Dexamethasone alters F-actin architecture and promotes cross-linked actin network formation in human trabecular meshwork tissue

Elevated intraocular pressure is an important risk factor for the development of glaucoma, a leading cause of irreversible blindness. This ocular hypertension is due to increased hydrodynamic resistance to the drainage of aqueous humor through specialized outflow tissues, including the trabecular meshwork (TM) and the endothelial lining of Schlemm's canal. We know that glucocorticoid therapy can cause increased outflow resistance and glaucoma in susceptible individuals, that the cytoskeleton helps regulate aqueous outflow resistance, and that glucocorticoid treatment alters the actin cytoskeleton of cultured TM cells. Our purpose was to characterize the actin cytoskeleton of cells in outflow pathway tissues in situ, to characterize changes in the cytoskeleton due to dexamethasone treatment in situ, and to compare these with changes observed in cell culture. Human ocular anterior segments were perfused with or without 10(-7) M dexamethasone, and F-actin architecture was investigated by confocal laser scanning microscopy. We found that outflow pathway cells contained stress fibers, peripheral actin staining, and occasional actin "tangles." Dexamethasone treatment caused elevated IOP in several eyes and increased overall actin staining, with more actin tangles and the formation of cross-linked actin networks (CLANs). The actin architecture in TM tissues was remarkably similar to that seen in cultured TM cells. Although CLANs have been reported previously in cultured cells, this is the first report of CLANs in tissue. These cytoskeletal changes may be associated with increased aqueous humor outflow resistance after ocular glucocorticoid treatment.

Biomechanics of Schlemm's canal endothelial cells: influence on F-actin architecture

Aqueous humor drains from the eye through Schlemm's canal, a small endothelial-lined collecting duct. Schlemm's canal endothelial cells may be important in controlling the pressure within the eye (and hence are of interest in glaucoma), and are subject to an unusual combination of shear stress and a basal-to-apical pressure gradient. We sought to characterize this biomechanical environment and determine its effects on F-actin architecture in situ. A theoretical model of flow in Schlemm's canal was used to estimate shear stresses applied to endothelial cells by flowing aqueous humor. Alignment of Schlemm's canal endothelial cells in human eyes was quantified by scanning electron microscopy. F-actin architecture was visualized by fluorescent labeling and compared for closely adjacent cells exposed to different biomechanical environments. We found that, despite the relatively low flow rate of aqueous humor, shear stresses experienced by Schlemm's canal endothelial cells could reach those in the arterial system. Schlemm's canal endothelial cells showed a statistically significant preferential alignment, consistent with a shear-mediated effect. Schlemm's canal endothelial cells subjected to a basal-to-apical pressure gradient due to transendothelial flow showed a prominent marginal band of F-actin with relatively few cytoplasmic filaments. Adjacent cells not subject to this gradient showed little marginal F-actin, with a denser cytoplasmic random network. We conclude that Schlemm's canal endothelial cells experience physiologically significant levels of shear stress, promoting cell alignment. We speculate that this may help control the calibre of Schlemm's canal. F-actin distribution depends critically on the presence or absence of transendothelial flow and its associated pressure gradient. In the case of this pressure gradient, mechanical reinforcement around the cell periphery by F-actin seems to be critical.

Finite element modeling of optic nerve head biomechanics

PURPOSE

Biomechanical factors have been implicated in the development of glaucomatous optic neuropathy, particularly at the level of the lamina cribrosa. The goal of this study was to characterize the biomechanics of the optic nerve head using computer modeling techniques.

METHODS

Several models of the optic nerve head tissues (pre- and postlaminar neural tissue, lamina cribrosa, central retinal vessel, sclera, and pia mater) were constructed. Stresses, deformations, and strains were computed using finite element modeling for a range of normal and elevated intraocular pressures. Computed retinal surface deformations were compared with measured deformation patterns in enucleated human eyes. A sensitivity analysis was performed in which tissue properties and selected geometric features were varied.

RESULTS

Acute IOP-induced deformation of the vitreoretinal interface was highly dependent on optic cup shape but showed a characteristic "W-shaped" profile that did not match the deformation of the anterior surface of the lamina cribrosa. The central retinal vasculature had surprisingly little effect on optic nerve head biomechanics. At an IOP of 50 mm Hg, strains (fractional elongation) in the lamina cribrosa averaged 4% to 5.5%, dependent on model geometry, with maximum strains up to 7.7%. Strains in the lamina cribrosa were more dependent on scleral stiffness, scleral thickness, and scleral canal diameter than on lamina cribrosa stiffness and optic cup shape.

