A gimbal-mounted pressurization chamber for macroscopic and microscopic assessment of ocular tissues

Scleral structure and biomechanics

As the eye's main load-bearing connective tissue, the sclera is centrally important to vision. In addition to cooperatively maintaining refractive status with the cornea, the sclera must also provide stable mechanical support to vulnerable internal ocular structures such as the retina and optic nerve head. Moreover, it must achieve this under complex, dynamic loading conditions imposed by eye movements and fluid pressures. Recent years have seen significant advances in our knowledge of scleral biomechanics, its modulation with ageing and disease, and their relationship to the hierarchical structure of the collagen-rich scleral extracellular matrix (ECM) and its resident cells. This review focuses on notable recent structural and biomechanical studies, setting their findings in the context of the wider scleral literature. It reviews recent progress in the development of scattering and bioimaging methods to resolve scleral ECM structure at multiple scales. In vivo and ex vivo experimental methods to characterise scleral biomechanics are explored, along with computational techniques that combine structural and biomechanical data to simulate ocular behaviour and extract tissue material properties. Studies into alterations of scleral structure and biomechanics in myopia and glaucoma are presented, and their results reconciled with associated findings on changes in the ageing eye. Finally, new developments in scleral surgery and emerging minimally invasive therapies are highlighted that could offer new hope in the fight against escalating scleral-related vision disorder worldwide.

The pressure-induced deformation response of the human lamina cribrosa: Analysis of regional variations

The objective of this study was to measure the pressure-induced deformation response of the human lamina cribrosa (LC) and analyze for variations with age and anatomical region. The posterior scleral cup of 8 eyes from 6 human donors was mounted onto a custom inflation chamber. A laser-scanning microscope was used for second harmonic generation (SHG) imaging of the collagen structure in the posterior volume of the LC at pressures from 5mmHg to 45mmHg. The SHG volumes were analyzed by the Fast-Fourier Iterative Digital Volume Correlation (DVC) algorithm for the three dimensional (3D) displacement field. The components of the Green-Lagrange strain tensor and the in-plane principal and maximum shear strains were evaluated from the DVC displacement field for the central and peripheral regions of the LC and the nasal, temporal, inferior, and superior quadrants surrounding the central retinal artery and vein. Among the major findings were that older age was associated with lower strains, the maximum shear strain was larger in the peripheral than central region, and the maximum principal strain was lower in the nasal quadrant. The elliptical shape of the LC was also predictive of the biaxial strain ratio. Age-related and structure-related variations in the pressure-induced strains of the LC may contribute to the susceptibility and severity of optic nerve damage in glaucoma, and regional variations may explain the progression of axonal damage and tissue remodeling observed in the LC in glaucoma.

STATEMENT OF SIGNIFICANCE

Glaucoma causes vision loss through progressive damage of the retinal ganglion axons at the lamina cribrosa (LC), the connective tissue structure that supports the axons as they leave the eye. Mechanical characterization of the LC is challenging because of the complex 3D shape and inaccessibility of the tissue. We present a new method using digital volume correlation to map the 3D displacement and strain fields in the LC under inflation. We report for the first time significant regional variations in the strains that are consistent with the pattern of optic nerve damage in early glaucoma. Thus regional strain variations may be predictive of the progression of axonal damage in glaucoma.

Sequential-digital image correlation for mapping human posterior sclera and optic nerve head deformation

Optic nerve head (ONH) deformations may be involved in the onset or further development of glaucoma, including in patients with relatively normal intraocular pressures (IOPs). Characterizing posterior scleral deformations over physiological pressures may provide a better understanding of how changes in IOP lead to changes in the mechanical environment of the ONH and possibly retinal ganglion cell death. Pressure inflation measurement test protocols are commonly used to measure deformation of the peripapillary sclera with full-field noncontact optical methods. The purpose of this work was to develop and validate a new sequential 3D digital image correlation (S-DIC) approach for quantification of posterior scleral pressure induced deformation that improves z (in-depth) resolution of the DIC measurement without losing in-plane sensitivity, while also being able to contour and map deformations of the complex-shaped ONH. Our approach combines two orthogonal axes of parallax with standard 3D DIC methods using a single high-resolution camera. The enhanced capabilities of S-DIC with respect to standard 3D DIC has been demonstrated by carrying out a complete benchmark for shape, deformation, and strain measurement on an object of known complex geometry. Our S-DIC method provided a reconstruction accuracy of 0.17% and an uncertainty in z-position measurement of 8 μm. The developed methodology has also been applied to a human posterior scleral shell, including the full peripapillary sclera and optic nerve. The relatively inexpensive S-DIC approach may provide new information on the biomechanical deformations of the optic nerve head and, thus, the death of retinal ganglion cells in primary open angle glaucoma.

Biomechanics of the Posterior Eye: A Critical Role in Health and Disease

The posterior eye is a complex biomechanical structure. Delicate neural and vascular tissues of the retina, choroid, and optic nerve head that are critical for visual function are subjected to mechanical loading from intraocular pressure, intraocular and extraorbital muscles, and external forces on the eye. The surrounding sclera serves to counteract excessive deformation from these forces and thus to create a stable biomechanical environment for the ocular tissues. Additionally, the eye is a dynamic structure with connective tissue remodeling occurring as a result of aging and pathologies such as glaucoma and myopia. The material properties of these tissues and the distribution of stresses and strains in the posterior eye is an area of active research, relying on a combination of computational modeling, imaging, and biomechanical measurement approaches. Investigators are recognizing the increasing importance of the role of the collagen microstructure in these material properties and are undertaking microstructural measurements to drive microstructurally-informed models of ocular biomechanics. Here, we review notable findings and the consensus understanding on the biomechanics and microstructure of the posterior eye. Results from computational and numerical modeling studies and mechanical testing of ocular tissue are discussed. We conclude with some speculation as to future trends in this field.