Straight Iris Hooks and Tissue Displacement: A Finite Element Model of the Iris and Lens of the Human Eye

Abstract

Purpose:: To evaluate the effect of the angle of attack of standard, straight iris hooks for mechanical dilation of the iris and stabilization of the lens capsule.

Methods:: Finite element models of the human iris and lens capsule with a circular anterior capsulorhexis, as found during cataract surgery, were created with the finite element algorithm LARSA 2000. The iris was modeled as anisotropic, elastic, and incompressible with four-node shell elements with zero bending stiffness and orthotropic in-plane axial stiffness in a radial shell mesh of 1296 elements with 72 elements per arc. Similarly, the lens capsule was modeled using a mesh of four-node shell elements with isotropic, elastic, and incompressible material properties with a total of 3242 elements and 72 elements per arc. The zonules were modeled as axial tension-only springs, attached to the capsule in posterior, central, and anterior rings. With these models the effect of four, evenly-spaced iris hooks on displacement of the pupil margin and margin of the capsulorhexis was studied. Attack angles of 5⁰, 25⁰, and 45⁰ relative to the plane of the iris were compared. To dilate the pupil with hooks, the model assumed a constant horizontal displacement of the pupillary border of 2.5 mm. To stabilize the lens capsule with hooks, the model assumed 0.1 mm horizontal displacement of the capsulorhexis border.

Results:: Vertical displacement, or lift, of the margin of the pupil varied with the approach angle of the hook: 0.22 mm with a 5⁰ hook angle, 1.17 mm with a 25⁰ hook angle, and 2.5 mm with a 45⁰ angle. The total displacements of the margin of the pupil with hook angles of 5⁰, 25⁰, and 45⁰ are 2.51 mm, 2.76 mm and 3.54 mm respectively. When four hooks were placed on the capsulorhexis margin, the vertical displacement of the anterior capsule margin also depended on the attack angle of the hooks: 0.009 mm with a 5⁰ angle, 0.047 mm with a 25⁰ angle, and 0.1 mm with a 45⁰ attack. The total displacements of the capsule margin with hook angles of 5⁰, 25⁰, 45⁰ are 0.1 mm, 0.11 mm, and 0.141 mm respectively.

Conclusions:: The three angles of attack - 5°, 25°, and 45° from the horizontal plane - did indeed provide differing results. The deformations of both the iris and the lens capsule increase as the angle of attack increases. The greatest deformations were caused by the 45° angle of attack.

Keywords: cataract • anterior segment • computational modeling