ANATOMICAL STUDY
Surgical Anatomy of Facial Nerve for Revision Transmastoid Surgery Ela Cömert, MD, PhD,* Ayhan Cömert, MD,† Ümit Tunçel, MD,* and İbrahim Tekdemir, MD† Abstract: We analyze the relationships of the 3 segments of the facial nerve with respect to constant anatomic structures that can be identified during revision surgery via translabyrinthine approach. This study was conducted on 15 formalin-fixed cadavers whose facial nerves were dissected bilaterally under operative microscope via translabyrinthine approach. The distances between the round window niche and the midpoint of the tympanic segment and the beginning of the mastoid segment were 6.64 ± 1.79 mm and 3.99 ± 0.79 mm, respectively. The distances between the tympanic ostium of the eustachian tube and the first and the second genu were 7.02 ± 0.62 mm and 12.25 ± 1.24 mm, respectively. We used the superior semicircular canal, the tympanic ostium of the eustachian tube, and the round window niche as landmarks to identify the facial nerve during revision surgery. Our study also showed that the auricular branch may also be originated from the posterior surface of the facial nerve. Key Words: Facial nerve, surgical revision, otologic surgical procedures (J Craniofac Surg 2014;25: 619–622)
O
ne of the most important structures that should be identified during transmastoid surgery is the facial nerve. The anatomy of the facial nerve has been previously described in detail.1,2 After entering the internal acoustic canal, the facial nerve passes into the fallopian canal. From the translabyrinthine approach, the first part of the facial nerve, the labyrinthine segment, runs toward the geniculate ganglion. It turns posteriorly to form the first genu, and the greater petrosal nerve exits from this ganglion. The second part of the nerve, the tympanic or horizontal segment courses just superior to the oval window, turns at an angle of 125 degrees, forming the second genu. In the region of the pyramidal eminence, the nerve to the stapedius muscle diverges and the facial nerve runs as the mastoid or vertical segment toward the stylomastoid foramen. The chorda tympani nerve and the auricular branch arise from this segment.
From the *Ankara Oncology Education and Research Hospital, Clinic of Otolaryngology, Ankara, Turkey; and †Department of Anatomy, Ankara University Faculty of Medicine, Ankara, Turkey. Received September 30, 2013. Accepted for publication October 28, 2013. Address correspondence and reprint requests to Ayhan Cömert, MD, Department of Anatomy, Ankara University Faculty of Medicine, 06100 Sihhiye, Ankara, Turkey; E-mail: [email protected] The manuscript has not been previously presented in a meeting. This research received no specific grant from any funding agency, commercial or not-for-profit sectors. The authors report no conflicts of interest. Copyright © 2014 by Mutaz B. Habal, MD ISSN: 1049-2275 DOI: 10.1097/SCS.0000000000000499
Dehiscence or variations in the fallopian canal and repeated surgery render the facial nerve vulnerable to inadvertent injury by the surgeon. Revision surgeries for chronic otitis media, facial decompression, and recurrent vestibular schwannoma are much more complicated than the primary surgery and may have high risk for facial nerve injury. We performed a study in which the relationship of the facial nerve with respect to constant anatomic structures that could be identified during revision surgery was noted after the whole 3 segments of the facial nerve were skeletonized via translabyrinthine approach.
MATERIALS AND METHODS Fifteen formalin-fixed human cadavers were dissected bilaterally for the study. Five of the cadavers were female and 10 of them were male. Both sides of all cadavers were dissected under the operating microscope (Opmi 99; Carl Zeiss, Gottingen, Germany) with the aid of a microdrill. After postauricular incision, a simple mastoidectomy was performed on each temporal bone to identify the mastoid segment of the facial nerve. An extended facial recess approach was then used to access the entire length of the mastoid and the tympanic segment. Next, the posterior external auditory canal was drilled away, and the ossicles were taken out. Then, labyrinthectomy was performed, and the labyrinthine segment and the intracanalicular portion of the facial nerve were identified. Finally, the entire length of the facial nerve from the fundus of the internal acoustic canal to the stylomastoid foramen was exposed. Direct measurements of the shortest distances between the points described below were measured with the measuring probe and read against the digital caliper. The angles were measured with goniometry (error 0.50). All data were measured independently by 2 authors and repeated twice, and their average values were calculated. We also noted the length of each segment and angles between the segments. For the purposes of this study, the following anatomic landmarks were defined (Fig. 1): C, mastoid cortex; SF, stylomastoid foramen; M, beginning of the mastoid segment; MM, midpoint of the mastoid segment; SS, sigmoid sinus; EAC, posterior wall of the external acoustic canal; P, promontorium; T, beginning of the tympanic segment; TT, midpoint of the tympanic segment; RWN, middle of the round window niche; COC, cochleariform process; 1G, first genu; 2G, second genu; L, beginning of the labyrinthine segment; LL, midpoint of the labyrinthine segment; SSC, most superior point of the superior semicircular canal; ET, most posterosuperior point of the tympanic ostium of the eustachian tube. The length of the labyrinthine segment was measured from the fundus of the internal acoustic canal to the geniculate ganglion. The length of the tympanic segment was measured from the geniculate ganglion, not included in the measurement, to the posterior wall of the tympanum, at the point where the facial nerve coursed anterior and inferior to the lateral semicircular canal. The length of the mastoid segment was measured from the point inferior to the lateral semicircular canal to the stylomastoid foramen. The segment of the facial nerve anterior and inferior to the lateral semicircular canal was measured individually.
