The patients with advanced malignancy and those undergoing revision parotid surgery were not included.
Surgical technique
All the parotidectomies were performed under general anesthesia with the endotracheal tube placed at the opposite angle of the mouth. The patient was laid supine with neck extended and head slightly rotated to the opposite side. Modified Blair incision was made, starting in the preauricular crease, continuing inferiorly and curving around the earlobe backward onto the mastoid process, and extending toward the hyoid in a skin crease. With the lobule reflected superiorly, the superficial musculoaponeurotic flap was elevated over the parotid, and a subplatysmal plane in the neck was elevated anteriorly up to the edge of the masseter muscle. The posterior flap was elevated in the plane above the deep cervical fascia covering the sternocleidomastoid muscle. The greater auricular nerve was identified and its anterior branch was transected sharply close to the gland. The parotid gland was freed from the cartilaginous EAC and the anterior border of the sternocleidomastoid muscle which was retracted to expose the PBDM. The pinna was retracted as far upward as possible allowing the cartilage of the external auditory meatus (EAM) to be identified. At this point, the TP was clearly visualized protruding downward, more prominent than the rest of the tragal cartilage due to its pointed appearance. The periosteum over the mastoid process was elevated in an anterior direction to view the tympanic plate. A V-shaped sulcus was found opening forward – the tympanomastoid suture. After identification of all the four landmarks, i.e., TP, TMS, PDGM, and junction of the bony and cartilaginous EAM, the FNT was identified in the fibrofatty tissue lying between the TP and the PDGM. Following identification of the FNT, calipers were used to measure the shortest distance from the edge of the FNT to each of the landmarks. A similar procedure was followed in cadavers.
Results
Of the 15 patients who underwent parotidectomy, more than half of the patients (53.3%) were male and the rest were female (46.7%). Four (26.7%) patients were in the age group of 20–34 years, 8 (53.3%) in 35–49 years, and 3 (29%) in 50–64 years. Pleomorphic adenoma was the most common in 11 (73.3%), Warthin's tumor in 2 (13.3%), and retention cyst in another 2 (13.3%) cases. Of the 15 cadavers, 13 (86.7%) were male and the rest 2 (13.3%) were female. The distance of various landmarks from the facial nerve as observed in our study is shown in Table 1.
;)
Table 1: Distance of various landmarks from facial nerve trunk
During surgery, the FNT was found to be 5.4 ± 1.45 (3–9 mm) from the TMS, 9.5 ± 2.45 mm (6–14 mm) from the TP, 7.4 ± 2.42 mm (5–11 mm) from the PBDM, and 10.6 ± 2.42 (7–14 mm) from the junction between the bony and cartilaginous EAM. These distances in the cadaver were 5.1 ± 1.53 mm (3–8 mm), 10.5 ± 2.51 mm (6–15 mm), 7.1 ± 1.41 mm (5–10 mm), and 9.6 ± 1.47 mm (8–13 mm), respectively.
DISCUSSION
Surgeons depend on the use of anatomical landmarks for identifying various structures. Good landmarks are those which are easy to identify, easy to palpate, and remain in a relatively constant position throughout the procedure, thus allowing for a safe and fast identification of anatomical structures.[9] Bony structures are more suitable than soft tissue or cartilaginous landmarks because of their rigid and reliable anatomical location.[5] For identification of the facial nerve during parotid surgery, a number of reference points have been used. These include the TP, stylomastoid foramen, TMS, PBDM, stylomastoid artery, retromandibular vein, transverse process of the atlas, styloid process, angle of the mandible, junction of the bony and cartilaginous EAM, and peripheral branches of the facial nerve. There seems to be little agreement over the most reliable and appropriate landmark.[10]
TP refers to the medial most pointed end of the cartilage of the EAM. The nerve exits the stylomastoid foramen approximately 1 cm below and 1 cm medial to this point.[11] However, different observers interpret the definition and direction of the TP differently because it is mobile, asymmetrical, and has a blunt irregular tip.[5] Stylomastoid foramen is anatomically a very constant landmark, but during surgery, it can be difficult to find since it is surrounded by thick fascia which is continuous with the skull base, and excessive dissection in this area can damage the facial nerve.
