|Year : 2017 | Volume
| Issue : 2 | Page : 108-112
To study the anatomy of tympanomastoid segment of facial nerve and its variations in human cadaveric temporal bone
Nitika Gupta1, Rohan Gupta2, IP Singh2, Sunil Kotwal2, Anil Suri2, Sunanda Raina3
1 Department of Otorhinolaryngology and Head and Neck Surgery, King George's Medical University, Lucknow, India
2 Department of Otorhinolaryngology and Head and Neck Surgery, Government Medical College, Jammu, Jammu and Kashmir, India
3 Department of Anatomy, Government Medical College, Jammu, Jammu and Kashmir, India
|Date of Web Publication||14-Jun-2017|
Department of ENT and Head and Neck Surgery, SMGS Hospital, Government Medical College, Shalamar Road, Jammu - 180 001, Jammu and Kashmir
Source of Support: None, Conflict of Interest: None
Introduction: The facial nerve is one of the most significant and vulnerable structures in the temporal bone. Its dysfunction affects both voluntary and voluntary motion leading to noticeable disfigurement and emotional distress to those suffering from it. Iatrogenic facial paralysis is known to be a feared complication of ear surgery, and its incidence is reported to be 0.6–3.6% in all otologic surgical procedures, which increases to 4–10% in revision cases. Objective: The aim of this study is to study the anatomical aspects of tympanomastoid segment of the facial nerve. Materials and Methods: Thirty wet human cadaveric temporal bones were dissected in the temporal bone dissection laboratory in the Department of ENT, Government Medical College, Jammu. The length of the tympanomastoid segment of the facial nerve, its relationship with important bony landmarks, and the presence of any anomaly or variation in its course were studied. Observations and Results: The mean length of the tympanic segment of the facial nerve was 10.44±1.428 mm while that of the mastoid segment was 13.33±2.202 mm. The angle formed between the tympanic and mastoid segments of the facial nerve varied between 95°and 120°with the mean angle being 104.66°±7.760°.
Keywords: Facial nerve, mastoid segment, temporal bone, tympanic segment
|How to cite this article:|
Gupta N, Gupta R, Singh I P, Kotwal S, Suri A, Raina S. To study the anatomy of tympanomastoid segment of facial nerve and its variations in human cadaveric temporal bone. Indian J Otol 2017;23:108-12
|How to cite this URL:|
Gupta N, Gupta R, Singh I P, Kotwal S, Suri A, Raina S. To study the anatomy of tympanomastoid segment of facial nerve and its variations in human cadaveric temporal bone. Indian J Otol [serial online] 2017 [cited 2021 Apr 11];23:108-12. Available from: https://www.indianjotol.org/text.asp?2017/23/2/108/208015
| Introduction|| |
The adult temporal bone, an amalgam of the squamous part, petrous part, mastoid part, tympanic part, and styloid process, is a fascinating, intricate, and complex structure. A great number of neurologic, vascular, sensory, and supporting structures crowd the ~ 16 cm 3 space within it. One of the most sensitive structures in the temporal bone and the bête-noir of the ENT surgeons' worldwide is the facial nerve.
Facial nerve was first recognized and described by Galen (130–200 BC), it is the nerve of the second branchial arch and is composed of about 10,000 motor, sensory and parasympathetic fibers. For years, it was believed that anomalies of the facial canal and facial nerve do not occur and the nerve route appeared to be protected in its bony canal against outside influences and injury. Therefore, occasionally, whenever the facial nerve was injured along its course, it was presumed that the damage was always due to carelessness or lack of surgical skill. But now, it is well documented that the route of the facial canal, as it traverses the temporal bone, is somewhat variable and at times displays bony dehiscences, which are of vital concern to the otologists.
The most common site for the facial nerve injury during ear surgery is the tympanic segment, but if the anatomical landmarks are followed and extra cautions are taken, iatrogenic injury to the facial nerve can be prevented.
It is also an important observation that the otologists having inadequate familiarity with facial nerve also have a tendency to perform incomplete surgery in various, yet serious, ear diseases such as chronic suppurative otitis media.
