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A. Alarcon, R. G. Anera, L. Jimenez del Barco, J. J. Castro; Use of Canonical Representation of the Corneal Surface after LASIK. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5092.
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We have used an intrinsic corneal system of coordinates to calculate the characteristic corneal parameters (radii of curvature and conic constants) and compared them to parameters derived directly from the topographies. We have assessed the utility of using these intrinsic corneal parameters.
We have worked with 90 eyes operated on LASIK (lasik in situ keratomileusis) to correct myopia and astigmatism. From elevation maps of the corneal surface (Orbscan 3.12, Bausch&Lomb, Inc.), the main corneal parameters for the intrinsic corneal system of coordinates were calculated. These parameters were compared to parameters provided by VOLPro 6.89 software (Sarver&Associates, Inc.). Also, we have compared VOLPro parameters to parameters provided by Orbscan corneal topographer.
With corneal parameters referred to the intrinsic corneal system, we have achieved a significant reduction of the standard error for pre- and post-surgical asphericity values (0.32 for intrinsic values and 1.56 for VOLPro) and a smaller difference between the two main meridians (0.05±0.04 between the two meridians for intrinsic asphericity and 0.90±1.21 for VOLPro asphericity). We have also observed that changes for intrinsic curvature radius due to surgery (1.16±0.51mm ) are larger than changes in VOLPro curvature radius (0.83±0.49mm). On the other hand, changes in the asphericity are greater in VOLPro parameters (1.22±1.47) than intrinsic parameters (0.59±0.30). Also, we have proved that the prediction of the theoretical post-surgical corneal asphericity is better using these intrinsic parameters (the relative difference is 21% for intrinsic values and 81% for VOLPro values). This relative difference depends on the level of astigmatism. Finally, we have found that the relative difference is lower using the conic model in VOLPro (37%), also compared to topography results (the relative difference using a topographer parameters was 45%).
The use of corneal parameters referenced to the intrinsic main axes of the cornea allows a better characterization of the corneal geometry. The application of these parameters reduces the differences between the real experimental and the theoretically predicted values.
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