 Correction of irregular corneal astigmatism using topography-based flying-spot-mode excimer laser photoablationBerthold Seitz, M.D., E.B.O.D., Achim Langenbucher, Ph. D., Murat M. Kus, M.D., Michael Harrer, B.S.
Department of Ophthalmology, University of Erlangen-Nürnberg
(Chairman: Prof. Dr. G.O.H. Naumann)
Schwabachanlage 6, D-91054 Erlangen, Germany
AbstractPurpose: To introduce a novel approach for the correction of asymmetric or irregular corneal astigmatism. Methods: 1. Corneal topography analysis (TMS-1, Tomey, Tennenlohe, Germany) of the irregular PMMA surface providing 6400 samples. 2. Decomposition of topography height data into orthogonal Zernike polynomials of radial order n=16. 3. Definition of a regular spherical or spherocylindrical target surface by restriction of Zernike representation to parabolic (and zero order astigmatic) components. 4. Calculation of differential height data between original and target surface for each of 6400 samples. 5. Interpolation and transformation of position data from a polar into a Cartesian grid with data reduction. 6. Calculation of the ablation profile comparing original and target surface. 7. Calculation of sleeping (i.e. stationary) time at each respective grid position considering ablation rate of the tissue, energy profile of the circular laser spot (1.0mm), repetition rate of the laser (20/s) and laser spot overlap (80%). 8. Computer-based time-controlled tissue ablation using an 193nm excimer laser (modified MEL60, Aesculap-Meditec, Jena, Germany) in "flying spot mode". 9. Comparison of topography analysis of the ablated surface with the original surface and with the calculated target surface. Results: Before/after treatment, surface regularity index (SRI) was 1.19/0.39, surface asymmetry index (SAI) was 1.84/0.14 and difference between flattest and steepest hemimeridians in the 3mm zone was 8.7/1.1 diopters. Total time duration for the correction (8896 laser pulses, diameter 8.0mm) was 488s. Mean and maximum sleeping time at each of 408 positions was 1.09± 0.69s and 3.24s. Mean and maximum ablation depth at each position was 42± 27m m and 125m m. Root mean square error comparing target surface and ablated surface was 2.7m m. Conclusion: Tying corneal topography into the laser software offers a straight-forward concept for correction of irregular corneal astigmatism after Zernike decomposition of topography height data. For clinical implementation, however, uneven epithelial hyperplasia, which may play a major role in patients with irregular astigmatism, has to be compensated for. In addition, influences of epithelial and stromal wound healing on regression of the initial laser effect have to be considered for creation of the final algorithms. No sponsoring organization. No proprietary interests. Corresponding author and reprint requests: Dr. Berthold Seitz, Augenklinik mit Poliklinik der Universität Erlangen-Nürnberg, Schwabachanlage 6, D-91054 Erlangen, Germany. Phone: -49-9131-854477 Fax: -49-9131-854436 |
