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

Background: Slipped muscles are complications of strabismus surgery that ar e encountered more frequently after recessions of the inferior and medial r ectus muscles. Methods: We obtained multipositional high-resolution magneti c resonance imaging of 10 orbits of 6 normal subjects, 9 orbits of 5 patien ts with thyroid-associated eye disease, and 4 orbits of 2 patients with thy roid-associated eye disease on the day after rectus muscle recessions using a suspension technique. The are of contact and the distance between the in sertion 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 an d lateral rectus muscles, respectively (P = .022 for the vertical rectus mu scles; P = .05 for the horizontal rectus muscles, paired ttest), and that t heir insertions may be found several millimeters posterior to their points of tangency in extreme rotation of the globe in their respective field of a ction, The appearance of the suspended recessed inferior rectus muscle on t he 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 rectu s muscles may explain one of the mechanisms accounting for the increased in cidence of slipped muscles encountered after recessions of these muscles. ( J AAPOS 2000;4:84-93).