|Year : 2011 | Volume
| Issue : 2 | Page : 71-74
Temporal bone dissection skill: A necessity for life otologic surgeries?
Samuel A Adoga1, Nuhu D Maan1, Benjamin T Ugwu2, Babatanko M Umar3, George O Nwaorgu4
1 Department of Otorhinolaryngology, Jos University Teaching Hospital, Jos, Nigeria
2 Department of Surgery, Jos University Teaching Hospital, Jos, Nigeria
3 Department of Anatomy, University of Jos, Jos, Nigeria
4 Department of Otorhinolaryngology, University of Ibadan and University College Hospital, Ibadan, Oyo State, Nigeria
|Date of Web Publication||20-Dec-2011|
Samuel A Adoga
Department of Otorhinolaryngology, Jos University Teaching Hospital, Jos
Source of Support: None, Conflict of Interest: None
Background: The anatomy of the temporal bone is complex and not easily learned. Hours of study in the temporal bone laboratory are required for a good grasp by the intending otologic surgeon in order to avoid predictable complications. Aim and Objective: This study aims at highlighting the steps involved toward acquiring the necessary skills and understanding this complex anatomy before embarking on life otological surgeries (temporal bones surgeries and cochlear implant) in our center. Materials and Methods: This was a prospective study of cadaver temporal bone dissection conducted over a period of 3 months. A total of 10 dry, formalin-fixed cadavers were used for the dissection. A team of doctors headed by a consultant otolaryngologist carried out the dissections on the cadavers. The landmark of importance for the dissections was the McEwen's triangle. From this starting point, various otologic surgeries were performed hands-on on the cadavers using the appropriate burs and their sizes. Anatomic features encountered during the dissection were noted and recorded. Results: The 10 cadavers (100%) were all adult males. The youngest and oldest cadavers were aged 25 and 45 years, respectively, with an overall mean age of 38.9 years. The interval between death and embalmment varied from 5 to 79 days. The suprameatal crest, dural plate, aditus and antrum were all present in the 20 temporal bones dissected. Cribrifossae and wide marrow mastoid cavity were noted in 17 (85%) temporal bones each, highly pneumatized mastoid, herald air cells and incus were seen in 14 (70%) each, tympanic remnant was seen in 13 (65%) and stapes in 6 (30%). Conclusion: Temporal bone dissection provides an avenue in understanding the anatomic features and the variations that may pose a challenge in cochlear implant and other otologic surgeries and it enhances the dexterity of the otologic surgeon.
Keywords: Cadaver, Dissection, Necessity, Nigeria, Temporal bone
|How to cite this article:|
Adoga SA, Maan ND, Ugwu BT, Umar BM, Nwaorgu GO. Temporal bone dissection skill: A necessity for life otologic surgeries?. Indian J Otol 2011;17:71-4
|How to cite this URL:|
Adoga SA, Maan ND, Ugwu BT, Umar BM, Nwaorgu GO. Temporal bone dissection skill: A necessity for life otologic surgeries?. Indian J Otol [serial online] 2011 [cited 2019 Mar 22];17:71-4. Available from: http://www.indianjotol.org/text.asp?2011/17/2/71/91041
| Introduction|| |
The temporal bone anatomy is complex. Hours of study are required for the three-dimensional conceptualization by intending otologic surgeons.
Unfamiliarity with the surgical anatomy leads to predictable complications during surgery. ,,, Mastoidectomies, tympanoplasties, cochlear implantation, etc. are performed through acquisition of temporal bone dissection skills.  Temporal bones' dearth and low awareness for will to body donor program hampered the acquisition of otologic skills in some developing countries. ,
Twenty temporal bones were dissected over a period of 3 months as a sequel to cochlear implantation and other temporal bone surgeries in our center. This formed the basis for this study.
| Materials and Methods|| |
We obtained the institutional permission from the ethics board of the Faculty of the Medical Sciences, University of Jos, Nigeria, to carry out this prospective study. We then dissected 20 consecutive temporal bones from 10 formal in-fixed cadavers after soaking them in water for about 30 minutes prior to dissection. The landmarks for the dissections were the spine of Henle, the "inferior temporal line" or the suprameatal crest at the point of insertion of the inferior temporalis muscles and a tangent through the posterior wall of the external acoustic meatus of right and left temporal bones to meet the inferior temporal lines. This forms the McEwen's triangle which lies directly over the mastoid antrum as depicted in [Figure 1].
