The Effect of Axial Length on Extraocular Muscle Leverage

Abstract

Magnetic resonance imaging was used to determine the effect of axial length (AL) on globe rotational axis and horizontal extraocular muscle leverage during horizontal duction. Prospective observational case series. At a single academic center, 36 orthophoric adults with a wide range of ALs underwent high-resolution axial orbital magnetic resonance imaging 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 extraocular muscle insertions and rotational axes. 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 (R2= 0.06) or anteroposterior (R2= 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 (R2= 0.24, P < .001) and LR (R2= 0.32, P < .001) lever arms significantly increased by about 0.3mm per 1.0-mm of increased AL, with a corresponding reduction in predicted per-millimeter effect of surgical repositioning of their insertions. Regardless of AL, the globe rotates about a point nasal and anterior to its geometric center, giving the LR more leverage than the MR. This eccentricity may diminish the effect of tendon repositioning in moderate to highly myopic patients, with reductions in per-mill imeter dose/response predicted with longer AL.