Abstract

P 171

Determination of the refractive power by analysing Zernike polynomials based on topological measurements and wave front simulations

Guth H.¹, Hellmann A.¹, Scherer K. P.¹, Sieber I.¹, Stiller P.¹, Wackernagel W.^2 
1Institute for Applied Computer Science, Forschungszentrum Karlsruhe GmbH, ²Department of Ophthalmology, Medical University Graz

Objective: In order to implant the optimal IOL into the patient’s eye in the course of a cataract surgery, the correct refraction power for the IOL has to be determined. To do this, the curvature of the front surface of the cornea is usually regarded. However, if a refractive surgery at the cornea (PRK, LASIK, etc.) has been performed, the curvature conditions of the anterior and posterior cornea surfaces are changed. Therefore the computation of the overall refractive power has to be updated, respecting the intervention based changement of corneal curvature.
Methods: For the geometry analysis of both cornea surfaces, on the one hand spheres are fitted to the topological measured data; on the other hand the coefficients of Zernike polynomials are adapted to these measured data. As a further method, the topologies of both cornea surfaces are used to perform a wave front simulation, which is analysed using Zernike polynomials.
Results: With the estimated curvatures of the two cornea surfaces and the well-known refractive indices, the overall refraction power could be determined. Additionally, on the basis of geometry analyses of the cornea surfaces using Zernike polynomials, the single refraction powers have been determined and so the overall power could be computed. Using the adapted Zernike coefficients of the wave front simulation, the overall power of the cornea could be derived immediately. The methods specified above were applied both to computed "ideal" cornea forms and to measured cornea topologies of real patient eyes.
Conclusions: The presented methods are suitable to determine the overall refraction power of the cornea. Based on Zernike coefficients, potentially existing cornea astigmatism can be detected, which could be respected by selecting a special IOL. Regarding wave front analysis, even higher order aberrations could be taken into account.

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