TMS is regarded by some as the most reliable landmark[512] It is closest to the FNT,[5] its relation to the facial nerve is reliable and allows the nerve to be identified close to the foramen where it is least subject to displacement.[12] However, others feel that the TMS is obscured by the strong tendon of the sternocleidomastoid muscle and using it as a landmark increases the complexity of the surgery because it requires elevation of the periosteum around the ear canal and dissection inferiorly to reach the TMS.[101314] The styloid process is not considered to be a reliable landmark because it lies deeper to the nerve and it can be small or absent in a proportion of cases.[11] According to Shaheen,[15] a surgeon who finds himself accidentally confronting the styloid process evidently strayed too deeply. The anatomical variations of the stylomastoid artery (being medial to the FNT in 63% and lateral in 37%)[16] and of the retromandibular vein[17] preclude their use as a pointer to the FNT.
The mastoid process has also been described as a landmark, but it lies deep to the sternocleidomastoid muscle and is hence mainly a palpatory landmark. Salame et al.[18] determined the distance between the FNT at the stylomastoid foramen and the tip of the mastoid process and found it to be 17.22 ± 3.18 mm. The junction between the bony and the cartilaginous EAM is a little over 1 cm from the FNT and is subject to movement and thus cannot be counted as a reliable landmark.[9]
The FNT lies approximately 1 cm above and parallel to the upper border of the digastric muscle near its insertion at the mastoid tip. The muscle is easy to identify being just deep to the sternocleidomastoid muscle and also by the direction of the muscle fibers which run toward the mastoid tip. According to Maran,[11] an angle is formed where the PBDM crosses the tympanic plate of the petrous bone and the facial nerve bisects this angle. However, according to de Ru et al.,[5] the attachment of the muscle may be more anterior in its insertion to the mastoid process and more posterior in others, thus altering its relation to the FNT. The transverse process of the atlas was found by Pather and Osman[10] to be the safest and the most reliable landmark for identification of the FNT as it can be easily palpated and does not require a deep and complex dissection.
Joseph et al.[19] have described the Trident landmark in which the facial nerve forms the central prong of a trident, whereas the base of the styloid process forms the upper point and the superior border of the origin of the PBDM forms the lower point of the landmark. However, looking for the styloid process can be risky (vide supra).
Thus, a study of these landmarks defining their exact relationship to the facial nerve is warranted. The present study was aimed to review four of the most commonly used landmarks in the identification of the FNT, namely the TP, the TMS, the PBDM, and the junction of the bony and cartilaginous EAM.
In our study, the distance between the TP and the FNT ranged from 6 to 14 mm, with a mean of 9.5 ± 2.45 mm in surgeries [Figure 1], whereas in cadavers, it was 10.5 ± 2.51 mm. This distance has variedly been reported by different authors. Lesser distances of 3.2–9.9 mm and 6.9 ± 1.8 mm have been reported by de Ru et al.[5] and Rea et al.,[9] respectively, in cadavers; greater distances have been reported by others, namely 24.3–49.2 mm by Pather and Osman[10] (cadavers) and 13.5–19 mm (live) and 14–21 mm (cadavers) by Saha et al.[8] Hence, it is inferred that TP is not a reliable landmark since it is flexible and subject to retraction.[20]
Figure 1: The relation of facial nerve trunk (FNT) and tragal pointer (TP) during parotidectomy
The distance of FNT from the TMS was found to be 5.4 ± 1.45 mm (3–9 mm) in live [Figure 2] and 5.1 ± 1.53 mm (3–8 mm) in cadavers [Figure 3]. This is consistent with the findings of Bushey et al.[21] who reported the mean distance to be 4.9 mm. Others have reported lesser distances of 2.7 mm,[5] and 2.5 mm.[9] The TMS is thus the nearest landmark to the FNT. This is in agreement with others who observed that it is not only nearest to the FNT but also its position is invariable and it leads to the stylomastoid foramen, thus allowing the nerve to be located close to the stylomastoid foramen where it is least likely to be displaced.[522] It can be identified in cadavers without any difficulty, but in live surgery, it is obscured by the tendon of the sternocleidomastoid muscle and its exposure requires a deep dissection which could be difficult and risky.[13]
Figure 2: The relation of facial nerve trunk (FNT) and tympanomastoid suture (TMS) during surgery
Figure 3: The relation of facial nerve trunk (FNT) and tympanomastoid suture (TMS) in cadaver
The distance of the junction of the bony and cartilaginous EAM was found to be 10.6 ± 2.42 mm (7–14 mm) in live surgeries and 9.6 ± 1–53 mm (8–13 mm) in cadavers. This is consistent with the observations of Rea et al.[9] who found this distance to be 10.9 ± 1.7 mm in cadavers. However, Pather and Osman found it to be 7.3–21.9 mm (mean = 13.4 mm). This variation excludes it as a reliable landmark since it is far from the FNT and also because being cartilaginous it is subject to movement.
