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C.J. Roberts, A.M. Mahmoud, M.D. Twa, R.G. Lembach, P. Vinciguerra; Curvature Gradient vs Eccentricity: Higher–Order vs Lower–Order Assessment of Corneal Shape and Their Correlation to Spherical Aberration Induction After LASIK . Invest. Ophthalmol. Vis. Sci. 2006;47(13):570.
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© ARVO (1962-2015); The Authors (2016-present)
To compare eccentricity and a new parameter, curvature gradient, in correlating corneal shape to spherical aberration induction after LASIK.
A retrospective analysis was performed on 60 eyes of 30 subjects who had previously undergone LASIK in a controlled trial. One eye was treated with a Bausch & Lomb laser (BL), and the contralateral eye treated with a VISX S3 laser (V). The eyes treated with a VISX laser had significantly greater spherical aberration induction at 6 months post–op than the eyes treated with the B&L laser.1 For the current study, the pre–op and 6 month post–op corneal topography data (Keratron Scout) were exported to custom software for analysis. A best–fit ellipsoid was calculated over a 6mm region of interest for each tangential curvature map, producing central radius of curvature and eccentricity (ε). Curvature and eccentricity correspond to a second order calculation. Since spherical aberration is a higher–order term, a new higher–order shape term was defined, curvature gradient, which is the derivative of curvature. A new curvature gradient map was created from each tangential curvature map in the dataset. The average of all points in the curvature gradient map was defined as the "curvature gradient index." The pre–op and post–op eccentricity and curvature gradient index were statistically compared between lasers using a student t test.
No statistically significant differences were found between lasers pre–operatively with either ε2 (p < 0.53) or curvature gradient index (p < 0.34), or post–operatively in ε2 (p < 0.21). A statistically significantly difference was found in curvature gradient index (BL: 0.68 ± 0.64, V: 1.37 ± 0.71; p < 0.0001) between lasers post–operatively. The eyes with the greatest spherical aberration induction (V) had the greatest curvature gradient index.
Curvature Gradient, which is a higher order term, correlates to the induction of spherical aberration in LASIK. Eccentricity, which is lower–order term, has serious limitations. Reference:
1. Twa M, Lembach R, Bullimore M, Roberts C: "A Prospective Randomized Clinical Trial of Laser In–Situ Keratomileusis Using Two Different Lasers." American Journal of Ophthalmology, 2005; 140(2):173–83.
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