The skull base may be directly affected by a pathologic process or can be used as a pathway to approach lesions (12
). Tumors and other lesions may arise intracranially or extracranially and can involve any of the intracranial fossae, nasal cavity, paranasal sinuses, orbits, pterygopalatine and infratemporal fossae, pharynx and parapharyngeal space, and craniocervical regions. Profound anatomical knowledge is the foundation for cranial base surgery and extensive dissection work in the laboratory is crucial to achieve adequate anatomical proficiency and three-dimensional mastery of the relations between the structures. The modern skull base surgeon must master both intracranial, extracranial, and endonasal surgical anatomy.
The cranial base is divided into three regions (anterior, middle, and posterior) with different anatomical relationships and distinct surgical approaches. There is an extensive connection between the intracranial and the extracranial surfaces of the cranial base through a number of foramina and canals (Table 131.1
). Important neurovascular structures travel along those pathways and are a route for intracranial and extracranial spread of tumors.
Anterior Cranial Base
The intracranial surface of the anterior cranial base is formed by three different bones: frontal, ethmoid, and sphenoid (12
). The frontal bones compose the majority of the anterior cranial base contributing to its lateral part. The orbital process of the frontal bone articulates posteriorly with the lesser wing of the sphenoid bone. Those two bones constitute the roof of the orbit and the optic canal, which transmits the optic nerve and the ophthalmic artery. Posterolaterally, the optic canals are bounded by the anterior clinoid processes, which are connected to the sphenoid sinus by the optic struts running under the optic nerves. The frontal sinus is located anteriorly between the external and the internal walls of the frontal bone. The internal cortical surface (posterior table of the frontal sinus) corresponds to the anterior limit of the anterior cranial base. The anterior cranial base faces the frontal lobes with the gyri recti
medially and the orbital gyri
laterally. In the midline, the superior sagittal sinus continues to the floor of the anterior cranial base where it connects with a small emissary vein at the foramen cecum. The fronto-orbital artery is a branch of the anterior cerebral artery that travels along the inferior and medial surface of the frontal lobe. Because of its proximity with the anterior cranial base, procedures in this region present an increased risk of injury of the
fronto-orbital branch. The olfactory bulbs are situated over the cribriform plates, and the olfactory tracts course posterolaterally over the surface of the brain as they cross over the optic nerves.
The midline of the anterior cranial base is related to the nasal cavity, ethmoid cells, and sphenoid sinus. The ethmoid bone forms the anterior two-thirds of the midline anterior cranial base. The regions of the ethmoid bone related to the intracranial surface from medial to lateral are the crista galli
, cribriform plate, and fovea ethmoidalis
. The crista galli separates the anterior half of the cribriform plates in the midline and is attached to the falx cerebri
. Anterior to the crista galli, the foramen cecum transmits an emissary vein responsible for the venous drainage from the nasal cavity to the superior sagittal sinus. Besides the potential risk of intracranial dissemination of nasal infections, congenital lesions such as nasal dermoids, gliomas, and meningoceles can communicate intracranially through the foramen (13
). The thin lateral lamella of the cribriform plate continues laterally as the fovea ethmoidalis
or roof of the ethmoid sinus. The depth of the lateral lamella is an important risk factor for iatrogenic cerebrospinal fluid (CSF) leak during transethmoidal procedures. The olfactory filaments pass through the cribriform plate from the nasal cavity to the intracranial olfactory bulbs and are a route for intracranial spread of sinonasal malignancy. The posterior third of the midline anterior cranial base is formed by the planum sphenoidale
, which corresponds to the roof of the sphenoid sinus.
At the junction of the ethmoid sinus and orbit, the anterior and posterior ethmoidal foramina along the frontoethmoidal suture line transmit the anterior and posterior ethmoidal arteries, respectively. The anterior ethmoid artery is located between the second and third ethmoid septations in a coronal plane that is tangential to the posterior surface of the globe. The posterior ethmoid artery is roughly at the junction of the fovea ethmoidalis and planum sphenoidale. These arteries diverge as they cross the roof of the ethmoid and often need to be identified and ligated/coagulated during procedures in the anterior cranial base.
