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Anteromedial Corridors to the Cranial Nerves
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will allow lateral mobilization of the ICA and access to
the underlying petrous bone and the Dorello canal. The
petrous apex can be drilled laterally to the medial anterior margin of the internal auditory canal. Medial to lateral
access can be facilitated by drilling the clivus at the petroclival junction.
32.1.6 Median Approaches to
Posterior Cranial Fossa—CN II, CN III,
and CNs VI–XII
Cranial Nerve VII: Facial Nerve
The facial nerve (CN VII) emerges from the lateral
brainstem at the pontomedullary junction and traverses
the pontomedullary cistern to enter the internal auditory
meatus, where it courses anterolaterally through the
petrous temporal bone.
Cranial Nerve VIII: Vestibulocochlear
Nerve
The vestibulocochlear nerve (CN VIII) emerges from the
lateral brainstem at the cerebellopontine angle. It traverses the pontomedullary cistern to travel with the facial
nerve through the internal auditory meatus and petrous
temporal bone.
Cranial Nerve IX: Glossopharyngeal
Nerve
The glossopharyngeal nerve (CN IX) emerges from the
anterolateral medulla between the inferior olive and the
inferior cerebellar peduncle and traverses the pontomedullary cistern to exit through the jugular foramen.
Cranial Nerve X: Vagus Nerve
The vagus nerve (CN X) emerges from the anterolateral
medulla as several rootlets that converge as it traverses
the pontomedullary cistern to exit through the jugular
foramen.
brainstem. These include the transsellar/subsellar posterior clinoid, transclival, and transodontoid approaches.
The median posterior clinoid approach is a caudal extension of the transsellar and transtuberculum/transplanum approaches described above.
of the clivus is bounded posteriorly by the dorsum sellae in the midline and the posterior clinoids laterally.
Removal of these bony structures allows direct access to
the interpeduncular cistern and basilar artery directly
posterior to the upper clivus. This was described in the
setting of pituitary transposition described above.
retrosellar space, the basilar artery is identified in the
interpeduncular cistern. The posterior cerebral, superior
cerebellar, and posterior communicating arteries, CN II,
CN III, and CN VI, and the membrane of Liliequist may be
identified (Fig. 32.17).
Extradural removal of the posterior clinoids and dorsum sellae can be achieved via a subsellar approach. This
approach provides access to the middle clivus, which
extends from the Dorello canal to the jugular foramen,
and does not require the combined transtuberculum/
transplanum approach. The sellar face is opened and removed. The bone overlying the SIS is removed. This will
allow the pituitary to be mobilized superiorly en bloc.
Inferior and lateral to the sellae, the ascending paraclival
carotid arteries are identified, and the lateral limits of
the approach are marked. The sellar dura is not opened
and the pituitary is elevated en bloc. Once the pituitary
is elevated, the dorsum sellae and posterior clinoids are
removed (Fig. 32.12a, b). The middle third of the clivus
under the sellae turcica and between the paraclival ICA
is removed, allowing access to the retrosellar/retroclival
space. Opening the dura provides access to the basilar artery, prepontine cistern, CN VI medially, and CN V, CN VII,
and CN VIII laterally.
ACom
OC
ON
PS
PCA
PT
10
The upper third
7
In the
III
Cranial Nerve XI: Accessory Nerve
The accessory motor nerve (CN XI) is composed of spinal rootlets that enter the skull through the foramen
magnum and join with cranial rootlets that emerge from
the anterolateral medulla. The cranial and spinal roots
traverse the pontomedullary cistern to exit through the
jugular foramen.
Cranial Nerve XII: Hypoglossal Nerve
The hypoglossal nerve (CN XII) emerges as several rootlets from the anterior medulla in the preolivary sulcus
between the olive and pyramid. It courses through the
pontomedullary cistern to exit through the hypoglossal
foramen.
Several median EEAs provide medial access to CNs at
the level of the medial skull base, skull base cisterns, and
VBJ
VI
Fig. 32.17 Panoramic view demonstrating complete
panclival and parasellar/suprasellar exposure following removal
of the osseous and bony framework of the sellae. The optic
chiasm (OC), optic nerves (ON), oculomotor nerves (CN III),
and trochlear nerve (CN VI) can be seen. After opening the
membrane of Liliequist and the diaphragma, the pituitary
gland (PT) is completely mobile and displaced posteriorly.