CONCLUSIONS

Computed levels of strain in the lamina cribrosa are biologically significant and capable of contributing to the development of glaucomatous optic neuropathy, even without considering the probable accentuating effect of the lamina cribrosa's microarchitecture. Depending on optic cup shape, IOP-induced deformation of the vitreoretinal interface may not match lamina cribrosa deformation. This finding implies that scanning laser tomography has limited ability to estimate lamina cribrosa deformation when imaging the anterior topography of the optic nerve head. Biomechanical effects in the lamina cribrosa depend strongly on scleral properties.

Reconstruction of human optic nerve heads for finite element modeling

PURPOSE

Glaucoma is a common ocular disease whose pathogenesis is hypothesized to involve biomechanical damage to optic nerve tissues. Here we describe a method for the construction of patient-specific models that can be used to evaluate the biomechanical environment within the optic nerve head. We validate the method using a virtual eye, and demonstrate its use in computing optic nerve head biomechanics.

METHODS

Human eyes were imaged and the optic nerve head region was processed to allow serial plastic histologic sections to be cut. These sections were photographed, unwarped and aligned so as to reconstruct three-dimensional patient-specific models incorporating sclera, pre- and post-laminar nerve, lamina cribrosa, and pia mater. Deformations, stresses and strains were computed in the resulting model using finite element techniques.

RESULTS

The approach successfully reconstructed patient-specific optic nerve head models. Reconstruction of a virtual eye showed excellent agreement between the true and reconstructed geometries, and between the deformations and strains computed on the true and reconstructed geometries. A sample reconstruction showed reasonable agreement between computed and measured retinal surface deformations.

CONCLUSION

The technique presented here is viable and can be used to accurately compute human optic nerve head biomechanics.

Human Saphenous Vein Coronary Artery Bypass Graft Morphology, Geometry and Hemodynamics

Coronary artery bypass graft (CABG) failure has been linked to graft hemodynamics, in particular wall shear stress. This study characterizes the morphology, geometry and wall shear stress patterns in human CABGs. The intimal thickness (IT) in 49 human saphenous vein CABGs was measured by digital light microscopy. The geometry of six saphenous vein CABGs was replicated by post-mortem infusion of Batson's #17 anatomical corrosion casting compound. Graft hemodynamics were evaluated in two flow models, fabricated from the casts, under steady (Re = 110) and pulsatile flow (Re = 110, alpha = 2) conditions. Saphenous vein CABGs in situ for more than 2 months had, on average, the greatest IT on the hood and suture sites of the distal anastomosis. Floor thickening was highly variable and significantly less than IT at the hood, suture site and graft body. All casts showed an indentation along the floor and 5/6 casts displayed a sharp local curvature on the hood. In both flow models, a large increase in wall shear rate occurred on the hood, just proximal to the toe. The local geometry of the hood created this large spatial gradient in wall shear stress which is a likely factor in hood intimal hyperplasia.

Finite element modeling of optic nerve head biomechanics

PURPOSE

Biomechanical factors have been implicated in the development of glaucomatous optic neuropathy, particularly at the level of the lamina cribrosa. The goal of this study was to characterize the biomechanics of the optic nerve head using computer modeling techniques.

METHODS

Several models of the optic nerve head tissues (pre- and postlaminar neural tissue, lamina cribrosa, central retinal vessel, sclera, and pia mater) were constructed. Stresses, deformations, and strains were computed using finite element modeling for a range of normal and elevated intraocular pressures. Computed retinal surface deformations were compared with measured deformation patterns in enucleated human eyes. A sensitivity analysis was performed in which tissue properties and selected geometric features were varied.

RESULTS

Acute IOP-induced deformation of the vitreoretinal interface was highly dependent on optic cup shape but showed a characteristic "W-shaped" profile that did not match the deformation of the anterior surface of the lamina cribrosa. The central retinal vasculature had surprisingly little effect on optic nerve head biomechanics. At an IOP of 50 mm Hg, strains (fractional elongation) in the lamina cribrosa averaged 4% to 5.5%, dependent on model geometry, with maximum strains up to 7.7%. Strains in the lamina cribrosa were more dependent on scleral stiffness, scleral thickness, and scleral canal diameter than on lamina cribrosa stiffness and optic cup shape.

