EyeWorld India March 2017 Issue

14 EWAP FEATURE March 2017 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 reading with a keratometer or topographer, the power of the cornea would now be overestimated. The opposite is true with hyperopic photoablation, 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 overestimating 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 surgery 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 increasingly 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 intraoperative 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 mathematical 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 intraoperative aberrometry can help significantly 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 members 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 fluctuate 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 affects those assumptions can determine 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 negative spherical aberration, like the Tecnis lens (Abbott Medical Optics, Abbott Park, Illinois) or SN 60WF (Alcon, Fort Worth, Texas). Additionally, 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 photoablation tends to create negative spherical 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 corneas 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 order aberrations in the cornea as a result of their surgery,” Dr. Masket said. “If one adds a diffractive lens, this could contribute to higher order How previous refractive - from page 12