J. C. Downs

Scleral Biomechanics in the Aging Monkey Eye

PURPOSE To investigate the age-related differences in the inhomogeneous, anisotropic, nonlinear biomechanical properties of posterior sclera from old (22.9 +/- 5.3 years) and young (1.5 +/- 0.7 years) rhesus monkeys. METHODS The posterior scleral shell of each eye was mounted on a custom-built pressurization apparatus, then intraocular pressure (IOP) was elevated from 5 to 45 mm Hg while the 3D displacements of the scleral surface were measured with speckle interferometry. Each scleral shells geometry was digitally reconstructed from data generated by a 3-D digitizer (topography) and 20-MHz ultrasound (thickness). An inverse finite element (FE) method incorporating a fiber-reinforced constitutive model was used to extract a unique set of biomechanical properties for each eye. Displacements, thickness, stress, strain, tangent modulus, structural stiffness, and preferred collagen fiber orientation were mapped for each posterior sclera. RESULTS The model yielded 3-D deformations of posterior sclera that matched well with those observed experimentally. The posterior sclera exhibited inhomogeneous, anisotropic, nonlinear mechanical behavior. The sclera was significantly thinner (P = 0.038) and tangent modulus and structural stiffness were significantly higher in old monkeys (P < 0.0001). On average, scleral collagen fibers were circumferentially oriented around the optic nerve head (ONH). No difference was found in the preferred collagen fiber orientation and fiber concentration factor between age groups. CONCLUSIONS Posterior sclera of old monkeys is significantly stiffer than that of young monkeys and is therefore subject to higher stresses but lower strains at all levels of IOP. Age-related stiffening of the sclera may significantly influence ONH biomechanics and potentially contribute to age-related susceptibility to glaucomatous vision loss.

Comparison of clinical and three-dimensional histomorphometric optic disc margin anatomy.

PURPOSE To investigate the anatomic basis of the optic disc margin in the normal monkey eye by colocalizing optic disc photographs to three-dimensional (3D) histomorphometric reconstructions of the same optic nerve head. METHODS Optic disc photographs from 28 normal monkey eyes were overlaid onto 3D central retinal vessel reconstructions generated as part of postmortem optic nerve histomorphometric reconstructions for each eye. Within each reconstruction, the Bruchs membrane opening (BMO) was delineated. Alignment was achieved by matching the clinical vessel outline to the vessel reconstruction with parallel viewing software. An experienced observer viewed stereophotographs and marked the disc margin onto clinical photographs with custom software. Alignment of the delineated disc margin to the histomorphometrically defined BMO was qualitatively assessed within each image. RESULTS In 20 eyes, BMO aligned well to the disc margin delineation. In four eyes, alignment improved after repeated colocalization. Careful review of the histomorphometric reconstructions identified that in most cases Bruchs membrane extended beyond the termination of the border tissue of Elschnig, most substantially in the superior and nasal sectors. Misalignments could be explained by inaccurate BMO marking or where Bruchs membrane terminated externally to the inferior edge of the border tissue; this latter structure aligned to the disc margin. CONCLUSIONS BMO was a clinically detectable entity and represented the disc margin in most eyes in this study. The 3D architecture of the border tissue combined with the presence of an overhang of Bruchs membrane makes an important contribution to disc margin anatomy.

The effect of acute intraocular pressure elevation on peripapillary retinal thickness, retinal nerve fiber layer thickness, and retardance.

PURPOSE To determine whether acutely elevated intraocular pressure (IOP) alters peripapillary retinal thickness, retinal nerve fiber layer thickness (RNFLT), or retardance. METHODS Nine adult nonhuman primates were studied while under isoflurane anesthesia. Retinal and RNFLTs were measured by spectral domain optical coherence tomography 30 minutes after IOP was set to 10 mm Hg and 60 minutes after IOP was set to 45 mm Hg. RNFL retardance was measured by scanning laser polarimetry in 10-minute intervals for 30 minutes while IOP was 10 mm Hg, then for 60 minutes while IOP was 45 mm Hg, then for another 30 minutes after IOP was returned to 10 mm Hg. RESULTS RNFLT measured 1120 microm from the ONH center decreased from 118.1 +/- 9.3 microm at an IOP of 10 mm Hg to 116.5 +/- 8.4 microm at 45 mm Hg, or by 1.4% +/- 1.8% (P < 0.0001). There was a significant interaction between IOP and eccentricity (P = 0.0006). Within 800 microm of the ONH center, the RNFL was 4.9% +/- 3.4% thinner 60 minutes after IOP elevation to 45 mm Hg (P < 0.001), but unchanged for larger eccentricities. The same pattern was observed for retinal thickness, with 1.1% +/- 0.8% thinning overall at 45 mm Hg (P < 0.0001), and a significant effect of eccentricity (P < 0.0001) whereby the retina was 4.8% +/- 1.2% thinner (P < 0.001) within 800 microm, but unchanged beyond that. Retardance increased by a maximum of 2.2% +/- 1.1% 60 minutes after IOP was increased to 45 mm Hg (P = 0.0031). CONCLUSIONS The effects of acute IOP elevation on retinal thickness, RNFL thickness and retardance were minor, limited to the immediate ONH surround and unlikely to have meaningful clinical impact.

Effects of strain rate on the mechanical properties of posterior rabbit sclera

We investigate the strain rate dependence of posterior rabbit sclera. The material properties of the sclera can serve as a direct estimate of the properties of the lamina cribrosa and other ocular load bearing tissues that cannot be tested directly. Knowledge of the behavior of these tissues is necessary to develop computational models of the posterior pole to investigate the role of increased intraocular pressure (IOP) in glaucoma. We tested posterior rabbit sclera in tension to determine the effects of two different strain rates (0.1%/sec and 10%/sec) on its material properties. Modulus (1-5% strain) showed a significant increase (256%) from 8.22 MPa at 0.1%/sec to 29.3 MPa at 10%/sec. Strain energy density (0-10% strain) also showed a significant increase (151%) from 0.0621 MPa at 0.1%/sec to 0.156 MPa at 10%/sec. The material properties of posterior rabbit sclera exhibit high strain rate sensitivity (a viscoelastic trait) and slight nonlinearity, but a strong nonlinear response was not observed.