Identify the facial nerve

The facial nerve is inferior to the lateral semi-circular canal, as it passes from its vertical segment through to its horizontal segment. We know that the facial nerve is never lateral to the lateral semi-circular canal. It is always medial. And that is best seen by rotating the temporal bone.

The line of the short process of the incus is virtually that of the facial nerve at its knee point or genu. The first genu is seen at the anterior end of the first dissection, and here the nerve passes very sharply before it enters the internal auditory meatus. The second genu passes from the horizontal tympanic segment to the vertical segment or mastoid segment.

So the nerve will be inferior to the lateral canal, it’s roughly along the line of sight of the incus, and is anterior and deep to the sigmoid sinus. If you orientate the bone back into the surgical position, and safely drill along the anticipated line of the mastoid or vertical segment of the facial nerve, you can expect to see it within the bone, before the drill engages it. So make sure that the dissection is carried well into the mastoid and inferiorly to the surface of the digastric ridge.

Keep a thin layer of bone over the external ear canal so that if possible both the line of sight down the external ear canal and the posterior portion of the dissection and the anterior rim of the dissection, ie the posterior part of the external ear canal are both visible. This is the best orientation.

You have now set up an orientation where drilling along this line of sight will take you directly to the facial nerve. The facial nerve is most safely found in its vertical or mastoid segment, not at the genu.

Since the anterior wall of the external ear canal is relatively thin, drill along the anticipated line of sight of the nerve, using long brushstrokes keeping away from the genu. Try to find the nerve in its vertical segment prior to finding it at its genu. Once you have a much clearer impression of a redder change of colour, there is much better evidence that you are approaching the facial nerve.

Use the facial nerve monitor probe again and you should hear the facial movements and also see them marked on the screen. If you move away from the nerve to another piece of bone, you won’t have the same effect. Change to a slightly smaller burr, so it will fit in the region.