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FIGURE 1. Microscopic photograph of a right temporal bone showing the labyrinthine and tympanic segments; the angle between the superior semicircular canal, the first genu, and the beginning of the labyrinthine segment; and some of the points and distances measured. 1G indicates first genu; 2G, second genu; asterisk, stapes; double asterisk, carotid artery; ET, tympanic ostium of the eustachian tube; IAC, internal acoustic canal; L, beginning of the labyrinthine segment; LL, midpoint of the labyrinthine segment; M, beginning of the mastoid segment; SS, sigmoid sinus; SSC, superior semicircular canal; T, beginning of the tympanic segment; TT, midpoint of the tympanic segment.
Means, SDs, and ranges were computed for the various measured distances. The Statistical Package for the Social Sciences software (version 16.0 for Windows; Statistical Package for the Social Sciences, Inc, an IBM Company, Chicago, IL) was used for statistical analysis. The Mann-Whitney test was used to evaluate differences between the left and right sides. A P value of less than 0.05 was considered statistically significant.
RESULTS In all the 30 temporal bones, all the segments of the facial nerve, the greater petrosal nerve, the chorda tympani, and the auricular branch of the facial nerve could be exposed without injury. After entering the fallopian canal, the labyrinthine segment coursed superiorly, anteriorly, and laterally through the geniculate ganglion. After the geniculate ganglion, the tympanic segment turned inferiorly, posteriorly, and laterally. Then, it passed inferior and anterior to the lateral semicircular canal, and the mastoid segment coursed inferiorly through the stylomastoid foramen. The segments of the facial nerve and the points used for measurements are shown in Figures 1 and 2. No difference in these measurements was identified between the right and left sides (P > 0.05). The lengths of the segments of the facial nerve are presented in Table 1. The distances between the labyrinthine and the tympanic segment and specific anatomic structures are listed in Table 2. The distances between the mastoid segment and specific anatomic structures are shown in Table 3. The angles of the first and the second genu were 28 ± 3 degrees (range, 24–35 degrees) and 123 ± 4 degrees (range, 120–135 degrees), respectively. An additional angle was defined between the most superior point of the superior semicircular canal, the tip of the first genu, and the beginning of the labyrinthine segment and measured as 57 ± 4 degrees (range, 52–65 degrees). The angle between the facial nerve and the chorda tympani nerve was 21 ± 4 degrees (range, 15–28 degrees).
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FIGURE 2. Photograph of a right temporal bone showing the beginning and ending points of the segments and some of the points and distances measured. 1G indicates first genu; 2G, second genu; asterisk, upper edge of the lateral semicircular canal that corresponds to the ending of the tympanic segment; ET, tympanic orifice of the eustachian tube; M, beginning of the mastoid segment; MM, midpoint of the mastoid segment; P, promontorium; RWN, round window niche; SF, stylomastoid foramen; SS, sigmoid sinus; T, beginning of the tympanic segment; TT, midpoint of the tympanic segment.
The chorda tympani nerve was originating from the anterior surface of the mastoid segment in all temporal bones. The auricular branch arised from the mastoid segment, proximal to the point at which the chorda tympani originated. The distance between the stylomastoid foramen and the branching point of the chorda tympani was measured as 4.83 ± 0.75 mm (range, 3.96–6.81 mm). The distance between the stylomastoid foramen and the auricular branch origin was measured as 8.18 ± 0.9 mm (range, 6.44–9.16 mm). In both sides of 1 cadaver, the auricular branch was originating from the posterior surface of the mastoid segment. It crossed the facial nerve laterally and reached the posterior wall of the external acoustic canal (Fig. 3), whereas in the rest of the cadavers, the auricular branch was originating from the anterior surface of the mastoid segment.
DISCUSSION The anatomy of the facial nerve at the fundus of the internal acoustic canal has been previously described in detail and used to identify the labyrinthine segment.3 Most techniques used to access the internal acoustic canal are avoided when the facial nerve is the only structure to be exposed to prevent injury to the other structures of the internal acoustic canal.4 Landmarks for the labyrinthine segment are not as apparent as with other segments of the nerve. Without finding the internal acoustic canal, identification of the labyrinthine
TABLE 1. Length of Segments of the Facial Nerve
LS TS ILP MS
Length, mm
SD, mm
Range, mm
5.08 9.59 4.52 14.67
1.04 0.9 0.56 1.83
3.79–5.39 8.43–10.75 3.83–5.14 12.23–17.27
ILP, the portion of the facial nerve that lies anteroinferior to the lateral semicircular canal (length of the second genu); LS, labyrinthine segment; MS, mastoid segment; TS, tympanic segment.
