Microsurgical Anatomy of the Cranial Base

Abstract

The skull base represents one of most complex anatomical structures in the human body; as such, it has been the subject of significant anatomical and clinical research and discussion for centuries. The skull base consists of both an intracranial surface, which is related to the brain, and an extracranial or craniofacial surface, which is related to the nasal cavity sinuses, orbits, infra-temporal and pterygopalatine fossae, and the parapharyngeal and infrapetrosal spaces. In addition, the osseous segment (skull) is divided into the cranium and the facial skeleton. The cranium is further subdivided into the calvarium and the cranial base. In this chapter, through cadaveric anatomy, we will particularly focus on developing a comprehensive 360-degree perspective of the skull base, integrating the intracranial (dorsal) and extracranial (ventral) views.

FormalPara Key Points

• 1. The dorsal and ventral view of the skull base corresponds to the same anatomical structures, only from a different perspective.

• 2. The frontozygomatic suture is the projection of the superior orbital fissure.

• 3. In the coronal plane, the root of the zygoma projects from lateral to medial to the foramen spinosum.

• 4. The superior turbinate projects to the elements of the upper clivus and interpeduncular fossa elements.

• 5. The middle turbinate projects to the middle clivus and prepontine cistern

• 6. The lower turbinate projects to the lower clivus and the medullary cistern

7.1 Introduction

The anatomy of the skull base has been well described by several anatomists; most notably, Prof. Albert Rhoton Jr. who is widely considered as the definitive authority; we are indebted to him for his dedication and teaching. Through cadaveric dissections, he described with great detail every anatomical structure of the cavernous sinus, orbit, anterior floor, temporal bone, middle fossa, jugular foramen, foramen magnum, nasal cavity, and paranasal sinuses with the intent to better understand the anatomical basis of surgical skull base approaches with hope of enhancing their safety and efficacy [1].

Although technological advances, such as neuronavigation, have attempted to simplify the anatomical basis of surgical approaches, they have not obviated the absolute need for the unique knowledge gained from precise cadaveric anatomic dissections as a prerequisite to perfecting surgical technique [2, 3]. It is only with cadaveric dissections that one can achieve a comprehensive perspective of the relative position and orientation of critical microsurgical anatomical structures [4].

In this chapter, through cadaveric dissections, we will incrementally develop and present an anatomic comprehensive 360-degree perspective of the skull base, integrating the intracranial (dorsal) and extracranial (ventral) views.

7.2 The Intracranial Anatomy of the Skull Base (Dorsal View)

The osseous segment (skull) is divided into rostral (cranium) and caudal (facial skeletal) components (Fig. 7.1). The cranium is in turn subdivided into the calvarium and the cranial base. The skull base has an internal intracranial surface, which is related with the brain, and an extracranial or craniofacial external surface, which is related to the nasal cavity and sinuses, orbits, infra temporal and pterygopalatine fossae, and the parapharyngeal and infrapetrosal spaces (Figs. 7.2, 7.3). Both surfaces are connected by canals, foramina, and fissures through which neural and vascular structures traverse. Both the intracranial and extracranial base surfaces are divided into anterior, middle, and posterior parts, each of which have central and paired lateral portions [5] (Figs. 7.4, 7.5, 7.6 and 7.7).

Fig. 7.1

(a) Lateral view of the skull which divides the osseous segments into the cranium and facial skeleton. (b) Facial skeleton. Ethm. Perp ethmoidal perpendicular, Sup. Orb. Fiss superior orbital fissure

Fig. 7.2

(a) Dorsal or intracranial view of the skull base. (b) Ventral or extracranial view of the skull base. Ant. Clin. Procc anterior clinoid process, For foramen, Inf inferior

Fig. 7.3

(a) Closeup view of the ventral extracranial surface of the skull base. (b) Ventral extracranial view of the infratemporal fossa. (c) Ventral view of the jugular foramen and surrounding region. For foramen, Inf. Orb. Fiss inferior orbital fissure, Pterigomax. Fiss pterygomaxillary fissure

Fig. 7.4

(a) Dorsal intracranial view of Anterior cranial base. (b) Dorsal intracranial view of the middle cranial base. (c) Dorsal intracranial view of the posterior cranial base. Ant anterior, Arc arcuate, Clin clinoid, Emin eminence, For foramen, Hypogl hypoglossal, IAC internal acoustic meatus Impress impression, Intrajug intrajugular, Post posterior, Proc process, Sig sigmoid, Trig trigeminal

Fig. 7.5

Segmental layered views of the structures of the skull base. (a) superficial (in the plane of the cerebral hemispheres) sagittal view of the skull base. (b) Multi-layered (planes) view extending from the orbital surface (left side) to the brainstem (right side) of the skull base. (c) Multi-layered (planes) lateral view extending anteriorly from the extraconal orbital plane to the optic tract posteriorly and its somatotopic insertion into the calcarine cortex. A artery, Ant anterior, CVJ craniovertebral junction , For foramen, Infraorb infraorbital, Infratemp infratemporal, N nerve, Post posterior, Ph. I.C.A Parapharyngeal internal carotid artery, Post posterior, Pterygopal pterygopalgtine

Fig. 7.6

A layered axial dorsal view of the skull base with segmental peeling of the dura (a) Intradural view of the key anatomic structures with the basal dura of the cranial base left intact. (b) The dural layer of the left side was removed, exposing the following structures: the constituents of the anterior cranial base, the periorbita and ethmoidal cells in the middle cranial fossa, the cavernous sinus, and respective triangles; and in the posterior fossa, the tentorial surface of the cerebellum is observed. (c) Segmental peeling is continued in the anterior, middle, and posterior fossae; in the anterior fossa, the periorbita is removed, thereby exposing the intraconal access via the apertures between the extraoccular muscles. In the middle fossa and posterior fossa the triangles of the cavernous sinus and structures of the posterior fossa are exposed on the left side. Ant anterior, Ant. Clin anterior clinoid, Arc. Emin arcuate eminence, CVJ craniovertebral junction, Ethm ethmoidal, For foramen, Midd. Menin. A middle meningeal artery, Pet. N petrosal nerve, Sup. Orb. Fiss superior orbital fissure, Sup. Sem. C superior semicircular canal, Sup. Pet superior petrosal, Tr triangle

Fig. 7.7

(a) Posterior view of the cranial base . (b) The brainstem was removed and the cranial nerves are exposed. A.I.C.A anteroinferior cerebellar artery, Ant. Clin anterior clinoid, CN cranial nerve, Coll colliculus, CVJ craniovertebral junction, For foramen, Midd, Cer. P middle cerebellar peduncle, C cave, Gr greater, Sup. Orb. Fiss superior orbital fissure, P.I.C.A posterior cerebral artery, Post. Cer. A posterior cerebellar artery, S.C.A superior cerebellar artery, Tr triangle

The intracranial surface can be segmented into components; the sphenoid ridge, joined medially by the chiasmatic sulcus, divides the anterior and middle cranial bases; the petrous ridges, joined by the dorsum sellae and posterior clinoid processes, divide the middle and posterior cranial bases. In essence, there is an inherent symmetry of the sphenoid ridge anteriorly, dividing the middle fossa and anterior fossa, and the petrous ridge posteriorly, dividing the middle fossa and the posterior fossa (Fig. 7.4).

