Modeling Passive Mechanical Interaction Between Aqueous Humor and Iris

[+] Author and Article Information
Jeffrey J. Heys

Department of Chemical Engineering, University of Colorado, Boulder, CO 80309-0424

Victor H. Barocas

Department of Biomedical Engineering, University of Minnesota, Minneapolis, MN 55455e-mail: baroc001@umn.edu

Michael J. Taravella

Department of Ophthalmology, University of Colorado Health Sciences Center, Denver, CO 80262

J Biomech Eng 123(6), 540-547 (Jun 29, 2001) (8 pages) doi:10.1115/1.1411972 History: Received August 22, 1999; Revised June 29, 2001
Copyright © 2001 by ASME
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Cross section of the anterior segment of the eye showing the path of AH. The AH is secreted by the ciliary body and exits the eye through the trabecular meshwork. The posterior chamber is between the iris and lens, and the anterior chamber is between the cornea and iris.
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Model domain including the structure that each boundary represents and the iris in the rest position. The fluid domain is shown in gray, and the elastic solid domain is black.
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Corneal displacement in response to a nonphysiological IOP increase from 2.1 kPa (16 mmHg, dotted line) to 67 kPa (500 mmHg). Because the corneal displacement is quite small even at high IOP, this artificial case was used to illustrate how the model equations incorporate corneal expansion.
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A typical mesh used in the model with 250 elements that results in approximately 4000 degrees of freedom. The mesh is highly refined in the pinch region between the iris and lens. The mesh is deformed slightly due to the movement of the iris.
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(a) An ultrasound biomicrograph of the peripheral iris of a healthy eye, and (b) an ultrasound biomicrograph of the iris taken from an off-axis angle to allow the spatial relationship between iris tip and lens to be shown. Visible in the ultrasounds are the cornea (C), iris (I), and lens (L). In (b), the iris is in apparent contact with the lens due to the ∼50 μm resolution limit of the instrument. (Biomicrograph courtesy of C. J. Pavlin, K. Harasiewicz, and F. S. Foster.)
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The model prediction for the iris contour and flow profile for a healthy eye using normal boundary conditions. The velocity in the pinch region is approximately three orders of magnitude higher than elsewhere in the eye.
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A flow profile with the pupil diameter reduced to 2 mm. The iris now has a more convex contour and the pressure drop between the anterior and posterior chambers is doubled.
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The model prediction of IOP in a normal eye during blinking with every third time step shown. The blinking rate is set for every 2.8 s, typical for males. The inset shows that there is a small, steady pressure rise between blinks.
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Iris bombé is a condition in which an inflammatory membrane seals the iris to the lens. (a) The extreme anterior bowing of the iris can be observed using ultrasound biomicroscopy. (b) This same behavior can be modeled by attaching the iris to the lens. The pressure difference between the anterior and posterior chamber is 1.7 kPa (13 mmHg). Simulations were run with no AH flow for simplicity. (Biomicrograph courtesy of J. S. Schuman.)




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