June 2019

CORNEA

Presentation spotlight
IOL power calculations incorporating intraoperative aberrometry


by Stefanie Petrou Binder, MD EyeWorld Contributing Writer

A study involving nearly 2,000 eyes suggests that calculations incorporating intraoperative aberrometry (IA) outperform preoperative calculations, with the difference even more pronounced when IA suggested a different IOL power from preoperative measurements.

Evolving beyond formulae

The evolution of IOL power calculations has resulted in improved, more reliable refractive outcomes in cataract surgery. But achieving the predicted postoperative spherical equivalent is still difficult in a high percentage of eyes, with just over half of eyes demonstrating a residual refractive cylinder of ≤0.50 D, according to one study,1 only 55% of cases reaching actual emmetropia according to another study, 2 and long axial lengths presenting a continued challenge to obtain target outcomes.3
Intraoperative aberrometry, through its use of an aphakic refraction-based calculation, has been shown to help improve refractive outcomes in cataract surgery, according to Michael Breen, OD, who spoke at the 36th Congress of the European Society of Cataract and Refractive Surgeons. While IA has shown encouraging outcomes, he said extensive data is lacking on how it could improve refractive outcomes in long eyes.
IA in patients with bilateral cataracts undergoing toric IOL implantation increased the proportion of eyes with postoperative refractive astigmatism of 0.50 D or less and reduced the mean postoperative refractive astigmatism at 1 month, compared to standard methods, according to a study.4 An unrelated study found 67% of eyes with prior myopic LASIK or PRK came within ±0.5 D of IA’s predicted outcome, compared to 46% with conventional preoperative methodology.5

Assessing IA

In a retrospective analysis, Dr. Breen looked for differences between the absolute prediction error using an IA driven calculation for IOL power determination and the absolute prediction error that would have resulted had the surgeon’s preoperative plan been followed.
He collected postoperative data from the ORA System (Alcon) from multiple centers in the U.S. The ORA Analyzor database stores patient preoperative, intraoperative, and postoperative data on a secure server. The database had more than 1 million cataract surgeries recorded, roughly 300,000 of which included postoperative data. Of those, 35,766 entries had the specified inclusion criteria, 1,786 of which had axial lengths over 26.5 mm and a single-piece acrylic IOL platform.
Dr. Breen hypothesized that the absolute prediction error would be lower with IA and the proportion of eyes with postoperative absolute prediction error ≤0.50 D of the predicted postoperative SE would be higher with IA compared to the preoperative plan. Ten percent of the eyes were randomly selected for hypothesis development and 90% were used for hypothesis confirmatory analysis.
None of the eyes had a history of surgery, prior refractive surgery, or ocular disease. IA calculations were driven by the measured aphakic spherical equivalent (SE).
“The calculation we did for prediction error was fairly basic,” Dr. Breen said. “It was the IA predicted postoperative SE minus the actual postoperative SE. The absolute prediction error for the preoperative calculation was the difference between the power implanted and the IOL power based on the preoperative calculation, actual postoperative SE, and the back-calculated postoperative SE prediction error had the preoperative IOL power been implanted,” he said.
The results showed that both the mean and median preoperative prediction errors were significantly greater than the IA prediction error. “There were significant differences in both PEs, that is the mean absolute and the median absolute PE, not in all the cases but especially in the cases where there was a difference in lens power that was calculated by IA compared to the preoperative, comprising about 51% of the cases,” Dr. Breen said. “These differences in prediction error can be misleading sometimes because they look like they are very small differences. But they result in a big difference in the eyes that end up within a half diopter of predicted postoperative spherical equivalent,” he said.
The difference in mean absolute value of the prediction error was 0.12 between intraoperative aberrometry and preoperative formula-calculated value (P<.0001). The difference in the absolute median error was also 0.12. As far as the percentage of eyes with a prediction error ≤0.50 D, in the completed data set the IA percentage was 82.4% while the preop calculation would have resulted in 68.3%.
“In those cases where the aberrometry recommendation was different from the preoperative plan, the differences were more pronounced,” he explained. “The mean absolute value of the prediction error for cases in which the IA recommended an IOL power that differed from the preoperative IOL power calculation (n=913) showed a difference of 0.19 (P<.0001) between intraoperative aberrometry and the preop formula calculation. The absolute median error in cases in which the IA recommendation differed from the preoperative IOL power calculation (n=913) showed a difference of 0.21 (P<.0001),” he said. In these cases, the percentage of eye with an absolute prediction error ≤0.50 D was 86.7% for IA compared to 59.6% had the preop calculated IOL power been implanted.
Additional analyses are needed to determine whether the level of experience of a surgeon influences the results. Dr. Breen thinks the differences may be even more pronounced for more experienced surgeons or when using more advanced power calculation formulas, which might narrow the gap.

About the doctor

Michael Breen, OD

Head, Medical Science, Surgical and Vision Care
North America Clinical Development and Medical Affairs
Alcon

References

1. Holland E, et al. The AcrySof Toric intraocular lens in subjects with cataracts and corneal astigmatism: a randomized, subject-masked, parallel-group, 1-year study. Ophthalmology. 2010;117:2104–11.
2. Behndig A, et al. Aiming for emmetropia after cataract surgery: Swedish National Cataract Register study. J Cataract Refract Surg. 2012;38:1181–6.
3. Wang L, et al. Optimizing intraocular lens power calculations in eyes with axial lengths above 25.0 mm. J Cataract Refract Surg. 2011;37:2018–27.
4. Woodcock MG, et al. Intraoperative aberrometry versus standard preoperative biometry and a toric IOL calculator for bilateral toric IOL implantation with a femtosecond laser: One-month results. J Cataract Refract Surg. 2016;42:817–25.
5. Ianchulev T, et al. Intraoperative refractive biometry for predicting intraocular lens power calculation after prior myopic refractive surgery. Ophthalmology. 2014;121:56–60.

Financial interests

Breen
: Alcon

Contact information

Breen: michael.breen@alcon.com

IOL power calculations incorporating intraoperative aberrometry IOL power calculations incorporating intraoperative aberrometry
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