Abstract
A detailed understanding of the anatomy of the skull base is essential for the surgical treatment of chordomas and chondrosarcomas. The skull base can be divided into the anterior, middle, and posterior skull base. The sphenoid bone and sinus are central to understanding the endoscopic approaches for these lesions. The clivus can be divided into three segments with regard to the associated neurovascular structures and related endoscopic approaches. The superior clivus extends down to the floor of the sella and is associated posteriorly with the midbrain, the superior cerebellar artery, and the oculomotor nerve. The middle clivus extends from the floor of the sella to the roof of the choana and is associated with the pons, the abducens nerve, and the anterior cerebellar artery. The inferior clivus extends below the roof of the choana and is associated with the medulla, the hypoglossal nerve, and the posterior inferior cerebellar artery.
Keywords
Chondrosarcoma, Chordoma, Clivus, Endoscopic anatomy, Skull base anatomy, Surgical anatomy
Outline
Introduction 89
General Skull Base Anatomy 2 89
Essential Endoscopic Endonasal Anatomy: The Sphenoid Bone, Sphenoid Sinus, and Pituitary Gland 96
Anatomy of the Clivus and Environs 98
General 98
Surgical Subdivision of the Clivus 99
Upper Clivus 100
Middle Clivus 102
Lower Clivus 103
References 105
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Introduction
This chapter will review the anatomy of the base of the skull as it pertains to approaches used for the treatment of chordomas and chondrosarcomas of this region.
Since chordomas and chondrosarcomas usually arise within the clivus and the petroclival synchondrosis, respectively, the clivus and its surrounding structures will be the focus of our discussion. Furthermore, given the growing use of endoscopic endonasal approaches for the treatment of both chordomas and chondrosarcomas, our anatomic analysis will further focus on endoscopic endonasal anatomy. We begin our review with a general discussion of skull base anatomy (anterior, middle, and posterior skull base), then proceed to some basic principles of endoscopic endonasal anatomy, and conclude with analysis of the complex anatomy of the clival region.
General Skull Base Anatomy
Anterior Skull Base ( Fig. 9.1 )
The anterior cranial fossa is enclosed by the frontal bone. Most of the floor of the anterior cranial fossa is composed of the orbital process of the frontal bone, which is mainly convex and engraved by markings from the frontal gyri on either side. The central portion of the anterior skull base is a deeper invagination of the ethmoid bone. The cribriform plate lies in the center and is perforated by multiple olfactory nerve filaments. The crista galli protrudes from the midline as the main anterior bony attachment of the falx. Slightly anterior lies the foramen cecum, which is usually plugged by fibrous tissue, but occasionally may transmit an anterior nasal emissary vein, which can be a route of intracranial spread of infections or tumors from the nasal cavity. On either side of the cribriform plate, the roof of the ethmoid sinus is formed by the fovea ethmoidalis of the frontal bone. Anterolaterally, the anterior ethmoid artery and nerve are transmitted through the cribroethmoid foramen, whereas the posterior ethmoid artery is transmitted posteriorly. Notably, the two vessels have a constant configuration, deviating from each other as they pass from lateral to medial.
The sphenoid bone forms the posterior part of the anterior cranial fossa. The sphenoid bone is analyzed in detail later in this chapter. Immediately behind the cribriform plate lies the planum sphenoidale, which marks the roof of the sphenoid sinus. It is bordered laterally by the lesser wings of the sphenoid bone, which medially and posteriorly form the anterior clinoid processes, important landmarks for the clinoidal internal carotid arteries and the optic nerves. Posteriorly the planum is bordered by the prechiasmatic sulcus, an important landmark for the optic chiasm. Above the cribriform plate and the planum sphenoidale lie the olfactory bulb and tract, respectively, which run in the olfactory sulcus. The latter separates the gyrus rectus from the orbital gyri of the frontal lobe.