© 2014 Mutaz B. Habal, MD
Copyright © 2014 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.
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TABLE 2. Distances Between Labyrinthine and Tympanic Segment and Several Important Anatomic Structures
TT-P (length) TT-RWN (length) COC-1G (length) ET-1G (length) ET-2G (length) LL-SSC (length) LL-TT (depth)
Distance, mm
SD, mm
Range, mm
6.22 6.64 4.29 7.02 12.95 2.28 6.33
1.85 1.79 0.91 0.82 2.84 0.44 1.21
4.23–9.74 4.29–9.18 3.39–6.12 6.25–9.12 8.22–15.88 1.41–2.69 5.74–7.58
1G, first genu; 2G, beginning of the second genu; COC, cochleariform process; ET, eustachian tube; LL, midpoint of the labyrinthine segment; P, promontorium; RWN, round window niche; SSC, superior border of the superior semicircular canal; TT, midpoint of the tympanic segment.
segment is based on the angle between the tympanic and labyrinthine segments and the depth of the labyrinthine segment with regard to the tympanic segment. Our study confirms that the angle of the first genu is 28 degrees, and in addition to the current literature, we find that the depth of the labyrinthine segment is 6.33 ± 1.21 mm (range, 5.74– 7.58 mm) with regard to the tympanic segment. The superior semicircular canal and the superior ampullary nerve were previously used to identify the facial nerve over the fundus of the internal acoustic canal.5 During our dissections, we observed that the most superior aspect of the superior semicircular canal could be defined even after labyrinthectomy. We used it as a landmark for the labyrinthine segment of the facial nerve. We measured the distance between the most superior point of the superior semicircular canal and the midpoint of the labyrinthine segment (mean ± SD, 2.28 ± 0.44 mm) and the angle between the most superior point of the superior semicircular canal, the first genu, and the beginning of the labyrinthine segment (mean ± SD, 57 ± 4 degrees). Precise identification of the first genu and the superior edge of the superior semicircular canal would lead to a layout of the whole course of the labyrinthine segment. The surgical approach to the facial nerve through the mastoid process is the only one that is widely used currently, and the facial nerve can be exposed from the geniculate ganglion to the stylomastoid foramen. Surgical anatomy of the perigeniculate area of the facial nerve has been studied mostly from the middle cranial fossa approach.6–8 The structures used to identify the first genu on the anterosuperior aspect of
TABLE 3. Distances Between Mastoid Segment and Several Important Anatomic Structures
C-SF (depth) C-MM (depth) C-M (depth) SS-SF (length) SS-MM (length) SS-M (length) EAC-MM (length) RWN-M (length) RWN-M (depth)
Distance, mm
SD, mm
Range, mm
15.92 17.62 17.59 10.79 8.93 9.41 3.42 3.99 8.58
3.32 3.16 2.27 2.84 1.99 2.19 0.44 0.76 1.74
12.33–21.79 14.32–23.29 14.63–20.38 5.38–14.18 6.20–10.81 5.69–12.52 2.80–4.55 3.29–5.47 3.25–5.47
C, mastoid cortex; EAC, posterior wall of the external auditory canal; M, beginning of the mastoid segment; MM, midpoint of the mastoid segment; RWN, round window niche; SF, stylomastoid foramen; SS, sigmoid sinus.
FIGURE 3. Photograph of a right temporal bone showing the auricular branch originating from the posterior surface of the mastoid segment. Arrowheads indicate the course of the chorda tympani; arrows, the course of the auricular branch; asterisk, incus; DR, digastrics ridge; EAC, posterior wall of the external acoustic canal; LSC, lateral semicircular canal; PSC, posterior semicircular canal; SS, sigmoid sinus; SSC, superior semicircular canal.
