Evidence for a pulley of the inferior oblique muscle.

PURPOSE This study was undertaken to investigate evidence for a connective tissue pulley constraining the path of the inferior oblique (IO) muscle. METHODS From magnetic resonance images, the cross-sectional area, path, and orbital relationships of the human IO were determined in multiple gaze positions. Rectus pulleys were directly imaged with intravenous gadodiamide contrast. Images were compared with serial histologic sections of IO muscles of humans and monkeys. RESULTS The IO path from origin to the lateral border of the inferior rectus (IR) muscle was straight. Lateral to the IR, the IO curved to follow the globe. At the point of IR crossing, the IO moved anteriorly from infraduction to supraduction by approximately 53% of the IR insertion's travel. Gaze-related change in IO cross section was demonstrable near the IR center. The gaze-related inflection in IO path corresponded to its encirclement by a pulley consisting of a dense ring of collagen, stiffened by elastin and smooth muscle, and united with the IR pulley. Orbital layer fibers of the IO inserted on its pulley, the lateral rectus (LR) pulley, and associated connective tissues. CONCLUSIONS Like the rectus muscles, the human and monkey IO has a connective tissue pulley serving as its functional origin. The position of the IO pulley is influenced by its coupling to the actively moving IR pulley, whereas in turn the IO orbital layer inserts on and presumably shifts the IR and LR pulleys. These intercouplings facilitate implementation by rectus extraocular muscle suspensions of a commutative ocular motor plant.