Base of skull
The base of the skull, also known as the cranial base, is the inferior surface of the cranium that forms the floor of the cranial cavity, providing structural support for the brain while allowing the passage of neurovascular structures and the spinal cord.[1] It is a complex, irregular region composed of five primary bones: the unpaired ethmoid, sphenoid, occipital, and frontal bones, along with the paired temporal bones.[2] This bony platform separates the intracranial contents from the extracranial spaces and is characterized by varying depths and numerous foramina that facilitate the transit of cranial nerves, arteries, veins, and other vital elements.[2] The base of the skull is anatomically divided into three distinct fossae, corresponding to the major compartments of the brain.[2] The anterior cranial fossa, the shallowest division, is formed anteriorly by the frontal bone and posteriorly by the lesser wings of the sphenoid and the ethmoid bone; it supports the frontal lobes of the brain and includes the cribriform plate for olfactory nerve passage.[1] The middle cranial fossa, deeper and butterfly-shaped, extends from the lesser wings of the sphenoid to the petrous ridges of the temporal bones, housing the temporal lobes and featuring key openings like the optic canal, superior orbital fissure, foramen rotundum, foramen ovale, and foramen spinosum for cranial nerves II–VI and associated vessels.[3] The posterior cranial fossa, the deepest section, is bounded anteriorly by the petrous temporal bones and posteriorly by the occipital bone, accommodating the cerebellum and brainstem with major foramina such as the foramen magnum (for the spinal cord and vertebral arteries), jugular foramen (for cranial nerves IX–XI and the internal jugular vein), and hypoglossal canal (for cranial nerve XII).[2] These divisions and foramina underscore the base of the skull's critical role in protecting neural and vascular pathways, making it a frequent site of pathology in trauma, tumors, and infections.[2] For instance, basilar skull fractures, often resulting from high-impact head injuries, can disrupt these passages, leading to complications like cerebrospinal fluid rhinorrhea or vascular damage such as epidural hematomas from middle meningeal artery rupture.[1] The region's intricate anatomy also poses unique challenges in surgical approaches, particularly for skull base tumors, where precise navigation of neurovascular elements is essential.[3]Overview
Definition and boundaries
The base of the skull, also referred to as the cranial base, constitutes the inferior aspect of the cranium and serves as the floor of the cranial cavity, providing structural support for the brain while permitting the passage of neurovascular elements. This region effectively separates the intracranial contents from the underlying facial skeleton (viscerocranium) and neck structures, forming a critical barrier that maintains compartmentalization between the neurocranium and extracranial spaces.[2] Externally, the boundaries of the skull base are delineated anteriorly by the frontal bone, which interfaces with the nasal cavity; posteriorly by the occipital bone, encompassing the foramen magnum as the primary exit for the spinal cord; and laterally by the temporal bones, which contribute to the lateral margins and petrous ridges. These external limits integrate the skull base with adjacent skeletal elements, ensuring continuity between the cranium and the orofacial and cervical regions. Internally, the endocranial surface presents a complex topography characterized by fossae and ridges that create impressions accommodating the contours of the frontal and temporal lobes, as well as the cerebellum, while also featuring sulci for the dural venous sinuses that facilitate cerebral venous drainage.[2] In relation to the overall cranium, the skull base is distinctly positioned as the foundational floor of the neurocranium, contrasting with the calvaria—the domed superior vault formed primarily by the frontal, parietal, and occipital bones—that provides overhead protection for the brain. This architectural distinction underscores the skull base's role in both supporting and insulating neural structures from inferior threats.[2]Divisions
The base of the skull is subdivided into three primary fossae: the anterior, middle, and posterior cranial fossae, which collectively form distinct compartments that accommodate specific regions of the brain and facilitate neurovascular passage.[4][5][6] These divisions are defined by bony ridges and sulci that create natural boundaries, allowing for compartmentalized protection and support of intracranial structures. The anterior cranial fossa is the shallowest and most anterior of the three, formed primarily by the frontal bone, ethmoid bone, and parts of the sphenoid bone. It houses the anteroinferior portions of the frontal lobes of the cerebrum and is bounded posteriorly by the lesser wings of the sphenoid bone laterally and the limbus of the sphenoid bone medially.[4] This fossa plays a key role in supporting the frontal brain regions involved in executive functions and olfaction. The middle cranial fossa, located centrally and deeper than the anterior, is shaped by the sphenoid bone and the temporal bones. It accommodates the temporal lobes of the cerebrum laterally and the pituitary gland centrally within the sella turcica, with anterior boundaries formed by the lesser wings of the sphenoid and the limbus, and posterior boundaries defined by the superior borders of the petrous parts of the temporal bones medially and laterally, along with the dorsum sellae.