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O. Baca, A. Barroso, R. Velasco, D. Viggiano, R. Bueno-García; Normal Patterns in Elevation Maps of Scanning Slit Corneal Topography Before Refractive Surgery . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2551.
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Purpose: To find and classify the normal anterior and posterior elevation maps in patients candidates to LASIK. Methods: We evaluated 226 eyes of 113 patients before undergoing LASIK surgery. Anterior and posterior corneal elevation maps were assessed using Orbscan II® topographies. Specific parameters evaluated on the Orbscan II topographies were mean anterior and posterior elevation measured in microns using the apex-fixed best fit corneal curvature method, central and thinnest pachymetry and morphology of the color-code elevation maps in order to evaluate and classify the patterns. Distribution of patterns depended on the presence and magnitude of astigmatism and the spherical aberration associated. We stratified three groups of data in anterior and posterior maps: spherical refraction without astigmatism (group 1), astigmatism > 1.00 diopters (group 2) and astigmatism > 1.00 diopters (group 3). Besides, in posterior elevation we studied four subgroups depending on mean elevation measure: 40 microns. Results: We determined five topographical patterns in anterior and posterior elevations maps: complete positive band pattern, incomplete positive band pattern, central island pattern, butterfly' s wings pattern and diffuse pattern. In group one 79.2% of cases corresponded to central island pattern, 12.8% to incomplete positive band pattern and 4.8% to diffuse pattern. In group two 43.9% corresponded to incomplete positive band pattern, 24.4% to complete positive band pattern, 19.51% to central island pattern and 6.8% to butterfly' s wings pattern. In group three 71. 87% corresponded to complete positive band pattern, 18. 75 % to incomplete positive band pattern, 6.25% to butterfly' s wings pattern and 3.13% to central island pattern. The mean anterior elevation measure in group 1 was 5.41 ± 2.53 microns in hyperopia and 9.35 ± 5.49 microns in myopia; in group 2 was 6.63 ± 3.69 microns and in group three was 8.10 ± 4.23 microns. The mean posterior elevation measure in subgroup 1 was 17.67 ± 5.46 microns, in subgroup 2 was 27.35 ± 1.04 microns, in subgroup 3 was 34.03 ± 2.95 microns and in subgroup 4 was 46.80 ± 2.35 microns. Mean central pachymetry for all groups was 555 ± 30 microns and thinnest pachymetry was 547 ± 30 microns. Conclusions: The maps of elevation topography in the normal cornea have a characteristic distribution depending on the refractive error associated. When using in combination with the anterior and posterior elevation measure and clinical and pachymetrical findings may be helpful in the preoperative assessing of patients before refractive surgery.
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