Rectus extraocular muscle paths and decompression surgery for Graves orbitopathy: mechanism of motility disturbances.

PURPOSE: To study possible causes of motility disturbances that may result from orbital decompression surgery in patients with Graves orbitopathy and especially the role of rectus extraocular muscle paths.

METHODS: Sixteen patients with Graves orbitopathy were studied before and 3 to 6 months after translid (6 patients) and coronal (10 patients) orbital decompression surgery for disfiguring proptosis. Ocular motility changes were measured by comparing maximum ductions and severity of diplopia, and the positions and the displacements of the anterior rectus muscle paths were objectively measured using cine magnetic resonance imaging (MRI).

RESULTS: Averaged preoperative rectus muscle path positions were not different from those in normal subjects. Averaged postoperative muscle path positions were generally the same as preoperative paths. The only significant exceptions were centrifugal (outward from the orbital axis) displacements of the inferior rectus (IR) muscle path after translid surgery, and of the medial rectus (MR) muscle path after coronal surgery. The amount of IR path displacement with translid surgery was directly correlated with range of depression and with severity of vertical diplopia. The amount of MR path displacement with coronal surgery was inversely correlated with range of abduction and directly correlated with severity of horizontal diplopia.

CONCLUSIONS: The anterior orbital connective tissue seems to form a "functional skeleton" that is usually (except as noted for IR and MR) capable of keeping the rectus muscle paths aligned after decompression surgery and preserving the normal functions of rectus muscle pulleys. The centrifugal displacement of the IR and MR may increase the elastic component of the muscle force, leading to the specific patterns of motility disturbance that may occur in some patients after translid and coronal surgery. These findings suggest that standard surgical management of Graves orbitopathy should be supplemented.