CONCLUSIONS

Computed levels of strain in the lamina cribrosa are biologically significant and capable of contributing to the development of glaucomatous optic neuropathy, even without considering the probable accentuating effect of the lamina cribrosa's microarchitecture. Depending on optic cup shape, IOP-induced deformation of the vitreoretinal interface may not match lamina cribrosa deformation. This finding implies that scanning laser tomography has limited ability to estimate lamina cribrosa deformation when imaging the anterior topography of the optic nerve head. Biomechanical effects in the lamina cribrosa depend strongly on scleral properties.

Ocular biomechanics and biotransport

The eye transduces light, and we usually do not think of it as a biomechanical structure. Yet it is actually a pressurized, thick-walled shell that has an internal and external musculature, a remarkably complex internal vascular system, dedicated fluid production and drainage tissues, and a variety of specialized fluid and solute transport systems. Biomechanics is particularly involved in accommodation (focusing near and far), as well as in common disorders such as glaucoma, macular degeneration, myopia, and presbyopia. In this review, we give a (necessarily brief) overview of many of the interesting biomechanical aspects of the eye, concluding with a list of open problems.

Intimal thickness is not associated with wall shear stress patterns in the human right coronary artery

OBJECTIVE

Low wall shear stress has been implicated in atherogenesis throughout the arterial tree, including the right coronary artery (RCA). The objective of this study was to determine the level of covariation of intimal thickness and wall shear stress in the human RCA.

METHODS AND RESULTS

Postmortem histological measurements of intimal thickness were compared with wall shear stresses calculated from computational flow modeling in 4 human right coronary arteries. A statistically significant correlation between intimal thickness and wall shear stress was found in only 1 of the 4 arteries studied.

CONCLUSIONS

Wall shear stress does not appear to be related to intimal thickness in the 4 RCAs studied.

Experimental and numerical studies of adenovirus delivery to outflow tissues of perfused human anterior segments

PURPOSE

To investigate the efficacy of two different methods of adenoviral transfer of genes to trabecular meshwork (TM) and Schlemm's canal (SC) cells in cultured human anterior segments, using both experimental and numerical analyses.

METHODS

Replication-deficient adenoviruses having coding sequence for beta-galactosidase (beta-gal) under the control of the cytomegalovirus promoter were used. Efficiency of gene transfer over time was verified by infecting cultured human TM cells and assaying for beta-gal activity. Next, ostensibly normal paired human eyes were prepared by standard techniques and perfused for 2 to 5 days to measure baseline facilities. Eyes were then infected by one of two methods: standard transcorneal puncture, or injection into a 1 mm diameter silastic segment of supply tubing immediately upstream of the perfusion dish. In both cases, the nominal total dose was 2 x 10(8) viral particles. Five days after viral injection, eyes were harvested and fixed, and wedges from each of four quadrants were examined histologically. Sections were assayed for beta-gal activity and/or stained with toluidine blue. In a parallel study, flow and viral transport within perfused anterior segments were numerically simulated for conditions that approximated those used experimentally.

RESULTS

Eyes receiving viral particles by transcorneal injection showed variable levels of beta-gal activity and highly variable TM cellular morphology, ranging from excellent preservation to cellular lysis. Eyes receiving an equivalent viral dose via the supply tubing showed higher transfer efficiency, as judged by almost complete TM cell loss (indicative of viral toxicity) and intense extracellular beta-gal activity from the residual cytoplasm. At lower doses (1/3 to 1/1000 of that used in transcorneal injection) beta-gal activity was still present, while TM cell morphology was good at the lower viral doses. Computer modeling showed that the region beneath the cornea was nearly stagnant, and consequently virus introduced into this region by transcorneal injection was delivered very slowly to the TM. This caused the effective delivered viral dose to be low and sensitively dependent on the volume and shape of the transcorneally injected virus bolus.

CONCLUSIONS

Injection of adenovirus into supply tubing led to consistent delivery of reporter gene and approximately 300-fold greater efficiency of gene transfer compared to the transcorneal injection method, and is therefore the preferred method for introducing viral particles into perfused anterior segments. These findings were consistent with computer modeling of flow and mass transport in perfused anterior segments. Although these quantitative results are specific to adenovirus, this general trend should hold for a wide range of perfused compounds.

