EyeWorld Asia-Pacific June 2012 Issue

36 June 2012 EWAP REFRACTIVE An eye with an LRI to correct astigmatism Source: Samuel Masket, MD How the femtosecond laser changes astigmatism management A stigmatism is extremely common in patients undergoing cataract surgery and is a critical factor in determining outcomes, quality of vision, and patient satisfaction. It’s estimated that 70% of patients have half a diopter or more of astigmatism, and 40% have at least 1 D, yet not every cataract surgeon practices astigmatism management. Limbal relaxing incisions (LRIs) for the treatment of astigmatism are difficult for many surgeons and the results are variable, causing some surgeons to abandon the technique altogether. But now ophthalmologists have in their armamentarium the femtosecond laser for cataract surgery, a disruptive technology that solves many of the problems surrounding manually made LRIs. “The femtosecond laser provides reproducibility, reduced variability compared with manual incisions, and it’s almost impossible to perforate the eye,” said Eric Donnenfeld, MD , co-chairman of cornea, Nassau University Medical Center, East Meadow, NY, USA. “It’s controlled by OCT [optical coherence tomography] data, so you can preset your desired depth of incision and get reproducible incisions that you can’t get with a manual LRI.” The problem with LRIs “We don’t have a good, consistent way to do LRIs,” said Kerry D. Solomon, MD , director, Carolina Eyecare Research Institute, Carolina Eyecare Physicians, Charleston, SC, USA. “I know it’s not consistent because everyone does it differently.” Questions surgeons face that may discourage them from learning the LRI technique include which type of diamond knife to use, which nomogram is appropriate, how deep should the incision be, and how does one determine the depth of the cornea. “I think it’s confusing for some surgeons,” Dr. Solomon said. “Because the results are inconsistent, I think there are a great many surgeons who say ‘Forget it.’” There’s also an awareness issue, said Dr. Solomon. When ophthalmologists first began performing corneal refractive surgery, less than a diopter of astigmatism was considered not clinically significant. However, with the invention of LASIK and PRK, refractive surgeons began to realize that by treating lower amounts of astigmatism, say 0.75 D to 0.50 D, 20/20 (6/6) rates increased to above 90% of patients. “There isn’t a LASIK or PRK surgeon out there now who wouldn’t treat astigmatism,” he said. “If you ask them how much astigmatism to treat, how much is clinically significant, almost all of them would say any amount. But with cataract surgery, there are still a lot of surgeons who say less than a diopter is not clinically significant.” The femtosecond technique Technically speaking, the femtosecond laser does not make an LRI, it makes an arcuate incision. To use this technique with the femtosecond laser for cataract surgery, the first step is to take a pre-op topography and a manual keratometry, followed by a keratometry using, for example, an IOLMaster (Carl Zeiss Meditec, Dublin, Calif., USA/Jena, Germany). After the readings are collected, they are put into an LRI calculator such as www.lricalculator.com (Abbott Medical Optics, AMO, Santa Ana, Calif., USA), which tells the surgeon how much cylinder to treat. “We then take the Eric Donnenfeld nomogram and reduce it by one third because these incisions are more central and therefore more effective,” Dr. Donnenfeld Treating astigmatism with arcuate incisions by Faith A. Hayden EyeWorld Staff Writer explained. “That tells us exactly where to place the incisions and how long they should be.” Surgeons bring that information into the OR and plug it into the femtosecond laser, which allows them to input the length of the incision and its axis, as well as select between one and two incisions. “We then do an intraoperative OCT that shows us the corneal thickness in the area of the incisions, and then we adjust the incision to 85% depth,” Dr. Donnenfeld said. “The 85% depth gives us a reproducible result. When we’re doing the cataract surgery, we open the incision with a Sinskey hook.” As Dr. Solomon pointed out, though, the incision doesn’t have to be opened at the time of surgery. When combined with toric IOLs, the arcuate incisions are created but not opened. There is some variability in keratometry measurements between devices (IOLMaster, topography, auto Ks, manual Ks). Which device is correct? If post-op Dr. Solomon has found that the measurements that were used for toric IOL calculations were correct and the chosen IOL has reduced the residual astigmatism to less than half to a quarter diopter, he doesn’t open the arcs. “But if I have more than half a diopter of residual astigmatism in the same meridian, then I have arcuate incisions already created that haven’t been opened, so the full effect hasn’t been realized,” he explained. “I can open these with a slit lamp a week to months later. It’s something we’ve been doing with both toric lenses and presbyopia- correcting lenses.” Another advantage of using the femtosecond laser is the ability to create intrastromal ablations, which can’t be performed with a manual LRI.