the temporal bone are the arcuate eminence and the greater superficial petrosal nerve.8 By transmastoid approach, the most commonly used structure to identify the geniculate ganglion is the cochleariform process.9 We used the cochleariform process and also the tympanic ostium of the eustachian tube as landmarks for the geniculate ganglion. The most common site for facial nerve injury during ear surgery is the tympanic segment. Its course was described by Tos.1 It is directed 70 degrees laterally and 10 degrees inferiorly near the horizontal semicircular canal. This part of the nerve courses just superior to the oval window and forms the superomedial boundary of the mesotympanum.2 The second genu passes just anterior to the lateral prominence of the horizontal semicircular canal, and the nerve follows the anterior aspect of the lateral semicircular canal for 4.52 ± 0.56 mm. The landmarks that were previously used for identifying the tympanic and mastoid segments included the external acoustic meatus, the oval window niche, the sulcus tympanicus, the promontorium, the cochleariform process, the sigmoid sinus, the tympanic membrane, the digastric ridge, the stylomastoid foramen, the pyramidal eminence, the lateral semicircular canal, the mastoid cortex, and the short process of incus.9–15 In addition, we measured the distances between the round window niche, the tympanic opening of the eustachian tube, and the tympanic segment because most of these structures can be observed during revision surgery of the middle ear. The angle of the second genu, the depth of the mastoid segment, and the distances between the sigmoid sinus and the mastoid segment are compatible with the current data in the literature.10,13,16 It is difficult to compare some of our results with the present literature because we mainly based on constant landmarks that can also be identified after extensive mastoid and tympanic surgery. Most of the anatomic structures that are previously described can be removed or displaced at the previous surgery. Although the oval window, the lateral semicircular canal, the cochleariform process, and the ossicles are good landmarks for initial surgery, these are even difficult to identify and usually of no help in identifying the facial nerve during revision surgery. The chorda tympani originates in a variable position along the mastoid segment of the facial nerve.17 Our measurements between the stylomastoid foramen to the departure point of the chorda tympani and the auricular branch origin to the chorda tympani nerve origin are compatible with the current literature.15,17,18 In the study of Eshraghi et al,17 the auricular branch was demonstrated as originating from the anterior surface of the facial nerve superior to the origin of the chorda tympani nerve. Our study shows that the auricular branch may also be
© 2014 Mutaz B. Habal, MD
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originated from the posterior surface of the mastoid segment, and drilling of the posterior surface of the facial nerve may result in a sensory deficit in the posterior external acoustic canal and the conchal bowl. The incidence of facial nerve dehiscence among patients who underwent revision mastoid surgery for chronic otitis media was reported as 28%.19 Intraoperative facial nerve monitoring was recommended in avoiding injuries to the nerve, especially in cases of revision surgery and congenital malformations.20 Surgical complications secondary to the lack of consistently identifiable surgical landmarks with respect to the facial nerve represent an important drawback to both transmastoid and translabyrinthine revision surgery. This study improved the surgical approach by describing the anatomic location of the 3 segments of the facial nerve with regard to more constant landmarks for revision transmastoid surgery.
CONCLUSIONS Most of the anatomic structures such as the oval window, the lateral semicircular canal, the cochleariform process, and the ossicles are good landmarks for initial surgery; these are even difficult to identify and usually of no help in identifying the facial nerve during revision surgery. We measured the distance between the most superior point of the superior semicircular canal and the midpoint of the labyrinthine segment and the angle between the most superior point of the superior semicircular canal, the first genu, and the beginning of the labyrinthine segment to determine the labyrinthine segment. We measured the distances between the round window niche, the tympanic opening of the eustachian tube, and the tympanic segment. Our study also shows that the auricular branch may also be originated from the posterior surface of the mastoid segment, and drilling of the posterior surface of the facial nerve may result in a sensory deficit in the posterior external acoustic canal and the conchal bowl.
REFERENCES 1. Tos M. Anatomy. In: Tos M, ed. Manual of Middle Ear Surgery. Stuttgart, Germany: Thieme, 1993:48–49 2. Lambert PR, Canalis RF. Anatomy and embryology of the auditory and vestibular system. In: Canalis RF, Lambert PR, eds. The Ear Comprehensive Otology. Philadelphia: Lippincott Williams & Wilkins, 2000:41–46 3. House WF. Acoustic neuroma (monograph I). Arch Otolaryngol 1964;80:733–740
