EyeWorld Asia-Pacific September 2015 Issue

September 2015 11 EWAP FEATURE him to measure IOL power using Placido topography, as well as giving a wavefront measurement of the eye. “Lastly, my Pentacam allows me to measure both the anterior astigmatism as well as the posterior astigmatism—the true net corneal power,” he said. Dr. Mah uses the axis from his topography readings and tends to take the power of keratometry from the optical biometry measurements. He also does a tomography to make sure that the patient is a LASIK or PRK candidate in case some enhancement is needed. Manual and auto Ks Dr. Garg said that manual and auto Ks can prove helpful in providing confirmation of other diagnostics. Dr. Hovanesian does not tend to use manual or auto Ks because he gets much more reliable results from the other machines. These only measure a couple of points on the cornea, he said. Meanwhile, topography is able to measure thousands of points, and the surgeon is getting a qualitative map. Surgically induced astigmatism (SIA) Calculating surgically induced astigmatism is important. “Surgical calculators rely on one’s SIA when determining the final IOL orientation,” Dr. Garg said. “If you are just estimating your SIA, this can introduce error.” “It’s essential to calculate the SIA because it’s part of your equation,” Dr. Hovanesian said. There is a tool by Warren Hill, MD, that makes this calculation easy, where a surgeon can measure pre- and postoperative surgical astigmatism and determine what the SIA is. “For most people, if you want to guess, it’s going to be about a half diopter of flattening wherever the clear corneal incision is,” Dr. Hovanesian said. Manifest refractive error in toric IOL planning Manifest refractive error can help give physicians information about what the patient is used to. “This becomes important when considering toricity and the possibility of flipping the axis of astigmatism,” Dr. Garg said. “Also, it is important to counsel patients about the astigmatism we are treating.” Patients need to understand the difference between corneal astigmatism and manifest astigmatism, he said. “I find it useful to show patients their diagnostic tests so they understand that I am treating their corneal astigmatism, with a goal to minimize their refractive error,” he said. Toric IOL calculator Although many physicians may be able to implant toric IOLs without the use of a toric calculator, this is another tool that can be used to ensure accuracy and can make measurements and preoperative assessments easier. “Figuring out toricity in a case that is on axis is not difficult,” Dr. Garg said. “However, for patients whose astigmatism is not on axis the toric calculator is very useful.” The determination of the final axis involves vector analysis. Dr. Garg prefers to use the Abbott Medical Optics toric calculator (Abbott Park, Ill.). “This calculator not only provides a choice of IOL powers but also compensates for the spherical power of the IOL in the final toric calculation,” he said. Dr. Mah likes the fact that the three major companies with toric IOLs commercially available in the U.S. have toric calculators. “I think one thing that’s important for the beginning toric IOL surgeon is to use the commercially available toric IOL calculators,” he said. Additionally, Dr. Mah uses the Barrett Toric Calculator, which is available on the APACRS website. “As information becomes available, such as posterior corneal power, that can be adopted into the newer IOL calculators,” he said. Alcon (Fort Worth, Texas), Abbott Medical Optics, and Bausch + Lomb (Bridgewater, NJ) all have online calculators that are free, Dr. Hovanesian said. Although he generally does his own calculations, he said these calculators become valuable when there is an unusual angle. “They allow you to put in the patient’s astigmatism, the intended lens, and the surgically induced astigmatism,” he said. “What they do not include is the posterior corneal astigmatism.” Posterior corneal astigmatism Posterior corneal astigmatism is something that a number of physicians are factoring into their measurements. This can be done with specific tools and also with a general estimate. To account for posterior corneal cylinder, Dr. Garg said he generally undercorrects with-the-rule cylinder and slightly overcorrects against-the-rule cylinder. “I have recently started looking at the Barrett Toric Calculator, which takes into account posterior corneal cylinder,” he said. “Additionally, I routinely use intraoperative aberrometry for my toric IOL cases.” Dr. Hovanesian said that posterior corneal astigmatism plays an important role for these patients. “It’s important to consider posterior corneal astigmatism because for nearly 90% of patients there is about a half diopter of corneal astigmatism that behaves as though it is steep at 180,” he said. Using an intraoperative wavefront aberrometry system like one from Clarity Medical Systems (Pleasanton, Calif.) or the ORA (Alcon) can measure all the aberrations of the eye, including posterior corneal astigmatism. There are a couple of ways to factor in posterior corneal astigmatism, Dr. Mah said. Posterior corneal astigmatism can be calculated by using a diagnostic machine, or a physician can just incorporate the “fudge factor,” which is around 0.5 D to 0.75 D, he said. “I think there’s an impression that in order to start down the pathway of doing premium IOLs and toric IOLs, everyone has to invest in new equipment and spend a lot of money in order to get good results,” Dr. Mah said. However, he thinks that good results can be obtained without some of these machines, although these technologies can be helpful in improving outcomes, he said. Toric IOLs are a benefit to patients, and surgeons need to look closely at adopting them, he said. EWAP Editors’ note: Drs. Hovanesian and Mah have financial interests with Alcon, Abbott Medical Optics, and Bausch + Lomb. Dr. Garg has financial interests with Abbott Medical Optics. Contact information Garg : gargs@uci.edu Hovanesian : johnhova@gmail.com Mah : Mah.Francis@scrippshealth.org