7.2.1 The Anterior Cranial Base (Fig. 7.8)

The intracranial surface is formed by a medial and lateral part. The medial part is related superiorly with the gyrus rectus and inferiorly with the nasal cavity and sphenoid sinus. It is comprised, from anterior to posterior, of the following structures:

• 1. Frontal sinus: It drains from the infundibulum to the hiatus semilunaris of the middle meatus; posterior to this is the foramen cecum through which the emissary vein passes and communicates with the superior sagittal sinus and venous plexuses of the nose, diploe, and orbit;

• 2. Crista galli: It is the upper part of the perpendicular plate of the ethmoid and is attached to the falx;

• 3. Cribriform plate: It supports the olfactory bulb and is formed by multiple foramina, allowing the passage of the olfactory nerves; the anterior and posterior ethmoidal canals mark the anterior and posterior limits of the cribriform plate. The anterior and posterior ethmoidal canals divide the medial segment of anterior fossa into the respective segments: frontal, cribriform, and planum. Similarly, the subcranial region lateral to the ethmoidal canals within the orbit is divided into into bulbar, retrobulbar, and apical regions. The anterior ethmoidal artery and nerve pass through the anterior ethmoidal canal and the posterior ethmoidal artery and nerve pass through posterior ethmoidal canal;

• 4. Planum and limbus sphenoidale: The planum is the anterior horizontal part of the superior surface of the body of the sphenoid bone and is located between the cribiform plate and the limbus sphenoidale. The limbus is situated between the planum and the anterior border of the chiasmatic sulcus.

• 5. Chiasmatic sulcus: It is situated in the superior surface of the body of the sphenoid bone, between the limbus sphenoidale and the tuberculum sellae and is related with the optic chiasm, which lies above and behind the sulcus. The sulcus ends laterally along both sides of the optic foramen [5].

Fig. 7.8

Axial view of segmentally peeled anterior cranial base. (a) Dorsal intracranial intradural view and related osseous structures. (b) Subcranial extradural view of the orbit and ethmoidal cells. (c) Progressive view of the intraconal space of the orbit. (d) Coronal osseous view of the orbit. (e) Dorsal Superior view of the left orbit. (f) Lateral view of the right orbit. A artery, Ant anterior, Ant. Clin anterior clinoid, Cil ciliary, Crib cribriform, CN cranial nerve, Ethm ethmoidal, Falc falciform, Inf inferior, Inf. Orb. Fiss inferior orbital fissure, Lac lacrimal, Lat lateral, Lig ligament, Long Longitudinal, For foramen, ICA internal carotid artery, M muscle, N nerve, Ophth ophthalmic, Post posterior, Post. Clin posterior clinoid, Supraorb supraorbital, Sup superior, Sup. Orb. Fiss superior orbital fissure

The intracranial segment of the anterior cranial fossa lateral to the ethmoidal canals consists superiorly of the orbital gyri and inferiorly of the orbit and optic nerve. It is formed by the frontal bone and the lesser wing of the sphenoid bone, which transitions medially into the anterior clinoid process and the optic canal.

7.2.2 The Orbit

The orbit is formed by the following (Fig. 7.8):

• 1. seven bones: frontal, zygomatic, sphenoid, lacrimal, ethmoid, palatine bones, and maxilla;

• 2. seven muscles: the inferior oblique muscle, inserted in the medial wall of the orbit, the four recti, the superior oblique muscle which is inserted at the annulus tendinosus, and the levator palpabrae superioris muscle;

• 3. one ciliary ganglion and seven nerves: trochlear, lacrimal, nasociliary, frontal branches of the ophthalmic division, abducens, optic, and oculomotor nerve (superior and inferior division); and

• 4. vessels: ophthalmic artery, which has three branches inside the orbit, and the superior ophthalmic vein [6].

Orbital Anatomic Relationships:

The orbit communicates posteriorly with the anterior and middle cranial fossae and inferiorly with the pterygopalatine and infratemporal fossae. The optic canal articulates the orbit with the sellar region, and contains the optic nerve and ophthalmic artery, which is situated along the superomedial corner of the orbital apex. The superior orbital fissure (SOF) connects the cavernous sinus to orbit. The annulus of Zinn surrounds the optic foramen and the SOF. The trochlear, frontal and lacrimal nerves course outside the annulus of Zinn. The oculomotor, abducens, optic and nasociliary nerves course within it. The inferior orbital fissure separates the medial wall and floor of the orbit and articulates the orbit with the pterygopalatine fossa. The structures coursing inside it are the zygomatic branch of the maxillary nerve, ascending branches of the pterygopalatine ganglion, branches of the internal maxillary artery, and branches of the inferior ophthalmic vein [7].

7.2.3 The Middle Cranial Base (MCB): (Figs. 7.4 and 7.9)

The medial part of MCB is formed by the body of the sphenoid bone and consists of the following:

• 1. tuberculum sellae: It is located behind the chiasmatic sulcus, anterior to the pituitary fossa, medial to the supraclinoid carotid, and inferior to the optic chiasm;

• 2. pituitary fossa: It is situated on the body of the sphenoid bone, medial to the carotid sulcus and the middle clinoid process (MCP). The MCP is connected to the to the anterior clinoid process (ACP) via the carotidoclinoid ligament which can ossify and form a caroticoclinoid foramen; the tuberculum sellae and the chiasmatic sulcus anteriorly, and the dorsum sellae with the posterior clinoid process (PCP) posteriorly.

• 3. dorsum sellae and the posterior clinoid processes (PCP): The dorsum sellae constitutes the posterior wall of the pituitary fossa and, together with the basilar part of the occipital bone, forms the clivus. It ends lateral to the PCP. The lateral segment of the MCB is formed by the greater wings of the sphenoid, the petrosal, and squamosal parts of the temporal bone. The anatomically critical structures located within the MCB include: (1) the clinoidal region, (2) the superior orbital fissure, (3) the cavernous sinus, (4) the middle fossa, and (5) the petrous bone [8].

Fig. 7.9

Dorsal layered axial view of intracranial middle and posterior fossa of the cranial base. (a) Intradural and osseous structures with relationship to the middle and posterior fossa. (b and c) Peeling of the dura of the middle cranial fossa and removal of the free edge of the tentorium on the left to expose the cavernous sinus and the triangles of the middle fossa. Ant. Clin anterior clinoid, Arc. Emin arcuate eminence, CN cranial nerve, For foramen, Interped interpeduncular, Gen. Gan geniculate ganglion , Gr. Pet. N greater petrosal nerve, Inf. Alv. inferior alveolar, Infraorbit infraorbital, Infratemp infratemporal, Mid. Men. A middle meningeal artery, N nerve, Pet. Car. A petrosal carotid artery, Sup. Pet superior petrosal, Supraorb supraorbital, Tens. Tymp. M tensor tympanic muscle, Tr triangle, Trig. Imp trigeminal impression

7.2.3.1 The Clinoidal Region

The anterior clinoid process (ACP) is formed by the medial end of the lesser wing of the sphenoid bone and forms the roof, floor, and medial wall of the optic canal (Fig. 7.10). The ACP has three sites of continuity with the sphenoid bone, one lateral and two medial: (1) The base laterally is attached to the medial edge of the sphenoid crest; (2) The base medially attaches to the body of the sphenoid bone through the anterior roof of the lesser wing and forms the roof of the optic canal; and (3) The optic strut connects the posterior root of the lesser wing with the sphenoid body, which forms the floor of the optic canal. In addition, the optic strut separates the optic canal from the superior orbital fissure.