Figure 131.1 Endoscopic view with a 0-degree endoscope during a dissection. A: Surface anatomy of the sphenoid sinus after removal of the anterior wall. B: Surrounding structures after bony removal.
The surface anatomy of the sphenoid sinus is important for endonasal approaches to the pituitary and surrounding areas (Fig. 131.1
). The degree of sphenoid pneumatization and patterns of septations vary greatly. When there are multiple septations, lateral septations always deviate toward the internal carotid artery (ICA) and care must be exercised when removing the septations. The sella is bounded by the clival recess inferiorly, cavernous sinus and ICA laterally, and optic canal superolaterally. The clival recess is bounded by the paraclival ICA and petrous apex laterally. The sixth cranial nerve courses superolaterally behind the paraclival ICA and is at risk of injury when drilling posterior to the paraclival ICA just below the sellar floor.
Middle Cranial Base
The intracranial surface of the middle cranial base is formed by the sphenoid and temporal bones. The limit between the anterior and the middle cranial bases is the sphenoid ridge joined medially by the chiasmatic sulcus. The limit between the middle and the posterior cranial bases is the petrous ridge joined medially by the dorsum sellae
and the posterior clinoid process (12
The intracranial surface of the middle cranial base can be divided in two regions: medial and lateral. The medial
part of the middle cranial base is composed by the body of the sphenoid bone. The greater wing of the sphenoid bone and the temporal bone (squamosal and petrosal segments) form the lateral portion of the middle cranial base, containing the middle cranial fossa.
The temporal bone has a pyramidal shape, the sides of which form the middle fossa floor (superior face), the anterior limit of the posterior fossa (posterior face), muscle attachments of neck and infratemporal fossa (anteroinferior face), and the muscular-cutaneous-covered side of the head (lateral), which forms the base of the pyramid. The temporal bone consists of four embryologically distinct components: the squamous, mastoid, petrous, and tympanic part.
The greater and lesser petrosal nerves course across the upper surface of the petrous bone. The carotid canal extends upward and medially and provides passage to the ICA. Medially, Meckel’s cave forms an impression on the upper surface of the petrous bone. The roof of the carotid canal opens below the trigeminal ganglion near the distal end of the carotid canal. The arcuate eminence approximates the position of the superior semicircular canal. A thin lamina of bone, the tegmen tympani, covers the middle ear and ossicles on the anterolateral side of the arcuate eminence. The internal auditory canal can be identified below the floor of the middle fossa by drilling along a line approximately 60 degrees medial to the arcuate eminence, near the middle portion of the angle between the greater petrosal nerve and arcuate eminence (12
The area below the middle cranial fossa includes the infratemporal fossa, parapharyngeal space, infrapetrosal space, and pterygopalatine fossa. The boundaries of the infratemporal fossa are the medial pterygoid muscle and the pterygoid process medially, the mandible laterally, the posterior wall of the maxillary sinus anteriorly, the greater wing of the sphenoid superiorly, and the medial pterygoid muscle joining the mandible and the pterygoid fascia posteriorly. The fossa opens into the neck below. The infratemporal fossa contains the branches of mandibular nerve, the maxillary artery, and the pterygoid muscles and venous plexus. The mandibular nerve exits the cranial base through the foramen ovale. The pterygoid venous plexus connects through the middle fossa foramina and inferior orbital fissure with the cavernous sinus and empties into the retromandibular and facial veins (12
). From a lateral infratemporal approach, a plane is formed by the lateral pterygoid plate, foramen ovale (third division of the trigeminal nerve), foramen spinosum (middle meningeal artery), and the spine of the sphenoid. On a deeper level, the eustachian tube overlays the petrous carotid canal. These are useful landmarks for locating the petrous ICA.
