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K. Lim, V. Poukens, J.L. Demer; Fascicular Specialization in the Human Medial Rectus (MR) Muscle: Evidence for Functional Independence of Global (GL) and Orbital Layers (OL) . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5066.
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© ARVO (1962-2015); The Authors (2016-present)
Connective tissue pulleys form functional mechanical origins of extraocular muscles, and receive insertions from an unknown number of muscle fibers. We investigated insertions and anatomic relationships of naturally–occurring fiber fascicles within the human MR to clarify their interactions and relationship to the pulley.
A 17 mos old human orbit was fixed and removed with the orbital bones intact, serially sectioned en bloc in the coronal plane at 10 µm intervals, and stained with Masson trichrome and van Gieson’s elastin stains. Serial sections of the MR were digitally photographed for color segmentation of muscle and collagen content using Adobe Photoshop and Image J. We computed average collagen fraction in 6 regions in OL and 3 in GL. In mid–orbit, we selected 8 representative fascicles likely to be in OL and 6 likely to be in GL, also sampling the OL–GL border. Fascicles were traced anteriorly to their insertions. Collagen content and muscle fiber number of one OL fascicle were analyzed along its length. In one OL fascicle, every fiber was traced to its termination.
Fascicles in OL remained distinct from each other and from the GL throughout the muscle length examined; GL fascicles remained less distinct but did not join OL fascicles. Serial tracing showed that 7 of 8 fascicles initially classified as OL inserted into the pulley via short tendons distributed over a wide anteroposterior extent, the exception being a global surface fascicle that inserted on sclera. Of 6 fascicles initially classified as GL, 5 bypassed the pulley without insertion, and one located at the OL–GL border inserted on the pulley. In an OL fascicle, collagen fraction increased from 4 to 100% and muscle fibers decreased from 166 to zero as traced anteriorly. Collagen content in OL increased from 29±5 (SD)% in mid orbit to 65±9% in anterior orbit, but slightly decreased from 26±6% to 23±1% in GL. Tracing of every fiber in an OL fascicle demonstrated terminations on pulley tendons without myomyous junctions by 30/32 fibers, while 2 fiber terminations could not be determined.
Fibers in the human MR OL lack myomyous or GL junctions, but nearly all insert into the pulley via a broad distribution of short tendons and dense intercalated collagen. Fibers in the GL generally do not insert in pulley tissues, and are associated with less collagen. These features support the distinct role of the OL in anteroposterior positioning of connective tissues proposed in the active pulley hypothesis, as well as substantial mechanical independence of the OL and GL.
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