The inflation response of the posterior bovine sclera

An in vitro inflation test method was developed to characterize the mechanical behavior of the bovine posterior sclera. The method used digital image correlation to provide a spatially resolved, full-field deformation map of the surface of the posterior sclera in response to controlled pressurization. A series of experiments were performed in the range of 2-6 kPa (15-45 mmHg) to characterize the load-unload displacement response at various pressure rates and the time-dependent displacement response at different applied pressures. The magnitude of the displacement was largest in the peripapillary region, mainly between the apex and the optic nerve head. Further, the results showed that bovine scleral tissue exhibited nonlinear and viscoelastic behavior characterized by a rate-dependent displacement response, hysteresis during unloading and creep. The creep rate was insensitive to the applied pressure, suggesting that the tissue can be modeled as a quasilinear viscoelastic material in the physiological pressure range of 2-6 kPa

Hydrostatic pressure does not cause detectable changes to survival of human retinal ganglion

Purpose: Elevated intraocular pressure (IOP) is a major risk factor for glaucoma. One consequence of raised IOP is that ocular tissues are subjected to increased hydrostatic pressure (HP). The effect of raised HP on stress pathway signaling and retinal ganglion cell (RGC) survival in the human retina was investigated. Methods: A chamber was designed to expose cells to increased HP (constant and fluctuating). Accurate pressure control (10-100mmHg) was achieved using mass flow controllers. Human organotypic retinal cultures (HORCs) from donor eyes (<24h post mortem) were cultured in serum-free DMEM/HamF12. Increased HP was compared to simulated ischemia (oxygen glucose deprivation, OGD). Cell death and apoptosis were measured by LDH and TUNEL assays, RGC marker expression by qRT-PCR (THY-1) and RGC number by immunohistochemistry (NeuN). Activated p38 and JNK were detected by Western blot. Results: Exposure of HORCs to constant (60mmHg) or fluctuating (10-100mmHg; 1 cycle/min) pressure for 24 or 48h caused no loss of structural integrity, LDH release, decrease in RGC marker expression (THY-1) or loss of RGCs compared with controls. In addition, there was no increase in TUNEL-positive NeuN-labelled cells at either time-point indicating no increase in apoptosis of RGCs. OGD increased apoptosis, reduced RGC marker expression and RGC number and caused elevated LDH release at 24h. p38 and JNK phosphorylation remained unchanged in HORCs exposed to fluctuating pressure (10-100mmHg; 1 cycle/min) for 15, 30, 60 and 90min durations, whereas OGD (3h) increased activation of p38 and JNK, remaining elevated for 90min post-OGD. Conclusions: Directly applied HP had no detectable impact on RGC survival and stress-signalling in HORCs. Simulated ischemia, however, activated stress pathways and caused RGC death. These results show that direct HP does not cause degeneration of RGCs in the ex vivo human retina

Quantitative mapping of collagen fiber orientation in non-glaucoma and glaucoma posterior human scleras

Purpose. The posterior sclera has a major biomechanical influence on the optic nerve head, and may therefore be important in glaucoma. Scleral material properties are influenced significantly by collagen fiber architecture. Here we quantitatively map fiber orientation in non-glaucoma and glaucoma posterior human sclerae. Methods. Wide-angle x-ray scattering quantified fiber orientation at 0.5-mm intervals across seven non-glaucoma post-mortem human sclerae, and five sclerae with glaucoma history and confirmed axon loss. Multiphoton microscopy provided semiquantitative depth-profiling in the peripapillary sclera. Results. Midposterior fiber orientation was either uniaxial (one preferred direction) or biaxial (two directions). The peripapillary sclera was characterized by a ring of fibers located mainly in the mid-/outer stromal depth and encompassing ∼50% of the total tissue thickness. Fiber anisotropy was 37% higher in the peripapillary sclera compared with midposterior, varied up to 4-fold with position around the scleral canal, and was consistently lowest in the superior-nasal quadrant. Mean fiber anisotropy was significantly lower in the superior-temporal (P < 0.01) and inferior-nasal (P < 0.05) peripapillary scleral quadrants in glaucoma compared with non-glaucoma eyes. Conclusions. The collagen fiber architecture of the posterior human sclera is highly anisotropic and inhomogeneous. Regional differences in peripapillary fiber anisotropy between non-glaucoma and glaucoma eyes may represent adaptive changes in response to elevated IOP and/or glaucoma, or baseline structural properties that associate with predisposition to glaucomatous axon damage. Quantitative fiber orientation data will benefit numerical eye models aimed at predicting the sclera's influence on nerve head biomechanics, and thereby its possible role in glaucoma