We used the largest cutting burr of 6 mm to mark out the McEwen's triangle above. Starting at the junction where the two lines met, which overlay the mastoid antrum, the tegmen mastoidum was deepened so that the dural plate, sinodural angle and the posterior wall of the external meatus became thinned out while maintaining copious irrigation to keep the bone dust from obscuring our views. The dissection was continued until the aditus to mastoid antrum was met. Thereafter, the operating microscope was used to aid visualization in order to locate the incus at the fossa incudus without damaging it. A smaller drill bit of 3 mm cutting burr was further used for drilling the marrow until the horizontal semicircular canal was viewed. Thereafter, a less traumatic approach using 2 mm diamond burr was used to continue drilling. With a slightly angulated needle, the herald (retrofacial) air cells were sorted and drilled through to visualize the facial recess bound superiorly by the incus, medially by the facial nerve and laterally by the chorda tympani without damage to any of them or the annulus of the tympanic membrane.
This step brings into view the incudostapedial joint, the promontory or medial wall of the middle ear and part of the round window membrane because the niche obscures the full view of the posterior part of the middle ear. Next, the niche was drilled out, exposing the full view of the round window membrane. Finally, an inferior cochleostomy was performed by drilling the inferior part of the round window membrane into the scala tympani using 1 mm diamond burr. Other mastoid surgeries such as facial nerve exploration, canal wall up and wall down, atticotomy and fenestration of lateral semicircular canal were performed hands-on on these specimens.
The important landmarks noted during the dissections included: the inferior temporal lines (suprameatal crests), the spine of Henle, cribriform fossae, dural plates, degrees of pneumatization, presence of aditus to the mastoid antrum, ossicles (malleus, incus, stapes), horizontal semicircular canals, herald air cells, facial nerves, tympanic membranes and the mastoid cavities. These were identified and recorded.
The equipment used included: temporal bone dissection microscope, House-Urban temporal bone holder, NSK Volvorere GX (NSK Nakanishi Inc., Japan), high-speed (reverse and forward cutting) drill up to 35,000 rpm with serial no. 12964253, cutting burrs of 6 mm, 3 mm, diamond burrs of 2 mm and 1 mm, slightly angulated needle, curette, micro scissors (all from Jull Surgical, New Delhi, India), drip stand, operating goggle and apron.
| Results|| |
The results of the 17 important surgical anatomical features noted during the dissection of the 20 cadaver temporal bones are as shown in the histogram given in [Figure 2].
|Figure 2: Histogram showing the number of anatomical features actually present against the anatomical l temporal bone dissected. a. Suprameatal crest, b. Spine of Henle, c. Cribriform fossae, d. Dural plate, e. Jugular plate, f. Degree of pneumatisation, g. Aditus to mastoid antrum, h. Ossicles , i. Horizontal semi circular canal, j. herald air cells, k. facial nerve (fallopian) canal, m. Facial recess, n. tympanic membranes remnant, o. Mastoid cavity, p. Oval windows, q. Round window|
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East represents normal (n=20); West represents actual anatomic features present in each cadaver temporal bone dissected except in the degree of pneumatization where East, West and North represent highly, moderately and poorly pneumatized, respectively; and in Ossicles, East, West and North represent incus, stapes and malleus, respectively.
| Discussion|| |
The age of the cadavers ranged from 25 to 45 years, with an overall mean of 38.9 years. The interval between death and embalmment varied between 5 and 79 days. In the interval, the cadavers were frozen before embalmment with formalin was subsequently carried out. All the cadavers used for our dissection were of male gender. This finding is in discordance with that of Fernando et al.  who reported the age range of 55-80 years with a mean age of 67 years in Brazil, which is higher than that obtained in our study. They had five females and a male cadaver which is in sharp contrast to our study.
Most of the normal anatomic features such as suprameatal crest, dural plate, aditus to mastoid antrum, lateral or horizontal semicircular canal, facial nerve (Fallopian) canal, etc. were consistently present in the 20 (100%) temporal bones dissected from the 10 cadavers. Twenty canal wall up and canal wall down mastoidectomies were alternately performed. However, variations were noted in the other anatomic structures, notably the cribrifossae and wide marrow mastoid cavity which constituted 17 (85%) each, highly pneumatized mastoid, herald air cells and incus present in 14 (70%) each, tympanic remnant in 13 (65%), stapes in 6 (30%), spine of Henle, poorly pneumatized mastoid and malleus in 4 (20%) each, narrow mastoid cavity in 3 (15%) and moderately pneumatized mastoid in 2 (10%), as depicted in [Figure 2].
