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R Kono, RA Clark, JL Demer; Active Pulleys: Magnetic Resonance Imaging (MRI) of Rectus Muscle Paths in Tertiary Gazes . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1460.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The orbital layer of each rectus extraocular muscle (EOM) inserts on the corresponding connective tissue pulley, while the global layer inserts on the eyeball. The active pulley hypothesis proposes that rectus EOM pulleys make large, gaze-related translations along the EOM axes in coordination with the scleral insertions to implement a linear ocular motor plant. This study was designed to measure gaze-related shifts in EOM pulley locations. Methods: MRI was performed in 8 normal volunteers in 2 mm thickness coronal planes perpendicular to the orbital axis for 9 cardinal gaze directions. Intravenous gadodiamide contrast was administered to define EOM tendons anterior to the globe equator. Paths of EOMs, defined by their area centroids, were transformed into a normalized oculocentric coordinate system. Sharp inflections in EOM paths in secondary and tertiary gaze positions defined pulley locations, which were then correlated with gaze direction and compared with theoretical predictions. Results: Rectus pulley positions were consistent with a central primary position. In tertiary gaze positions, each of the four rectus pulleys translated posteriorly with EOM contraction and anteriorly with EOM relaxation by a significant (P < 0.05 ) amount predicted by the active pulley hypothesis, but more than 100 times greater than the translation predicted by a passive pulley model. Each pulley moved the same distance and in the same direction as the scleral insertion, even though histological examination and other data show that the pulley is not anatomically coupled to the underlying sclera. The anteroposterior shift of the medial rectus pulley was 14 mm over a horizontal gaze range of 60 degrees. Conclusion: The major prediction of the active pulley hypothesis, coordinated gaze-related anteroposterior shifting of EOM pulleys, is quantitatively supported by changes in path inflections among tertiary gaze positions for each of the four rectus EOMs. Human rectus pulleys are actively positioned to shift the ocular rotational axis to attain commutative behavior of the ocular motor plant.
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