Application of an organotypic ocular perfusion model to assess intravitreal drug distribution in human and animal eyes

Intravitreal (ITV) drug delivery is a new cornerstone for retinal therapeutics. Yet, predicting the disposition of formulations in the human eye remains a major translational hurdle. A prominent, but poorly understood, issue in pre-clinical ITV toxicity studies is unintended particle movements to the anterior chamber (AC). These particles can accumulate in the AC to dangerously raise intraocular pressure. Yet, anatomical differences, and the inability to obtain equivalent human data, make investigating this issue extremely challenging. We have developed an organotypic perfusion strategy to re-establish intraocular fluid flow, while maintaining homeostatic pressure and pH. Here, we used this approach with suitably sized microbeads to profile anterior and posterior ITV particle movements in live versus perfused porcine eyes, and in human donor eyes. Small-molecule suspensions were then tested with the system after exhibiting differing behaviours in vivo. Aggregate particle size is supported as an important determinant of particle movements in the human eye, and we note these data are consistent with a poroelastic model of bidirectional vitreous transport. Together, this approach uses ocular fluid dynamics to permit, to our knowledge, the first direct comparisons between particle behaviours from human ITV injections and animal models, with potential to speed pre-clinical development of retinal therapeutics.

Transpupillary collagen photocrosslinking for targeted modulation of ocular biomechanics

The central vision-threatening event in glaucoma is dysfunction and loss of retinal ganglion cells (RGCs), thought to be promoted by local tissue deformations. Here, we sought to reduce tissue deformation near the optic nerve head by selectively stiffening the peripapillary sclera, i.e. the scleral region immediately adjacent to the optic nerve head. Previous scleral stiffening studies to treat glaucoma or myopia have used either pan-scleral stiffening (not regionally selective) or regionally selective stiffening with limited access to the posterior globe. We present a method for selectively stiffening the peripapillary sclera using a transpupillary annular light beam to activate methylene blue administered by retrobulbar injection. Unlike prior approaches to photocrosslinking in the eye, this approach avoids the damaging effects of ultraviolet light by employing red light. This targeted photocrosslinking approach successfully stiffened the peripapillary sclera at 6 weeks post-treatment, as measured by whole globe inflation testing. Specifically, strain was reduced by 47% when comparing treated vs. untreated sclera within the same eye (n = 7, p=0.0064) and by 54% when comparing the peripapillary sclera of treated vs. untreated eyes (n = 7, p<0.0001). Post-treatment characterization of RGCs (optic nerve axon counts/density, and grading), retinal function (electroretinography), and retinal histology revealed that photocrosslinking was associated with some ocular toxicity. We conclude that a transpupillary photocrosslinking approach enables selective scleral stiffening targeted to the peripapillary region that may be useful in future treatments of glaucoma.

Improving Stem Cell Delivery to the Trabecular Meshwork Using Magnetic Nanoparticles

Glaucoma is a major cause of blindness and is frequently associated with elevated intraocular pressure. The trabecular meshwork (TM), the tissue that primarily regulates intraocular pressure, is known to have reduced cellularity in glaucoma. Thus, stem cells, if properly delivered to the TM, may offer a novel therapeutic option for intraocular pressure control in glaucoma patients. For this purpose, targeted delivery of stem cells to the TM is desired. Here, we used magnetic nanoparticles (Prussian blue nanocubes [PBNCs]) to label mesenchymal stem cells and to magnetically steer them to the TM following injection into the eye's anterior chamber. PBNC-labeled stem cells showed increased delivery to the TM vs. unlabeled cells after only 15-minute exposure to a magnetic field. Further, PBNC-labeled mesenchymal stem cells could be delivered to the entire circumference of the TM, which was not possible without magnetic steering. PBNCs did not affect mesenchymal stem cell viability or multipotency. We conclude that this labeling approach allows for targeted, relatively high-efficiency delivery of stem cells to the TM in clinically translatable time-scales, which are necessary steps towards regenerative medicine therapies for control of ocular hypertension in glaucoma patients.

Quantitative analysis of three-dimensional fibrillar collagen microstructure within the normal, aged and glaucomatous human optic nerve head

The aim of this study was to quantify connective tissue fibre orientation and alignment in young, old and glaucomatous human optic nerve heads (ONH) to understand ONH microstructure and predisposition to glaucomatous optic neuropathy. Transverse (seven healthy, three glaucomatous) and longitudinal (14 healthy) human ONH cryosections were imaged by both second harmonic generation microscopy and small angle light scattering (SALS) in order to quantify preferred fibre orientation (PFO) and degree of fibre alignment (DOFA). DOFA was highest within the peripapillary sclera (ppsclera), with relatively low values in the lamina cribrosa (LC). Elderly ppsclera DOFA was higher than that in young ppsclera (p < 0.00007), and generally higher than in glaucoma ppsclera. In all LCs, a majority of fibres had preferential orientation horizontally across the nasal–temporal axis. In all glaucomatous LCs, PFO was significantly different from controls in a minimum of seven out of 12 LC regions (p < 0.05). Additionally, higher fibre alignment was observed in the glaucomatous inferior–temporal LC (p < 0.017). The differences between young and elderly ONH fibre alignment within regions suggest that age-related microstructural changes occur within the structure. The additional differences in fibre alignment observed within the glaucomatous LC may reflect an inherent susceptibility to glaucomatous optic neuropathy, or may be a consequence of ONH remodelling and/or collapse.

Differential effects of tyrosine-rich amelogenin peptide on chondrogenic and osteogenic differentiation of adult chondrocytes

Current approaches to treat osteoarthritis (OA) are insufficient. Autologous chondrocyte implantation (ACI) has been used for the past decade to treat patients with OA or focal cartilage defects. However, a number of complications have been reported post-ACI, including athrofibrosis and symptomatic hypertrophy. Thus, a long-term ACI strategy should ideally incorporate methods to ‘prime’ autologous chondrocytes to form a cartilage-specific matrix and suppress hypertrophic mineralization. The objective of this study is to examine the effects of tyrosine-rich amelogenin peptide (TRAP; an isoform of the developmental protein amelogenin) on human articular cartilage cell (HAC) chondrogenic differentiation and hypertrophic mineralization in vitro. Effects of chemically synthesized TRAP on HAC chondrogenic differentiation were determined by assessing: (1) sGAG production; (2) Alcian blue staining for proteoglycans; (3) collagen type II immunostaining; and (4) expression of the chondrogenic genes SOX9, ACAN and COL2A1. Hypertrophic mineralization was assayed by: (1) ALP expression; (2) Alizarin red staining for Ca⁺²-rich bone nodules; (3) OC immunostaining; and (4) expression of the osteogenic/hypertrophic genes Ihh and BSP. Chemically synthesized TRAP was found to suppress terminal osteogenic differentiation of HACs cultured in hypertrophic mineralization-like conditions, an effect mediated via down-regulation of the Ihh gene. Moreover, TRAP was found to augment chondrogenic differentiation of HACs via induction of SOX9 gene expression when cells were cultured in pro-chondrogenic media. The results obtained from this proof-of-concept study motivate further studies on the use of TRAP as part of a preconditioning regimen in autologous chondrocyte implantation procedures for OA patients and patients suffering from focal cartilage defects.

