Lecture #127: Anatomy: The Orbit and Extraocular Muscles

What fills most of the orbital space?

The orbit is primarily filled with adipose tissue, which cushions the eye and allows smooth movement within the bony cavity.

What is a blowout fracture of the orbit?

A blowout fracture occurs when increased orbital pressure forces orbital contents through weak areas of the orbital walls—typically the medial wall (ethmoid) or floor (maxilla). This may trap muscles or soft tissue and affect eye movement.

Which orbital walls are most likely to fail in a blowout fracture?

The medial wall (ethmoid bone, lamina papyracea) and orbital floor (maxilla) are the thinnest and most vulnerable to collapse under pressure from trauma.

What is the function of the levator palpebrae superioris?

The levator elevates the upper eyelid. It originates from the lesser wing of the sphenoid and inserts into the superior tarsus. Its smooth muscle portion (Müller’s muscle) provides sympathetic tone that helps keep the eyelid elevated.

What is the origin of all four rectus muscles?

All four rectus muscles (superior, inferior, medial, lateral) originate from the common tendinous ring at the apex of the orbit and insert just posterior to the cornea on the sclera.

What is the unique course of the superior oblique muscle?

The superior oblique originates from the sphenoid, passes through the trochlea (a fibrocartilaginous pulley), then inserts deep to the superior rectus on the posterolateral sclera.

What is the origin and path of the inferior oblique muscle?

The inferior oblique originates from the anterior medial orbital wall (unlike all others) and travels posterolaterally beneath the eyeball to insert deep to the lateral rectus.

What is the innervation pattern of the extraocular muscles?

Extraocular innervation follows: SO4 (superior oblique = CN IV), LR6 (lateral rectus = CN VI), and AO3 (all others = CN III). Sympathetic fibers innervate Müller’s muscle in the upper eyelid.

What movements occur on the vertical axis of the eye?

Abduction (movement away from the midline) and adduction (movement toward the midline) occur along the vertical axis.

What movements occur on the horizontal axis of the eye?

Elevation and depression occur on the horizontal axis, moving the eye up or down respectively.

What movements occur around the anterior-posterior axis of the eye?

Intorsion (inward rotation) and extorsion (outward rotation) occur around the AP axis, helping maintain visual stability during head tilting.

What is the difference between a muscle's action and its function?

Action describes what a muscle does when acting alone. Function describes the muscle’s true contribution during coordinated eye movements, which depends on eye position and pairing with other muscles.

Which muscles elevate the abducted eye?

The superior rectus is the primary elevator of an abducted eye because its line of pull aligns with the eye’s axis when abducted.

Which muscles depress the abducted eye?

The inferior rectus strongly depresses the abducted eye due to alignment of its force vector when the eye is turned laterally.

Which muscles elevate the adducted eye?

The inferior oblique elevates the adducted eye because its path allows it to pull the eye upward when it is turned medially.

Which muscles depress the adducted eye?

The superior oblique depresses the adducted eye, as its tendon’s pull becomes a downward force when the eye is turned medially.

Why do specific muscles control elevation and depression depending on adduction vs. abduction?

Because the rectus and oblique muscles attach at angles that change how their pull translates into movement. Adduction alters the vertical pull of the obliques; abduction aligns vertical pull with the recti.

What is torsion in eye movement?

Torsion refers to rotation of the eye around its long (AP) axis. It accompanies many eye movements due to the misalignment between the orbital axis and the visual axis.

Which muscles cause intorsion of the eye?

Intorsion is primarily produced by the superior oblique and secondarily by the superior rectus, especially in primary position.

Which muscles cause extorsion of the eye?

Extorsion is produced by the inferior oblique and secondarily by the inferior rectus, helping stabilize the eye during head tilt.

How do the superior rectus and inferior oblique cooperate in eye elevation?

They coordinate to elevate the eye without unwanted torsion. As the superior rectus tends to intort, the inferior oblique extorts, balancing torsional forces.

How do the superior oblique and inferior rectus cooperate in eye depression?

The superior oblique tends to intort the eye during depression, while the inferior rectus tends to extort, neutralizing torsional motion for straight downward gaze.

How is elevation tested clinically in the right eye?

The eye is abducted first (testing SR) then elevated; when adducted, elevation tests the IO. This isolates each muscle by aligning or misaligning their force vectors.

What is the purpose of the “H test” in extraocular muscle evaluation?

The H test positions the eye so that each muscle’s primary action is isolated, allowing accurate assessment of extraocular muscle function and nerve integrity.

What is the classic presentation of an oculomotor (CN III) lesion?

The eye appears “down and out” due to unopposed action of lateral rectus and superior oblique, often with ptosis and pupillary dilation.

What is Bielschowsky’s sign and what does it indicate?

Bielschowsky’s sign is increased vertical deviation of the eye when the head is tilted toward the side of a superior oblique palsy. Head tilt worsens diplopia due to loss of intorsion.

Why does increased orbital pressure require fractures for expansion?

The orbit is bounded by rigid bone and filled with noncompressible structures. Pressure from trauma must exit through weak walls, producing blowout fractures.

Why is primary vs. non-primary eye position important for muscle function?

Muscle force vectors change when the eye is adducted or abducted. Actions described in textbooks assume primary position, but real functional testing requires moving the eye out of primary position.

Why is torsion important during head tilt?

Torsion stabilizes the visual field when the head tilts, preventing the world from appearing tilted by counter-rotating the eyes in the opposite direction.