Abstract
Cadaveric dissections helps aspiring ear surgeon to learn anatomy of temporal bone so as to avoid injury to the many vital structures. Objectives are to study the course of facial nerve and its variation in 50 cases of temporal bone dissection. To study the various parameter of the tympanomastoid segments of facial nerve, its relation with the important middle ear structures and their variations. To study the incidence of anatomical variations such as dehiscence of bony canal of facial nerve, bony overhang in the oval window region. The Current study was conducted at Dept. of ENT Temporal bone lab. 50 temporal bones were dissected to study the various parameters of the tympano-mastoid segments of the facial nerve, its relations with the important middle ear structures and their anomalies. The present study was carried out from Jan 2016 to Jan 2019. Out of 50 bones dissected, 44 (88%) bones were well pneumatised and 6(12%) bones were sclerotic. Length of the tympanic segment varied from 7.8 to 11.88 mm with a mean of 9.47 mm (± 1.06 mm). In 17 bones (34%) length of the mastoid segment was 13.1–14 mm, in 12 bones (24%) the length was 14.1–15 mm, while in 10 bones (20%) it was 12–13 mm. In 6 bones (12%) it was 15.1–16 mm and < 12 mm in 5 other bones (10%). The distance of second genu from outer cortex varied from 17.87–23.67 mm with a mean of 19.64 mm (± 1.42 mm).Distance of chorda tympani from stylomastoid foramina varied from 3.4–7.8 mm with a mean of 5.69 mm (± 1.51 mm). The study has provided knowledge of anatomy of temporal bone and its pneumatization pattern, the anatomy of the facial nerve, its variations and their incidence, relation of various middle ear landmarks with the facial nerve and proficiency in dissection.
Keywords: Temporal bone, Anatomy, Facial nerve, Dissection
Introduction
Many facial expressions like joy, sorrow and other human expressions is a result of 7000 motor fibres of facial nerve firing in unison and a time. Facial nerve dysfunction causes disfigurement, emotional distress leading to detriment in social interaction. Inability to smile due to a surgeon’s faulty technique is an unforgivable crime. Only by cadaveric dissections can the aspiring ear surgeon learn to safely traverse the perilous anatomy of temporal bone so as to avoid injury to the many vital structures concealed in an area no larger than an olive [1]. The facial canal, as it traverse the temporal bone, may display congenital bony dehiscence, variation, and anomalies in its usual course and in rare instance, may include a large persisting embryonic artery or vein each of this feature can have clinical and surgical significance.
The otologic surgeon wants to be familial with these variations and must suspect and identify them in order to avoid injury to the facial nerve [2].
Aims and Objectives
(1) Objectives are to study the course of facial nerve and its variation in 50 cases of temporal bone dissection.
(2) To study the various parameter of the tympanomastoid segments of facial nerve, its relation with the important middle ear structures and their variations.
(3) To study the incidence of anatomical variations such as dehiscence of bony canal of facial nerve, bony overhang in the oval window region.
Methods
The Current study was conducted at Dept. Of E.N.T, Temporal bone lab.50 temporal bones were dissected to study the various parameters of the tympano-mastoid segments of the facial nerve, its relations with the important middle ear structures and their anomalies. The present study was carried out from Jan 2016 to Jan 2019. Study procedures done were modified radical mastoidectomy and Facial nerve decompression.
-
I.
Inclusion criteria:
-
1.
Temporal bone of adult.
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II.
Exclusion criteria:
Diseased bone and cholesteatoma bone.
Results
Type of Bone
Out of 50 bones dissected, 44 (88%) bones were well pneumatised and 6(12%) bones were sclerotic. As shown in Fig. 1.
Fig. 1.
Type of Bone
Tympanic Segment
Length of the tympanic segment varied from 7.8–11.88 mm with a mean of 9.47 mm (± 1.06 mm). In 20 bones (40%) the length of the tympanic segment was 8.6–9.5 mm, In 13 bones (26%) it was 9.6–10.5 mm, while in 8 bones (16%) it was 7.5–8.5 mm. In 7(14%) bones it was 10.6–11.5 mm, in 2 bones (4%) it was > 11.5 mm as shown in Table 1.
Table 1.
Length of tympanic segment
| Length (mm) | No. of bones (%) |
|---|---|
| 7.5–8.5 mm | 8 (16) |
| 8.6–9.5 | 20 (40) |
| 9.6–10.5 | 13 (26) |
| 10.6–11.5 | 7 (14) |
| > 11.5 | 2 (04) |
In all the 50 temporal bones tympanic segment of the facial nerve, turned posterior from the geniculate ganglion, the processus cochleaformis and tensor tympani were anterior to the nerve. The nerve passed above promontory and the oval window niche was inferior to the nerve. The nerve passed below the horizontal semi circular canal in all 50 temporal bones.