[5] This compartment is essential for housing auditory and memory-related brain areas while providing space for the hypophyseal structures. The posterior cranial fossa represents the deepest and most posterior division, constructed from the occipital bone, temporal bones, and contributions from the sphenoid bone. It contains the cerebellum and brainstem, bounded anteriorly by the dorsum sellae of the sphenoid medially and the superior aspects of the petrous temporal bones laterally.[6] This fossa is critical for enclosing structures responsible for coordination, balance, and vital autonomic functions. Transitions between these fossae are marked by prominent bony ridges and sulci, such as the sphenoidal ridges (formed by the lesser wings of the sphenoid), which separate the anterior from the middle fossa, and the petrous ridges of the temporal bones, which delineate the middle from the posterior fossa.[7] These features ensure structural integrity and guide the spatial organization of the intracranial cavity.Anatomical structure
Constituent bones
The base of the skull is formed by portions of five primary bones: the frontal, ethmoid, sphenoid, temporal (paired), and occipital bones, which collectively create a stable platform supporting the brain and facilitating its compartmentalization into fossae.[2] These bones articulate through sutures to form a continuous bony structure that varies in thickness and contour across the anterior, middle, and posterior regions.[8] The frontal bone contributes to the anterior cranial fossa through its orbital plates, which form the roof of the orbits and provide a smooth, concave surface for the overlying frontal lobes of the brain.[9] These plates are thin and horizontally oriented, extending posteriorly from the supraorbital margins to articulate with adjacent bones.[10] The ethmoid bone also participates in the anterior cranial fossa via its cribriform plate, a horizontal sieve-like structure that lies between the orbital plates of the frontal bone and the body of the sphenoid bone.[2] Additionally, the perpendicular plate of the ethmoid bone descends medially to form part of the nasal septum, anchoring the ethmoid's contribution to the skull base.[11] The sphenoid bone is a central, keystone element spanning all three cranial fossae, with its body forming the core of the base and housing the sella turcica—a saddle-shaped depression that accommodates the pituitary gland.[12] The lesser wings project anteriorly to contribute to the anterior fossa and orbital roofs, while the greater wings extend laterally to form the floor of the middle fossa; posteriorly, the body and pterygoid processes help delineate the posterior fossa boundaries.[13] Each temporal bone, paired on either side, contributes to the middle and posterior cranial fossae through its petrous and squamous parts; the petrous portion, a dense, pyramid-shaped structure, forms the medial wall of the middle fossa and extends into the posterior fossa, providing robust support for the temporal lobes and brainstem.[2] The squamous part lies superiorly, forming a flatter contribution to the lateral aspect of the middle fossa.[14] The occipital bone dominates the posterior cranial fossa with its basilar part, a quadrilateral plate that slopes anteriorly to articulate with the sphenoid and temporal bones, creating the clivus—a smooth incline supporting the brainstem.[15] The squamous part forms the posterior wall of the fossa, while the paired occipital condyles on the basilar region's inferior surface enable articulation with the cervical vertebrae.[16] These bones integrate seamlessly through fibrous sutures, such as the frontoethmoidal, sphenofrontal, and occipitomastoid sutures, ensuring a unified, load-bearing platform that withstands intracranial pressures and transmits forces from the head to the spine.[2]Sutures and articulations
The base of the skull features a network of fibrous joints known as sutures and cartilaginous joints called synchondroses that interconnect its constituent bones, ensuring structural stability while permitting controlled expansion during early growth phases.[2] These articulations are classified by their morphological characteristics, with serrate sutures exhibiting interlocking, saw-tooth-like edges for enhanced interlocking strength, as seen in the frontosphenoidal suture uniting the frontal and sphenoid bones.[17] Plane sutures involve relatively flat opposing surfaces, exemplified by the spheno-occipital synchondrosis, which functions similarly despite its cartilaginous composition.[2] Schindylesis represents a specialized type where a thin bony sheet fits into a fissure of an adjacent bone, such as in the ethmosphenoidal articulation between the ethmoid and sphenoid bones.[17] Prominent sutures specific to the skull base include the sphenofrontal suture, which obliquely joins the lesser wings of the sphenoid to the frontal bone along the anterior cranial fossa; the frontoethmoidal suture, connecting the frontal bone to the ethmoid's cribriform plate; and the sphenoethmoidal suture, linking the sphenoid body to the ethmoid posteriorly.