Analysis of mastoid portion of facial nerve course in temporal bone using computed tomography

Abstract

<b>Introduction:</b> The facial nerve (FN) follows a complex route in the temporal bone. Successful temporal bone surgery requires knowledge of its course which can be achieved using imaging methods such as computed tomography. This investigation aims to analyze the FN course in its mastoid portion and second genu and the frequency of its atypical course.</br></br> <b>Material and methods:</b> This is a retrospective study that enrolled 104 CT scans of temporal bones of patients followed up in the outpatient clinic of the Otorhinolaryngology Department of Dentistry Faculty of the Medical University of Warsaw between 2020 and 2022. FN courses were classified as straight, bulging, or letter “S”-like. Other parameters estimated: </br> • position of the second genu according to the prominence of the lateral semicircular canal (classified as lateral, medial, or middle) and its distance from a line adjacent to the lateral semicircular canal was measured;</br> • the distance between the short process of the incus and the outermost point of the second genu;</br> • the course of the mastoid portion of FN and the location of SG in comparison to the tympanic portion of n. VII. The course was later classified as lateral, medial, or middle.</br></br> <b>Results:</b> Among the 104 assessed temporal bones, the course of the mastoid portion of FN was classified as bulging in 47 cases (45.2%). Straight and letter “S”-like courses were present in 41 (39.4%) and 16 (15.4%) cases, respectively. Deviation of the second genu according to the prominence of the lateral semicircular canal was medial in 86 cases (82.7%), and middle in 4 cases (3.8%). In the rest, second genu was paramedian to the prominence of the later circular canal. The mean distance between the short process of the incus and the outermost point of SG was 4.9 mm (0.73 mm). The mastoid portion was located laterally, medially, and in the middle of the tympanic portion plane in, respectively, 4.8%, 83.7%, and 11.5%. The second genu was located laterally, medially, and in the middle of the tympanic portion plane in 1.9%, 73.1%, and 25%, respectively. In conclusion, seven temporal bones (6.7%) of five patients presented with a lateral displacement of FN in relation to the established anatomical landmarks.</br></br> <b>Conclusions:</b> Although the atypical course of FN, including the lateral displacement of the mastoid portion, is rare, screening for it is crucial before surgical exploration.