Kolb had suggested that knowledge is created through experience and temporal bone dissection allows scenarios to be conducted in an arena where failure is allowed, and the budding otologists can learn from these mistakes without causing harm to the actual patients. The present study was therefore conducted to gain a thorough knowledge of the intricate, convoluted course of the facial nerve, its anatomic relationship to other vital structures, and to acquire this necessary skill.
| Materials and Methods|| |
The present study was conducted in the temporal bone dissection laboratory in the Department of Otorhinolaryngology and Head and Neck surgery, S.M.G.S Hospital, Government Medical College (GMC), Jammu, for a period of 1-year from November 1, 2013, to October 31, 2014, on thirty adults' cadaveric temporal bones, obtained from the Department of Anatomy, GMC, Jammu.
The side of the chosen bone was identified, and the specimen was mounted onto the temporal bone holder in surgeon's view position. All the unnecessary soft tissue was removed, and the following landmarks were clearly identified on the lateral surface of the temporal bone, that is, the spine of Henle, posterosuperiorly, the suprameatal triangle (MacEwen's triangle) behind the spine of Henle, and the superior temporal line (supramastoid crest), superiorly.
The transmastoid approach was utilized for surgical exposure of the facial nerve in its entire horizontal and vertical course. Once the exposure of the facial nerve was accomplished, the bone over the entire nerve accessible by this approach was thinned down to eggshell consistency with the diamond burr and constant suction irrigation.
After the complete exposure of the horizontal and vertical segment of facial nerve, measurements were made with a divider and a measuring probe and read against the millimeter scale. The length of tympanic and mastoid segment of facial nerve, its relationship and distance from important bony landmarks, angle of second genu, depth of facial nerve at the level second genu and at stylomastoid foramen, dehiscence of Fallopian canal More Details (if any), and facial nerve anomalies (if any) were recorded as per the attached pro forma.
| Observations and Results|| |
Of the total of 30 dissected temporal bones in the present study, there were 17 (56.66%) right-sided and 13 (43.33%) left-sided temporal bones.
The mean length of the tympanic segment of the facial nerve in the present study was 10.44±1.428 mm, with the length varying from 8 to 13 mm [Table 1] and [Figure 1]a.
|Figure 1: (a) The tympanic segment of the facial nerve. (b) The relation of the tympanic segment with the cochleariform process. (c) The relation of the facial nerve with the incudostapedial joint and short process of incus. (d) The relation of the facial nerve with the annulus. (e) The mastoid segment of the facial nerve|
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The tympanic segment of the facial nerve, at its proximal end (anteriorly), was found to be above and medial to the cochleariform process in 28 (93.33%) specimens, while it was adjacent to the cochleariform process in 2 (6.66%) specimens [Figure 1]b. It turned posteriorly from the cochleariform process, with its middle portion observed to lie above the oval window in all the thirty (100%) dissected temporal bone specimens [Figure 1]c. It then passed under the lateral semicircular canal in all the thirty (100%) dissected temporal bone specimens. The tympanic segment of the facial nerve was observed to incline inferiorly and descend obliquely making a 30°–35°angle with the horizontal in 28 (93.33%) temporal bones, while in two (6.67%) temporal bones, the course was almost horizontal from the geniculate ganglion to the second genu. Dehiscence of this segment of the nerve was not observed in any of the specimens.
The angle formed by the second (tympanic) and third (mastoid) portions of the facial nerve varied between 95°and 120°with the mean angle being 104.66°±7.760°. The maximum number of temporal bone specimens, i.e., 8 (26.67%) had an angle of 95°which was closely followed by seven (23.33%) temporal bones with an angle 105°[Table 2]. In the present study, the length of the mastoid segment of facial nerve was ranged between 9 and 17 mm with the mean length calculated to be 13.44±2.202 mm [Table 3] and [Figure 1]d and [Figure 1]e.
The mastoid segment descended vertically up to the stylomastoid foramen in 25 dissected specimens, i.e., 83.33% in three (10%) specimens, the nerve was observed to descend slightly medially, while in two (6.675) specimens, the nerve descended slightly laterally. Dehiscence of this segment was not observed in any of the temporal bones.
