Better measurements can

Improving cataract outcomes

ELP remains top challenge, better measurements can improve accuracy. Howard Larkin reports

Predicting effective lens position (ELP) remains the biggest obstacle to improving cataract surgery refractive outcomes, in part because it is just that: a “prediction” and not a direct measurement. However, adopting best practices in measuring biometry and corneal power, verifying preoperative data and using the best available intraocular lens (IOL) power formulas help reduce other sources of error, improving cataract surgery refractive outcomes, Douglas D Koch MD told Refractive Surgery Day at AAO 2019 in San Francisco, USA.

Preoperative and intraoperative OCT are improving the understanding of ELP and are improving ELP prediction, but no studies using these methods as yet show superior IOL power calculation accuracy, said Dr Koch, of Baylor College of Medicine, Houston, Texas, USA. One big issue is that ELP shifts unpredictably postoperatively, with nearly 18% of patients experiencing a change in anterior chamber depth of 1.0mm or more between one hour and two months after surgery (Hirnschall et al. JCRS 2018;44:1310-1316).

“We may nail [ELP] on the operating room table, but it may change as the patient heals as we have all seen,” Dr Koch said.

SHORT EYE = BIGGER ERROR ELP is especially problematic in short eyes, reducing the accuracy of IOL power calculations, Dr Koch said. In eyes with axial length shorter than 22.0mm, Dr Koch’s group

We may nail [ELP] on the operating room table, but it may change as the patient heals as we have all seen Douglas D Koch MD Preoperative and intraoperative OCT are improving the understanding of ELP and are improving ELP prediction... Douglas D Koch MD

found the percentage of patients within 0.5D of target post-op varied from 63% to 78% for seven formulas tested, which was not statistically significant (Goekce S et al. JCRS 2017;43(7):802-7). That compares with 91% for eyes 21.0mm to 29.0mm using Warren Hill MD’s RBF formula, which is the overall best performance Dr Koch has seen reported, and 70-to-80% overall in clinical practice and the literature.

With very short eyes, “you have a high power IOL in a small chamber with variable lens thickness and anterior chamber depth preoperatively, and a small shift gives you a lot of refractive change”, Dr Koch explained.

Conversely, lower power IOLs in longer eyes may contribute to better outcomes because it reduces the impact of ELP shift on refractive errors. Outcomes are better with long eyes using modifications Dr Koch and colleagues developed for the Holladay 1 and 2 formulas, and the Barret Universal 2 and the Hill RBF 2 doing an excellent job also (Wang L, Koch DD. JCRS 2018;44:1396-1397).

In a database of 1,664 eyes 25.0mm or longer provided by Dr Hill, substituting the modified axial length formula for the manufacturers’ using the Holladay 1 formula reduced mean refractive error from +0.23D to -0.08D, and increased the percentage within 0.5D of target from 77.2% to 92.9%. Hyperopic outcomes and mean absolute error also were reduced significantly, Dr Koch said.

By comparison, in clinical practice and the literature, 70-to-80% of surgeries are within 0.5 dioptre of predicted refraction, and outcomes are worse for complex eyes (Melles RB et al, Ophthalmology 2018;125:160-178). For post-LASIK eyes with no pre-refractive surgery data, 70% of eyes within 0.5D is about the best current IOL formulae can deliver, Dr Koch said.

REDUCING CORNEA MEASUREMENT ERROR While predicting ELP may be the largest source of post-op refractive error, difficulties in measuring corneal power also contribute, Dr Koch said. Taking multiple corneal measurements and verifying the quality of raw data go a long way toward improving the accuracy of corneal power estimates.

“It is also the one step we can do the most about in practice.”

Other best practices Dr Koch recommended for increasing the accuracy of IOL power calculations are to use accurate measurement devices, such as optical or swept source biometry, and multizone LEDs for K readings. Educating patients about the fallibility of calculations and their impact on outcomes helps keep expectations in line.

“We are approaching an upper limit for accuracy with preoperative measurements,” Dr Koch said. Postop IOL power adjustment will soon make it possible to correct IOL power prediction errors, whether by changing lens curvature or localised refractive index changes, and Dr Koch believes it will become mainstream.

“These are exciting new technologies that will improve our outcomes.”