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A. M. Mahmoud, C. J. Roberts; The Influence of Altered Posterior Angle of Incidence on Effective Posterior Power Calculation Before and After LASIK. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4194.
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© ARVO (1962-2015); The Authors (2016-present)
To examine the error in topographic calculations of posterior corneal power which are based on the assumption that incoming light rays are parallel. In reality, these rays have already been refracted by the anterior surface.
Orbscan data from 118 eyes of 59 subjects were retrospectively identified, and the posterior surface was reprocessed with custom software. Two scans of each eye had been acquired pre-operatively and 1 month post-LASIK for myopia. Anterior tangential power (aTAN) using n=1.376, posterior tangential power (pTAN), and pachymetry data were extracted. Ray tracing was used to generate Total Corneal Power (TCP) and Anterior Corneal Power (ACP) data, which were used to calculate Effective Posterior Power (EPP) data as TCP - ACP, which means the power contribution due to corneal thickness was included in the "effective" parameter. The central zone averages over a 4 mm diameter were calculated. From the Gaussian Equivalent Power equation, GEP = F1 + F2 - (d/n)*F1*F2, the Gaussian Effective Posterior Power was calculated as GEPP = F2 - (d/n)*F1*F2. For this study, aTAN was used for F1, pTAN (parallel incoming rays) was used for F2, d was pachymetry, and n=1.376. The formula for GEPP was rearranged to solve for the Gaussian Posterior Power (GPP) without the contribution of corneal thickness, F2 = GEPP / (1 - (d/n)*F1). aTAN was used for F1 and EPP was used instead of GEPP to make this power formulation a function of refracted rays. All statistical comparisons were done with t-tests.
There are no significant differences pre to post-LASIK for EPP and GPP, both based on refracted rays. There are significant differences (p < 0.0001) pre to post-LASIK for GEPP and pTAN, both based on parallel rays, 0.05±0.08 D and 0.04±0.08 D, respectively. The comparisons of EPP to GEPP and GPP to pTAN all show significant differences (p<0.001) for both pre and post-LASIK. The mean differences for EPP - GEPP are 0.11±0.10 D and 0.17±0.10 D for pre and post-LASIK, respectively. The mean differences for GPP - pTAN are 0.12±0.10 D and 0.18±0.10 D for pre and post-LASIK, respectively.
The reduction of corneal thickness after myopic LASIK is balanced by the decrease in anterior curvature which reduces the refraction of the rays striking the posterior surface, resulting in no overall posterior power difference. However, the shape of the posterior surface does change, based on the analysis with parallel rays which reflect diopters of curvature, not power. Formulas which use refracted incoming rays provide a better measurement of posterior power than those which assume incoming parallel rays.
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