Abstract
Recent studies have shown that each rectus and inferior oblique extraocular muscle passes through a connective tissue sleeve that acts as a pulley, determining muscle pulling direction. Pulleys are condensations of collagen and elastin, suspended from the orbit and from other connective tissues by collagen, elastin, and smooth muscle bands. These soft pulleys function as mechanical origins of the striated extraocular muscles (with respect to muscle pulling directions), and so are active tissues that are clinically relevant and surgically accessible.This paper reviews recent findings regarding the pulleys, including neural control of pulley smooth muscles. Clinical correlations are provided by binocular alignment measures, magnetic resonance images, and binocular, three-dimensional eye movement recordings, in patients with well-defined strabismus syndromes. Computer modeling using the Orbit™ biosimulation program conceptually unifies these data and predicts effects of pulley abnormalities and surgery.Location, structure and composition of extraocular pulleys are very similar across subjects, and comparable across primates. The pulleys are located between the globe equator and the globe-optic nerve junction in primary position, and make only small, but significant, movements with gaze change. Connective tissue bands joining adjacent pulleys are particularly dense between the superior and lateral rectus muscles, and between the medial rectus and the inferior rectus-inferior oblique complex. Smooth muscle is abundant in pulley suspensions, and is innervated by sympathetic, parasympathetic, and nitroxidergic neurons. Extraocular muscle soft pulleys determine the outcome of transposition surgery, and abnormal pulley locations may cause “A” and “V” pattern strabismus.Extraocular soft pulleys, and related connective and smooth muscle tissues, are involved in the etiology and treatment of strabismus. Modern techniques are essential to understanding these complex structures and applying this knowledge to strabismus diagnosis and treatment planning.
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