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      Analysis of photoastigmatic keratectomy with the cross-cylinder ablation

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          Abstract

          Aim:

          The aim was to evaluate the safety and efficacy of the “cross-cylinder” technique in the correction of astigmatism.

          Setting and Design:

          A prospective interventional study from a university eye department was conducted.

          Material and Methods:

          The photoastigmatic refractive keratectomy (PARK) using the “cross-cylinder” technique was performed in 102 eyes of 84 patients with at least 0.75 D of astigmatism. The study population was divided into two groups: in the first group the preoperative astigmatic power ranged from –0.75 D to –3.00 D (group 1), in the second group it ranged from –3.25 D to –6.00 D (group 2). Group 1 included 82 eyes of 67 patients (29 males and 38 females) with a mean cylinder power of –1.90 ± 0.63 D, group 2 included 20 eyes of 17 patients (13 males and 4 females) with a mean cylinder power of -4.28 ± 0.76 D. All eyes were targeted for emmetropia. The results were evaluated using Calossi's vector analysis method. Six-month postoperative outcomes are presented.

          Results:

          Six months after PARK the mean sphere for the entire cohort was +0.28 ± 0.75 D (range +2.5 to –2 D), the mean cylindrical power was +0.33 ± 0.51 D (range +2.5 to –1.25 D) and the mean spherical equivalent refraction was +0.73 ± 0.81 D (range +1.75 to –2 D).

          Conclusions:

          The cross-cylinder technique may be safely used with predictable results for the correction of astigmatism.

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          Most cited references32

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          Power vector analysis of the optical outcome of refractive surgery.

          To demonstrate the power vector method of representing and analyzing spherocylindrical refractive errors. School of Optometry, Indiana University, Bloomington, Indiana, USA. Manifest and keratometric refractive errors were expressed as power vectors suitable for plotting as points in a 3-dimensional dioptric space. The 3 Cartesian coordinates (x, y, z) of each power vector correspond to the powers of 3 lenses that, in combination, fulfill a refractive prescription: a spherical lens of power M, a Jackson crossed cylinder of power J0 with axes at 90 degrees and 180 degrees, and a Jackson crossed cylinder of power J45 with axes at 45 degrees and 135 degrees. The Pythagorean length of the power vector, B, is a measure of overall blurring strength of a spherocylindrical lens or refractive error. Changes in refractive error due to surgery were computed by the ordinary rules of vector subtraction. Frequency distributions of blur strength (B) clearly demonstrate the effectiveness of refractive surgery in reducing the overall blurring effect of uncorrected refractive error. Power vector analysis also revealed a reduction in the astigmatic component of these refractive errors. Paired comparisons revealed that the change in manifest astigmatism due to surgery was well correlated with the change in keratometric astigmatism. Power vectors aid the visualization of complex changes in refractive error by tracing a trajectory in a uniform dioptric space. The Cartesian components of a power vector are mutually independent, which simplifies mathematical and statistical analysis of refractive errors. Power vectors also provide a natural link to a more comprehensive optical description of ocular refractive imperfections in terms of wavefront aberration functions and their description by Zernike polynomials.
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            Astigmatism analysis by the Alpins method.

            N Alpins (2000)
            To determine the effectiveness of correcting astigmatism by laser refractive surgery by a vectorial astigmatism outcome analysis that uses 3 fundamental vectors: target induced astigmatism vector (TIA), surgically induced astigmatism vector, and difference vector, as described by the Alpins method. A data set of 100 eyes that had laser in situ keratomileusis to correct myopia and astigmatism (minimum preoperative refractive astigmatism 0.75 diopter) was analyzed. The data included preoperative and 3 month postoperative values for manifest refraction and standard keratometry. Using the ASSORT or VectrAK analysis program, individual and aggregate data analyses were performed using simple, polar, and vector analysis of astigmatism and an analysis of spherical change. Statistical analysis of the results was used for means and confidence limits, as well as to examine the differences between corneal and refractive astigmatism outcomes. At an individual patient level, the angle of error was found to be significant, suggesting variable factors at work, such as healing or alignment. A systematic error of undercorrection of astigmatism is prevalent in the treatment of these 100 patients by a factor of between 15% and 30%, depending on whether refractive or corneal values are examined. Spherical correction showed systematic undercorrection of 11%, and parallel indices demonstrated it to be more effective than the astigmatic correction. This method of astigmatism analysis enables the examination of results of astigmatism treatment measured by both refractive and corneal measurements using vector analysis. By examining individual vector relationships to the TIA (ie, the correction index, index of success, and flattening index), a comprehensive astigmatism analysis is completed. Each index provides information necessary for understanding any astigmatic change. Astigmatic outcome parameters are more favorable when measured by subjective refractive than objective corneal methods.
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              Multivariate analysis of refractive data: mathematics and statistics of spherocylinders.

              To develop methods for multivariate statistical analysis of spherocylinders and to use these methods to compare autorefraction and manifest subjective refraction in 50 healthy eyes. Department of Ophthalmology, Arhus University Hospital, Arhus, Denmark. A method was developed to transform a spherocylinder to a suitable format as a spherical equivalent power (SEP) and 2 polar values, separated by an arch of 45 degrees, a so-called power-vector format. The accuracy of autorefraction was defined as the difference between autorefraction and manifest refraction, using the described power vector. These entities were subjected to multivariate analysis using the Hotelling T2 test. A method of graphic analysis was developed, using matrix algebra and computation of eigenvectors and eigenvalues. For individual data, the variation was considerably larger for the SEP than for the astigmatism. For aggregate data, univariate, bivariate, and trivariate statistical analysis did not demonstrate significant average differences between the 2 refraction methods. No refractive components and no combinations of refractive components displayed significant mean differences. The study confirmed our clinical experience that the astigmatism derived from autorefraction is nearly identical to manifest refraction, while the sphere needs some adjustment. In groups of healthy eyes, autorefraction can be used as a substitute for manifest refraction. Statistical analysis of spherocylinders, including evaluation of refractive procedures, can be performed in an exact manner with multivariate statistics.
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                Author and article information

                Journal
                Indian J Ophthalmol
                Indian J Ophthalmol
                IJO
                Indian Journal of Ophthalmology
                Medknow Publications & Media Pvt Ltd (India )
                0301-4738
                1998-3689
                Jul-Aug 2012
                : 60
                : 4
                : 283-287
                Affiliations
                [1 ]Department of Medicine and Surgery, University of Salerno, Salerno, Italy
                [2 ]Eye Department, 2 nd University of Naples, Naples, Italy
                [3 ]Centro Grandi Apparecchiature, 2 nd University of Naples, Naples, Italy
                [4 ]St Paul's Eye Unit, Royal Liverpool University Hospital, Liverpool, UK
                [5 ]Department of Medicine and Ageing Science, Ophthalmic Clinic, University “G. d’Annunzio” of Chieti-Pescara, Italy
                Author notes
                Correspondence to: Dr. Maddalena De Bernardo, Centro Grandi Apparecchiature, 2 nd University of Naples, Via De Crecchio 16, 80134 Napoli, Italy. E-mail: maddalenadebernardo@ 123456alice.it
                Article
                IJO-60-283
                10.4103/0301-4738.98707
                3442463
                22824597
                8e351307-79c3-4750-a71c-d23e522168c4
                Copyright: © Indian Journal of Ophthalmology

                This is an open-access article distributed under the terms of the Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                : 01 September 2011
                : 13 January 2012
                Categories
                Original Article

                Ophthalmology & Optometry
                refractive surgery,cross-cylinder technique,photoastigmatic refractive keratectomy

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