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Sara Jouzdani, Rouzbeh Amini, Victor Barocas; Computational Model of Iris-Aqueous Humor Interaction During Pupilary Dilation. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1513.
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To examine dynamic behavior of aqueous humor flow and changes in iris contour during dilation in a computational model. We hypothesized that iris motion during dilation caused anterior bowing of the iris, which resulted in narrowing of the anterior chamber angle and decreased the iris-lens contact.
We developed a mathematical model of the anterior segment to simulate contraction of the dilator muscle during pupillary dilation. An axisymmetric model treating the anterior segment of the eye was generated based on the average dimensions of a normal human eye. Aqueous humor was modeled as an incompressible Newtonian fluid, and the iris was modeled as an incompressible neo-Hookean solid with the dilator localized on the posterior surface of the iris. Active dilator contraction was applied by imposing stress in the dilator region. The pupil diameter was controlled by the force balance between the dilator muscle stress and the elasticity of the iris tissue in a transient model. Iris-lens contact and the pressure difference between the anterior and the posterior chambers were calculated.
At the beginning of simulated dilation, the iris tip moved toward the anterior surface of the lens, causing an increase in the apparent contact from 0.42 mm to 0.49 mm. However, as the iris bowed anteriorly, the iris-lens contact decreased gradually from 0.5mm to 0.32mm. During the simulated dilation, the iris tip was tightly pinned to the lens surface and blocked flow from posterior chamber to the anterior chamber. As a result, the pressure difference between the posterior and anterior chambers rose by a factor of 10. Anterior chamber angle and iris chord length decreased significantly from 50° and 4.5mm to 40° and 4mm, respectively.
Our results were consistent with clinical studies which have shown that the anterior chamber angle and iris chord length decreased, and iris concavity increased during dilation. This phenomenon is attributed primarily by pupillary block and trapping of the aqueous humor in the posterior chamber.
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