ACom, anterior communicating artery; PS, pituitary stalk; PCA,
posterior cerebral artery; VBJ, vertebrobasilar junction.
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Anteromedial Corridors to the Cranial Nerves
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The transsellar/subsellar posterior clinoid approach
is a transclival approach through the upper and middle clivus. The median transclival approach through the
lower clivus extends from the jugular foramen to the
cervicomedullary junction and foramen magnum.
10
This
will typically occur as part of a panclival exposure. The
transclival approach requires modification to the initial
bilateral sphenoid sinus exposure. The nasal septum is
completely detached from the sphenoid rostrum and
wide bilateral sphenoidotomies are performed to allow
identification of the key rostral anatomic landmarks,
including the bilateral carotid canal, medial pterygoid
plate, pterygoid canal, and vidian nerve (Fig. 32.18). Additionally, the key anatomic boundaries are identified
within the fossa of Rosenmüller, including the floor of
the sphenoid rostrally, the basopharyngeal fascia, the
Eustachian tubes laterally, the soft palate caudally, and
the nasopharyngeal mucosa posteriorly (Fig. 32.19). The
basopharyngeal fascia is removed from the floor of the
sphenoid sinus and the rostrum of the clivus. The sphenoid sinus floor is removed to the level of the clivus. The
vidian artery and nerve in the vidian canal are key anatomic landmarks (Fig. 32.20). The vidian canal courses
to the anterior genu of the ICA. Removal of the clivus superior to the vidian canal is limited to the midline clivus
between the carotid canals. If removal of the clivus inferior and lateral to the anterior genu of the ICA is necessary, the clivus is drilled in a caudal to rostral direction
using the vidian canal as the superior limit. The midline
dural opening occurs in short segments to ensure hemostasis. The lateral dural opening extends under the
horizontal segment of the petrous ICA to the Eustachian
tube as it disappears obliquely into the skull base at the
lateral boundary of the fossa of Rosenmüller. A lateral
dural opening superior to the petrous ICA will expose
CN VI entering the Dorello canal medial, superior, and
dorsal to the anterior genu of the ICA. The transclival
approach provides access to CN II and CN III in the upper
retroclival space, CN VI in the medial middle retroclival
space, CN V, CN VII, and CN VIII in the lateral middle
retroclival space, and CNs IX–XII in the lower retroclival
space. The vertebral arteries, vertebrobasilar junction,
FR
VN
PVC
Fig. 32.18 Exposure of the critical oblique foramina. The
scissors are on the foramen rotundum (FR), and the vidian
nerve (VN) and palatovaginal canal (PVC) are seen inferomedial
in an oblique line. Note the branches of the greater palatine
artery.
V2
Fig. 32.19 Confl uence of V2, the vidian nerve (VN), and the
Eustachian tube (ET) at the level of foramen lacerum (FL). Note
as one triangulates on these three structures (blue triangle)
and traces them posteriorly, they converge on Meckel’s cave.
MS, maxillary strut, ICA, internal carotid artery, V2, maxillary
division of CN V.
356
MS
ET
VN
ICA
I
C
A
VN
FL
Fig. 32.20 The vidian canal (VC) and vidian nerve (VN) are
exposed after circumferential drilling has been completed and
the plane deepened to the point where the vidian nerve inserts
inferior to the genu of the ICA, marking the transition from the
petrous to the paraclival ICA.
VC
Anteromedial Corridors to the Cranial Nerves
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basilar artery, and superior cerebellar and posterior
cerebral arteries are also exposed in the transclival approach (Fig. 32.17).
The median transodontoid approach provides access
to the extradural craniocervical junction.
11
This approach
can be performed as a caudal extension of the transclival
approach or as an independent approach that does not
require sphenoidotomies. The nasopharyngeal mucosa, paraspinal muscles, and atlanto-occipital membrane
are identified and partially resected to expose the lower
clivus and anterior arch of C1. For access to the foramen
magnum, the superior portion of the anterior arch of C1
is removed to expose the tip of the odontoid, and the medial occipital condyles are removed. Care is taken to avoid
entering the joint capsule. For transodontoid access, the
entire anterior arch of C1 and the odontoid are removed.
The dura overlying the brain stem is opened to provide
access to CN XII. Craniocervical stability is maintained by
avoiding disruption of bilateral alar ligaments and bilateral occipital condyle joint capsules.
