SURGICAL IMPLICATIONS OF THE RECTUS EXTRAOCULAR-MUSCLE PULLEYS

Purpose: Magnetic resonance imaging (MRI) shows that the paths of rect us extraocular muscle bellies remain fixed in the orbit during large o cular rotations, and across large surgical transpositions of their ins ertions. This stability of muscle paths is due to their passage throug h pulleys which are coupled to the orbit and located in a coronal plan e anterior to the muscle bellies near the equator of the globe. Autops y studies have shown the pulleys to be fibroelastic sleeves consisting of dense bands of collagen and elastin, suspended from the orbit and adjacent extraocular muscle sleeves by bands of similar composition. I mmunohistochemical studies have revealed substantial smooth muscle in the pulley suspensions and in posterior Tenon's fascia. The pulleys mu scles in the sense of determining extraocular muscle pulling directuso ns. This study was conducted to determine the theoretical effects of t he pulleys on the outcome of rectus transposition surgery. Methods: Th e functional and anatomical evidence for the existence of the rectus e xtraocular muscle pulleys was reviewed. In two patients, binocular ali gnment data were collected using the Hess screen test before and after vertical rectus transposition surgeries for lateral rectus paralysis. Paths of the rectus extraocular muscles were determined using high re solution MRI. The Orbit(TM) 1.5 extraocular biosimulation program was employed to compute theoretical binocular alignment and muscle paths, under alternative conditions including or omitting the pulleys, Result s: Pulleys are required to account for observed paths of rectus extrao cular muscles following transposition surgery. In the absence of pulle ys, transposition of the superior and inferior rectus muscles to the l ateral rectus insertion for abducens paralysis would result in bizarre ocular misalignments not observed clinically, Conclusions: The human orbit contains specialized musculofibroelastic tissues in and just pos terior to Tenon's fascia, which serve as pulleys, determining actions of rectus extraocular muscles. These pulleys are located in a roughly coronal plane just posterior to the equator of the globe, Unimpaired p ulley function is essential to effective muscle transposition surgery.