Fluid and structure coupling analysis of the interaction between aqueous humor and iris

Abstract

Background: Glaucoma is the primary cause of irreversible blindness worldwide associated with high intraocular pressure (IOP). Elevated intraocular pressure will affect the normal aqueous humor outflow, resulting in deformation of iris. However, the deformation ability of iris is closely related to its material properties. Meanwhile, the passive deformation of the iris aggravates the pupillary block and angle closure. The nature of the interaction mechanism of iris deformation and aqueous humor fluid flow has not been fully understood and has been somewhat a controversial issue. The purpose here was to study the effect of IOP, localization, and temperature on the flow of the aqueous humor and the deformation of iris interacted by aqueous humor fluid flow.

Methods: Based on mechanisms of aqueous physiology and fluid dynamics, 3D model of anterior chamber (AC) was constructed with the human anatomical parameters as a reference. A 3D idealized standard geometry of anterior segment of human eye was performed. Enlarge the size of the idealization geometry model 5 times to create a simulation device by using 3D printing technology. In this paper, particle image velocimetry technology is applied to measure the characteristic of fluid outflow in different inlet velocity based on the device. Numerically calculations were made by using ANSYS 14.0 Finite Element Analysis. Compare of the velocity distributions to confirm the validity of the model. The fluid structure interaction (FSI) analysis was carried out in the valid geometry model to study the aqueous flow and iris change.

Results: In this paper, the validity of the model is verified through computation and comparison. The results indicated that changes of gravity direction of model significantly affected the fluid dynamics parameters and the temperature distribution in anterior chamber. Increased pressure and the vertical position increase the velocity of the aqueous humor fluid flow, with the value increased of 0.015 and 0.035 mm/s. The results act on the iris showed that, gravity direction from horizontal to vertical decrease the equivalent stress in the normal IOP model, while almost invariably in the high IOP model. With the increased of the iris elasticity modulus, the equivalent strain and the total deformation of iris is decreased. The maximal value of equivalent strain of iris in high IOP model is higher than that of in normal IOP model. The maximum deformation of iris is lower in the high IOP model than in the normal IOP model.

Conclusion: The valid model of idealization geometry of human eye could be helpful to study the relationship between localization, iris deformation and IOP. So far the FSI analysis was carried out in that idealization geometry model of anterior segment to study aqueous flow and iris change.

Keywords: Aqueous humor; Fluid–structure interaction; Intraocular pressure; Iris deformation; Mechanical properties.

Fig. 1

The main outflow pathway of aqueous humor from generation to outflow. The aqueous humor is actively secreted by the ciliary body, and it processes into the posterior chamber, passes through the pupil, enters the anterior chamber, and eventually exits, primarily through the trabecular meshwork

Fig. 2

a A two-dimensional representation of the idealization model and boundary conditions. b Three-dimensional representation of the aqueous humor model domain based on a and created by software SolidWorks2014, and the corresponding structure model (cornea and iris) is shown in c. In a P is pressure, V is stream velocity, T is temperature. V = 0 means no-slip boundary condition on cornea surface, lens surface and iris root

Fig. 3

a A three-dimensional model of simulation device manufacted by 3D printing based on the idealization geometry in Fig. 2, magnified in 5 times. b The device model designed an inlet pipe, an outlet pipe, a silicone pad, two buffer spaces, a sink and many small holes (1, 2, 3, and 4). The fluid is infused by the inlet pipe into the inlet buffer space, through 2 circle of small holes (3, 4) evenly into the posterior chamber from all directions, passes through the pupil, enters the anterior chamber, uniform inflow of small holes (1, 2) and outlet buffer space, and eventually exits through the outlet pipe. The silicone pad can prevent water leakage. And the function of the water sink is to avoid the refraction of the laser. The device produced by 3D printing is shown in c and d

Fig. 4

Connection diagram of experimental apparatus

Fig. 5

Velocity distribution measured at different speed

Fig. 6

Scatter plots created by the data in Table 2

Fig. 7

The velocity vector (a), streamline (b), pressure (c), and temperature (d) distribution with the gravity in the −y direction and the outlet pressure in 27 mmHg, which simulating the high IOP patient in supine position

Fig. 8

The velocity vector (a), streamline (b), pressure (c), and temperature (d) distribution in −x gravity direction and the outlet pressure in 27 mmHg, which simulating the high IOP patient in sitting position

Fig. 9

Distributions of iris and cornea interacted in the fluid flow with the gravity in −y direction and the outlet pressure in 27 mmHg

Fig. 10

A 3D model built with a heating tube and a macro distance adjusting part, based on the model in Fig. 3a

Fig. 11

Line charts created by the data in Table 4