EyeWorld Asia-Pacific June 2014 Issue

12 June 2014 EWAP FEAturE placing incisions with the laser in every case is always the same, he said. “It’s never the same from one patient to the next with the doctor.” The pilot study with the Verion Image Guided System (Alcon, Fort Worth, Texas, U.S.) did show a significant improvement in prediction error over manual cataract surgery and the Optiwave Refractive Analysis (WaveTec, Aliso Viejo, Calif., U.S.) intraoperative aberrometry. The study also showed that results with AK incisions were competitive with toric IOLs. Eric D. Donnenfeld, MD , clinical professor of ophthalmology, New York University Medical Center, New York, NY, U.S., thinks that for astigmatism correction, the femtosecond lasers add a level of predictability, reliability, and safety that can never be achieved with a manual limbal relaxing incision. “The fact that the majority of ophthalmologists never perform a manual limbal relaxing incision speaks to the point that many ophthalmologists are not comfortable with using a diamond knife to incise a cornea,” Dr. Donnenfeld said. He said recent surveys show only about 25% of ophthalmologists currently do limbal relaxing incisions. “The arcuate incisions with the laser add a level of safety that can’t be achieved manually,” he said. Dr. Donnenfeld said that several of the lasers have OCT visualization that show how deep the incisions go and significantly help to avoid perforations. “The incisions can be set at a specific depth and that’s achieved,” he said. George Stamatelatos, OD , senior clinical optometrist, New Vision Clinics, Melbourne, Australia, likewise pointed to this as a strength. “With the OCT that’s available on the femtosecond lasers, you can get the precise depth that you want,” Dr. Stamatelatos said. When you take an average thickness of the cornea in the peripheral zone, it is just an average, whereas with the femtosecond you know that you’re going to go to 85% of the actual depth. Another advantage that Dr. Donnenfeld believes will take the femtosecond cataract approach into the mainstream is that it is controlled by the computer and doesn’t rely on a certain skill level. “The novice surgeon can do just as well as the experienced surgeon,” Dr. Donnenfeld said. The laser incisions are adjustable after surgery, allowing outcomes to be titrated, Dr. Donnenfeld said. Akin to the intact serrations in a postage stamp, the incision can be put in place by the laser. The stamp isn’t separated until these serrations are pulled apart. “I will make the incisions with a laser and then open these one incision at a time,” he said. This almost eliminates the possibility of overcorrection and allows him to titrate the results to the desired effect. In addition, he said that the femtosecond allows for the possibility of creating intrastromal relaxing incisions that could never be done manually. Noel Alpins, MD , medical director, New Vision Clinics, also believes femtosecond lasers can be beneficial in correcting astigmatism. He cited a small series of LRIs that he did with the laser after first gaining access. “In about three out of 10 patients we did LRIs straight off the bat,” Dr. Alpins said. “All of the patients had a good reduction of corneal astigmatism, as well as refractive cylinder.” In his view, the real benefit of using the femtosecond laser is that practitioners can precisely determine the depth of the incision. “You get exactly what you dial into the machine,” he said. Measurement obstacles He stressed, however, one variable people are not thinking about that needs to be considered is the white-to-white measurement. “If the white-to-white is 10 versus 12, then that incision can be anywhere between 1 and 3 mm from the limbus,” he explained. “You can have a lot of variability in the distance between that incision and the distance to the limbus.” When this is done manually, practitioners tend to put the incision just inside the limbus. “I think a lot of the refinement yet to come in femtosecond LRIs is to hone down the white-to-white and to know where the incision is in relationship to the physical surgical limbus,” Dr. Alpins said. Another obstacle in attaining best correction of corneal astigmatism with femtosecond lasers is determining the true power of such astigmatism. Dr. Alpins said that simulated keratometry, used to help ascertain corneal astigmatism, relies on only one ring of topography readings— the seventh ring, located roughly over the 3-mm zone. “The thing about using only one ring when the topographers have up to 32 rings is that it’s a bit of a lucky shot,” Dr. Alpins said, adding that in a fat cornea the ring will sit over the 4 mm zone and in a steep one over the 2 mm zone. “It doesn’t measure the same point in the cornea every time if you use only one ring,” he said. To try to obtain better results, Dr. Alpins has been using the parameter corneal topographic astigmatism (CorT). For this, he considered all of the rings and all of the data captured on corneal topography and took the vectorial average of them all. “What we showed was the CorT value of the anterior cornea was so much more accurate than the SimK, manual keratometry, paraxial curvature matching and corneal wavefront,” Dr. Alpins said. “It was the best of them all.” Dr. Alpins is now using three machines to ascertain total corneal power—the Sirius Corneal Topographer (CSO, Florence, Italy), Pentacam (Oculus, Arlington, Wash., U.S.), and Ziemer (Port, Switzerland). Together these offer a CorT number that is more accurate not just for the anterior cornea but for the whole cornea, Dr. Alpins said, adding that this will give practitioners the number needed for LRI and toric IOL placement. Douglas D. Koch, MD , professor and the Allen, Mosbacher, and Law Chair in Ophthalmology, Cullen Eye Institute, Baylor College of Medicine, contends there’s a variability introduced into femtosecond measurements by two things that don’t currently get considered. “One of them was pointed out by Dr. Holladay in terms of toric lenses, and that is the fact that there is less effective toricity as the anterior chamber deepens and as the IOL power diminishes,” Dr. Koch said. With astigmatism measurement in general, there is a lot of variability in anterior corneal measurement. continued on page 14 Operating - from page 11