The anterior skull base is intimately related to the paranasal sinuses and the orbit. The ethmoid sinuses lie between the orbits and directly inferior to the central anterior skull base. Directly posterior is the sphenoid sinus, which is analyzed in detail later. The frontal sinus lies anteriorly between the thin posterior plate and thicker anterior plate of the frontal bone. The orbit is roughly pyramidal in shape, with the orbital apex lying closer to the medial wall than the lateral. The orbit is related superiorly to the frontal sinus, medially to the ethmoid sinus, and inferiorly to the maxillary sinus. The medial wall is formed from anterior to posterior by the orbital process of the frontal bone and the lacrimal, ethmoid, and sphenoid bones. The anterior and posterior ethmoidal foramina, transmitting the corresponding anterior and posterior ethmoidal arteries, are found 24 and 12 mm posterior to the anterior lacrimal crest, respectively, whereas the optic foramen, transmitting the optic nerve, is found approximately 6 mm behind the posterior ethmoidal foramen. The floor of the orbit is formed by the maxilla and by the zygoma anterolaterally. The infraorbital groove transmitting the infraorbital nerve is found on the orbital floor. The lateral wall is formed by the frontal and zygomatic bones anteriorly and the greater and lesser wings of the sphenoid bone posteriorly, separated by the superior orbital fissure. It is essential to realize that although the anterior lateral wall is related to the temporalis muscle, the posterior lateral wall is related to the middle fossa and the temporal lobe. The orbital roof is mainly formed by the orbital process of the frontal bone, whereas the lesser wing of the sphenoid contributes to some extent posteriorly. The orbital apex is composed of three main portals: the superior and inferior orbital fissures and the optic canal. The annulus of Zinn, the common tendinous attachment of the orbital muscles, separates the orbital apex into lateral and medial compartments. The lateral superior orbital fissure transmits the lacrimal, frontal, and trochlear nerves and the superior ophthalmic vein. The superior and inferior divisions of the oculomotor nerve and the nasociliary and abducens nerves are transmitted within the annulus of Zinn in the medial compartment of the superior orbital fissure. Superomedially within the annulus of Zinn, the optic canal transmits the optic nerve and ophthalmic artery. The ophthalmic artery arises from the supraclinoidal internal carotid artery (ICA) just medial to the anterior clinoid process. After running inferiorly and laterally to the optic nerve, the ophthalmic artery within the orbital apex crosses superiorly and gives off the supratrochlear and frontal arteries (important for vascularized pericranial flaps) as well as the anterior and posterior ethmoidal arteries described earlier. Notably, in 7%–13% of cases, the optic canal can be completely encompassed within the posterior ethmoidal cells (Onodi cells), and the optic nerves can be injured during endonasal approaches if this is not recognized on preoperative imaging. The inferior ophthalmic vein runs in the inferior orbital fissure to join the superior ophthalmic vein at the orbital apex to drain into the cavernous sinus.
Middle Skull Base ( Fig. 9.2 )
The sphenoid bone forms the central portion of the middle skull base. The sella turcica, a rounded depression that encircles the pituitary gland, lies in the middle of the sphenoid bone. The sella is separated from the prechiasmal sulcus anteriorly by the tuberculum sellae, a transverse crest between the two depressions. The sella is bordered posteriorly by the dorsum sellae, and the posterior clinoid processes. The sides of the sphenoid bone slope laterally to the middle fossa, which is grooved by the ICA as it transitions from its petrous segment over the foramen lacerum to its cavernous segment. The lateral aspect of the middle skull base is roughly triangular and is limited by the sphenoid ridge anteriorly and the petrous ridge posteriorly. The floor consists of the greater wing of the sphenoid anteriorly, the petrous ridge of the temporal bone posteriorly, and the squamous temporal bone laterally.
There are several important foramina in the middle cranial fossa. The superior orbital fissure connects the middle fossa and the cavernous sinus to the orbit and its contents. The foramen rotundum lies immediately inferior and lateral to the superior orbital fissure and transmits the maxillary division of the trigeminal nerve. The two are separated by a bony island—the maxillary strut of the sphenoid bone. The foramen rotundum has an average length of approximately 4 mm and actually is more of a canal than a foramen. Posterolateral to the foramen rotundum, the foramen ovale transmits the mandibular division of the trigeminal nerve and may also contain an accessory meningeal artery, the lesser superficial petrosal nerve (LSPN), and emissary veins to the pterygoid plexus. These neurovascular structures may also exit or enter the skull base through their own independent foramina. Immediately posterolateral to the foramen ovale, the foramen spinosum transmits the middle meningeal artery. The foramen lacerum lies medial to the foramen ovale and is formed by the confluence of the petrous, sphenoid, and occipital bones. It is usually filled with fibrocartilage and roofed by the lacerum segment of the ICA. The petrous carotid canal is found immediately posterior and slightly lateral to the foramen lacerum. The inconstant foramen of Vesalius is seen within the mandibular strut, an island of bone bridging the foramen ovale and foramen lacerum. It transmits an emissary vein from the pterygoid plexus to the cavernous sinus. Immediately posterolateral to the foramen spinosum, the innominate foramen transmits the LSPN, which courses toward the foramen ovale. Posteromedial and parallel to LSPN, the greater superficial petrosal nerve (GSPN) exits through its homonymous foramen and courses parallel to the petrous ridge toward the foramen lacerum. There, the GSPN joins the deep petrosal nerve and enters the vidian canal to become the nerve of the vidian canal. The GSPN can be traced posteriorly to the geniculate ganglion of the facial nerve. Of note, the course of GSPN is an important landmark of the horizontal segment of the petrous carotid, which lies just inferior and medial to this nerve.