32.1.7 Paramedian Approaches to
Posterior Cranial Fossa—CN V2,
CN V3, CNs VI–XII
The paramedian approaches to the posterior cranial fossa provide access to the middle and lower CNs. These
include the infrapetrous, transcondylar, and jugular foramen approaches. The paramedian infrapetrous approach
is an infratemporal approach to the posterior cranial fossa.8 Following the transpterygoid and quadrangular space
approach described above, the V2, the vidian canal, and
the anterior genu of the ICA are identified. The medial
pterygoid plate is removed to the level of the foramen
rotundum in the middle cranial fossa. The lateral pterygoid plate is removed and V3 is identified along the
lateral edge. V3 is followed to the foramen ovale, guiding removal of the intervening bone. The Eustachian tube
is identified and the medial portion of the cartilaginous
segment is resected. The bone and cartilage between the
Eustachian tube and the horizontal petrous segment of
the ICA medial to V3 is removed to reach the inferior surface of the petrous apex. The horizontal petrous and ascending paraclival segments of the ICA are identified and
skeletonized. Further bone removal proceeds under the
petrous ICA into the petrous apex as necessary. CN VI can
be identified posteriorly, and CN VII and CN VIII laterally.
The paramedian transcondylar approach is an inferior
extension of the infrapetrous approach and a lateral extension of the transclival approach, both described above
(see chapter on transcondylar approach). Removal of the
clivus inferior to the petrous ICA continues caudally to
the foramen magnum and medial occipital condyle, and
laterally into the condyle. The hypoglossal canal is located
rostrolateral to the occipital condyle. CN XII can be identified exiting the hypoglossal canal inferiorly.
The paramedian jugular foramen approach is a lateral extension of the transcondylar approach. The fossa
of Rosenmüller is followed laterally to identify the position of the Eustachian tube, as this is a key anatomic
landmark that determines the position of the ascending
parapharyngeal segment of the ICA as it enters the petrous carotid canal. Identification of the parapharyngeal
ICA at the level of the entrance to the petrous carotid
canal allows identification of the jugular fossa immediately posterolateral. CNs IX–XI can be identified in the
jugular foramen.
32.2 Tips and Tricks
EEAs to the ventral skull base provide median and
paramedian corridors to access CNs I–XII. The relationship of critical neurovascular structures to the target
lesion is the basis for determining the appropriate corridor. The fundamental principle that will guide this
decision is to avoid crossing the plane of a CN or artery
to access a lesion. The anteromedial approaches presented here should be combined with transcranial approaches as necessary in an individualized manner to
provide the best access to a particular skull base lesion
and affected CNs.
References
1. Kassam AB, Prevedello DM, Carrau RL, et al. Endoscopic endonasal
skull base surgery: analysis of complications in the authors’ initial
800 patients. J Neurosurg 2011;114(6):1544–1568
2. Bockmühl U, Carrau RL, Otto BA, Prevedello DM, Kassam AB. The
sinonasal corridor. In: Draf W, Carrau RL, Bockmühl U, Kassam AB,
Vajkoczy P, eds. Endonasal Endoscopic Surgery of the Skull Base
Tumors: An Interdisciplinary Approach. Stuttgart, New York, Delhi, Rio: Thieme; 2015:174–187
3. Kassam A, Snyderman CH, Mintz A, Gardner P, Carrau RL. Expanded endonasal approach: the rostrocaudal axis. Part I. Crista galli to
the sella turcica. Neurosurg Focus 2005;19(1):E3
4. Draf W. Endonasal frontal sinus drainage type I–III according to
Draf. In: Kountakis S, Senior B, Draf W, eds. The Frontal Sinus.
