Inferior oblique muscle

The inferior oblique is one of the two oblique extraocular muscles and is unique because it is the only extraocular muscle that originates from the anterior part of the orbit, rather than the posterior aspect.

Origin and Insertion

• Origin: From the orbital surface of the maxilla, just lateral to the lacrimal fossa, in the anteromedial part of the orbital floor.

• Course: The muscle passes laterally and posteriorly across the floor of the orbit, running beneath the inferior rectus.

• Insertion: Into the posterior part of the sclera, in the inferolateral quadrant, beneath the lateral rectus, and behind the equator of the eyeball.

Nerve Supply

• Oculomotor nerve (cranial nerve III) — inferior division.

Actions

• Primary action: Extorsion — rotates the upper pole of the eyeball away from the nose.

• Secondary actions: Elevation and abduction (moves the eye upward and outward).

Functional Note

When the eye is in the primary position (looking straight ahead), the inferior oblique runs backward and outward beneath the eyeball, forming an angle of about 51° with the visual axis.

Because of this oblique alignment, it pulls the inferior surface of the eyeball frontward, medially, and outward, → producing elevation, abduction, and extorsion.

Clinical Correlation

To test the inferior oblique, the examiner asks the patient to first adduct the eye (look medially), then look upward. This aligns the visual axis with the muscle’s long axis, isolating its elevating action. In the adducted position, the inferior oblique acts as the main elevator of the eyeball.