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4. Eren E, Basoglu MS, Gürcan Bingölballi A, et al. Conquering the castle: a novel technique for the middle fossa approach in facial decompression. Otolaryngol Head Neck Surg 2012;147:907–911 5. Sanna M, Saleh E, Russo A, et al. Identification of the facial nerve in the translabyrinthine approach: an alternative technique. Otolaryngol Head Neck Surg 2001;124:105–106 6. Lee HK, Lee EH, Lee WS, et al. Microsurgical anatomy of the perigeniculate ganglion area as seen from a translabyrinthine approach. Ann Otol Rhinol Laryngol 2000;109:255–257 7. Bento RF, Pirana S, Sweet R, et al. The role of the middle fossa approach in the management of traumatic facial paralysis. Ear Nose Throat J 2004;83:817–823 8. Rupa V, Weider DJ, Glasner S, et al. Geniculate ganglion: anatomic study with surgical implications. Am J Otol 1992;13:470–473 9. Nikolaidis V, Nalbadian M, Psifidis A, et al. The tympanic segment of the facial nerve: anatomical study. Clin Anat 2009;22:307–310 10. Boemo RL, Navarrete ML, Lareo S, et al. Anatomical relationship between the position of the sigmoid sinus, tympanic membrane and digastric ridge with the mastoid segment of the facial nerve. Eur Arch Otorhinolaryngol 2008;265:389–392 11. Feng Y, Zhang YQ, Liu M, et al. Sectional anatomy aid for improvement of decompression surgery approach to vertical segment of facial nerve. J Craniofac Surg 2012;23:906–908 12. Yadav SP, Ranga A, Sirohiwal BL, et al. Surgical anatomy of tympano-mastoid segment of facial nerve. Indian J Otolaryngol Head Neck Surg 2006;58:27–30 13. Kharat RD, Golhar SV, Patil CY. Study of intratemporal course of facial nerve and its variations—25 temporal bones dissection. Indian J Otolaryngol Head Neck Surg 2009;61:39–42 14. Adad B, Rasgon BM, Ackerson L. Relationship of the facial nerve to the tympanic annulus: a direct anatomic examination. Laryngoscope 1999;109:1189–1192 15. Calli C, Pinar E, Oncel S, et al. Measurements of the facial recess anatomy: implications for sparing the facial nerve and chorda tympani during posterior tympanotomy. Ear Nose Throat J 2010;89: 490–494 16. Kudo H, Nori S. Topography of the facial nerve in the human temporal bone. Acta Anat (Basel) 1974;90:467–480 17. Eshraghi AA, Buchman CA, Telischi FF. Sensory auricular branch of the facial nerve. Otol Neurotol 2002;23:393–396 18. Muren C, Wadin K, Wilbrand HF. Anatomic variations of the chorda tympani canal. Acta Otolaryngol 1990;110:262–265 19. Berçin S, Kutluhan A, Bozdemir K, et al. Results of revision mastoidectomy. Acta Otolaryngol 2009;129:138–141 20. House JR III, Luxford WM. Facial nerve injury in cochlear implantation. Otolaryngol Head Neck Surg 1993;109:1078–1082
© 2014 Mutaz B. Habal, MD
Copyright © 2014 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.
Surgical Anatomy of Facial Nerve for Revision Transmastoid Surgery Ela Cömert, MD, PhD,* Ayhan Cömert, MD,† Ümit Tunçel, MD,* and İbrahim Tekdemir, MD† Abstract: We analyze the relationships of the 3 segments of the facial nerve with respect to constant anatomic structures that can be identified during revision surgery via translabyrinthine approach. This study was conducted on 15 formalin-fixed cadavers whose facial nerves were dissected bilaterally under operative microscope via translabyrinthine approach. The distances between the round window niche and the midpoint of the tympanic segment and the beginning of the mastoid segment were 6.64 ± 1.79 mm and 3.99 ± 0.79 mm, respectively. The distances between the tympanic ostium of the eustachian tube and the first and the second genu were 7.02 ± 0.62 mm and 12.25 ± 1.24 mm, respectively. We used the superior semicircular canal, the tympanic ostium of the eustachian tube, and the round window niche as landmarks to identify the facial nerve during revision surgery. Our study also showed that the auricular branch may also be originated from the posterior surface of the facial nerve. Key Words: Facial nerve, surgical revision, otologic surgical procedures (J Craniofac Surg 2014;25: 619–622)
O
ne of the most important structures that should be identified during transmastoid surgery is the facial nerve. The anatomy of the facial nerve has been previously described in detail.1,2 After entering the internal acoustic canal, the facial nerve passes into the fallopian canal. From the translabyrinthine approach, the first part of the facial nerve, the labyrinthine segment, runs toward the geniculate ganglion. It turns posteriorly to form the first genu, and the greater petrosal nerve exits from this ganglion. The second part of the nerve, the tympanic or horizontal segment courses just superior to the oval window, turns at an angle of 125 degrees, forming the second genu. In the region of the pyramidal eminence, the nerve to the stapedius muscle diverges and the facial nerve runs as the mastoid or vertical segment toward the stylomastoid foramen. The chorda tympani nerve and the auricular branch arise from this segment.
From the *Ankara Oncology Education and Research Hospital, Clinic of Otolaryngology, Ankara, Turkey; and †Department of Anatomy, Ankara University Faculty of Medicine, Ankara, Turkey. Received September 30, 2013. Accepted for publication October 28, 2013. Address correspondence and reprint requests to Ayhan Cömert, MD, Department of Anatomy, Ankara University Faculty of Medicine, 06100 Sihhiye, Ankara, Turkey; E-mail: [email protected] The manuscript has not been previously presented in a meeting. This research received no specific grant from any funding agency, commercial or not-for-profit sectors. The authors report no conflicts of interest. Copyright © 2014 by Mutaz B. Habal, MD ISSN: 1049-2275 DOI: 10.1097/SCS.0000000000000499
Dehiscence or variations in the fallopian canal and repeated surgery render the facial nerve vulnerable to inadvertent injury by the surgeon. Revision surgeries for chronic otitis media, facial decompression, and recurrent vestibular schwannoma are much more complicated than the primary surgery and may have high risk for facial nerve injury. We performed a study in which the relationship of the facial nerve with respect to constant anatomic structures that could be identified during revision surgery was noted after the whole 3 segments of the facial nerve were skeletonized via translabyrinthine approach.