Berlin, Heidelberg, New York: Springer; 2005:219–232
5. Ong YK, Solares A, Carrau RL, Prevedello DM, Kassam AB. Preservation of olfactory function following endoscopic resection of select
malignancies of the nasal vault. Surgical Techniques Development
2012;2:e5
6. de Lara D, Ditzel Filho LFS, Prevedello DM, et al. Endonasal endoscopic approaches to the paramedian skull base. World Neurosurg
2014; 82(6, Suppl):S121–S129
7. Kassam AB, Prevedello DM, Thomas A, et al. Endoscopic endonasal
pituitary transposition for a transdorsum sellae approach to the
interpeduncular cistern. Neurosurgery 2008;62(3, Suppl 1):57–
72; discussion 72-4
8. Kassam AB, Gardner P, Snyderman C, Mintz A, Carrau R. Expanded endonasal approach: fully endoscopic, completely transnasal approach to the middle third of the clivus, petrous bone,
middle cranial fossa, and infratemporal fossa. Neurosurg Focus
2005;19(1):E6
9. Kassam AB, Prevedello DM, Carrau RL, et al. The front door to
meckel’s cave: an anteromedial corridor via expanded endoscopic endonasal approach- technical considerations and clinical
series. Neurosurgery 2009; 64(3, Suppl):ons71–ons82, discussion
ons82–ons83
10. Kassam A, Snyderman CH, Mintz A, Gardner P, Carrau RL. Expanded endonasal approach: the rostrocaudal axis. Part II.
Posterior clinoids to the foramen magnum. Neurosurg Focus
2005;19(1):E4
11. Kassam AB, Snyderman C, Gardner P, Carrau R, Spiro R. The
expanded endonasal approach: a fully endoscopic transnasal
approach and resection of the odontoid process: technical case
report. Neurosurgery 2005; 57(1, Suppl):E213, discussion E213
357
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Chapter 33
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33.1 Parapharyngeal ICA and
the Eustachian Tube 360
Bony Landmarks
33.2 Petrous ICA and the
Vidian Canal 360
33.3 Paraclival ICA and the Clival
Recess and the Foramen
Rotundum 361
33.4 Tips and Tricks 364
Bony Landmarks
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33 Bony Landmarks
Ali Jamshidi, Leo F. S. Ditzel Filho, Edward Kerr, Brad A. Otto, Ricardo L. Carrau, Daniel M. Prevedello
Introduction
Surgical expertise in skull base surgery is built upon
one’s ability to access challenging areas of the cranial vault while minimizing morbidity and maximizing
the degree of tumor extirpation. Therefore, the principle of endoscopic endonasal approaches (EEA) to the
skull base is based upon the concept of enhancing a
surgeon’s effectiveness in removing tumors by allowing for more direct access to them; a trajectory that
avoids traversing vital neurovascular structures is a key
element to this modality. To achieve this goal, a keen
understanding of the anatomic relationships between
cranial nerves, dural sinuses, arterial structures, and
the surrounding bony network is critical. To emphasize
the relevance of the osseous anatomy, the authors have
organized the bony landmarks of the skull base as appreciated from the EEA perspective by discussing them
in relation to the internal carotid artery (ICA) as well as
to cranial nerves.
The ICA is the most critical arterial structure of the skull
base because injuries to this vessel and its branches are
associated with the most devastating complications.
When considered from an EEA perspective, the ICA can
be divided into six segments: parapharyngeal, petrous,
paraclival, parasellar, paraclinoid, and intradural.
1–4
5
33.1 Parapharyngeal ICA and
the Eustachian Tube
The parapharyngeal ICA is relevant in EEA once it is superior to the hard palate. This segment is considered to
begin at the common carotid bifurcation and ends at the
external orifice of the carotid canal in the petrous bone.
From an EEA perspective, this ICA segment is situated
in the depth of the posterolateral aspect of the fossa of
Rosenmüller, which can be found at the superior–posterior aspect of the torus tubarius (Fig. 33.1). In other words,
the Eustachian tube (ET) is anterior and medial to this
segment. Typically, the ICA can be found at the junction
of the bony and cartilaginous portion of the ET at the level
of the carotid canal.
icance in this location.
6
There are no cranial nerves of signif-
33.2 Petrous ICA and the Vidian
Canal
The petrous segment of the ICA begins at the external
orifice of the carotid canal in the petrous bone and ends
at the posterolateral aspect of the foramen lacerum.
This segment of the ICA runs in an inferior-to-superior,
5
Tor. tu b.
Foss. Rm.
Eust. tub.
Sella
Fig. 33.1 Using a 0-degree
endoscope, the choana can be
visualized posterior to the inferior
turbinates and muscular palate.
The sphenoid rostrum (Sphen. ros.)
and the sella mark the midline. The
Rosenmüller fossa (Foss. Rm.) is
located superior and posterior to
the torus tubarius (Tor. tub.) (the
projection of the cartilaginous portion
of the Eustachian tube [Eust. tub.]).
