ASCRS IOL calculator power predictions depend on accurate input

Methods using surgically induced change in refraction only and no prior data produce more accurate, consistent results than historical methods.

IOL power calculations that use surgically induced changes in refraction or no previous data proved more accurate than methods using keratometry values prior to corneal refractive surgery and surgically induced changes in refraction, a study found.

The authors used the American Society of Cataract and Refractive Surgery IOL power calculator to gauge the accuracy of IOL power prediction methods after previous LASIK or PRK.

Making accurate IOL power calculations after previous corneal refractive surgery is difficult and time-consuming, Li Wang, MD, PhD, and colleagues Warren Hill, MD, and Douglas D. Koch, MD, said in the study.

“We found that the method using no prior data at all or using only part of the previous data was superior to the method using pre-LASIK or pre-PRK Ks and the change in manifest refraction,” Dr. Wang told Ocular Surgery News. “Rely on methods that use no prior data at all or the method of using changes in manifest refraction only. Pay more attention to methods in those two categories.”

Power calculation methods

The Web-based ASCRS calculator generates IOL power for eyes undergoing cataract surgery with previous LASIK, PRK or radial keratotomy. It classifies methods using keratometry values prior to LASIK or PRK and change in manifest refraction resulting from LASIK or PRK; surgically induced change in refraction; or no previous refractive data.

IOL power is calculated using the Aramberri double-K method modification of the Holladay 1 formula, which uses corneal power prior to LASIK or PRK to estimate effective lens position. A default value of 43.86 D is used when corneal power prior to LASIK or PRK is not available.

“If we have a pre-LASIK corneal power, the Holladay 1 uses that value to calculate the lens position,” Dr. Wang said. “If the patient does not have that value, the formula we are using, the so-called double-K Holladay 1 formula, uses a default value.”

The clinical history method has been considered the standard approach to calculating IOL power in eyes with previous refractive surgery. However, that method is reliable only when historical data are accurate; inaccurate historical data can skew final IOL power calculations, Dr. Wang said.

A 1 D inaccuracy in historical data yields a 1 D error in final refraction at the corneal plane, she said.

Patients and parameters

The retrospective study, published in the Journal of Cataract and Refractive Surgery, included 72 eyes of 57 patients with a mean age of 58 years. Refractive data prior to LASIK or PRK were available for 70 eyes, and keratometry values prior to LASIK or PRK were available for 62 eyes.

All patients underwent implantation of an AcrySof SN60WF aspheric IOL (Alcon), had no intraoperative or postoperative complications, and had postoperative corrected distance visual acuity of 20/32 or better.

Outcome measures included mean arithmetic IOL prediction error, mean absolute IOL prediction error, variance in mean arithmetic IOL prediction error and percentage of eyes within a certain refractive prediction error (IOL prediction error of 0.71 D or refractive error at the spectacle plane of 0.5 D, and IOL prediction error of 1.43 D or refractive error at the spectacle plane of 1 D, assuming that 1 D of IOL prediction error resulted in 0.7 D of refractive error at the spectacle plane).

IOL power prediction errors

Study results showed that mean arithmetic IOL prediction errors were as follows:

• –0.66 D to –0.3 D for IOL power calculation methods using keratometry values prior to LASIK or PRK and surgically induced changes in refraction
• –0.25 D to 0.49 D for methods using surgically induced changes in refraction
• –0.24 D to 0.18 D for methods using no previous data

Mean absolute IOL prediction errors were as follows:

• 1.1 D to 1.31 D for methods using values prior to LASIK or PRK and surgically induced changes in refraction
• 0.62 D to 0.71 D for methods using surgically induced changes in refraction
• 0.57 D to 0.69 D for methods using no previous data

Percentage of eyes in each group achieving ±0.5 D of the predicted refraction and ±1 D of the predicted refraction were as follows:

• 37% to 44% of eyes within 0.5 D and 60% to 69% of eyes within 1 D in the group using keratometry values prior to LASIK or PRK and surgically induced changes in refraction
• 57% to 67% of eyes within 0.5 D and 86% to 91% of eyes within 1 D in the group using surgically induced changes in refraction
• 58% to 60% of eyes within 0.5 D and 90% to 96% of eyes within 1 D in the group using no previous data

Further study is devoted to gauging the accuracy of IOL calculation methods in eyes with previous hyperopic laser vision correction, Dr. Wang said.

“We are collecting those data because hyperopic eyes are not as numerous as the myopic eyes. Also, the change in refraction is normally smaller in hyperopic eyes. We just have a smaller sample for those groups,” she said. – by Matt Hasson

• Li Wang, MD, PhD, can be reached at Cullen Eye Institute, Baylor College of Medicine, 6565 Fannin St., NC-205, Houston, TX 77030; 713-798-7946; fax: 713-798-3027; e-mail: liw@bcm.edu.

One of the most significant challenges for refractive cataract surgeons is to provide the best possible uncorrected vision to patients undergoing cataract surgery following previous LASIK or PRK. These patients are accustomed to good uncorrected visual acuity, and the refractive results following cataract surgery have been variable. The authors have reviewed their results with the ASCRS website that allows physicians to use varied sources of data, including post-refractive surgery keratometry and historical information. The authors conclude that the surgically induced variables provide better outcomes than the historical method. This study documents the usefulness of the ASCRS website and the importance of using surgically induced changes in the cornea.

– Eric D. Donnenfeld, MD
OSN Cornea/External Disease Board Member