MATERIALS AND METHODS Fifteen formalin-fixed human cadavers were dissected bilaterally for the study. Five of the cadavers were female and 10 of them were male. Both sides of all cadavers were dissected under the operating microscope (Opmi 99; Carl Zeiss, Gottingen, Germany) with the aid of a microdrill. After postauricular incision, a simple mastoidectomy was performed on each temporal bone to identify the mastoid segment of the facial nerve. An extended facial recess approach was then used to access the entire length of the mastoid and the tympanic segment. Next, the posterior external auditory canal was drilled away, and the ossicles were taken out. Then, labyrinthectomy was performed, and the labyrinthine segment and the intracanalicular portion of the facial nerve were identified. Finally, the entire length of the facial nerve from the fundus of the internal acoustic canal to the stylomastoid foramen was exposed. Direct measurements of the shortest distances between the points described below were measured with the measuring probe and read against the digital caliper. The angles were measured with goniometry (error 0.50). All data were measured independently by 2 authors and repeated twice, and their average values were calculated. We also noted the length of each segment and angles between the segments. For the purposes of this study, the following anatomic landmarks were defined (Fig. 1): C, mastoid cortex; SF, stylomastoid foramen; M, beginning of the mastoid segment; MM, midpoint of the mastoid segment; SS, sigmoid sinus; EAC, posterior wall of the external acoustic canal; P, promontorium; T, beginning of the tympanic segment; TT, midpoint of the tympanic segment; RWN, middle of the round window niche; COC, cochleariform process; 1G, first genu; 2G, second genu; L, beginning of the labyrinthine segment; LL, midpoint of the labyrinthine segment; SSC, most superior point of the superior semicircular canal; ET, most posterosuperior point of the tympanic ostium of the eustachian tube. The length of the labyrinthine segment was measured from the fundus of the internal acoustic canal to the geniculate ganglion. The length of the tympanic segment was measured from the geniculate ganglion, not included in the measurement, to the posterior wall of the tympanum, at the point where the facial nerve coursed anterior and inferior to the lateral semicircular canal. The length of the mastoid segment was measured from the point inferior to the lateral semicircular canal to the stylomastoid foramen. The segment of the facial nerve anterior and inferior to the lateral semicircular canal was measured individually.
The Journal of Craniofacial Surgery • Volume 25, Number 2, March 2014
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FIGURE 1. Microscopic photograph of a right temporal bone showing the labyrinthine and tympanic segments; the angle between the superior semicircular canal, the first genu, and the beginning of the labyrinthine segment; and some of the points and distances measured. 1G indicates first genu; 2G, second genu; asterisk, stapes; double asterisk, carotid artery; ET, tympanic ostium of the eustachian tube; IAC, internal acoustic canal; L, beginning of the labyrinthine segment; LL, midpoint of the labyrinthine segment; M, beginning of the mastoid segment; SS, sigmoid sinus; SSC, superior semicircular canal; T, beginning of the tympanic segment; TT, midpoint of the tympanic segment.
Means, SDs, and ranges were computed for the various measured distances. The Statistical Package for the Social Sciences software (version 16.0 for Windows; Statistical Package for the Social Sciences, Inc, an IBM Company, Chicago, IL) was used for statistical analysis. The Mann-Whitney test was used to evaluate differences between the left and right sides. A P value of less than 0.05 was considered statistically significant.
RESULTS In all the 30 temporal bones, all the segments of the facial nerve, the greater petrosal nerve, the chorda tympani, and the auricular branch of the facial nerve could be exposed without injury. After entering the fallopian canal, the labyrinthine segment coursed superiorly, anteriorly, and laterally through the geniculate ganglion. After the geniculate ganglion, the tympanic segment turned inferiorly, posteriorly, and laterally. Then, it passed inferior and anterior to the lateral semicircular canal, and the mastoid segment coursed inferiorly through the stylomastoid foramen. The segments of the facial nerve and the points used for measurements are shown in Figures 1 and 2. No difference in these measurements was identified between the right and left sides (P > 0.05). The lengths of the segments of the facial nerve are presented in Table 1. The distances between the labyrinthine and the tympanic segment and specific anatomic structures are listed in Table 2. The distances between the mastoid segment and specific anatomic structures are shown in Table 3. The angles of the first and the second genu were 28 ± 3 degrees (range, 24–35 degrees) and 123 ± 4 degrees (range, 120–135 degrees), respectively. An additional angle was defined between the most superior point of the superior semicircular canal, the tip of the first genu, and the beginning of the labyrinthine segment and measured as 57 ± 4 degrees (range, 52–65 degrees). The angle between the facial nerve and the chorda tympani nerve was 21 ± 4 degrees (range, 15–28 degrees).
