A novel method of identifying the internal acoustic canal in the middle fossa approach in a cadaveric study-the rule of 2s.

Objective Multiple landmarks and anatomic relationships exist to identify internal acoustic canal (IAC) in middle fossa approach for removing intracanalicular schwannomas. We attempted to identify a reproducible, practical method to quickly identify the IAC that would be applicable when an expanded middle fossa approach is required. Design Middle fossa approach was performed on 10 cadavers (21 dissections). In the first head, temporal and suboccipital craniotomies were performed to identify landmarks and formulate a hypothesis. Porous acusticus (PA) was identified and IAC was circumferentially skeletonized into middle fossa. Orientation of IAC in the middle fossa was evaluated in relation to foramen spinosum (FS), foramen ovale (FO), petrous ridge, and petrous apex. Consistency of this relationship was tested in the remaining heads. Results The opening of PA (point A) was consistently found at a mean of 2.38 cm posterolateral to the petrous apex along the petrous ridge (range 2.1 to 2.8). A line was drawn from the FO to FS and extrapolated posteriorly. The IAC (point B) was found a mean distance of 2.39 cm from FS along the FS-FO line (range 2.1 to 2.8). The course of IAC was consistently found by connecting point A to point B. Conclusion A novel, practical, and reproducible method is described to identify the IAC via the expanded middle fossa approach.