It was thus a further challenge to the ocular motor field when magnetic resonance imaging (MRI) studies revealed that the ON acts as a significant mechanical load during ocular rotations.
14 When adduction exceeds about 26 degrees,
15 and in some people, even a smaller angle,
16 the redundancy in typical ON length is geometrically exhausted so that its path becomes maximally straightened, and the ON begins to act as a tether.
14,17–19 In healthy people, this tethering in adduction stretches the ON
16 and translates the eyeball nasally but does not retract it posteriorly.
16,19,20 However, in patients with primary open angle glaucoma (POAG) who have either had only normal
20 or elevated levels of intraocular pressure,
19 ON traction in adduction causes significant globe retraction. Not surprisingly, infrared imaging discloses deformation of the optic disc during horizontal eye rotation,
21 particularly in adduction beyond exceeding the threshold of tethering.
15 It is also notable that lesser but still significant optic disc tilting occurs even in abduction where there is no tethering of the ON.
15,21 See-saw deformation of the optic disc during horizontal eye rotation is exaggerated in patients with papilledema in whom the pressure of intracranial fluid that fills the optic nerve sheath (ONS) is abnormally elevated,
22 but even in this case adduction causes greater folding in the peripapillary retina than does abduction.
23 An FEM of modest 13 degrees ab- and adductions has been implemented by assumed forces exerted at the scleral insertions of the horizontal rectus EOMs as the globe rotates about an assumed fixed center within homogeneous but freely flowing orbital fat filling the orbit, without explicit EOMs or discrete connective tissues.
24 This simplified FEM predicts that transverse forces in the orbit cause significant shearing forces even in the sinuous ON that tilt the optic disc and peripapillary tissues during adduction more than abduction, remarkably predicting ON tractional force at about the same absolute level as reported for horizontal rectus EOM tensions: about 15 gm in adduction and 9 gm in abduction.
24