Annulus of Zinn
The annulus of Zinn is the fibrous ring formed by the common origin of the 4 rectus muscles (Fig 1-6). The ring encircles the optic foramen and the central portion of the superior orbital fissure. The superior origin of the lateral rectus muscle separates the superior orbital fissure into 2 compartments. The portion of the orbital apex enclosed by the annulus is called the oculomotor foramen. This opening transmits CN III (upper and lower divisions), CN VI, and the nasociliary branch of the ophthalmic division of CN V (trigeminal). The superior and lateral aspect of the superior orbital fissure external to the muscle cone transmits CN IV as well as the frontal and lacrimal branches of the ophthalmic division of CN V. Cranial nerve IV is the only nerve that innervates an extraocular muscle and does not pass directly into the muscle cone when entering the orbit. A retrobulbar block therefore spares this muscle’s action. Cranial nerves III and VI pass directly into the muscle cone through the oculomotor foramen. The superior ophthalmic vein passes through the superior and lateral portion of the superior orbital fissure outside the oculomotor foramen.
Figure 1-6 Anterior view of the right orbital apex showing the distribution of nerves as they enter through the superior orbital fissure and optic canal. This view also shows the annulus of Zinn, the fibrous ring formed by the common origin of the 4 rectus muscles.
(Modified with permission from Nerad JA. Techniques in Ophthalmic Plastic Surgery. Philadelphia: Saunders; 2010.)
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.