ИСТИНА

Tonometry includes the mechanical loading of the eyeball by loading devices of different forms and weights and the measuring of the resulting geometrical changes. The aim of the investigation is to obtain efficient information about the mechanical characteristics and mechanical state of the individual human eye from these data. To solve the above-mentioned inverse problem, a simple model of the eyeball loaded by an external device, which represents the eye shell as the composition of an isotropic, linearly elastic homogenous two-dimensional soft shell (cornea) and an elastic element (sclera), is developed. The boundary-value problem for the system of four nonlinear differential equations was solved numerically. The solution turned out that for certain shapes of the loading device several solutions may exist, one of which only being physically significant. By varying the central tension, among possible shapes of the loaded cornea, the shape for which the distance between the cornea and sclera is minimal was chosen. Using the solution obtained, it is possible to simulate the tonometry test for various tonometer shapes and elastic moduli present in the model and get some useful consequences for clinical testing. The calculations showed that for loading the cornea by devices of the so-called impression type (convex stamp or thin rod) certain important characteristics depend on the elastic modulus of the sclera alone, whereas for loading by the applanation tonometer (flat stamp) the elastic modulus of the cornea is also important. The approach developed enables us to find the intraocular pressure and the two elastic constants basing on standard measurements by tonometers of two different types: applanation and impression tonometers. Separately from the individual elastic properties of the eye (at least scleral stiffness) the true intraocular pressure cannot be estimated even approximately. Moreover, two elastic parameters are additional characteristics of the eye that can be used in diagnostics. The method proposed is being tested in clinics.