The parapharyngeal internal carotid
artery is found in the posterior–lateral
depth of the Foss. Rm.
Sphen. ros.
360
Bony Landmarks
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posterior-to-anterior, and lateral-to-medial direction
toward the foramen lacerum. Important landmarks for
the localization of this segment of the ICA from anterior-to-posterior and medial-to-lateral spatial orientation
are the second and third segments of the trigeminal
nerve (V2 and V3) and their respective foramen, as well
as the foramen spinosum
7
; all of these foramina and
their respective traversing structures are superior to the
petrous segment (Fig. 33.2). In addition, the bony aspect
of the ET, found posterior to the carotid canal’s opening in
the nasopharynx, shares an osseous wall with the petrous
carotid artery; the junction between the cartilaginous
and bony aspect of the ET serve as landmark for this segment of the ICA, as it is less than 3 mm from this point.
7
The vidian canal serves as the most anterior, superior,
and medial bony landmark for the anterior genu of the
petrous ICA at the foramen lacerum from an EEA perspec-
8
tive
(Fig. 33.3). The canal is situated directly in front of
the foramen lacerum.
as a landmark for the transition of the petrous segment of
the ICA to the paraclival segment in the sagittal plane.
9
Therefore, the vidian canal serves
10
Following the vidian nerve posteriorly allows the surgeon to identify the horizontal position of the petrous
ICA. Thus, in the cases where a tumor distorts normal
anatomy, the surgeon can navigate toward the vidian
nerve to establish a safe area for dissection.
The vidian canal is formed by the intersection of the
medial pterygoid plate with the floor of the sphenoid
sinus; this beak-shaped area of bone is known as the
pterygoid wedge.
5
The canal can be found at the immediate superolateral aspect of the pterygoid wedge, which
is formed by following the articulation of the vomer with
the sphenoid rostrum posterolaterally toward the medial
pterygoid plate. The pterygoid plates can be found inferior and lateral to the vidian canal.6 Immediately superior
and lateral to the vidian canal is the foramen rotundum
and inferior and medial to it is the palatovaginal canal,
which is also referred to as the palatosphenoidal canal.
8
The foramen ovale and its associated exiting nerve (V3)
can be found superiorly and posteriorly after removing
the lateral pterygoid muscle from the lateral pterygoid
plate.
To access the vidian canal and to visualize its nerve, the
pterygopalatine fossa contents must be mobilized. To do
so, the periosteum between the palatovaginal canal and
the vomer-sphenoidal suture is dissected and the sphenoid process of the palatine bone is removed to transect
and/or lateralize the neurovascular contents of the palatovaginal canal, namely the pharyngeal artery branches
and the palatovaginal nerve.
8
Further dissection allows
for the pterygopalatine fossa to be lateralized to expose
the medial aperture of the vidian canal.
The distal aspect of the petrous carotid artery and the
middle fossa can be reached by beginning to drill the
inferomedial aspect of the canal followed by its superior
aspect so that the vidian nerve can be transposed superiorly; the inferior and lateral aspect of the canal is then
drilled sequentially.
11
During transpterygoid approaches,
drilling the vidian canal in this fashion protects inadvertent injury to the ICA and allows the vidian nerve to be
fully retracted, rather than transected to avoid potential
morbidity from keratoconjunctivitis.
33.3 Paraclival ICA and the
Clival Recess and the Foramen
Rotundum
The paraclival segment begins at the posterolateral aspect
of the foramen lacerum and extends superiorly toward
the medial petrous apex at the level of the petroclival fissure in the axial plane.
and intracavernous components, with the petrolingual
ligament serving as the landmark between these two
segments; inferior to the petrolingual ligament, the paraclival ICA is medial to the Meckel’s cave.
and V3 are lateral to the paraclival ICA. This segment of
5
This segment has extracavernous
12
Therefore, V2
For. spin.
Ant. clin.
For. lac.
Op. can.
Fig. 33.2 An endocranial view of
the middle cranial fossa and anterior
cranial fossa. The petrous internal
carotid artery (ICA) enters the carotid
canal at the external orifi ce and this
segment terminates at the foramen
lacerum (For. lac.). The foramen
rotundum (For. rot.), the foramen
ovale (For. ova.), and the foramen
spinosum (For. spin.) are all important
landmarks for the petrous ICA. The
anterior clinoid (Ant. clin.) connects to
the ventral surface of the skull base by
the optic strut, which forms the fl oor
of the optic canal (Op. can.).
