Iris Morphological and Biomechanical Factors Influencing Angle Closure During Pupil Dilation

Purpose To use finite element (FE) analysis to assess what morphological and biomechanical factors of the iris and of the anterior chamber are more likely to influence angle narrowing during pupil dilation. Methods The study consisted of 1,344 FE models comprising of the cornea, sclera, lens and iris (stroma, sphincter and dilator tissues) to simulate pupil dilation and to assess changes in angle. For each model, we varied the following parameters: anterior chamber depth (ACD = 2 –4 mm) and width (ACW = 10–12 mm), iris convexity (IC = 0–0.3 mm), thickness (IT = 0.3–0.5 mm), stiffness ( E = 4–24 kPa) and Poisson’s ratio ( v = 0–0.3), and simulated pupil dilation. We evaluated for the change in anterior chamber angle (△∠) and the final dilated anterior chamber angles (∠ f ) from baseline to dilation for each parameter. Results The final dilated AC angles decreased with a smaller ACD (∠ f = 53.4°±12.3° to 21.3°±14.9°), smaller ACW (∠ f = 48.2°±13.5° to 26.2°±18.2°), larger IT (∠ f = 52.6°±12.3° to 24.4°±15.1°), larger IC (∠ f = 45.0°±19.2° to 33.9°±16.5°), larger E (∠ f = 40.3°±17.3° to 37.4°±19.2°) and larger v (∠ f = 42.7°±17.7° to 34.2°±18.1°). The change in AC angle increased with larger ACD (△∠ = 9.37°±11.1° to 15.4°±9.3°), smaller ACW (△∠ = 7.4°±6.8° to 16.4°±11.5°), larger IT (△∠ = 5.3°±7.1° to 19.3°±10.2°), smaller IC (△∠ = 5.4°±8.2° to 19.5°±10.2°), larger E (△∠ = 10.9°±12.2° to 13.1°±8.8°) and larger v (△∠ = 8.1°±9.4° to 16.6°±10.4°). Conclusions This parametric study offered valuable insights into the factors that could influence angle closure. The morphology of the iris (IT and IC) and its innate biomechanical behavior ( E and v ) were crucial in influencing the way the iris deformed during dilation, and angle closure was further exacerbated by decreased AC biometry (ACD and ACW).