PubMed · 7722895
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P J Parden. 1995. Vector analysis.. https://doi.org/10.1016/s0886-3350(13)80462-2
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BACKGROUND: The refractive outcome of thermokeratoplasty for astigmatism correction depends upon the optical zone diameter (OZD) and coagulation angle (CA; distance between two coagulation spots around the flat meridian). METHODS: Astigmatism was induced in 36 spherical human eye-bank eyes (Ho:YAG laser, 15 Hz, 20 mJ/pulse, 25 pulses per coagulation spot) with different OZDs and CAs. Thirty eyes received free-hand laser application (marked positions). Six eyes were treated using a suction mask, providing a constant OZD of 8.1 mm and an angle of 22.5 degrees between the laser spots. To compare the results, silicone replicas of the eyes were made pre- and postoperatively and analyzed by video-topometery. RESULTS: With an average decrease of 4 D/mm the refractive change is inversely linear to the OZD. Changes of the CA showed significant variations between 22.5 degrees and 45 degrees, only. Mask guidance leads to the same amount of average refractive change. The deviation is +/- 3.4 D for the freehand application but only +/- 0.8 D for mask guidance. CONCLUSION: The data obtained in this in vitro study are transferred to a treatment nomogram describing the immediate postoperative refractive change, not identical with the final refraction. The results of the study are limited by the fact that astigmatism was induced in spherical corneas, rather than correcting preexisting ones.
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.