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A. J. Lang, T. Icenogle, A. Chayet, E. Barragan; Determining the Center of the Ablation in Topographies. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3541.
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Suggest the topographic display / analysis software settings to most correctly locate the center of LASIK ablations in postoperative elevation maps.
For the purposes of postop evaluation of LASIK outcomes for potential re-treatment and secondary procedures such as post-LASIK intracorneal implants for presbyopia, it is necessary to determine the true location of the LASIK ablation center in the postop topography. This analysis focuses on elevation data and acknowledges that curvature maps may provide further useful information. The apparent ablation location is a function of types of reference sphere that is subtracted to reveal the ablation pattern in the much stronger anterior corneal curvature. We manually identify the apparent ablation centers in postop and preop Pentacam (Oculus, Inc.) elevation data, using reference spheres with centered on the corneal apex or allowed to "float" to the X (nasal / temporal) and Y (inferior / superior) position best fitting the general anterior corneal curvature, in subjects undergoing myopic LASIK. With the assumption that the postop minus preop difference map represents the best estimate of the true ablation center, we ask which postop analysis setting yields the least magnitude of the vector difference (e.g., radial distance) between the difference map ablation center and the postop map ablation center. We also assessed the radial difference between the center in the pachymetry difference map and in the postop elevation map. MatLab simulations are also used to investigate the ablation center error as a function of topographic analysis parameters.
Elevation maps from 12 eyes were assessed, ranging from -1D targeted myopic correction to approximately -5D correction, with a mean correction of -2.9D. The elevation difference maps were calculated with the same reference sphere; i.e., without float and using the same radius of curvature. The mean radial difference between ablation centers in the elevation difference map and in the postop map without float was 0.21 +/- 0.31 mm, and with respect to the "floated" postop map 0.68 +/- 0.44 mm. The difference is statistically significant (p = 0.02). The mean radial difference between ablation centers in the pachymetry difference map and in the postop elevation map without float was 0.77 +/- 0.47 mm, and with respect to the "floated" postop elevation map 0.66 +/- 0.36 mm. The difference is not statistically significant (p = 0.41).
With the above assumptions, it appears that the best estimate of the true ablation center in the postop elevation map is given by subtracting a reference sphere without float.
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