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Svetlana M. Bauer, Liudmila A. Karamshina, Andrei B. Kachanov, Eva B. Voronkova; Biomechanical Modeling of the Applanation Tonometry after Refractive Surgery. Invest. Ophthalmol. Vis. Sci. 2012;53(14):6803.
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To study the effect of corneal multilayer structure on the intraocular pressure (IOP) measurements obtained using Goldmann’s and Maklakoff’s applanation tonometeres (GAT & MAT) before and after refractive surgery.To analyze how the appearance of an additional layer, which is observed after LASIK surgery, may influence corneal stiffness and IOP measurements.
Mathematical models of the GAT and MAT methods were developed. The corneo-scleral eye shell was modeled as conjugated transversally isotropic spherical shells with different material and geometric characteristics.The corneal thickness was divided into four layers with different elastic properties. For each corneal laeyr the modulus of elasticity in the radial direction was assumed to be 20 times smaller than one's in the meridional/circumferentional directions. For the simulated sclera radial-to-merional moduli ratio was assumed equal to 100. Nonlinear analyses were carried out using the commercial software ANSYS.The influence of elastic properties of corneal layers on the diameter of the contact area tonometer-cornea (MAT) and on the applied load (GAT) was analyzed. In order to estimate the effect of multilayer structure of the cornea on IOP changes during measurements only elastic parameters of corneal layers were varied. Axisymmetric nonlinear calculations for various values of elastic moduli of the cornea under constant real IOP value were carried out. For each calculation run the averaging of elastic moduli of the cornea layers was fulfilled and the cornea was treated as a single-layer structure with an average elastic modulus. The results obtained with the multilayer corneal model for MAT (with the weight of 10 g) and GAT before and after refractive surgery were compared with the results for a single-layer corneal model with an averaged elastic modulus .
In case of the multilayer corneal model larger contact area of the plummet and the cornea during tonometry and lower tonometric pressure values were obtained. The applied load in the Goldmann’s tonometry was lower for the multilayer corneal model compared to the one-layer model.
Lower IOP values and more precise estimation of the tonometric pressure can be achieved with the multilayer corneal model. After LASIK surgery the additional layer appeared reduces corneal stiffness and, hence, the IOP-values measured using both Goldmann’s and Maklakoff’s tonometers.
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