Skull Base: Foramina and Cranial Nerve Exits

Overview

The internal surface of the cranial base is stepped into three fossae — anterior, middle, and posterior — each descending toward the foramen magnum. The foramina piercing this floor transmit the cranial nerves and vessels that supply the face, teeth, and jaws. Mastery of these openings underlies the interpretation of skull-base fractures, the spread of dental infection, and the rationale for regional nerve blocks.

Anterior Cranial Fossa

Houses the frontal lobes.

• Cribriform plate of the ethmoid: transmits the olfactory nerve (CN I) filaments. Fracture here causes anosmia and CSF rhinorrhoea.
• Anterior and posterior ethmoidal foramina: ethmoidal vessels and nerves.

Middle Cranial Fossa

Houses the temporal lobes and the pituitary in the sella turcica; the richest fossa for foramina, dominated by branches of the trigeminal nerve.

• Optic canal: optic nerve (CN II) and ophthalmic artery.
• Superior orbital fissure: CN III, IV, VI, the ophthalmic division of the trigeminal (CN V1), and the ophthalmic veins.
• Foramen rotundum: the maxillary nerve (CN V2) — en route to the pterygopalatine fossa and ultimately the upper teeth.
• Foramen ovale: the mandibular nerve (CN V3) and accessory meningeal artery — the target territory for the lower teeth.
• Foramen spinosum: middle meningeal artery.
• Foramen lacerum: crossed superiorly by the internal carotid artery.

Posterior Cranial Fossa

Houses the brainstem and cerebellum.

• Internal acoustic meatus: facial (CN VII) and vestibulocochlear (CN VIII) nerves.
• Jugular foramen: CN IX, X, XI and the internal jugular vein.
• Hypoglossal canal: hypoglossal nerve (CN XII).
• Foramen magnum: medulla-spinal cord junction, vertebral arteries, and the spinal roots of CN XI.

Trigeminal Divisions and the Teeth

The skull-base foramina explain the entire sensory map of the teeth:

• CN V2 leaves via foramen rotundum, enters the pterygopalatine fossa, and gives the superior alveolar nerves to the upper teeth and the greater/lesser palatine and nasopalatine nerves to the palate.
• CN V3 leaves via foramen ovale and gives the inferior alveolar nerve (to the lower teeth, entering the mandibular foramen), the lingual nerve, and the buccal and auriculotemporal nerves.

Clinical Relevance

• Regional anaesthesia: an inferior alveolar nerve block deposits anaesthetic near the mandibular foramen where CN V3's dental branch enters the mandible; maxillary blocks target the nerve trunks derived from foramen rotundum in the pterygopalatine fossa.
• Spread of infection: emissary veins and the pterygoid venous plexus create routes by which dental and facial infection can reach the cavernous sinus, risking cavernous sinus thrombosis.
• Skull-base fractures: CSF rhinorrhoea (cribriform plate) or otorrhoea (petrous temporal) signals a breach of the meninges.
• Trigeminal neuralgia: lancinating pain in a V2 or V3 distribution can mimic dental pain and is a frequent diagnostic trap.

Study Points

• Assign each cranial nerve to its foramen of exit.
• Trace CN V2 and CN V3 from the middle cranial fossa to the upper and lower teeth respectively.
• Explain the anatomical basis of inferior alveolar and maxillary nerve blocks.