Rate of change in central corneal thickness: a viability indicator for conventional drainage tissues in organ culture

Organ culture of human anterior segments is a powerful tool for understanding trabecular meshwork biology. However, data from a significant percentage of cultured anterior segments are unusable because tissues fail to meet quality control requirements, such as having adequate trabecular meshwork histology. The purpose of the present study was to evaluate a novel, real time method for assessing the viability of conventional drainage tissues in the human anterior segment perfusion model. Twenty-two human anterior segments were perfusion cultured using standard techniques for one week while measuring outflow facility and central corneal thickness (CCT). After perfusion-fixation, toludine blue-stained histological sections of drainage tissues from all four quadrants of each anterior segment were graded and endothelial cell nuclei from cornea centers were stained with 4',6-diamidino-2-phenylindole and counted. We found that most anterior segments with a stable outflow facility had a CCT that decreased over time, while anterior segments with an unstable outflow facility had CCT measurements that failed to decrease over time (P<0.01). When comparing CCT measurements to histological appearance of outflow tissues, we found that in 11/11 cases, anterior segments with an acceptable histological score had a negative CCT slope (P<0.01). Conversely in 3/4 instances, anterior segments with an unacceptable histological score had a positive CCT slope. Lastly, we observed a significant relationship between CCT measurements and corneal endothelial density (P<0.01). Thus, the simple procedure of measuring CCT during anterior segment perfusion provides a second useful measure to assess the viability of the anterior segment during the perfusion process.

Protective ocular mechanisms in woodpeckers

AIMS

Woodpeckers possess mechanisms protecting the eye from shaking/impact. Mechanisms available to woodpeckers but not humans may help explain some eye injuries in Shaken Baby syndrome (SBS).

METHODS

Gross dissection and histologic examination of eyes and orbits of seven woodpeckers.

RESULTS

All birds showed restricted axial globe movement due to the tight fit within the orbit and fascial connections between the orbital rim and sclera. The sclera was reinforced with cartilage and bone, the optic nerve lacked redundancy, and the vitreous lacked attachments to the posterior pole retina.

CONCLUSIONS

Woodpecker eyes differ from human infants by an inability of the globe to move axially in the orbit, the sclera to deform, and the vitreous to shear the retina. These findings support current hypotheses that abusive acceleration-deceleration-induced ocular injury in human infants may be related to translation of vitreous within the globe and the globe within the orbit. The woodpecker presents a natural model resistant to mechanical forces that have some similarity to SBS.

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.

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.

Measurement of Gd-DTPA diffusion through PVA hydrogel using a novel magnetic resonance imaging method

Polyvinyl alcohol-cryogel (PVA-C) is a hydrogel that is an excellent tissue mimic. In order to characterize mass transfer in this material, as well as to demonstrate in principle the ability to noninvasively measure solute diffusion in tissue, we measured the diffusion coefficient of the magnetic resonance (MR) contrast agent gadolinium diethylene triaminopentaacetic acid (Gd-DTPA) through PVA-C using a clinical MR imager. The method involved filling thick-walled rectangular PVA-C "cups" with known concentrations of Gd-DTPA solutions. Then by using a fast inversion recovery spin echo MR imaging protocol, a signal "null" contour was created in the MR image that corresponded to a second, known concentration of Gd-DTPA. By collecting a series of MR images through the PVA-C wall as a function of time, the displacement of this second known isoconcentration contour could be tracked. Application of Fick's second law of diffusion yielded the diffusion coefficient. Seven separate experiments were performed using various combinations of initial concentrations of Gd-DTPA within the PVA-C cups (3.2, 25.6, or 125 mM) and tracked isoconcentrations contours (0.096, 0.182, or 0.435 mM Gd-DTPA). The experimental results and the predictions of Fick's law were in excellent agreement. The diffusivity of Gd-DTPA through 10% PVA hydrogel was found to be (2.6 +/- 0.04) x 10(-10) m(2)/s (mean +/- s.e.m.). Separate permeability studies showed that the diffusion coefficient of Gd-DTPA through this hydrogel did not change with an applied pressure of up to 7.1 kPa. Accurate measurements could be made within 30 min if suitable Gd-DTPA concentrations were selected. Due to the excellent repeatability and fast data acquisition time, this technique is very promising for future in vivo studies of species transport in tissue.

Computational blood flow modeling based on in vivo measurements

Study of the relationship between hemodynamics and atherogenesis requires accurate three-dimensional descriptions of in vivo arterial geometries. Common methods for obtaining such geometries include in vivo medical imaging and postmortem preparations (vessel casts, pressure-fixed vessels). We sought to determine the relative accuracy of these methods. The aorto-iliac (A/I) region of six rabbits was imaged in vivo using contrast-enhanced magnetic resonance imaging (MRI). After sacrifice, the geometry of the A/I region was preserved via vascular casts in four animals, and ex situ pressure fixation (while preserving dimensions) in the remaining two animals. The MR images and postmortem preparations were used to build computer representations of the A/I bifurcations, which were then used as input for computational blood flow analyses. Substantial differences were seen between MRI-based models and postmortem preparations. Bifurcation angles were consistently larger in postmortem specimens, and vessel dimensions were consistently smaller in pressure-fixed specimens. In vivo MRI-based models underpredicted aortic dimensions immediately proximal to the bifurcation, causing appreciable variation in the aorto-iliac parent/child area ratio. This had an important effect on wall shear stress and separation patterns on the "hips" of the bifurcation, with mean wall shear stress differences ranging from 15% to 35%, depending on the model. The above results, as well as consideration of known and probable sources of error, suggests that in vivo MRI best replicates overall vessel geometry (vessel paths and bifurcation angle). However, vascular casting seems to better capture detailed vessel cross-sectional dimensions and shape. It is important to accurately characterize the local aorto-iliac area ratio when studying in vivo bifurcation hemodynamics.

