IOL power calculations advance for accuracy

Lens-calculation formulas continue evolving as surgeons learn the factors needed for better refractive outcomes.

By Vanessa Caceres

Determining the correct power of a patient’s intraocular lens (IOL) isn’t as simple as focusing binoculars. It involves extensive measurement and calculation, especially when calculating for those picking up the whole tab, the premium IOL patients.

These higher levels of calculations didn’t always exist.

“In 1977, the state-of-the-art for estimating IOL power for emmetropia was to simply add +19.0D to the pre-cataractous refraction,” recalls Warren E. Hill, MD, East Valley Ophthalmology, Mesa, Ariz., on his web site. Even a decade later, it was considered acceptable to be within plus or minus 1D of the intended target refraction, Dr. Hill remembers.

Evolution

Today, with greater understanding of the factors affecting IOL power — and a growing patient demand for accuracy — surgeons and researchers are integrating this newfound understanding into IOL calculations. The knowledge they are incorporating has evolved over the past 10 years, enabling surgeons to get within either plus/minus 1D or 0.5D of the intended refraction more frequently.

“There’ve been a number of areas where we have learned a lot,” says Douglas D. Koch, MD, the Allen, Mosbacher and Law chair in ophthalmology, Cullen Eye Institute, Baylor College of Medicine, Houston. “We’ve confirmed that the determination of the effective lens position [ELP] is a critical issue, and all of the formulas are doing their best to estimate that.

“[But] In the absence of a more direct way of measuring ELP, it remains a major a source of error.”

Another evolutionary area considered in toric IOL calculations is the posterior cornea, about which Dr. Koch and colleagues originally reported.¹ “We realized that the posterior cornea not only contributes to astigmatism but also to our measurements for the spherical power of the IOL, which assumes posterior curvature based on a fixed ratio that is sometimes inaccurate,” he says. Also, without considering posterior corneal astigmatism, a patient’s postoperative uncorrected vision can be somewhat degraded.

“Doug’s paper shows us that the posterior cornea astigmatism is about .25D or .37D against the rule,” says Jack T. Holladay, MD, clinical professor of ophthalmology, Baylor College of Medicine, Houston.

“The bottom line of his paper and the Baylor nomogram is that if you’ve got with-the-rule astigmatism, you drop down one toric step size. If you have against-the-rule astigmatism, you go up one toric step size.”

Adds Dr. Koch: “For eyes with higher amounts of with-the-rule anterior corneal astigmatism, the posterior cornea contributes even more in the opposite direction, so we need to back off the power even more.”

However, Dr. Holladay explains, Dr. Koch’s work also stated that this general rule of thumb doesn’t apply 10% to 15% of the time, which is why measurements with Scheimpflug technology—to accurately assess the posterior cornea—have become more important.

D. Rex Hamilton, MD, associate clinical professor of ophthalmology and director of the Laser Refractive Center, Stein Eye Institute, University of California, Los Angeles, and colleagues found similar results to Dr. Koch’s findings regarding posterior astigmatism. “We noted that, on average, the residual refractive astigmatism following toric IOL implantation matched the preoperative posterior corneal astigmatism that was unaccounted for.”²

Says Dr. Hill: “Having some strategy regarding the posterior corneal astigmatism for the toric IOL is required for best outcomes.” Surgeons can also factor in the influence of the posterior cornea by using the Barrett Toric Calculator, which does this automatically; or it can be manually added to Dr. Koch’s Baylor nomogram.

Further evolution

Another area that has helped IOL formulas evolve in recent years has been the correction of longer axial length with the Wang-Koch adjustment, says Dr. Hamilton.³ The adjustment is part of the common IOL power calculation formulas currently used, and it’s a critical step because without it, the surgeon will likely get hyperopic surprises in eyes longer than 25 mm. “The longer the length, the bigger the surprise,” Dr. Hamilton says.

“Our standard calculations for long eyes are inaccurate,” explains Dr. Koch. “They were based on assumptions about speed of light in a long eye that are incorrect.” Besides the Wang-Koch formula, there are also newer adjustment formulas for eyes with longer axial lengths, he adds.

Surgeons also have discovered that short eyes (< 22 mm) require special attention, Dr. Koch says. For higher lens powers, the IOL may sit more anteriorly than expected, which requires adjustment.

Higher-order aberrations appear to have more influence on IOL calculation accuracy than previously thought, Dr. Koch says. “This applies to eyes with previous corneal refractive surgery and normal eyes. It’s a point of further investigation.”

Finally, IOL calculation formulas have evolved to include more variables that lead to more accurate results, including axial length, keratometry readings, anterior chamber depth, lens thickness, and horizontal corneal diameter, notes Dr. Holladay, who created the Holladay 1 and Holladay 2 formulas, the latter of which includes the additional variables. “Those variables give us better accuracy. It’s more knowledge about the anatomy of the eye to determine ELP.”

Is There One Best Formula?

The surgeons interviewed may have their personal favorite IOL calculation formulas, but it’s not a blanket approach. Surgeons must combine their personal A constants with newer technology to aim for better precision.

For example, Dr. Holladay explains that a formula like the Holladay 2 will get about 80% of patients within plus or minus .5D of the intended power. That’s in contrast to his Holladay 1. “If you don’t personalize your lens constant and you use an older formula, it’ll be 60% to 70%,” he says. However, with the use of intraoperative aberrometry or newer technology, surgeons can bump their intended target up to 90%, he says.

“There are pros and cons with all the formulas,” says Dr. Koch. He still gets fabulous results with the Holladay 1; he uses the Haigis formula and, to a certain extent, the Olsen formula for short eyes. He also finds the Barrett formula helpful for normal and short eyes.

Dr. Hill looks to the Olsen formula for moderate myopia to high axial hyperopia; the Barrett Universal II formula for mild axial hyperopia to extreme axial myopia; he notes that neither formula requires axial length correction. “Holladay 2 is also a good choice but gives improved outcomes if the axial length is adjusted using the Wang-Koch method for eyes with an axial length greater than 26 mm,” he says. In toric IOL patients, Dr. Hill uses the Barrett Toric calculator.

Future evolution

In the future, IOL formulas could lose their luster. “The most recent original work in IOL power selection does not involve formulas but rather a sophisticated form of pattern recognition and data interpolation,” Dr. Hill says. “Initial results suggest this approach is on par with the very best theoretical formulas, and in some situations may actually perform better. This may be the next stage of evolution.”

Dr. Hamilton also sees IOL formulas losing their status, even becoming obsolete, thanks to personalization with intraoperative aberrometry systems. “If we can personalize our constants and feel confident about them, there’s not a real reason to do preoperative measurements.”

Dr. Holladay believes Scheimpflug imaging will become a larger part of preoperative planning because of its ability to fully measure the back and the front of the eye, rather than assuming what the back surface is. “Scheimpflug will give us both surfaces, and we’ll get a better measure of the total power, which you need for better prediction of the IOL power to get a desired refraction.” OM

For more information about Scheimpflug imaging, and to read about the new Holladay Report, please turn to page 54.

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