February 04, 2016
3 min read

Intraoperative aberrometry enhances IOL power calculation in astigmatic eyes

Surgeon relates when to use or not to use intraoperative aberrometry for best outcomes.

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With careful use, intraoperative aberrometry provides valuable and otherwise unattainable data to improve refractive outcomes in cataract surgery, according to one surgeon.

“We don’t know what the surgically induced astigmatism is going to be until we make our incision, and although there are devices out there that are starting to measure posterior corneal curvature better, I think aberrometry combines those two hard-to-obtain pieces of information the best,” John P. Berdahl, MD, said at OSN New York 2015.

Intraoperative aberrometry measures corneas with corneal curvature accurately, making it an effective and reliable method for IOL selection and alignment, Berdahl said.

John P. Berdahl

When to trust aberrometry

One reason to trust aberrometry is its usefulness in measuring surgically induced astigmatism, according to Berdahl.

“We all believe that our surgically induced astigmatism is 0.3 D or 0.4 D of flattening on axis, but when I look at my standard deviation, it’s about 0.5 D. And when you look at the literature, it’s about 0.7 D. That means that 68% of the time, you’re within ±0.7 D, and that is a big issue,” Berdahl said. “So I think we’re good on average, but any individual may be different.”

Literature suggests that in routine cases, surgeons achieve their refractive goals within 0.5 D of the intended target about 60% of the time, but with aberrometry in monofocal IOL cases, the rate is about 84%, and with aberrometry in advanced technology IOL cases, the rate is about 86%, according to Berdahl.

“On a virgin eye that has not had any prior corneal surgery, I probably change my lens power 10% of the time — not that frequently. And those are usually small tweaks and changes when I’m using intraoperative aberrometry,” Berdahl said.

In post-refractive situations, “aberrometry wins” against a variety of well-established IOL power calculation methods, Berdahl said, with standard deviation being “much smaller” when aberrometry is used.

In cases of astigmatism, Berdahl said, “On average, people have about 0.3 D to 0.4 D of against-the-rule astigmatism on the posterior cornea. So if we apply the Baylor nomogram, we’re going to be better about four out of five times, but a little less than 20% of the time, people have with-the-rule astigmatism on their posterior cornea. So if you follow the nomogram, one out of five times you’re going to make people worse than they would have been if you had just ignored it in the first place,” Berdahl said. Rather, measuring, through the wavefront, the posterior cornea with aberrometry in these cases is better than making an estimate based on population-based statistics, he said.

If after surgery the patient still has residual astigmatism, Berdahl suggested using astigmatismfix.com, a free website, to help calculate the optimal surgical plan to reduce the residual astigmatism. In an analysis of more than 7,000 patients with toric IOL placement and residual astigmatism, Berdahl and colleagues found that the ideal axis and the intended axis in patients undergoing toric IOL placement was different in 76% of cases.

“Basically what that means is that your preoperative measurements weren’t right if you have residual astigmatism,” Berdahl said.

When to question aberrometry

In certain instances, use of aberrometry is not as trustworthy but can still be helpful, Berdahl said. For example, in patients with high hyperopia in whom effective lens position matters more than in patients whose eyes are longer, the aberrometer only estimates effective lens position based on preoperative calculations. Readings on highly irregular corneas and pseudophakic sphere readings are less reliable with aberrometry, he said. And for post-RK patients, whose eyes are unstable, Berdahl said, “I still use it, but I don’t trust it as much.”

The surgeon still needs to pay attention to the readings, though. If incorrect biometric data are entered, then the readings will not be right.

“You have to make sure the numbers make sense,” he said. – by Kate Sherrer

Disclosure: Berdahl reports he is a lecturer and consultant for Alcon.