Anatomy of the Orbits and Extraocular Muscles

1. Overview

2. Bony Boundaries

Orbital Rim (Base)

Four Walls

• Orbital part of the frontal bone (main contributor) + lesser wing of sphenoid posteriorly
• Features: trochlear fovea anteromedially (attachment for the superior oblique pulley) and lacrimal fossa anterolaterally (for the orbital part of lacrimal gland)
• Separates orbit from the anterior cranial fossa

• Maxilla (frontal process), lacrimal bone, ethmoid (orbital plate - the largest contributor, contains ethmoidal air cells), sphenoid (posteriorly)
• Thinnest wall (lamina papyracea of ethmoid) - fractures here allow air into the orbit
• Contains the lacrimal groove (bounded by anterior lacrimal crest of maxilla and posterior lacrimal crest of lacrimal bone)
• Anterior and posterior ethmoidal foramina are at the junction of medial wall and roof (transmit anterior/posterior ethmoidal nerves and vessels)
• The two medial walls are parallel to each other

• Orbital surface of maxilla (main contributor) + small parts of zygomatic and palatine bones
• Also forms the roof of the maxillary sinus
• The inferior orbital fissure runs along the posterolateral boundary of the floor
• Most susceptible to blowout fractures, often trapping the inferior rectus and inferior oblique muscles

• Anteriorly: zygomatic bone; posteriorly: greater wing of sphenoid
• Thickest, strongest wall
• The superior orbital fissure lies between the greater and lesser wings of sphenoid (between the roof and lateral wall)

3. Key Openings

Superior Orbital Fissure (SOF)

• Located between the greater and lesser wings of the sphenoid, between the roof and lateral wall
• Transmits: CN III (oculomotor), CN IV (trochlear), CN V1 (ophthalmic - lacrimal, frontal, nasociliary branches), CN VI (abducens), superior ophthalmic vein, orbital branch of the middle meningeal artery, recurrent branch of lacrimal artery
• SOF syndrome: ophthalmoplegia + ptosis + fixed dilated pupil, WITHOUT vision loss

Inferior Orbital Fissure (IOF)

• Between the greater wing of sphenoid, orbital surface of maxilla, and orbital process of palatine bone
• Transmits: CN V2 branches (infraorbital nerve + zygomatic nerve), branches from sphenopalatine ganglion to lacrimal gland, infraorbital artery and vein, ophthalmic vein branch

Optic Canal

• Formed by the lesser wing of sphenoid
• Runs from the middle cranial fossa to the orbital apex
• Transmits: optic nerve (CN II) + ophthalmic artery
• Orbital apex syndrome: complete ophthalmoplegia + ptosis + fixed dilated pupil + visual loss (due to CN II, III, IV, V1, VI involvement)

4. Contents of the Orbit

5. Extraocular Muscles

The Six Muscles

Detailed Muscle Table

The Annulus of Zinn

Spiral of Tillaux

The Trochlea

• Located at the angle between the superior and medial walls of the orbit
• Only cartilage in the orbit
• The tendon of the superior oblique passes through it, changing direction to insert on the posterior upper temporal quadrant of the globe
• A displaced trochlea causes diplopia on downward gaze
• Trochleitis causes peritrochlear pain without extraocular movement deficit

6. Muscle Actions and Eye Positions

• When the eye is abducted 23°: visual axis aligns with superior/inferior rectus - these act as pure elevators/depressors
• When the eye is adducted 51°: visual axis aligns with superior/inferior oblique - these act as pure depressors/elevators

• Superior rectus is best tested with eye abducted (looking out)
• Superior oblique is best tested with eye adducted (looking in)

Cardinal Directions of Gaze (Yoke Muscles)

7. Levator Palpebrae Superioris

• Origin: Lesser wing of sphenoid
• Insertion: Superior tarsal plate and skin of upper lid
• Innervation: CN III (oculomotor nerve)
• Action: Elevates the upper eyelid
• Horner's syndrome (loss of sympathetic innervation to superior tarsal muscle/Muller's muscle) gives partial ptosis; complete ptosis implies CN III lesion

8. Clinical Correlates