Fig. 7.10

The clinoidal region (a and b). Ant. Clin anterior clinoid, Ant. Petroclin. Lig anterior petroclinoid ligament, CN cranial nerve, Falc. Lig Falciform ligament, Hypo hypophyseal, ICA internal carotid artery, Interclin. Lig interclinoid ligament, Post. Clin posterior clinoid, Post. Petroclin. Lig posterior petroclinoid ligament, Sph sphenoidal, Tr Triangle

The clinoidal segment of the internal carotid artery (ICA) courses underneath the ACP and posteriorly courses upward along its medial edge. The anterior clinoid is the site of attachment of the anterior petroclinoid and interclinoid dural folds (ligaments). In addition, the falciform ligament extends from the ACP across the roof of the optic canal to the planum sphenoidale [9].

7.2.3.2 The Superior Orbital Fissure (SOF)

The SOF is located between the optic strut and the maxillary strut and is formed between the lesser and greater wings of the sphenoid. The neurovascular elements passing through the SOF are the oculomotor, trochlear, ophthalmic , and abducens nerves, recurrent meningeal artery, and the superior and inferior ophthalmic veins. The orbitomeningeal band is found on the lateral edge of the superior orbital fissure, and in this area, cranial nerves are not present (only an artery) [10]. The orbitomeningeal band forms the apex of the SOF and represents an important surgical landmark for skull base approaches.

7.2.3.3 The Cavernous Sinus (CS)

The cavernous sinus is located on each side of the sella and sphenoid sinus, and extends it from the SOF to the dorsum sellae (Fig. 7.11). The inferior border of its posterior wall is located between Meckel’s cave and the dorsum sellae. The roof is formed anteriorly by the dural lining of the ACP and posteriorly by the oculomotor triangle. The lateral wall extends from Meckel’s cave to the SOF. The medial wall is formed by the dura of the lateral wall of the sella. The cavernous sinus is formed by dural walls (the cavernous sinus itself represents the space contained within the dural folds of the periosteal and meningeal layers of dura) that contain a venous space, internal carotid artery, and cranial nerves. The oculomotor, trochlear, and ophthalmic nerves course along the lateral wall. The abducens nerve courses along the medial side of the ophthalmic nerve and the internal carotid artery; it is not attached to a dural wall. The CS receives draining veins from the orbit, sylvian fissure, and middle and anterior fossae, and communicates with the basilar, superior and inferior petrosal, and intercavernous sinuses [11].

Fig. 7.11

Layered segmental sagittal view (left side) of the middle fossa, infratemporal fossa, Cavernous Sinus and SOF. (a) Intracranial, extradural view of the pericavernous and middle cranial fossa. (b) removal of the overlying osseous structures and dura exposing the lateral wall of the cavernous sinus, SOF and infratemporal fossa. (c) Magnified view of the region and exposure of the respective triangles of the cavernous sinus. Anterolat anterolateral, Clin. Car. A clinoidal carotid artery, For foramen, Gen. Gan geniculate ganglion, Gr. Pet. N greater petrosal nerve, Infratemp infratemporal, Mid. Men. A middle meningeal artery, N nerve, Pet. Car. A petrous carotid artery, Posterolat, posterolateral, Posteromed posteromedial, Pterygopal pterygopalatine, Sup. Sem. C superior semicircular canal, Sup. Orb. Fiss superior orbital fissure, Supraclin. Car. A supraclinoid carotid artery, Tens. Tymp. M tensor tympanic muscle, Tr triangle, Trig. Gang trigeminal ganglion

7.2.3.4 The Triangles of Cavernous Sinus and the Middle Floor

The cavernous sinus and the middle cranial fossa can be represented as each possessing four triangles, and two in the paraclival area (Table 7.1).

Table 7.1 Triangles of the Cavemous Sinus and the Middle Cranial Fossa

The bones of the greater wing of the sphenoid, the squamosal and the petrosal parts of the temporal squama, comprise the intracranial surface of the middle fossa. The tensor tympani muscle and the eustachian tube are located medial to the foramen spinosum and represent important landmarks over the lateral surface of the internal carotid artery. The horizontal segment of the petrous carotid is situated between the posteromedial and posterolateral triangles. Additionally, the greater and lesser petrosal nerves intimately travel along the petrous bone and the petrosal horizontal segment of the ICA. The greater petrosal nerve originates from the geniculate ganglion and runs parallel with the petrous carotid artery, joining with the deep petrosal nerve to form the vidian nerve, which in turn communicates with a branch to the sphenopalatine ganglia. The greater petrosal nerve corresponds to the pterygopalatine ganglion. The cochlea lies between the angle of the labyrinthine segment of the facial nerve and the greater petrosal nerve and is lateral to the genu of the petrous carotid artery [8].

The anatomical structures located along the petrous surface of the middle fossa from medial to lateral are: (1) the trigeminal impression, (2) the depression forming the roof of the internal acoustic meatus, (3) the arcuate eminence, which is related with the superior semicircular canal, and (4) the tegmen tympani, which forms the roofs of the mastoid antrum and tympanic cavities and the canal for the tensor tympani [12].

7.2.4 The Posterior Floor

7.2.4.1 Neurovascular Complex of the Posterior Fossa

Three neurovascular complexes are located in the posterior cranial base and are segmented based on their relationship with the cerebellar arteries: (1) The upper complex contains the superior cerebellar artery, midbrain, cerebellomesencephalic fissure, superior cerebellar peduncle, tentorial surface of the cerebellum, and the oculomotor, trochlear, and trigeminal nerves; (2) The middle complex includes the anterior inferior cerebellar artery, pons, middle cerebellar peduncle, cerebellopontine fissure, petrosal surface of the cerebellum, and the abducens, facial, and vestibulocochlear nerves; and (3) The lower complex presents the posteroinferior cerebellar artery, medulla, inferior cerebellar peduncle, cerebellomedullary fissure, suboccipital surface of the cerebellum, and the glossopharyngeal, vagus, spinal accessory, and hypoglossal nerves [13].

7.2.4.2 The Cerebellopontine Angle (CPA)

The CPA is located in the lateral segment of the posterior fossa and is limited on either side by the superior and inferior limbs of the angular cerebellopontine fissure (Fig. 7.12). The borders are the petrosal cerebellar surface folding around the pons and middle cerebellar peduncle [13]. The neurovascular structures are the fifth, seventh, and eighth cranial nerves, the anterior inferior cerebellar artery, the superior petrosal vein, and the flocculus. The CPA is related with two relevant anatomical regions: the petroclival region medially and the jugular region inferiorly [13, 14].

Fig. 7.12

Cerebellopontine angle (CPA). (a) Anatomical view of the CPA. (b) Surgical view of the CPA. Cer. Pon. Fiss cerebellopontine fissure, Pet. Fiss petrous fissure

7.2.4.3 Temporal Bone

The temporal bone is formed by the following structures: squamosal mastoid, petrosal, tympanic, and styloid (Fig. 7.13). The mastoid part is trabeculated, pneumatized and contains the antrum. The relevant landmarks of the temporal bone are the following: 1) The supramastoid crest, which represents the end of the superior temporal line and is at the level of the middle cranial fossa; and 2) The suprameatal triangle, which is a depression located between the supramastoid crest and posterior edge of the external meatus and projects to the mastoid antrum and the horizontal semicircular canal. The mastoid antrum is a wide space inside the mastoid and communicates with the tympanic cavity; it is related with the lateral and posterior semicircular canals and the facial canal. The tegmen tympani forms the roof and the suprameatal triangle forms the lateral wall of the antrum.

