The Pulley System
The 4 rectus muscles are surrounded by fibroelastic pulleys that maintain the position of the EOMs relative to the orbit. The pulleys consist of collagen, elastin, and smooth muscle, enabling them to contract and relax. Dynamic magnetic resonance imaging (MRI) studies show that, in some cases, the pulleys act mechanically as the rectus muscle origins. The pulleys may also serve to stabilize the muscle path, preventing sideslipping or movement perpendicular to the muscle axis (see Fig 3-4). Anteriorly, the pulleys merge with the intermuscular septum, which fuses with the conjunctiva 3 mm posterior to the limbus. The posterior section of the intermuscular septum separates the intraconal fat pads from the extraconal fat pads (see Fig 3-5). Numerous extensions from the EOM sheaths attach to the orbit and help support the globe.
The inferior oblique muscle originates inferonasally from the periosteum of the maxillary bone, near the orbital rim, adjacent to the anterior lacrimal crest, and it continues laterally, entering its connective tissue pulley inferior to the inferior rectus muscle, at the site where the inferior oblique muscle also penetrates the Tenon capsule. The inferior oblique pulley and inferior rectus pulley join to form the Lockwood ligament (see Fig 3-7). Attached to the conjoined inferior oblique and inferior rectus pulley complex is the dense neurofibrovascular bundle containing the inferior oblique motor nerve.
The active pulley hypothesis proposes that the pulley positions are shifted by the contraction of the orbital layer against the elasticity of the pulley suspension. This concept remains controversial: whether there is actual innervational control of the pulleys is still debated. However, high-resolution MRI scans have shown that the pulleys are located only a short distance from the globe center; therefore, small shifts in pulley position would confer large shifts in EOM pulling direction. Normal pulleys shift only slightly in the coronal plane, even during large eye movements. Heterotopy (malpositioning) of the rectus pulleys may cause some cases of incomitant strabismus and A or V patterns (see Chapter 10), and these anomalies can mimic oblique muscle dysfunction by misdirecting the forces of the rectus muscles. Bony abnormalities, such as those seen with craniosynostosis, can also alter the direction of pull of rectus muscles by causing malpositioning of the pulleys.
The pulley model and its implications have been challenged by other high-resolution MRI studies, which show that during eye movements into eccentric fields, the posterior portions of rectus muscles shift. These findings are consistent with the more traditional model of eye muscle function, which is the “restrained shortest-path model.”
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.