The skull base (SB) is the osseous foundation of the cranial vault. It contains many openings that allow communication between the extracranial and intracranial structures. This communication is crucial in normal physiologic processes yet may also arrow spread of disease. This article provides a comprehensive review of SB anatomy including important landmarks and anatomic variants relevant to SB surgery. We also illustrate the diverse pathologies affecting the SB.
Key points
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The skull base (SB) is the osseous foundation of the cranial vault containing many openings that allow communication between the extracranial and intracranial structures.
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The sphenoid is a complex bony structure that is likened to a bird with unfurled wings.
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Ossification of the dural fold that stretches between the anterior and middle clinoid processes may form a caroticoclinoid foramen (ring).
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Pituitary macroadenomas are by far the most frequently encountered intrasellar pathology, followed by meningiomas and craniopharyngiomas.
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The most common cerebellopontine angle lesions include vestibular schwannomas, meningiomas, and epidermoid cysts.
Video content accompanies this article at http://www.oralmaxsurgery.theclinics.com .
Introduction
The skull base (SB) is the osseous foundation of the cranial vault that separates the intracranial and extracranial structures. It contains many openings and apertures that transmit critical neurovascular structures and allow communication between intracranial and extracranial contents. This communication is crucial for normal physiologic processes, however, may also allow spread of disease between the extracranial and intracranial environments. Knowledge of imaging anatomy is important in narrowing the differential diagnosis of SB lesions, staging, and in surgical planning. In this chapter, we provide a comprehensive review of SB imaging anatomy including important landmarks and anatomic variants of clinical relevance to endoscopic endonasal and open transbasal approaches to the SB. In addition, we illustrate the diverse array of pathologies affecting the SB using cases from our clinical practices.
Skull Base Anatomy
The SB is comprised of five bones: two paired frontal and temporal bones and unpaired sphenoid, ethmoid, and occipital bones ( Fig. 1 A). The SB has two surfaces: endocranial and exocranial ( Fig. 1 B). The endocranial surface serves as the floor of the cranium where the brain and neurovascular structures are seated. The exocranial surface is situated above multiple neck compartments. The anterior exocranial surface of the SB is superiorly related to the orbits and sinonasal structures. The central exocranial surface of the SB lies above multiple compartments of the suprahyoid neck, including the roof of the pharyngeal mucosal and the parapharyngeal, masticator, and parotid spaces. The posterior exocranial surface of the SB is rostral to the pharyngeal mucosal, the carotid, retropharyngeal, perivertebral spaces, and the craniocervical junction.
When viewed from above, the endocranial surface may be divided into three compartments namely anterior skull base (ASB), central skull base (CSB), and posterior skull base (PSB). The designation of the boundaries ( Table 1 ) between these compartments is arbitrary. This approach may oversimplify the separation between compartments and may not perfectly incorporate the three-dimensional structures of the SB. However, this comprehensively organizes the anatomy and pathologic conditions of the SB for practical discussion.
| ASB | Anterior | Paired frontal bones |
| Posterior | Lesser wings of the sphenoid and planum sphenoidale | |
| CSB | Anterior | Lesser wings of the sphenoid bone Anterior clinoid processes Anterior margin of the chiasmatic groove |
| Posterior | Petrous ridges of the temporal bone Dorsum sellae of the sphenoid bone |
|
| Lateral | Sphenoidal angle inner (two-thirds) surface of parietal bone Squamous temporal bone Greater wings of the sphenoid bone |
|
| PSB | Anterior | Dorsum sellae of the sphenoid bone Superior margin of the petrous temporal bone |
| Posterior | Squamous part of the occipital bone | |
| Lateral | Mastoid part of the temporal bone |
The ASB houses the frontal lobes and the olfactory bulbs and is bounded anteriorly and laterally by the paired frontal bones. The lesser wing of the sphenoid and the planum sphenoidale define the posterior border of the ASB ( Fig. 1 C). The chiasmatic sulcus, a shallow shelf between the medial aspects of the optic nerve canals between the anterior border of the sella and planum sphenoidale is regarded by some authors as part of the ASB, while others consider it to be a CSB structure. Fig. 2 A–C summarizes the neurovascular structures of the SB.