Effects of TGF-beta2 in perfused human eyes

PURPOSE

TGF-beta2 is known to be present at elevated levels in the aqueous humor of patients with primary open-angle glaucoma (POAG). Studies have shown that TGF-beta2 influences cultured trabecular meshwork (TM) cells, but the effects of this cytokine on intact TM and outflow facility have not been studied. The purpose of this study was to investigate whether TGF-beta2 treatment induces changes in outflow facility and morphologic changes in the TM tissue and whether these changes are comparable to those previously recorded in glaucomatous eyes.

METHODS

Baseline facility was measured in paired human eyes (n = 8 pairs), with a constant-flow anterior segment culture system. Medium perfusing experimental eyes was then supplemented with activated human recombinant TGF-beta2 (3.0 ng/mL, comparable to or slightly greater than measured aqueous humor levels in patients with POAG), and facility was measured for at least 8 days. At the conclusion of the perfusion, eyes were fixed and processed for light microscopy, transmission electron microscopy, and immunolabeling studies.

RESULTS

TGF-beta2 perfusion reduced outflow facility by 27% (P = 0.03) and promoted focal accumulation of fine fibrillar extracellular material in multilayered structures under the inner wall of Schlemm's canal. In treated eyes, Schlemm's canal was 27% shorter (P = 0.02), and the length of the inner wall apparently available for fluid flow was 33% less (P = 0.001), both compared with paired control eyes.

CONCLUSIONS

TGF-beta2 reduces outflow facility when perfused into cultured human anterior segments. Furthermore, TGF-beta2 affects the extracellular matrix of the trabecular meshwork in a manner that is consistent with the observed reduction in outflow facility. Although the distribution of accumulated fibrillar material was different in these perfused eyes than that in POAG, the difference could be due to variation in biomechanical environment for TM cells in cultured anterior segments compared with the living eye. Overall, these results support the hypothesis that elevated TGF-beta2 levels in the aqueous humor play a role in the pathogenesis of the ocular hypertension in POAG.

The relationship between wall shear stress distributions and intimal thickening in the human abdominal aorta

PURPOSE

The goal of this work was to determine wall shear stress (WSS) patterns in the human abdominal aorta and to compare these patterns to measurements of intimal thickness (IT) from autopsy samples.

METHODS

The WSS was experimentally measured using the laser photochromic dye tracer technique in an anatomically faithful in vitro model based on CT scans of the abdominal aorta in a healthy 35-year-old subject. IT was quantified as a function of circumferential and axial position using light microscopy in ten human autopsy specimens.

RESULTS

The histomorphometric analysis suggests that IT increases with age and that the distribution of intimal thickening changes with age. The lowest WSS in the flow model was found on the posterior wall inferior to the inferior mesenteric artery, and coincided with the region of most prominent IT in the autopsy samples. Local geometrical features in the flow model, such as the expansion at the inferior mesenteric artery (common in younger individuals), strongly influenced WSS patterns. The WSS was found to correlate negatively with IT (r2 = 0.3099; P = 0.0047).

CONCLUSION

Low WSS in the abdominal aorta is co-localized with IT and may be related to atherogenesis. Also, rates of IT in the abdominal aorta are possibly influenced by age-related geometrical changes.

Computational modeling of arterial biomechanics: insights into pathogenesis and treatment of vascular disease

We review how advances in computational techniques are improving our understanding of the biomechanical behavior of the healthy and diseased cardiovascular system. Numerical modeling of biomechanics is being used in a wide variety of ways, including assessment of effects of mural and hemodynamically induced stresses on atherogenesis, development of risk measures for aneurysm rupture, improvement in interpretation of medical images, and quantification of oxygen transport in diseased and healthy arteries. Although not amenable to routine clinical use, numerical modeling of cardiovascular biomechanics is a powerful research tool.

Perfusion with the olfactomedin domain of myocilin does not affect outflow facility

PURPOSE

Mutations in the MYOC gene coding for myocilin are associated with elevated intraocular pressure (IOP), and recombinant myocilin, produced in a prokaryotic expression system, has been reported to affect aqueous outflow facility. This study was conducted to test whether perfusion with a fragment of recombinant myocilin (containing the full-length olfactomedin domain), produced in a eukaryotic expression system, affects facility.

