Morphological and biomechanical analyses of the human healthy and glaucomatous aqueous outflow pathway: Imaging-to-modeling

Abstract

Background and objective: Intraocular pressure (IOP) is maintained via a dynamic balance between the production of aqueous humor and its drainage through the trabecular meshwork (TM), juxtacanalicular connective tissue (JCT), and Schlemm's canal (SC) endothelium of the conventional outflow pathway. Primary open angle glaucoma (POAG) is often associated with IOP elevation that occurs due to an abnormally high outflow resistance across the outflow pathway. Outflow tissues are viscoelastic and actively interact with aqueous humor dynamics through a two-way fluid-structure interaction coupling. While glaucoma affects the morphology and stiffness of the outflow tissues, their biomechanics and hydrodynamics in glaucoma eyes remain largely unknown. This research aims to develop an image-to-model method allowing the biomechanics and hydrodynamics of the conventional aqueous outflow pathway to be studied.

Methods: We used a combination of X-ray computed tomography and scanning electron microscopy to reconstruct high-fidelity, eye-specific, 3D microstructural finite element models of the healthy and glaucoma outflow tissues in cellularized and decellularized conditions. The viscoelastic TM/JCT/SC complex finite element models with embedded viscoelastic beam elements were subjected to a physiological IOP load boundary; the stresses/strains and the flow state were calculated using fluid-structure interaction and computational fluid dynamics.

Results: Based on the resultant hydrodynamics parameters across the outflow pathway, the primary site of outflow resistance in healthy eyes was in the JCT and immediate vicinity of the SC inner wall, while the majority of the outflow resistance in the glaucoma eyes occurred in the TM. The TM and JCT in the glaucoma eyes showed 1.32-fold and 1.13-fold larger beam thickness and smaller trabecular space size (2.24-fold and 1.50-fold) compared to the healthy eyes.

Conclusions: Characterizing the accurate morphology of the outflow tissues may significantly contribute to constructing more accurate, robust, and reliable models, that can eventually help to better understand the dynamic IOP regulation, hydrodynamics of the aqueous humor, and outflow resistance dynamic in the human eyes. This model demonstrates proof of concept for determining changes to outflow resistance in healthy and glaucomatous tissues and thus may be utilized in larger cohorts of donor tissues where disease specificity, race, age, and gender of the eye donors may be accounted for.

Keywords: Computational fluid dynamics; Fluid-structure interaction; Juxtacanalicular tissue; Schlemm's canal; Trabecular meshwork.

Fig. 1.

(a) The original grayscale image of the TM/JCT/SC complex converted into a stack of (b) binary images. (c) The trabecular beam thickness was measured as the average thickness of the trabecular structure in the binary images. (d) The diameter of the largest sphere that fits inside the gap in between the trabecular beams was reported as the trabecular space. The same measurement process was done for the healthy and glaucoma decellularized outflow tissues.

Fig. 2.

FE model of the TM/JCT/SC complex with aqueous humor with the embedded beam elements for the (a) healthy cellularized, (b) healthy decellularized, (c) glaucoma cellularized, and (d) glaucoma decellularized. The SC and JCT have the thicknesses of ~2.2 μm and ~14 μm, respectively. The μm-sized pores with the average size of ~1.3 μm were distributed in the SC inner wall with the pore density of 835 pores/mm².

Fig. 3.

Physiologic inlet IOP load boundary. The volumetric average pressure in the SC lumen of the healthy cellularized/decellularized and glaucoma cellularized/decellularized FSI models were used as the outlet load boundary for the CFD simulations.

Fig. 4.

The (a) solid minimum angle, (b) solid maximum angle, (c) solid element aspect ratio, and (d) solid element time step in the TM/JCT/SC complex FE models.

Fig. 5.

The DNA content comparison between the glaucoma cellularized and decellularized outflow tissues.

Fig. 6.

Representative scanning electron micrographs for the healthy and glaucoma cellularized and decellularized-NH4OH human TM (magnifications ×200, ×500 and ×1000). The TM and corneal endothelium (CE) are labelled in ×200 image. * represents where the iris was removed to allow visualization of the TM. ► represents the trabecular space and ● represents the trabecular beam. Scale bars = 200 μm (×200); 100 μm (×500); 50 μm (×1000).

Fig. 7.

X-ray computed tomography images of the healthy and glaucoma cellular and decellular-NH₄OH human TM/JCT/SC complex shown in (a) front-on and (b) cross-sectional orientations. Red dashed boxes indicate region of interest for zoomed-in views. The TM and SC are indicated by curly bracket and black arrow, respectively. ((a): Scale bar = 200 μm; inset scale bar = 50 μm and (b): Scale bar = 200 μm).

Fig. 8.

The (a) trabecular beam thickness, (b) trabecular space size, and (c) porosity in the healthy and glaucoma decellularized outflow tissues.

Fig. 9.

The (a) 1^st principal stress and (b) strain in the healthy cellularized, healthy decellularized, glaucoma cellularized, and glaucoma decellularized TM/JCT/SC complex FE models.

Fig. 10.

The (a) maximum shear stress and (b) strain in the healthy cellularized, healthy decellularized, glaucoma cellularized, and glaucoma decellularized TM/JCT/SC complex FE models.

Fig. 11.

The aqueous humor pressure in the healthy cellularized, healthy decellularized, glaucoma cellularized, and glaucoma decellularized TM/JCT/SC complex (a) FSI and (b) CFD models.

Fig. 12.

The aqueous humor maximum shear stress in the healthy cellularized, healthy decellularized, glaucoma cellularized, and glaucoma decellularized TM/JCT/SC complex (a) FSI and (b) CFD models.

Fig. 13.

Displacement in the (a) healthy cellularized and (b) healthy decellularized TM/JCT/SC complex FE models.