Localization of the internal auditory canal by identifying the intersection of the posterior border of the trigeminal ganglion and the superior petrosal sinus in cadavers

The identification of the internal auditory canal (IAC) has relied on visualization of the arcuate eminence (AE). However, it is not uncommon that the topographic markers on the middle cranial base are featureless and difficult to identify, including the AE. “Point T”, the intersection of the posterior border of the trigeminal ganglion (TG) and the superior petrosal sinus (SPS) has been presented as a marker to localize the IAC. Thirty-four sides from 17 dry skulls and five formalin-fixed latex-injected cadaver heads were studied. In the dry skull, the imaginary line of the IAC was defined by connecting the uppermost point of the rim of the external auditory canal and the uppermost point of the porus acousticus on the petrosal ridge. Point T was defined as the posterior margin of the trigeminal impression on the petrosal ridge. For cadaver heads, a standard middle fossa approach was performed, and the line of the IAC was defined by joining the tip of Bill’s Bar and the midpoint of the dura on the porus acousticus. Point T was expressed as the intersection of the posterior border of the TG and the SPS. The distance between point T and the medial end of the IAC was termed “segment TI”, and the angle spanning from segment TI to the IAC was “angle theta (θ)”. In dry skulls, segment TI (mean ± standard deviation [SD]) measured 9.74 ± 0.71 mm and angle θ was 135.56 ± 3.21°; in cadaver heads, segment TI measured 10.25 ± 0.58 mm and angle θ measured 133.43 ± 2.00°. An alternative for localization of the IAC is proposed when the AE is difficult to identify in the middle cranial fossa. As a mnemonic, the IAC can be located by identifying point T first, and then tracing 1 cm posteriorly along the SPS and turning laterally 90° plus half of 90° (135° total).