METHODS

293 EBNA cells were transfected by a vector containing the BM40 signal peptide, a human cDNA coding for myocilin, and a polyhistidine tag (HisTag) sequence. Recombinant protein was isolated by Ni-chelate chromatography, and characterized, and perfused into cultured anterior segments of human and porcine eyes.

RESULTS

Recombinant myocilin was secreted as a approximately 55-kDa intact protein and two fragments arising from cleavage of the recombinant protein at amino acid 215. The C-terminal fragment, containing the entire olfactomedin domain, was successfully isolated. When perfused into human and porcine eyes, this C-terminal fragment did not appreciably affect outflow facility.

CONCLUSIONS

Although the olfactomedin domain appears to be important for the function of myocilin, perfusion with a recombinant myocilin fragment containing this domain does not change outflow facility. It is possible that both the olfactomedin and N-terminal domains (including the leucine zipper) must be present for myocilin to have full function. Alternatively, posttranslational modifications of myocilin may have a major impact on protein function.

Effects of cardiac motion on right coronary artery hemodynamics

The purpose of this work was to investigate the effects of physiologically realistic cardiac-induced motion on hemodynamics in human right coronary arteries. The blood flow patterns were numerically simulated in a modeled right coronary artery (RCA) having a uniform circular cross section of 2.48 mm diam. Arterial motion was specified based on biplane cineangiograms, and incorporated physiologically realistic bending and torsion. Simulations were carried out with steady and pulsatile inflow conditions (mean ReD=233, alpha=1.82) in both fixed and moving RCA models, to evaluate the relative importance of RCA motion, flow pulsation, and the interaction between motion and flow pulsation. RCA motion with a steady inlet flow rate caused variations in wall shear stress (WSS) magnitude up to 150% of the inlet Poiseuille value. There was significant spatial variability in the magnitude of this motion-induced WSS variation. However, the time-averaged WSS distribution was similar to that predicted in a static model representing the time-averaged geometry. Furthermore, the effects of flow pulsatility dominated RCA motion-induced effects; specifically, there were only modest differences in the WSS history between simulations conducted in fixed and moving RCA models with pulsatile inflow. RCA motion has little effect on time-averaged WSS patterns. It has a larger effect on the temporal variation of WSS, but even this effect is overshadowed by the variations in WSS due to flow pulsation. The hemodynamic effects of RCA motion can, therefore, be ignored as a first approximation in modeling studies.

The pore density in the inner wall endothelium of Schlemm's canal of glaucomatous eyes

PURPOSE

In a prior study, it has been reported that glaucomatous eyes have a significantly lower density of pores in the inner wall of Schlemm's canal than do normal eyes. However, in that study the glaucomatous eyes were fixed at much lower flow rates than the normal eyes, and that is now known to affect inner wall pore density. The objective of the current study was to compare the inner wall's pore density in glaucomatous and normal eyes, accounting for the effects of fixation conditions.

METHODS

Outflow facility was measured in enucleated glaucomatous human eyes. Eyes were fixed under constant flow conditions, microdissected to expose the inner wall of Schlemm's canal, and prepared for scanning electron microscopy (SEM). The density and diameter of the two subpopulations of pores in the inner wall, intracellular and intercellular (or "border") pores, were measured. Data were compared with those in previous studies of normal eyes.

RESULTS

As previously reported, pore density decreased with increasing postmortem time and increased with increasing volume of fixative passed through the outflow pathway and with increasing fixation time. Linear regression analysis indicated that glaucomatous eyes had less than one fifth the number of pores than normal eyes have, after accounting for the influence of volume of fixative perfused through the eyes (835 pores/mm(2) in normal eyes versus 160 pores/mm(2) in glaucomatous eyes). A nonlinear regression of pore density versus fixative volume produced a pore density at zero fixative volume that was not statistically different from zero. If true, this implies that all (or nearly all) inner wall pores observed by SEM are fixation artifacts. The density of intracellular pores and the diameter of these pores correlated with the density and diameter of the border pores, respectively.

CONCLUSIONS

Inner wall pores are reduced in glaucomatous eyes. If pores are physiological structures, the elevated intraocular pressure characteristic of glaucoma may be explained by decreased porosity of the inner wall endothelium. Both border and intracellular pores seem to be induced in a similar fashion by fixation. The unlikely possibility that all inner wall pores are fixation-induced cannot be excluded. If so, a fundamental reassessment of the mechanism by which aqueous humor crosses the inner wall endothelium is necessary.

