Aqueous Humor Dynamics Modeling And Research In Human Eyes Based On Lattice Boltzmann Method

The production,circulation,and drainage of aqueous humor are essential to the normal physiological function of the eye.The formation of intraocular pressure is associated with the outflow resistance of aqueous humor.Normal intraocular pressure not only stabilizes eye morphology,but also plays an important role in physiological process.When the outflow of aqueous humor is blocked(increased resistance or blockage of the outflow passage),intraocular pressure rises,which increases the risk of causing eye diseases.Therefore,a systematic and indepth study of the hydrodynamic process in the anterior segment is necessary,which contributes to giving a profound insight of ocular diseases and providing a theoretical reference for ocular drug administration or surgical control and treatment.Due to a direct measurement of aqueous humor is not yet available,numerical simulation becomes a promising choice.The mesoscopic lattice Boltzmann method has the advantages of simple algorithm,high parallelism,and easy handling of complex geometric boundaries.It is especially suitable for the study of human eye fluid mechanics,a flow field with complex anatomical structure and multi-factor interaction.Therefore,in this paper,the lattice Boltzmann method is used to study the hydrodynamic problems in the anterior segment of the human eye in detail.On the basis of the model,the effect of intraocular lens implantation on the hydrodynamics of anterior segment is also investigated.The main contents in this thesis are listed as follows:(1)A coupled lattice Boltzmann model for simulating the dynamics of aqueous humor in the human eye is presented,which involves incompressible Navier-Stokes flow,heat convection and diffusion,and Darcy seepage flow.The incompressible Navier-Stokes equations are coupled with the heat convection and diffusion equation by the Boussinesq approximation,and it is coupled with Darcy's law for aqueous humor flow through the velocity boundary condition,which provides an important negative feedback mechanism for the model.Verifying simulations for the healthy eye indicate that the model is stable and robust for simulating the dynamics of aqueous humor due to its negative feedback mechanism.(2)With the present model,further investigations on aqueous humor flow have been carried out systematically,including the effects of heat convection and buoyancy,aqueous humor production rate,permeability of trabecular mesh,viscosity of aqueous humor and pupil size on intraocular pressure.The simulated results show agreement with clinical practices or those reported in the published literatures.Specifically,the simulations for aqueous humor flow in the healthy eye indicate that heat convection and diffusion can significantly affect the flow patterns,and is essential for the normal physiological functions of aqueous humor.With simulating the effects of various factors on the intraocular pressure,we find that the intraocular pressure increases with the increase of the aqueous humor production rate or aqueous humor viscosity,while it decreases with the increase of the permeability of trabecular mesh.It's interesting that all these factors have more significant influences on the intraocular pressure in pathologic eyes than healthy ones.The intraocular pressure is insensitive to the viscosity of the aqueous humor,which may be one of the causes that the viscosity would not have been considered as a factor for controlling the intraocular pressure.(3)On the basis of the present model,the simulation of the effect of an iris-fixed intraocular lens implantation on the flow of aqueous humor in the anterior segment are carried out.The key physical parameters of aqueous humor flow under natural convection are estimated.The flow fields,as well as,the shear stress acting on the cornea and iris for the cases with or without an irisfixed intraocular lens are compared.The numerical simulation results are basically consistent with the existing research results.Then the effects of a perforated intraocular lens,intraocular lens placement and intraocular lens implantation at different anterior chamber depths in standing and supine positions on aqueous fluid flow are studied.The results show that implantation of irisfixated phakic intraocular lens has no significant effect on the maximum velocity of aqueous humor and the maximum shear stress acting on the cornea and iris.When the intraocular lens is close to the iris(less than 0.05 mm from the iris),the maximum shear stress acting on the iris increases sharply,more than twice than that in the normal eye.It is recommended that patients with intraocular lens implantation be examined on a regular basis.The maximum velocity of aqueous humor and the maximum shear stress acting on the cornea and iris increase with the increase of anterior chamber depth and gradually approache the corresponding value in the normal eye.Therefore,intraocular lens implantation is not recommended for individuals with shallow anterior chamber depth.Due to the advantages of the lattice Boltzmann method in modeling complex fluid systems,the present model can be expected to apply to the dynamics of the aqueous humor more deeply and obtain some more interesting results,which can provide important theoretical guideline for the control and treatment of ocular diseases with drugs or surgery.