Jacobson's nerve clues to the round window niche

The round window niche is used as a primary guide to cochleostomy for cochlear implantation, but sometimes the niche is not visible through the dissected facial recess. In such cases, Jacobson's nerve may be a guide to the niche. The objectives of this study were to depict the distance from Jacobson's nerve to the lip of the round window niche, and how this distance may relate to orientation of the manubrium as viewed through the external ear canal. Also, are these distances related to mastoid pneumatisation size? The study involved post-mortem anatomic dissection of 41 bequeathed adult crania (82 temporal bones). Viewing with an operative microscope, distances were measured with a fenestrometer-type instrument. Mastoid sizes were determined radiographically. Jacobson's nerve was identifiable in 81 of 82 temporal bones. Distances from Jacobson's nerve to the lip of round window niche averaged 2.1 mm, range zero to 3.2 mm. Distances from Jacobson's nerve to the round window niche were not obviously associated with either mastoid size or orientation of the manubrium in the head. If the main trunk of Jacobson's nerve is visible on the promontory (as it is in > or =95% of cases), with 95 per cent certainty it is within 3.3 mm of the lip of the round window niche. Distances from Jacobson's nerve to the lip of the round window niche do not correlate with either manubrium orientation as viewed through the external ear canal, or the extent of mastoid pneumatisation.