Dehiscence in the tympanic segment was observed in two bones:
Dehiscence in Tympanic Segment
In the first case dehiscence was noted in the tympanic segment. It was approx. 2 × 1 mm, oval in shape with smooth margin and the sheath of facial nerve was normal.
In the second case dehiscence was noted in the tympanic segment, it was approx. 1 × 1 mm, which was oval in shape with smooth margin and the sheath of facial nerve was normal.
Bony Overhang in Oval Window
Bony overhang in oval window seen in 8 case its 2 mm in 20 cases it’s in range of 2 to 2.5 mm and 14 cases it found in range of 2.5 to 3 mm in 8 cases there was no significant bony overhang.
Length of Mastoid Segment
The length of mastoid segment varied from 10 to 16 mm, with a mean of 13.59 mm (± 1.27 mm). In all 50 temporal bones, the mastoid segment of facial nerve was anterior to the tip of mastoid and medial to tympanomastoid suture.
In 17 bones (34%) length of the mastoid segment was 13.1–14 mm, in 12 bones (24%) the length was 14.1–15 mm, while in 10 bones (20%) it was 12–13 mm. In 6 bones (12%) it was 15.1–16 mm and < 12 mm in 5 other bones (10%). As shown in Table 2.
Table 2.
Length of vertical segment
| Length (mm) | No. of bones (%) |
|---|---|
| < 12 | 5 (10) |
| 12.1–13 | 10 (20) |
| 13.1–14 | 17 (34) |
| 14.1–15 | 12 (24) |
| 15.1–16 | 6 (12) |
Distance of Second Genu from Outer Cortex
The distance of second genu from outer cortex varied from 17.87–23.67 mm with a mean of 19.64 mm (± 1.42 mm). The distance of second genu from outer cortex was 18.6–19.5 mm in 14 bones (28%) each. In 14 bones (28%) it was 19.6–20.5 mm, while in 10 bones (20%) the distance was 17–18.5 mm. In 6 bones (12%) the distance was found to be 20.6–21.5 mm and in 4 bones (8%) it was 21.6–22.5 mm, while in 1 bone (2%), it was 22.6–23.5 mm. Another 1 bone (2%) it was > 23.5 mm. As shown in Table 3.
Table 3.
Distance of second genu from outer cortex
| Length (mm) | Percentage (%) |
|---|---|
| 17.5–18.5 mm | 10 (20) |
| 18.6–19.5 | 14 (28) |
| 19.6–20.5 | 14 (28) |
| 20.6–21.5 | 06 (12) |
| 21.6–22.5 | 4 (08) |
| 22.6–23.5 | 1(2) |
| > 23.5 | 1 (2) |
The angle between tympanic and mastoid segment varied between 95 and 125 degree with a mean of 109.2 degree (± 11.05 mm).
Distance of Chorda Tympani from Stylomastoid Foramen
Distance of chorda tympani from stylomastoid foramina varied from 3.4–7.8 mm with a mean of 5.69 mm (± 1.51 mm). In 12 bones (24%) distance of chorda tympani from stylomastoid foramina was 4.1–5.0 mm, In 11 bones (22%) it was 7.1–8.0 mm, in 10 bones (20%) it was 5.1–6.0, in 9 (18%) bones it was 6.1–7.0 while in 8 (16%) bones it was 3.1–4.0 mm. As shown in Table 4.
Table 4.
Distance of chorda tympani from stylomastoid foramen
| Length (mm) | Percentage (%) |
|---|---|
| 3.1–4 mm | 8 (16) |
| 4.1–5 | 12 (24) |
| 5.1–6 | 10 (20) |
| 6.1–7 | 9 (18) |
| 7.1–8 | 11 (22) |
Discussion
Facial expressions controlled by facial nerve, is an important means of social communication in human beings. The facial nerve surpasses all other nerves in the human body with respect to the length and tortuousity of its intra-osseous course through the temporal bone. The disease of the middle ear, may involve the facial nerve, leading on to its paresis or paralysis, more so, if there is any developmental anomaly. With the advent of microscope in the ear surgery, there is a renewed interest amongst otologists to study the microscopic anatomy. It is now well established that in developmental anatomy lay the means of understanding the previously baffling aspects of adult structures of the human ear and temporal bone [3]. The bony architecture itself and the neurovascular structures in and around it make the temporal bone a difficult region to perform surgery. The localization of these important structures, such as the facial nerve and related surrounding relations in this bony framework might be challenging for the inexperienced surgeon. This difficulty may partly be overcome by pre-operative radiological evaluation. However, careful dissection in light of various landmarks is still a major tool for successful surgery. Therefore, the landmarks of the temporal bone with their relation among each other should be very well known. If the anatomical landmarks are followed and extra caution taken, iatrogenic injury to the facial nerve can be prevented,
The mean length of tympanic and mastoid segment was found to be 9.47 ± 1.06 mm (range 7.8–11.88 mm), 13.59 mm ± 1.27(range 10 to 16 mm) respectively, which is considerably at variance to different cranial configuration of various races. As shown in Table 5.
