New Approach for the Calculation of the Intraocular Lens Power Based on the Fictitious Corneal Refractive Index Estimation

Abstract

Purpose. To identify the sources of error in predictability beyond the effective lens position and to develop two new thick lens equations. Methods. Retrospective observational case series with 43 eyes. Information related to the actual lens position, corneal radii measured with specular reflection and Scheimpflug-based technologies, and the characteristics of the implanted lenses (radii and thickness) were used for obtaining the fictitious indexes that better predicted the postoperative spherical equivalent (SE) when the real effective lens position (ELP) was known. These fictitious indexes were used to develop two thick lens equations that were compared with the predictability of SRK/T and Barrett Universal II. Results. The SE relative to the intended target was correlated to the difference between real ELP and the value estimated by SRK/T (ΔELP) (r = −0.47, p=0.002), but this only predicted 22% of variability in a linear regression model. The fictitious index for the specular reflection (nk) and Scheimpflug-based devices (nc) were significantly correlated with axial length. Including both indexes fitted to axial length in the prediction model with the ΔELP increased the r-square of the model up to 83% and 39%, respectively. Equations derived from these fictitious indexes reduced the mean SE in comparison to SRK/T and Barrett Universal II. Conclusions. The predictability with the trifocal IOL evaluated is not explained by an error in the ELP. An adjustment fitting the fictitious index with the axial length improves the predictability without false estimations of the ELP.

Determining the Theoretical Effective Lens Position of Thick Intraocular Lenses for Machine Learning–Based IOL Power Calculation and Simulation

“…However, knowing the geometry of the IOL makes it possible using a thick lens paraxial model to relate the optically estimated position of an IOL to its estimated anatomic position. Fernández et al 22 studied the relationship between the measured ALP and the back-calculated ELP and demonstrated the differences between both values, induced by assumptions made in theoretical eye models.…”

“… 25 , 26 This finding is reflected in a recent study 27 in which there was no significant difference in the accuracy of thick lens IOL power formula based on calculated versus manufacturer's IOL data for eyes with ALs of 22 mm and more. Fernández et al 22 suggested modifying the refractive index of the cornea to correct errors beyond the ELP prediction, including assumptions from the biometers.…”

New method to assess the accuracy of intraocular lens power calculation formulas according to ocular biometric parameters

“…A ccuracy in intraocular lens (IOL) power calculation has become a topic arousing great interest, because nowadays crystalline-lens replacement can be considered not only for the surgical treatment of an ocular disease but also for a refractive surgery procedure. 1 New formulas and updates to existing ones have been proposed in the last years, but refractive-status prediction errors greater than ± 0.50 diopters (D) still occur in a rather high percentage of cases-which ranges between 8% and 50% of the eyes, depending on the specific study or sample stratification, [2][3][4][5][6][7][8][9] -with the best accuracy obtained for multifocal IOL implantation procedures, and the vast majority of surgeons being clustered around 78%. 5,10 The major part of the methods for IOL power calculation is based on the vergence formula, and the main difference lies in the estimation of the effective lens position including from 2 up to 7 variables, some of them corresponding to eye parameters.…”

Commentary: Newer insights into methods of intraocular lens power calculation

“…These include regression and vergence methods, employing “thin lens” and “thick lens” models as well as ray-tracing methods. [ 3 4 5 ]…”

“…This provides us with a set of formulae, where each is more appropriate for a given range of axial length than the other. [ 3 4 5 ] The Hoffer-Q formula is most accurate in eyes with a shorter AL while SRK/T and Haigis formulae are better suited for eyes with longer AL. The accuracy of these formulae may differ according to the ACD, even in eyes with the same AL and keratometry.…”