Choosing the Patient
Since the irregular cornea created by keratoconus often decreases best-corrected acuity and prevents patients from wearing contact lenses, surgeons say the goal of the therapeutic Intacs procedure is to regularize the corneal contour enough so that patients can once again wear contacts or spectacles. The procedure also aims at stabilizing the cone through the flattening effects of the Intacs.

"The best patients for Intacs have mild to moderate keratoconus," says Mark Swanson, MD, who practices in Agua Prieta, Mexico. Dr. Swanson is also an official trainer of Intacs surgeons for Addition Technology. "K readings over 60 D don't do as well as lower ones. We can stabilize the cone and make the cornea a better optical surface, but we can't flatten the cornea more than 10 D. Sometimes we only flatten it 5 or 6 D with the Intacs, but we still make it more regular. In some cases we can treat 4 to 6 D of irregular astigmatism, but, especially in those cases in which the corneas are very steep, we don't have as much of an effect." He adds that the other group of patients who don't do well are those with very thin corneas, less than 200 µm, because their cones are severe.

David Hardten, MD

Some surgeons place CK spots to help enhance the effect of the Intacs procedure.

Teaneck, N.J., surgeon Peter Hersh adds that the ideal candidate wouldn't be someone who's actually a penetrating keratoplasty candidate, such as those with central, visually significant scars or excessive thinning in the mid-periphery where the surgeon is going to place the segments.

Dr. Hersh says the Intacs have several effects. "The Intacs provide biomechanical support, flattening and regularization of the cornea," he says. "So, by putting in Intacs you're giving more peripheral support, causing a change in the corneal biomechanics that gives a secondary flattening of the cone, and also, by virtue of getting this 'new limbus,' tending to regularize the corneal topography as well, reducing the irregular astigmatism." He says the procedure also hefts up the corneal periphery, where the tissue may be somewhat thinner. "Whether that, in and of itself, is having an effect is hard to say," he adds.

Techniques
Since there's a lack of large-scale, well-controlled studies on Intacs for keratoconus to dictate the definitive method of performing the procedure, surgeons approach the surgery in different ways. Here are the current techniques surgeons are using:

 • Placing the segments. Dr. Swanson recommends first placing the entry incision on the cornea's steep axis. This is the method that first-time Intacs users are taught.

"You really need to know where to insert those segments," he says. "If we insert them on the flattest axis, we will do better work on the cornea, especially a keratoconic cornea." He recommends following elevation topography when using this steep-axis technique.

Dr. Swanson also says that managing the sizes of the two segments, which involves sometimes mismatching them, can lead to better results. Currently in the United States, there are three approved segment thicknesses: 0.25 mm; 0.30 mm and 0.35 mm. Outside of the United States, surgeons also have access to 0.40-mm, 0.45-mm and small, 0.210-mm segments.

As an example of a mismatched segment implantation, Dr. Swanson describes a cone that's located inferiorly with the cornea's steep axis on the horizontal plane. In this case, he'll make his incision on the horizontal axis. He'll then place one thinner segment under the cone and a thicker one opposite it. "That way, we're going to pull up the opposite center, distributing the forces with a thicker one above. We're going to distribute those forces 360 degrees."

He says he believes this approach yields better results and fewer optical aberrations than placing only one thick segment inferiorly or a thick one inferiorly and a thinner one superiorly. He says these other approaches "flatten the cornea but don't move the optical center, leaving the aberrations the same."

Though Dr. Swanson swears by the steep-axis technique, Dr. Hersh places his incision in the flat axis, and says he achieves good results, too.

"Surgeons use different strategies," says Dr. Hersh. "Some will mismatch the Intacs between top and bottom, especially in cases that have more of a pellucid-type pattern. Some surgeons will surround the cone, while some will go on the top and bottom of it. I don't think we really know at this point if there's a significant difference between the various techniques. I just don't think there's enough data out there. There's such variability in these corneas that it's difficult to ascertain what the best methodology is."

Dr. Hersh describes his technique as surrounding the cone with the Intacs and placing a tight suture in the flattest meridian.