Other important anatomic landmarks of the middle skull base, from medial to lateral, include the trigeminal impression on the petrous ridge, which accommodates cranial nerve V in its course from the posterior cranial fossa to the Meckel cave. The internal carotid runs in its bony canal just under this area, which may be dehiscent. The trigeminal impression is followed posterolaterally by the trigeminal prominence, and then by the meatal impression, which corresponds to the underlying internal acoustic canal. Proceeding more lateral, one finds the arcuate eminence, which corresponds to an elevation created by the underlying superior semicircular canal, and subsequently, the flattened paper-thin roof of the tympanic cavity and mastoid air cells—the tegmen tympani and tegmen mastoideum, respectively.
An essential component of the middle skull base is the cavernous sinus, a large venous compartment encased within two layers of the dura, which surrounds the sella turcica and is continuous anteriorly with the superior orbital fissure. It is surrounded by five walls of dura. The anterior wall is formed by the periosteal layer of dura overlying the sella anteriorly as it separates from its meningeal layer laterally, which actually forms the medial wall as it surrounds the pituitary gland. The posterior wall faces the posterior cranial fossa and is formed by an extension of the periosteal layer between petrous apex and dorsum sella. The roof, formed by the oculomotor triangle posteriorly and the clinoidal triangle anteriorly, is continuous with the diaphragma and lateral wall. The lateral and medial walls meet at the inferior limit or floor of the cavernous sinus, demarcated by the maxillary division of the trigeminal nerve and the Meckel cave posteriorly. The lateral wall is essentially a continuation of the outer periosteal dural layer of the middle fossa. The meningeal layer of the middle fossa and the periosteal layers come together at the petroclinoidal fold, which corresponds to the transition of the lateral wall to the roof. Multiple venous structures drain into the cavernous sinus, including the basilar plexus, the ophthalmic veins, the sphenoparietal sinus, and veins from the foramen rotundum, foramen ovale, and the foramen of Vesalius. The cavernous sinus then drains into the superior and inferior petrosal sinuses, which then drain into the transverse and sigmoid sinuses, respectively. The cavernous ICA has a short ascending segment, a posterior genu, a horizontal segment, and an anterior genu that continues as the paraclinoidal carotid. Two major branches arise from the cavernous carotid: (1) the meningohypophyseal trunk, which arises from the posterior genu and gives off the inferior hypophyseal arteries, the tentorial arteries of Bernasconi and Cassinari, and the dorsal meningeal artery and (2) the inferolateral trunk, which arises from the horizontal segment and supplies the lateral wall and the first two divisions of the trigeminal nerve. These three branches may also arise independently from the ICA. The inferolateral trunk courses, between the abducens medially and V1 laterally, to the lateral wall of the cavernous sinus. An inconstant third branch is the McConnell capsular artery, arising medially from the horizontal segment, just anterior to the inferolateral trunk, to supply the superior, anterior, and inferior capsules of the pituitary.
From an endoscopic standpoint, given that the cavernous ICA forms a major boundary when operating within the cavernous sinus, it is instructive to divide the cavernous sinus into venous compartments with regard to their relationship to the carotid. As such, we have a superior, an inferior, a posterior, and a lateral compartment. Although such a compartmentalization may be more relevant for pituitary adenomas, similar principles apply for chordomas or chondrosarcomas with cavernous sinus invasion. The superior compartment lies between the horizontal cavernous segment of the carotid and the roof of the cavernous sinus. Within this compartment, the paraclinoidal carotid is found anteromedially as it courses superiorly and posteriorly, the dura of the oculomotor triangle is found posterolaterally, and the interclinoidal ligament is found superomedially. Conversely, below the horizontal carotid and between the posterior genu and the anterior or sphenoidal wall of the cavernous sinus lies the inferior compartment, which houses the sympathetic plexus and the abducens nerve coursing inferolaterally to the horizontal segment of the carotid. The abducens is also found just posterior and lateral to the short ascending segment of the cavernous carotid as it enters the posterior compartment of the cavernous sinus, which is defined as the area behind the posterior genu. The lateral compartment is located lateral to the horizontal segment of the cavernous carotid; it contains the abducens nerve coursing within the sinus, as well as cranial nerves III, IV, and V1 contained within the lateral wall of the cavernous sinus as they course toward the superior orbital fissure. Although cranial nerves V2 and V3 abut the cavernous sinus, they are not contained within its wall.