The 20 temporal bones in our study had intact Fallopian canal More Details and this might be due to the relatively few number of temporal bones dissected even though availability of cadaver is difficult; this is not in tandem with the work of Ozbek  who had reported 55 cases out of 118 in his study. Coker and co-workers  found that the most common site of dehiscence and iatrogenic injury during mastoid surgery in the tympanic segment was over the oval window. Identifying the Fallopian canal dehiscence is very important in order to avoid iatrogenic injury to the facial nerve during operation along the course of the facial nerve.
In this study, we had 70% well-pneumatized, 10% moderately pneumatized and 20% sclerotic mastoids, but we did not compare the bilaterality of mastoid air cells. This is only a slight variation from the findings of Grooves et al. where they had 80% cellular mastoid and 20% diploeic and sclerotic mastoid air cells. The high degree of pneumatization makes the dissection easier, but may predispose the individual to rare complications like pneumocephalus after nose blowing post-cochlear implant surgery, as noted by Hagr  in Canada. It also tends to make landmark distances longer as opined by Aslan and co-workers  in Ankara, Turkey, where the distances measured from facial nerve to external auditory canal, sigmoid sinus, posterior dural fossa and lateral surface of the mastoid were longer. The poorly (sclerotic) pneumatized mastoids have many arguable etiologies, but the more consistent ones include previous otitis media and Eustachian tube More Details dysfunction as observed by Pakira et al. and Lee et al. in India and Korea, respectively, where they noted that these two conditions make mastoid air cells become sclerotic.
The consequence upon dissection of sclerotic mastoid is slower progression in order to remain within the landmark and avoid causing iatrogenic injury to the underlying structures. Aslan et al., in Manisa, Turkey, noted the correlation between pneumatization of the cells surrounding the sinodural angle and the distances between sigmoid sinus and facial nerve at the midpoint of its vertical distance in poorly pneumatized mastoid where they obtained decreased values of the sinodural angles during dissection of the mastoid at three different points, namely, the sinodural angle area, inter-sinofacial area and mastoid apex. However, generally speaking, Lee et al.,  in Korea, observed that most of the structures like the facial nerve and its canals maintain their anatomic position despite the few marrow spaces. The anterior projection of the sigmoid sinus into the mastoid antrum produced narrow angle mastoid cavities in 3 (15%) of our temporal bones. This made the access to facial recess during dissection challenging because the facial nerve canal was closer to the posterior wall of the external acoustic meatus. This finding concurs with that of Lee and co-workers,  also in Korea, who noted that mastoid pneumatization is the most important factor that influences the access to facial recess during mastoid surgery.
The tympanic membrane remnants were present in 13 (65%) and absent in 7 (35%) cases. This absence may have been due to previous cases of otitis media or destruction by other disease processes. Benitez et al. and Terao et al. observed tympanic membrane destruction in otitis media of Egyptian mummy and five temporal bones of leukemic patients.
The ossicles, especially incus, were present in 14 (70%), stapes in 6 (30%) and malleus in 4 (20%) of our cases. These may have been absent due to diseases or traumatic dislodgement as noted by Benitez,  Terao,  Wysocki  and co-workers.
The mastoid antrum was consistently found within the McEwen's triangle in all the 20 (100%) temporal bones. This has made it truly the first important landmark in the commencement of mastoid surgery as observed long ago by Catherine  and Pickard. 
Overall, 20 canal wall up and canal wall down dissections were carried out providing good surgical experience in temporal bone surgeries.
[Figure 3] illustrates a dissected temporal bone.
The formalin-fixed soft tissue did not allow for easy soft tissue work like various otologic skin incisions on the cadaver. All the temporal bones were of male adults, hence gender and pediatric differences of temporal bone dissections could not be studied.
| Conclusion|| |
Temporal bone dissection provides the avenue in understanding the anatomic features, the variations that may pose a challenge in cochlear implantation and other temporal bone surgeries, as it enhances the dexterity of the otologic surgeon. The use of fresh cadaver will provide the nearest ideal to human and more anatomic features will be visualized to enhance greater dexterity and navigation to make our surgical skills better.
| Acknowledgment|| |
We acknowledge the various contributions and the invaluable assistance by the technicians in Anatomy Department, Faculty of Medical Sciences, and University of Jos, Nigeria.
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[Figure 1], [Figure 2], [Figure 3]