Three-dimensional location of human rectus pulleys by path inflections in secondary gaze positions

PURPOSE. Connective tissue pulleys serve as the functional mechanical origi ns of the extraocular muscles (EOMs). Anterior to these pulleys, EOM paths shift with gaze to follow the scleral insertions, whereas posterior EOM pat hs are stable in the orbit. Inflections in EOM paths produced by gaze shift s can be used to define the functional location of pulleys in three dimensi ons (3-D). METHODS. Contiguous magnetic resonance images in planes perpendicular to th e orbital axis spanned the anteroposterior extents of 22 orbits of 11 norma l adults with the eyes in central gaze, elevation, depression, abduction, a nd adduction. Mean EOM cross-sectional area centroids represented in a norm alized, oculocentric coordinate system were plotted over the length of each EOM to determine paths. Path inflections were identified to define pulley locations in 3-D. RESULTS. All rectus EOM paths exhibited in secondary gaze positions distinc t inflections 3 to 9 mm posterior to globe center, which were consistent ac ross Subjects. The globe center and the lateral rectus pulley translated sy stematically in the orbit with lateral gaze, whereas other pulleys remained stable relative to the orbit. CONCLUSIONS. Distinct inflections in rectus EOM paths in secondary gaze pos itions confirm the existence of pulleys and define their locations in 3-D. The globe and lateral rectus pulley translate systematically with gaze posi tion. The EOM pulleys may simplify neural control of eye movements by imple menting a commutative ocular motor plant in which commands for 3-D eye velo city are effectively independent of eye position.