Finite-element modeling of fluid-structure interactions affecting iris profile

Mechanical interaction between aqueous humor, iris, and intraocular structures can alter the iris profile from its normal curvature. Glaucoma, a group of diseases that can lead to vision loss, can involve excessive iris deflection in either the anterior (angle-closure glaucoma) or posterior (pigmentary glaucoma) direction.; A finite-element model was used to investigate the fluid-solid mechanics in the anterior segment of the eye. The aqueous humor was modeled as an incompressible Newtonian fluid while the iris was modeled as an incompressible neo-Hookean solid. Model geometry and boundary conditions used physiological parameters and descriptions found in the literature. Active muscle stresses were incorporated to account for both sphincter and dilator muscles in the iris. Numerical discretization was accomplished using the standard Galerkin finite element method and solution using DASPK, a differential-algebraic system solver.; A steady-state model incorporating sphincter contraction was used to simulate pupillary block, the predominant mechanism of angle-closure glaucoma. Results were consistent with Mapstone's pupil-blocking force analysis and compared favorably with pharmacologically-induced iris force measurements. Assessment of certain anatomical risk factors was completed by quantifying the contribution to angle closure. Lens thickness and position were factors that directly contributed to pupillary block while anterior chamber diameter affected angle-closure susceptibility. Anterior chamber depth decreased with severity of angle closure, also consistent with clinical observations, but could not differentiate between different contributing mechanisms.; Transient pupil dilation simulations showed an initial anterior iris deflection due to incompressibility of the aqueous humor in the posterior chamber, an effect that increased with dilation rate while independent of dilation mechanism.; Accommodative microfluctuation simulations showed that the iris response exhibited the same waveform as the stimulus for small-amplitude microfluctuations generally associated with the high-frequency component. Low-frequency microfluctuations with relatively larger amplitudes elicited an asymmetrical response, indicating that the forces generated by the lens movement significantly affected the aqueous-iris mechanical interaction.