A numerical study of blood flow patterns in anatomically realistic and simplified end-to-side anastomoses

PURPOSE

Recently, some numerical and experimental studies of blood flow in large arteries have attempted to accurately replicate in vivo arterial geometries, while others have utilized simplified models. The objective of this study was to determine how much an anatomically realistic geometry can be simplified without the loss of significant hemodynamic information.

METHOD

A human femoral-popliteal bypass graft was used to reconstruct an anatomically faithful finite element model of an end-to-side anastomosis. Nonideal geometric features of the model were removed in sequential steps to produce a series of successively simplified models. Blood flow patterns were numerically computed for each geometry, and the flow and wall shear stress fields were analyzed to determine the significance of each level of geometric simplification.

RESULTS

The removal of small local surface features and out-of-plane curvature did not significantly change the flow and wall shear stress distributions in the end-to-side anastomosis. Local changes in arterial caliber played a more significant role, depending upon the location and extent of the change. The graft-to-host artery diameter ratio was found to be a strong determinant of wall shear stress patterns in regions that are typically associated with disease processes.

CONCLUSIONS

For the specific case of an end-to-side anastomosis, simplified models provide sufficient information for comparing hemodynamics with qualitative or averaged disease locations, provided the "primary" geometric features are well replicated. The ratio of the graft-to-host artery diameter was shown to be the most important geometric feature. "Secondary" geometric features such as local arterial caliber changes, out-of-plane curvature, and small-scale surface topology are less important determinants of the wall shear stress patterns. However, if patient-specific disease information is available for the same arterial geometry, accurate replication of both primary and secondary geometric features is likely required.

Effects of ethacrynic acid on Schlemm's canal inner wall and outflow facility in human eyes

PURPOSE

The role of the inner wall of Schlemm's canal in determining aqueous outflow facility is poorly understood. To quantify the relationship between inner wall pore characteristics and aqueous outflow facility in human eyes, both control eyes and eyes in which facility had been pharmacologically increased by ethacrynic acid (ECA) infusion were studied.

METHODS

Outflow facility was measured in enucleated human eyes before and after delivery of 0.25 mM ECA (one eye of each of 6 pairs) or 2.5 mM ECA (one eye of each of 13 pairs). ECA, and vehicle in contralateral eyes, was delivered into Schlemm's canal by retroperfusion, thereby largely avoiding drug exposure to the trabecular meshwork. After facility measurement, eyes were fixed under conditions of either constant pressure (physiological intraocular pressure, 13 pairs) or "equal flow" (6 pairs) and were microdissected to expose the inner wall of Schlemm's canal. The density and diameter of intercellular and intracellular inner wall pores were measured using scanning electron microscopy.

RESULTS

Retroperfusion with 2.5 mM ECA increased facility by 73% (P < 0.001), whereas 0.25 mM ECA increased facility by 19% (not statistically significant). The density of intercellular pores in the inner wall of Schlemm's canal was increased by 520% in 2.5 mM ECA-retroperfused eyes (P < 0.00004), whereas intracellular pore density remained approximately constant. Large pores (size > or = 1.1 microm) were particularly enhanced in ECA retroperfused eyes. The net change in facility due to ECA was not correlated with changes in pore density or other inner wall pore statistics.

CONCLUSIONS

Our data are most consistent with a model in which pores in the inner wall of Schlemm's canal indirectly influence facility. However, measured changes in facility due to changes in inner wall properties did not agree with quantitative predictions of the pore funneling theory, suggesting that changes in facility may instead be due to gel leakage from the extracellular spaces of the juxtacanalicular tissue. More definitive experiments are required to confirm this hypothesis.

Enthacrynic and acid effects on inner wall pores in living monkeys

PURPOSE

The influence of the inner wall of Schlemm's canal on aqueous outflow facility remains poorly understood. We examined the relationship between inner wall pore characteristics and outflow facility in living primate eyes in which facility had been pharmacologically increased by ethacrynic acid (ECA) infusion and in contralateral control eyes.

METHODS

Outflow facility (two-level constant pressure perfusion) was measured in eight pairs of living monkey eyes before and after administration of a bolus dose of either 0.125 mM ECA or vehicle. After exsanguination, eyes were fixed in situ under constant-pressure conditions (mean fixation pressure approximately 19 mm Hg). The density and diameter of inner wall pores and the number and area of platelet aggregates on the inner wall of Schlemm's canal were measured by scanning electron microscopy.

RESULTS

In ECA-treated eyes, outflow facility increased 63% (P < 0.0001), intracellular pore density decreased 46% (P = 0.0094), intracellular pore size increased 27% (P = 0.049), platelet aggregate density increased 158% (P < 0.0001), and area covered by platelets increased 210% (P = 0.012) relative to contralateral controls. Although the average density and size of intercellular pores were essentially unaffected by ECA, an increased density of large (> or = 1.90 microm) intercellular pores was seen in ECA-treated eyes. The density of intracellular pores increased with the duration of fixative perfusion. Other than a weak negative correlation between outflow facility and intracellular pore density in ECA-treated eyes (P = 0.052), facility was not correlated with inner wall pore features.

CONCLUSIONS

Our data are most consistent with a scenario in which ECA promotes formation of large intercellular pores in the inner wall of Schlemm's canal, which are then masked by platelet aggregates. Masking of intercellular pores, combined with fixation-induced alteration of inner wall pore density, greatly complicates attempts to relate facility to inner wall structure and suggests that in vivo pore density is smaller than in fixed tissue. Additionally, facility-influencing effects of ECA on the juxtacanalicular tissue cannot be excluded.

Accuracy of computational hemodynamics in complex arterial geometries reconstructed from magnetic resonance imaging

PURPOSE

Combining computational blood flow modeling with three-dimensional medical imaging provides a new approach for studying links between hemodynamic factors and arterial disease. Although this provides patient-specific hemodynamic information, it is subject to several potential errors. This study quantifies some of these errors and identifies optimal reconstruction methodologies.

METHODS

A carotid artery bifurcation phantom of known geometry was imaged using a commercial magnetic resonance (MR) imager. Three-dimensional models were reconstructed from the images using several reconstruction techniques, and steady and unsteady blood flow simulations were performed. The carotid bifurcation from a healthy, human volunteer was then imaged in vivo, and geometric models were reconstructed.

