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J. L. Demer, V. Poukens, H. Ying, X. Shan, J. Tian, D. S. Zee; Length and Connectivity of Global Layer (GL) Fibers in Primate Superior Oblique (SO) Muscle. Invest. Ophthalmol. Vis. Sci. 2009;50(13):653.
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Tension in individual extraocular muscle (EOM) fibers can be related to overall EOM tension if the lengths and possible series-parallel arrangements of the fibers are known. Under normal conditions, EOM fibers are too small and numerous to trace them individually over any significant length. After trochlear neurectomy, however, some GL fibers in the SO become so markedly hypertrophic that they can be individually identified and studied. We exploited post-denervation hypertrophy to trace individual SO fibers throughout the length of the EOM.
An adult monkey was perfused with formalin 12 mos following unilateral intracranial trochlear neurectomy (IOVS 2007; 48: 2602 - 2625). The ipsilateral orbit was embedded whole in paraffin, serially sectioned at 10 micron thickness, and stained with Masson trichrome. A group of 14 markedly hypertrophic GL fibers was identified in the SO midbelly. Each fiber was traced from high magnification micrographs posteriorly to the orbital apex, and anteriorly near trochlear entry, for a total length of ~19 mm.
All fibers remained within the GL. Six fibers ran the entire ~19 mm SO length without myomyous junctions, and all fibers were undivided in their posterior 13 mm. Moving anteriorly, there were 2 sets of primary myomyous division into secondary fiber pairs ~14 mm anterior to the origin, one secondary fiber dividing again 1 mm further anteriorly. Complex myomyous junctions occurred only 17 - 18 mm anterior to the origin, which is anterior to the globe-optic nerve junction, and within the sheath surrounding the SO: two pairs of fibers joined into single fibers, a secondary and tertiary branch of the same fiber rejoined, and two primary and one secondary fiber joined.
Hypertrophic GL fibers in primate SO extend most or all of retrotrochlear SO length, and do not insert on orbital layer fibers. About half of hypertrophic GL fibers are involved in complex, interwoven myomyous junctions limited to a short extent in the anterior orbit. This suggests that the tension exerted by normal GL fibers is largely transmitted as oculorotary force to the eye. However, some non-linear force interaction is to be expected in short anterior segments of GL fibers that are involved in myomyous junctions.
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