The Effect of Axial Length on Extraocular Muscle Leverage

PURPOSE: Magnetic resonance imaging (MRI) was used to determine the effect of axial length (AL) on globe rotational axis and horizontal extraocular muscle (EOM) leverage during horizontal duction. DESIGN: Prospective observational case series. METHODS: At a single academic center, 36 orthophoric adults with a wide range of ALs underwent high-resolution axial orbital MRI in target-controlled adduction and abduction. ALs were measured in planes containing maximum globe cross-sections. Area centroids were calculated to determine globe centers. Rotational axes in orbital coordinates were calculated from displacements of lens centers and globe-optic nerve attachments. Lever arms were calculated as distances between published EOM insertions and rotational axes. RESULTS: ALs averaged 26.3±0.3mm (standard error, range 21.5-33.4mm). Rotational axes from adduction to abduction averaged 1.1±0.2mm medial and 1.1±0.2mm anterior to the globe’s geometric center in adduction. Linear regression demonstrated no significant correlation between AL and rotational axis horizontal (R(2)=0.06) or anteroposterior (R(2)= 0.07) position. Medial rectus (MR) lever arms averaged 12.0±0.2mm and lateral rectus (LR) lever arms averaged 12.8±0.2mm. Both MR (R(2)=0.24, p