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S.T. Awwad, R.W. Bowman, H.D. Cavanagh, S.M. Verity, J.P. McCulley; Optimizing Corneal Power Calculation After Myopic Keratorefractive Surgery . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3616.
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© ARVO (1962-2015); The Authors (2016-present)
To devise a formula that accurately predicts the refractive corneal power after myopic LASIK/PRK
A retrospective review was conducted on 18 eyes of 15 patients with a history of myopic LASIK or PRK and subsequent phacoemulsification with intraocular lens (IOL) implantation. Data analyzed included keratometry (KpreLASIK) and manifest spherical equivalence (SE) prior to LASIK/PRK, post–LASIK/PRK SE and corneal topography, axial length (range of 23.64 to 28.1 mm), power of IOL implanted, and SE one month after phacoemulsification. Post LASIK/PRK average corneal power (AveK) was defined as the average of the topographic data points that make the steep and flat meridians, as determined by the TMS–4 topography system. The refractive corneal power after LASIK/PRK (RCPbackcalc) was back–calculated for every eye using the double–K SRK/T formula. Multifactorial regression analyses were performed using RCPbackcalc as an independent variable and using each of AveK, simulated keratometry (SimK), effective refractive power (EffRP), and with either (KpreLASIK – AveK) or (SEpostLASIK – SEpreLASIK). Corneal power was then calculated for every eye from each formula, as well as from the historical K and EffRPadj methods, and the resultant value was subtracted from the corresponding RCPbackcalc. The mean and standard deviations of the absolute differences were then compared for each formula.
The adjusted AveK was found to be the best predictor of postLASIK refractive corneal power. The simplified regression formulae were: RCP= AveK–0.18 x (KpreLASIK – AveK), and RCP=AveK – 0.15 x (SEpostLASIK – SEpreLASIK) producing a mean absolute difference of 0.27+/– 0.19 D and 0.28+/–0.19 D, respectively. Regressions based on SimK resulted in mean absolute deviations of at least 0.67+/–0.62 D (P=0.043), while those based on EffRP resulted in deviations of at least 1.34 +/– 1.03 D (P=0.004). EffRPadj yielded a deviation of 1.04 +/– 0.98 D (P=0.018), and historical K method produced a deviation of 1.03 +/– 0.62 D (P<0.001). No deviation was more than 1 D for the regressions based on AveK, as opposed to 6 (33%) for EffRPadj, and 7 (39%) for the historical K method (P=0.019 and P=0.008, respectively).
Refractive corneal power after myopic LASIK/PRK can be accurately predicted based on Average Corneal Power (AveK) and the LASIK induced change in either the spherical equivalence or the corneal keratometries. Regressions based on SimK are less accurate, while the historical k and the EffRPadj methods as well as regressions based on EffRP are the least accurate on an aggregate basis.
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