[2] In the posterior region, the occipitomastoid suture binds the occipital bone to the mastoid process of the temporal bone, while the petro-occipital suture secures the petrous portion of the temporal bone to the occipital bone, both contributing to the robust architecture of the posterior cranial fossa.[17] These interconnections distribute mechanical stresses effectively across the skull base.[2] Synchondroses in the skull base, such as the spheno-occipital synchondrosis, are temporary cartilaginous joints that bridge the sphenoid and occipital bones, facilitating longitudinal growth before eventual ossification to reinforce stability in maturity.[2] This joint's persistence as cartilage longer than typical sutures underscores its role in accommodating brain expansion.[17] The skull base also forms a critical articulation with the cervical spine via the atlanto-occipital joint, a paired synovial condyloid joint where the occipital condyles of the occipital bone engage the superior facets of the atlas (C1 vertebra), enabling nodding motions of the head while maintaining overall cranial poise.[2]Foramina and openings
The base of the skull contains numerous foramina, fissures, and canals that serve as passages through the bony structure. These openings are distributed across the anterior, middle, and posterior cranial fossae, as well as associated regions, and vary in size and shape depending on their anatomical position. Their precise locations and dimensions are critical for understanding the structural integrity of the skull base.[1] In the anterior cranial fossa, the primary openings are the multiple small foramina within the cribriform plate of the ethmoid bone, forming a sieve-like structure on the floor of the fossa. These foramina are irregularly spaced and measure approximately 1-2 mm in diameter each, collectively spanning the midline ethmoid region.[1][18] The middle cranial fossa hosts several prominent foramina and fissures. The optic canal is a short, funnel-shaped passage in the lesser wing of the sphenoid bone, located at the anterior-lateral corner of the sella turcica, with a diameter of about 5-6 mm. Adjacent to it, the superior orbital fissure is a cleft between the greater and lesser wings of the sphenoid, measuring roughly 20 mm in length. More posteriorly on the sphenoid floor, the foramen rotundum is a round opening, approximately 3 mm in diameter, situated inferior to the superior orbital fissure; the foramen ovale is an oval-shaped aperture, about 7 mm long and 3 mm wide, located posterolateral to the rotundum; and the foramen spinosum is a smaller, round foramen, around 2.5 mm in diameter, positioned posterior to the ovale. The carotid canal, formed by the temporal bone, is a zigzag passage entering anteromedial to the styloid process and exiting near the apex of the petrous temporal bone, with a variable diameter of 4-7 mm. The vidian canal, also known as the pterygoid canal, runs through the base of the pterygoid process of the sphenoid bone, inferior and medial to the foramen rotundum, measuring about 1.5-2 mm in diameter.[1][18][19][20] The posterior cranial fossa includes larger, more robust openings, as well as the internal auditory meatus, a short canal in the petrous part of the temporal bone on the posterior surface within the fossa, with a diameter of around 4 mm. The foramen magnum is the largest aperture in the skull base, a midline oval in the occipital bone, measuring approximately 30-35 mm in length and 25-30 mm in width. Laterally, the jugular foramen is an irregular, keyhole-shaped passage formed at the junction of the temporal and occipital bones, with dimensions varying from 10-15 mm in its transverse diameter. The hypoglossal canal is a paired, short tunnel in the occipital bone, anterolateral to the foramen magnum, about 6-8 mm long and 4-5 mm wide. The condyloid canal, also termed the posterior condylar canal, is an inconstant emissary passage in the lateral part of the occipital bone near the condylar fossa, typically 2-3 mm in diameter when present.[1][18][21][22]| Fossa/Region | Opening | Bone(s) Involved | Location | Approximate Size |
|---|---|---|---|---|
| Anterior | Cribriform plate foramina | Ethmoid | Floor, midline | 1-2 mm each (multiple) |
| Middle | Optic canal | Sphenoid (lesser wing) | Anterior-lateral sella turcica | 5-6 mm diameter |
| Middle | Superior orbital fissure | Sphenoid (greater/lesser wings) | Anterior wall, lateral to optic canal | 20 mm length |
| Middle | Foramen rotundum | Sphenoid | Floor, inferior to superior orbital fissure | 3 mm diameter |
| Middle | Foramen ovale | Sphenoid | Floor, posterolateral to rotundum | 7 mm x 3 mm |
| Middle | Foramen spinosum | Sphenoid | Floor, posterior to ovale | 2.5 mm diameter |
| Middle | Carotid canal | Temporal | Anteromedial to styloid, petrous apex | 4-7 mm diameter |
| Middle | Vidian (pterygoid) canal | Sphenoid (pterygoid process) | Inferior-medial to rotundum | 1.5-2 mm diameter |
| Posterior | Internal auditory meatus | Temporal (petrous) | Posterior surface within fossa | 4 mm diameter |
| Posterior | Foramen magnum | Occipital | Midline floor | 30-35 mm x 25-30 mm |
| Posterior | Jugular foramen | Temporal, occipital | Lateral floor | 10-15 mm transverse |
| Posterior | Hypoglossal canal | Occipital | Anterolateral to magnum | 6-8 mm x 4-5 mm |
| Posterior | Condyloid canal | Occipital | Lateral condylar fossa | 2-3 mm (inconstant) |