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FIGURE 2. Photograph of a right temporal bone showing the beginning and ending points of the segments and some of the points and distances measured. 1G indicates first genu; 2G, second genu; asterisk, upper edge of the lateral semicircular canal that corresponds to the ending of the tympanic segment; ET, tympanic orifice of the eustachian tube; M, beginning of the mastoid segment; MM, midpoint of the mastoid segment; P, promontorium; RWN, round window niche; SF, stylomastoid foramen; SS, sigmoid sinus; T, beginning of the tympanic segment; TT, midpoint of the tympanic segment.
The chorda tympani nerve was originating from the anterior surface of the mastoid segment in all temporal bones. The auricular branch arised from the mastoid segment, proximal to the point at which the chorda tympani originated. The distance between the stylomastoid foramen and the branching point of the chorda tympani was measured as 4.83 ± 0.75 mm (range, 3.96–6.81 mm). The distance between the stylomastoid foramen and the auricular branch origin was measured as 8.18 ± 0.9 mm (range, 6.44–9.16 mm). In both sides of 1 cadaver, the auricular branch was originating from the posterior surface of the mastoid segment. It crossed the facial nerve laterally and reached the posterior wall of the external acoustic canal (Fig. 3), whereas in the rest of the cadavers, the auricular branch was originating from the anterior surface of the mastoid segment.
DISCUSSION The anatomy of the facial nerve at the fundus of the internal acoustic canal has been previously described in detail and used to identify the labyrinthine segment.3 Most techniques used to access the internal acoustic canal are avoided when the facial nerve is the only structure to be exposed to prevent injury to the other structures of the internal acoustic canal.4 Landmarks for the labyrinthine segment are not as apparent as with other segments of the nerve. Without finding the internal acoustic canal, identification of the labyrinthine
TABLE 1. Length of Segments of the Facial Nerve
LS TS ILP MS
Length, mm
SD, mm
Range, mm
5.08 9.59 4.52 14.67
1.04 0.9 0.56 1.83
3.79–5.39 8.43–10.75 3.83–5.14 12.23–17.27
ILP, the portion of the facial nerve that lies anteroinferior to the lateral semicircular canal (length of the second genu); LS, labyrinthine segment; MS, mastoid segment; TS, tympanic segment.
© 2014 Mutaz B. Habal, MD
Copyright © 2014 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.
The Journal of Craniofacial Surgery • Volume 25, Number 2, March 2014
Surgical Anatomy of Facial Nerve
TABLE 2. Distances Between Labyrinthine and Tympanic Segment and Several Important Anatomic Structures
TT-P (length) TT-RWN (length) COC-1G (length) ET-1G (length) ET-2G (length) LL-SSC (length) LL-TT (depth)
Distance, mm
SD, mm
Range, mm
6.22 6.64 4.29 7.02 12.95 2.28 6.33
1.85 1.79 0.91 0.82 2.84 0.44 1.21
4.23–9.74 4.29–9.18 3.39–6.12 6.25–9.12 8.22–15.88 1.41–2.69 5.74–7.58
1G, first genu; 2G, beginning of the second genu; COC, cochleariform process; ET, eustachian tube; LL, midpoint of the labyrinthine segment; P, promontorium; RWN, round window niche; SSC, superior border of the superior semicircular canal; TT, midpoint of the tympanic segment.
segment is based on the angle between the tympanic and labyrinthine segments and the depth of the labyrinthine segment with regard to the tympanic segment. Our study confirms that the angle of the first genu is 28 degrees, and in addition to the current literature, we find that the depth of the labyrinthine segment is 6.33 ± 1.21 mm (range, 5.74– 7.58 mm) with regard to the tympanic segment. The superior semicircular canal and the superior ampullary nerve were previously used to identify the facial nerve over the fundus of the internal acoustic canal.5 During our dissections, we observed that the most superior aspect of the superior semicircular canal could be defined even after labyrinthectomy. We used it as a landmark for the labyrinthine segment of the facial nerve. We measured the distance between the most superior point of the superior semicircular canal and the midpoint of the labyrinthine segment (mean ± SD, 2.28 ± 0.44 mm) and the angle between the most superior point of the superior semicircular canal, the first genu, and the beginning of the labyrinthine segment (mean ± SD, 57 ± 4 degrees). Precise identification of the first genu and the superior edge of the superior semicircular canal would lead to a layout of the whole course of the labyrinthine segment. The surgical approach to the facial nerve through the mastoid process is the only one that is widely used currently, and the facial nerve can be exposed from the geniculate ganglion to the stylomastoid foramen. Surgical anatomy of the perigeniculate area of the facial nerve has been studied mostly from the middle cranial fossa approach.6–8 The structures used to identify the first genu on the anterosuperior aspect of
TABLE 3. Distances Between Mastoid Segment and Several Important Anatomic Structures
C-SF (depth) C-MM (depth) C-M (depth) SS-SF (length) SS-MM (length) SS-M (length) EAC-MM (length) RWN-M (length) RWN-M (depth)
Distance, mm
SD, mm
Range, mm
15.92 17.62 17.59 10.79 8.93 9.41 3.42 3.99 8.58
3.32 3.16 2.27 2.84 1.99 2.19 0.44 0.76 1.74
12.33–21.79 14.32–23.29 14.63–20.38 5.38–14.18 6.20–10.81 5.69–12.52 2.80–4.55 3.29–5.47 3.25–5.47
C, mastoid cortex; EAC, posterior wall of the external auditory canal; M, beginning of the mastoid segment; MM, midpoint of the mastoid segment; RWN, round window niche; SF, stylomastoid foramen; SS, sigmoid sinus.