RESULTS

Reconstructed models of the phantom showed good agreement with the gold standard geometry, with a mean error of approximately 15% between the computed wall shear stress fields. Reconstructed models of the in vivo carotid bifurcation were unacceptably noisy, unless lumenal profile smoothing and approximating surface splines were used.

CONCLUSIONS

All reconstruction methods gave acceptable results for the phantom model, but in vivo models appear to require smoothing. If proper attention is paid to smoothing and geometric fidelity issues, models reconstructed from MR images appear to be suitable for use in computational studies of in vivo hemodynamics.

Two pore types in the inner-wall endothelium of Schlemm's canal

PURPOSE

It has been reported that fixation conditions significantly influence the apparent pore density in the inner-wall endothelium of Schlemm's canal. In the present study, the manner in which fixation conditions affect the two subtypes of inner-wall pores, intracellular pores and intercellular (or border) pores, was investigated.

METHODS

Outflow facility was measured in enucleated human eyes. Eyes were fixed under constant flow" or constant pressure conditions, microdissected to expose the inner wall of Schlemm's canal, and prepared for scanning electron microscopy. The density and diameter of the two subtypes of pores in the inner wall were measured.

RESULTS

Intracellular pore density decreased with increasing postmortem time (P < 0.001) and increased with increasing volume of fixative passed through the outflow pathway (P < 0.001), whereas border pore density showed no dependence on these parameters (P > 0.25 and P > 0.15, respectively). Border pore density increased with increasing fixation pressure (P < 0.005), even though intracellular pore density showed no such dependence (P > 0.4). No correlation was found between outflow facility and the predictions of Poiseuille's law, Sampson's law, or the funneling theory for the hydraulic conductivity of the intracellular pores (P > 0.35) or the border pores (P > 0.1).

CONCLUSIONS

The intracellular and border pores form two morphologically and functionally distinct populations in the inner wall of Schlemm's canal. The dependence of intracellular pore density on postmortem time and on volume of fixative passed through the outflow pathway suggests that these pores are artifacts of tissue fixation or processing conditions. That border pores do not depend on such conditions and that their presence is correlative with perfusion pressure suggests that this population may be nonartifactual. New histologic techniques for examining the inner wall of Schlemm's canal are necessary to determine the in vivo state of inner-wall pores and how they influence outflow facility.

Study of regional deformation of the optic nerve head using scanning laser tomography

PURPOSE

Previous studies have suggested that IOP-induced deformation of the optic nerve head (ONH) at the level of the lamina cribrosa may contribute to axonal damage in glaucomatous optic neuropathy. Our purpose was to introduce a novel enucleated eye model for characterizing acute IOP-induced changes in ONH topography, and to develop improved analytical methods for detection of regional topographic change in the ONH.

METHODS

Using a specially designed experimental apparatus, enucleated human eyes were progressively pressurized to 5, 15, 30, and 50 mmHg. Seven topographic images of the optic disc were taken at each pressure by a scanning laser tomographer (Heidelberg Retina Tomograph-HRT). The dependence of ONH topography on IOP was quantified for the entire nerve using standard HRT indices of ONH topographic change. The supero-inferior and nasal-temporal hemifields were also analyzed. A new method of analysis was developed which computes the location of the point of maximum slope within a 10 degrees sector of the ONH, as well as the magnitude of this slope. This method, termed "Inflection Point Analysis," was designed to be robust to the potential artefacts of image translation, reference plane location, and the subjective determination of ONH limits.

RESULTS

The results of three eyes are presented to illustrate the techniques. In our enucleated eye model, average ONH depth progressively increased with IOP, showing a maximum average posterior displacement of 36 microm as IOP was changed from 5 to 50 mmHg. Significant regional variability in ONH displacement was observed, which both Inflection Point Analysis and standard HRT parameters were able to detect. Inflection point analysis showed several advantages over standard HRT parameters: it was insensitive to artefacts due to tilt, was able to objectively delineate the boundary between the optic cup and neuroretinal rim, and was able to sensitively track changes in the location of this margin.

CONCLUSIONS

Scanning laser tomography is capable of detecting regional variation in the deformation of the ONH in response to acute changes in IOP. Our enucleated eye model and Inflection Point Analysis are promising tools for basic studies of ONH deformation in response to IOP. More extensive studies of both enucleated and in vivo eyes are required to determine the potential of Inflection Point Analysis for studying and tracking the progression of glaucomatous optic neuropathy.

Flow waveform effects on end-to-side anastomotic flow patterns

PURPOSE

Restenosis due to distal anastomotic intimal hyperplasia, a leading cause of arterial bypass graft failure, is thought to be promoted by hemodynamic effects, specifically 'abnormal' wall shear stress patterns. The purpose of this study was to quantify the effects of flow waveform on peri-anastomotic flow and wall shear stress patterns.

METHODS

Blood flow and wall shear stress patterns were numerically computed in a representative three-dimensional anastomosis using femoral, iliac and coronary flow waveforms suitable for humans at rest. Numerical results were validated against experimental data.

RESULTS

Peri-anastomotic wall shear stress patterns were influenced by a complex interplay between secondary flow effects and unsteadiness. Peripheral flow waveforms (iliac, femoral) produced large temporal and spatial wall shear stress gradients on the host artery bed. In comparison, the coronary flow waveform produced normalized bed wall shear stress gradients that were a factor of 2-3 less than for the peripheral waveforms, even though average bed wall shear stress magnitudes were similar for the two waveforms.

CONCLUSIONS

If anastomotic intimal hyperplasia is promoted by large spatial and/or temporal gradients of wall shear stress, as has been proposed, this study predicts that there will be markedly less intimal hyperplasia on the host artery bed of coronary bypass grafts than for peripheral bypass grafts. This information, in conjunction with a comparative histopathologic study of intimal hyperplasia distribution, could help determine specific wall shear stress factors promoting intimal hyperplasia.

Computational blood flow modelling: errors associated with reconstructing finite element models from magnetic resonance images

Construction of computational blood flow models from magnetic resonance (MR) scans of real arteries is a powerful tool for studying arterial hemodynamics. In this report we experimentally determine a lower bound for errors associated with such an approach, and present techniques for minimizing such errors. A known, simple three-dimensional geometry (cylindrical tube) was imaged using a commercial MR scanner, and the resulting images were used to construct finite element flow models. Computed wall-shear stresses were compared to known values and peak errors of 40-60% were found. These errors can be attributed to limited spatial resolution, image segmentation and model construction. A simple smoothing technique markedly reduced these peak errors. We conclude that smoothing is required in the construction of arterial models from in vivo MR images. If used appropriately, such images can be used to construct acceptably accurate computational models of realistic arterial geometries.

