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Subspecialties Refractive, Cataract

One IOL Formula… To Rule Them All

There a number of IOL power calculation formulae out there in the wild, but there’s still debate as to which one’s the best under which circumstance, and often, that answer isn’t clear. There can be such great differences between the types of lenses – and the conditions under which they are implanted – that ophthalmologists frequently have to choose formulae based on each one’s strengths and weaknesses. But if the surgeon is not be aware of (or does not have access to) the full spectrum of formulae, suboptimal outcomes are risked. So, to overcome these potential obstacles, researchers from the Wilmer and Jules Stein Eye Institutes (Baltimore and Los Angeles) devised a new graphical calculation method that combined the most accurate portions of several existing formulae into a single super formula (1).

The team used a computer simulation to generate 3D surfaces based on five existing IOL formulae, then analyzed each surface to determine the points at which the IOL powers from each formula showed the greatest difference. Finally, they generated a 3D “super surface” that incorporated the ideal portions of four different formulae to form the super formula. Once the formula was established, it was tested on a set of 100 eyes, which had their IOL powers calculated using each of the five original formulae and the super formula. In every case, the super formula localized to the correct portion of the super surface and chose the most appropriate IOL power value for the patient in question. How did the standard formulae compare? Their values deviated from those chosen by the super formula in 16 to 48 percent of cases (see Table 1).

Table 1. Percentage of cases in which each standard formula’s calculated IOL power deviated from that of the super formula by more than 0.5 D.
Formula Deviation from super formula by >0.5 D
Holladay I 16%
Holladay I with Koch adjustment 22%
SRK/T 24%
Hoffer Q 30%
Haigis 48%

As premium lenses diversify and patient expectations increase, it’s becoming more challenging for doctors to meet those expectations consistently. The researchers hope that their IOL super formula – which is able to evolve with new developments and new data – will help surgeons to both understand the relative merits and weaknesses of individual formulae and optimize individual calculations to improve clinical outcomes for their patients.

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  1. JG Ladas, et al., “A 3-D ‘super surface’ combining modern intraocular lens formulas to generate a ‘super formula’ and maximize accuracy”, JAMA Ophthalmol, [Epub ahead of print] (2015). PMID: 26469147.
About the Author
Michael Schubert

While obtaining degrees in biology from the University of Alberta and biochemistry from Penn State College of Medicine, I worked as a freelance science and medical writer. I was able to hone my skills in research, presentation and scientific writing by assembling grants and journal articles, speaking at international conferences, and consulting on topics ranging from medical education to comic book science. As much as I’ve enjoyed designing new bacteria and plausible superheroes, though, I’m more pleased than ever to be at Texere, using my writing and editing skills to create great content for a professional audience.

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