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M. Gomez-Valcarcel, J. Tejedor, A. Guirao; Comparison of Corneal Shape and Aberrations in Keratectasia After Myopic and Hyperopic Treatments. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1039. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To study topographic and optical aberrations patterns in corneas with keratectasia, looking for differences between myopic and hyperopic treatments.
Two eyes of two patients treated (Technolas 217z100, Bausch & Lomb) with myopic LASIK and hyperopic PRK were studied. Both were diagnosed postoperatively with keratectasia. Corneal thickness, ablation diameter, flap (epithelium), and attempted correction were: 557 microns, 6.5 mm, 120 microns, and - 5.50 -2.50 x 170, for the myopic LASIK patient; and 513 microns, 6.5 mm, 60 microns, +5.5 D, for the hyperopic PRK patient. Postoperative refractions were: -2.25 -4.00 x 180, and -5.50 -0.50 x 104, for the myopic and hyperopic patient, respectively. Corneal topography was measured (Orbscan, Orbtek, Inc.) before and after surgery, for both the anterior and the posterior corneal surface. Pachymetry maps were obtained. Corneal aberrations were calculated by ray-tracing from the corneal elevations for a 6-mm pupil. Pre and postoperatively corneal shape and aberrations were compared.
Myopic patient: the anterior corneal surface experienced an irregular steepening that led to curvature and astigmatism values similar to those before surgery. Corneal asphericity, and therefore spherical aberration, changed only slightly. Asymmetries induced in the corneal surface produced an important amount of high-order aberrations (RMS = 0.6 microns, pre; 2.6 microns, post), such as coma and trefoil, both oriented along the axis of the astigmatic correction attempted. Postoperative pachymetry showed a decentered zone of corneal thinning. Hyperopic patient: the cornea suffered a central bulging responsible of an overcorrection of 5 D and, in consequence, a postoperative important myopia. High-order aberrations greatly increased (0.5 to 1.7 microns), particularly spherical aberration. The pachymetry map was irregular after surgery. The posterior surface changed moderately in both patients.
We studied a case of ectasia after hyperopic PRK, which has rarely been reported. The main topographic change associated with ectasia is central bulging: for myopia, the steepening corresponds with the area of corneal thinning, whereas for hyperopia the thinning is around the induced bulging. The change in curvature towards myopia produces an important overcorrection or undercorrection in the hyperopic or myopic treatment, respectively. High-order aberrations largely increase in corneas with ectasia.
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