October 2018

COVER FEATURE

Revisiting astigmatism
Maximizing posterior corneal astigmatism for toric IOL calculations


by Vanessa Caceres EyeWorld Contributing Writer


The Abulafia-Koch/Hill-RBF artificial intelligence toric calculator using a new posterior corneal astigmatism algorithm
Source: Warren Hill, MD


Measurement of the posterior surface continues to evolve, improve

The technology and formulas to measure posterior corneal astigmatism continue to evolve as cataract surgeons increasingly realize their importance for patients with astigmatism.
The recognition of posterior corneal astigmatism has helped with surgical astigmatism.
“Posterior corneal astigmatism, by and large, creates against-the-rule refractive astigmatism,” said Douglas Koch, MD, professor and Allen, Mosbacher, and Law Chair in Ophthalmology, Baylor College of Medicine, Houston. Dr. Koch is a pioneer in recognizing posterior corneal astigmatism. “[The posterior cornea] does this because the posterior surface of most corneas is steep vertically. Because the posterior cornea is a minus lens, this creates greater net refractive power along the horizontal meridian, which is of course against-the-rule refractive astigmatism.”
The end result is that in patients who have with-the-rule astigmatism on the anterior surface, total corneal astigmatism will be lower because of the counteracting effect of the posterior corneal astigmatism.
There’s person-to-person variability, Dr. Koch added. “The goal is to measure each patient individually so we can calculate and more accurately treat the astigmatism with cataract surgery,” he said.
Information about posterior corneal astigmatism as shared by Dr. Koch and Graham Barrett, FRANZCO, Perth, Australia, assists with toric IOL planning, said
Warren Hill, MD, adjunct professor of ophthalmology and visual sciences, Case Western Reserve University, Cleveland, and East Valley Ophthalmology, Mesa, Arizona. “The posterior cornea exerts an influence on both the meridian of alignment for the toric IOL and the net corneal astigmatism to be corrected,” he said.

Measuring posterior cornea astigmatism

With the recognition of the importance of posterior corneal astigmatism, toric calculators were developed that are now commonly incorporated into the manufacturers’ calculators, Dr. Koch said. “They’re all based on regression, and all of them work fairly well,” he said. However, he said that posterior corneal astigmatism can still be challenging—“oddly enough, even though it’s right there in front of us.”
Dr. Koch praises some of the newer technological advances that include Scheimpflug technology, such as the Galilei (Ziemer, Port, Switzerland), and the inclusion of swept-source OCT with devices like the IOLMaster 700 (Carl Zeiss Meditec, Jena, Germany) as helping to target and measure astigmatism. “We’re getting reasonable data, especially for against-the-rule astigmatism on the anterior cornea. It’s still a work in progress for with-the-rule astigmatism,” Dr. Koch said. However, he said that there are also encouraging data from the Cassini (Cassini Technologies, The Hague, the Netherlands), which uses reflection technology—color LEDs for anterior astigmatism and seven white LEDs that reflect off the posterior cornea.
“We’re making progress, but I’m surprised we are not further ahead. That said, I expect some major progress in the next year or two,” Dr. Koch said.
Although there are devices that measure posterior corneal astigmatism, Dr. Hill noted that at the present time, regression algorithms based on population mean values appear to do a better job overall to predict this value when doing toric IOL calculations.
“I think that the two most accurate regression algorithms for estimating posterior corneal astigmatism are the one contained within the Barrett toric calculator and the more recent Abulafia-Koch calculator,” he said. The Barrett calculator can be found on various sites online, including on the ASCRS website. The Abulafia-Koch calculator was added to the EyeSuite i9 software release on the LENSTAR biometer (Haag-Streit, Koniz, Switzerland), Dr. Hill said.

Aiming for anterior accuracy

Before lasering in on just posterior corneal astigmatism, Dr. Koch thinks it is important for cataract surgeons to step back and first aim for the best possible anterior surface astigmatism measurements. “I see a lot of variability in anterior corneal measurements. We should not get so hung up on the posterior that we neglect the careful vetting of anterior measurements that needs to be done,” he said.
For better anterior surface measurements, Dr. Koch shared a few pearls.
• Do at least two measurements with different devices. His group does three or four measurements. “One of the devices can be a biometer, particularly biometers that have multiple LEDs, like the IOLMaster 700 and the LENSTAR, both of which have more than six [LEDs],” he said.
• Perform topography. “I like reflection topography for the anterior surface. It is more accurate than elevation measurements, and Placido rings give valuable information about surface quality,” he said.
• Repeat topography when necessary. “Make sure that all of your measurements closely match. If they don’t, re-evaluate the corneal surface and repeat the measurements,” Dr. Koch advised.

Final pearls

Drs. Koch and Hill reflected on other important issues for posterior corneal astigmatism, such as whether preop measurements suffice and when posterior measurements aren’t necessary.
“Good preop measurements, combined with an advanced toric calculator such as the Barrett or the Abulafia-Koch/Hill-RBF calculator, are capable of a 90% accuracy of 0.50 D or less of residual refractive astigmatism,” Dr. Hill said. “Correctly used, intraoperative aberrometry can also achieve values such as this for residual refractive astigmatism.”
Most of the time, Dr. Koch does not think that intraoperative aberrometry improves the accuracy of toric IOL selection and alignment. “For posterior corneal astigmatism in particular, its magnitude in a normal eye may be below the threshold of noise for intraoperative aberrometry so I am not sure that it adds more at this point than our current nomograms,” he said.
It is important not to insert values for total corneal astigmatism into toric IOL calculators that use anterior corneal measurements to estimate posterior corneal astigmatism. “If a measuring device provides a true, or net, corneal power, the posterior cornea has already been incorporated into this value,” Dr. Hill said. “Adding a posterior corneal algorithm would account for the posterior cornea a second time and make the calculation inaccurate.”
“If you have against-the-rule astigmatism, the posterior cornea often doesn’t contribute a lot, typically around 0.2 D,” Dr. Koch said. “But you have to add in lens tilt. Postoperative IOL tilt (which can be predicted from preoperative crystalline lens tilt1,2) introduces up to 0.2 D of additional against-the-rule refractive astigmatism.3
Measurements will continue to improve and evolve, including the impact of lens tilt, Dr. Koch predicted. “In the future, our preoperative measurements will include total corneal astigmatism and crystalline lens tilt. Knowing the latter and the type of IOL and its meridional alignment, we will be able to predict the refractive impact of IOL tilt. These steps will take us to a new level of accuracy in optimizing our treatment of patients’ astigmatism,” he said.

References

1. Hirnschall N, et al. Prediction of postoperative intraocular lens tilt using swept-source optical coherence tomography. J Cataract Refract Surg. 2017;43:732–736.
2. Wang L, et al. Evaluation of crystalline lens and intraocular lens tilt using a swept-source optical coherence tomography biometer. J Cataract Refract Surg. Accepted for publication.
3. Weikert MP, et al. Astigmatism induced by intraocular lens tilt evaluated via ray tracing. J Cataract Refract Surg. 2018; 44:745–749.

Editors’ note: Dr. Hill has financial interests with Alcon (Fort Worth, Texas) and Haag-Streit. Dr. Koch has financial interests with Alcon, Johnson & Johnson Vision (Santa Ana, California), and Carl Zeiss Meditec.

Contact information

Hill
: hill@doctor-hill.com
Koch: dkoch@bcm.edu

Maximizing posterior corneal astigmatism for toric IOL calculations Maximizing posterior corneal astigmatism for toric IOL calculations
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