Superior oblique muscle

Origin and insertion

The superior oblique muscle originates from the body of sphenoid bone, medial to the origin of the levator palpebrae superioris muscle and superomedial to the optic canal. In contrast to the other extraocular muscles, superior oblique and inferior oblique do not originate from the common tendinous ring. Instead superior oblique originates externally, superomedially to the tendinous ring. 

From its sphenoid attachment, the superior oblique muscle runs anteriorly, parallel to the medial wall of the orbit. Near its insertion, the muscle tendon hooks around a cartilaginous pulley, called the trochlea of superior oblique. This cartilaginous loop is found attached to the nasal part of frontal bone. Located between the tendon and the trochlea is a thin separating synovial sheet. 

From the trochlea, the tendon takes a sharp posterolateral turn before inserting onto the posterio-superolateral scleral surface of the eye. This insertion is located behind the equator of the eyeball, between the attachments of the superior rectus and lateral rectus muscles.

Relations

The superior oblique muscle is situated in the upper medial portion of the orbit, adjacent to its medial wall. The muscle is located superior to medial rectus and medial to both the superior rectus and levator palpebrae superioris muscles.

The eyeball and all of the extraocular muscles are enveloped by a muscular fascia, derived from the fascial sheath of the eyeball (Tenon’s capsule). This is an important clinical point due to the fact that the subtendinous space is a common location for the injection of local anesthetics during various surgical procedures in this region. 

Innervation

Superior oblique is the only extraocular muscle that receives its innervation through the trochlear nerve (CN IV). Trochlear nerve is the only cranial nerve that emerges from the posterior aspect of the brainstem. It takes a fairly long path through the endocranium, and enters the orbit via the superior orbital fissure to innervate the superior oblique muscle. 

In clinical practice, it's important to keep in mind the fact that except for the superior oblique and lateral rectus, all extraocular muscles are supplied by oculomotor nerve (CN III).

Blood supply

Superior oblique is vascularized by small branches of the anterior ciliary arteries which are derived from the ophthalmic artery. 

Function

Before getting into the actions of this muscle, it's important to highlight that in reality, almost all movements of the eyeball involve actions of at least three muscles. To get a clear picture of the outside world, the movements of the eyeballs need to be controlled, precise and well-coordinated. 

The belly of superior oblique muscle is located posterior to the eye. However, due to the fact that the tendon of the muscle traverses the trochlea of the orbit, the muscle tendon approaches the eyeball from the front and attaches its superior aspect. This allows the superior oblique to direct the gaze in several different directions while providing stability for the eyeball by counteracting head movements. When the eyeball is in a neutral position, superior oblique is responsible for abduction (directed laterally from the nose), depression (inferiorly) and internal rotation (movement of the superior pole of the eye medially) of the eyeball. Note that these functions are not equally efficient in every position of the eye; internal rotation (intorsion) of the eyeball is particularly effective when the eye is in the abducted position, while depression of the eyeball is strongest when the eye is in the adducted position. The superior oblique muscle functions antagonistically to the inferior oblique muscle. 

The following quiz and videos can help you learn the muscles that move the eyes.