To understand the actions of the superior oblique muscle, it is helpful to imagine the eyeball as a sphere that is constrained – like the trackball of a computer mouse – in such a way that only specific rotational movements are possible.

The tendon attaches to the top (superior aspect) of the eyeball at an angle of 51 degrees concerning the primary position of the eye (looking straight forward).

When the eye is in the primary position (looking straight ahead), contraction of the superior oblique produces depression and intorsion in roughly equal amounts.

In fact, each of the six extraocular muscles exerts rotational forces in all three planes (elevation-depression, adduction-abduction, intorsion-extorsion) to varying degrees, depending on which way the eye is looking.

The recent discovery of soft tissue pulleys in the orbit – similar to the trochlea, but anatomically more subtle and previously missed – has completely changed (and greatly simplified) our understanding of the actions of the extraocular muscles.