The crista galli ( Fig. 3 A) is an important landmark in ASB surgery, particularly in endoscopic endonasal approaches. The falx cerebri attaches to the frontal ridge (see Fig. 3 A) and crista galli. The cribriform plate is porous to allow for transmission of the olfactory nerves originating from the olfactory bulb and terminating in the nasal cavity. The Keros classification is used to ascertain the depth of the olfactory sulcus ( Fig. 3 B), which contains the anterior ethmoidal artery ( Fig. 3 C). The CSB is made up of the sphenoid and temporal bones ( Fig. 4 A–E). The posterior boundary of the CSB is defined by the dorsum sellae and posterior clinoid processes medially and the petrous ridges laterally. The CSB contains the temporal lobes and many important neurovascular structures including the internal carotid arteries and cranial nerves. The CSB anatomy and pathology may be divided into the sella and parasellar structures. Important parasellar structures include the cavernous sinuses and their tributaries, the internal carotid arteries and accompanying venous plexus (ie, the venous plexus of Rectorzik), and multiple cranial nerves (see Fig. 4 D).
The sphenoid is a complex bony structure that is likened to a bird with unfurled wings. (see [CR] and Fig. 4 A). The body of the sphenoid bone is at the center stage in the discussion of anatomy and pathology of the CSB. The lesser (LSW) and greater (GSW) sphenoid wings contain openings and apertures that transmit critical neurovascular structures ( Table 2 ). The GSW forms the floor of the CSB. The LSW provides an important communication between the exocranial and intracranial environments through the orbital fissures that has important implications in the spread of disease. The parts of the sphenoid bone are summarized (see Fig. 4 A–C).
| Apertures/Foramen | Location | Transmitted Structure(s) |
|---|---|---|
| Cribriform plate | Anterior cranial fossa (Ethmoid bone) | CN I Olfactory n. |
| Optic canal | Anterior cranial fossa (Sphenoid bone) | CN II Optic n. |
| Superior orbital fissure | Middle cranial fossa (Sphenoid bone) | CN III Oculomotor n. CN IV Trochlear n. CN V1 Ophthalmic div. Trigeminal n. CN VI Abducens n. |
| Inferior orbital fissure | Orbit | CN V2 Maxillary div. Trigeminal n. Infraorbital a. and v. |
| Foramen rotundum | Middle cranial fossa (Sphenoid bone) | CN V2 Maxillary div. Trigeminal n. |
| Foramen ovale | Middle cranial fossa (Sphenoid bone) | CN V3 Mandibular div. Trigeminal n. |
| Foramen spinosum | Middle cranial fossa (Sphenoid bone) | Middle meningeal a. |
| Foramen lacerum | Middle cranial fossa (Sphenoid bone) | Internal carotid a. |
| Internal auditory meatus | Posterior cranial fossa (Petrous part of the Temporal bone) | CN VII Facial n. CN VIII Vestibulocochlear n. |
| Jugular foramen | Posterior cranial fossa (B/w Temporal and Occipital bones) | CN IX Glossopharyngeal n. CN X Vagus n. CN XI Accessory nerve Sigmoid sinus to inferior jugular v. |
| Hypoglossal canal | Posterior cranial fossa (Occipital bone) | CN XII Hypoglossal n. Anterior condylar vein |
The vidian and palatovaginal canals run along the floor of the sphenoid sinus (see Fig. 4 B) and are important landmarks in endoscopic SB surgery. The temporal bone (see Fig. 4 E) is another complex osseous structure in the SB which may be involved in various pathologic conditions.
The PSB is composed of the posterior temporal and occipital bones. The latter forms the posterior boundary of the PSB. The PSB houses the cerebellum and brainstem and contains the major dural venous sinuses including the jugular bulbs. The internal auditory canal contains the canalicular segments of CN VII and CN VIII ( Fig. 5 A ). The foramen magnum in the PSB is the largest opening in the SB that transmits the brainstem and cervicomedullary structures. The temporal and occipital bones form the jugular foramen. The jugular spine splits the jugular foramen into two compartments: the pars nervosa and pars vascularis. The pars nervosa is the anteromedial compartment that transmits the glossopharyngeal nerve and its tympanic branch (Jacobsen nerve). The pars vascularis, which is larger than its counterpart, contains CN X (vagus) and CN XI (spinal accessory) ( Fig. 5 B). The anterior condylar canal (AKA hypoglossal canal) ( Fig. 5 C and D) harbors CN XII and the anterior condylar vein.
The apertures of the skull allow for transmission of the cranial nerves and vessels (see Table 2 ).
The lesser sphenoid wing forms the optic canal that transmits the ophthalmic artery, traveling sympathetic fibers, and the optic nerve before joining the optic chiasm. This canal is a pathway from the orbit to the middle cranial fossa that allows spread of disease.
