Track 20. Biomechanics of Organs 20.1.

6768 Mo, 12:00-12:15 (P9) The Relationship between the hydrodynamic patterns of aqueous humor outflow and outflow resistance D.R. Overby 1 , S.R. Hofmann 1 , S.A. Kasper 2, Z. Lu 2, H. Gong 2. 1Tulane University, New Orleans, LA, USA, 2Boston University School of Medicine, Boston, MA, USA

Homeostatic regulation of intraocular pressure (lOP) is critical for vision, and loss of lOP regulation is implicated in blindness associated with glaucoma. The primary factor controlling lOP is the resistance to aqueous humor outflow from the anterior chamber of the eye, however there is little hydrodynamic understanding of how outflow resistance is generated within outflow pathway tissues. Rather than being uniform, aqueous humor outflow patterns are segmental, flowing along preferred routes such that only a fraction of the total area of outflow pathway tissue is involved in hydrodynamic filtration at any given instant. This area is termed the effective filtration area. We hypothesize that the effective filtration area influences the magnitude of outflow resistance by narrowing the hydrodynamic patterns of outflow and thereby increasing the effective resistance of the tissues near the inner wall endothelium of Schlemm's canal. To investigate this hypothesis, we examine the relationship between effective filtration area and outflow resistance for different experimental conditions designed to either increase or decrease outflow resistance. Enucleated bovine eyes are perfused with fluorescent tracer microparticles (0.5 ~tm, 0.002% v/v) to outline the hydrodynamic patterns of outflow, while outflow resistance is recorded for a user-defined lOP (7 to 45 mmHg). Eyes are either perfused with a resistance-decreasing drug (50 ~tM Y-27632) or perfused at elevated lOP to increase outflow resistance. Outflow pathway tissues are then perfusion-fixed and processed for confocal microscopy to examine tracer labeling patterns along the inner wall endothelium of the aqueous plexus (the bovine equivalent of Schlemm's canal) and to measure the effective filtration area. Increasing lOP from 7 to 45 mmHg coincided with a 2-fold increase in outflow resis- tance (p<0.034, n=21) and a 30 to 50% reduction in effective filtration area (p<0.036). Conversely, perfusion with Y-27632, lead to a 30% reduction in outflow resistance (p = 0.03, n = 7) and a 2.5-fold increase in effective filtration area (p=0.002). These data support our hypothesis and suggest that the morphologic factors controlling the effective filtration area are likely important determinants of aqueous humor outflow resistance.

6131 Mo, 12:15-12:30 (P9) Effect o f cyclic b iomechanical s t ress on f luid f low through convent ional dra inage tissues R.E Ramos, W.D. Stamer. Ophthalmology and Vision Science, The University of Arizona, Tucson, AZ, USA

The conventional drainage tissue is pressure-sensitive and responsible for the regulation fluid flow out of the human eye, and thus intraocular pressure. The anterior segment perfusion model enables the study of conventional drainage tissue in organ culture. Since cyclic intraocular pressure fluctuations associated with each heartbeat (average 2.7 mmHg) occur in vive, the aim of the present study was to introduce cyclic pressure pulsations to this model system, and to determine effects on fluid flow through the conventional drainage pathway. Two pumps, a syringe pump and a pulsatile blood pump, were used in tandem to simulate the magnitude and frequency of intraocular pressure oscillations in vive. A fixed resistor (steel eye) was connected between the pumps and pressure transducer to initially set the magnitude for pressure pulsations. Post- mortem human eyes (n = 5) were perfused at constant inflow rate (2.5 ~tl/min) until a stable baseline within a physiological range was reached. Intraocular pulsations were then introduced to the system and intra-chamber pressure was recorded over time. Viability of outflow tissues was evaluated using standard real-time and post-perfusion techniques. Cyclic intraocular pressure pulsations were replicated in vitro with intra-chamber oscillations having a magnitude of 2.7mmHg and frequency of 1 Hz. In post-mortem human anterior segments, ocular pulsations resulted in an immediate decrease (within 24 hours) in pressure, and thus outflow rate. A new baseline of outflow was established within 72 hours, decreasing on average by 28±12% of the original baseline. Results suggest that cyclic intra-chamber pressure oscillations stimulate a contraction of trabecular meshwork tissues that cause a decrease in outflow facility. The absence of ciliary muscle tone to counterbalance a trabecular response in these preparations may explain the increased intra-chamber pressure in response to pressure pulsations.

