February 2018

COVER FEATURE

Laser vision correction
Current considerations for optimizing topographic ablations


by Michelle Stephenson EyeWorld Contributing Writer




This patient is 1 year postop, –7.75 D in left eye (–7.0 D in right eye). Topo-guided patient is 20/15, 20/15, 20/15+2 OU. This is a pre-
and postop higher order aberration map, seeing the improvement in Zernike values. Source: Michael Gordon, MD

After gaining popularity globally, topographic ablations are gaining traction in the U.S.

Topography-guided laser ablation has gained popularity internationally and is starting to gain popularity in the United States. It has slowly evolved from a treatment for damaged corneas to a treatment for keratoconus and a tool for enhancing laser refractive surgery outcomes.
“Since the initial clinical trial when everyone was excited about the results of topographic treatments, surgeons have realized that topographic ablations are not as easy as they seem,” said Michael Gordon, MD, San Diego. “They may not be getting the expected results, or it may take too much time to plan. I think the adoption of topographic ablations has suffered because of this. There are so many variants now that people are confused about what procedure to use on which patients.”
Ronald Krueger, MD, Cleveland, has been using topographic ablations since their FDA approval several years ago. “Over the course of time what I have learned about optimizing these ablations is that I will try to treat as close to the topographic axis of astigmatism as I can,” he said. “However, where the topographic axis deviates from the refractive axis, I will use additional tools to choose the proper axis of astigmatism that I should be treating. One needs to compare the manifest refraction with the measured astigmatism from topography to determine the modified refraction, which is then used for laser entry. Surgeons must make their own determination based on what the topography says and what the refraction says as to what they want to come up with the final product of astigmatism axis and magnitude that they treat. When it comes to axis, I try to stay as close to the topographic axis as I can, and in about 75% of my cases, I’m on that axis. But because there is posterior astigmatism and internal astigmatism, I will use other tools to help determine whether I should be treating closer to the manifest refraction axis than the topographic axis because of those differences. Sometimes I will treat between the topography axis and the manifest axis. But 75% of the time, I’m on the topographic axis.”

Normalizing keratoconus or other irregular corneas

“Topographic ablations are designed for abnormal corneas,” said Karl Stonecipher, MD, Greensboro, North Carolina. “This topography- guided treatment to normalize the cornea has been combined with crosslinking to lock it into place. With topographic ablations, we are taking your God-given optical system and normalizing the topography to a reference shape.
“Today, there is controversy about whether to use topography-guided refractive laser vision correction or topographic modified refraction laser vision correction,” Dr. Stonecipher said. “Contoura [Alcon, Fort Worth, Texas] was originally used to fix sphere and cylinder, then we normalize topography. Now, we are normalizing the cornea with this topographic modified refraction and treating the sphere and cylinder. The question is, how do we treat keratoconus that is not forme fruste keratoconus? Should we do this topographic modified refraction and do laser vision correction? Or should we do what we’re calling TCAT, or topographic treatments, to normalize the cornea and laser vision correction on top of that at the same time?”

Post-refractive surgery patients

According to Dr. Stonecipher, a wavefront-optimized patient with residual topographic abnormalities should be treated with TCAT. “If you have a patient who has undergone TCAT, and he or she has a perfectly normal cornea but residual refractive error, you want to use wavefront-optimized,” he said.
Dr. Stonecipher and his daughter recently published an article on the influences of enhancement with this laser system, and they evaluated 4,029 cases. The enhancement rate was 0.74%. “You’re not enhancing many people because we are typically getting them to 20/20,” he said. “Now, we’re trying to figure out how to give them better than normal vision. We’re hoping to emulate what happened in the study, which is that approximately one-third of the patients got better vision than what they had before in their glasses or contact lenses.”
According to Dr. Gordon, in the United States, surgeons can only treat corneas that have a Q value of 0 to –1. “Most patients who have had previous myopic surgery have a Q value greater than 0, meaning it’s on the plus side. In this case, you have to reduce the spherical aberration on the target to 0 in order to do it. It’s like doing a hyperopic treatment,” he said. “You have to do a C12 C4 neutralization. You have to look at how much myopia you’re going to induce, just by trying to minimize the amount of positive spherical aberration.”
Dr. Krueger treats previously treated eyes on an off-label basis. “In those particular cases, I’m often likely to correct the aberrations and some of the astigmatism without necessarily getting it all, as far as astigmatism or myopia goes,” he said. “In other words, for irregular eyes, the key is to get rid of most of the aberrations. If you do that successfully, if there is a little residual myopia and astigmatism, you can always do a secondary procedure to eliminate that. I sometimes try to go for it all if it seems reasonable for me to do it, but there are a lot of factors that have to do with the change in the spherical refraction that occurs when you’re treating aberrations. If I look at the profile of what I’m treating and I make my modified correction to be zero myopia and zero astigmatism and only aberrations, I can look at the Zernike terms and look at the term of C12. C12 is the spherical aberration term, and I can determine if that’s a high value or a low value. If it’s a higher value, I can click in sphere into my modified ablation. Right now, it’s at zero, so I can start clicking in some minus sphere or plus sphere to see how the C4 term, which has to do with spherical correction, changes to match what the spherical aberration term says. When I match the C4 and the C12, I know that the correction of those aberrations is going to change the sphere by that much, and that’s how I have to modify what I plug into the treatment so as to not overcorrect or undercorrect.”

The future

Dr. Gordon thinks that surgeons will realize that it doesn’t take much longer to plan topographic ablations and that they do provide better results. This realization will increase their popularity.
Dr. Krueger is already seeing signs of its increase in popularity. “I heard a statistic that one-third of centers that have a WaveLight laser [Alcon] have the topographic unit,” he said. “It looked like of all the treatments being done in the U.S. with the WaveLight laser, more than 10% were being topographically guided, and I thought that was good for something that was brand new, more expensive, and a little more complex to figure out. I think there may be further growth in the area. However, I ultimately think that the continued success with this platform will fuel a company like Alcon to go to the next step, which is to link wavefront-guided with topography-guided with biometry and create an all-in-one treatment. If you have that capability, some of the things that we have had to use additional tools to figure out can be done automatically with those three technologies combined. I think that will make the precision of customized treatment much better than anything we have seen before. Then I think the acceptance will be even greater.”

Editors’ note: Drs. Gordon and Krueger have financial interests with Alcon. Dr. Stonecipher has no financial interests related to his comments.

Contact information

Gordon
: mgordon786@gmail.com
Krueger: krueger@ccf.org
Stonecipher: stonenc@aol.com

Current considerations for optimizing topographic ablations Current considerations for optimizing topographic ablations
Ophthalmology News - EyeWorld Magazine
283 110
220 154
,
2018-02-08T05:29:20Z
True, 2