Annulus of Zinn

The annulus of Zinn is a fibrous ring that overlies the optic canal and the medial aspect of the superior orbital fissure and is continuous with the dura of the middle cranial fossa. It is divided into the superior Lockwood tendon and the inferior tendon of Zinn.

Six of the seven extraocular muscles arise from the annulus of Zinn: The four rectus muscles, the levator palpebrae superioris muscle, and the superior oblique muscle. The inferior oblique muscle arises from the medial orbital floor lateral to the lacrimal sac.

Cranial nerves II, III, nasocilliary branch of V1, and VI enter through the annulus of Zinn, while Cranial nerves IV and the frontal and lacrimal branches of the ophthalmic division of the trigeminal nerve (V1) enter the orbit through the superior orbital fissure lateral to the annulus of Zinn.

The ophthalmic artery also passes through the annulus of Zinn, while the superior ophthalmic vein enters the orbit through the superior orbital fissure lateral to the annulus of Zinn. The diagram, adapted from Petruzzelli and Hampson, shows the right orbit. 2=cranial nerve II; 3i=inferior division of cranial nerve III; 3s=superior division of cranial nerve III; 4=cranial nerve IV; 6=cranial nerve VI.

References

• Aviv RI, Casselman J. Orbital imaging: Part 1. Normal anatomy. Clin Radiol. 2005 Mar;60(3):279-87
• Petruzzelli GJ and Hampson CM. Orbit Anatomy. eMedicine. Mar 11, 2008.