Recent advancements of optical interferometry applied to medicine

Abstract

Nonetheless the large diffusion of laser excimer ablative procedures to correct vision problems by corneal surgery (PTK, PRK, LASIX) still there are problems to achieve the targeted correction (TC) in every treated eye. We have registered and published cases of reduced TC, attributing the problem to the fact that excimer laser devices are not flexible enough to adapt to the intrinsic characteristics of each cornea. The latest generation of excimer lasers enables personalizing the procedures combining laser beam-scanning with optical methods of acquiring in real time the corneal map using wave front analysis techniques. None the less we feel this is not a complete solution to the problem, since the elasticity of the corneal tissue should be also taken into account. Corneal tissue elasticity is related to the infra ocular pressure (IOP), age and other factors and shows aging effects also. Observing our records of more than 2500 cornea surgeries we believe that corneal tissue elasticity has an essential role in the achievement of TC. In order to demonstrate this assumption we developed some experiments aiming also to define mathematical model that can enable simulating the cornea behavior during and after excimer laser ablation surgery. We have firstly investigated the possibilities of mapping the cornea surface with a modified Twymann-Green interferometer (TGI). Experiments were carried on a bovine cornea at rest and under a static stress. Then we hive determined experimentally the resonant frequency of the same bovine cornea. Finally we have investigated the behavior of the same bovine cornea applying at force vibrating at resonance frequency. The corneal surface deformation was detected in real time using for the first time worldwide PulsESPI, a laser device that enables real time deformation analysis exploiting electronic speckle pattern interferometry (ESPI). This approach may enable defining a quite reliable simulation model of the cornea based on its geometry and its elastic factors. We suggest using finite element method (FEM) to analyze the corneal surfaces deformed statically and at resonance condition in order to determine elasticity coefficients to be adopted in a proposed preliminary model.