Sphenoid bone

Os sphenoidale

Also known as: sphenoid, keystone of the cranium

A single, bat- or butterfly-shaped bone in the centre of the cranial base that articulates with every other cranial bone. It carries the sella turcica for the pituitary and many of the key foramina of the middle cranial fossa.

Overview

The sphenoid has a central body with paired greater wings, lesser wings and pterygoid processes. The body contains the sphenoidal sinuses and on its upper surface the sella turcica — the hypophyseal fossa (housing the pituitary gland) bounded in front by the tuberculum sellae and behind by the dorsum sellae; the anterior and posterior clinoid processes anchor the tentorium cerebelli. The cavernous sinuses lie on each side of the body.

The lesser wings form the posterior edge of the anterior cranial fossa and contain the optic canals. The cleft between lesser and greater wings is the superior orbital fissure. The greater wings form the floor of the middle cranial fossa, part of the lateral orbital wall and, externally, part of the temple at the pterion; each is pierced by the foramen rotundum, foramen ovale and foramen spinosum (in that order anterior to posterior). The pterygoid processes descend from the junction of body and greater wing as medial and lateral pterygoid plates (origins of the pterygoid muscles), with the pterygoid hamulus (tensor veli palatini pulley) on the medial plate and the pterygoid canal at their root.

Location

Middle of the cranial base, between frontal/ethmoid in front and occipital/temporal behind.

Parts & subdivisions

Body and sella turcica

Hypophyseal fossa for the pituitary gland; sphenoidal sinuses within the body.

Optic canal (lesser wing)

Optic nerve (CN II) and ophthalmic artery.

Superior orbital fissure

Oculomotor (III), trochlear (IV), ophthalmic division of trigeminal (V1: lacrimal, frontal, nasociliary), abducens (VI), superior ophthalmic vein, sympathetic fibres.

Foramen rotundum (greater wing)

Maxillary nerve (V2) to the pterygopalatine fossa.

Foramen ovale (greater wing)

Mandibular nerve (V3), accessory meningeal artery, lesser petrosal nerve, emissary vein ("OVALE").

Foramen spinosum (greater wing)

Middle meningeal artery and vein, meningeal branch of V3 (nervus spinosus).

Pterygoid processes

Medial and lateral pterygoid plates; pterygoid canal (nerve of pterygoid canal = greater + deep petrosal nerves).

Clinoid processes

Anterior (lesser wing) and posterior (dorsum sellae) bony projections; tentorium attachment.

Anatomy

Articulations

All other cranial bones (frontal, parietals, occipital, temporals, ethmoid) and the vomer, palatines, zygomatics and occasionally maxillae.

Ossification

Mostly endochondral (body, lesser wings, roots of greater wings); the lateral greater wings and lateral pterygoid plates are intramembranous. Pre- and post-sphenoid parts fuse ≈8 months fetal; the spheno-occipital synchondrosis fuses at ≈13–18 years.

Clinical significance

Pituitary adenoma

Enlarges the sella turcica (ballooned sella on X-ray) and compresses the optic chiasm above it → bitemporal hemianopia. Resected by the transsphenoidal route through the sphenoidal sinus.

Superior orbital fissure syndrome

Fracture or mass at the fissure → ophthalmoplegia (III, IV, VI), ptosis, fixed dilated pupil and forehead numbness (V1); with optic canal involvement it becomes orbital apex syndrome.

Mnemonics

Foramina of the greater wing (anterior → posterior)

"Standing Room Only": Superior orbital fissure → foramen Rotundum (V2) → foramen Ovale (V3) → (foramen Spinosum, middle meningeal artery).

Superior orbital fissure contents

"Live Frankly To See Absolutely No Insult": Lacrimal, Frontal, Trochlear, Superior division of III, Abducens, Nasociliary, Inferior division of III.