Abstract
Purpose :
To use finite element (FE) analysis to assess the effect of crypts on physiological iris movement and its significance on angle closure during pupil dilation.
Methods :
The study consisted of FE models comprising of the cornea, sclera, lens and iris (anterior border layer (ABL), stroma, sphincter and dilator tissues) to simulate pupil dilation and to assess changes in iris shape. We designed a first biphasic model for pupil dilation on a model consisting of the following biometry: anterior chamber depth (ACD) = 3mm, anterior chamber width (ACW) = 11.5mm, iris convexity (IC) = 0.25mm, iris thickness (IT) = 0.5mm, with the following biomechanical properties: iris stiffness Eiris = 10kPa, dilator and sphincter stiffness Emuscle = 20kPa, a uniaxial dilator muscle contraction force of 40kPa and ABL hydraulic permeability of 10-5mm2/Pa*s. This model was modified to include the presence of either 4 small crypts or 12 large crypts of surface areas 0.0052mm2 and 0.034mm2 per crypt. Dilator muscle contraction was from 0–0.2s and aqueous outflow from the iris continued until equilibrium was reached (biphasic transient analysis). We also used a second model with a higher risk of angle closure (ACD = 2.3mm, ACW = 11mm, IC = 0.1mm, IT = 0.45mm, ABL hydraulic permeability 5x10-6mm2/Pa*s) and modified it to include the presence of 4 small crypts. Anterior chamber angles (ACAs) were calculated using the positions of the iris boundary node and second adjacent node.
Results :
The ACAs decreased for all cases (Fig 1). Immediately after dilation, the ACAs decreased from 45.28° to 17.38°, 18.97°, 21.56° for the scenarios with no crypts, small crypts and large crypts respectively, and all ACAs increased to the equilibrium state of 23.05° when t = ∞. In the second scenario, when the ABL permeability decreased, the ACA collapsed from 38.87° to 0°, and the ACA remained closed (Fig 2A). When crypts were added, aqueous humor left the stroma matrix, preventing the collapse of the ACA, which was found to be 7.19° (Fig 2B and 2C).
Conclusions :
Our models suggest that iris crypts facilitate aqueous outflow, and in the cases of patients with a high risk of angle closure, could prevent angle closure by providing bypass when the anterior boarder layer limits outflow. We believe that further insights into the macroscopic and microscopic nature of the ABL and iris stroma would be crucial in developing a preventive measure for angle closure.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.