Large-Deformation and Time-Dependent Mechanical Properties of the Human Conventional Aqueous Outflow Pathway

Abstract

Purpose : Intraocular pressure (IOP) is incredibly dynamic with relatively large fluctuations that cause both a large-deformation hyperelastic and a time-dependent viscoelastic biomechanical environment in the conventional aqueous outflow pathway. The aqueous humor actively interacts with the trabecular meshwork (TM), juxtacanalicular tissue (JCT), and Schlemm’s canal (SC) inner wall via a dynamic fluid-structure interaction (FSI) coupling.

Methods : A quadrant of the anterior segment from a normal human donor eye was dynamically pressurized from the SC lumen, and imaged using a customized green-light optical coherence tomography (OCT) setup (Figure a). The TM/JCT/SC complex finite element (FE) with embedded collagen fibrils was reconstructed based on the segmented boundary nodes in the OCT imaging data (Figure b). The hyperviscoelastic mechanical properties of the outflow tissues’ extracellular matrix with embedded viscoelastic collagen fibrils were calculated using the inverse FE method coupled with an optimization algorithm. Then, the 3D microstructural FE model of the TM, with adjacent JCT and SC inner wall of the same quadrant was constructed using optical coherence microscopy imaging data and subjected to a pressure boundary condition from the SC lumen. The resultant deformation/strain in the outflow tissues was calculated using the FSI method (Figure c), and compared to the digital volume correlation (DVC) data (Figure d).

Results : The average shear modulus of 0.92 MPa was found in the TM, larger than the JCT (0.47 MPa) and SC inner wall (0.85 MPa). The average time-dependent shear modulus was larger in the SC inner wall (97.65 MPa) compared to the TM (84.38 MPa) and JCT (56.30 MPa). The collagen fibrils in the TM showed stiffer mechanical response compared to the JCT. The resultant deformation/strain in the FSI model was in good agreement with the DVC data (Figure).

Conclusions : The conventional aqueous outflow pathway is subjected to a rate-dependent IOP load-boundary with large fluctuations. This necessitates addressing the biomechanics of the outflow tissues using hyperviscoelastic material-model. This will also with accurate flow analyses using FSI in future studies, so will allow studying the effects of different drugs or treatments on the hydrodynamics of the aqueous humor and biomechanics of the conventional aqueous outflow pathway.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.