Eyeworld

EW FEATURE 76 Revisiting astigmatism • October 2018 AT A GLANCE • Posterior corneal astigmatism is becoming a useful measurement for toric IOL planning. • Device manufacturers continue to improve the ability to measure posterior corneal astigmatism, and new toric IOL nomograms incorporate calculators that can determine it based on regression formulas. • Surgeons must first aim for accurate anterior surface astigmatism measurements. • Solid preop measurements, along with the use of appropriate calcula- tors, are more important to deter- mine posterior corneal astigmatism than intraop aberrometry. by Vanessa Caceres EyeWorld Contributing Writer of the counteracting effect of the posterior corneal astigmatism. There's person-to-person vari- ability, Dr. Koch added. "The goal is to measure each patient individually so we can calculate and more ac- curately 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 scienc- es, Case Western Reserve University, Cleveland, and East Valley Oph- thalmology, 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 im- portance of posterior corneal astigmatism, toric calculators were developed that are now commonly incorporated into the manufac- turers' calculators, Dr. Koch said. "They're all based on regression, and all of them work fairly well," he said. However, he said that posteri- or 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 help- ing to target and measure astig- matism. "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 poste- rior cornea. Measurement of the posterior surface continues to evolve, improve T he technology and formu- las to measure posterior cor- neal astigmatism continue to evolve as cataract sur- geons 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 poste- rior 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 Maximizing posterior corneal astigmatism for toric IOL calculations The Abulafia-Koch/Hill-RBF artificial intelligence toric calculator using a new posterior corneal astigmatism algorithm Source: Warren Hill, MD "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 astig- matism, 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 calcu- lator was added to the EyeSuite i9 software release on the LENSTAR biometer (Haag-Streit, Koniz, Swit- zerland), Dr. Hill said. Aiming for anterior accuracy Before lasering in on just posteri- or 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 ante- rior measurements that needs to be done," he said. For better anterior surface mea- surements, 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 reflec- tion topography for the anterior surface. It is more accurate than elevation measurements, and Placido rings give valuable infor- mation about surface quality," he said. • Repeat topography when neces- sary. "Make sure that all of your measurements closely match. If they don't, re-evaluate the corneal surface and repeat the measure- ments," Dr. Koch advised. Final pearls Drs. Koch and Hill reflected on other important issues for posterior corneal astigmatism, such as wheth- er 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. "Correct- ly used, intraoperative aberrometry

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