PURPOSE: To determine the effect of rectus extraocular muscle (EOM) transposition with posterior fixation (PF), we employed magnetic resonance imaging (MRI) to demonstrate pulley inflections in EOM paths before and after surgery in patients with paralytic strabismus.

DESIGN: Consecutive interventional case series.

METHODS: Five consecutive patients (three males and two females with a mean age 52 years, range 33 to 77 years) with paralytic strabismus were studied prospectively before and more than 6 weeks after EOM transposition and PF by means of contiguous cross-sectional MRI obtained in planes perpendicular to the long axis of the orbit. Muscle paths were determined in three dimensions (3-D) for each EOM by analysis of cross-sectional area centroids in normalized, oculocentric coordinate systems.

RESULTS: Four patients underwent full tendon transposition with PF of the vertical rectus EOMs. One other patient underwent full tendon transposition without PF of the horizontal rectus EOMs superiorly. For transpositions with PF, there was a large displacement of EOM path in central (straight ahead) gaze beginning in the posterior orbit. After surgical transposition, clear inflections representing pulley locations of the superior, medial, and lateral rectus paths occurred in central gaze. There was no clear path inflection for the inferior rectus in central gaze, but there was a small inflection in adduction. After all transpositions, the globe center shifted away from the transposed insertions.

CONCLUSIONS: Rectus EOM transpositions with PF shift EOM pulleys posteriorly and in the directions of the transposed EOM tendons, while translating the globe center. These changes may explain the superior effectiveness of PF in increasing duction towards the transposition.