Inferior oblique muscle

Origin and insertion

The thin inferior oblique muscle lies near the anterior border of the floor of the orbit. Similar to the superior oblique muscle, but unlike the recti muscles, inferior oblique does not originate from the common tendinous ring (annulus of Zinn). Instead, inferior oblique arises from the anterior aspect of the orbit.

Specifically, from the medial (orbital) surface of the maxilla just lateral to the nasolacrimal fossa. From there it ascends obliquely and posterolaterally, coursing between the lateral rectus muscle and the floor of the orbit. The muscle inserts onto the inferolateral half of the sclera, posterior to the equator of the eyeball.

Relations

The inferior oblique muscle inserts at the posterior quadrant of the eyeball (inferolateral part), between the lateral and inferior recti muscles, and slightly posterior to the insertion of the superior oblique muscle. As the inserting tendons of the inferior and superior oblique muscles pass each other, they lie in the same oblique vertical plane.

The fascial sheath of inferior oblique blends with the thickened sheath of the inferior rectus muscle. These two then blend with the medial and lateral check ligaments, which are triangular sheet expansions of the medial and lateral recti muscles. This fusion of the inferior rectus, inferior oblique and the check ligaments, forms the suspensory ligament of the eyeball, a hammock-like sling that provides support to the eyeball. Additionally, the fused sheaths of the inferior rectus and inferior oblique give off an anterior expansion called the inferior check ligament which attaches itself on the tarsus of the inferior eyelid.

Innervation

The inferior oblique muscle is supplied by the inferior division of the oculomotor nerve (CN III), which provides it with general somatic efferent fibers.

Blood supply

Inferior oblique is supplied by the ophthalmic artery and infraorbital artery, these arise from the internal carotid and maxillary arteries, respectively.

Function

Due to its oblique course and attachment on the posterolateral side of the eyeball, contraction of the inferior oblique muscle pulls the eyeball in a direction posterior to its vertical axis, and therefore rotates the eye laterally around this axis. Thus the inferior oblique simultaneously exhibits three actions on the eyeball:

• Elevates the anterior part of the eyeball; by depressing the posterior part of the eyeball, particularly when the eye is adducted
• Abducts the eyeball laterally
• Externally rotates the eyeball

By elevating and abducting the eyeball the inferior oblique rotates the visual axis upwards and outwards. This is significant for functions such as looking up from reading, in which the eyes move from a state of convergence.

All extraocular muscles work either in synergy or opposition to one another. For example, the inferior oblique muscle works in synergy with the superior rectus muscle to elevate the pupil, but at the same time their rotatory actions oppose and neutralize each other. The complimentary action is important to ensure that no rotation of the eyeball occurs during its elevation. In addition to these functions, the inferior oblique muscle also acts with the aforementioned check ligaments and the retrobulbar fat to prevent retraction of the eyeball by the rectus muscles. These structures provide support for the eyeball and help retain its position.