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Qi Wei, Soh Youn Suh, Robert A. Clark, Joseph Demer; Biomechanical Modeling of Sagging Eye Syndrome (SES). Invest. Ophthalmol. Vis. Sci. 2020;61(7):1146.
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© ARVO (1962-2015); The Authors (2016-present)
We developed a computational model of the ocular plant to examine factors contributing to SES, the commonest cause of acquired diplopia in older adults. We studied SES associated with pure hypertropia, asking if displaced pulleys alone are consistent with absence of appreciable esotropia (ET) in central gaze.
We associated each rectus EOM with 4 springs to model pulleys that move actively under influence of the orbital layers as a function of gaze. Spring stiffnesses were adjusted to match simulated pulley positions to MRI measurements (Chaudhuri & Demer, JAMA Ophthalmol. 131:619, 2013). The hypotropic eye lateral rectus (LR) pulley was more inferior (7.7±1.1mm) than in the hypertropic eye (4.8±3.4mm). Muscle innervations with hypertropic eye fixating were first estimated. When the hypotropic eye was following, we applied Hering’s law & used hypertropic eye innervations to drive the hypotropic eye. Inferior rectus (IR) muscle recession was simulated to examine treatment effect.
Simulated pulley locations & EOM lengths in the hypertropic & hypotropic eyes matched average published MRI measurements of pulleys in SES. In central gaze, the model predicted 2 deg hypotropia & 1 deg exotropia with V pattern & excyclotorsion. There was a small esotropia in infraversion. Such deviations are expected mechanically because of the greater inferior displacement of the hypotropic than hypertropic LR, and esotropia was absent except in infraversion. While 5mm IR recession corrected the hypotropia in central gaze, it vertically overcorrected in other gaze positions, but slightly overcorrected the ET in infraversion.
The computational ocular motor plant model with actively controlled pulleys accurately simulated hypertropia in SES caused by asymmetric LR pulley sag without large angle ET, consistent with typical clinical findings. A neuro-biomechanical model may be useful to systematically examine SES mechanisms and quantitatively assess surgical treatment effectiveness.
This is a 2020 ARVO Annual Meeting abstract.
A. 3D model of the orbit. Central and four secondary vertical gazes (black dots). B. Simulated Hess screen representation Pre-op in blue, post 5 mm inferior rectus recession in red.
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