Oxygen mass transfer calculations in large arteries

The purpose of this study was to model the transport of oxygen in large arteries, including the physiologically important effects of oxygen transport by hemoglobin, coupling of transport between oxygen in the blood and in wall tissue, and metabolic consumption of oxygen by the wall. Numerical calculations were carried out in an 89 percent area reduction axisymmetric stenosis model for several wall thicknesses. The effects of different boundary conditions, different schemes for linearizing the oxyhemoglobin saturation curve, and different Schmidt numbers were all examined by comparing results against a reference solution obtained from solving the full nonlinear governing equations with physiologic values of Schmidt number. Our results showed that for parameters typical of oxygen mass transfer in the large arteries, oxygen transport was primarily determined by wall-side effects, specifically oxygen consumption by wall tissue and wall-side mass transfer resistance. Hemodynamic factors played a secondary role, producing maximum local variations in intimal oxygen tension on the order of only 5-6 mmHg. For purposes of modeling blood-side oxygen transport only, accurate results were obtained through use of a computationally efficient linearized form of the convection-diffusion equation, so long as blood-side oxygen tensions remained in the physiologic range for large arteries. Neglect of oxygen binding by hemoglobin led to large errors, while arbitrary reduction of the Schmidt number led to more modest errors. We conclude that further studies of oxygen transport in large arteries must couple blood-side oxygen mass transport to transport in the wall, and accurately model local oxygen consumption within the wall.

Factors affecting the pores of the inner wall endothelium of Schlemm's canal

PURPOSE

A linear relationship between the density of pores in the inner wall of Schlemm's canal and aqueous outflow facility has been reported previously in a study in which investigators examined only eyes fixed at constant pressure, so that fixative flow rates differed from eye to eye. Because pores may form as a function of flow rate, the purpose in the current study was to verify the previous findings, using constant flow perfusions.

METHODS

Outflow facility was measured in enucleated human eyes. Eyes were fixed under either constant flow or constant pressure conditions, microdissected to expose the inner wall of Schlemm's canal, and prepared for scanning electron microscopy. The density and diameter of pores in the inner wall were measured.

RESULTS

Statistical analysis showed no correlation between outflow facility and either the density or the diameter of pores. Pore density decreased significantly during the hours after death. Examining only eyes for which experimentation was started within 20 hours of death, we found that pore density increased significantly with the volume of fixative that had been perfused through the outflow pathway.

CONCLUSIONS

The correlation found by Allingham et al between outflow facility and pore density in the inner wall endothelium was not confirmed. However, the relationship between pore density and volume of fixative perfused is consistent with and may be responsible for the finding in the previous study. Because fixation conditions can influence the apparent pore density in the inner wall endothelium significantly, the conclusion reached previously, that pores contribute only 10% of the aqueous outflow resistance, may require reevaluation.

Combined analysis of spatial and velocity displacement artifacts in phase contrast measurements of complex flows

MR phase contrast (PC) velocity imaging is a promising tool for quantifying blood flow velocity in vivo. PC velocity imaging is, however, susceptible to artifacts that result from the displacement of spins during the finite duration pulse sequences. Such displacement artifacts can lead to errors in velocity measurements, especially in the presence of oblique and accelerating flows, which are common throughout the cardiovascular system. By tracking particles (representing spins) through a computed velocity field, and assuming that spatial and velocity encodings occur at discrete times during the pulse sequence, we simulate the separate and combined effects of oblique and acceleration artifacts on PC velocity images. We demonstrate, both by simulation and MR measurement, the errors associated with such artifacts in PC velocity measurements in a representative flow geometry. Using example particle trajectories, we provide a fluid dynamic basis for characteristic phase-velocity image distortions that can arise when imaging complex, physiologically relevant flows.

Particle volumetric residence time calculations in arterial geometries

The quantification of particle (platelet) residence times in arterial geometries is relevant to the pathogenesis of several arterial diseases. In this manuscript, the concept of "volumetric residence time" (VRT) is introduced. The VRT takes into account where particles accumulate and how long they remain there, and is well-suited to characterizing particle distributions in the complex geometries typical of the cardiovascular system. A technique for the calculation of volumetric residence time is described, which assumes that platelets are neutrally buoyant passive tracer particles, and which tracks small Lagrangian fluid elements containing a uniform concentration of platelets. This approach is used to quantify particle (platelet) residence times in the region of a modeled stenosis with a 45 percent area reduction. Residence time distributions are computed for a representative population of platelets, and for a subpopulation assumed to be "activated" by exposure to shear stresses above a threshold value. For activated platelets, high particle residence times were observed just distal to the apex of the stenosis throat, which can be explained by the presence of high shear stresses and low velocities in the throat immediately adjacent to the vessel wall. Interestingly, the separation zone distal to the stenosis showed only modestly elevated residence times, due to its highly mobile and transient nature. This calculation demonstrates the utility of the VRT concept for cardiovascular studies, particularly if a subpopulation of all particles is to be tracked. We conclude that the volumetric residence time is a useful tool.

MR measurement and numerical simulation of steady flow in an end-to-side anastomosis model

Magnetic resonance phase contrast velocity imaging ('MR PC velocimetry') is a powerful tool for measuring blood velocity. While it has been demonstrated that MR PC velocimetry can accurately measure velocity profiles in simple, unidirectional flows, errors can arise when measuring the more complex, multidirectional flows of clinical importance. In this work, we have compared MR PC velocity measurements with numerical simulations of steady flow in a physiologically relevant model of an end-to-side anastomosis, which produces many of the complex flow features expected in vivo. MR PC velocity profiles, measured using both 2D and 3D pulse sequences, showed very good agreement with numerically computed profiles, although at 2D PC data showed slightly larger errors than the 3D PC data in regions of high accelerations and oblique flow, likely due to displacement artifacts. We conclude that, under ideal conditions, MR phase contrast velocimetry can measure complex flow patterns to within 15% accuracy, but that care should be taken when using 2D pulse sequences to measure such complex flows.

