Eyeworld

EW FEATURE 36 by Maxine Lipner EyeWorld Senior Contributing Writer Assessing femtosecond favorability Does the laser make AK more predictable? F or some, astigmatic keratotomy (AK) has entered the 21st century with use of the femtosec- ond laser to craft incisions. But does the laser really enhance AK predictability? Eric D. Donnenfeld, MD, clinical professor of ophthalmology, New York University Medical Center, thinks so. "The advantages are the precision and accuracy of the laser and the use of the OCT guidance to make certain that the incisions are of the right depth," he said. "In ad- dition, the incisions are symmetrical and reduce the incidence of irregular astigmatism because they follow a normal contour rather than a manual LRI, which cannot be placed as accurately." What's more, the incisions can be performed with low energy, which produces a small effect, he said. "Then the incisions can be opened sequentially at a later date to titrate the effect." This allows AK, for the first time, to become adjustable postoperatively. "You can open one incision at the time of surgery and open a second postoperatively as needed, which allows more accurate results, but more importantly in many cases almost completely guar- antees that you won't flip the axis," he said. "The incidence of overcor- rections is dramatically improved as these incisions are art forms rather than science, and even with the laser there is an inherent unpre- dictability." There is also the ability to do intrastromal AK incisions, some- thing that would not be possible manually, Dr. Donnenfeld said. These offer less discomfort because nerves are not being cut and inci- sions do not splay open or change over time, he said. He finds the laser's precision unmatched. "In our experience, there's no doubt that the femtosec- ond laser increases the precision, accuracy, and safety of astigmatic incisions," Dr. Donnenfeld said, adding that 75% of practitioners do not perform AKs because they do not feel comfortable with the manual procedure. "Adding the precision of the femtosecond laser I think will create the opportunity for more doctors to treat astigmatism with this technology," he said. Picture of precision Stephen G. Slade, MD, director, Laser Center of Houston, agrees that femtosecond AK is more precise. He emphasized the fact that there is an image-guided component with the femtosecond incisions. "Everyone immediately starts thinking about the cutting ability of the femtosecond laser and forgets that they're image-guided," Dr. Slade said. "With AKs it tells you exactly where you're going to cut, the depth at that point, and where you have positioned the AK." By comparison, those perform- ing manual AKs are eyeballing it from a microscope, he said. With some femtosecond devices there is even the ability to take a picture of the patient while sitting up so that there is no cyclotorsion, he said. "You could plan your surgery the night before on those images," he said. "It brings up the image and shows you exactly where to put the AK—if the patient cyclotorts a bit, it compensates for this and places it to the micron." By contrast, a manual ink mark is 2 to 3 degrees wide, he said. In terms of the actual cutting, Dr. Slade sees several advantages of the femtosecond laser over a hand- held diamond knife. "You can adjust the depth directly, you can angle it anyway you want, you can make it deeper on the sides—you can customize it," he said. He finds the consistency of the technique a welcome contrast to the variability in patient corneas. "Some corneas are thick, some are thin, then gradually gets to the appropri- ate depth, but that depth may never be at the one that you intended." As a result, manual AK techniques lead to variability in incision architecture and depth, he said. By contrast, OCT-guided fem- tosecond procedures are extremely precise. "You're now dealing with transitioning from human-measured variables to computer-measured and applied variables," Dr. Thompson said. "If you're going for a 45-degree arc, you are going to get it precisely, and if you're going for a OCT-guided 85% depth, you're going to get that precisely." Dr. Thompson likes the fact that the AK effect is titratable with the femtosecond laser since practitioners can open incisions as needed. He combines this with the use of intraoperative aberrometry. "I take my aberrometry meas- urement in surgery and if it is less than the preoperative measure- Refractive results of femtosecond laser cataract surgery June 2014 AT A GLANCE • Femtosecond image-guided AKs are positioned to exact specifications; manual incisions may simply be eyeballed. • The titratability of femtosecond incisions ensures that practitioners will not flip the astigmatism axis. • Many rely on femtosecond AK for lower astigmatism and toric lenses for greater amounts. some are floppy, and some are stiff," Dr. Slade said. "Inherently astig- matic keratotomy on the cornea has a lot of variability because of the medium." The femtosecond lasers, how- ever, do exactly the same thing every time, he said. "Now we have an invariable method operating on a variable medium—that's why the femtosecond laser results with AK are better than we get with blades," he said. Vance Thompson, MD, associ- ate professor of ophthalmology, University of South Dakota School of Medicine, Sioux Falls, S.D., like- wise believes that the femtosecond laser reduces unpredictability. "When we have a diamond blade enter the corneal collagen, there's a compressive issue where there's tissue being pushed in front of the blade," Dr. Thompson said. "Oftentimes the beginning of the incision is a slope that's shallow and Femtosecond incisions can be performed with low energy and opened sequentially at a later date to titrate the effect. Source: Eric D. Donnenfeld, MD 32-41 Feature_EW June 2014-DL_Layout 1 6/3/14 12:31 PM Page 36

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