Mechanical interaction between aqueous humor, iris, and intraocular structures can alter the iris profile from its normal curvature. In particular, significant changes to the iris profile occur during accommodation as the anterior lens movement forces the iris into greater posterior bowing. We extended a previous mathematical model of the anterior segment and investigated the response of this coupled fluid–solid system due to accommodative microfluctuations. The results showed that the system 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 a response different from the stimulus, indicating that the forces generated by the lens movement significantly affected the aqueous–iris mechanical interaction.

*accommodative microfluctuations*have been measured by infrared optometry to have frequencies ranging between 0 and 6 Hz, with power spectra showing two dominant frequency bands (Campbell, Robson, & Westheimer, 1959): a low-frequency component (LFC) composed of frequencies less than 0.5 Hz and a high-frequency component (HFC) typically centered near 2.0 Hz. The definition of the HFC varies between reported studies. Campbell et al. (1959) reported the HFC to be found between 1.3 and 2.2 Hz, whereas Kotulak and Schor (1986b) reported it to be found between 1.5 and 2.5 Hz. Winn, Pugh, Gilmartin, and Owens (1990) showed a correlation between arterial pulse and the HFC, defined as the frequency band between 1.0 and 2.3 Hz.

**v**is the velocity,

**T**is the stress tensor,

*ρ*is the density,

*P*is the pressure,

**I**is the identity matrix, and

*μ*is the viscosity. The density and viscosity of aqueous humor are similar to those of water (Beswick & McCulloch, 1956; Scott, 1988; Vass et al., 2004).

**F**is the deformation tensor,

**is the Cauchy stress tensor,**

*σ**G*is the shear modulus, and

**u**is the displacement. A shear modulus of 9 kPa was used in the model based on bovine iris tissue (Heys & Barocas, 1999).

*μ*l/min (Caprioli, 1992). Pressure-dependent outflow was modeled as:

*Q*

_{PD}is the pressure-dependent outflow,

*k*is the fluid conductivity through the trabecular meshwork, IOP is the intraocular pressure, and

*P*

_{vein}is the episcleral venous pressure. In the simulation, values of 0.3

*μ*l·min

^{−1}·mmHg

^{−1}and 9 mmHg were used for

*k*and

*P*

_{vein}, respectively (Kaufman, 1996). Pressure-independent outflow through the uveoscleral pathway was also included at the same boundary at a rate of 0.4

*μ*l/min (Kaufman, 1996).

*μ*l/min, this model results in an IOP of 16 mmHg. Coupling of fluid and solid mechanics was accomplished by imposing no-slip and stress balance conditions at the aqueous–iris interface.

*r*and

*z*are radial and axial coordinates of the anterior lens surface and

*a*and

*c*are experimentally determined parameters. Equation 8 can be recast as:

*r*

_{0},

*z*

_{0}) are the coordinates of a point on the peripheral anterior lens that is assumed to remain fixed during accommodation. This assumption was justified by the fact that the human eye accommodates primarily by varying lens curvature and not position. Based on measurements of Cook and Koretz (1991), we chose this point to be approximately 4 mm from the corneal axis. Under steady conditions, the initial position of the lens was set such that the calculated anterior chamber depth was approximately 3 mm, consistent with data for a normal eye (Fontana & Brubaker, 1980).

*a*and

*c*as time-dependent functions. The function

*c*(

*t*) was determined in the model according to how anterior chamber depth varied with time. The function

*a*(

*t*) was subsequently calculated by combining Equations 8 and 9 to yield:

*μ*m, the approximate resolution of 50 MHz ultrasound imaging (Liebmann & Ritch, 1996). Curvature of the iris profile was defined as the maximum distance between the posterior surface of the iris and the line connecting the iris root and pupil margin (Liebmann, Tello, Chew, Cohen, & Ritch, 1995). The pressure difference between the anterior and posterior chambers (Δ

*P*

_{AP}) was determined by noting pressure values representative of the respective regions, justified because pressure variation within each chamber was much less than Δ

*P*

_{AP}.

*P*

_{AP}.

*P*

_{AP}, and iris curvature during the first 20 s of LFC microfluctuations relative to the steady case (i.e., without any microfluctuation). Three different cases are shown to illustrate the effects of varying amplitude and frequency within the defined ranges for LFC microfluctuations. For all LFC cases tested, the system reached steady oscillatory response after 20 s.

*P*

_{AP}, and iris curvature all exhibited sinusoidal waveforms similar to that of the microfluctuation. At larger amplitudes, however, the response waveform became asymmetric. This is demonstrated further in Figure 3, which plots the maximum and minimum deviations in iris curvature from the steady case as a function of LFC microfluctuation amplitude and frequency.

*P*

_{AP}(not shown) exhibited the same sinusoidal response as iris curvature. Variations in amplitude or frequency within the HFC range did not affect the general waveform of iris–lens contact, Δ

*P*

_{AP}, and iris curvature. An increase in either amplitude or frequency magnified the mechanical response due to a faster lens velocity.

*P*

_{AP}, the system exhibited a sinusoidal response. After sufficient time, the system recovered and exhibited the same oscillatory response as the cases without the initial accommodation.

*P*

_{AP}directly affected the iris profile through pressure forces. During steady conditions, Δ

*P*

_{AP}was determined by the flow resistance in the iris–lens gap and bulk aqueous flow (Silver & Quigley, 2004). During anterior lens movement, Δ

*P*

_{AP}was further affected by the associated fluid outflow to the posterior segment (as necessitated by volume conservation of the lens), which caused a depressurization in the posterior chamber (Figure 5). Finally, there was also localized pressurization as fluid was squeezed out of the iris–lens gap (Movie 1). The complicated interplay between iris and aqueous humor mechanics during lens movement dictated the overall Δ

*P*

_{AP}. In all cases, changes in iris curvature were reflective of the respective changes in Δ

*P*

_{AP}.

*P*

_{AP}, and consequently, iris curvature, were effectively “blunted” ( Figure 2). The overall waveform at larger amplitudes became asymmetric, with high Δ

*P*

_{AP}and iris curvature values being attenuated. Decreasing the frequency further dampened high Δ

*P*

_{AP}and iris curvature values, as the lens velocity during microfluctuations was lower.

*P*

_{AP}sufficiently to complicate the aqueous–iris mechanical interaction.

*P*

_{AP}. Iris–lens contact, however, changed only by a factor of 2, and the total contact area changed by less than 10%. Because it is likely that the iris modulus varies with pupil dilation, the aqueous–iris mechanical response could be further complicated during accommodation. Efforts are currently underway to investigate the aqueous–iris response to multiple stimuli, such as the “near triad”—simultaneous positive accommodation and pupillary constriction during viewing of a near object.