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Posterior corneal power measurements may enhance accuracy of IOL calculations
Intraoperative posterior corneal astigmatism measurements may give clinicians a more accurate reading of the eye’s total refractive power.
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Clinicians should factor higher-order aberrations and posterior corneal curvature into IOL power calculations, a lecturer said.
“Measuring posterior corneal power should improve the accuracy of IOL power calculations. It makes total sense. If we could measure the entire cornea, we ought to be able to nail the corneal element,” Douglas D. Koch, MD, said during the Kelman Lecture in the Innovator Session at the American Society of Cataract and Refractive Surgery meeting in Chicago.
Rising patient expectations will spur the development of more thorough and innovative measurement methods, Koch said.
“Right now, our patients are waiting for us to solve these problems and provide them with the best possible vision for their lifetimes. I think the future is extremely promising. We have a lot to be done but we’re up to this task,” he said.
Koch presented a new toric IOL nomogram that accounts for posterior corneal astigmatism and is proving to be more predictable than the standard approach that accounts for only anterior corneal stigmatism, he said.
Koch’s colleagues in this work are Li Wang, MD, PhD; Mitchell P. Weikert, MD; Richard B. Jenkins, MD; and Elizabeth Yeu, MD.
Higher-order aberrations, depth of focus
Higher-order corneal aberrations can increase depth of focus in eyes with a history of corneal refractive surgery, Koch said.
“Higher-order aberrations of the cornea offer us an opportunity because, if we understand them better … we’re going to be able to predict the quality of vision and depth of focus for each of our patients,” he said. “We’re going to be able to match the IOL to the corneal higher-order aberrations to achieve the desired optical outcome. And we’re going to be able to design new IOLs that have higher-order aberrations to maximize the desired optical outcome.”
Currently, higher-order aberrations are not used to select dioptric IOL power for healthy eyes, he said.
“But I think this will change in the future,” Koch said. “Progress is being made. Currently, I think that spherical aberration is sort of lost in the noise of the errors and other measurement variability. However, corneal higher-order aberrations do play a critical role in calculating IOL power in eyes that have undergone corneal refractive surgery. But we have a long way to go.”
One downside is the potential for higher-order aberrations to diminish image quality, Koch said.
“I say potential because it depends very much upon the mix and magnitude of the various aberrations,” he said.
Posterior corneal astigmatism
Posterior corneal astigmatism should be factored into IOL calculations for eyes that have undergone refractive surgery, Koch said.
“We should try to measure these patients preoperatively. Posterior corneal astigmatism measurements are not perfect, but they’re getting better,” he said. “I think they’re useful, and we use a combined dual Scheimpflug-Placido topography regularly in toric IOL patients. Intraoperatively now we’re seeing some benefits of detecting this astigmatism by measuring the entire refractive power of the eye on the table after the crystalline lens has been removed.”
Calculations of corneal astigmatism based solely on anterior corneal surface measurements may result in over- or under-correction, Koch said.
Map from the Galilei dual Scheimpflug analyzer. The simulated keratometry astigmatism is 1.69 D at 78°, posterior corneal astigmatism is –0.71 D at 82°, and the total corneal power astigmatism using the ray tracing method is 1.18 D at 75°.
Image: Koch DD
“If you calculate corneal astigmatism from measurements on the anterior corneal surface only, in the with-the-rule eyes, you’re going to potentially get a mean over-correction of 0.5 D. In the against-the-rule eyes, you’re going to get a mean under-correction of 0.3 D,” Koch said.
In eyes that have undergone refractive surgery, a formula based on RTVue Fourier-domain optical coherence tomography (Optovue) that does not rely on previous keratometry readings or refraction shows great promise, Koch said.
“The early results of the OCT formula are, in fact, very encouraging,” he said. “All of these things are works in progress, including these other formulas, as we continue to refine them.”
In a toric IOL study involving preoperative measurement of the anterior cornea only and Koch’s new nomogram, all eyes were within 1 D of targeted correction and most eyes were within 0.5 D. – by Matt Hasson