Fig. 7.13

Left-sided layered surgical view of the mastoid and petrous bones. (a) Surface exposure of the mastoid seen from a surgical view. Three important landmarks are the asterion, mastoid tip, and the supramastoid crest. (b) Mastoid antrum was opened and the lateral semicircular canal is observed. The semicircular canal is a relevant landmark to locate the facial nerve. (c) Deeper plane dissection with drilling and removal of the petrous bone resulting in exposure of the semicircular canal, facial nerve, and Trautmann’s triangle. CN cranial nerve, Jug jugular, Lat lateral, M muscle, Occipitomast occipitomastoid, Parietomast parietomastoid, Post posterior, Sup superior, Supramast supramastoid, V vein

There are three important anatomical landmarks which are useful when performing a mastoidectomy: (1) the asterion is a craniometric point formed by the lambdoid, parietomastoid and occipitomastoid sutures, (2) the mastoid tip is related inferiorly with the jugular bulb and the occipital condyle, and (3) the supramastoid crest, which has been described previously.

A mastoidectomy exposes the following structures: (1) The sinodural angle, which is located between the sigmoid sinus and middle fossa dura and is useful when localizing the tentorium and the superior petrosal sinus; (2) The otic capsule or bony labyrinth, which is a solid bone that contains the vestibule, semicircular canals, and cochlea; and (3) The Trautmann’s triangle, which is formed by the sigmoid and superior petrosal sinuses, the otic capsule, and the jugular bulb [12].

The petrous portion of the temporal bone is compact and contains the cochlea, the vestibule, the semicircular, facial, and carotid canals; in addition, it forms the jugular fossa and the petroclival fissure. The semicircular canals are situated posterosuperior to the vestibule. The lateral semicircular canal is situated above the tympanic segment of the facial nerve. The posterior semicircular canal lies behind and lateral to the internal acoustic meatus. The superior semicircular canal projects toward the floor of the middle fossa en route to the arcuate eminence [12].

7.2.4.4 The Jugular Foramen (JF)

The jugular foramen is situated at the lower end of the petroclival fissure and is divided into three compartments: two venous compartments and a neural, or intrajugular, compartment (Fig. 7.14). The posterolateral or sigmoid segment receives drainage from the sigmoid sinus, and the anteromedial or the petrosal segment receives drainage from the inferior petrosal sinus. The intrajugular segment, separates the sigmoid and petrosal segments, where the glossopharyngeal, vagus, and accessory nerves course through. The JF is located approximately 5 mm below the porus acousticus of the internal auditory canal and 5 mm above the intracranial orifice of the hypoglossal canal. The jugular tubercle, a rounded prominence, is situated approximately 8 mm medial to the jugular foramen. The otic capsule is situated superior to the dome of the jugular bulb. The stylomastoid foramen is located lateral to the outer orifice of the jugular foramen.

Fig. 7.14

Left lateral (sagittal view) layered view of the pericondylar region. (a) Panoramic view, (b) Closeup view of the posterolateral condylar region. (c) Magnified view following condyle resection with exposure of the jugular bulb rostrally and hypoglossal canal and nerve caudally. Ac acoustic, CN cranial nerve, For foramen, Hypo hypoglossal, Int internal, Stylomast stylomastoid, Trans transverse, Procc process, V vein

The structures that course within the JF are the jugular bulb, the inferior petrosal sinus, meningeal branches, the glossopharyngeal, vagus, and accessory nerves with their ganglia, the tympanic branch of the glossopharyngeal nerve (Jacobson’s nerve), the auricular branch of the vagus nerve (Arnold’s nerve), and the cochlear aqueduct [15].

7.2.5 Craniocervical Junction (CVJ)

The craniocervical junction is anatomically complex and is situated between the condyles, lower clivus and foramen magnum and C1 and C2 vertebrae, and the medulla and spinal cord, respectively (Fig. 7.15). There are three muscular layers: (1) The superficial layer, which consists of the sternocleidomastoid and trapezius muscles, (2) The middle layer, which consists of the splenius capitis, semispinalis capitis, and longissimus capitis muscles, and (3) The deep layer, which is composed of the superior and inferior oblique muscles, as well as the major and minor rectus capitis posterior muscles. The suboccipital triangle is formed by the rectus capitis posterior major, superior oblique, and the inferior oblique muscles. In addition, it contains the V3 segment of the vertebral artery, the C1 nerve, and the venous plexus [16].

Fig. 7.15

Layered dissections of the posterior and posterolateral region of the cranial base. (a) Superficial, middle (b and c), and deep (c) layers of the para nuchal muscles of the region. (d) Suboccipital triangle. (e and f) craniocervical junction. A artery, Atl atlantoid, Cap capitis, CN cranial nerve, Con condyle, CVJ craniovertebral junction, Em emissary, Sternocleidomas sternocleidomastoid, For foramen, Gang ganglion, Gr greater, Inf inferior, Lig ligament, Long longissimus, Maj major, M muscle, N nerve, Obl oblique, Occ occipital, Plex plexus, Post posterior, Rec rectus, Semispin semispinalis, Splen splenium, Sup superior, Tr triangle, V vein

The osseous structures of the CVJ include the following: (1) the foramen magnum, (2) the occipital condyles, which are oval in shape, and are directed forward and medially. The articular surfaces are convex when adjacent to the atlas. The margins of condyles are attached to the atlanto-occipital joints, with each medial side attached to the alar ligament. The hypoglossal canal is situated inside the condyle and is directed forward and laterally from the posterior cranial fossa, (3) Atlas—C1, and (4) Axis—C2 [17].

The atlas and axis are connected by the following: (1) the atlantoaxial joint, which is formed by four synovial joints, (2) the cruciform ligament, which is formed by the transverse ligament and the vertical ligament, extending from the clivus to the body of the axis, (3) the anterior longitudinal ligament, and (4) the posterior longitudinal ligament. The atlas and the occipital bone articulate with the atlanto-occipital joints, articular capsule and by the anterior and posterior atlanto-occipital membranes [16].

The axis and occipital bones are formed by four fibrous bands: (1) the tectorial membrane, which is a cephalic extension of the posterior longitudinal ligament, (2) the alar ligaments, which extend from the dens to the medial surfaces of the condyles, and (3) the apical ligament, which courses from the tip of the dens to the anterior margin of the foramen magnum.

The neurovascular elements located within the CVJ are the caudal extension of the brainstem, cerebellum and fourth ventricle , the rostral part of the spinal cord, and the lower cranial and upper cervical nerves [18].

7.3 The Ventral Anatomy of the Skull Base (Endonasal Perspective)

In this section, we will describe the microsurgical anatomy of the ventral skull base as seen through the endonasal view.

7.3.1 Sinonasal Anatomy

The soft tissue and osseous landmarks consist of the following structures (Figs. 7.16 and 7.17):

• 1. The nasal septum is formed by quadrangular cartilage, vomer, and perpendicular plate,

• 2. Turbinates: There are three turbinates along the lateral wall of the nasal cavity. The inferior turbinate (IT) attaches to the conchal crest of the perpendicular plate of the palatine bone. The IT points directly to the lower third of the clivus and is a critical landmark as it projects to the supracondylar groove and is useful in locating CN XII, vertebral artery, and supraolivary fossa in the brainstem. The middle turbinate (MT) is formed by a body, anterior buttress, posterior buttress, and basal lamella. The anterior buttress attaches the MT to the agger nasi cells, while the posterior buttress attaches to the ethmoidal crest of the perpendicular plate of the palatine bone. The sphenopalatine foramen is positioned above the posterior end of MT. The basal lamella of MT is attached to the lamina papyracea and to the cribriform plate by the vertical portion. The ethmoid complex is divided by the basal lamella into the anterior and posterior ethmoid cells. The vertical lamella divides the anterior skull base into the lamina cribossa medially and fovea ethmoidallis laterally. The uncinate process and the bulla ethmoidalis are lateral to the MT. The MT is a critical landmark and points to the middle third of the clivus; it is also helpful in locating both the abducens nerve at its origin at the vertebrobasilar junction. The superior turbinate (ST), in turn, points to the upper third of the clivus and a landmark to locate the third nerve in a deeper plane and the sphenoid ostium in the superficial plane which is situated medial and posteroinferior to the ST.

