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A.M. Mahmoud, C.J. Roberts, D. Castellano, R.G. Lembach; Functional Optical Zone Size Measured after LASIK . Invest. Ophthalmol. Vis. Sci. 2004;45(13):221.
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Purpose: To develop a software technique to define and extract the functional optical zone from topographic data after refractive surgery. Methods: A retrospective study of 52 eyes 29 patients who received Keratron Scout corneal topography (Optikon, Rome, Italy) and Zywave wavefront measurements (Bausch&Lomb, Rochester, New York) three months post–LASIK using one of three lasers was conducted. The acquired data were processed using The Ohio State University Corneal Topography Tool (OSUCTT) to generate refractive power maps. Next, a region growing algorithm was developed and applied to determine the size and location of the functional optical zone (FOZ). First, the flattest area bounded by a 2 mm diameter circle was located and used as a seed point for the region growing. Then, an algorithm based on a bayesian classifier was used to define the FOZ. The centroid of this region and its distance from the center of the map (decentration) was calculated. Also, the average refractive power inside (ref_oz), outside (ref_out) of the FOZ, and the difference (ref_diff = ref_oz – ref_out) were calculated. Linear regression analysis was performed to analyze the relationship of decentration to coma and ref_diff to spherical aberration. Coma was calculated from the wavefront measurements as being the square root of the sum of the squares of Z3–1 and Z31. The term Z40 was used to represent spherical aberration. Results: Decentration of the FOZ significantly correlated with coma (p < 0.01, r2=0.18). Ref_diff significantly correlated with spherical aberration (p < 0.01, r2=0.33). Average FOZ was 28.5 mm2 with a major axis of 6.35 mm and a minor axis of 5.71mm. Conclusions: Size, decentration, and relative magnitude of FOZ calculated from refractive power maps based on topographic measurements provides important information about optical aberrations induced by refractive surgery.
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