In all the dissected specimens, the nerve was found to exit the temporal bone through the stylomastoid foramen, which opens at the base of the petrosa between the mastoid and the styloid process.
| Discussion|| |
The great pioneer otologist, Bezold, in his textbook of otology, warned about “the danger to the patient of an incompetent operator, who does not know the many anatomical details crowded together in the narrow space of the temporal bone and their extreme variability, which is much greater here than in any other region of the body.”
The patient who suffers from facial paralysis experiences not only functional consequences but also a psychological impact of a change in self-image and impaired communicative ability. In fact, a 1991 poll revealed that the level of discomfort that Americans felt on meeting those with facial paralysis was second only to that associated with interacting with the mentally ill.
Throughout the era characterized by frequent operations for mastoiditis, one of the principal operative hazards was traumatic injury to the endotemporal segment of the facial nerve, resulting in temporary or permanent facial paralysis. Although facial nerve injury is at times unavoidable because of the extent of disease, most cases of postoperative facial paralysis occur as a result of unrecognized facial nerve trauma at the hands of an unskilled otologic surgeon. Unfortunately, minor variations and major anomalies often occur in the course of the facial nerve, predisposing the nerve to inadvertent surgical injury. Moreover, normal surgical landmarks are often distorted in the diseased mastoid, and hence positive identification of vital structures is mandatory to perform a successful procedure. Therefore, the surgeon should use as many of the available anatomical landmarks  which act as a frame of reference for the facial nerve.
In the present study, a total of thirty wet human temporal bones were dissected, out of which 17 (56.66%) were right-sided bones and 13 (43.33%) were left-sided bones.
The length of the tympanic or horizontal part of the facial nerve was measured and found to be 10.4±1.428 mm (8–13 mm). It was in agreement with the studies by Măru et al., Kharat et al., Sentürk et al., Yadav et al., and Guerrier  who reported the length of the tympanic segment of the facial nerve to be 10.25±0.75 mm (9.15–12.03 mm), 9.28±1.13 mm (7–12 mm), 9.56±1.54 mm (7.30–13.5 mm), 11.1 mm±0.88 (9–13 mm), and 11 mm, respectively. Schaitkin and May, Wilbrand, Botman and Jongkees, and Rulon and Hallberg  reported the length of the tympanic segment of the facial nerve to vary from 8 to 11 mm. Findings of the present study were also comparable to a study of Dimopoulos et al. and Proctor  who reported the tympanic segment of the facial nerve to be 11.4±1.24 mm and 10–12 mm long, respectively, while Jepsen, Kudo and Nori, and Ezzat et al. observed its length to be 12–13 mm, 12 mm (8.67–15.60 mm), and 12.50±1.04 mm, respectively. Observed mean length of the tympanic segment of the facial nerve by Botros  was 8 mm, which is less as compared to the present study.
The cochleariform process is a consistent landmark for identifying the tympanic segment of the facial nerve, even when other landmarks are obscured or have been destroyed by pathology. Its location is like an anatomical guide to identify and localize the facial nerve at any stage of operation. In the present study, the anterior part of the tympanic segment of the facial nerve in 28 (93.33%) specimens was observed to lie slightly above and medial to the cochleariform process, while in only two (6.66%) specimens, the cochleariform process was adjacent to the tympanic segment of the facial nerve. These findings were consistent with Proctor, Kharat et al., and Bibas et al. who have also reported the anterior end of the tympanic segment to lie above and medial to the cochleariform process but were slightly different to that observed in the study of Yadav et al. and Măru et al. who noticed the tympanic segment of the facial nerve to lie slightly above and medial to the cochleariform process in 68% and 60% of the specimens and adjacent to it in 32% and 40% specimens, respectively.
The middle portion of the tympanic segment nerve was found to lie above the oval window and stapes with no bony overhang in all the thirty (100%) temporal bones in the present study, and these findings were consistent with the study of Bibas et al., Ezzat et al., and Yadav et al. Măru et al. in a study on 35 temporal bones also observed similar relation but with two cases having tympanic segment covering more than half of the oval window while Kharat et al. in a study of 25 temporal bones established the same relation with one case of overhang of the facial nerve in the region of the oval window, measuring about 2 mm × 1.5 mm.