The distance of FNT from the PBDM was found to be 7.4 ± 2.42 mm (5–11 mm) during surgeries [Figure 4] and 7.1 ± 1.41 mm (5–10 mm) in cadavers. Similar observations were made by Saha et al. In cadavers, this distance has been reported by others as 4.5 mm,[5] 5.5 mm,[9] and 14.6 mm.[10] During parotidectomy, lateral retraction of the sternocleidomastoid muscle exposes the PBDM, and it can be easily recognized by the direction of its fibers that run to the mastoid tip. de Ru et al.[5] observed that its insertion to the mastoid process may be anterior or posterior which could alter its relation to the FNT. In large tumors of the parotid gland, the PBDM may become adherent to the tumor or the surrounding structures rendering the dissection difficult. We did not encounter any displacement of PBDM in our study since most of the tumors were benign and not of very large size. Similar observations have been made by others.[58] We found this to be a reliable and consistent landmark for identification of the FNT.
Figure 4: The relation of facial nerve trunk (FNT) and posterior belly of the digastric muscle (PBD) during surgery'
Apart from the variations in the proximity of the different landmarks to the FNT as observed in various studies including ours, differences have also been observed between the sexes, the two sides of the face, and between the dentulous and edentulous jaws.[1023] Thus, the use of these absolute values is not advocated in routine surgical practice. In view of these observations, we concur with the view of Pather and Osman[10] that the closest landmark is not necessarily the best or the most identifiable landmark and that more than one landmark may need to be used for the safe identification of the FNT. In case of difficulty, we must try to expose as many landmarks as safely possible after separating the posterior aspect of the parotid gland from its posterior attachments, namely the EAM, the mastoid process, and the sternocleidomastoid muscle. This will expose a small area from the EAM above to the mastoid process below, and gentle dissection in the fibrofatty tissue to a depth of about 1 cm will expose the facial nerve using the aforementioned landmarks. Needless to mention that the surgeon must be thoroughly conversant with all possible anomalies of the facial nerve which may be pushed deep, inferiorly or superiorly by the tumor. It may also divide within the stylomastoid foramen where it appears as five separate branches.[4]
For identification of the facial nerve, nerve monitoring, since its introduction in the early 1990s, is widely being used during parotid surgery, but its usefulness in routine procedures is controversial. Terrel et al.[24] are of the opinion that its advantages (improved postoperative facial nerve function, reduced operating time, better surgeon comfort, ability to check nerve integrity after surgery, etc.) and disadvantages (false-positive or false-negative result, electrode injury, etc.) need to be considered together with the additional costs in deciding whether routine use of continuous monitoring is useful and cost-effective adjunct to parotid surgery. The use of nerve monitoring does not obviate the need for a sound anatomical knowledge of the various landmarks described herein for identification of the facial nerve during parotid surgery.
