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 posteri or-most point of the tympanic segment of the facial nerve (TSFN) relative t o the posterior-most point of the short process of the incus, using ultrahi gh-resolution computed tomography (CT) of the temporal bone. Study Design, Setting, and Patients: Included were patients who had been re ferred 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 s lice in which the posterior-most point of the TSFN was identified. Its shor test distance to the TSFN was measured. Afterward, the length of the TSFN t hat 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 proces s of the incus and the FN in the previous measurement. The measurements wer e performed on 30 normal temporal bones. Results: The shortest distance mea sured between the posterior-most point of the short process of the incus an d the TSFN was, on average, 3.31 mm for all 30 ears (range 2-4.8 mm). The l ength 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 h ow much posterior to the posterior-most point of the short process of the i ncus the facial nerve is located.