September 2018

CATARACT

Device focus
New methods for IOL calculations


by Michelle Stephenson EyeWorld Contributing Writer

IOL calculation pearls

Mitchell Weikert, MD,
discusses IOL calculation
methods.


Several new IOL calculation formulas are gaining popularity. According to Douglas Koch, MD, Houston, formulas can be divided into four categories: vergence formulas, which can be subclassified according to the number of variables they take into account; ray-tracing formulas; artificial intelligence formulas; and regression formulas.
“In addition to superb new vergence formulas, particularly the Barrett Universal II, two approaches that are getting a lot of attention now are the ray-tracing formulas and the artificial intelligence approaches,” Dr. Koch said. “Ultimately, I think the ray-tracing formulas will be what we end up using, but they have yet to demonstrate superior outcomes, primarily because they don’t have a better way of calculating effective lens position than any of the other approaches.”
Ray-tracing formulas are based on pure optics, and they take into account such factors as irregular astigmatism of the cornea, cornea asphericity, and IOL parameters, such as anterior and posterior curvature and spherical aberration. “Theoretically, that would give them an advantage and would give them a universal appeal because they could be used in normal eyes but also in eyes with complex corneas that make vergence approaches theoretically more difficult,” he said.
Artificial intelligence approaches, like Radial Basis Function (RBF) from Warren Hill, MD, use big data. “This is an ingenious method of generating internal data relationships that look at different combinations of data and, by doing that, can slot where any given patient’s eye might fall. Using just three key data points—corneal curvature, anterior chamber depth, and axial length—one can determine for that set of measurements the optimal IOL power,” Dr. Koch explained.
According to Elizabeth Yeu, MD, Norfolk, Virginia, the Barrett Universal formula has improved her outcomes, particularly for eyes shorter than 22 mm and eyes longer than 25.5 mm. “In very long eyes, the Barrett Universal performs on its own extremely well. I think that formula has become better over time. While it’s not artificial intelligence, it is refined based on results, so there is some level of artificial tweaking that has occurred since its introduction years ago,” she said.
She noted that the Hill-RBF and the Barrett Universal are her two go-to formulas. “We’ve had a little bit of a sneak peek because Dr. Hill incorporates our patient outcomes information as part of his data systems used to refine the RBF formula. We had a first user’s look at it to see if it was further refined in its results as compared to the original RBF, and it has proven to lead to better outcomes,” she said.

IOL calculation preferences

For standard cases, Dr. Koch uses the Barrett Universal II, the Hill-RBF, and the Holladay 1. Dr. Yeu agreed. “For average axial length, I use Holladay 1 because I still love it. I also use Hill-RBF and Barrett Universal,” she said.
For high myopes, Dr. Koch has been using the Holladay 1 with the Wang-Koch modification. “However, I think that the Barrett and the Hill-RBF are also doing a good job now with the long eyes. For the short eyes, we’re still stymied. We published a paper that showed roughly 70–75% accuracy within 0.5 D for short eyes, and none of the formulas stood out.1 In fact, the old Holladay 1 did as well as, if not better than, any of the other formulas. So the short eyes remain a challenging niche that we have not sorted out yet,” he said.
Dr. Yeu said the Hill-RBF and the Barrett Universal are comparable for very high myopes with an axial length of more than 27 mm. “For eyes shorter than 22 mm, the Barrett Universal, in my hands, has been the most accurate, with the Hill-RBF as a backup to make sure that it doesn’t deviate too much. For patients who are 25.5 to 27 mm, the optimized axial length formula by Wang-Koch using the Holladay 1 is extremely accurate, but the Barrett Universal and the Hill-RBF are equally as accurate,” she said.
For post-refractive surgery patients, Dr. Koch likes the Barrett True K formula, OCT-based, and Haigis. “When we have refractive data, we like the Masket formula as well,” he added.
Dr. Yeu said that the Barrett True K formula, which is now part of the ASCRS post-refractive calculator, has taken her predictability of a 0.5 D spherical equivalent to 86–88%. “It is better now than it used to be because if you look at the ASCRS calculator, while you do not need to fill in every cell because the different cells exist for various devices, it certainly is helpful. The ones I do fill in every time are the four single millimeter zones of the Atlas [Carl Zeiss Meditec, Jena, Germany], as well as the K value of my LENSTAR [Haag-Streit, Koniz, Switzerland]. I don’t use historical information. I use the basic biometry information from my LENSTAR. With that information alone, the average predicted IOL power is extremely accurate using the Barrett True K as well as the other information populated by the ASCRS post-refractive calculator. If you have access to intraoperative aberrometry, which now exists only in ORA [Alcon, Fort Worth, Texas], use it,” she advised.

The future

Dr. Koch thinks that ray-tracing formulas are where cataract surgery is headed. “We have to figure out a better way to determine the effective lens position, and I’m becoming more and more of a skeptic about our ability to predict that. I think there is going to be a certain number of eyes in which the effective lens position is going to surprise us and where our accuracy is not going to be as good as we want. While we need better approaches for estimating effective lens position, we’re still going to rely postoperatively on corneal refractive surgery and the ability to modify IOLs postoperatively, such as those we have seen with RxSight [Aliso Viejo, California] and Perfect Lens [Irvine, California].”

Reference

1. Gökce SE, et al. Intraocular lens power calculations in short eyes using 7 formulas. J Cataract Refract Surg. 2017;43:892–897.

Editors’ note: Dr. Yeu has financial interests with Alcon, Johnson & Johnson Vision (Santa Ana, California), and Carl Zeiss Meditec. Dr. Koch has financial interests with Carl Zeiss Meditec and Perfect Lens.

Contact information

Koch: dkoch@bcm.edu
Yeu: eyeu@vec2020.com

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