2Surgical Anatomy of the Temporal Bone

FSurface and cortical landmarks

The flap is elevated to expose the mastoid cortex landmarks, the spine of Henle, the linea temporalis and the mastoid tip, with at least 3 cm of bone exposed above and beyond the mastoid for device seating. The spine of Henle (suprameatal spine) sits at the posterosuperior margin of the bony external canal and is a key surface guide to the underlying antrum. The linea temporalis approximates the level of the tegmen (the bony roof over the middle ear and mastoid), and the sigmoid sinus lies posteriorly; both are danger boundaries the cortical mastoidectomy must respect. Saucerization of the superior and posterior bony margins is deliberately avoided, because intact margins protect the connecting leads and provide a platform to stabilise the receiver-stimulator.[2009][2016]

FInto the mastoid: antrum, lateral canal, incus

A simple cortical mastoidectomy is deepened until the antrum is entered and the lateral (horizontal) semicircular canal is identified, the cardinal deep landmark of the mastoid. The fossa incudis and the short process of the incus are then uncovered, with the short process seen refracted through bone; this is the immediate landmark for the facial recess. The plane of the short process of the incus leads directly to the facial recess, so the incus orientation effectively aims the surgeon's tympanotomy. The mastoid antrum and lateral canal also orient the surgeon to the second genu of the facial nerve as it turns from horizontal to vertical.[2016][2009]

TThe facial recess triangle

The facial recess (posterior tympanotomy) is a triangular window bounded medially by the mastoid (vertical) segment of the facial nerve, laterally by the chorda tympani, and superiorly by the bone of the fossa incudis. The recess is opened by a successive drill-out of the chorda-facial angle, working from the fossa incudis downward and always stroking the bur parallel to the course of the facial nerve, never letting it pass above the fossa incudis. The chorda tympani can usually be preserved, but a small recess or a device needing wide exposure may force its sacrifice; care is then taken not to tear the tympanic membrane where the chorda enters at the annulus. Opening the recess gives the surgeon a view through to the incudostapedial joint and the cochlear windows, the gateway to the scala tympani. The anatomy of the facial recess is fully developed at birth, which is why infants can be implanted even though the mastoid itself continues to grow.[2016][2009][2007]

CThe target: promontory, round window and scala tympani

Looking through the recess, the promontory is the bulge of the basal turn of the cochlea, and the round window niche lies posteroinferior to it on the medial wall. If the round window is hard to find, the rule of thumb is that it is never more than 2 mm from the inferior margin of the oval window and usually directly inferior to it; misreading this can send the array into a hypotympanic air cell. The round window niche is bounded by the subiculum inferiorly and the ponticulus separates it from the oval window region; the true membrane is usually obscured by a perforable mucosal veil that can mimic a defect. The scala tympani is entered either directly through the round window membrane or via a cochleostomy drilled just anteroinferior to it; postmortem trauma studies support a site directly inferior to the round window membrane, often requiring removal of the bony subicular overhang. The cochleostomy fenestra is made with a small diamond bur and kept only slightly larger than the electrode, with great care not to rest the bur on an exposed facial nerve in the floor of the recess.[2009][2005][2010]