Making IOL power calculation simpler, more accurate

February 1, 2014

Calculating intraocular lens (IOL) powers has become simpler and more accurate in eyes with long and short axial lengths with use of intraoperative aberrometry and an optimized method of regression analysis.

 

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Calculating intraocular lens (IOL) powers has become simpler and more accurate in eyes with long and short axial lengths with use of intraoperative aberrometry and an optimized method of regression analysis.

 

Dr. Donnenfeld

By Lynda Charters; Reviewed by Eric D. Donnenfeld, MD

New York City-Calculating intraocular lens (IOL) powers has become simpler and more accurate in the most difficult cases-eyes with long and short axial lengths with use of intraoperative aberrometry and an optimized method of regression analysis.

“IOL calculation is paramount for obtaining good refractive results,” said Eric Donnenfeld, MD.

Dr. Donnenfeld reviewed the literature on refractive outcomes after IOL implantation and said he discovered that one recent large study of over 17,500 eyes conducted by Behndig and colleagues (J Cat Refract Surg 2012; 38: 1181-6) found that the mean average refractive prediction error was 0.5 D. Only 55% of eyes achieved this refractive target, and that percentage was a global one in a population that included eyes with all axial lengths.

“Calculating the IOL power in long and short eyes is less predictable,” said Dr. Donnenfeld, founding partner of Ophthalmic Consultants of Long Island and Connecticut, clinical professor of ophthalmology, New York University Medical Center, New York, and a trustee of Dartmouth Medical School.

He reported the postoperative refractive results using a new method of calculating IOL powers for eyes with long and short axial lengths using the ORA intraoperative aberrometry (WaveTec Vision) and a refined method of regression analysis.

The IOL power formula

The ORA aberrometer measures the aphakic refraction and not the axial length. Dr. Donnenfeld explained that the formula incorporates the aphakic spherical equivalent into a refractive vergence formula. Regression coefficients are then applied as part of the calculation process used to calculate the effective lens position.

Each lens has a unique set of regression coefficients so that each lens must be studied. As more cases are added to the study, he noted, the regression analysis can be increasingly refined.

Dr. Donnenfeld and his colleagues have refined the optimized lens models for different axial lengths, specifically in six axial length groups: 21.99 mm or less (short eyes), 22.00 to 22.99 mm, 23.00 to 23.99 mm, 24.00 to 24.99 mm, 25.00 to 25.99 mm, and 26.00 mm or more (long eyes).

Retrospective study

The investigators conducted a retrospective multicenter analysis of outcomes achieved with the SN60WF IOL (AcrySof IQ IOL, Alcon). The study included two groups: group 1, in which they evaluated a pre-refined optimization process that was not specific for the six axial length groups; and group 2, in which they evaluated a post-refined optimization process using a formula that was refined for each axial length group.

In both groups, special attention was paid to the long and short eyes. The IOL power calculations were performed using the ORA aberrometer, which Dr. Donnenfeld said he believes is an “indispensable” operating room tool for determining refractive outcomes and honing his refractive results.

He reported that in group 1 (prerefined optimization) with 3,046 eyes, 80% had 0.5 D or less of ametropia with a mean average refractive prediction error of 0.32 D.

“(However), with those eyes, the prediction value of intraoperative aberrometry and the various formulas used to calculate the IOL powers broke down,” Dr. Donnenfeld said.

When the investigators looked specifically at the short eyes, the results before optimization showed that the outcomes were within 0.5 D of emmetropia in 60% of the 119 short eyes and in about 79% of the 189 long eyes in that group.

In group 2 (post-refined optimization) that included 142 short eyes and 227 long eyes, he pointed out a notable increase in the percentage of short eyes that were within 0.5 D or less of emmetropia, i.e., 73%; 81% of the long eyes reached that benchmark.

Globally, in 4,184 eyes that were analyzed post-refined optimization, 82% of eyes were within 0.5 D of emmetropia, he reported.

“This is an extraordinarily good result and much better than that reported in the literature,” Dr. Donnenfeld said.

When the data was optimized further with the newest intraoperative aberrometry with VerifEye, that global figure increases to 86%, he added.

“This technology is valuable for improving refractive outcomes in some of the most difficult cases managed on a daily basis, the high hyperopes and high myopes,” Dr. Donnenfeld said.

WaveTec’s refined optimization process, used with measurements provided by the ORA aberrometer, improved the refractive outcomes in eyes with unusual axial lengths that were implanted with the SN60WF IOL, he said. Improvements were also seen globally in eyes that did not have short or long axial lengths.

ORA with VerifEye (WaveTec Vision)-hardware recently introduced into the ORA aberrometer-using the same refined optimized process, provided improved outcomes compared to the use of ORA alone.

“The case for the use of intraoperative aberrometry is now indisputable because it allows analysis of posterior corneal astigmatism, the cylinder induced by incisions, and provides results that look at the visual axis rather than the corneal vertex,” Dr. Donnenfeld said.

 

Eric D. Donnenfeld, MD

E: ericdonnenfeld@gmail.com

Dr. Donnenfeld is a consultant to WaveTec Vision.

 

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