Purchase this article with an account.
Ashraf M. Mahmoud, Li Wang, Mitchell P. Weikert, Douglas D. Koch, Cynthia J. Roberts; Factors Influencing IOL Power Calculations After Refractive Surgery. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5757.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
To investigate the relationship between SimK’s and estimates of total corneal power, both pre- and post-LASIK.
Data from 19 eyes of 10 subjects were retrospectively identified. One scan of each eye had been acquired pre- and post-LASIK from a Dual Scheimpflug-Placido based tomographer (GALILEI, Ziemer Group). For each time point, the following values were recorded: Anterior Axial and Tangential Curvature, Posterior Axial and Tangential Curvature, Pachymetry, Total Corneal Power (TCP), SimK’s and Posterior/Anterior radius of curvature (ROC) ratio (Ratio). All values are the average over the central 4.0 mm diameter. Gaussian Equivalent Power (GEP) was calculated using GEP = F1 + F2 - (d/n)*F1*F2, where F1 is anterior power adjusted to n=1.376, F2 is posterior power, and d is pachymetry. GEP is referenced to the 1st principal plane, while TCP is referenced to the anterior surface and SimK’s to the posterior surface. Both GEP and TCP were adjusted to the posterior surface, adjGEP and adjTCP, respectively, and then compared to SimK average (SA) pre- and post-operatively. Also, Ratio was compared pre- to post-operatively.
Pre-operatively, the mean difference for SA-adjGEP is 0.48±0.19 D, SA-adjTCP is 0.9±0.09 D, and adjGEP-adjTCP is 0.43±0.18 D. Post-operatively, the mean SA-adjGEP is 0.64±0.30 D, SA-adjTCP is 1.50±0.22 D, and adjGEP-adjTCP is 0.86±0.35 D. Ratio is 0.823±0.010 pre-operatively and 0.753±0.023 post-operatively. All comparisons were significantly different between pre and post-op (p < 0.0001) with greater disparity and variability between SA and corneal power post-operatively.
Anterior Axial curvature and therefore SimKs use the index of refraction n = 1.3375 derived using the Gullstrand Eye Model based on spherical surfaces: anterior ROC = 7.7 mm, posterior ROC = 6.8 mm, ratio = 0.883, and pachymetry = 0.5 mm. The significant change in posterior/anterior ROC ratio post-op, with more than double the range as pre-LASIK, can lead to errors in IOL power calculations using SimK’s after refractive surgery. TCP takes into account individual parameters, and likely provides a better estimate of IOL power than SimK’s after refractive surgery. However, the reference plane for the power measurement must be taken into account.
This PDF is available to Subscribers Only