Obtaining correct information about the mechanical state of the eyeball based on the ocular response analyzer (ORA) data

In the Ocular Response Analyzer (ORA), used to measure intraocular pressure and eval-uate the viscoelastic properties of the cornea, the cornea is subjected to a rapid non-stationary action of a narrow, axisymmetric jet of compressed air. The information generated by the ORA is based on a statistical comparison of the obtained data with the results of intraocular pressure measurements taken with a Goldmann applanation tonometer, without analyzing the physical processes occurring within the device. For this analysis, we developed a mathematical model in which the cornea is considered a viscoelastic Voigt solid and the scleral region a zero-di-mensional object elastically responding to changes in intraocular pressure. The present paper is devoted to the further development of this approach, focusing on taking into account the specific operating characteristics of the device, when estimating the actual mechanical prop-erties of the eyeball using the parameters provided by the ORA. A relationship is established between the plenum pressure used for instrument calibration and the pressure acting on the cornea. It is shown that for an eye with normal elastic characteristics, the ORA yields an intra-ocular pressure value that matches the Goldmann tonometer reading, provided the corneal viscoelasticity parameter (strain relaxation time) is not too large. For larger values of this pa-rameter, an algorithm is proposed for a more refined calculation of intraocular pressure. Cal-culations show that the corrections introduced by taking into account the influence of individual values of the mechanical characteristics of the eyeball on intraocular pressure, although they lead to some refinement, are comparable to the errors in the measurement process itself, which gives only a rough estimate of this value. What makes the device indispensable is the ability to determine the corneal viscoelasticity parameter, for which two algorithms are proposed. This parameter depends only on the mechanical properties of the cornea, which distinguishes it from the corneal hysteresis directly reported by the ORA and traditionally used to estimate its viscoelastic properties: the latter also depends on pressure. It can be expected that the visco-elasticity parameter, having a clear physical meaning, will also prove effective for diagnostics.