IOL Formulas Improve Refractive Accuracy
Today’s IOL formulas have improved visual outcomes by helping cataract surgeons increase accuracy, and the best one to use often depends on surgeon preference. “We’ve moved toward utilization of advanced formulas,” said George Waring IV, MD. “In a recent review, the Barrett Universal performed best across the full spectrum of axial lengths. As a result, we’ve moved toward utilization of the Barrett Universal. The Hill-RBF is a radial basis function advanced formula, which also represents the new-generation formulas and performs very well. It is becoming more robust with more data inputs over time. We have been utilizing this with great success. Historically, we utilized an optimized SRK/T formula for normal axial lengths, and this performed very well. However, it required adjustments for short and long eyes. We would routinely utilize the Wang-Koch-Maloney adjustment for long eyes and the Hoffer Q for short eyes. Not only does the Barrett Universal outperform the other formulas, it’s also convenient in that it performs well across the full range, so that we can use a single formula.”
John Berdahl, MD, said his go-to formula is the Hill-RBF. “I use that in every situation that it’s within bounds. If it’s out of bounds, I use the Barrett Universal formula, and usually those line up well,” he said. Doug Koch, MD, uses the Barrett, the Hill-RBF, and the Holladay 1 for all cases. “In long eyes, I use the Holladay 1 with a Wang-Koch axial length modification, which I still find to be my most accurate way to calculate IOL power in axial myopes more than 26 mm. In short eyes, I add the Holladay 2, which I think is helpful. Unfortunately, we have found that none of the formulas are as accurate as we had hoped in short eyes. Sometimes I will take the average of them in order to get the best result,” he said.
Post-refractive surgery eyes
Eyes that have undergone refractive surgery present a unique challenge. “For these eyes, we developed an algorithm that includes a weighted mean of the available post-refractive calculator inputs,” Dr. Waring said. “We use this routinely with ORA intraoperative aberrometry [Alcon] adjustment. We have also done work with the Pentacam AXL [Oculus], which is a hybrid high-resolution tomography unit and optical biometer that incorporates tomography-driven keratometry with optical biometry in a single unit. In addition, the device has advanced formula outputs, including post-refractive outputs. The Holladay software in this device is unique in that to the best of my knowledge, it’s the only available device that mathematically calculates and estimates preoperative, pre-refractive keratometric values without prior information. That allows the user to input this into multiple double-K formulas for post-refractive patients without prior records. Furthermore, it has the Barrett True K, which also performs very well. In our algorithm, we utilize the ASCRS post-refractive calculator, which incorporates multiple data points. We’ve also had great success with the Fourier domain high-resolution OCT algorithms in post-refractive patients.
“Additionally, we have performed a pilot study evaluating the post-refractive outputs from the Pentacam AXL with intraoperative aberrometry and have found and presented excellent correlation,” Dr. Waring said. “As a result, we think that we may be utilizing intraoperative aberrometry less and less in the future with the increasing robustness of the advanced formulas.”
According to Dr. Waring, it is imperative for surgeons to check their outcomes. “We need to track our refractive outcomes to understand how their inputs and the formulas are performing. We also need to make sure that the formulas are optimized appropriately with our surgeon factors and/or A constants. We think that surgically induced astigmatism has less of an impact on these variables than we once thought, but it’s appropriate to be evaluated, understood, and taken into account when optimizing outcomes overall. It’s been shown that the biometric accuracy is improved after ocular surface optimization or, in other words, in healthy eyes relative to dry eyes. Therefore, we recommend ocular surface optimization in this age group given the prevalence of dry eye in cataract patients,” he said.
About the doctors
John Berdahl, MD
Vision Sioux Falls, South Dakota
Douglas D. Koch, MD
Professor and Allen, Mosbacher, and Law Chair in Ophthalmology
Cullen Eye Institute
Baylor College of Medicine
George Waring IV, MD, FACS
Waring Vision Institute
Mt. Pleasant, South Carolina
Berdahl: Alcon, Johnson & Johnson Vision, Carl Zeiss Meditec
Koch: Alcon, Johnson & Johnson Vision, Carl Zeiss Meditec