Cadaveric Temporal bone dissection: a Study of the Variations in the Course of the Facial Nerve by Measuring the Second Genu Angle

Objective: To study the variations in the course of the facial nerve by studying the second genu angle (S.G.A.) aiming to provide a more detailed description of the anatomy of the nerve, thus avoiding its injury during middle ear surgery. Material and Methods: 35 adult temporal bones were studied by dissecting them at the temporal bone laboratory in Ain Shams University Specialized Hospital (ASUSH). The variations in the course of the facial nerve were studied by measuring the S.G.A., the length of the tympanic segment of the nerve as well as the distance between the S.G.A. and the oval window niche., in an attempt to find the relationship between these 3 variables. Results: The study conveyed on 35 temporal bones showed that 28 (80%) had an obtuse S.G.A. of the facial nerve, 5 (15%) had a right S.G.A. and 2 (5%) had an acute S.G.A. with an overall mean of the S.G.A. was 116.97° + 18.07. The average length of the tympanic segment of the facial nerve in the 35 temporal bones was 12.47 mm + 0.435. The distance from the oval window niche to the S.G.A. in all the specimens varied from 2mm to 4.1 mm (mean= 3.314 + 0.435 mm). A relationship was found between the length of the facial nerve, the S.G.A. and the distance of the facial nerve from the oval window niche. The shorter the tympanic segment of the facial nerve, the more obtuse the S.G.A is and the closer the nerve to the oval window niche is. On the other hand the longer the tympanic segment of the facial nerve, the more acute the S.G.A is and the more the distance of the nerve from the oval window niche is. Conclusion: The measurement of the S.G.A. and the distance between the S.G.A. and the oval window niche are variable depending on the length of the tympanic segment of the facial nerve. This might help the otologic surgeon to anticipate the position of the facial nerve to avoid its injury during middle ear surgery.