PubMed · 8972356
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G W Hall. 1996. Optical zone size.. https://doi.org/10.1016/s0886-3350(96)80053-8
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PURPOSE: To present a novel experimental approach for treating irregular corneal astigmatism. METHODS: After decomposition of topographic analysis data into orthogonal Zernike polynomials, a regular target surface was defined, and the ablation profile was calculated. In a polymethylmethacrylate with an irregular surface, computer-controlled ablation was performed using a 193-nm excimer laser in "flying-spot mode." RESULTS: The difference between flattest and steepest hemimeridians in the 3-mm zone, surface regularity index, and surface asymmetry index were reduced from 8.7 to 1.1 diopters, 1.19 to 0.39, and 1.84 to 0.14, respectively. CONCLUSION: Tying corneal topography into laser software offers a straightforward concept for correction of irregular corneal astigmatism in an experimental model.
PURPOSE: To develop a method to predict the refractive power of the cornea from corneal topography. METHODS: We reviewed preoperative and postoperative cycloplegic refraction, keratometry, and corneal topography in 40 eyes of 40 patients who had undergone photorefractive keratectomy, radial keratotomy, myopic keratomileusis in situ, or hyperopic lamellar keratoplasty. For each axial dioptric power map, we calculated the aspheric ellipsoid that best fit that map. Central corneal points were weighted more heavily than peripheral points, based on the Stiles-Crawford effect. The equation of the best-fit ellipsoid yielded the spherical and astigmatic power and axis for each cornea preoperatively and postoperatively. RESULTS: The preoperative corneal spherical and astigmatic powers measured by the best-fit method were consistent with the spherical and astigmatic powers measured by keratometry and simulated keratometry. The change in corneal spherical power predicted by the best-fit method was significantly (P < .05) more accurate at predicting the change in spherical equivalent refraction than change either in spherical equivalent keratometry or in spherical equivalent simulated keratometry. The prediction of the astigmatic change was less precise than that of the spherical, but the best-fit method was the most accurate. CONCLUSIONS: The best-fit method is more accurate than simulated keratometry and standard keratometry are in evaluating corneal refractive power after refractive surgery. An improved method of calculating corneal refractive power may facilitate subjective refraction after refractive surgery, improve the accuracy of intraocular lens power calculation for eyes that have had previous refractive surgery, and improve ablation profiles for excimer laser refractive surgery.
PURPOSE: To determine the effect of successful pterygium surgery on astigmatism and other corneal topographic indices. METHODS: Preoperative and postoperative refraction and corneal-topography analysis were performed on 16 consecutive eyes of 12 patients who underwent successful pterygium surgery, with statistical comparison of the pre- and postoperative visual acuity, refractive astigmatism, topographic astigmatism, Surface-Regularity Index, and Surface-Asymmetry Index. RESULTS: Successful pterygium surgery caused statistically significant improvements in visual acuity, refractive and topographic astigmatism, the Surface-Regularity Index, and the surface asymmetry index. CONCLUSION: Corneal-topography analysis is an important component of the evaluation of patients with pterygium, revealing significant abnormalities that are improved by successful removal of the pterygium.