The pterygopalatine fossa is located between the maxillary sinus in the front, the pterygoid process behind, the palatine bone medially, and the body of the sphenoid bone above. The fossa opens laterally through the pterygomaxillary fissure into the infratemporal fossa and medially through the sphenopalatine foramen to the nasal cavity. Both the foramen rotundum for the maxillary nerve and the pterygoid canal for the vidian nerve open through the posterior wall of the fossa. The fossa contains branches of the maxillary nerve, vidian nerve, the pterygopalatine ganglion, and the pterygopalatine segment of the maxillary artery.
The parapharyngeal space is predominantly a fat-filled space, but also contains the eustachian tube, pharyngeal branches of the ascending pharyngeal and facial arteries, and branches from the glossopharyngeal nerve.
Endonasally, the medial aspect of the middle fossa (Meckel’s cave) is anterior to the paraclival ICA and superior to the petrous segment of the ICA. The second division of the trigeminal nerve (foramen rotundum) and the vidian nerve (pterygoid canal) are helpful landmarks. Meckel’s cave is bounded by the lateral cavernous sinus superiorly containing the third, fourth, ophthalmic branch of fifth, and sixth cranial nerves. Foramen ovale is just posterior to the pterygoid base.
Posterior Cranial Fossa and Craniocervical Junction
The posterior cranial fossa may be approached posterior, inferior, and medial to the temporal bone. The sigmoid sinus defines the posterior margin of the petrous temporal bone. The infrapetrosal space contains the jugular bulb and lower end of the inferior petrosal sinus; the branches of the ascending pharyngeal artery; the glossopharyngeal, vagus, and accessory nerves; and the opening of the carotid canal through which the carotid artery passes. Below the torcula and transverse sinuses, the occipital bone protects the posterior fossa and cerebellum down to the foramen magnum. From an endonasal view, another portion of the occipital bone, the clivus, protects the brainstem and can be divided into three segments: superior (posterior clinoid to floor of sella), middle (floor of sella to floor of sphenoid sinus), and inferior (floor of sphenoid sinus to foramen magnum). Intracranially, the superior clivus is associated with the third cranial nerve, the middle clivus is associated with the sixth cranial nerve, and the inferior clivus is associated with the lower cranial nerves. Inferolaterally, the hypoglossal foramen is bounded superiorly by the jugular tubercle and the occipital condyle inferiorly. Finally, the occipital condyle is a “ball and socket” joint that articulates with the atlas of the cervical spine.
It is important to understand the layers of the scalp in order to preserve function and plan reconstruction with pedicled scalp flaps. The scalp has five layers designated by the acronym SCALP: Skin, subcutaneous tissues, Aponeurosis (galea), Loose areolar layer, and Periosteum. Together, the loose areolar layer and periosteum comprise the pericranial flap whereas the galeopericranial flap includes the galeal layer. The pericranial flap is supplied by the supratrochlear
and supraorbital vessels, which exit from foramina or notch along the superior orbital rim. Laterally, the galea is continuous with the superficial temporal fascia. A temporoparietal flap, derived from this fascia, receives its blood supply from the superficial temporal artery. The temporalis muscle, another important reconstructive flap, is covered by the deep temporal fascia. The deep temporal arteries, terminal branches of the internal maxillary artery, supply the muscle on its deep surface.
In many respects, surgical approaches to the skull base are determined by the vascular anatomy. The ICA has five segments: parapharyngeal, petrous, paraclival, cavernous, and supraclinoid. The ophthalmic artery branches off of the ICA just after it exits the cavernous sinus and dural rings and runs inferolateral to the optic nerve inside the optic canal. The vertebral arteries usually enter the posterior fossa through the dura between the lateral lamina of C1 (after exiting the transverse foramen) and lateral foramen magnum. The circle of Willis comprises the ICAs, anterior cerebral arteries, anterior communicating artery, posterior cerebral arteries, and posterior communicating artery. A patent circle of Willis is predictive of collateral cerebral blood flow (CBF), but anatomical variations are common.