May 2019


Is it time to change how we talk about IOL calculation formulas?

by Liz Hillman EyeWorld Senior Staff Writer

First, second, third, and fourth generation: Most cataract surgeons are familiar with describing IOL power calculation formulas in such terms. But this is something that Douglas Koch, MD, Mitchell Weikert, MD, and others say the field should get away from, favoring instead a reclassification of the formulas based on how they work.
The proposed reclassification divides the formulas into the following categories: vergence, artificial intelligence, ray tracing, and combination.
Overall, such a reclassification “will help us communicate better,” Dr. Weikert said.
Dr. Koch said that he and colleagues Li Wang, MD, Warren Hill, MD, and Adi Abulafia, MD, recommended reclassifying formulas based on their mechanism—and abandoning the “generation” terminology—in the first of a 2017 four-part editorial series in the Journal of Cataract & Refractive Surgery.1 In the past, he said, while experts in the field had often talked about IOL calculation formulas in terms of function, the common lingo defaulted to generations.
One reason to move away from this, Dr. Koch and Dr. Weikert explained, is because some people may assume that later generations are better than earlier ones, which isn’t necessarily the case. Dr. Weikert also said that as the formulas are built upon and refined, the generation lingo starts to break down.
Talking about the formulas based on how they work gives surgeons a better understanding of the limits and the assumptions that go into them. If you know how the formulas are derived and how they come about, it can help formulate your decision making. It can also help people make connections when they understand the common etiology, Dr. Weikert said. Dr. Koch said he uses the Holladay 1, Barrett, Hill-RBF, and Olsen. If using the “generations” classification, some might question his use of a third-generation formula (the Holladay 1). Dr. Koch, in its defense, said, “it’s a two-variable vergence formula that happens to be so good I still use it.”
“Also to Dr. Weikert’s point, if we understand the mechanism, then we are better able to tailor our formula choice for any given patient,” Dr. Koch said.
Dr. Koch said he thinks that the shift in classification would be adopted more quickly if the editors of medical journals started requiring authors to refer to IOL calculation formulas based on their mechanism instead of generations.

IOL calculation formulas: functional classification
Source: Mitchell Weikert, MD

About the doctors

Douglas Koch, MD

Professor and Allen, Mosbacher, and Law Chair in Ophthalmology
Baylor College of Medicine

Mitchell Weikert, MD
Associate professor
Department of Ophthalmology
Baylor College of Medicine Houston


1. Koch DD, et al. Pursuing perfection in intraocular lens calculations: I. Logical approach for classifying IOL calculation formulas. J Cataract Refract Surg. 2017;43:717–718.

Financial interests

: Alcon, Carl Zeiss Meditec
Weikert: None

Contact information


Is it time to change how we talk about IOL calculation formulas? Is it time to change how we talk about IOL calculation formulas?
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