Aim: To measure corneal and scleral radii of curvature in response to intraocular pressure (IOP). Methods: Using digital photographic profile images of 16 fresh porcine eyes, the curvatures of the cornea and sclera were determined in response to five consecutive incremental 100 ml saline intravitreal injections. IOP was measured and ocular rigidity calculated. Elastic moduli of the cornea and sclera were estimated. Results: Intraocular pressure and the radius of curvature of the sclera increased linearly with increasing volume. There was no statistical change in corneal curvature. The elasticity of the cornea and sclera was constant during the 15-50 mm Hg increase in IOP. The estimated range of the elastic moduli of the cornea and sclera were, respectively 0.07-0.29 MPa and 0.2 MPa to 0.5 MPa. The scleral rigidity ranged from 0.0017 to 0.0022. Conclusions: The elastic moduli of the cornea and sclera are independent of IOP. The modulus of elasticity of the sclera is higher than that of the cornea. Elevation of IOP changes the curvature of the sclera but not that of the cornea. Porcine scleral rigidity is similar to human scleral rigidity. Scleral curvature could be a novel method for measuring IOP. F riedenwald 1 was the first to define scleral rigidity in terms of ocular volume and intraocular pressure (IOP). He realised that the sclera was important in understanding the effects of IOP on the optic nerve, and that the manifestations of glaucoma and the severity of its sequelae may be dependent on scleral rigidity. Recently, a non-linear finite element sensitivity study demonstrated that the stiffness of the sclera was the most important factor for determining the vulnerability of the optic nerve head to increasing IOP.2 This study demonstrated that even large strains of neural tissue had less effect on the optic nerve head than the indirect effects of IOP on the sclera. The effect of increasing IOP on scleral deformation has not been quantified. As the eyeball is a biological and a curved structure, a sensitive method is required to measure the small deformational changes induced by fluctuations in IOP. This study investigates the change in radii of curvatures of the fresh intact porcine cornea and sclera with IOP. Ocular rigidity is calculated and the ranges of elastic moduli of the cornea and sclera are estimated. METHODSSixteen porcine eyes were obtained from the local abattoir; eyes were collected was within 4 h of death and then transported on ice. All eyes were from animals aged 5.5 months. Experiments were completed between 3-6 h postmortem. During the preparation and experiment, samples were kept moist with saline solution. The extraocular muscles and extraneous fat were carefully removed from each eyeball and each specimen weighed on a digital balance before and after experimentation.At 20˚C, the eyeballs were placed, with their optic axes horizontal, on a specially designed base made of hollow, clear perspex tubing with gradation along the edge, to enable them to be maintained in a secure position duri...

The elasticity of the porcine cornea and sclera could be approximated to constant over the physiological range, but showed a tendency to non-linearity. The modulus of elasticity for the sclera is 3 to 3.5 times that of the cornea. The cornea is more resistant than the sclera to IOP changes within the physiological range.

Objective: To investigate the fluctuations in intraocular pressure during the day and to see if these are associated with changes in corneal shape and in the patterns of ocular aberrations. Methods: Intraocular pressure, corneal curvature, refractive error, spherical equivalent and aberrations (defocus (sphere); cylinder (astigmatism); coma, trefoil and third order spherical aberration) were measured in 17 healthy subjects three times during the day. The first measurement was made between 9:00 and 9:30, the second at midday (12:30-13:00) and the third in the afternoon (17:00-17:30). Aberrations, corneal shape, refractive error and pupil size (for which correction was made) were measured with an Irx3 Dynamic Wavefront Aberrometer. Intraocular pressures were measured using a non-contact tonometer (Cambridge Instruments Inc.) and calibrated with the Goldmann applanation tonometer. Results: Variations in intraocular pressures were unrelated to age or refractive error. Statistically significant differences in intraocular pressure between morning and midday as well as between midday and afternoon were found. Intraocular pressure variations between midday and afternoon were associated with changes in spherical equivalent, corneal radius of curvature and aberrations (defocus, cylinder, coma, trefoil and spherical aberration) over the same time period. Aberration patterns varied between individuals, and no association was found between two eyes of the same subject. Conclusions: Changes in intraocular pressure have no noticeable effect on image quality. This could be because the eye has a compensating mechanism to correct for any effect of ocular dynamics on corneal shape and refractive status. Such a mechanism may also affect the pattern of aberrations or it may be that aberrations alter in a way that offsets any potentially detrimental effects of intraocular pressure change on the retinal image. Variations in patterns of aberrations and how they may be related to ocular dynamics need to be investigated further before attempts at correction are made.

Purpose The elastic moduli of the cornea, sclera, and limbus for different corneal eccentricities (e) and varying levels of intraocular pressure (IOP) were modelled in order to determine how the rheological properties, especially those of the limbus, need to alter to maintain optical image quality when the eye is subjected to small variations in IOP. Methods Finite element analysis (FEA) was used to construct eyeball models with four different corneal eccentricities (e ¼ 0, 0.33, 0.5, 0.65). Three values for Young's modulus of the cornea were tested in all models (0.2 megapascal (MPa), 1.2 and 10 MPa). For each corneal modulus, scleral moduli of 3, 4, 5, 7, and 10 times that of the corneal modulus were selected. The limbal modulus was varied to optimise image quality of the eye model subjected to IOP variations of 70.8 mmHg for three different levels of IOP (8,16, and 32 mmHg). Results The elastic modulus of the limbal ring increases with an increase in corneal modulus and rises to a peak when the ratio of scleral to corneal moduli is between 5 and 7 depending on corneal eccentricity. Different levels of IOP produce only slight differences in the relative moduli required to maintain optical image quality. Conclusions The significance of a peak in the value of Young's modulus of the limbus is not clear but suggests that there may be an optimal limbal modulus that must be balanced with the moduli of cornea and sclera for preservation of image quality.