• 3. The palatine bone—the PB forms: (a) the medial wall of the pterygopalatine fossa, (b) the part of the floor and lateral wall of the nasal cavity, (d) the roof of the mouth, (e) the floor of the orbit, (f) the anterior wall of the pterygoid fossa, and (g) the anterior edge of the inferior orbital fissure. The sphenopalatine foramen sits between the orbital and sphenoid processes of the palatine bone.

• 4. The ethmoid bone (EB)—The EB is comprised of the following: (a) the crista galli, (b) the cribriform plate, (c) the perpendicular ethmoidal plate, which is situated in the medial portion, and (d) the two lateral masses or labyrinths (anterior, middle and posterior ethmoidal cells) and the lamina papyracea. The cribriform plate is located on the roof of the nasal cavity through which the olfactory nerve fibers travel through the perforations on the way to the superior turbinate, nasal septum, and a portion of the middle turbinate; it also supports the olfactory bulb. The perpendicular plate is a thin, flattened lamina, that descends from cribriform plate; it joins inferiorly with the quadrangular cartilage, anteriorly with the spine of the frontal bone and the crest of the nasal bones, and posteriorly with the sphenoidal crest and the vomer (upper and lower halves, respectively; all of which form the nasal septum). The agger nasi is the most anterior ethmoidal air cell of the agger nasi region, which is normally identified in 90% of patients. The agger nasi region is a small ridge with the lacrimal bone located anteriorly and the axilla superiorly; the axilla is the lateral attachment of the nasal wall of the middle turbinate. The onodi cell is a posterior ethmoid cell and in approximately 7–25% of cases, there is a bony impression of the optic nerve. The uncinate process is a bony lamina situated in the lateral nasal wall from the maxillary ostium to the frontal recess. Resection of the uncinate process allows for visualization of the ostium of the maxillary sinus, and rostrally it also serves as an access to the orbit. The ethmoid bulla is the largest anterior ethmoidal air cell and when removed, it exposes the lamina papyracea. It lies posterior to the uncinate process, anterior to the basal lamella, and superior to the infundibulum. The hiatus semilunaris is located between the posterior free edge of the uncinate and the ethmoid bulla, and communicates with the ethmoid infundibulum. The frontoethmoidal suture has two important landmarks, the anterior and posterior ethmoidal foramina, in which the respective anterior and posterior ethmoidal arteries and nerves traverse. The lamina papyracea forms the lateral surface of the ethmoidal cells and is delineated by the fronthoethmoidal and the ethmoidomaxillary suture. The ethmoidomaxillary suture projects posteriorly towards the lateral opticocarotid recess.

• 5. The sphenoid bone (SB)—The SB is a rectangular cavity which comprises the lesser and greater wing, body, two pterygoid processes and the sphenoidal sinus. The sphenoidal sinus is formed by the following:

  • (a) The anterior wall, which is located posterior to the ethmoid sinuses and is formed by the sphenoidal crest, ostium sphenoidale and the rostrum. The ostium sphenoidale is located posterior to the superior turbinate and is 15 mm superior to the choana. There are three types of pneumatization in the sphenoid sinus: conchal, presellar, and sellar. The pneumatization may extend to the greater sphenoid wing creating a lateral sphenoid recess, which is defined as the segment lateral to the medial pterygoid plate rostrally.

  • (b) The sphenoid floor consists of the following: (1) the palatovaginal canal, (2) the vidian canal, (3) the foramen rotundum, and (4) the pterygoid plate;

  • (c) The lateral wall contains the following: (1) the maxillary strut, (2) the lateral recess, (3) the V2 prominence, and (4) the mandibular strut, which is situated between the maxillary and mandibular nerves and is related with the horizontal part of the petrosal carotid artery;

  • (d) The posterior wall contains the following: (1) the sella, (2) the lingual strut, (3) the carotid prominences, represented by the paraclinoidal, parasellar, and paraclival segments, (4) the lateral opticocarotid recess (LOCR), which is a triangle-shaped bony pneumatization situated laterally between the paraclinoidal carotid artery and the optic nerve; this depression is the result of the pneumatization of the optic strut and its relationship with the ACP, (5) the medial opticocarotid recess (MOCR) is a teardrop-shaped osseous indentation identified at the confluence of the sella, lateral tuberculum recess, paraclinoidal carotid artery, and optic canal. Entry at the level of the MOCR allows for simultaneous access to the carotid canal, optic canal, suprasellar region, anterior cranial base, and cavernous sinus; and

  • (e) The roof of the sphenoid sinus contains the following: (1) the tuberculum sellae, which is located anterior and superior to the sella and corresponds with the superior intercavernous sinus; its intracranial projection is juxtaposed with the tuberculum sellae and the chiasmatic sulcus; (2) the limbus of the sphenoid, which is a bony ridge situated between the planum and the tuberculum sellae. The dural projection of the limbus corresponds to the interfalciform ligament, which is the continuation of the dural falciform fold and from a surgical view, represents the transition between the anterior fossa and suprasellar cistern. The interfalciform ligament is critical landmark as it approximates the position of the chiasm, (3) the planum, which extends as a covering from the posterior ethmoidal canal to the limbus, (4) the lateral tubercular crest, and (5) the supraoptic recess, which is a depression situated above the optic canal and corresponds intracranially to the superior root of the anterior clinoid [5, 19,20,21,22].

Fig. 7.16

Endonasal view progressively travelling superficially from the anterior paranasal cavity to the deeper nasopharyngeal cavity. (a) The middle and inferior turbinates. (b) Anterior adhesion of the middle turbinate. (c) The middle turbinate medialized with the uncinate process and bulla ethmoidalis exposed. (d) The sphenoid ostium covered by the superior turbinate rostrally and the choana caudally. (e) The posterior attachment of the middle turbinate is the ethmoidal crest of the palatine bone while the posterior adhesion of the inferior turbinate is the conchal crest of the palatine bone. (f) Nasopharyngeal region. Torus tubarius is a relevant landmark to locate the parapharyngeal internal carotid artery. Lac lacrimal, ET eustachian tube, Inf inferior, Midd middle, Ost ostium, Pal palatine, Phar pharyngeal, Post posterior, Proc process, Ros rosenmuller, Sph sphenoid, Sup superior, Turb turbinate, Unc uncinate

Fig. 7.17

Sphenoid sinus. (a and b) Anterior face of the sphenoid sinus. (c) Sphenoid sinus. (d) Posterior, lateral and roof of the sphenoid sinus. Ethm ethmoidal, Imp impression, LOCR lateral opticocarotid recess, MOCR medial opticcarotid recess, Nas nasal, Sph sphenoid , Opt optic, Ost ostium, P. Clin. ICA paraclinoid internal carotid artery, Palatovag palatovaginal, Perp perpendicular, Procc process, Sup superior, Turb turbinate

7.3.2 Endonasal Surface of the Anterior Cranial Base: Median Plane

The anterior endonasal surface is formed by the frontal, ethmoid, and sphenoid bones, and is divided into median and lateral segments by the basal lamella (Figs. 7.18 and 7.19). The median segment extends from the frontal sinus, the crista galli, the cribriform plate, the perpendicular plate of ethmoid bone, the planum, and the limbus to tuberculum sellae of the sphenoid body. The anterior endonasal cranial base faces superiorly with the frontal lobes and the suprasellar region [5].

