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R. Amini, V. H. Barocas; Computer Simulation of Iris Contour in Corneal Indentation. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3924.
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To evaluate the possible role of whole-globe deformation in the angle opening observed when the cornea is indented.
Two finite-element computer models were used to simulate indentation of the cornea. First, a simple was used to predict how the globe deforms when the cornea is indented. Second, the deformation at the iris root from the model was used as an input to our detailed model of aqueous humor and iris, and the motion of the iris was calculated.
The model predicted posterior rotation of the iris root for a narrow indenter (e.g., a gonioscope or small eye cup), which in turn opened up the angle. An important feature of this result is that the pressure in the posterior chamber is predicted to be higher than that of the anterior chamber, in contrast to what occurs in posterior iris motion during accommodation. There is no "reverse pupillary block" during indentation, but rather simple motion of the iris. For a large eye cup, the model predicted anterior rotation of the iris with the corresponding changes the other results. The quantitative results were highly dependent on how the external support for the globe was represented, but qualitative results were largely unchanged.
The results suggest that the proposed mechanism is a reasonable explanation for the opening of the angle in indentation gonioscopy and in indentation ultrasound studies. The results suggest that blinking may have a similar effect, pumping fluid from the posterior to the anterior chamber every few seconds, which would be consistent with the observation that the iris tends to drift forward when blinking is prevented, as in an ultrasound examination.
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