ZERNIKE DECOMPOSITION OF CORNEAL TOPOGRAPHY DATA FOR CORRECTION OF CORNEAL SURFACE IRREGULARITIES WITH THE EXCIMER-LASER

Background A retinal image performance distorted by an asymmetric or i rregular corneal surface cannot be compensated for with spherocylindri c glasses completely. The best corrected visual acuity is markedly dec reased and contact lens fitting often impossible. The purpose of this study was to calculate the differential height between corneal topogra phy raw data and any regular surface with mathematical methods in orde r to to ablate the differential height with a computer-controlled lase r beam, thereafter. Methods A Zernike decomposition of radial degree n =16 was realized within a clinically relevant central corneal area of 8 mm in diameter based on corneal topography raw height data of a comm ercially available topographer (TMS-1, Tomey, Erlangen). Any target su rface could be defined by varying weighting of the Zernike coefficient s. The calculated differential height ablation between the raw data an d the target surface given in a polar grid was transformed to a Cartes ian grid to evaluate the sleeping time at each grid position consideri ng the characteristic ablation curve for the intended ablation of the height difference. Subsequently, differential height ablation was simu lated using an automated laser beam control for a modified excimer las er (MEL60, Aesculap-Meditec, Jena). We developed software tools for Ze rnike decomposition of corneal topography raw height data and time-reg ulated automatic laser beam control of the grid positions in the highe r programming language C (Borland C++ 3.1, Borland Inc., Munchen). Res ults Definition of a target surface can be realized alternatively by s electing a set of Zernike coefficients or defining a spherical or sphe rocylindrical surface by superposition of parabolic terms in a fixed p roportion creating a best-fit target surface to the raw data. In orgin ally ,,relatively flat'' areas, the differential height profile indica tes a ,,relatively deep'' ablation resulting in relative steepening to wards the periphery of the ablation zone. The resolution of the mechan ical unit of the laser beam control consisting of two linear stepping motors is 9 mu m in the focal plane with a reproducibility of 5 mu m T he software unit is guiding the laser beam in a meandering fashion wit hin the ablation area considering the calculated sleeping time for eac h grid position. Mean overlay, of the 1 mm laser spots is 70%. The las er beam diameter of 1 mm effects a peripheral transition zone of 0.5 m m. Conclusions Zernike decomposition of corneal topography height data is an efficient tool for localizing and quantifying superficial irreg ularities and for directly calculating an ablation profile from create d differential height data. With an automatical laser beam control a w ell-defined laser ablation of superficial corneal irregularities is po ssible, subsequently.