Abstract
Purpose: :
To examine the distinctive effects of dynamic pupillary block, iris volume change, and anatomical shape of the dilator on the narrowing of the anterior chamber angle during dilation.
Methods: :
A mathematical model of the anterior segment was developed to simulate contraction of the dilator muscle. The aqueous humor and iris were modeled as an incompressible Newtonian fluid and a neo-Hookean solid, respectively. To change the pupil diameter from 3 mm to 6 mm during 12 seconds, active dilator contraction was applied by imposing stress in the dilator region. Dilation was simulated with changing three parameters: (1) a compressible vs. an incompressible iris to identify the effects of iris volume change, (2) a thin dilator (4 μm, 1% of full thickness) vs. a thick dilator covering the full thickness iris to identify the effects of dilator anatomy, and (3) in the presence vs. absence of pupillary block to identify the effect of dynamic motion of aqueous humor from the posterior to anterior chamber. The changes in the apparent iris-lens contact and AOD500 were calculated for each case.
Results: :
When the entire thickness of the iris was modeled as the contracting dilator muscle and in the presence of pupillary block, the iris curvature initially increased from -0.03 mm to +0.01 mm when iris lens gap was at its minimum and eventually decreased to -0.06 mm as the iris fully dilated for incompressible model. AOD500 was decreased by 5% and 6% as pupil diameter reached 4 mm for the thick dilator case in the presence and in the absence of pupillary block, respectively. The predicted changes were more pronounced when the dilator was modeled as a thin layer on the posterior surface of the iris. For example, AOD500 decreased by 6% and 12% (pupil diameter reaching 4 mm for both case) in the presence and in the absence of pupillary block, respectively. Dilation simulation with a compressible iris with a thin dilator in the presence of pupillary block predicted that as the pupil diameter increased from 3 mm to 4 mm, the iris-lens contact decreased from 0.4 mm to 0.29 mm.
Conclusions: :
Our simulations predicted the most dramatic change in AOD500 when the dilator muscle was modeled as a thin layer in the posterior surface of the iris, the iris was molded as an incompressible material, and the pupillary block was prevented. In addition, the iris incompressibility (i.e. lack or iris volume change) led to a narrower anterior chamber angle following dilation simulation, a result consistent with clinical observation (Quigley et al., J Glaucoma, 18:173-9, 2009). The change in AOD500 was more pronounced when the iris was incompressible, while the decrease in iris lens contact was more pronounced in the presence of pupillary block.
Keywords: iris • anterior chamber • aqueous