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Research Papers

Reverse Pupillary Block Slows Iris Contour Recovery From Corneoscleral Indentation

[+] Author and Article Information
Rouzbeh Amini

Department of Biomedical Engineering, University of Minnesota, 7-105 Hasselmo Hall, 312 Church Street SE, Minneapolis, MN 55455amin0035@umn.edu

Victor H. Barocas1

Department of Biomedical Engineering, University of Minnesota, 7-105 Hasselmo Hall, 312 Church Street SE, Minneapolis, MN 55455baroc001@umn.edu

1

Corresponding author.

J Biomech Eng 132(7), 071010 (May 26, 2010) (6 pages) doi:10.1115/1.4001256 History: Received October 02, 2009; Revised February 05, 2010; Posted February 11, 2010; Published May 26, 2010; Online May 26, 2010

Corneoscleral indentation changes the iris contour and alters the angle between the iris and cornea. Although this effect has long been observed, the mechanism by which it occurs remains poorly understood. Previous theoretical research has shown that corneoscleral indentation can deform the eye globe and consequently rotate the iris root. In this work, we studied the fluid-structure interaction between the iris and aqueous humor, driven by iris root rotation. The iris root rotation obtained from our previous whole-globe model was used as a boundary condition for a fluid-structure interaction finite element model of the anterior eye. We studied the effect of two parameters-rotation angle and indentation speed-on the iris contour and aqueous humor dynamics. We found that posterior rotation of the iris root caused posterior bowing of the iris. After the iris root was returned to its original orientation, the aqueous humor was trapped in the anterior chamber because the iris tip pinned against the lens (reverse pupillary block). After 0.5–2 min of simulation, aqueous humor secretion into the posterior chamber and outflow from the anterior chamber allowed the system to return to its original steady state flow condition. The faster or farther the iris root rotated, the longer it took to return to steady state. Reverse pupillary block following corneoscleral indentation is a possible explanation for the clinical observation that prevention of blinking causes the iris to drift forward.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Anterior segment anatomy and aqueous humor flow (Courtesy: National Eye Institute, National Institutes of Health, Bethesda, MD)

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Figure 2

Axisymmetric model of the anterior segment. The aqueous humor and iris domains colored light gray and dark gray, respectively. The iris concavity (13) (CD) is defined as the maximum distance between the iris pigment epithelium and a reference plane (AB), which connects the most peripheral points of the iris pigment epithelium to the most central ones.

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Figure 3

Iris profile and aqueous humor pressure distribution (a) before indentation (t=0), (b) iris root rotated 10 deg (t=50 ms), (c) iris root held on the maximum rotated configuration for 50 ms (t=100 ms), and (d) iris rotated back to pre-indentation configuration (t=300 ms).

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Figure 4

Changes in (a) ACA, (b) apparent iris-lens contact, (c) pressure difference between the anterior and posterior chambers, (d) flow through iris-lens gap, (e) iris concavity, and (f) pupil diameter versus time. I=posterior rotation of the iris root during indentation; II=hold; III=anterior rotation of iris root after pressure removed; IV=beginning of recovery.

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Figure 5

Recovery of (a) ACA, (b) apparent iris-lens contact, (c) pressure difference between the anterior and posterior chambers, (d) flow through iris-lens gap, (e) iris concavity, and (f) pupil diameter to their pre-indentation values. Arrow in (d) shows that the pupil remained blocked (negligible gap flow) for almost 55 s after the iris root returns to its pre-indentation position.

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Figure 6

(a) Pressure difference between the anterior and posterior chambers versus time with different iris root rotation. Lines are 6 deg, 8 deg, 10 deg, and 12 deg maximum rotation. (b) Recovery time versus rotation angle. (c) Pressure difference between the anterior and posterior chambers versus time with different indentation duration. Lines are 50%, 67%, 100%, and 200% of base case rotation speed. (d) Recovery time versus indentation speed.

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Figure 7

Change in the posterior chamber volume versus recovery time. Data points correspond to different cases shown in Fig. 6.

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