What is the annulus of Zinn?

The annulus of Zinn (common tendinous ring) is a fibrous ring at the orbital apex that surrounds the optic canal. Five of the six extraocular muscles originate from this structure (all four rectus muscles and the superior oblique). The inferior oblique is the exception, originating from the anterior orbital floor.

What is the spiral of Tillaux?

The spiral of Tillaux describes the progressive increase in insertion distance from the limbus for the four rectus muscles. From closest to farthest: medial rectus (5.5 mm), inferior rectus (6.5 mm), lateral rectus (6.9 mm), and superior rectus (7.7 mm). This knowledge is important for strabismus surgery and for understanding how each muscle's leverage changes based on its insertion point.

Why is the inferior oblique's origin unique?

The inferior oblique is the only extraocular muscle that originates from the anterior orbital floor (near the nasolacrimal fossa) rather than from the annulus of Zinn at the orbital apex. This anterior origin gives it a different angle of approach to the globe and contributes to its unique combination of actions (extorsion, elevation, and abduction).

Extraocular Muscle Anatomy: Origins, Insertions, and Actions

The Six Extraocular Muscles

Each eye has six extraocular muscles (EOMs) that coordinate to produce precise eye movements. Four are rectus muscles (straight muscles that insert anterior to the equator of the globe) and two are oblique muscles (muscles that approach the globe at an angle).

Origins

The four rectus muscles and the superior oblique originate from the annulus of Zinn (common tendinous ring) at the orbital apex, surrounding the optic canal. The inferior oblique is the exception: it originates from the maxillary bone near the nasolacrimal fossa, making it the only EOM that does not originate from the orbital apex.

Insertions

The rectus muscles insert on the sclera at varying distances from the limbus. The spiral of Tillaux describes the progressive distance of each rectus insertion from the limbus:

Medial rectus: 5.5 mm (closest to limbus)
Inferior rectus: 6.5 mm
Lateral rectus: 6.9 mm
Superior rectus: 7.7 mm (farthest from limbus)

The oblique muscles insert behind the equator of the globe, on the posterior sclera.

Primary Actions

Muscle	Primary Action	Innervation
Medial rectus	Adduction (inward)	CN III
Lateral rectus	Abduction (outward)	CN VI
Superior rectus	Elevation (upward)	CN III
Inferior rectus	Depression (downward)	CN III
Superior oblique	Intorsion, depression, abduction	CN IV
Inferior oblique	Extorsion, elevation, abduction	CN III

Each muscle also has secondary and tertiary actions. The oblique muscles have particularly complex actions because of their angled insertion behind the equator.

The superior oblique's primary action (intorsion) is often tested clinically by asking the patient to look down and in. In a CN IV palsy, the patient has difficulty looking down while the eye is adducted and often tilts their head away from the affected side to compensate for the torsional misalignment.

Blood Supply

The EOMs receive their blood supply primarily from the muscular branches of the ophthalmic artery. Each rectus muscle also contributes anterior ciliary arteries that supply the anterior segment. The medial, inferior, and lateral rectus muscles each contribute two anterior ciliary arteries, while the lateral rectus contributes one.

This arterial contribution is clinically relevant during strabismus surgery: operating on more than two rectus muscles simultaneously risks compromising the anterior segment blood supply.

Forgetting that the inferior oblique is the only EOM that does not originate from the annulus of Zinn. It originates from the anterior orbital floor near the lacrimal fossa. This unique origin is a common exam question and is clinically relevant for surgical approaches.

Key Takeaways

Five of six EOMs originate from the annulus of Zinn; the inferior oblique originates from the orbital floor
Rectus muscle insertions follow the spiral of Tillaux: MR (5.5 mm), IR (6.5 mm), LR (6.9 mm), SR (7.7 mm)
Primary actions: MR adducts, LR abducts, SR elevates, IR depresses
Innervation follows LR6(SO4)3: lateral rectus by CN VI, superior oblique by CN IV, all others by CN III
Rectus muscles contribute anterior ciliary arteries to the anterior segment blood supply