Extraocular Muscle Action
The width of the insertion of each EOM serves to stabilize the eye and mitigate changes in action that would otherwise occur in different eye positions. For example, when the eye looks upward, the insertion of the medial rectus muscle also shifts upward. But this also tightens the inferior fibers and slackens the superior fibers, in effect shifting the net vector from the muscle downward toward its original position. See also Activity 3-1 in Chapter 3.
Figure 4-1 The 6 cardinal positions, which correspond to the primary fields of action of the extraocular muscles. RSR, right superior rectus; LIO, left inferior oblique; LSR, left superior rectus; RIO, right inferior oblique; RLR, right lateral rectus; LMR, left medial rectus; LLR, left lateral rectus; RMR, right medial rectus; RIR, right inferior rectus; LSO, left superior oblique; LIR, left inferior rectus; RSO, right superior oblique.
Regardless of eye position or whether the insertion points have been moved surgically, the rectus muscles are still constrained to pass through the same openings in the Tenon capsule as they course from the orbital apex to the eye. The orbital connective tissue sheaths surrounding these openings have been described as muscle pulleys and have a specialized structure, including smooth muscle, that may provide active modulation of muscle action. See Chapter 3 for further discussion of muscle pulleys.
Arc of contact
In primary position, each muscle wraps around the globe for several millimeters before reaching its insertion on the sclera. The length of muscle in contact with the globe is called the arc of contact (see Chapter 3, Table 3-1, which gives the arc of contact for the EOMs). The point where the muscle first contacts the globe is the effective insertion of the muscle. As the muscle contracts and the eye rotates toward the muscle, the effective insertion moves forward on the globe, toward the scleral insertion point, as the arc of contact decreases. The muscle remains tangential to the globe at its effective insertion, maintaining the same torque through much of the eye movement.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.