The superior orbital fissure (see Fig. 4 C) is a cleft formed by the lesser and greater wings of the sphenoid bone. This transmits the superior and inferior ophthalmic veins, CN III (oculomotor nerve), CN IV (trochlear nerve), ophthalmic division (V1—lacrimal, frontal, and nasociliary branches) of the trigeminal nerve, CN VI (abducens nerve), orbital branch of the middle meningeal artery, recurrent branch of lacrimal artery, and the superior and inferior ophthalmic veins. The greater sphenoid wing, body of the maxillary bone at the orbital floor, and the palatine bones form the inferior orbital fissure (see Fig. 4 C). This transmits the infraorbital and zygomatic branches of the maxillary division (V2) of the trigeminal nerve, the infraorbital artery and vein, inferior ophthalmic vein, and emissary veins to the pterygoid venous plexus.
The foramen rotundum (see Fig. 2 C) is a circular aperture of the greater wing of the sphenoid beneath the anterior clinoid process. This contains the maxillary division (V2) of the trigeminal nerve. The foramen ovale (see Fig. 2 C) is an oval-shaped opening within the greater wing of the sphenoid, lateral to the dorsum sellae. This carries the mandibular division (V3) of the trigeminal nerve that innervates the muscles of mastication.
The foramen spinosum (see Fig. 2 C) is located inferolateral to the foramen ovale and is also an opening created by the greater wing of the sphenoid. This foramen transmits the middle meningeal artery (MMA), a vessel that supplies the lateral dural surfaces of the brain. Because of its location beneath the articulation point of four skull bones called the pterion, skull fracture may cause a tear to this artery and result in epidural hematoma. The MMA may be embolized to treat chronic subdural hematomas.
Landmarks and Anatomic Variants of Clinical Importance and Relevance to Skull Base Surgery
Understanding the relationship of SB lesions to important anatomic landmarks and recognizing anatomic variants is crucial in planning the surgical approach. Knowledge of relevant anatomy and anatomic variants may help avoid complications.
ASB
The crista galli has important clinical implications in ASB surgery particularly in endoscopic transcribriform approaches. Knowledge of dimensions of the crista galli is important in preoperative planning of access to the lesion and instrumentation. The crista galli, being part of the ethmoid bone, may be pneumatized and accumulate fluid ( Fig. 6 ) that can get infected and simulate rhinosinusitis. Mucoceles may expand the crista galli and produce bone resorption and new bone formation. Alterations in the morphology of bony landmarks (such as the foramen cecum and frontal ridge) (see Fig. 3 A) may serve as important clues to the diagnosis and extent of disease. Lesions such as meningoencephalocele may produce expansion of the foramen cecum and erosion of the crista galli.
The olfactory recess contains the olfactory bulb and anterior ethmoidal artery (see Fig. 3 C). The depth of the olfactory fossa is defined by the height of the lateral lamella. The Keros classification is the most common criterion used to assess the risk of injury to the anterior cranial base. It provides helpful information on the depth of the olfactory fossa in surgical planning and in avoiding iatrogenic injury to the cribriform plate and medial ethmoid roof. , Preoperative CT is necessary for identifying the anterior ethmoidal artery and avoiding injury to this vessel during endoscopic sinus surgery.
CSB
The vidian canal (see Fig. 4 B) is an important landmark for identification of the petrous segment of the internal carotid artery (ICA), particularly during extended endoscopic endonasal approaches to cranial base surgery. The vidian canal lies along the medial pterygoid plateau and harbors the vidian artery, vein, and nerve. It has a critical role in planning endoscopic SB surgeries because of its relationship to the ICA. The vidian canal is found to be below the level of the anterior genu of the petrous (ICA) in most patients (89%), and less commonly at the same level or above the petrous ICA (7% and 4%, respectively). This implies that a working room inferior and medial to the vidian canal might not always be safe because this could be located superior to the level of the anterior genu of the petrous ICA. Because the vidian nerve lies immediately underneath the sphenoid sinus mucosa, bony dehiscence of the vidian canal may have implications regarding potential injury to the vidian nerve.
The palatovaginal canal runs along the floor of the sphenoid sinus, medial to the vidian canal (see Fig. 4 B). The constant relationship of the external orifice of the vidian and palatovaginal canals and sphenopalatine artery can be used as an anatomic landmark when performing endoscopic vidian neurotomy.
The intimate relationship of the anterior clinoid process (ACP) to the clinoid segment of the ICA makes treatment of vascular and neoplastic lesions related to the ACP challenging. Preoperative CT is useful in detecting anatomic variations of the ACP. Appropriate modifications may be required when performing extradural anterior clinoidectomy to ensure a safe approach. Pneumatization of the ACP should not be mistaken for flowvoids in the ICA ( Fig. 7 A ).
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