Schlemm's canal and primary open angle glaucoma: correlation between Schlemm's canal dimensions and outflow facility

The purpose of the study was to examine the correlation between outflow facility and morphometric measurements of Schlemm's canal (SC) in normal and glaucomatous human eyes. Outflow facility was measured in ten normal and five glaucomatous (POAG) human eyes prior to perfusion-fixation at a pressure of 15 mmHg. Two separate sections per quadrant (eight sections per eye) were analysed using a morphometric analysis system. SC cross sectional area, perimeter, and inner wall length were measured. Inner wall endothelial nuclei were counted. When pooled data were examined, a statistically significant correlation was found between facility and SC area (P < or = 0.01). Most importantly, POAG eyes had a significantly smaller SC cross-sectional area, SC perimeter and SC inner wall length compared to normal eyes. If examined as a reduction in SC filtering area, this decrease in SC inner wall length alone could account for approximately 41-55% of the difference in outflow facility observed between normal and POAG eyes. The dimensions of Schlemm's canal in glaucomatous human eyes were significantly smaller than those in normal eyes. This reduction in SC dimensions may account for approximately half of the decrease in outflow facility observed in POAG eyes.

Retroperfusion studies of the aqueous outflow system. Part 2: Studies in human eyes

PURPOSE

To extend the retroperfusion technique to allow the delivery of drugs into Schlemm's canal in enucleated human eyes and to use this technique to gain insights into the function of the inner wall of Schlemm's canal.

METHODS

Using our previously developed retroperfusion technique, the anterior chamber of enucleated human eyes was held at a small negative pressure (-0.75 mm Hg), and fluid was allowed to flow retrograde from the limbal vessels, through the collector channels, and into Schlemm's canal. In this manner, the sulfhydryl agent N-ethyl maleimide (NEM) or the fixative agent glutaraldehyde was delivered to the inner wall of Schlemm's canal in normal and glaucomatous human eyes. Facility changes caused by retroperfusion were measured and correlated with histologic studies of the inner wall of Schlemm's canal.

RESULTS

Retroperfusion effectively delivers fluid from the scleral surface into the lumen of Schlemm's canal. Retroperfusion with vehicle alone does not alter facility or change outflow pathway morphology. Retroperfusion with NEM causes an approximately 35% facility increase and concomitant inner wall openings. Retroperfusion with glutaraldehyde in normal eyes and eyes with primary open-angle glaucoma causes a facility decrease of 53% and 64%, respectively, and localized fixation of the inner wall of Schlemm's canal. The magnitude of the facility changes caused by retroperfusion were similar to those seen using conventional forward perfusion of NEM and glutaraldehyde.

CONCLUSIONS

Retroperfusion is a viable technique for the delivery of drugs or other agents into Schlemm's canal in enucleated human eyes. Retroperfusion-induced changes in outflow facility are correlated strongly with morphologically observed alterations in inner wall structure. The majority of outflow resistance is localized to the inner wall of Schlemm's canal or the immediately adjacent 10-microns region of the juxtacanalicular tissue in normal eyes and in eyes with primary open-angle glaucoma. Inner wall giant vacuoles and pores likely persist for sometime, even after fixation at zero or negative pressure.

Accuracy of MR phase contrast velocity measurements for unsteady flow

The accuracy of MR phase contrast (PC) velocity measurements for unsteady flow has been quantitatively assessed. Spatially resolved velocity fields were measured in a long straight tube using a gated PC technique, and the resulting MR PC velocity data were compared with velocities derived from the analytic Womersley solution to the Navier-Stokes equations governing fluid flow. The overall root-mean-square (rms) difference between the measured and analytic velocities was 1.6 cm s-1 for nominally sinusoidal flow waveforms with peak velocities ranging from 51.6 cm s-1 to 59.8 cm s-1. This rms difference corresponded to 7.5% of the mean fluid velocity, which is similar to the cited accuracy of approximately 5% for MR PC velocimetry for steady flows. Linear regression between the PC velocity measurements and the velocities obtained using the analytic expression was highly significant (r2 = 0.997) and yielded a slope of 0.998, close to the expected value of 1. We conclude that the gated MR PC velocity measurements in unsteady flow are accurate.

Simulation of non-Newtonian blood flow in an end-to-side anastomosis

In order to quantify the importance of non-Newtonian blood rheology on anastomotic flow patterns, the characteristics of Newtonian and non-Newtonian blood flows were compared in a 2-D, 45 degree end-to-side anastomosis model under both steady and unsteady flow conditions. All flows were assumed to be two-dimensional, and were simulated numerically using parameters consistent with blood flow in the femoral artery. A novel, purely viscous constitutive relation, based on a generalized form of the power law relation, was developed to model the non-Newtonian rheology of blood. The resulting wall shear stress patterns indicate that for the parameter values under consideration, non-Newtonian blood rheology has a significant effect on steady flow wall shear stresses, but no significant effect on unsteady flow wall shear stresses. Based on these and other simulations, a parameter is formulated that gives an indication of the importance of non-Newtonian blood rheology under a given set of flow conditions. In addition, an argument is presented for allowing the conclusions from this two-dimensional study to be extended to three-dimensional blood flow.

Deformation of the lamina cribrosa by elevated intraocular pressure

The purpose of this study was to determine the mechanical response of the lamina cribrosa (LC) to elevated intraocular pressure (IOP) so as to identify possible mechanisms of optic nerve damage in early glaucoma. Ten pairs of normal human eyes were fixed after 24 hours' exposure to 50 mm Hg pressure (experimental eyes) or 5 mm Hg pressure (contralateral control eyes). Photomicrographs of the central region of the optic nerve head (ONH) were taken to examine the LC morphologically and to measure the dimensions of the LC. It was found that elevated IOP caused the LC to deflect posteriorly without affecting its thickness. The majority of the posterior displacement in the LC occurred near the periphery of the ONH. This shape change is consistent with a model of force distribution within the LC in which shear stresses are dominant; such stresses are maximal at the periphery and minimal at the centre of the ONH. These findings support a model in which mechanical forces, specifically shearing stresses within the peripheral lamina, play a direct role in the pathology of glaucomatous optic neuropathy.

