The anatomic relationship between the second genu of the facial nerve and the incus: a high-resolution computed tomography study

OBJECTIVE: This study defines the three-dimensional location of the posterior-most point of the tympanic segment of the facial nerve (TSFN) relative to the posterior-most point of the short process of the incus, using ultrahigh-resolution computed tomography (CT) of the temporal bone. STUDY DESIGN, SETTING, AND PATIENTS: Included were patients who had been referred for CT of the temporal bone for various reasons other than suspected pathologic conditions of the middle ear. The decision whether to include a patient in the study was based on the referring physician's referral note for the examination. INTERVENTION: All of the patients underwent axial CT of the temporal bone, with consecutive slices of 1.1 mm width and 0.5-mm increments. MAIN OUTCOME MEASURES: The posterior-most point of the short process of the incus was identified on axial CT and was superimposed onto a lower axial slice in which the posterior-most point of the TSFN was identified. Its shortest distance to the TSFN was measured. Afterward, the length of the TSFN that remained posterior to the perpendicular of the posterior-most point of the short process of the incus was measured by measuring the length of TSFN that could be seen posterior to the point of bisection of the line marking the shortest distance between the posterior-most point of the short process of the incus and the FN in the previous measurement. The measurements were performed on 30 normal temporal bones. RESULTS: The shortest distance measured between the posterior-most point of the short process of the incus and the TSFN was, on average, 3.31 mm for all 30 ears (range 2-4.8 mm). The length of TSFN that could be seen posterior to the perpendicular of the most posterior point of the short process of the incus was, on average, 2.70 mm (range 1.80-3.90 mm). CONCLUSIONS: In addition to defining the spatial location of the posterior-most point of the TSFN, this study also offers a practical method by which the surgeon can evaluate, preoperatively, how far medial to the incus and how much posterior to the posterior-most point of the short process of the incus the facial nerve is located.