Iris biomechanics in health and disease.

Abstract

Computational models of the eye have been studied by various investigators. The main purpose of developing a computational model is to provide a better understanding of the normal function of the eye as well as the abnormalities causing ocular diseases. For instance, by using computational methods, new insights have been brought to the pathophysiology and anatomical risk factors of angle-closure glaucoma, a mysterious eye disease closely related to the mechanics of the iris. Unlike the clinical research, computational studies are neither hindered by experimental difficulties nor by patient health risks. We developed computational models of the ocular tissues at three different levels to understand the mechanisms by which ocular globe deformation, iris-aqueous-humor interaction, and detailed iris structure affect the iris configuration. These models include: a finite-element model of the whole ocular globe consisting of the iris, cornea, sclera, and limbus, a finite-element model of iris-aqueous-humor interaction in the anterior eye, and a finite element model of the iris with its active and passive constituent tissues. Our whole-globe simulation showed that corneoscleral indentation, a diagnostic and/or treatment method in various glaucoma-related complications, would lead to changes in the anterior chamber angle. Our model showed that the limbus, due to its unique mechanical properties, plays an important role in the deformation of the whole ocular globe. Simulations performed using our anterior-segment model showed that the rapid changes (~sec) in the iris-aqueous-humor system due to corneoscleral indentation may lead to long recovery times (~min). We showed that a similar long recovery mechanism prevents the iris from drifting forward during normal blinking. Finally, simulations based on the detailed iris anatomy showed that the posterior location of the dilator muscle could contribute to the iris anterior bowing following dilation even in the absence of the aqueous humor pressure difference. Clinical studies have emphasized the key role of the iris shape and configuration in physiology and pathophysiology of the eye. In the course of our research, we showed that iris configuration is ultimately affected by many parameters including deformation of the whole ocular globe, interaction with aqueous humor flow, and activation of its constituent muscles.