Contribution of different anatomical and physiologic factors to iris contour and anterior chamber angle changes during pupil dilation: Theoretical analysis

PURPOSE. To investigate the contribution of three anatomical and physiologic factors (dilator thickness, dynamic pupillary block, and iris compressibility) to changes in iris configuration and anterior chamber angle during pupil dilation. METHODS. A mathematical model of the anterior segment based on the average values of ocular dimensions was developed to simulate pupil dilation. To change the pupil diameter from 3.0 to 5.4 mm in 10 seconds, active dilator contraction was applied by imposing stress in the dilator region. Three sets of parameters were varied in the simulations: (1) a thin (4 μm, 1% of full thickness) versus a thick dilator (covering the full thickness iris) to quantify the effects of dilator anatomy, (2) in the presence (+PB) versus absence of pupillary block (-PB) to quantify the effect of dynamic motion of aqueous humor from the posterior to the anterior chamber, and (3) a compressible versus an incompressible iris to quantify the effects of iris volume change. Changes in the apparent iris-lens contact and angle open distance (AOD500) were calculated for each case. RESULTS. The thin case predicted a significant increase (average 700%) in iris curvature compared with the thick case (average 70%), showing that the anatomy of dilator plays an important role in iris deformation during dilation. In the presence of pupillary block (+PB), AOD500 decreased 25% and 36% for the compressible and incompressible iris, respectively. CONCLUSIONS. Iris bowing during dilation was driven primarily by posterior location of the dilator muscle and by dynamic pupillary block, but the effect of pupillary block was not as large as that of the dilator anatomy according to the quantified values of AOD500. Incompressibility of the iris, in contrast, had a relatively small effect on iris curvature but a large effect on AOD500; thus, we conclude that all three effects are important.