There are multiple important anatomic relationships when it comes to the neural structures of the cavernous sinus. Only the abducens nerve and the carotid sympathetic plexus course within the sinus itself. All other nerves are related to the lateral cavernous sinus wall. The abducens nerve, entering the cavernous sinus at the inferior aspect of the posterior compartment and just lateral to the posterior ascending cavernous carotid as described previously, courses just inferolaterally and then laterally to the horizontal segment of the carotid, lying just inferior and medial to V1 on its way to the superior orbital fissure. The oculomotor nerve, coursing from the interpeduncular fossa, enters the roof of the cavernous sinus within the oculomotor triangle, which is formed by the anterior petroclinoidal ligament laterally (extending from petrous apex to anterior clinoid), the posterior petroclinoidal ligament posteromedially (extending from the petrous apex to posterior clinoid), and the interclinoidal ligament anteromedially (extending between the anterior and posterior clinoid processes). These three dural folds essentially form the posterior roof of the cavernous sinus. The oculomotor nerve courses just lateral and parallel to the interclinoidal ligament and is surrounded by a thin invagination of the oculomotor triangle dura containing arachnoid and CSF (also known as the oculomotor cistern) before being incorporated into the lateral wall of the cavernous sinus under the clinoidal triangle, which forms the anterior roof of the cavernous sinus. The trochlear nerve enters the cavernous sinus in the posterolateral corner of the oculomotor triangle, close to the junction of the anterior and posterior petroclinoidal ligaments. Here, it lies posterior and lateral to cranial nerve III. It is then incorporated in the lateral wall of the cavernous sinus between the oculomotor nerve superiorly and V1 inferiorly, with which it forms the supra- and infratrochlear triangles, respectively.
Posterior Skull Base ( Fig. 9.3 )
The anterior wall of the posterior fossa is formed by the posterior surface of the petrous bone laterally and the clivus in the midline. The posterior and lateral confines are formed by the occipital bone, whereas the roof is formed by the tentorium cerebelli. The floor ends at the level of the foramen magnum.
The posterior surface of the petrous bone has two important foramina: (1) the internal auditory canal transmitting the seventh and eighth nerve complex and the labyrinthine branches of the anterior inferior cerebellar artery to the inner ear and (2) the vestibular aqueduct, which transmits the endolymphatic duct and which is found lateral to the internal auditory canal. The jugular foramen is found below these two foramina and is formed by the junction of the jugular process of the petrous bone with the occipital bone. The jugular foramen has a posterolateral compartment, the pars venosa, which contains the jugular bulb and cranial nerves X and XI, and an anteromedial compartment, the pars nervosa, which contains the glossopharyngeal nerve, the inferior petrosal sinus as it courses toward the jugular bulb, and, usually, a posterior meningeal branch from the ascending pharyngeal artery. Inferior and medial to the jugular foramen lies the hypoglossal canal with the hypoglossal nerve.
The vertebral arteries give off the posterior inferior cerebellar arteries ventral to the lower cranial nerves before joining to form the basilar artery at the pontomedullary junction. The basilar artery gives off the anterior inferior cerebellar artery just distal to the vertebrobasilar junction. The anterior inferior cerebellar arteries loop close to the internal acoustic canal and are intimately related to the seventh and eighth nerve complex. The superior cerebellar arteries arise from the distal basilar artery just before it bifurcates into the two posterior cerebral arteries.
From the external surface, encountered when performing a far lateral craniotomy, the posterior skull base is protected by the mastoid tip and the suboccipital musculature. The sternocleidomastoid muscle arises from the mastoid tip, whereas the digastric muscle arises just medial, from the mastoid groove. The occipital artery usually courses medial to the mastoid tip. Deep to the sternocleidomastoid lie the splenius capitis and longissimus capitis muscles, while posteriorly in the midline lays the trapezius with the semispinalis capitis arising deep to it. A very important anatomic landmark is the suboccipital triangle containing the vertebral artery and its venous plexus. The suboccipital triangle is exposed when these muscles are reflected off the superior nuchal line. The muscles forming this triangle are the superior and inferior obliquus capitis and the rectus capitis posterior major. Identification of the vertebral artery within this triangle allows access to the occipital condyle, which can be drilled to provide ventral access to the foramen magnum, lower clivus, and craniocervical junction.
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