The effect of changing intraocular pressure on the corneal and scleral curvatures in the fresh porcine eye

Abstract

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...

Noninvasive Measurement of Scleral Stiffness and Tangent Modulus in Porcine Eyes

“…20 This suggested that the sclera is stiffer than the cornea, providing key structural support to the eye. The scleral tangent moduli of porcine eyes measured in this study are comparable to the results reported by Pierscoionek et al 26 using the inflation test (0.2-0.5 MPa, the testing IOP ranged from 15-50 mm Hg). However, they are significantly less than those measured with the tensile test reported by Wollensak and Spoerl 25 (5.95 MPa at 8% strain).…”

Corneal Biomechanical Properties from Two-Dimensional Corneal Flap Extensiometry: Application to UV-Riboflavin Cross-Linking

“…The strain upon the applied stress is measured from the lateral elongation, axial apex displacement, 4 shift of mercury droplets attached on the corneal surface, 5,7 or from changes in the corneal radius of curvature. 10,11 A new technique to estimate the corneal biomechanical properties, by measuring the corneal deformation upon air puff applanation, has recently been suggested. 12,13 In all in vitro biomechanical measurements, corneal hydration plays a role as it affects the tissue's mechanical response.…”

“…The most widespread applied method is strip extensiometry, [1][2][3][4] followed by corneal button inflation, 5,6 and whole globe inflation. [4][5][6][7][8][9][10] In these techniques, a load is applied (typically along one axis in strip extensiometry, or radially by increasing IOP in inflation techniques). The strain upon the applied stress is measured from the lateral elongation, axial apex displacement, 4 shift of mercury droplets attached on the corneal surface, 5,7 or from changes in the corneal radius of curvature.…”

Scleral birefringence as measured by polarization-sensitive optical coherence tomography and ocular biometric parameters of human eyes in vivo

“…The short-term alteration of IOP did not significantly affect the scleral birefringence in the case of porcine eyes ex vivo. Although it has been known that human and porcine eyes have similar properties of the sclera in some aspects [35][36][37][38], additional studies would be required to show whether the outcome of porcine eyes is interchangeable to human eyes. If it was exchangeable, our result would suggest that the short-term alteration of IOP does not explain the significant negative relationship between the IOP and scleral birefringence of human eyes in vivo.…”