Fig. 7.18

Layered endonasal view of the ventral skull base. (a) View at the level of the ethmoidal cells, perpendicular plate of the ethmoid and the superior part of the septum. (b) Following removal of the bulla ethmoidalis , ethmoidal cells and uncinate process, exposing the lamina papyracea laterally and the anterior and posterior ethmoidal artery canals rostrally. (c) Following bilateral frontal sinusotomies, complete ethmoidectomies exposure of the cribriform plate and dura of the anterior cranial base is demonstrated. (d) Following dural opening of the anterior cranial base, exposure of the basal surface of the frontal lobe and falx is visualized. A artery, Ant anterior, C canal, Ethm ethmoidal, Front frontal, Gyr gyrus, Lam lamina, Med medial, Of orbitofrontal, Olf olfactory, Orb orbital, Pap papyracea, Perp perpendicular, Post posterior

Fig. 7.19

Layered dissection of the ventral skull base following complete posterior ethmoidectomy and superior sphenoidectomy. (a, b, c and d) progressive osseous and dural peeling exposing critical deeper structures. (e and f) Intracranial intradural anatomy seen from the sphenoid sinus. A artery, AIS anterior intercavernous sinus, C canal, CN cranial nerve, Falc falciform, Front frontal, Gyr gyrus, Hypo hypophyseal, Inter interhemispheric, Lig ligament, LOCR lateral opticocarotid recess, Med medial, MOCR medial opticocarotid recess, Of orbitofrontal, Orb orbital, Opht ophthalmic, P. Clin. ICA paraclinoid internal carotid artery, Prom prominence, Sup superior, SOCR supra opticocarotid recess, Tuber tuberculum

7.3.3 Endonasal Surface of the Middle Cranial Base. Sella: The Median Plane (Fig. 7.20)

The anatomy of sphenoid bone was described previously. The ACP can be isolated from a ventral view by locating (Fig. 7.20): (1) the LOCR , which corresponds to the optic strut, and (2) the supraoptic recess, which corresponds to the anterior root of the lesser wing of the sphenoid, with the ACP representing the osseous segment in between these two spaces.

Fig. 7.20

Layered dissection of the parasellar region. (a and b) progressive exposure and peeling of the dural layers of the pituitary fossa. (c) Exposure of the pituitary gland and cavernous sinus. (d) Exposure of the interpeduncular fossa. A artery, Bas basilar, A. Ch anterior choroidal , Ant anterior, CN cranial nerve, Cin cinereum, Hypo hypophyseal, Inter-falc inter falciform, Inter intercavernous, L. Circ long circumflex, Lig ligament, Mam mamillary, Menin meningeal, Opht ophthalmic, P. Co posterior communicating, Periost periosteal, Pit pituitary, P. Clin. ICA paraclinoid internal carotid artery, P. Clival. ICA paraclival internal carotid artery, P. Sellar. ICA parasellar internal carotid artery, Premam premamillary, SCA superior cerebellar artery, Sup superior, Tuber tuberculum

The pituitary gland is covered by two layers of dura: an outer periosteal layer and inner meningeal layer. As these two layers travel laterally, they separate creating a space between them to form part of the cavernous sinus. The outer periosteal layer continues laterally to form the anterior sphenoidal wall of the cavernous sinus, and the inner meningeal layer stays attached to the gland and turns posteriorly toward the dorsum sellae to form the medial wall of the cavernous sinus. The meningeal layer is present in the superior and lateral portions of the sellae. The respective superior and inferior intracavernous sinuses are situated in both dural layers. The pituitary capsule forms a thin but real adherent layer and the pituitary ligaments are numerous fibrous projections connecting the pituitary capsule to the medial cavernous sinus wall. The inferior hypophyseal artery originates from the parasellar segment of the ICA at the meningohypophyseal trunk and courses medially toward the posterior gland; the superior hypophyseal artery arises from the medial surface of the intradural segment of the internal carotid artery (at the level of the carotid cave) and has three branches, respectively, supplying the anterior lobe of the pituitary, chiasm and stalk; the hypophyseal arteries form an anastomotic network along the stalk and infrachiasmatic racemic plexus. The capsular artery (McConnells’s artery) originates from the superior portion of the parasellar segment of the ICA. The segments of the internal carotid located in this region are: parasellar, paraclinoid (carotid cave) and intradural carotid [23].

The pituitary gland is circumferentially surrounded by the following: (a) posteriorly, the dorsum sellae, basilar plexus, posterior clinoid, and the interpeduncular cistern, (b) superiorly, the infundibular recess and floor of the third ventricle, (c) inferiorly, the clival recess, and (d) laterally, the cavernous sinus [21,22,23].

7.3.4 Endonasal Surface of the Posterior Cranial Base. The Clival Region

The clivus separates the sphenoid sinus and nasopharynx from the posterior fossa. The clivus is divided into superior, middle and inferior thirds (Figs. 7.21 and 7.22). The superior third or “sellar” segment, extends from the dorsum sellae and posterior clinoid to the floor of the sella, and was described previously.

Fig. 7.21

Layered dissection of the clivus and retroclival region. (a) Soft tissues, dural, and osseous structures of the clivus . (b and c) Clival segments. (d, e and f) Exposure of the brainstem through the middle clivus. A artery, BA basilar artery, AICA anteroinferior cerebellar artery, C canal, Cav cavernous, Cer cerebral, CN cranial nerve, inf inferior, CST corticospinal tract, LOCR lateral opticocarotid recess, For foramen, Inf inferior, Imp impression, Lac lacerum, Mag magnum, Midd middle, Petr petrous, P. Clival. ICA paraclival internal carotid artery, SCA superior cerebellar artery, Sup superior, Subst substantia, Trans. Pont. F transverse pontine fibers

Fig. 7.22

Inferior Clivus. (a, b, c and d) Soft tissues and osseous structures of the inferior clivus. (e and f) Exposure of the brainstem from the nasal cavity. A artery, Atl atlantoid, AICA anteroinferior cerebellar artery, Ant anterior, BA basilar artery, Cap capitis, Cav cavernous, CN cranial nerve, CST corticospinal tract, Fiss fissure, For foramen, Inf inferior, Lab labyrinthic, Long longissimus, Longit longitudinal, Mag magnum, Mem membrane, Mid middle, M muscle, Occip occipital, Pal palatine, Petrocliv petroclival, Ross rossenmuller, Tuber tuberculum, V vein, VBJ vertebrobasilar junction

The middle third or “sphenoidal” segment extends from the floor of the sella to the level of foramen lacerum. This division is related with the following:

• 1. Paraclival carotid arteries; this intercarotid space could have 1 of 3 shapes, namely trapezoid, square, or hourglass [24];

• 2. The petroclival fissure has the inferior petrosal sinus on its upper surface and travels from the cavernous sinus to the jugular foramen;

• 3. The foramen lacerum and the choana, which are located at approximately the same level. The foramen lacerum is located lateral to the floor of the sphenoid sinus at the sphenopetroclival confluence. The upper portion of the foramen lacerum is occupied by the petrous and paraclival segments of the ICA and the lower portion is composed of fibrocartilaginous tissue formed by the pharyngobasilar fascia, muscles, eustachian tube, and petroclival synchondrosis;

• 4. Upper medulla and lower part of the pons; and

• 5. Prepontine cistern (abducens nerve and basilar artery).

