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      Calculation of Unknown Preoperative K Readings in Postrefractive Surgery Patients

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      1 , 1 , , 2
      Journal of Ophthalmology
      Hindawi

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          Abstract

          Purpose

          To determine the unknown preoperative K readings (Kpre) to be used in history-based methods, for intraocular lens (IOL) power calculation in patients who have undergone myopic photorefractive keratectomy (PRK).

          Methods

          A regression formula generated from the left eyes of 174 patients who had undergone PRK for myopia or for myopic astigmatism was compared with other methods in 168 right eyes. The Pearson index and paired t-test were utilized for statistical analysis.

          Results

          The differences between Kpre and those obtained with the other methods were as follows: 0.61 ± 0.94 D (range: −3.94 to 2.05 D, p < 0.01) subtracting the effective treatment, 0.01 ± 0.86 D (range: −2.61 to 2.34 D, p = 0.82) with Rosa's formula, −0.02 ± 1.31 D (range: −3.43 to 3.68 D, p = 0.82) with the current study formula, and −0.43 ± 1.40 D (range: −3.98 to 3.12 D, p < 0.01) utilizing a mean K (Km) of 43.5 D.

          Conclusions

          These formulas may permit the utilization of history-based methods, that is, the double-K method in calculating the IOL power following PRK when Kpre are unknown.

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          Most cited references28

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          Intraocular lens calculation after refractive surgery for myopia: Haigis-L formula.

          To describe the Haigis-L formula for the calculation of intraocular lenses (IOLs) after refractive laser surgery for myopia based on current biometry and keratometry and present clinical results. University Eye Hospital, Wuerzburg, Germany, and various clinics and private practices. The basic concepts of the new algorithm were described and summarized. The Haigis formula was analyzed with respect to its usability for eyes after laser surgery for myopia and modified accordingly. Correction curves for IOLMaster keratometry were derived from previous studies. The new formula was checked using the postoperative results of 187 cataract procedures in which 32 IOL types were implanted by 57 surgeons. Input data were current IOLMaster biometry as follows: axial length (AL), anterior chamber depth (ACD), and keratometry (corneal radii) measurements. Before IOL surgery, the mean spherical equivalent was -7.60 diopters (D)+/-3.90 (SD) (range -20.00 to -1.25 D); the mean AL, 27.02+/-2.01 mm (range 23.09 to 35.32 mm); the mean ACD, 3.52 +/- 0.36 mm (range 2.43 to 4.39 mm); and the mean of the measured corneal radii, 8.70+/-0.60 mm (range 7.28 to 10.96 mm). The mean arithmetic refractive prediction error was -0.04+/-0.70 D (range -2.30 to +2.40 D) and the median absolute error, 0.37 D (range +0.01 to +2.40 D). The percentages of correct refraction predictions within +/-2.00, +/-1.00, and +/-0.50 D were 98.4%, 84.0%, and 61.0%, respectively. The new formula would produce promising results in eyes without refractive history. Its refractive predictability fulfills the current criteria for normal eyes.
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            Intraocular lens power calculation after corneal refractive surgery: double-K method.

            To determine the accuracy of a method of calculating intraocular lens (IOL) power after corneal refractive surgery. Department of Ophthalmology, Hospital de Gipuzkoa, San Sebastián, Spain. The SRK/T formula was modified to use the pre refractive surgery K-value (Kpre) for the effective lens position (ELP) calculation and the post refractive surgery K-value (Kpost) for IOL power calculation by the vergence formula. The Kpre value was obtained by keratometry or topography and the Kpost, by the clinical history method. The formula was assessed in 9 cases of cataract surgery after laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK) in which all relevant data were available. Refractive results of the standard SRK/T and the double-K SRK/T were compared statistically. The mean IOL power for emmetropia and the achieved refraction (mean spherical equivalent [SE]), respectively, were +17.85 diopters (D) +/- 3.43 (SD) and +1.82 +/- 0.73 with the standard SRK/T and +20.25 +/- 3.55 D and +0.13 +/- 0.62 D with the double-K SRK/T. No case in the standard SRK/T group and 6 cases (66.66%) in the double-K group achieved a +/-0.5 D SE. Double-K modification of the SRK/T formula improved the accuracy of IOL power calculation after LASIK and PRK.
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              Underestimation of intraocular lens power for cataract surgery after myopic photorefractive keratectomy.

              To assess the validity of corneal power measurement and standard intraocular lens power (IOLP) calculation after photorefractive keratectomy (PRK). Nonrandomized, prospective, cross-sectional, clinical study. A total of 31 eyes of 21 females and 10 males with a mean age at the time of surgery of 32.3 +/- 6.6 years (range, 24.4-49.5 years). Subjective refractometry, standard keratometry, TMS-1 corneal topography analysis, and pachymetry were performed before and 15.8 +/- 10.4 months after PRK for myopia (n = 24, -1 .5 to -8.0 diopters [D], mean -5.4 +/- 1.9 D) or myopic astigmatism (n = 7, sphere -2.0 to -7.5 D, mean -4.4 +/- 1.9 D; cylinder -1.0 to -3.0 D, mean -1.9 +/- 0.7 D). The IOLP calculations were done using two different formulas (SRK/T and HAIGIS). Keratometric power (K) and topographic simulated keratometric power (TOPO) as measured (Kmeas, TOPOmeas) and as calculated according to the change of power of the anterior corneal surface or according to the spherical equivalent change after PRK (Kcalc, TOPOcalc), IOLP for emmetropia, and postoperative ametropia for calculated corneal powers were assessed in a model. After PRK, mean Kmeas and TOPOmeas were significantly greater (0.4-1.4 D, maximum 3.3 D) than mean KRcalc and TOPOcalc (P 1 D) than IOLP values using topographic readings (P < 0.0001). The theoretically induced mean refractive error after cataract surgery ranged from +0.4 to +1.4 (maximum, +3.1) D. Corneal power overestimation and IOLP underestimation correlated significantly with the spherical equivalent change after PRK (P = 0.001) and the intended ablation depth during PRK (P = 0.004). To avoid underestimation of IOLP and hyperopia after cataract surgery following PRK, measured corneal power values must be corrected. The calculation method using spherical equivalent change of refraction at the corneal plane seems to be the most appropriate method. In comparison with this method, direct power measurements underestimate corneal flattening after PRK by 24% on average. Use of conventional topography analysis seems to increase the risk of error. However, because this study is retrospective and theoretical, there is still a need for a large prospective investigation to validate the authors' findings.
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                Author and article information

                Contributors
                Journal
                J Ophthalmol
                J Ophthalmol
                JOPH
                Journal of Ophthalmology
                Hindawi
                2090-004X
                2090-0058
                2018
                11 February 2018
                : 2018
                : 3120941
                Affiliations
                1Department of Medicine and Surgery, University of Salerno, Salerno, Italy
                2Department of Ophthalmology, Heinrich Heine University Düsseldorf, Düsseldorf, Germany
                Author notes

                Academic Editor: Ciro Costagliola

                Author information
                http://orcid.org/0000-0002-2366-6556
                http://orcid.org/0000-0003-4566-0064
                Article
                10.1155/2018/3120941
                5828239
                29607215
                3a19af1a-2f81-4dd4-8fe2-89c39e617fb5
                Copyright © 2018 Nicola Rosa et al.

                This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                : 11 July 2017
                : 20 December 2017
                Categories
                Clinical Study

                Ophthalmology & Optometry
                Ophthalmology & Optometry

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