The posteroinferior part of the tympanic segment was found to pass under the lateral semicircular canal in all the thirty cases. These findings were consistent with those reported in literature by Bibas et al., Proctor, Măru et al., Yadav et al., Ezzat et al., and Kharat et al.
The tympanic segment was observed to course inferiorly and descends obliquely making an angle of 30°–35°with the horizontal in 28 (93.33%) temporal bones, while in two (6.66%) specimens, the trajectory of the nerve was almost horizontal. Findings of the present study were similar to those reported in the literature by Procter, Botman and Jongkees, and Rulon and Hallberg. Yadav et al. and Măru et al. also reported similar course being followed by the tympanic segment but with different numbers. The tympanic segment in 80% specimens, in their studies, were reported to incline inferiorly and descend obliquely, while in 20% specimens, the course was horizontal and parallel to the plane of the horizontal semicircular canal. Kharat et al. reported the degree of descent of the tympanic segment in relation to horizontal to be 30°in 100% cases, while Măru et al. found it to be varying between 5°and 37°.
Developmental dehiscence of the tympanic segment is not uncommon, making the nerve vulnerable during middle ear surgery, especially around the oval window. In the present study, dehiscence of the tympanic segment was not seen in any of the temporal bone dissection specimens. These findings were contrary to those reported by Yadav et al. and Kharat et al. who observed bony dehiscence in the tympanic segment in 12% of specimens in their respective studies and Măru et al. who reported the rate of facial canal dehiscence in the tympanic segment to be 14.28% (5 of 35 cases), with four of the five cases involving the lateral aspect of the facial canal in the oval window area. Kim et al., Bayazit et al., and Li and Cao  also reported dehiscence involving the tympanic segment with slightly varying rates, i.e., 8.6%, 8.9%, and 11.4% in their respective studies. Baxter  in 535 temporal bones study found that 55% of the facial nerves were dehiscent, with 91% being located in the tympanic segment. Eighty-three percent of these were located adjacent to the oval window involving the lateral, inferior, and medial portions of the canal, with the nerve protruding from its canal in 26%. Dietzel, who had examined 211 temporal bones, also found a high incidence of 57%.
The vertical portion of the facial nerve extended from the second turn to the stylomastoid foramen, and its mean length in the present study was found to be 13.44±2.202 mm (range: 9–17 mm).
Findings similar to the present study were also observed by Procter, Kharat et al., Dimopoulos et al., Kullman et al., Măru et al., and Şentürk et al. who in their respective studies observed the mean length of the mastoid segment to be 13 mm, 13.7±1.45 mm, 13.9±1.97 mm, 12.2 mm, 13.78±1.12 mm, and 12.28±1.90 mm. Yadav et al. and Qiu et al. recorded the length of the mastoid segment in their respective studies to be 15.4±2.4 mm (10–20 mm) and 16.2 mm which were much longer as compared to the present study.
The mastoid segment of the facial nerve in the present study descended vertically up to the stylomastoid foramen in 25 dissected specimens, i.e., 83.33% in three (10%) specimens the nerve was observed to descend slightly medially, while in two (6.675) specimens, the nerve descended slightly laterally. Similar findings were also reported by Nager and Proctor. Yadav et al. in their study reported that in 60% of specimens, the mastoid segment of the nerve was showing a vertical descent up to stylomastoid foramen, in 20%, it was observed to descend slightly laterally, and in 20%, the nerve had a medial descent, while Sendulski et al. reported that 55% descended medially, 16% laterally, and 29% in sagittal plane.
In the present study, dehiscence involving the mastoid segment was not observed in any of the dissected temporal bone specimens. These findings were concurrent with the study of those reported by Proctor, Kharat et al., Măru et al., and Yadav et al.
| Conclusion|| |
The present study has provided knowledge of the anatomy of the tympanic and mastoid segment of the facial nerve, its variations in course and length and relation of various middle ear landmarks with the facial nerve. The otologic surgeon must be aware of all these details to avoid any disaster and alleviate the fear regarding facial nerve.
“The surgeon learned in anatomy, with the knowledge and skill learned in the dead house may safely traverse the perilous narrow ocean of the operation in the fallopian aqueduct.”
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Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3]
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