The inferior third or “nasopharyngeal” segment extends from the foramen lacerum to the foramen magnum and corresponds to the nasopharynx. The basopharyngeal fascia covers the inferior clivus and inferior to it lies the longissumus capitis muscle, rectus capitis anterior muscles, median raphe, anterior longitudinal ligament, pharyngeal tubercle, occipital condyles, supracondylar groove, and the anterior edge of the foramen magnum. In addition, it harbors the neurovascular elements of the premedullary cistern, which comprise of the following: (a) the vertebral arteries and proximal segments of the posterior inferior cerebellar arteries, (b) the median anterior medullary vein, (c) the cranial nerves IX-XII, (d) the olives, and (e) the medullary pyramids [25,26,27].

7.3.5 Endonasal Surface of the Craniocervical Junction

The soft tissues juxtaposed along the basopharyngeal fascia and regional paraclival musculature include the following: (a) the longus capitis muscles, which courses from the tubercles on the lateral processes of C3-C6 and inserts on the clivus lateral to the pharyngeal tubercle, (b) the longus colli muscle, which extends from the anterior tubercles of the transverse process of C3, C4 and C5 vertebrae and inserts at the anterior tubercle of C1, (c) the rectus capitis anterior muscle, which is situated behind the longus capitis and courses from the anterior surface of the lateral mass and the transverse process of C1 to the anterior surface of the condyle, and (d) the midline raphe (Fig. 7.23).

Fig. 7.23

Endonasal view of the craniocervical junction. (a and b) Removal of the soft tissues layers to expose the osseous elements of the craniocervical junction. (c and d) Removal of the osseous components to expose the underlying neurovascular elements and cisterns of the craniocervical junction. A artery, Ant anterior, CST corticospinal, For foramen, Lig ligament, Mag magnum, Med medial, Tuber tuberculum, V vein

The osseous boundaries are the following: (a) the anterior edge of the foramen magnum, (b) the condyles, (c) the lower clivus, (d) the transverse process and posterior arch of the atlas, and (e) the odontoid. The superficial connective system is formed by the following supporting ligaments: (a) the anterior longitudinal ligament and (b) the atlanto-occipital membrane. The deep connective system is formed by the following: (a) the alar ligaments, (b) the apical ligament, and (c) the cruciform ligament.

The relevant neurovascular elements are the following: (1) the inferior lower medulla (medullary pyramids and olive), (2) the vertebral and anterior spinal arteries, (3) the anterior spinal vein, and (4) the cervical cord with the C1-C2 ventral roots [28, 29].

7.3.6 Endonasal Surface of the Anterior Cranial Base: Lateral Plane (Lateral to the Parclinoidal and Cavernous ICA)

The lateral segment consists of the orbit and optic canal. The medial wall (lamina papyracea) and the inferior wall (roof of the maxillary sinus) of the orbit can be viewed from an endoscopic endonasal perspective (Fig. 7.24). The osseous structures that need to be traversed to reach the lamina papyracea, from medial to lateral, are the middle turbinate, uncinate process, bulla ethmoidal, and the anterior and posterior ethmoidal cells. There are two sutures that anchor the lamina papyracea: (1) an inferiorly located maxilloethmoidal suture and (2) a superiorly located frontoethmoidal suture. The floor of the orbit can be reached from medial to lateral by accessing the middle and inferior turbinate, and medial wall of maxillary sinus.

Fig. 7.24

Endonasal view of a layered dissection of the left lateral wall of the sphenoid sinus exposing cavernous sinus, Meckel’s cave, petrous region, lateral sphenoid recess, and middle cranial base. (a) Relevant key osseaous landmarks (prominences and recesses) (b and c). Removal of the osseous framework to expose the respective dural layers. (d and e) Opening of the dural layers to sequentially expose the cavernous sinus, (f) Petrous region, and (g and h) Meckel’s cave. C canal, Cav cavernous, CN cranial nerve, Dist distal, For foramen, Gang ganglion, Lac lacerum, Ling lingual, LOCR lateral opticocarotid recess, Mand mandibular, Max maxillary, N nerve, SOF superior orbital fissure, P. Clin. ICA paraclinoid internal carotid artery, P. Clival. ICA paraclival internal carotid artery, P. Sellar. ICA parasellar internal carotid artery, Periost periosteal, Prom prominence, Prox proximal, Rec recess, Symp sympathetic, Trig trigeminal

The soft tissue and neurovascular elements along the ventral lateral plane are the following: (1) the superior oblique muscle, lateral rectus muscle and the inferior rectus muscle; (2) the nasociliary nerve, branches of CN III, ciliary nerve, and optic nerve; and (3) the ciliary arteries, ethmoidal arteries and the third segment of ophthalmic artery [22].

7.3.7 Endonasal Surface of the Middle Cranial Base. Parasellar Region, Meckel’s Cave and Petrous Region: Lateral Plane

The anatomical structures of the lateral plane are the following: (1) the cavernous sinus, (2) the anteromedial and anterolateral triangles, (3) the quadrangular space, (4) the Meckel’s cave, (5) the lateral recess of the sphenoid sinus, and (6) the petrous apex (Fig. 7.24).

The lateral wall of the sphenoid sinus contains the following foramina: (a) the foramen rotundum, which is situated superolateral to the vidian canal and can be identified by following the infraorbital nerve, (b) the vidian foramen, which is located inferomedial to the foramen rotundum and has been described previously, and (c) the palatovaginal canal, which is related to the vidian canal medially (the vidian canal is lateral and suprior to the PVC). The bony struts are represented by the following: (1) the lingual strut, which is located within the angle between the paraclival carotid artery and petrous carotid artery and projects to Meckel’s cave, (2) the maxillary strut, which is a bony prominence that separates the superior orbital fissure from the foramen rotundum and the mandibular strut, which is a structure that is located between the maxillary and mandibular nerve and serves to localize the horizontal segment of petrous carotid artery. The bony impressions are the following: (1) the maxillary nerve, which is visualized in the lateral recess, (2) the paraclival carotid artery, and (3) the vidian canal impression [30].

The cavernous sinus is located as it spans from the superior orbital fissure (rostral and anterior) to the petrous apex (caudal and posterior), and contains from superior to inferior, the oculomotor (CN III), trochlear (CN IV), ophthalmic (CN V1), and abducens (VI) nerves, the latter which runs underneath V1. There are several triangles which have been historically described in the region to provide access: (1) the anteromedial triangle, which is bordered superiorly by V1, inferiorly by V2, and anteriorly by an imaginary line connecting the superior orbital fissure and foramen rotundum, (2) the anterolateral triangle, which is bordered superiorly by V2, inferiorly by V3, and anteriorly by an imaginary line connecting the foramina of rotundum and ovale.

The quadrangular space (QS) is limited medially by the paraclival carotid artery and inferiorly by the petrous carotid artery, superiorly and obliquely by the abducens nerve (the surrogate for which is V1), and laterally by V2. The size of the quadrangular space depends on the shape of the intercarotid space, i.e. the angle of the genu at foramen of lacerum [31]. Meckel’s cave is located at the center of this space [32].

7.3.8 Subcranial Fossae: The Pterygopalatine and Infratemporal

The pterygopalatine fossa takes the form of an inverted pyramid located between the posterior wall of the maxillary sinus and the pterygoid plates. Its boundaries are the pterygoid plates posteriorly, the perpendicular plate of the palatine bone medially, and the posterior wall of the maxillary sinus anteriorly, the pterygomaxillary fissure laterally, the middle cranial fossa and greater sphenoid wing superiorly, and the pyramidal process of the palatine bone inferiorly (Fig. 7.25).

