Purpose: Magnetic resonance imaging (MRI) shows that the paths of rectus extraocular muscle bellies remain fixed in the orbit during large ocular rotations, and across large surgical transpositions of their insertions. This stability of muscle paths is due to their passage through pulleys which are coupled to the orbit and located in a coronal plane anterior to the muscle bellies near the equator of the globe. Autopsy 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. Immunohistochemical studies have revealed substantial smooth muscle in the pulley suspensions and in posterior Tenon's fascia. The pulleys function as mechanical origins of the rectus extraocular muscles in the sense of determining extraocular muscle pulling directusons. This study was conducted to determine the theoretical effects of the pulleys on the outcome of rectus transposition surgery.
Methods: The functional and anatomical evidence for the existence of the rectus extraocular muscle pulleys was reviewed. In two patients, binocular alignment 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 resolution MRI. The OrbitTM 1.5 extraocular biosimulation program was employed to compute theoretical binocular alignment and muscle paths, under alternative conditions including or omitting the pulleys.
Results: Pulleys are required to account for observed paths of rectus extraocular muscles following transposition surgery. In the absence of pulleys, transposition of the superior and inferior rectus muscles to the lateral 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 posterior 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 pulley function is essential to effective muscle transposition surgery.