The effect of wall distensibility on flow in a two-dimensional end-to-side anastomosis

The development of intimal hyperplasia at the distal anastomosis is the major cause of long-term bypass graft failure. To evaluate the suspected role of hemodynamic factors in the pathogenesis of distal intimal hyperplasia, an understanding of anastomotic flow patterns is essential. Due to the complexity of arterial flow, model studies typically make simplifying assumptions, such as treating the artery and graft walls as rigid. In the present study this restriction is relaxed to consider the effects of vessel wall distensibility on anastomotic flow patterns. Flow was simulated in an idealized 2-D distensible end-to-side anastomosis model, using parameters appropriate for the distal circulation and assuming a purely elastic artery wall. A novel numerical approach was developed in which the wall velocities are solved simultaneously with the fluid and pressure fields, while the wall displacements are treated via an iterative update. Both the rigid and distensible cases indicated the presence of elevated temporal variations and low average magnitudes of wall shear stress at sites known to be susceptible to the development of intimal hyperplasia. At these same sites, large spatial gradients of wall shear stress were also noted. Comparison between distensible-walled and corresponding rigid-walled simulations showed moderate changes in wall shear stress at isolated locations, primarily the bed, toe and heel. For example, in the case of a distensible geometry and a physiologic pressure waveform, the heel experienced a 38 percent increase in cycle-averaged shear stress, with a corresponding 15 percent reduction in shear stress variability, both relative to the corresponding values in the rigid-walled case.(ABSTRACT TRUNCATED AT 250 WORDS)

An improved ocular perfusion system

We describe the design, construction and testing of an improved ocular perfusion system for outflow facility measurement. The essential feature of the system is the measurement of a differential pressure across a calibrated length of tubing, which provides a stable and accurate reading of facility and hence also of flowrate. Perfusion of calibrated lengths of tubing showed excellent accuracy, while perfusions of enucleated eyes showed very low noise levels. The system is relatively inexpensive, easy to construct and maintain, insensitive to vibrations and other noise, and accurate. It is therefore felt to be superior to conventional "hanging bucket" ocular perfusion systems.

A numerical simulation of flow in a two-dimensional end-to-side anastomosis model

In order to understand the possible role that hemodynamic factors may play in the pathogenesis of distal anastomotic intimal hyperplasia, we carried out numerical simulations of the flow field within a two-dimensional 45 degree rigid-walled end-to-side model anastomosis. The numerical code was tested and compared with experimental (photochromic dye tracer) studies using steady and near-sinusoidal waveforms, and agreement was generally very good. Using a normal human superficial femoral artery waveform, numerical simulations indicated elevated instantaneous wall shear stress magnitudes at the toe and heel of the graft-host junction and along the host artery bed. These sites also experienced highly variable wall shear stress behavior over the cardiac cycle, as well as elevated spatial gradients of wall shear stress. These observations provide additional evidence that intimal hyperplasia may be correlated to wall shear stresses over the cardiac cycle, high wall shear stress gradients, or a combination of the three. The limitations of the present work (especially in regard to the two-dimensional nature of the flow simulations) are discussed, and results are compared to previous observations about distal anastomotic intimal hyperplasia.

Retroperfusion studies of the aqueous outflow system. Part I: Evaluation of technique using N-ethyl maleimide

PURPOSE

The goal of this study was to develop a new technique to deliver drugs or other agents to the lumen of the angular aqueous plexus/Schlemm's canal (AAP/SC) while bypassing the trabecular meshwork, thereby gaining insight into AAP/SC inner wall function.

METHODS

The anterior chamber is held at a small negative pressure and fluid is allowed to flow retrograde from the limbal vessels, through the collector channels, and into the AAP/SC ("retroperfusion"). Facility measurements are combined with histologic and tracer studies in bovine eyes.

RESULTS

(1) Retroperfusion with a saline solution does not alter facility or change outflow pathway morphology; (2) fluid is able to move retrograde from the scleral surface and enter the lumen of the AAP; and (3) retroperfusion with N-ethyl maleimide causes a dose-dependent increase in washout rate and concomitant inner wall breaks.

CONCLUSIONS

It is hypothesized that the observed increase in washout is due to leakage of extracellular materials through breaks in the inner wall.

The relationship between pore density and outflow facility in human eyes

The inner wall (IW) endothelial lining of Schlemm's canal was examined in six normal human eyes and four eyes with primary open angle glaucoma (POAG). Outflow facility was measured using constant pressure perfusion at 15 mmHg, eyes were fixed at 15 mmHg, and the IW endothelial lining was isolated and examined by scanning electron microscopy. Pore density, pore diameter, and bulge density were recorded by quadrant, and pore size and density were used to estimate IW endothelial facility, resistance, and hydraulic conductivity (facility per unit area). In POAG eyes, pores were less common (489 +/- 172 vs 1437 +/- 423 pores/mm2; P less than .005) and appeared to be more unevenly distributed than in normal eyes. A regional analysis of pore density (by quadrant) failed to detect a significant difference between quadrants of normal or POAG eyes. Pore density was correlated with measured outflow facility in normal eyes alone (P less than .02) and when normal eyes were pooled with POAG eyes (P less than .001). The percentage of total resistance attributed to the IW endothelium was 5.8% in normals compared to 9.5% in POAG eyes. This indicates there is a greater pressure drop across the IW endothelium in POAG eyes, suggesting that an intrinsic difference in IW endothelial function exists between normal and glaucomatous eyes. However, this difference alone does not account for the decreased outflow facility in POAG eyes. IW endothelial hydraulic conductivity is markedly higher than that of other vascular endothelia. We hypothesize that this may protect the IW endothelial lining of Schlemm's canal from mechanical stress induced by the relatively high rate of transcellular fluid flow.

Modulation of outflow resistance by the pores of the inner wall endothelium

The juxtacanalicular connective tissue (JCT) is widely believed to generate the bulk of aqueous humor outflow resistance, while the pores of the inner wall endothelium are thought to generate at most 10% of this resistance in humans. However, the hydrodynamic interaction of these two components of the aqueous outflow system, which arises because of their spatial proximity, has only recently been considered. Modelling the JCT as a homogeneously distributed porous material upstream of a low porosity filter (the inner wall endothelium), the pores of the inner wall are found to cause a "funneling effect," in which the aqueous humor flows preferentially through those regions of the JCT nearest the inner wall pores. The bulk of the pressure drop occurs in the immediate proximity of the pores (within three pore radii). This greatly increases the apparent flow resistance of the JCT. For a set of parameters characterizing the normal eye, this enhancement is approximately 30-fold. The conclusion of this study is that changes in inner wall porosity may greatly affect aqueous outflow resistance, despite the low flow resistance of the inner wall pores themselves.

