Anatomy of the Skull Base

Skull Base Overview - Cranial Floor Show

• Floor of the cranial cavity, separating brain from facial structures & upper aerodigestive tract.
• Comprises three distinct regions or fossae, arranged like steps:
  • Anterior Cranial Fossa (ACF): Houses frontal lobes. Formed by frontal, ethmoid, and sphenoid (lesser wings) bones.
  • Middle Cranial Fossa (MCF): Butterfly-shaped; lodges temporal lobes & pituitary. Formed by sphenoid (greater wings, body) & temporal bones.
  • Posterior Cranial Fossa (PCF): Largest & deepest; contains cerebellum & brainstem. Formed by occipital, temporal, and parts of sphenoid & parietal bones.

⭐ The clivus, a bony slope from the dorsum sellae to the foramen magnum, is a critical landmark of the posterior cranial fossa, related to the pons and medulla oblongata.

Anterior Cranial Fossa - Frontal Lobe's Perch

• Bony Make-up: Orbital plates (Frontal); Cribriform plate, Crista galli (Ethmoid); Lesser wings, Planum sphenoidale (Sphenoid).
• Contents: Houses frontal lobes, olfactory bulbs & tracts.
• Key Foramina & Structures:
  • Cribriform Plate: Olfactory nerve fascicles (CN I).
  • Anterior Ethmoidal Foramen: Anterior ethmoidal artery, vein, nerve.
  • Posterior Ethmoidal Foramen: Posterior ethmoidal artery, vein, nerve.
  • Foramen Cecum: Emissary vein (variable; to sup. sagittal sinus).
• Posterior Boundary: Posterior edge of lesser sphenoid wings (contains optic canal medially with CN II, ophthalmic artery).
• Clinical Pearls:
  • Trauma: CSF rhinorrhea, anosmia, periorbital ecchymosis ("panda eyes").
  • Common tumors: Meningioma, esthesioneuroblastoma.

  ⭐ Fractures of the cribriform plate are a classic cause of traumatic anosmia and CSF rhinorrhea.

Middle Cranial Fossa - Sphenoid's Busy Hub

• Key Bone: Sphenoid (body, lesser & greater wings).
• Boundaries:
  • Anterior: Lesser sphenoid wing.
  • Posterior: Petrous temporal, dorsum sellae.
  • Floor: Greater sphenoid wing, squamous temporal.
• Central Feature: Sella turcica (houses pituitary gland).
• Key Foramina & Transmitted Structures:
  • Optic Canal: CN II, ophthalmic artery.
  • Superior Orbital Fissure (SOF): CN III, IV, V1 (ophthalmic), VI; sup. ophthalmic vein.
  • Foramen Rotundum: CN V2 (maxillary). 📌 Rotundum for V2 (Maxillary n.).
  • Foramen Ovale: CN V3 (mandibular), accessory meningeal a., lesser petrosal n., emissary v. 📌 MALE in OVALE: Mandibular n., Accessory meningeal a., Lesser petrosal n., Emissary v.
  • Foramen Spinosum: Middle meningeal artery & vein, nervus spinosus. 📌 Spinosum = Small hole for Middle Meningeal Artery.
  • Foramen Lacerum: Cartilage-filled; ICA passes superiorly.
• Cavernous Sinus: Lateral to sella.
  • Contents: ICA, CN III, IV, V1, V2, VI.
  • 📌 Mnemonic: O TOM CAt (Oculomotor, Trochlear, Ophthalmic, Maxillary, Carotid (Internal), Abducens).

  ⭐ Infections from the 'danger triangle' of the face can spread to the cavernous sinus, causing thrombosis - a neurosurgical emergency.

Posterior Cranial Fossa - Brainstem's Grand Exit

• Largest & deepest fossa, housing vital structures.
• Boundaries:
  • Anterior: Dorsum sellae, superior petrous ridge of temporal bone.
  • Posterior: Squamous occipital bone.
  • Lateral: Petrous & mastoid temporal, lateral parts of occipital bone.
• Floor: Formed by occipital bone, posterior sphenoid body, petromastoid temporal.
• Key Foramina & Contents:
  • Foramen Magnum: Medulla, vertebral arteries, meninges, spinal roots of CN XI.
  • Jugular Foramen: CN IX, X, XI; Internal Jugular Vein (IJV). 📌 "Jugular 9, 10, 11"
  • Hypoglossal Canal: CN XII.
  • Internal Acoustic Meatus: CN VII, VIII, labyrinthine artery.
• Contents: Brainstem (pons, medulla), cerebellum.

⭐ The jugular foramen is divided by a fibrous or bony septum into an anteromedial pars nervosa (CN IX, X, XI) and a posterolateral pars vascularis (IJV).

High‑Yield Points - ⚡ Biggest Takeaways

• Cribriform plate: Transmits CN I; fracture risks CSF rhinorrhea and anosmia.
• Optic canal: Carries CN II and ophthalmic artery; crucial for vision.
• Superior orbital fissure: Passage for CN III, IV, V1, VI and superior ophthalmic vein.
• Foramen ovale for CN V3; foramen spinosum for middle meningeal artery.
• Jugular foramen: Transmits CN IX, X, XI and internal jugular vein.
• Internal acoustic meatus: Houses CN VII (facial) and CN VIII (vestibulocochlear).
• Clivus: Formed by basiocciput & basisphenoid; common site for chordomas.