Accurate Estimation of Intraocular Pressure and Corneal Material Behaviour Using a Non-Contact Method

Abstract

The present study is quantifying the effect of corneal parameters(including corneal geometry and material stiffness) with potential considerable influence on intraocular pressure (IOP) and corneal material estimation using finite element method to develop biomechanically-corrected IOP algorithm and biomechanically estimated material algorithm on the non-contact tonometry to estimate higher accurate IOP (with a reduced effect of CCT and age) compared to device’s IOP measurement and the in-vivo corneal material behaviour (with a reduced effect of IOP). The CorVis-ST (Oculus, Wetzlar, Germany) measures IOP using high-speed Scheimpflug technology, which can record the deformation of the cornea during the air pressure application and use this information to define the relationship between the true IOP and dynamic response parameters obtained from CorVis-ST. Hence, in this study the OCULUS CorVis-ST was used for the development of a precise method for estimation of intraocular pressure and corneal material behaviour. Numerical analysis using the finite element method (FEM) had been adapted to represent the operation of the IOP measurement by using the CorVis-ST. The analysis considered the important biomechanical parameters of the eye including IOP, central corneal thickness (CCT), corneal geometry (central radius of curvature, Rc; and anterior corneal asphericity, P), and corneal material behaviour. The numerical simulation results demonstrated higher association of IOP predictions with the first applanation pressure (AP1) rather than CCT and corneal material stiffness (related to age), and higher association of corneal material properties with the ratio between corneal displacement and AP1. The numerical simulation results for healthy and Keratoconic eyes were used as a base to develop algorithms for estimating the true IOP with a reduced effect of CCT and corneal material stiffness, and corneal material behaviour (stress-strain relationship) with a reduced effect of the true IOP. Biomechanically-corrected IOP (bIOP) algorithms for both healthy and keratoconic eyes were validated in clinical data (including healthy, KC, and refractive surgery data) with the aim of significantly reducing IOP dependence on CCT and corneal biomechanics and in experimental ex-vivo human eye tests to assess the accuracy of the bIOP algorithms. The results of experimental ex-vivo human eye tests showed that bIOP had a higher accuracy than the IOP measurement using the CorVis-ST and exhibited no significant correlation with CCT (p=0.756), whereas CVS-IOP was significantly correlated with CCT (p 0.05), In addition, no significant difference in bIOP was found between pre- and post-operative data (0.1±2.1 mmHg, p=0.80 for LASIK and 0.8±1.8 mm Hg, P=0.273 for SMILE), whereas there were significant decreases after surgeries in GAT-IOP (-3.2±3.4 mmHg and -3.2±2.1 mmHg, respectively; both p 0.05) in the values of IOP between healthy and KC patients, using the bIOP and bIOPkc algorithms, while there was a significant difference with CVS-IOP (p 0.05) and IOP (p>0.05) but was significantly correlated with age (p<0.01). The stiffness estimates and their variation with age were also significantly correlated (p<0.01) with stiffness estimates obtained in earlier studies on ex-vivo human tissue [1]. In addition, in KC eyes the β predications remain at approximately 80% of the normal cornea’s level. All developed algorithms for IOP and corneal material behaviour estimation demonstrated great success in significantly on providing close estimates of true IOP and corneal material behaviour and reducing the effect of corneal thickness and material stiffness on IOP measurement and the effect of IOP on the corneal material estimation.