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B. Audrey Nguyen, Matthew Aaron Reilly, Cynthia J Roberts; A nonlinear viscoelastic model of corneal and whole-eye motion of prostaglandin-analog treated subjects under loading by dynamic Scheimpflug analyzer. Invest. Ophthalmol. Vis. Sci. 2019;60(9):6833.
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An analytical model of corneal and whole-eye motion of the human eye under air-puff loading by a dynamic Scheimpflug analyzer was developed. The ocular components of the eye – the cornea and sclera – had been previously validated by a series of ex vivo human eye studies. In order to validate the extraocular components of the model and its ability to capture essential features of in vivo response to an air-puff, the effect of prostaglandin-analog (PGA) treatment for glaucomatous human subjects was analyzed using the model. PGA-treated subjects were selected because PGAs are known to increase uveoscleral outflow by affecting the sclera and generate atrophy of the orbital fat surrounding the globe.
This analytical model is a system of inertial masses representing the cornea and eye, and considers the extraocular tissues (fat and extraocular muscles) as a Kelvin-Voigt viscoelastic element, the cornea as three-parameter viscoelastic solid, and the sclera as a nonlinear stiffening spring. The values of spring and dashpot constants were estimated using an iterative approach to minimze the error between the simulation result and the in vivo data of corneal and whole-eye motion (WEM). From a separate study, 6 human subjects were selected as having at least a 10 mmHg drop in their intraocular pressure (IOP) as measured by Goldmann applanation tonometry (GAT).
The human subjects showed an unexpected significant decrease in WEM. The one-dimensional analytical model showed a significant increase (p<0.05) in the extraocular spring parameter after PGA treatment, as well as a significant decrease in scleral stiffness parameter. The two corneal spring terms remained unchanged (mean of -3.6%, 2.3% change respectively). While the corneal viscous term significantly increased, this has been shown to be strongly correlated with IOP changes.
The simulation results strongly suggest that the decrease in WEM in PGA-treated subjects is due to the coupling of reduced scleral stiffness and IOP. The reduced scleral stiffness results in the air-puff force being redirected into corneal deflection and scleral expansion, instead of into the rearward displacement of the globe in the presence of orbital fat atrophy.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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