Skull Base Surgery of the Posterior Fossa

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carotid artery is apparent with only a very thin or
no bony covering [ 21 , 33 ]. The posterior part of
the middle fossa floor contains the bony labyrinth
with its three components: the cochlea, vestibule,
and semicircular canals. The posterior part of the
middle fossa is more vulnerable to injury during
surgery given its intimate relationship with the
lateral part of the IAC. The cochlea is located at
the cochlear angle between the GSPN and the
labyrinthine part of the facial nerve just anterior
to the IAC fundus. The semicircular canals open
inferomedially into the vestibule, which is a
small cavity located in the posterolateral margin
of the IAC fundus.
Along the petrous ridge on the floor of the
middle fossa, a few bony landmarks can be iden-
tified: the petrous apex, trigeminal impression,
trigeminal prominence, meatal depression, arcu-
ate eminence (AE), and tegmen from medial to
lateral, respectively. The superior semicircular
canal (SSC) projects toward the middle fossa
floor and makes a bony elevation known as the
AE. In the anterior part of the middle fossa, the
foramen spinosum and foramen ovale are very
important landmarks that must be identified. The
foramen spinosum, with the middle meningeal
artery (MMA) passing through it, is located just
posterior and lateral to the foramen ovale. The

GSPN carries the preganglionic parasympathetic

fiber of the facial nerve to the lacrimal gland, and

some taste fibers from the soft palate originate at

the geniculate ganglion. It passes anteriorly in the

sphenopetrosal groove, where it runs beneath the

V3 division of the trigeminal nerve toward the

foramen lacerum, and then joins the lesser

petrosal nerve to form the vidian nerve in the vid-

ian canal (Fig. 11.4). The lateral extent of the

IAC is divided into four quadrants: Bill’s bar

(vertical crest) separates the facial nerve in the

superior- anterior quadrant from the superior ves-

tibular nerve in the superior-posterior quadrant;

the transverse crest separates the superior vestib-

ular nerve in the superior-posterior quadrant from

the inferior vestibular nerve in the inferior-poste-

rior quadrant. The cochlear nerve is located in the

inferior-posterior quadrant of the IAC.

IAC Localization Techniques

in the Petrous Bone

Four techniques are commonly reported in the lit-

erature to localize the IAC during MFA. The House

technique follows the GSPN posteriorly to the area

of the Gasserian ganglion (GG) and the labyrinthine

segment of the facial nerve medially toward porus

Fig. 11.3 Axial (a) and coronal (b) T1-weighted MRI
with gadolinium showing patient who presented with pro-
gressive vestibular symptoms and preserved hearing. The

cuts demonstrate a small VS located at the right IAC

extending from the fundus of the IAC to the prepontine

cistern

11 Vestibular Schwannomas