Fig. 7.25

Pterygopalatine and infratemporal fossa. (a) The sphenoid bone is opened. The posterior adhesion of the inferior turbinate attaches to the conchal crest of the palatine bone and medial wall of the maxilla is located medially. (b) The inferior turbinate was removed and the perpendicular plate of the palatine bone with its respective processes are observed. In addition, the maxillary sinus is opened. (c) The perpendicular plate of the palatine bone was removed and the pterygoid wedge is observed. (d) The posterior wall of the maxillary sinus was removed and the pterygopalatine is observed. (e) The infraorbital nerve divides the pterygopalatine fossa from the infratemporal fossa. (f) Close view of the pterygopalatine fossa. (g) Muscles and neurovascular elements of the pterygopalatine and infratemporal fossa. A artery, Auricolotemp auriculotempolar, Desc descend, Fiss fissure, For foramen, Gr greater, Inf inferior, Infra infraorbital, IMAX internal maxillary, Lat lateral, LOCR lateral opticocarotid recess, M muscle, Max maxillary, Med medial, N nerve, Ost ostium, Pal palatine, Perp perpendicular, Post posterior, Proc process, Pter pterygoid, Pterygomax pterygomaxillary, Rec recess, Seg segment, Sep septal, Sph sphenoid, Sphenopal sphenopalatine, Sup superior, Temp temporal

The pterygopalatine fossa contains the following:

• (a) The infraorbital nerve, which crosses the superior wall of the maxillary sinus along with its artery, after exiting the pterygopalatine fossa (PPF) via the inferior orbital fissure. This nerve separates the PPF from the infratemporal fossa and originates as a distal branch of V2.

• (b) The sphenopalatine ganglion has a triangular shape and lies posterior to the sphenopalatine artery, and has three main branches: (1) the descending palatine nerve inferiorly, (2) a branch of V2 superolaterally, and (3) the vidian nerve superomedially. The vidian nerve travels in a canal along the floor of the sphenoid sinus in a posterolateral direction, which meets with the second genu of the internal carotid artery (between the horizontal and vertical segments). The nerves in the PPF include the pharyngeal nerve, nasopalatine nerves, and posterior superior nasal nerve.

• (c) The third segment of the internal maxillary artery includes the following branches: (1) the posterior superior alveolar artery, (2) the infraorbital artery, (3) the descending palatine artery, (4) the pharyngeal artery, (5) the artery of the pterygoid canal, and (6) the sphenopalatine artery. The branches of the sphenopalatine artery include the posterior lateral nasal artery, which supplies the nasal turbinates, and the posterior septal artery.

• (d) The pterygoid venous plexus communicates to the orbit via the inferior orbital fissure and laterally to the infratemporal fossa.

The infratemporal fossa (IFT) is an anatomic space located under the floor of the middle cranial fossa, above the parapharyngeal space, bounded inferiorly by the alveolar process, anteriorly by the posterior surface of the maxilla, and posteriorly by the auricular tubercle of the temporal bone.

The anatomical elements in the IFT are the following: (1) the second segment of internal maxillary artery with its branches: pterygoid, posterior deep temporal , anterior deep temporal, masseteric, and buccinator arteries, (2) the otic ganglion and chorda tympani nerve, (3) the CN V2, V3, (4) the pterygoid plexus, (5) the medial and lateral pterygoid muscles, and (6) the ligaments (stylomandibular, pterygospinous, and sphenomandibular ligaments).

The IFT communicates with the following: (1) the temporal fossa, which contains the temporal muscle, nerve, and vessels, (2) the middle fossa, which is medial to the foramen ovale, and contains the mandibular division of the trigeminal nerve (V3) and the foramen spinosum, and (3) the orbital cavity through the infraorbital fissure and medially with the PPF via the pterygomaxillary fissure [33, 34].

7.3.9 Endonasal Surface of the Posterior Cranial Base: The Lateral Plane

The most relevant anatomical landmarks of the lateral plane are the following:

• 1. The lower clivus,

• 2. The condyle: The parapharyngeal carotid courses lateral to the condyle. The attachment of the alar ligament determines the inferior limit of the ventromedial condyle resection;

• 3. The jugular tubercle may not be prominent and may be flat or even indented on the surface. It is situated inferolateral to the clivus, and just superior to the hypoglossal canal and medial to its external orifice, posteromedial to the lower portion of the foramen lacerum, laterally is located the jugular foramen and the petroclival fissure is situated posterolateral. The lower cranial nerves (IX, X and XI) courses above the jugular tubercle toward the jugular foramen.

• 4. The hypoglossal canal divides the lateral inferior clival area into two compartments: tubercular (superior) and condylar (inferior). The tubercular compartment is the ventral representation of the jugular tubercle and its rostral point is located at the level of the pontomedullary junction. The intradurally relationship of the condyle is related with the vertebral artery;

• 5. The supracondylar groove is situated on the upper surface of the condyle formed by the insertion of the rectus capitis anterior muscle, anterior atlanto-occipital membrane, and synovial capsule joint. This supracondylar groove provides a reliable landmark for identifying the superior limit of the hypoglossal canal. The external orifice of the hypoglossal canal is located lateral to the supracondylar groove. The distance between the supracondylar groove and the articular surface of the occipital condyle averages 10 mm;

• 6. The petroclival fissure;

• 7. The jugular foramen; and

• 8. The foramen lacerum (Fig. 7.26) [4, 22, 35].

Fig. 7.26

Condylar and jugular region. (a) Soft tissues of the lower clivus. (b and c) The eustachian tube is medialized and cut. (d and e) The supracondylar grove is a relevant landmark to locate the hypoglossal canal. (f, g and h) The premedullary and the cerebellar medullary cistern are observed. A artery, Ant anterior, Atl. C Occ atlantoid occipital, Cer. Med cerebellar medullary, CN cranial nerve, Eust eustachian, ET eustachian tube, For foramen, Hypo hypoglossal, Inf inferior, Lac lacerum, Lev levator, Jug jugular, M muscle, Med medial, Ost ostium, Pet petrosal, Phar pharyngeal, Pal palatine, Pter pterygoid, Pontomed pontomedullary, Tens tensor, Tube r tuberculum

The neurovascular elements are the following:

• 1. Vasculature: vertebral, posteroinferior cerebellar, anterior spinal arteries, vertebrobasilar junction, and parapharyngeal internal carotid artery. Inferior petrosal sinus, medial anterior medullary and bridge veins;

• 2. Brainstem: medulla with the pyramids and the olives;

• 3. Cranial Nerves IX, X, XI, and XII; and

• 4. Cisterns: anterior and lateral premedullary cisterns;

7.4 Conclusion

Microsurgical anatomy of the cranial base is complex and involves an intricate system of osseous, foraminal, muscular, soft tissue, vascular, and neural structures. The anatomy is best appreciated in a 360-degree panoramic perspective. This is very difficult, if not impossible, to accomplish in surgical and cadaveric approaches, as a result we have been forced to interpret this anatomy through keyholes with limitations based on the vantage point, be they ventral or dorsal. While it is vital to understand this anatomy from both a dorsal (transcranial) and ventral (endonasal) perspective, in order to fully understand this structure, a schema needs to be developed to allow the operator to reconstruct and compile these vantage points into a 3-dimensional construct in order to move freely between the anatomy. As such, we recommend an intensive, long-term training of 1 to 2 years (or preferably, longer) in the cadaveric laboratory in order to learn and practice the art of these surgical approaches and techniques. The eventual goal of this study, as Professor Rhoton and Professor Yasargil have previously taught, is to perform gentle, precise, and accurate neurosurgery and to be able to navigate safely around and through the cerebrum and its osseous and soft tissue envelopes.