Coupled computational analysis of arterial LDL transport -- effects of hypertension

Hypertension, a risk factor for atherosclerosis, increases the uptake of low density lipoproteins (LDL) by the arterial wall. Our objective in this work was to use computational modeling to identify physical factors that could be partially responsible for this effect. Fluid flow and mass transfer patterns in the lumen and wall of an arterial model were computed in a coupled manner, replicating as closely as possible previous experimental studies in which LDL uptake into the artery wall was measured in straight, excised arterial segments. Under conditions of both flow and no-flow, simulations predicted an increase in concentration polarization of LDL at the artery wall when arterial pressure was increased from 120 to 160 mmHg. However, this led to only a slight increase in mean LDL concentration within the arterial wall. However, if the permeability of the endothelium to LDL was allowed to vary with intra-arterial pressure, then the simulations predicted that the uptake of LDL would be enhanced 1.9-2.6 fold at higher pressure. The magnitude of this increase was consistent with experimental data. We conclude that the concentration polarization effects, enhanced by elevated intra-arterial pressure, cannot explain the increase in LDL uptake seen under hypertensive conditions. Instead, the data are most consistent with a pressure-linked increase in endothelial permeability to LDL.

A numerical study of blood flow in coronary artery bypass graft side-to-side anastomoses

PURPOSE

When sequential grafts are used in multivessel coronary artery bypass grafting, the graft first supplies blood to one or more coronary arteries via a side-to-side anastomosis. We studied hemodynamics in idealized models of "parallel" and "diamond" side-to-side anastomoses, identifying features that might promote restenosis.

METHODS

Blood flow was computed in three representative anastomosis configurations: parallel side-to-side, diamond side-to-side, and end-to-side. We compared configurations and the effect of host-graft diameter ratio.

RESULTS

Hemodynamic patterns depended strongly on anastomosis geometry and graft/host diameter ratio. In the distal graft, the diamond configuration had large areas of low wall shear stress (WSS) and high spatial WSS gradients. In the proximal graft the unfavorable WSS patterns were comparable for all models, while the distal portion of the host artery the diamond model was best. Models with smaller host arteries had smaller regions of low WSS.

CONCLUSIONS

The parallel configuration was preferred over the diamond for maintaining graft patency, while the diamond configuration appeared best for maintaining host artery patency. Since graft patency is critical, parallel configurations seem hemodynamically advantageous. Larger graft/host ratios have better hemodynamic performance than smaller ones.

Computational modeling of mass transfer and links to atherosclerosis

In the context of atherogenesis, mass transport refers to the movement of atherogenic molecules from flowing blood into the artery wall, or vice versa. Although LDL transport clearly plays a role in atherosclerotic plaque development, it is much less clear whether abnormalities in mass transfer patterns are in themselves atherogenic. A powerful way of addressing this question is through computational modeling, which provides detailed descriptions of local mass transport features. Here we briefly review the strategy and some of the pros and cons of such a modeling approach, and then focus on results gained from studies in a variety of arterial geometries. The general picture is that zones of hypoxia (low oxygen transport from blood to wall) and elevated LDL tend to colocalize with each other, and with areas of atherosclerotic lesion development and/or intimal thickening. The picture is complicated by the fact that such zones also tend to have "abnormal" wall shear stress patterns, which are also believed to be atherogenic. Taken together, these results suggest, but do not prove, a role for mass transport in atherogenesis.

Computational analysis of coupled blood-wall arterial LDL transport

The transport of macromolecules, such as low density lipoproteins (LDLs), across the artery wall and their accumulation in the wall is a key step in atherogenesis. Our objective was to model fluid flow within both the lumen and wall of a constricted, axisymmetric tube simulating a stenosed artery, and to then use this flow pattern to study LDL mass transport from the blood to the artery wall. Coupled analysis of lumenal blood flow and transmural fluid flow was achieved through the solution of Brinkman's model, which is an extension of the Navier-Stokes equations for porous media. This coupled approach offers advantages over traditional analyses of this problem, which have used possibly unrealistic boundary conditions at the blood-wall interface; instead, we prescribe a more natural pressure boundary condition at the adventitial vasa vasorum, and allow variations in wall permeability due to the occurrence of plaque. Numerical complications due to the convection dominated mass transport process (low LDL diffusivity) are handled by the streamline upwind/Petrov-Galerkin (SUPG) finite element method. This new fluid-plus-porous-wall method was implemented for conditions typical of LDL transport in a stenosed artery with a 75 percent area reduction (Peclet number=2 x 10(8)). The results show an elevated LDL concentration at the downstream side of the stenosis. For the higher Darcian wall permeability thought to occur in regions containing atheromatous lesions, this leads to an increased transendothelial LDL flux downstream of the stenosis. Increased transmural filtration in such regions, when coupled with a concentration-dependent endothelial permeability to LDL, could be an important contributor to LDL infiltration into the arterial wall. Experimental work is needed to confirm these results.

