Skull Base Surgery of the Posterior Fossa

4

also forms the posterior wall of the sella turcica
(dorsum sella). The lateral walls of the posterior
fossa are formed by the mastoid and petrous por-
tions of the temporal bone, which are angled inward
to meet the relatively narrow clivus (Fig. 1.1a). The
posterior fossa is thus larger posteriorly, where the
cerebellum occupies the entire volume, but then
tapers anteriorly as the anatomical landscape transi-
tions to the brainstem and cranial nerves. Viewed
from above, its outline resembles a semicircle
topped by a rounded triangle. The relatively flat
floor of the posterior fossa exists along the postero-
lateral portion of the foramen magnum, whereas the
anterior part of the foramen magnum is the lower
edge of the upward sloping clivus.

Neural Foramina

The bones of the posterior fossa are lined with
dura mater, which often contains venous lakes
between its layers. Anteriorly and laterally, there
are five pairs of neural foramina into which a
sleeve of posterior fossa dura follows, forming
small or large CSF-filled dural caves (Fig. 1.1c).
The trigeminal nerve exits the posterior fossa
through an ostium formed by a depression of the
petrous temporal bone below and the superior
petrosal sinus above (Fig. 1.1b, d). The dura of
this foramen continues into the middle fossa to
form Meckel’s cave – a CSF-filled space that also
contains the trigeminal ganglion. More medially,
the abducens nerve enters a narrow dural sleeve
with CSF evagination (Dorello canal) that courses
between the petrous apex and clivus before enter-
ing the cavernous sinus. The facial, cochlear, and
vestibular nerves traverse the internal auditory
canal, which is tapered toward its lateral fundus
(Figs. 1.1b–d and 1.7e). Although the jugular
foramen, which is formed by an opening between
the petrous and occipital bones, is a large neural
foramen, it harbors relatively little CSF
(Figs. 1.1b–d and 1.2a, b). The hypoglossal canal
passes through the superior aspect of the occipital
condyle and travels anterolaterally (Figs. 1.1a–c
and 1.2a, d). Often there are duplicated inlets that
converge to a single outlet which then emerges
near the medial aspect of the jugular foramen.

Tentorium

The “lid” covering the posterior fossa “bowl” is

formed by the tentorium – an extension of dura

that separates the cerebrum from the cerebellum

with a central anterior opening (incisura)

(Fig. 1.1f). The tentorium is anchored to the

petrous ridge, where its layers separate to form

the superior petrosal sinus, and to the occipital

bone where it forms the transverse sinuses and

torcula (Fig. 1.1e). It slopes downward from the

incisura toward these lateral and posterior attach-

ments. Additional venous channels often course

between the layers of the tentorium. The falx

cerebri joins the tentorium in the midline at the

straight sinus, which communicates the vein of

Galen at the incisura with the torcula (Figs. 1.1f

and 1.5a). The tentorium extends anteriorly on

either side of the incisura as the anterior and pos-

terior petroclinoidal folds that attach to the ante-

rior and posterior clinoidal processes,

respectively. These two folds outline the poste-

rior roof of the cavernous sinus, where the oculo-

motor nerve enters (Fig. 1.1f). The trochlear

nerve is intimately related to the tentorial edge as

it enters the posterior cavernous sinus, just medial

to the anterior petroclinoidal fold (Fig. 1.4a).

Venous Sinuses

A rich anastomosing system of venous sinuses is

organized around the inner walls of the posterior

fossa [ 3 ]. Superiorly, the superior petrosal and

transverse sinuses course along the edges of the

tentorium. The two transverse sinuses meet at the

torcula, along with the straight sinus and the supe-

rior sagittal sinus which flank the falx cerebri. A

variable occipital sinus projects inferiorly from the

torcula along the falx cerebelli. The sigmoid sinus

begins at the junction of the transverse and petro-

sal sinuses and then courses in a sulcus of the mas-

toid bone (Fig. 1.1b), before emptying into the

jugular bulb inferior to the petrous bone (Figs. 1.1d

and 1.2a). The sigmoid sinus is more often larger

on the right side. The inferior petrosal sinus

courses along the petroclival fissure (Fig. 1.1b, d),

transmitting flow to the jugular bulb posteriorly

J. Basma and J. Sorenson