Extraocular Muscle Origins

The annulus of Zinn consists of superior and inferior orbital tendons and is the origin of the 4 rectus muscles (Fig 1-15). The upper tendon gives rise to the entire superior rectus muscle, as well as portions of the lateral and medial rectus muscles. The inferior tendon gives rise to the entire inferior rectus muscle and portions of the medial and lateral rectus muscles. The levator palpebrae superioris muscle arises from the lesser wing of the sphenoid bone, at the apex of the orbit, just superior to the annulus of Zinn (see the section “Levator palpebrae superioris muscle” later in the chapter).

The superior oblique muscle originates from the periosteum of the body of the sphenoid bone, above and medial to the optic foramen. The inferior oblique muscle originates anteriorly, from a shallow depression in the orbital plate of the maxillary bone, at the anteromedial corner of the orbital floor, near the fossa for the lacrimal sac. From its origin, the inferior oblique muscle then extends posteriorly, laterally, and superiorly to insert into the globe (see Table 1-1).

Table 1-1 Comparison of the Extraocular Muscles

Figure 1-15 Origin of the extraocular muscles. All extraocular muscles, except the inferior oblique, originate in the orbital apex. The 4 rectus muscles share a common fibrotendinous ring known as the annulus of Zinn. Note that the superior rectus and medial rectus are juxtaposed to the optic nerve sheath; this is the reason that patients with retrobulbar optic neuritis experience pain with movement of the eye.

(Reproduced with permission from Dutton JJ. Atlas of Clinical and Surgical Orbital Anatomy. 2nd ed. Philadelphia: Elsevier/Saunders; 2011, Fig 3-8.)

Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.