Cationic ferritin changes outflow facility in human eyes whereas anionic ferritin does not

PURPOSE

To determine the effect of charged moieties within the outflow pathway on aqueous outflow facility in human eyes.

METHODS

After baseline facility measurement in human eye bank eyes (n = 10 pairs), one eye of each pair received anterior chamber exchange and continued perfusion with medium containing 10 mg/ml cationic ferritin. Contralateral eyes were treated in a similar manner with anionic ferritin (10.0 or 102 mg/ml). Eyes were fixed by anterior chamber exchange and perfusion with universal fixative at 8 mm Hg (corresponding to a physiologic pressure of 15 mm Hg in vivo) and examined by transmission electron microscopy. In a second series of human eyes (n = 8 pairs), facility was measured before and after anterior chamber exchange, with a solution containing 0.1 U/ml neuraminidase.

RESULTS

Perfusion of eyes with anionic ferritin at either 10.0 or 102 mg/ml caused a negligible 2% increase in facility, whereas cationic ferritin perfusion reduced facility by 66% (P < 0.00001). Perfusion with fixative reduced facility by approximately 60% in both cationic and anionic ferritin-perfused eyes, relative to facilities after perfusion with ferritin. Transmission electron microscopy showed that the distribution of ferritin was segmentally variable. Cationic ferritin consistently labeled the luminal surface of the inner wall of Schlemm's canal, and variably labeled the juxtacanalicular connective tissue (JCT) and trabecular beam surfaces. Anionic ferritin was more prominent in the JCT and intertrabecular spaces and less so on the luminal surface of Schlemm's canal. By scanning electron microscopy, cationic ferritin was seen to accumulate at intercellular margins of the inner wall. Neuraminidase perfusion had no significant effect on outflow facility.

CONCLUSIONS

Cationic ferritin reduces outflow facility, presumably by binding to negatively charged sites in the outflow pathway. A possible mechanism is partial or complete blockage of intercellular clefts in the inner wall of Schlemm's canal by the ferritin that accumulates on the luminal surface of the inner wall. Although they are possible targets for ferritin binding, sialyl residues themselves seem to have little direct effect on outflow facility. Our data indicate that positively charged molecules, especially if they can interact with inner wall pores, have the potential to markedly alter outflow facility.

Requirements for mesh resolution in 3D computational hemodynamics

Computational techniques are widely used for studying large artery hemodynamics. Current trends favor analyzing flow in more anatomically realistic arteries. A significant obstacle to such analyses is generation of computational meshes that accurately resolve both the complex geometry and the physiologically relevant flow features. Here we examine, for a single arterial geometry, how velocity and wall shear stress patterns depend on mesh characteristics. A well-validated Navier-Stokes solver was used to simulate flow in an anatomically realistic human right coronary artery (RCA) using unstructured high-order tetrahedral finite element meshes. Velocities, wall shear stresses (WSS), and wall shear stress gradients were computed on a conventional "high-resolution" mesh series (60,000 to 160,000 velocity nodes) generated with a commercial meshing package. Similar calculations were then performed in a series of meshes generated through an adaptive mesh refinement (AMR) methodology. Mesh-independent velocity fields were not very difficult to obtain for both the conventional and adaptive mesh series. However, wall shear stress fields, and, in particular, wall shear stress gradient fields, were much more difficult to accurately resolve. The conventional (nonadaptive) mesh series did not show a consistent trend towards mesh-independence of WSS results. For the adaptive series, it required approximately 190,000 velocity nodes to reach an r.m.s. error in normalized WSS of less than 10 percent. Achieving mesh-independence in computed WSS fields requires a surprisingly large number of nodes, and is best approached through a systematic solution-adaptive mesh refinement technique. Calculations of WSS, and particularly WSS gradients, show appreciable errors even on meshes that appear to produce mesh-independent velocity fields.

