Location and stability of rectus muscle pulleys. Muscle paths as a function of gaze

The paths of the rectus extraocular muscles (EOMs) are constrained by pulleys, connective tissue sleeves mechanically coupled to the orbital walls. This study sought to investigate, using high-resolution magnetic resonance imaging (MRI), the location and stability of EOM pulleys in normal subjects and those with strabismus.

Multiple contiguous coronal MRI scans spanning the anterior-to-posterior extent of the orbit during primary gaze, upgaze, downgaze, adduction, and abduction were analyzed digitally to determine the paths of the rectus EOMs. Pulley locations were inferred from EOM paths.

Data for 10 orbits of six normal subjects established the normal paths of the rectus extraocular muscles in primary gaze. Muscle paths in primary position were highly uniform across normal subjects. In secondary gaze positions, rectus muscle paths at the level of the pulleys exhibited small but consistent shifts, relative to the orbit, opposite the direction of gaze, consistent with the expected mechanical effects of the intermuscular connective tissue suspensions of the pulleys. Twelve orbits of seven subjects with strabismus showed, as a group, no significant difference from normal in rectus muscle paths in primary gaze and no significant difference from normal in changes of muscle paths in secondary gaze. Two subjects with incomitant strabismus wer found to have grossly abnormal rectus muscle paths in primary gaze, suggesting heterotopic pulleys. Computer simulations of these heterotopic pulley locations accounted for the observed patterns of incomitant strabismus in both.

High-resolution MRI can determine the location and sideslip of rectus EOM pulleys. Pulley position is highly uniform across normal subjects, consistent with the notion that musculo-orbital tissue connections determine the pulling direction of the rectus EOMs. In normal subjects and subjects with strabismus, pulleys exhibit small shifts with eccentric gaze that are consistent with secondary intermuscular, but not musculo-global, mechanical couplings. Heterotopic pulley position is a potential cause of incomitant strabismus.