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Joseph L. Demer; Proportional Distribution of Human Lateral Rectus Muscle Compartments by Magnetic Resonance Imaging. Invest. Ophthalmol. Vis. Sci. 2012;53(14):2224.
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The nerve supply to the lateral rectus (LR) muscle has been recently discovered to be histologically segregated into superior and inferior zones innervating functionally differential fiber compartments, but the anatomic proportional contributions of these compartments in individual subjects has not been determined. This study used magnetic resonance imaging (MRI) to define regions of interest demarcating non-overlapping LR compartments for functional analysis.
Surface coil, quasi-coronal plane MRI (312 μm) perpendicular to the long orbital axis was obtained in target-controlled central gaze in 75 normal adult volunteers. LR cross sections were examined throughout the orbits to identify fissures and other features demarcating the compartmental border, from which the proportionate vertical dimensions of the superior and inferior compartments were determined.
Features demarcating the compartmental border were identifiable in 29 orbits of 23 subjects. The vertical dimension of the inferior compartment averaged 0.49 ± 0.10 (SD) of the total LR vertical dimension, giving a 95% confidence interval for the border point of 0.29-0.69. The distribution was slightly leftward skewed.
Extraocular muscles consist of parallel fiber groups that have a high degree of mechanical independence, so their actions are coordinated neurally by compartmentalized intramuscular nerves unlikely to cross grossly visible fissures. While MRI cannot identify structural demarcations, such as fissures, between LR compartments in every subject, functional assessments of the extreme 30% superior and inferior cross sections would provide statistical confidence for assessment of non-overlapping compartmental activity. This approach will enable functional MRI study of differential compartmental LR contribution to ocular motor behaviors such as ocular counter-rolling and vergence.
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