Structure-function correlations in the human medial rectus extraocular muscle pulleys.

PURPOSE Fibroelastic pulleys function like the trochlea to fix the position and pulling direction of the recti extraocular muscles within the orbit. This study characterized the fine structure of the human medial rectus muscle pulley. METHODS Human medial rectus muscle pulley tissue was dissected at autopsy, immersed in aldehyde fixative solution, and processed for and examined with light and electron microscopy. RESULTS Pulley structure were located within posterior Tenon's fascia, closely surrounding the medial rectus muscle. Pulleys were comprised of a dense collagen matrix with alternating bands of collagen fibers precisely arranged at right angles to one another. This three-dimensional organization most likely confers high tensile strength to the pulley. Elastin fibrils were interspersed in the collagen matrix. Fibroblasts and mast cells were scattered throughout the relatively acellular and avascular collagen latticework. Connective tissue and smooth muscle bundles suspended the pulley from the periorbita. Smooth muscle was distributed in small, discrete bundles attached deeply into the dense pulley tissue. CONCLUSIONS Fine structural observations confirm the existence and substantial structure of a pulley system in association with the medial rectus extraocular muscle. The presence of pulleys must be considered in models of the oculomotor plant. The cytoarchitecture and placement of pulleys suggest that they are internally rigid structures and are consistent with the idea that they determine functional origins for the extraocular muscles. However, the nature of the connective tissue-smooth muscle struts suspending the pulley system to the orbit supports the notion that the pulley position, and thus the vector force of the eye muscles, may be adjustable.