Varied spherical aberration profiles influence IOL choice

Lack of parity between wavefront aberration calculations raises questions about the accuracy and reliability of built-in calculation models, researcher says.

Wuqaas M. Munir

Wide variations between topographically derived wavefront aberration calculations significantly affected IOL selection, a study found.

The researchers compared wavefront aberration calculations using the Magellan corneal topographer (Nidek) and VOL-CT software (Sarver and Associates).

“The bottom line was that even though some folks are using corneal aberrations to select aspheric IOLs, that calculation may or may not be the same based on how those calculations are performed,” Wuqaas M. Munir, MD, the corresponding study author, said in an interview with Ocular Surgery News. “It leads us to question the manufacturers’ calculation models and raises the question of whether we need to re-look at some of these calculations that are being done by some of these instruments to ensure that they actually are accurate.”

The findings underscored the need to improve the standardization of wavefront aberration calculations between various instruments, the researchers said.

“I think the study applies particularly to those surgeons who are using corneal wavefront aberrations to choose appropriate aspheric IOLs for their patients,” Dr. Munir said. “I think it raises a flag of caution that if you’re relying on corneal wavefront aberrations to make these selections, it’s not clear whether these calculations are truly reliable, at least for the topographer that we looked at, and it’s something that we need to explore with other topographers as well. The jury is still out whether we have reliable criteria based on current technology to be able to do that.”

The study was published in the Journal of Refractive Surgery.

Measurement methods

The prospective study included 28 eyes of 28 patients. Mean patient age was 65.9 years. Eyes with corneal astigmatism of 4 D or more and eyes with poor topographic images resulting from dry eye syndrome or other factors were excluded from the study.

The Magellan corneal topographer was used to acquire corneal topography for all eyes. Investigators used 28 Zernike polynomial expansions calculated over a 4-mm to 6-mm pupil-centered zone to determine corneal higher-order aberrations.

“We looked at two different pupil sizes and centered our calculations on the same pupil offset,” Dr. Munir said. “We were able to replicate pupil size as well as the actual data. The only difference was the calculation model used. … We looked at 6 mm as well as 4 mm [pupils] and found that there were significant differences in the calculations.”

The researchers used aspheric IOL selection criteria set forth by Mark Packer, MD, and colleagues, Dr. Munir said.

Results and conclusions

Study results showed differences between the two systems’ calculations at 4 mm for fourth-order aberrations and sixth-order aberrations (both P < .05). The data also showed statistically significant differences between calculations at 6 mm for fifth-order aberrations and sixth-order aberrations.

However, investigators did not find statistically significant differences for third-order aberrations, mean total higher-order aberrations or spherical aberration.

Results showed that using Dr. Packer’s spherical aberration-based IOL selection criteria, IOL selection would have differed in 13 eyes if calculations had been made using the Nidek software as opposed to the VOL-CT software, the study authors said.

“We found that IOL selection actually differed based on how the calculations were performed,” Dr. Munir said. “In almost half the 28 eyes, there were significant differences in the IOL that would be selected based on those criteria.”

The results raised questions about the reliability and accuracy of calculation models built into the Nidek software, Dr. Munir said.

“I think there are lot of questions that it raises in terms of … how reliable or how repeatable the measurements are and how transparent [the] companies are in terms of revealing what the calculation models are, and making sure that they are valid calculations,” he said.

A similar analysis with eyes that have undergone previous cornea-based laser refractive surgery may be warranted, Dr. Munir said.

“This suggests that there’s a difference in the calculation model which, again, would imply that no matter what kind of cornea we looked at, there’s the potential that it would be true of refractive surgery patients as well. I think that needs to be looked at,” he said. – by Matt Hasson

Reference:

• Siegel NH, Munir WM. Effect of variation in derived corneal wavefront aberrations by calculation methods on intraocular lens selection. J Refract Surg. 2011;27(11):821-825.

• Wuqaas M. Munir, MD, can be reached at Boston University School of Medicine, Department of Ophthalmology, 85 E. Concord St., 8th floor, Boston, MA 02118; 617-414-2020; fax: 617-414-2929; email: wuqaas.munir@bmc.org.
• Disclosure: Dr. Munir has no relevant financial disclosures.