Safer Ear Surgery: Anatomical Study of Landmarks of the Tympanomastoid Segment of the Facial Nerve

Objectives: To study the variations of the tympanomastoid segment of the facial nerve in relation to fixed known anatomical landmarks in cadaveric temporal bone dissection to provide a more detailed description of the anatomy of the nerve. Materials and Methods: We studied 35 adult temporal bones by dissecting them at Temporal bone lab in A in Shams University Specialized Hospital. We studied the variations inthe course of the facial nerve by exposing the nerve completely throughout its course in the middle ear. The distance between geniculate ganglion and the cochleariform process, and the distance between the facial nerve and cochleariform process were measured. Furthermore, the length of the tympanic part of the facial nerve and the distance between the second genu angle and the oval .window niche were measured. The angulations of the second genu, measured using a simple protractor, were correlated with previous anatomical landmarks. The relation between the vertical mastoid segment exit to the digastric ridge was recorded. Results: The present study observed that the tympanic segment of the facial nerve extending from the geniculate ganglion to the horizontal semicircular canal lies above the cochleariform process, with a constant distance = 1mm in all the dissected temporal bones, while the distance of the geniculate ganglion (antero-superior) to cochleariform process was variable and ranged from 2-4 mm (mean=2.86 mm ± 1.5mm). When we measured the length of the tympanic part of the facial nerve, we found that the mean length of it equals 12.50± 1.04 mm. As regards the second genu angle (SGA), it was obtuse in 28 bones (80% ). On the other hand, it was a right angle in 5 bones (15%). . .and Was acute in two bones only (5%). Expectedly; there was a wide range of variations in the measurement of the SGA with an overall mean= 116.97° (± 18.07). We found that the distance between the SGA of the facial nerve and the oval window niche= 3.314 mm when the SGA = 116.97°. We were able to find a linear relation between the three variables. The more the tympanic part of the facial nerve is shorter, the more obtuse the angle is and the more the nerve comes closer to the oval window. On the other hand, the more the tympanic part of the facial nerve is longer, the more acute the angle is and the more the nerve is farther separated from the oval window. Completing the exploration of the fac ial nerve down to the digastric ridge, we found an angulation in the vertical part of the nerve in three specimens ( 12%), and we propose to name it a third genu. Furthermore, the exit of the facial nerve varied in relation to the digastric ridge; it was medial in 51%, or at the same level in 40%, but surprisingly it was lateral to it in 9%. Conclusion: Our results open a new way of thinking in the anatomical variations of the course of the facial nerve. Preoperative analysis of the specific facial nerve measurements reported in the present work provide a growing evidence to help to anticipate the position of the facial nerve accurately in relation to fixed anatomical landmarks in the mastoid, thus minimizing iatrogenic facial nerve injury in various ear operations.