Table 5.
Length of facial nerve segments of Japanese, Americans and Indian subjects
| Race | Tympanic segment (mm) | Mastoid segment (mm) | ||
|---|---|---|---|---|
| Max | Min | Max | Min | |
| Japanese (Kudo and Novi, 1970) | 15.6 | 8.67 | 15.7 | 11.8 |
| American (Rulon and Hallberg, 1962) | 11 | 8 | 14 | 9 |
| Indian (Present study) | 11.88 | 7.8 | 16 | 10 |
In the present study the length of tympanic segment varied from 7.5–11.88 mm with a mean length of 9.47 mm (± 1.06 mm). The minimum length noted was approximately 7.8 mm in 16% of specimens. Minimum length of 7 mm has been noted by Agarwal (1993) and Kharat et al. [1] which was similar to the present study.The maximum length of the tympanic segment was approximately 11.5 mm (seen in 4% of specimens) in the present study which is similar to the maximum limit of length shown in the study of Kharat et al. (2009) and Proctor B [1991] [4].
In all 50 temporal bones dissected, the tympanic segment of facial nerve turned posterior from geniculate ganglion. Tympanic segment of the nerve was found to lie above the oval window and below the horizontal semi-circular canal, above and medial to processus cochleariformis and tensor tympani in all 50 specimens. Similar findings have been reported by most of the other authors.
In the present study the length of mastoid segment varied from 10 to 16 mm with a mean length of 13.59 mm (± 1.27 mm). The minimum length noted was approximately 10 mm in 10% of specimens. Minimum length of 7 mm has been noted by Agarwal (1993), so it is more than Agarwal study [5]. Kharat et al. (2009) showed minimum length 10 mm, which was similar to the present study. The maximum length of the mastoid segment was approximately 16 mm (seen in 12% of specimens) in the present study which is similar to the maximum limit of length shown in the study Kharat et al. (2009) and Proctor B [1991], In the present study the degree of descent was noted as 30 degree in all specimens, which is same as that noted by all the authors except Proctor B, who showed degree of descent as 37 degree.
Dehiscence of the bony facial canal in the horizontal segment within the middle ear entails a risk of immediate or delayed facial paralysis during middle ear surgery. The facial canal containing the facial nerve over the oval window was observed to show bony dehiscence in 12% by some authors and some workers reported dehiscence in the range of 6 to 70% (Guild, Hough, Beddard and Saungers) [6–8]. In the present study, out of 50 temporal bones dissections, dehiscence in the tympanic segment, were found in 2 bones. In 2 bones dehiscence was noted at tympanic segment. The incidence of dehiscence comes to 4%. The incidence is low as compared to most authors, but it is approximately similar with the study of Yadav S.P.S. et al. (2006) and Kharat et al. (2009) [9].
In the present study, the distance of second genu from outer cortex varied from 17.87 mm with a mean of 23.67 mm (± 19.64 mm). The range of distance of second genu from outer cortex in present study is concurrent with the study of Yadav S.P.S. et al. (2006). Chorda tympani with Intratemporal origin have been found in the present study arise at a mean of 5.69 mm (range 3.4–7.8 mm) proximal to the stylomastoid foramen. This distance from stylomastoid foramen was similar with corresponding mean distance of 5 mm (range 1.0–11.0 mm) and 4.8 mm (range 0–10 mm) fond by Nager GT and Proctor B, and Muren C et al. respectively [10]. We agree with Wetmore SJ that the chorda tympani used as a landmark to identify the facial nerve in the mastoid by tracing it inferiorly to its origin [11].
Fowler (1961) has observed 7 patients, with the facial nerve was in a more lateral position than usual. This indicates the rarity of this variation but one should keep the possibility in mind to avoid injury to the facial nerve [12].
Over the year, the variations in the course of the mastoid segment of the facial nerve have become apparent. It does not always follow the vertical course as described in textbooks, but shows variations of surgical significance between the second genu and stylomastoid foramen.
In the present study of 50 temporal bones, 88% of bones were pneumatised and 12% were sclerotic. This finding is concomitant with the literature. The high degree of pneumatization makes the dissection easier but may predisposed the individual to pneumatocele because the bones were very thin whereas the poorly pneumatised makes the dissection slower in other to avoid injury to underlying structure and also not to miss the landmark; however most of the structures like the facial nerve and it canals maintain their anatomic position despite the few marrow spaces.
Acknowledgements
Study is conducted at ENT department of our institute thanks to all my seniors and entire colleague for their kind support.
Funding
No funding sources
Compliance with Ethical Standards
Conflict of interest
None declared
Ethical approval
This study approved by the institutional ethical committee
Footnotes
Publisher's Note
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References
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