Sensitivity of applanation tonometry readings to the geometrical parameters of the sclera and the shape of the cornea

Abstract

Abstract Purpose To study the effect of the geometrical parameters of a cornea (the thickness and difference of corneal shape from spherical segment) on applanation tonometry readings. Methods The mathematical model for Goldmann and Maklakov tonometers is developed. The corneoscleral shell is modeled as two conjugated transversal isotropic elliptic (with non‐zero eccentricity) shells (cornea and sclera) with different mechanical properties. The baseline (prior to loading) two‐segment shell is assumed to be filled with incompressible liquid under pressure. Simulated IOP measurements are obtained for Goldman and 5‐g and 10‐g Maklakov tonometers. To analyze the effect of the eyeball geometry on IOP readings the elastic constants of cornea and sclera are varied and geometrical parameters (such as central corneal thickness, cornea and sclera radii of curvature, scleral thickness, ratio of anterior‐posterior eye axis lengths vs. the equator diameter) are varied. Simulated calculations are compared with experimental data. Results The results are obtained over a wide range of parameters of the sclera and cornea. Conclusion If the shape of a cornea differs from spherical it could cause the measurement error of tonometry IOP readings ranged between 3% and 25%. Significant sensitivity of the Goldmann tonometer to corneal thickness compared to Maklakov's method may be explained with smaller contact zone and, hence, the larger influence of the flexural (bending) deformation, which depends on the thickness of a shell. Contrariwise the change of the radius of curvature of cornea affects greatly on the results obtained with Maklakov tonometer.