"For example," he explains, "if the corneal topography shows blue up top and red below, I like to surround the cone on both sides so I have an even force on either side. I will place them symmetrically to surround the cone. This allows me to adjust the suture tension, which I do with the guidance of intraoperative keratometry and topography, to get things as round as I can. I make the initial incision where I see the flattest area topographically, so I can then surround the cone and adjust the suture tension. This may be different than how other people are doing it, but we've been doing it this way and have achieved good results."

Peter Hersh, MD

In the surgical method of New Jersey's Peter Hersh, MD, adjusting suture tension can titrate the procedure's effect.

He says that eyes in which he's placed the Intacs superiorly and inferiorly are usually cases in which the cone is more central and in which the steep axis of refractive astigmatism is in the axis of his incision.

Minneapolis surgeon David Hardten bases his incision on the relationship of the apex of the cone to the center of the cornea.

"On the elevation topography map, I will draw a line from the central portion of the cornea to the apex of the cone," Dr. Hardten explains. "That's the center of my segment. Ninety degrees away from that point is the axis of my incision. I'll usually make the incision somewhere between the 7 o'clock and 1 o'clock positions, because I'm right-handed and it's a little easier for me to make it on the right side of the patient." Dr. Hardten says that, like Dr. Hersh, he too uses the thicker segments most often. "I haven't used the 0.25-mm segments much at all," he says. "I think the thicker segments work a little better, even if the patient is slightly hyperopic after the surgery."

Both Drs. Hersh and Hardten occasionally use conductive keratoplasty in conjunction with the Intacs to help achieve a greater effect.

"I've been doing CK before the segments to get some extra effect at the apex of the cone," says Dr. Hardten. "How many spots I place depends on how much room I have in that region, but it's usually between 10 and 15. I do that after I've made the incision but before I have tunneled or placed the segments. In some patients, I'll get more effect by placing additional CK inside the segments after they're placed, but I'll usually save this for a second operation as I can do this after the original surgery. This appears to be helpful in improving the irregular astigmatism by flattening the area that's right over the apex of the cone and shifting the steeper spots more centrally."

Los Angeles surgeon Brian Boxer Wachler says he's getting an intriguing effect from using only one segment to treat peripheral keratoconus, and will be presenting his data on this method at the upcoming meeting of the American Academy of Ophthalmology.

"We make the incision in the axis of refractive astigmatism," he says. "We'll place a single segment, for example in the lower part of the cornea, and get flattening over the cone. But our new finding is that we actually get steepening in the upper part of the cornea. This excessive flatness in the upper part of the cornea is part of the pathology, but it's been ignored." For central cones he still uses two segments.

As for why different techniques can work, Dr. Hersh has a theory.

"I think by supporting the mid-periphery, no matter what technique you use, the Intacs will tend to centralize the cone," he says. "I think that, in essence, the Intacs are going around the entire cornea, so you have a new limbus. I just don't think we know whether the insertion strategy makes a difference or not. The Intacs are basically subsuming 300 degrees of the 360-degree cornea, so exactly where they're placed, where those little 30-degree gaps are, I'm not sure makes a difference."

 • Manual vs. IntraLase. Drs. Swanson and Boxer Wachler have used the Intralase femtosecond laser to make the channels.

"I've done more tunnels manually, because the manual instruments are more readily available, give a reproducible technique, and I find they let me control many variables well, such as the depth and location," says Dr. Swanson. "The Intralase is good in that you don't have to worry about the depth of the tunnel. It will make them up to 400-µm deep, and no deeper. The company has announced that it will alter the algorithms to change this depth limit, but for now you can't go deeper." He says he prefers the manual instruments' ability to let the surgeon go down a certain percentage of the corneal depth, rather than an absolute number of microns like the IntraLase. "On a keratoconic cornea, it's not the same thickness 360 degrees around," he explains. "With manual dissection, you go a certain percentage of depth because you're dissecting lamellae."

Dr. Boxer Wachler says that the manner in which the Intralase makes the tunnel might be an issue. "The one concern I have with the Intralase is that it doesn't separate the lamellae, it photodisrupts across them, basically breaking across lamellar layers. With manual dissection, you maintain the integrity of the lamellar layers. Since we know the lamellae add to the structural integrity of the cornea, this photodisruption may impact the long-term stability [of the procedure]."