Steering evolution

One of the most critical tools for refractive cataract surgeons is arguably the intraocular lens (IOL) power calculation. It is our decoder for optimal IOL selection and a preoperative and intraoperative guide leading us to the best possible refractive outcome. Advances in the methodology and capabilities of these vital calculations have evolved tremendously in recent years, moving our calculations closer to prediction than estimation.

In 2008, Norrby concluded that the greatest sources of refractive errors are lens position, postoperative refraction, and axial length (AL).¹ The new optical biometry tools have increased the accuracy of measuring the AL, leaving the most room for error in optimal lens placement and refraction. Through case studies, Dr. Douglas Koch carefully details the concerns and challenges of IOL calculations and corneal measurements in his LXXIII Edward Jackson Memorial Lecture, “The Enigmatic Cornea and Intraocular Lens Calculations.”² Koch realizes that “a key step in improving the accuracy of our IOL calculations will be to obtain more accurate and comprehensive measurements of corneal power in normal corneas, abnormal corneas, and corneas that have been surgically modified.”²

Accurate preoperative measurements of the eye are crucial for a good postoperative result, as the IOL calculation is based on these precise measurements. At the 2015 annual meeting of the American Academy of Ophthalmology, biometry specialist Dr. Warren Hill presented findings of the mathematical approach of radial basis function (RBF).^A Hill described that the use of artificial intelligence and pattern recognition creates a new and more accurate way of selecting IOLs.^A RBF algorithms are used in a range of technologies, such as facial recognition software, thumbprint scanners, and electrocardiogram interpretation. Hill says, “In creating the Hill-RBF algorithm for IOL power selection, the objective was to discover if the power of an intraocular lens could be predicted significantly better than by using vergence formulas.”^A,B

The editorial board and contributing authors of the Journal of Cataract & Refractive Surgery have diligently kept up with the cutting-edge trends in power calculations. For the 50th anniversary of the publication of Fedorov’s paper describing the first theoretical IOL calculation formula,³ in 2017 the Journal of Cataract & Refractive Surgery published 4 Guest Editorials on the topic of perfecting IOL calculations.^4–7 In 4 parts, the authors discussed multiple facets of IOL calculation formulas, including measurement errors, IOL constants, criteria for outcomes analysis, and astigmatism analysis for IOL-based surgery. In the first part, the authors suggested that the terminology for classifying IOL calculations be classified by their method of calculating, and not by generational labels. The proposed classifications are historical/refraction based, regression, vergence, artificial intelligence, and ray tracing.⁴ In the second and third segments, measurement errors, validation criteria, and analyzing outcomes are discussed in detail.^5,6 In the fourth segment, astigmatism analysis is given a careful review and a surgically induced astigmatism calculator is posted online.⁷

In this issue, we feature 4 articles that cover various aspects of IOL power calculations and add to the pool of information. Sudhakar et al. (page 719) compares preoperative biometry and intraoperative aberrometry in short eyes. Predicted residual refractive error was calculated preoperatively using HofferQ,⁸ Holladay2,^C Haigis,⁹ Barrett Universal II,^D and Hill-RBF,^B and intraoperatively using intraoperative aberrometry. Their results show that intraoperative aberrometry is not significantly different from the best preoperative biometry methods available.

Liu et al. (page 725) adds to the study that Wang et al. conducted in 2011 and further investigates the accuracy of IOL power calculation in long eyes.¹⁰ Their study shows that the Barrett, Hill-RBF, and original and modified Wang-Koch AL adjustment formulas reduced the percentage of hyperopic outcomes, with the incidence of hyperopic outcomes being significantly lower with the original Wang-Koch formula.

Rong et al. (page 732) continues the research that Terzi et al. conducted in 2009, among others, and focuses on eyes with extreme myopia as they compare the accuracy of the Barrett Universal II, Haigis, and Olsen formulas.^11,12 Their results show that the accuracy of the Haigis formula in myopic eyes was affected by the AL and the corneal keratometry value, whereas the accuracy of the Barrett Universal II and Olsen formulas was affected by AL only.

Also featured in this issue is a pilot study by Hirnschall et al. (page 738) investigating a new method of evaluating refractive prediction errors using ray-tracing–based IOL power calculation. Ray tracing is an exciting new procedure that uses exact Snell’s law and provides a real simulation of the human eye. Ray-tracing calculation is a new trend that presumably offers a more precise corneal power calculation because minute corneal topography details are factored in while calculating the true geometrical position.

We are grateful for the authors of the JCRS IOL Guest Editorials, and all authors who continue to drive the field of IOL calculation toward greater accuracy. With the new iterations to the existing formulas, our prediction error will shrink and we will truly move from estimation to prediction. It is exciting to be part of a journal that is participating in the ophthalmological evolution.