For. rot.
For. ova.
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Bony Landmarks
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V2
Abd. n.
Sella
Fig. 33.3 The vidian canal is a critical
landmark when accessing the middle
cranial fossa during endoscopic
endonasal approach. The vidian nerve
(Vid. n.) can be traced posteriorly
from the pterygopalatine ganglion,
situated in the pterygopalatine fossa
(Pty. pal. fossa), as seen in this oblique
view of the right internal carotid
artery (ICA). Once the vidian canal is
identifi ed, it can be drilled as shown
in this specimen; the canal has been
drilled 270-degrees counterclockwise
from the 9 o’clock position (9) to the
6 o’clock position (6). Once the nerve
is identifi ed, it can be traced to the
anterior genu of the petrous segment
of the ICA as it enters the foramen
lacerum (For. lac.) to become the
paraclival segment of the ICA (Paracliv.
ICA). Abd. n., abducens nerve.
9
6
Pty. pal.
fossa
Vid. n. For. lac.
Pty. plate.
the ICA can be found by following the foramen rotundum
posteriorly and medially.
In addition, the paraclival segment can often be readily identified in a well-pneumatized sphenoid sinus; the
paraclival protuberance, which is seen lateral to the clival
recess (Fig. 33.4), overlies this portion of the ICA.5 Lateral and anterior to the paraclival protuberance is the
foramen rotundum (Fig. 33.4). In situations where the
sphenoid sinus is poorly pneumatized, removing the medial wall of the maxillary sinus to expose the infraorbital
nerve so that it can be followed to the maxillary nerve is
another strategy for locating the foramen rotundum and,
therefore, the paraclival ICA.
33.3.1 Parasellar ICA and the Sellar
Floor
The petroclival fissure marks the end of the paraclival segment and the beginning of the parasellar segment, which
is located entirely in the cavernous sinus; therefore, there
are no significant bony landmarks related to this segment
other than the sellar floor (Fig. 33.5). The bulge of the
sella turcica marks the floor of the sella; the clival recess
is directly inferior and usually slightly posterior to the
13
floor.
The abducens nerve travels immediately inferolateral to this segment of the ICA in the lateral compartment
of the cavernous sinus; the ophthalmic (V1), oculomotor,
and trochlear nerves traverse the lateral wall of the sinus
as they travel toward the superior orbital fissure.
Paracliv.
ICA
33.3.2 Paraclinoid ICA and
Opticocarotid Recesses
The parasellar segment ends at the proximal dural ring of
the ICA, where the paraclinoid segment begins. The osseous structures related to this segment that are critical for
identifying the paraclinoid ICA (Fig. 33.6) include the medial opticocarotid recess (MOCR), the lateral opticocarotid
recess (LOCR), lateral tubercular recess (LTR), and the distal osseous arch (DOA) of the carotid sulcus.
The degree of pneumatization dictates the prominence
of these bony structures from an endoscopic view. The
opticocarotid recesses (OCR) are considered consistent
osseous depressions that border the optic nerve and the
15
carotid.
The MOCR is the medial junction of the optic
canal—as the optic enters the subarachnoid space—and of
the paraclinoid carotid as the ICA exits the distal dural
ring, forming an osseous dimple. The lateral aspect of the
MOCR is the medial component of the LOCR where the
paraclinoid ICA meets the optic nerve at the optic canal;
therefore, the carotid is situated anterior and inferior to
these recesses. The LOCR is the pneumatized optic strut,
which forms the floor of the optic canal and the roof of
the superior orbital fissure. The optic strut therefore is
the structure that connects the sphenoid sinus to the
anterior clinoid; when it is well pneumatized, the LOCR
is the extension of the sphenoid sinus into the anterior
clinoid.
The LTR is the lateral portion of the tuberculum sellae
and demarcates the medial border of the paraclinoid ICA
5,14
362
LOCR
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Op. n.
Paracliv.
pro.
Plan. sphe
Tub. s el .
Sella
Cliv. rec.
Pos. eth.
Op. can.
SOF
For. rot.
Bony Landmarks
Fig. 33.4 In a well-pneumatized
specimen, the paraclival internal
carotid artery (ICA) can be identifi ed
relative to the paraclival protuberance,
which is the osseous landmark
overlying this segment of the ICA,
lateral to the clival recess (Cliv. rec.).