CONCLUSION
In order to prevent trauma to the facial nerve during parotid surgery, several landmarks have been described for its identification. Of the four commonly used landmarks which we studied, we found that the tympanomastoid suture with its invariable position to be nearest to the facial nerve trunk but its exposure needs a deep dissection which can be difficult and risky, thus precluding it as a perfect landmark. The posterior belly of digastric muscle which is easily exposed by retraction of the sternocleidomastoid muscle was found to be a consistent landmark. However considering the variations in distances of various landmarks, we conclude that more than one landmark be used for the safe identification of Facial nerve.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
REFERENCES
1. Witt RL. Facial nerve monitoring in parotid surgery: The standard of care? Otolaryngol Head Neck Surg. 1998;119:468–70
2. Martin H. The operative removal tumors of the parotid salivary gland Surgery. 1952;31:670–82
3. Robertson MS, Blake P. A method of using the tympanomastoid fissure to find the facial nerve at parotidectomy Aust N Z J Surg. 1984;54:369–73
4. Watkinson JC, Gaze MN, Wilson JA Stell and Maran Head and Neck Surgery.. 20004th ed. Oxford Butterman Heinmann
5. de Ru JA, van Benthem PP, Bleys RL, Lubsen H, Hordijk GJ. Landmarks for parotid gland surgery J Laryngol Otol. 2001;115:122–5
6. Davies JC, Agur AM, Fattah AY Anatomic Landmarks for Localization of the Branches of Facial Nerve. OA Anatomy. 2013:33
7. Janes RM. The treatment of tumours of the salivary glands by radical excision Can Med Assoc J. 1940;43:554–9
8. Saha S, Pal S, Sengupta M, Chowdhury K, Saha VP, Mondal L. Identification of facial nerve during parotidectomy: A combined anatomical and surgical study Indian J Otolaryngol Head Neck Surg. 2014;66:63–8
9. Rea PM, McGarry G, Shaw-Dunn J. The precision of four commonly used surgical landmarks for locating the facial nerve in anterograde parotidectomy in humans Ann Anat. 2010;192:27–32
10. Pather N, Osman M. Landmarks of the facial nerve: Implications for parotidectomy Surg Radiol Anat. 2006;28:170–5
11. Maran AG. Identification of the facial nerve in parotid surgery J R Coll Surg Edinb. 1973;18:58–9
12. Heeneman H. Identification of the facial nerve in parotid surgery Can J Otolaryngol. 1975;4:145–51
13. Browne HJ. Exposure of the facial nerve Br J Surg. 1988;75:724–5
14. Nishida M, Matsuura H. A landmark for facial nerve identification during parotid surgery J Oral Maxillofac Surg. 1993;51:451–3
15. Shaheen OH Problems in Head and Neck Surgery. 1984 London Bailliere Tindall
16. Upile T, Jerjes W, Nouraei SA, Singh SU, Kafas P, Sandison A, et al The stylomastoid artery as an anatomical landmark to the facial nerve during parotid surgery: A clinico-anatomic study World J Surg Oncol. 2009;7:71
17. Touré G, Vacher C. Relations of the facial nerve with the retromandibular vein: Anatomic study of 132 parotid glands Surg Radiol Anat. 2010;32:957–61
18. Salame K, Ouaknine GE, Arensberg B, Rochkind S. Microsurgical anatomy of the facial nerve trunk Clin Anat. 2002;15:93–9
19. Joseph ST, Sharankumar S, Sandya CJ, Sivakumar V, Sherry P, Krishnakumar T, et al Easy and safe method for facial nerve identification in parotid surgery J Neurol Surg B Skull Base. 2015;76:426–31
20. Trible WM. Symposium: Management of tumors of the parotid gland. III. Management of the facial nerve Laryngoscope. 1976;86:25–7
21. Bushey A, Quereshy F, Boice JG, Landers MA, Baur DA. Utilization of the tympanomastoid fissure for intraoperative identification of the facial nerve: A cadaver study J Oral Maxillofac Surg. 2011;69:2473–6
22. Brintnall ES, Tidrick RT, Huffman WC. A simplified and rapid anatomical approach to parotidectomy AMA Arch Surg. 1955;71:331–6
23. Zhong W, Ashwell K. Facial nerve trunk variations with surgical implications: A cadaveric study Int J Surg Open. 2015;1:35–40
24. Terrell JE, Kileny PR, Yian C, Esclamado RM, Bradford CR, Pillsbury MS, et al Clinical outcome of continuous facial nerve monitoring during primary parotidectomy Arch Otolaryngol Head Neck Surg. 1997;123:1081–7