FIGURE 3. Photograph of a right temporal bone showing the auricular branch originating from the posterior surface of the mastoid segment. Arrowheads indicate the course of the chorda tympani; arrows, the course of the auricular branch; asterisk, incus; DR, digastrics ridge; EAC, posterior wall of the external acoustic canal; LSC, lateral semicircular canal; PSC, posterior semicircular canal; SS, sigmoid sinus; SSC, superior semicircular canal.
the temporal bone are the arcuate eminence and the greater superficial petrosal nerve.8 By transmastoid approach, the most commonly used structure to identify the geniculate ganglion is the cochleariform process.9 We used the cochleariform process and also the tympanic ostium of the eustachian tube as landmarks for the geniculate ganglion. The most common site for facial nerve injury during ear surgery is the tympanic segment. Its course was described by Tos.1 It is directed 70 degrees laterally and 10 degrees inferiorly near the horizontal semicircular canal. This part of the nerve courses just superior to the oval window and forms the superomedial boundary of the mesotympanum.2 The second genu passes just anterior to the lateral prominence of the horizontal semicircular canal, and the nerve follows the anterior aspect of the lateral semicircular canal for 4.52 ± 0.56 mm. The landmarks that were previously used for identifying the tympanic and mastoid segments included the external acoustic meatus, the oval window niche, the sulcus tympanicus, the promontorium, the cochleariform process, the sigmoid sinus, the tympanic membrane, the digastric ridge, the stylomastoid foramen, the pyramidal eminence, the lateral semicircular canal, the mastoid cortex, and the short process of incus.9–15 In addition, we measured the distances between the round window niche, the tympanic opening of the eustachian tube, and the tympanic segment because most of these structures can be observed during revision surgery of the middle ear. The angle of the second genu, the depth of the mastoid segment, and the distances between the sigmoid sinus and the mastoid segment are compatible with the current data in the literature.10,13,16 It is difficult to compare some of our results with the present literature because we mainly based on constant landmarks that can also be identified after extensive mastoid and tympanic surgery. Most of the anatomic structures that are previously described can be removed or displaced at the previous surgery. Although the oval window, the lateral semicircular canal, the cochleariform process, and the ossicles are good landmarks for initial surgery, these are even difficult to identify and usually of no help in identifying the facial nerve during revision surgery. The chorda tympani originates in a variable position along the mastoid segment of the facial nerve.17 Our measurements between the stylomastoid foramen to the departure point of the chorda tympani and the auricular branch origin to the chorda tympani nerve origin are compatible with the current literature.15,17,18 In the study of Eshraghi et al,17 the auricular branch was demonstrated as originating from the anterior surface of the facial nerve superior to the origin of the chorda tympani nerve. Our study shows that the auricular branch may also be
© 2014 Mutaz B. Habal, MD
Copyright © 2014 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.
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originated from the posterior surface of the mastoid segment, and drilling of the posterior surface of the facial nerve may result in a sensory deficit in the posterior external acoustic canal and the conchal bowl. The incidence of facial nerve dehiscence among patients who underwent revision mastoid surgery for chronic otitis media was reported as 28%.19 Intraoperative facial nerve monitoring was recommended in avoiding injuries to the nerve, especially in cases of revision surgery and congenital malformations.20 Surgical complications secondary to the lack of consistently identifiable surgical landmarks with respect to the facial nerve represent an important drawback to both transmastoid and translabyrinthine revision surgery. This study improved the surgical approach by describing the anatomic location of the 3 segments of the facial nerve with regard to more constant landmarks for revision transmastoid surgery.
CONCLUSIONS Most of the anatomic structures such as the oval window, the lateral semicircular canal, the cochleariform process, and the ossicles are good landmarks for initial surgery; these are even difficult to identify and usually of no help in identifying the facial nerve during revision surgery. We measured the distance between the most superior point of the superior semicircular canal and the midpoint of the labyrinthine segment and the angle between the most superior point of the superior semicircular canal, the first genu, and the beginning of the labyrinthine segment to determine the labyrinthine segment. We measured the distances between the round window niche, the tympanic opening of the eustachian tube, and the tympanic segment. Our study also shows that the auricular branch may also be originated from the posterior surface of the mastoid segment, and drilling of the posterior surface of the facial nerve may result in a sensory deficit in the posterior external acoustic canal and the conchal bowl.
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© 2014 Mutaz B. Habal, MD
Copyright © 2014 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.