The other midline structures of the
posterior, middle, and anterior cranial
fossa can be seen as the clival recess,
sella (notice the superior and inferior
intercavernous sinus above and below
the sella), tuberculum sella (Tub.
sel.), and planum sphenoidale (Plan.
sphe.), respectively. For. rot., foramen
rotundum; LOCR, lateral opticocarotid
recess; Op. can., optic canal; Op.
n., optic nerve; Pos. eth., posterior
ethmoidal arteries; SOF, superior
orbital fi ssure.
P. cl in.
ICA
Sella floor
Cliv. rec.
Sella
face
Ca. pro.
P. cl iv.
ICA
P. se l.
ICA
LOCR
Fig. 33.5 The face of the sella
and the sella fl oor were removed to
expose the pituitary gland and the
superior and inferior intercavernous
sinuses. The parasellar internal carotid
artery (P. sel. ICA) is contained within
the cavernous sinus and its landmark
is the sella fl oor. The paraclinoid
ICA (P. clin. ICA) forms the carotid
protuberance (Ca. Pro.) that can be
seen in a well-pneumatized sphenoid
sinus. Cliv. rec. (clival recess); LOCR
(lateral opticocarotid recess); P. cliv.
ICA (paraclival ICA).
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Bony Landmarks
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LOCR
MOCR
Tub .
sel.
MOCR
DOA
P. clin.
Op. prot.
ICA
LOCR
Fig. 33.6 The lateral opticocarotid
recess (LOCR) and the medial
opticocarotid recess (MOCR) form
the major osseous landmarks for the
paraclinoid internal carotid artery (P.
clin. ICA) and the optic nerve (seen as
the optic protuberance [Op. prot.]).
The lateral tubercular recess (LTR)
contacts the superior–lateral aspect of
the sella and marks the medial border
of the P. clin. ICA. The distal osseous
arch (DOA) connects the LTR with the
LOCR and marks the distal dural ring.
Tub. sel., tuberculum sellae.
LTR
Sella
(Fig. 33.6); the LTR corresponds to the lateral tubercular
crest from an intracranial view. The bridge of bone that
connects the LTR with the medial component of LOCR is
the convex DOA, which is a landmark for the distal dural
ring. The superolateral aspect of the sella contacts the inferior border of the LTR and it is at this junction where the
paraclinoid ICA meets the sella.
33.3.3 Other Important Osseous
Landmarks
The superior orbital fissure (Fig. 33.7) serves as the junc-
tion between the middle cranial fossa and the orbit.
fissure is found lateral to and inferior to the optic canal.
As mentioned earlier, the optic strut is a shared osseous
ridge that forms both the floor of the optic canal and the
roof of the superior orbital fissure. The medial aspect of
the superior orbital fissure carries the most functionally
relevant structures, particularly the oculomotor nerve,
the trochlear nerve, and the abducens nerve; of all these
cranial nerves, the oculomotor is most superior when entering the superior orbital fissure immediately beneath
the optic strut. The foramen rotundum is inferior and
lateral to the superior orbital fissure and posterior to the
superior orbital fissure is the lateral compartment of the
cavernous sinus.
16
16
This
The maxillary strut (Fig. 33.7) is shaped as a trapezoid
and extends from the inferior lateral border of the superior orbital fissure, separating it from the foramen rotun-
17
The medial border of the maxillary strut is where
dum.
the lateral wall of the sphenoid sinus meets the pterygoid
process; the lateral border of this strut covers the dura
of the temporal lobe and the maxillary branch of the trigeminal nerve (V2). The maxillary recess corresponds
to the ventral view of the maxillary strut and forms the
floor of the superior orbital fissure and the anterior roof
of the foramen rotundum. Removing the maxillary recess
allows for access to the middle cranial fossa during EEA;
this space also corresponds to the anteromedial triangle
of the middle fossa (between V1 and V2) as seen from an
endocranial perspective.
33.4 Tips and Tricks
In the last few decades, advances in skull base surgery
have allowed for improved overall outcomes in patients,
and different technologies have helped to achieve this
goal. Regardless of these available tools, the safest
approach is the one that avoids traversing nerves and
arteries and that limits retracting excessively on neural
tissue. Because of the vast osseous network of the skull
base and the inherent need to drill these structures for
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