Computer simulation of blood flow patterns in arteries of various geometries

The purpose of this study is to illustrate the application of computer simulation to the study of blood flow through arteries and to demonstrate the relationship between geometry of the vessels and local flow patterns. A finite element computer program was developed to simulate steady and pulsatile blood flow by solving the continuity and Navier-Stokes equations. The accuracy of the computational method has been confirmed by comparing the numeric results to analytic solutions and to published experimental data from physical models. The results are presented as plots of the velocity vectors, streamlines, and pressure contours. The computational model has been applied to illustrate flow patterns in the following situations: pulsatile flow in a cylindric artery and an artery with an axisymmetric stenosis, steady flow in cylindric arteries with stenoses of varying severity and with different flow rates, steady flow in an artery containing a fusiform aneurysm, steady flow in a two-dimensional model of a symmetric Y-shaped bifurcation, and steady flow in a two-dimensional model of the carotid bifurcation. Regions that are commonly associated with arterial disease often coincide with zones of reversed or stagnant flow. In conclusion, the versatility and feasibility of computational simulation of blood flow is illustrated by this study. Although this mathematic model is a simplification of the real flow phenomena, it yields results that provide useful insights into the understanding of local blood flow patterns for a variety of complex geometries.

Effects of hydrogen peroxide-induced oxidative damage on outflow facility and washout in pig eyes

Previous studies show that hydrogen peroxide (H2O2) is present in the aqueous humor of many species and is capable of affecting outflow facility in animal model experiments. To study the hypothesis that oxidative damage to the outflow pathway may play a role in the pathogenesis of primary open-angle glaucoma, 3 mM H2O2 with 20 mM 3-aminotriazole and 1 mM carmustine (BCNU) in Dulbecco's phosphate-buffered saline (DPBS) was perfused into enucleated pig eyes at constant pressure. Baseline and experimental perfusions were done at two different pressures (7.5 and 30 mm Hg) to study the effect of pressure on the response to oxidative damage. Outflow facility in the baseline experiments (with DPBS only) was observed to increase nonlinearly with time during the perfusions, but could be linearized if plotted as a function of the volume perfused. Thus, a term "volumetric washout" (W) was introduced and defined as the fractional rate of change of outflow facility with respect to the volume perfused. This quantity was found to be independent of pressure in the baseline studies. Perfusion of H2O2 and inhibitors increased W at 7.5 mm Hg but decreased W at 30 mm Hg. These results indicate that oxidative damage increases outflow facility at normal pressure but decreases it at elevated pressure, suggesting that elevated pressure may increase the susceptibility of the outflow pathway to this form of insult.

Steady and pulsatile flow fields in an end-to-side arterial anastomosis model

We investigated the flow field within a rigid-walled in vitro model of an end-to-side 45 degree anastomosis in an attempt to identify possible hemodynamic factors that may contribute to the pathogenesis of distal anastomotic intimal hyperplasia. A high-resolution photochromic tracer technique was used to visualize the flow in orthogonal planes and to determine the axial wall shear stress profiles for both steady and pulsatile flows over a range of physiologically relevant conditions. The flow field showed qualitative similarities to those seen in curved vessel: rapidly moving fluid from the graft section affects the bed of the host vessel, that is, the wall opposite the anastomosis, eventually advancing down the host vessel in a spiraling motion. A small mobile separation zone was noted at the toe of the anastomosis. Comparison of wall shear stress profiles with previously reported preferential sites for the development of intimal hyperplasia supported a low wall shear stress and/or flow separation pathogenesis hypothesis. One notable exception was the bed of the host artery that appeared to be subjected to a complex hemodynamic environment.

Further studies on the flow of aqueous humor through microporous filters

It has recently been shown that aqueous humor is able to obstruct flow through Nuclepore polycarbonate filters having flow dimensions similar to those found in the juxtacanalicular tissue (JCT). We undertook studies designed to identify the component(s) of aqueous humor responsible for this obstruction and to determine the mechanism of blockage. We conclude that aqueous humor contains two components (one of which is specific to aqueous) which, when simultaneously present, hydrophobically bind to microporous filters and lead to filter blockage. Some implications for aqueous humor flow through the JCT are briefly discussed.

Two-dimensional gel electrophoresis of calf aqueous humor, serum, and filter-bound proteins

Recent studies have demonstrated that bovine and primate aqueous humor (AH) obstruct flow when perfused through artificial membranes with pore sizes similar to those found in the aqueous outflow pathway. Proteinaceous AH components were implicated in this phenomenon, which is not observed with serum diluted to comparable protein concentrations. In this study, we used two-dimensional gel electrophoresis to characterize the protein composition of calf AH and to identify those proteins binding to the filters and presumably causing this obstruction. Comparison of AH and serum under denaturing conditions showed quantitative and qualitative differences in their protein content. Among the most important: AH was seen to possess two protein subunit trains (approximately 28 kD and approximately 48 kD) not found in serum and two trains (approximately 28 kD and approximately 80 kD) with additionally charged components not found in serum. Serum, on the other hand, possesses one train (approximately 80-90 kD) not found in AH as well as a slightly greater relative amount of high-molecular weight protein subunits. The finding that hydrophobic filters retain more protein components than do hydrophilic filters suggests that the type and amount of protein adhering to them is determined largely by hydrophobic interactions. Whether such interactions occur in the outflow system, and if so, how they may relate to aqueous drainage remains to be determined.

Calculations of flow resistance in the juxtacanalicular meshwork

The structure of the juxtacanalicular meshwork (JCM) was analyzed morphometrically, and the resulting data were used to calculate the resistance to flow through this tissue. Two models of the JCM were presented and compared. In the first (Model A), aqueous humor was assumed to flow via open channels within a solid framework, while, in the second (Model B), these open spaces were assumed to be filled with extracellular matrix gel. An expression giving the resistance of such a gel as a function of gel concentration was presented and tested on corneal and scleral stroma. Morphometry of normal and glaucomatous human eyes showed that Model A underpredicted the resistance of the JCM by factors of 10-100, suggesting that a GAG or proteoglycan gel may control the flow resistance of this tissue. This was supported by Model B, which showed that measured bulk concentrations of GAGs were consistent with gel concentrations needed to account for the estimated resistance of the JCM in vivo. Some limitations and implications of Model B were discussed.

The hydrodynamic resistance of hyaluronic acid: estimates from sedimentation studies

The problem of estimating the permeability of hyaluronic acid as a function of concentration has been examined. A previously known method of obtaining permeability from sedimentation studies has been employed, and the results have been compared to those obtained from convection studies, in which solvent is forced through the hyaluronic acid. The two sets of results were seen to be at variance, which could be explained by a flow-induced polarization of the hyaluronic acid in the convection studies. The polarization phenomenon was described in terms of a convection-associated compaction balancing the hyaluronic acid's intrinsic resistance to compression. Based on these results and other arguments, it was suggested that data from sedimentation studies provide a more accurate estimate of hyaluronic acid permeability than do convection experiments. The implications of this finding in a physiological context were briefly discussed.