Skull Base Surgery of the Posterior Fossa

12

the temporal bone and turns vertically into the
carotid sulcus of the sphenoid, which is flanked
laterally by the lingual process of the sphenoid
bone and the petrolingual ligament. This vertical
paraclival portion then bends anteriorly below
the posterior clinoid process before turning
superiorly and posteriorly to exit the cavernous
sinus below the anterior clinoid process. The
cavernous sinus accepts venous drainage from
the inferior and superior ophthalmic veins
through the superior orbital fissure. It may drain
sylvian and cortical veins directly or via the
sphenoparietal sinus. It communicates with the
contralateral cavernous sinus through the basilar
plexus posteriorly and through the anterior and
posterior intercavernous sinus superiorly. The
cranial nerves enter the cavernous sinus at dispa-
rate angles and then converge at the superior
orbital fissure. Lesions may involve both the cav-
ernous sinus and posterior fossa through direct
extension. The posterior portion of the cavern-
ous sinus is often opened to expose the upper
basilar artery region of the posterior fossa
(Fig. 1.11f).

Clivus and Petroclival Region

The clivus (Latin, “slope”) forms the anterior
bony wall of the posterior fossa (Fig. 1.1a). The
superior third is formed by the sphenoid bone
(dorsum sellae). It is fused with the occipital
part of the clivus through a synchondrosis at the
level of the foramen lacerum. The occipital part
of the clivus can be conceptually divided into
middle and lower thirds which are at the level
of the internal auditory canal and jugular fora-
men, respectively. These divisions correspond to
Rhoton’s rule of three.
The junction of the petrous bone and clivus at
the petroclival fissure is marked by the inferior
petrosal sinus. The petrous apex and foramen lac-
erum are at the superior end of this fissure, and
the jugular tubercle and foramen are inferior to it
(Fig. 1.1a, b). The inferior petrosal sinus com-
municates superiorly with the posterior aspect of
the cavernous sinus (Fig. 1.2a). At this petroclival
venous confluence, the abducens nerve travels

through Dorello canal, which is roofed by the

petrosphenoidal ligament (Gruber’s ligament),

toward the inferior part of the cavernous sinus.

Given its anterior location, with anatomical

barriers that may include temporal bone struc-

tures and several cranial nerves, the petroclival

region is one of the most difficult areas to access.

Approaches include retrosigmoid, posterior

petrosal, anterior petrosal, orbitozygomatic, or

pretemporal transcavernous. The posterior petro-

sal approaches offer a shorter working distance

than the others, but the medial aspect of the

petrous apex can sometimes become a blind spot

due to the more lateral angle of attack. Anterior

endoscopic approaches to this area have fewer

neurovascular obstacles but are difficult expo-

sures nonetheless.

Internal Auditory Meatus

and Otological Structures

The internal auditory meatus is a CSF-filled

space that invaginates into the petrous bone,

gradually tapering from the wide medial porus to

the narrow lateral fundus. It is divided horizon-

tally at the fundus by the transverse crest, which

separates the facial and superior vestibular nerve

superiorly from the cochlear and inferior vestibu-

lar nerve below. The vertical crest (Bill’s bar)

separates the more anterior facial nerve from the

posterior superior vestibular nerve (Fig. 1.7e).

The labyrinthine artery is usually a branch of

AICA, an injury of which can cause hearing loss.

Superiorly, the suprameatal tubercle can be

drilled to access Meckel’s cave (Fig. 1.7b, c).

Drilling the posterior petrous temporal

bone, whether from a posterior or middle fossa

approach, may injure specialized structures for

the transduction of sound and motion. The inter-

nal auditory meatus is flanked by the labyrinth

posteriorly and the cochlea anteriorly (Figs. 1.1d,

1.8d, and 1.9c). The tympanic cavity lies between

the internal and external auditory canals and con-

tains the ossicles as well as a small segment of

the facial nerve. The superior semicircular canal

of the labyrinth protrudes toward the floor of

the middle fossa as the arcuate eminence, but

J. Basma and J. Sorenson