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EW FEATURE 50 All about IOL calculations • January 2017 and, as a result, it's more difficult to determine the true refractive power of the anterior corneal surface," he said. "You're not looking at unifor- mity of powers; you're looking at variability." Most formulas now try to use an average over the anterior corneal surface, but how you weigh various areas is tricky, Dr. Koch said. The second potential issue is that the posterior corneal power is unknown. "A basic assumption in IOL calculations is that you measure the front and you assume posterior power based on some fixed ratio compared to the front," Dr. Koch said. However, he noted that such an assumption is erroneous, some- times even in normal eyes, and in eyes that have undergone previous corneal refractive surgery, the rela- tionship between front and back is no longer valid. Physicians have to make an estimate of posterior corne- al power, while awaiting advances in technology that will allow them to comfortably obtain accurate mea- surements, Dr. Koch said. The third difficulty is that many lens calculation formulas use corneal power as one variable when calculat- ing effective lens position. This can be hard to factor in if the patient has had previous refractive surgery. "You would need to use formulas that don't take that into account or guess what the corneal power was prior to surgery," he said. Dr. Chang agreed that there are several factors to consider. First is that not all refractive surgery is the same. Different treatment modali- ties, degrees of refractive correction, ablation and blend zone sizes, and even centration can affect the size and location of the active optical zone, he said. Dr. Chang said that of his pa- tients coming in for cataract surgery, about 10% of them have had a previous refractive procedure. The physician must consider what surgery was done because pre- dictions will vary depending on the refractive procedure performed, Dr. Masket said. If it was laser-based sur- gery, or photoablative, it can create a series of problems, he said. If it was an incisional or RK-type surgery, it creates a different set of problems. If the patient has undergone photoablative surgery, the ratio of the front and back surface of the cornea is changed. In the unoper- ated eye, the anterior and posterior surfaces of the cornea are more or less parallel, Dr. Masket said, with the back surface having about a –6 D power. Standard corneal measuring devices have incorporated that into their readouts. However, the relationship between front and back changes after photoablative surgery. With myopic excimer laser surgery, he said, the front surface is significantly flattened, but the back surface isn't changed. "The back surface takes on a different role, so when you read with a device, it's making the same assumptions, but that's not the case," Dr. Masket said. When read- ing with a keratometer or topogra- pher, the power of the cornea would now be overestimated. The opposite is true with hyperopic photoabla- tion, which tends to underestimate the power of the cornea. "With standard measuring devices following photoablative refractive surgery, the relationship between the front and back surface of the cornea is altered, invalidating the power readings," Dr. Masket said. Regarding RK, keratometric devices do not allow measurement of the direct center of the cornea, Dr. Masket said. "The center cornea is flattened more than we read," he said, and this ends up overestimat- ing the power of the cornea. Dr. Masket said patients with previous refractive surgery are fairly common in his practice. There was an explosion of laser refractive sur- gery in the 1990s, he said, so many of those patients are now entering the age group where cataract surgery is indicated. Best formulas in these cases There are different categories of formulas, Dr. Koch said. Some require no knowledge of prior data, a situation that is becoming increas- ingly common. Of the formulas in this category, Dr. Koch likes to use the Barrett, Haigis, OCT, and Wang-Koch-Maloney formulas. A second category is formulas that use measurements obtained at the time of presentation for cataract surgery combined with knowledge of the refractive change produced by the LASIK procedure. In this category, Dr. Koch likes the Masket and Barrett formulas. He uses intraopera- tive aberrometry in all of these cases. Dr. Masket published the Masket regression formula in 2006, which he said still holds up in most cases as very accurate. "What we were able to determine is there is a mathemat- ical relationship between how much excimer laser ablation was done and what its effect is on IOL power calculation for both farsighted and nearsighted eyes," he said. He added that the Barrett True K and Haigis-L formulas also work well. Dr. Masket thinks that intraop- erative aberrometry can help sig- nificantly in people who have had hyperopic or myopic photoablation. These formulas are accessible via the ASCRS website, which includes the Post-Refractive IOL Calculator (iolcalc.ascrs.org), Dr. Masket said. This is convenient for ASCRS mem- bers and non-members to access and enter as much data as they can. RK doesn't alter the ratio of front to back, so the post-laser refractive formulas described are only for patients who have had LASIK or PRK, he added. The other problem in an RK cornea, Dr. Masket said, is that the cornea will fluctu- ate significantly in the early postop period in terms of corneal curvature. Soon after surgery, the cornea tends to flatten even more. "You have to follow the patient carefully with topography, etc. to know when the cornea has returned to its normal state," he said. The ASCRS calculator offers one place where all the formulas are present and surgeons can enter their data and get individual values and averages. Different formulas make certain assumptions, Dr. Chang said. How significantly refractive surgery af- fects those assumptions can deter- mine the relevance of any particular formula. "The ASCRS calculator has made it easier to see and compare the output of all the formulas," Dr. Chang said. IOLs to use Depending on the patient and the prior procedure, certain IOLs may be better options than others. One problem with the effect of laser treatment on the cornea is that the typical myopic ablation induces positive spherical aberration to the cornea, Dr. Masket said. Older style lasers induce even more, he added. It's therefore helpful to use lenses that have correction with some neg- ative spherical aberration, like the Tecnis lens (Abbott Medical Optics, Abbott Park, Illinois) or SN 60WF (Alcon, Fort Worth, Texas). Addi- tionally, Dr. Masket said that zero aberration lenses can be used in these cases, and he noted that Bausch + Lomb (Bridgewater, New Jersey) has several in that category. Conversely, hyperopic photoab- lation tends to create negative spher- ical aberration in the cornea. Zero spherical aberration lenses are good in that situation as are those with some positive spherical aberration. The case is similar for patients who have had RK because those cor- neas have an accentuated positive spherical aberration, Dr. Masket said. Such patients are better served with lenses that have negative spherical aberration or no spherical aberration built in. Dr. Masket said that presbyopic IOLs could be helpful in some of these cases, however, he noted that generally, post-RK corneas do not work well with these IOLs because the corneas tend to fluctuate over the course of the day, as well as progressively flatten over time. The incisions increase postop glare and nighttime vision difficulties, so a diffractive IOL will add to that problem. "With respect to patients who have had myopic or hyperopic LASIK, one of the other problems we may see is a greater degree of higher How previous continued from page 47 " With standard measuring devices following photoablative refractive surgery, the relationship between the front and back surface of the cornea is altered, invalidating the power readings. " –Samuel Masket, MD

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