Effects of peripapillary scleral stiffening on the deformation of the lamina cribrosa

Purpose: Scleral stiffening has been proposed as a treatment for glaucoma to protect the lamina cribrosa (LC) from excessive intraocular pressure–induced deformation. Here we experimentally evaluated the effects of moderate stiffening of the peripapillary sclera on the deformation of the LC. Methods: An annular sponge, saturated with 1.25% glutaraldehyde, was applied to the external surface of the peripapillary sclera for 5 minutes to stiffen the sclera. Tissue deformation was quantified in two groups of porcine eyes, using digital image correlation (DIC) or computed tomography imaging and digital volume correlation (DVC). In group A (n = 14), eyes were subjected to inflation testing before and after scleral stiffening. Digital image correlation was used to measure scleral deformation and quantify the magnitude of scleral stiffening. In group B (n = 5), the optic nerve head region was imaged using synchrotron radiation phase-contrast microcomputed tomography (PC μCT) at an isotropic spatial resolution of 3.2 μm. Digital volume correlation was used to compute the full-field three-dimensional deformation within the LC and evaluate the effects of peripapillary scleral cross-linking on LC biomechanics. Results: On average, scleral treatment with glutaraldehyde caused a 34 ± 14% stiffening of the peripapillary sclera measured at 17 mm Hg and a 47 ± 12% decrease in the maximum tensile strain in the LC measured at 15 mm Hg. The reduction in LC strains was not due to cross-linking of the LC. Conclusions: Peripapillary scleral stiffening is effective at reducing the magnitude of biomechanical strains within the LC. Its potential and future utilization in glaucoma axonal neuroprotection requires further investigation

Effects of the scleral collagen structure on the biomechanical response of the optic nerve head

The sclera is a fiber-reinforced material composed of dense superimposed lamellae of type I collagen fibrils embedded in a matrix of elastin and proteoglycan. Recent Wide-Angle X-ray Scattering (WAXS) experiments (Meek, 2009) showed that the collagen lamellae are strongly aligned circumferentially in the region closest to the optic nerve head (ONH). The collagen structure was more disperse and heterogeneous away from the peripapillary region. The collagen structure of the sclera directly influences its material stiffness properties and therefore the level of strain transmitted to the tissues of the ONH, which is the primary site of damage in glaucoma. The effects of the fiber structure on the ONH biomechanics have been studied on the monkey eye (Girard, 2009), but not on the human eye. Recent work evaluating the influence of the human sclera on ONH biomechanics approximated the scleral behavior as linear elastic (Sigal, 2009) or hyperelastic orthotropic (Eilaghi, 2009)

Modeling the effect of the experimentally-derived collagen structure on the mechanical anisotropy of the human sclera

The sclera is the main load-bearing structure of the eye. It must be sufficiently stiff to maintain the shape and dimensions of the eye under acute elevation of intraocular pressure (IOP). These properties stem from the fiber-reinforced structure of the sclera, which contains dense superimposed lamellae of type I collagen fibrils embedded in matrix of proteoglycans and elastin. Recently, wide-angle X-ray diffraction [1] (WAXS) was used to map the fibrillar arrangement and distribution of collagen over posterior human sclera [2]. The results showed that the peripapillary region, immediately adjacent to the optic nerve head (ONH) had a larger amount of collagen and a circumferential collagen structure. The collagen structure in the mid-posterior region was more heterogeneous. The collagen structure of the sclera directly influences its material stiffness properties and therefore the level of strain transmitted to the tissues of the optic nerve head, which is the primary site of damage in glaucoma. Models inspired from the microstructure are needed to evaluate the contribution of the collagen structure on the mechanical properties. Earlier modeling efforts have treated the sclera as a homogenous, isotropic, linear elastic [3] or hyperelastic material [4, 5]. Girard et al. recently added the effect of the collagen structure using a nonlinear anisotropic model [6]. The authors fit their model for the collagen orientation and distribution to mechanical inflation data of the posterior sclera