Eyeworld

EW FEATURE 62 forme fruste keratoconus, which is causing irregular astigmatism, then a PCRI could cause more harm than good and induce further instability into the cornea," Dr. Devgan said. Low risks Still, the risks of performing PCRIs are few, according to Dr. Mamalis. Putting the incision in the wrong place is one of them. It can be easy to get the axis confused, especially if it's an oblique axis, he said, so it's very critical to make sure that those incisions are put in the proper place, otherwise the astigmatism can be made even worse. "What I do, and what I insist that our residents do, is take a pic- ture of the corneal map and the planned area where we're going to be putting the incisions. I tape that up to the microscope in the operat- ing room and put it in the orienta- tion where we want to make the in- cision so that there's no question that we're putting the incision in the proper place," Dr. Mamalis said. Technical complications such as perforations in the cornea can occur as well, he said. For standard limbal relaxing incisions, most surgeons use either a diamond blade or a metal blade with a preset of approxi- mately 600 microns in depth. These leave quite a bit of leeway in terms of corneal thickness, so for surgeons who are not going to be routinely measuring corneal thickness and then setting their blade accordingly, the risk of a microperforation of the cornea is quite small, Dr. Mamalis said. The most common complica- tion is undercorrection, he said. "We'll often aim to correct a cer- tain amount of astigmatism and we'll find that we get 70-80% of it. It's pretty uncommon to get the full 100%," he explained. Overcorrection is also a risk, Dr. Devgan said. Diamond, metal, or laser? In terms of blades, Dr. Mamalis said there are some nice metal blades available, but he prefers diamond blades, especially those that are the preset depth and have "little rounded ski tips" on both sides that help to guide the incision. These give a nice, smooth incision and don't disrupt the corneal epithe- lium, he explained. Dr. Devgan said both types of blades are effective. Steel blades are the least expensive while diamonds are considered the standard since they tend to be sharper and produce more reliable and consistent inci- sions, he noted. However, the future may move toward femtosecond lasers for PCRIs since the accuracy is expected to be better, he said. Dr. Mamalis agreed. While he said he doesn't have experience with the femtosecond laser, he thinks it could potentially be a very accurate way to correct astigmatism because the depth of the incision is con- trolled by the measurement of the corneal thickness that takes place before the laser is used. But only time will tell. "It's too early to say if femtosecond lasers will produce better results than dia- monds," Dr. Devgan said. When toric lenses are best Up until a couple of months ago, the toric lenses available in the U.S. were not able to correct high degrees of astigmatism, Dr. Mamalis said. However, the U.S. FDA just approved higher power toric lenses, so sur- geons now have the ability to cor- rect a broader range of astigmatism, he said. The lenses that can correct extremely high levels of astigmatism available in Europe have not been approved yet. Toric lenses are great for pa- tients with regular, symmetric, and February 2011 Challenging refractive cases October 2011 Astigmatism continued from page 60 continued on page 63 52-67 Feature_EW October 2011-DL33_Layout 1 9/29/11 4:52 PM Page 62

• Cover