Eyeworld

EW REFRACTIVE 134 March 2018 Research highlight by Maxine Lipner EyeWorld Senior Contributing Writer Dr. Moshirfar believes in using more than one metric to determine SMILE eligibility. "On a day-to-day basis in our clinic, most clinicians rely on the Randleman criteria, topography, and the PTA score," he said. Developing a metric The study also highlighted the fact that SMILE was not free of ectasia risk in normal eyes, as some thought. "There were two eyes in this group that had normal topog- raphy, they had normal PTA scores and low risk of ectasia based on the Randleman criteria, but still devel- oped ectasia," Dr. Moshirfar said. "You can develop ectasia, even with SMILE, if the residual stromal bed is more than 300 microns." There are those, however, who make a case for the idea that you can do an even higher correction with SMILE, Dr. Moshirfar noted. The argument is that the SMILE cap, which typically is 100- to 120-mi- crons, can be as much as 140 mi- crons. "In the U.S., we're doing 120, but there are colleagues around the world who are doing 140 microns," he said. "The theory is that the cap still has the same tensile strength because you're not cutting the flap, you're creating a superior incision." The thinking is that with SMILE you don't need to respect the usual 250- to 300-micron stromal bed since you are not cutting the ante- rior stroma. "The SMILE surgeon says that 80 microns on top of the lenticule is not 80-microns strength, it's actually twice as strong because the anterior stroma is stronger than the posterior stroma," Dr. Moshirfar said. "Therefore that 80 microns is double the thickness in terms of strength, so it's 160 microns." Instead of leaving a 300-micron residual stroma bed, some think that a 220-micron bed is sufficient. However, Dr. Moshirfar stressed that the patients who developed ectasia here were 20/20, were all young, had only between –2 and –6 D of myopia, and didn't have a lot of astigmatism. "The cornea thickness among all of these eight cal keratoconus. Dr. Moshirfar thinks the results indicate that abnormal typography is the primary risk factor for ectasia in these cases. "It doesn't need to be keratoconus, but even more it's based on the asymmetry in topography," he said, adding that investigators should not rely on the existing PTA score mechanism. If a 40% PTA cutoff point was used in the SMILE cases studied here, only 25% of the eyes would have been ruled out for surgery, Dr. Moshirfar pointed out. "The other 75% would have still been a candidate for SMILE," he said. "That means six of these eyes would have still been labeled as good candidates." Marcony Santhiago, MD, has always emphasized that the PTA scores apply to normal corneas, Dr. Moshirfar stressed, adding that this metric was also meant for LASIK. With the Randleman scoring mechanism, Dr. Moshirfar noted that they were able to identify five of the eight eyes as not good can- didates for SMILE. "The other three rated as low risk," he said. In other words, 62% of the eyes would have been found to be poor candidates. don't destroy the basal nerves and nerve plexus, and as a result won't cause much dryness, he noted. In addition, some think that you could go deeper and remove more tissue in high myopes, which is attractive since predictability of LASIK goes down with higher corrections. Considering ectasia cases Dr. Moshirfar had conducted ectasia studies before and wanted to know if the same metrics currently being used for LASIK are also applicable for SMILE. These included the Randle- man Ectasia Risk Score System and the percent tissue altered (PTA). He went to the literature to see how commonly ectasia occurred in conjunction with SMILE. "To our surprise, there's not a lot of ectasia post-SMILE," he said. "There were only four articles and seven reported eyes that have developed ectasia out of eight eyes." He finds this impres- sive, noting that there have been more than 750,000 SMILE cases. Abnormal topography appeared to be a factor in almost all of these cases, with just one of the patients developing ectasia without subclini- Exploring how commonly ectasia occurred with SMILE W hen a patient un- dergoes LASIK, there can be an underlying worry about ectasia, but what about with small incision lenticule extraction (SMILE)? In a study published in Clinical Ophthalmology, investiga- tors considered cases of ectasia that occurred after SMILE, according to Majid Moshirfar, MD, professor of ophthalmology, Moran Eye Center, University of Utah, Salt Lake City. 1 One aspect of SMILE that makes it appealing to many is the idea that the cornea can be stronger with this technique. "It was attractive to me that if you don't do LASIK, you don't make that vertical inci- sion, which is almost 23 mm, if you assume that the flap is about 8.5 mm," Dr. Moshirfar said, adding that with SMILE you're only making a tiny hinge superiorly, which is about 5 mm. The fact that the large flap is not needed means that you When SMILE gets complicated SMILE 5.89 mm versus LASIK 23.14 mm incision Source: Majid Moshirfar, MD

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