Magnetic resonance imaging of the arc of contact of extraocular muscles: Implications regarding the incidence of slipped muscles

Abstract

Background: Slipped muscles are complications of strabismus surgery that are encountered more frequently after recessions of the inferior and medial rectus muscles. Methods: We obtained multipositional high-resolution magnetic resonance imaging of 10 orbits of 6 normal subjects, 9 orbits of 5 patients with thyroid-associated eye disease, and 4 orbits of 2 patients with thyroid-associated eye disease on the day after rectus muscle recessions using a suspension technique. The arc of contact and the distance between the insertion and the point of tangency of each of the extraocular muscles to the globe were measured in primary position as well as in the cardinal fields. Results: The data confirm that the inferior and medial rectus muscles have a significantly smaller wraparound effect on the globe than the superior and lateral rectus muscles, respectively ( P = .022 for the vertical rectus muscles; P = .05 for the horizontal rectus muscles, paired t test), and that their insertions may be found several millimeters posterior to their points of tangency in extreme rotation of the globe in their respective field of action. The appearance of the suspended recessed inferior rectus muscle on the first postoperative day and its decreased apposition to the globe raise concerns about the muscle losing contact with the globe in extreme downgaze. Conclusions: The small wraparound effect of the inferior and medial rectus muscles may explain one of the mechanisms accounting for the increased incidence of slipped muscles encountered after recessions of these muscles. (J AAPOS 2000;4:84–93)