Determination of the True Intraocular Pressure and Modulus of Elasticity of the Human Cornea in vivo

Abstract

The purpose of this study was to determine the true intraocular pressure and modulus of elasticity of the human cornea in vivo. The cornea was modeled as a shell, and the equations for the deformations of a shell due to applanating and intraocular pressures were combined to model the behavior of the cornea during applanation tonometry. At certain corneal dimensions called the calibration dimensions, the applanating and intraocular pressures are considered to be equal. This relationship was used to determine the modulus of elasticity of the cornea and the relationship between the applanating and intraocular pressures. The true intraocular pressure (IOPT) was found to be related to Goldmann's applanating pressure (IOPG) as IOPT = IOPG/K, where K is a correction factor. For the calibration corneal thickness of 0.52 mm, the modulus of elasticity E in MPa of the human cornea was found to be related to the true intraocular pressure IOPT in mmHg as E = 0.02291OPT. The generalization of the Imbert-Fick law that takes into account the effect of corneal dimensions and stiffness was found to be given by IOPT = 73.5W/(K A), where W is the applanating weight in gf (gram force) and A is the applanated area in mm2. The calculated true intraocular pressure and modulus of elasticity were found to agree with published experimental results. The mathematical model developed may therefore be used to improve results from applanation tonometry and to estimate the mechanical property of the cornea in vivo.

Varying Effects of Corneal Thickness on Intraocular Pressure Measurements with Different Tonometers

“…They found each millimetre of difference in corneal radius from an average of 7.85 mm caused 0.8 mmHg of error in GAT measurements. Orssengo and Pye 25 developed a theoretical mathematical model of the cornea, and proposed a correction factor between GAT measurement and true IOP according to corneal dimensions including central thickness and anterior radius of curvature. If applying Orssengo's and Shimmyo's formulae (Appendix) to calculate the corrected GAT values in our study cohort, 24,25 a 10 mm CCT change would result in a 0.59 mmHg (r ¼ 0.728) and a 0.78 mmHg (r ¼ 0.825) deviation in NCT measurements respectively, and a 0.45 mmHg (r ¼ 0.642) and a 0.65 mmHg (r ¼ 0.757) deviation in OBFT measurements respectively.…”

“…The corrected GAT value, representing the true intraocular hydrostatic pressure, was calculated according to the following formulae (see Table A1) derived from the studies of Ehlers et al, 5 Whitacre et al, 6 Doughty et al, 9 Shimmyo et al, 24 and Orssengo et al 25 Table A1 Corrected GAT Value…”

Application of structural analysis to the mechanical behaviour of the cornea

“…It is evident that a support orientation of 40 • provides the best match with the whole-eye model with an average difference of 0.7%. This approach is similar to that adopted by Orssengo & Pye (1999).…”

“…The cornea model has 640 shell elements and is provided with roller supports (with 40 • inclination) along all edge nodes. The modulus of elasticity and Poisson's ratio are taken as 0.0229 × IOP and 0.5, respectively, according to Orssengo & Pye (1999). The tonometer is modelled using 40 five-sided solid elements with two triangular and three rectangular faces as shown in figure 19.…”

Tonometry: an overview

“…As the forces of corneal resistance are transferred to the sleeve, the applanation pressures concord with the IOP [3]. As the area of applanation of tonopen is smaller than GAT (2.36mm 2 Vs 7.35mm 2 ) therefore, theoretically the difference between applanating pressure and IOP is reduced due to reduced corneal resistance of a smaller contact area [4]. Procedure: The tonopen probe tip should be covered with a new ocu-film tip cover for each patient.…”