Abstract
Purpose: :
To determine whether the cornea loses its stability during the applanation tonometry (AT) and, if it does, to analyze whether such collapse can result in the errors/misreading of the AT measurements.
Methods: :
Mathematical model of the AT methods was developed. Simulated corneoscleral shell was considered as two joined transversal isotropic shells with different mechanical properties. The cornea was approximated by an ellipsoid segment, whereas the sclera was assumed to be a part of spherical shell. Young's module of the cornea is significantly lower than that of the sclera. Radial modulus for each simulated eyeball tunic (cornea or sclera) are assumed to be significantly less than moduli in meridional or circumferential directions. The calculations were performed for a wide range of parameters including central corneal thickness (CCT), axial length (AL), radii of curvature, and elastic parameters of both the sclera and the cornea. The ratio of the polar and equatorial lengths of corneal semiaxes (K) was also varied to examine the effect of out-of-sphericity of the cornea shape on AT readings.
Results: :
The predicted Maklakoff intraocular pressure (IOP) readings, when IOP is measured by the area of the cornea flattened by a prescribed force, for corneas with different profile may alter by 8 mm Hg. Out-of-sphericity of the cornea shape could cause a measurement error of Goldmann AT (GAT) readings ranged between 3% and 25%. The change of CCT appeared to have greater effect on GAT readings than the change of the cornea and sclera radii of curvature. The cornea detachment from the tonometer during 10-g Maklakoff AT was revealed.
Conclusions: :
The developed mathematical model provides one of possible biomechanical explanation for the occurrence of false IOP readings during AT measurements. Corneas with flatted profile (K<1) and/or IOP lower than 15 mm Hg can cause corneal collapse during the AT procedure (detachment of cornea surface from the tonometer). We believe, that these observations could be helpful as general guidelines for all AT procedures, and, especially, for those after myopia refractive surgeries.
Keywords: intraocular pressure • computational modeling • stress response