5327 Mo, 14:00-14:15 (P11) Mathematical model ing o f aqueous humor f low in the eye A.A. Stein, I.N. Moiseeva, G.A. Lyubimov. Institute of Mechanics, Moscow University, Moscow, Russia

The problem of aqueous humor flow and intraocular pressure formation in the eye is investigated. In our lumped-parameter model the eye is considered as an

Biomechanics and Cell and Tissue Engineering - Anterior Segment $387

elastic shell filled with a fluid which inflows through the ciliary body epithelium and outflows through the trabecular and uveoscleral pathways. We assume that the inflow from the ciliary body capillaries depends on the pressure and osmotic pressure differences between the capillaries and the posterior chamber and is regulated by membrane-type relations. The outflow depends on the pressure difference between the aqueous humor and the episcleral veins. The model is analyzed in the stationary and nonstationary cases. The known data for the human eye, based on experimental studies, make it possible to estimate the model parameters and to conclude that the hydraulic and osmotic inflow mechanisms together can ensure the flow parameters observed, whereas the hydraulic mechanism alone is insufficient. The characteristic time of flow establishment after the outer conditions are disturbed is estimated. This time depends on the in- and outlet resistances and the shell stiffness, whereas the stationary intraocular pressure is independent of the elastic properties of the shell for a wide class of assumptions. The model describes the intraocular pressure response to various changes in the system parameters, for example, to an increase in the outflow resistance. The quasi-hydraulic approximation in which the osmotic inflow rate is assumed to be constant and the hydraulic- osmotic approximation in which the osmotic inflow component is regulated by a variable solute concentration difference on the inlet membrane with the active solute transport rate introduced explicitly are compared. The latter version of the model describes the increase in intraocular pressure and the decrease in flow-rate in response to outflow restrictions more realistically than the quasi- hydraulic one due to the presence of a convective mechanism that partially stabilizes the flow-rate.

5416 Mo, 14:15-14:30 ( P l l ) Biomechanical test ing of human trabecular meshwork cells and Schlemm's canal endothelial cells T. Juzkiw 1 , D.W.H. Chan 1 , W. Dai 1,4, C.R. Ethier 1,2,3. 1Mechanical and

2 Industrial Engineering, Institute of Biomaterials and Biomedical Engineering and 3Ophthalmology, University of Toronto, Toronto, ON, Canada, 4Visiting scholar from Xuanwu Hospital, Capital University of Medical Sciences, China

Purpose: Actin cytotoskeletal changes in human trabecular meshwork (TM) cells and Schlemm's canal (SC) endothelial cells have been observed in glaucomatous eyes. These changes may affect the biomechanical properties of the cells and influence resistance to aqueous outflow, with implications for the ocular hypertension characteristic of glaucoma. No data about the biomechanical properties of TM cells and SC cells are available. Here we use paramagnetic microbeads to directly measure the stiffness of cultured TM and SC cells. Methods: TM and SC cells were harvested from human donor eyes, grown to sub-confluence and serum starved before being used for experiments. A magnetic microneedle was then used to pull 4.5 um diameter tosyl-activated paramagnetic beads (Dynal Biotech) coated with a synthetic RGD peptide (In- tegra LifeSciences) and bound to integrins on the cells (Matthews et al., BBRC, 313: 758-764, 2004). An effective elastic modulus (stiffness) for each cell was obtained by fitting bead displacements to a lumped parameter model for the cell (Bausch et al., Biophysical Journal, 75: 2038-2049, 1998). Measurements were made on control TM cells; TM cells treated with actin altering agents Latrunculin-B (Lat-B) and Dexamethasone; control SC cells; control human umbilical vein endothelial cells (HUVEC; Cambrex Biosciences); and HUVECs treated with the microtubule disrupting agent nocodazole. Results: All cells showed viscoelastic behaviour. The mean stiffness of cul- tured TM cells was 0.0161±0.0022Pa-m (mean ±SEM; n=30 cells). The mean stiffness of cultured SC cells was 0.0184±0.0031 Pa-m (n = 13 cells), which was not significantly different from TM cells (p =0.77). A 37% reduction in TM cell stiffness and 63% reduction in HUVEC stiffness were observed when they were treated with Lat-B. The stiffness of HUVECs treated with nocodazole did not differ significantly from the untreated cells. Conclusions: TM and SC cells grown under static culture conditions have similar mechanical properties. They are less stiff than HUVECs but stiffer than previously reported values for fibroblasts. Lat-B reduces cell stiffness, but to a lesser degree than expected from observed changes in F-actin architecture. Acknowledgements: We acknowledge the financial support of the Canadian Institutes of Health Research (CRE).

6329 Mo, 14:30-14:45 ( P l l ) The inf luence o f refract ive surgery- induced modulat ion of corneal b iomechanics on corneal swel l ing response D.M. Grzybowski, N. Rogers, R. Lembach, C. Roberts. Ophthalmology, The Ohio State University, Columbus, Ohio, USA

Purpose: To measure changes in topography, pachymetry, and wavefront aberrations between normal and post-LASIK cornea in response to induced corneal edema. Methods: Five subjects (ten eyes) underwent induction of corneal edema by 2-hour nitrogen goggles exposure. Two subjects had previously undergone