Iris Morphological and Biomechanical Factors Influencing Angle Closure During Pupil Dilation

Abstract

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 (nu = 0-0.3), and simulated pupil dilation. We evaluated for the change in anterior chamber angle (delta f) and the final dilated anterior chamber angles (angle f) from baseline to dilation for each parameter. Results: The final dilated AC angles decreased with a smaller ACD (angle f = 53.4 degrees +/- 12.3 degrees to 21.3 degrees +/- 14.9 degrees), smaller ACW (angle f = 48.2 degrees +/- 13.5 degrees to 26.2 degrees +/- 18.2 degrees), larger IT (angle f = 52.6 degrees +/- 12.3 degrees to 24.4 degrees +/- 15.1 degrees), larger IC (angle f = 45.0 degrees +/- 19.2 degrees to 33.9 degrees +/- 16.5 degrees), larger E (angle f = 40.3 degrees +/- 17.3 degrees to 37.4 degrees +/- 19.2 degrees) and larger nu (angle f = 42.7 degrees +/- 17.7 degrees to 34.2 degrees +/- 18.1 degrees). The change in AC angle increased with larger ACD (Delta angle = 9.37 degrees +/- 11.1 degrees to 15.4 degrees +/- 9.3 degrees), smaller ACW (Delta angle = 7.4 degrees +/- 6.8 degrees to 16.4 degrees +/- 11.5 degrees), larger IT (Delta angle = 5.3 degrees +/- 7.1 degrees to 19.3 degrees +/- 10.2 degrees), smaller IC (Delta angle = 5.4 degrees +/- 8.2 degrees to 19.5 degrees +/- 10.2 degrees), larger E (Delta angle = 10.9 degrees +/- 12.2 degrees to 13.1 degrees +/- 8.8 degrees) and larger nu (Delta angle = 8.1 degrees +/- 9.4 degrees to 16.6 degrees +/- 10.4 degrees). 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 nu) were crucial in influencing the way the iris deformed during dilation, and angle closure was further exacerbated by decreased AC biometry (ACD and ACW).