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V. D. Sicam, A. Vrijling, J. J. Snellenburg, B. Braaf, M. J. V. Zaal, R. G. L. van der Heijde; Effect of Placido Ring Topography Skew Ray Error in Clinically Measured Higher Order Aberrations. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1036. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To validate the effect of skew ray errors (Sicam and van der Heijde, OVS 82(12) 2006) in corneal measurements of human eyes.
Corneal topography was measured for 9 post PKP and LKP corneas using the VU topographer (Prototype, VUmc Amsterdam, The Netherlands) and the Keratron Placido Topographer (Optikon 2000, Rome, Italy). Informed consent was obtained from the subjects in accordance with the tenets of the Declaration of Helsinki. The aberrations were characterized using standard Zernike convention. A theoretical model was developed to predict the effect of skew ray errors for non-rotation-symmetric aberrations (astigmatism, coma, trefoil and quadrafoil). Artificial surfaces (toric and coma) were also measured to verify the theoretical predictions.
Theoretical calculations showed that the suppression of the amplitude of non-rotation-symmetric corneal aberrations in Placido based topography is in the order of 4% for astigmatism, negligible for coma, 50% for trefoil and 80% for quadrafoil. There is no suppression of aberration amplitude expected for the VU topographer. The experimental results for the artificial surfaces agreed with the theoretical predictions: 2.90 ± 1.38 % underestimation for astigmatism and negligible for coma measured by the Keratron. No systematic underestimation was observed for the VU topographer. For post PKP and LKP corneas only the trefoil and quadrafoil aberrations showed significant difference between the measurements of the VU topographer and the Keratron, 49% (LOA = -23%,190%, p = 0.01) for the trefoil aberration and 85% (LOA -1%,246%, p= 0.008) for the quadrafoil aberration. Both trefoil and quadrafoil showed more than 0.5 diopter underestimation due to skew ray error.
Although the effect of skew ray error is not clinically significant for low astigmatism and coma aberration, the error produced in trefoil and quadrafoil is clinically relevant. E.g. a diagnosis of 0.5 diopter trefoil as measured by Placido ring topographers could actually be more than 1 diopter.
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