Purchase this article with an account.
J. E. Whitcomb, R. Amini, M. K. Al-Qaisi, T. Akkin, V. H. Barocas; Effect of the Posterior Location on the Iris Concavity During Dilation. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5549.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
To examine the effect of the dilator location on iris concavity during dilation in a computational and ex vivo porcine model. We hypothesized that during pupil dilation, anterior bowing of the iris can be caused by the posterior location of the dilator muscle even in the absence of aqueous humor pressure.
Isolated porcine irides 6 hours post-mortem were secured at the periphery and immersed in an oxygenated buffer. The dilator was pharmaceutically stimulated with 40 µL of 2.5% phenylephrine and 40 µL of 1% tropicamide. Images of the iris were taken before and after dilation by optical coherence tomography (OCT). A finite element model of a typical experiment of pupil dilation was developed. The geometry of the iris and location of the dilator was based on the initial OCT image. The iris was modeled as a neo-Hookean solid, and the active muscle component was applied only to the dilator region in the model.
The pupil diameter increased significantly after the dilator muscle was activated, from 3.96 ± 0.22 mm (mean ± 95 % CI, n = 28) to 5.58 ± 0.29 mm (p < 0.001). The OCT images showed an increase in the concavity from 0.292 ± 0.02 mm to 0.38 ± 0.03 mm (p = 0.04) and a decrease in the chord length of 2.96 ± 0.17 mm to 2.35 ± 0.14 mm (p = 0.002) after dilation. The computational model agreed with the correct experiment, showing an increase in the pupil diameter from 5.0 mm to 7.0 mm, an increase of concavity from 0.24mm to 0.45 mm, and a decrease in the chord length from 3.06 mm to 2.56 mm. When the simulations were repeated with the dilator distributed over the entire iris thickness, unlike the correct posteriorly located dilator, the iris concavity decreased following dilation.
The posterior location of the dilator can cause anterior bowing of the iris during dilation even in the absence of aqueous humor flow. This result suggests that the location of the dilator muscle may cause anterior bowing of the iris after aqueous humor pressure has been equalized between the anterior and posterior chambers, for example, as a result of laser iridotomy.
This PDF is available to Subscribers Only