superior oblique
Also called SO, superior oblique muscle
The extraocular muscle that runs forward along the upper inner wall of the orbit, passes through a cartilaginous pulley called the trochlea, and turns back and outward to insert behind the equator on the upper temporal part of the globe. Its primary action is intorsion, with depression and abduction as subsidiary actions1,3.
With the eye turned inward by about 51 degrees it becomes a pure depressor, which is its test position (down and in)1,2. Its antagonist is the inferior oblique, and the trochlear nerve, the fourth cranial nerve, supplies it and no other muscle2. A palsy produces a hypertropia worst in adduction and on tilt toward the same shoulder, with an excyclotorsion of the eye.
More detail
The trochlea acts as the muscle's working origin: because the tendon pulls from the front and inserts behind the equator, contraction rolls the top of the eye toward the nose and pulls the back of the globe upward, which turns the front downward. Texts give the angle between tendon and visual axis as 51 or 54 degrees1,2. With the eye turned outward the muscle becomes a pure intorter. Its yoke in down-and-in gaze is the inferior rectus of the other eye, and the nerve enters from the outer surface of the muscle rather than the side facing the globe2. The front fibers of the tendon intort and the back fibers depress, which lets surgery adjust torsion and depression separately1. A palsy also produces a head tilt to the opposite shoulder.
Sources (4)
- Kanski, Clinical Ophthalmology (2024), ch. 18
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 3 and ch. 4
- Adler's Physiology of the Eye, ch. 7
- Pickwell's Binocular Vision Anomalies, ch. 17
Reviewed October 2026