Factors influencing blood flow patterns in the human right coronary artery

Evidence suggests that atherogenesis is linked to local hemodynamic factors such as wall shear stress. We investigated the velocity and wall shear stress patterns within a human right coronary artery (RCA), an important site of atherosclerotic lesion development. Emphasis was placed on evaluating the effect of flow waveform and inlet flow velocity profile on the hemodynamics in the proximal, medial, and distal arterial regions. Using the finite-element method, velocity and wall shear stress patterns in a rigid, anatomically realistic model of a human RCA were computed. Steady flow simulations (ReD=500) were performed with three different inlet velocity profiles; pulsatile flow simulations utilized two different flow waveforms (both with Womersley parameter=1.82, mean ReD=233), as well as two of the three inlet profiles. Velocity profiles showed Dean-like secondary flow features that were remarkably sensitive to the local curvature of the RCA model. Particularly noteworthy was the "rotation" of these Dean-like profiles, which produced large local variations in wall shear stress along the sidewalls of the RCA model. Changes in the inlet velocity profiles did not produce significant changes in the arterial velocity and wall shear stress patterns. Pulsatile flow simulations exhibited remarkably similar cycle-average wall shear stress distributions regardless of waveform and inlet velocity profile. The oscillatory shear index was very small and was attributed to flow reversal in the waveform, rather than separation. Cumulatively, these results illustrate that geometric effects (particularly local three-dimensional curvature) dominate RCA hemodynamics, implying that studies attempting to link hemodynamics with atherogenesis should replicate the patient-specific RCA geometry.

Mass transport in an anatomically realistic human right coronary artery

The coronary arteries are a common site of atherosclerotic plaque formation, which has been putatively linked to hemodynamic and mass transport patterns. The purpose of this paper was to study mass transport patterns in a human right coronary artery (RCA) model, focusing on the effects of local geometric features on mass transfer from blood to artery walls. Using a previously developed characteristic/finite element scheme for solving advection-dominated transport problems, mass transfer calculations were performed in a rigid, anatomically realistic model of a human RCA. A qualitative and quantitative examination of the RCA geometry was also carried out. The concentration field within the RCA was seen to closely follow primary and secondary flow features. Local variations in mass transfer patterns due to geometric features were significant and much larger in magnitude than local variations in wall shear stress. We conclude that the complex secondary flows in a realistic arterial model can produce very substantial local variations in blood-wall mass transfer rates, and may be important in atherogenesis. Further, RCA mass transfer patterns are more sensitive to local geometric features than are wall shear stress patterns.

Effect of Healon and Viscoat on outflow facility in human cadaver eyes

PURPOSE

To compare the acute effects of Healon (sodium hyaluronate) and Viscoat (sodium chondroitin sulfate-sodium hyaluronate) on outflow facility in human cadaver eyes and determine which viscoelastic agent is least likely to cause an intraocular pressure (IOP) spike after cataract surgery.

SETTING

The Glaucoma Research Lab, University of Toronto, Ontario, Canada.

METHODS

In this prospective paired study, 15 pairs of human cadaver eyes were used. Following the construction of a 3.0 mm scleral tunnel, 0.25 cc of Healon was injected into the anterior chamber of 1 eye and 0.25 cc of Viscoat was injected into the contralateral eye. The viscoelastic agents were removed from both eyes in a standardized fashion and the scleral tunnels closed. The eyes were then perfused at a constant IOP of 8.0 mm Hg, corresponding to 16.0 mm Hg in vivo. Outflow facility (microL/minute [min]/mm Hg) was recorded every 15 minutes for 24 hours using standard methods.

RESULTS

Outflow facility in the Viscoat-treated eyes decreased appreciably for the first 3 hours, then recovered somewhat after 12 hours; facility in the Healon-treated eyes showed less of an overall decrease. Over the 24 hour perfusion period, mean outflow facility was 0.037 microL/min/mm Hg +/- 0.015 (SD) in the Viscoat-treated eyes and 0.060 +/- 0.012 microL/min/mm Hg in the Healon-treated eyes. Healon reduced outflow facility significantly less than Viscoat between 3.25 and 10.50 hours postoperatively (P < .05, 2-tailed t test).

CONCLUSIONS

Healon reduced outflow facility less than Viscoat between 3.25 and 10.50 hours postoperatively.