WAVEFRONT 101
Redefining Custom LASIK
More than a wavefront, custom LASIK is a chain of events. Outcomes are only as good as the weakest link.
By John A. Vukich, M.D.

When referring to custom LASIK, we often use the terms "wavefront" and "custom" interchangeably, but the most important fact for up-and-coming refractive surgeons to understand is that these words, although related, are not one and the same.

The wavefront measurement is only one variable in the custom treatment equation. While custom treatment can't be performed without a patient's wavefront data, the wavefront measurement alone does not provide all the necessary elements for improving vision. The surgeon needs to take those wavefront measurements through a process to deliver better vision.

We need to redefine custom correction to fully understand it. Custom LASIK started as wavefront-driven correction where the visual aberrations were characterized by more than just sphere and cylinder. Since then, we've expanded custom platforms to include much more than developing and recognizing wavefronts.

Series of Components

Today, custom treatment refers to the synchronized interaction among a series of components that culminates in customized laser vision correction.

In the United States, four platforms work with wavefront data. Each system works with a particular laser, specifically:

►VISX CustomVue uses the VISX S4 laser. Of the systems that offer customized treatment, VISX treats the widest range of aberrations, including myopia, high myopia, hyperopia and mixed astigmatism.

►Alcon CustomCornea wavefront system uses the LADAR-Vision laser.

►Bausch & Lomb Zyoptix wavefront system uses the Technolas 217z laser.

The fourth platform, the WaveLight Allegretto excimer laser system, also uses wavefront-derived data to guide treatment, but it differs from the others because it does not measure each patient's individual wavefront for a true custom ablation. Instead, the laser adds an aspheric profile to the ablation pattern in an effort to neutralize the induction of spherical aberration rather than proactively treat individual patient wavefront data. Because of this, the Allegretto laser
is referred to as "wavefront optimized."

In standard mode, all four excimer lasers are approved for up to –14.00D of myopia, but we tend to be more conservative in their application. This is one of the rare instances where we don't take full advantage of the privileges granted to us by the FDA. In terms of custom applications, the VISX platform recently was approved for expanded treatment of high myopia up to –12.00D.

What does this distinction mean for offering patients the best refractive outcomes? Our ability to provide a higher level of custom treatment gives us greater versatility, helping us reduce the likelihood of night vision- and contrast-related complaints, such as difficulty seeing clearly on foggy or rainy days.

With conventional LASIK procedures, the potential to add irregularity increases with the amount of refractive error you try to correct. In other words, the deeper the hole you dig, the more likely you'll have some irregularity to show for it. Whereas, when you use a custom platform to treat myopia ranging from –8.00D to –11.00D, you're probably not adding more aberrations. Keep in mind that you can't count on custom treatments to always reduce existing aberrations, but you can be sure you're not making them worse, which is always a possibility when treating high myopia with conventional excimer lasers.

Fourier is better able to reconstruct complex shapes than Zernike reconstruction.

Essential Components

The four essential components of a custom LASIK treatment are capture, analysis, registration and delivery. All custom platforms capture, analyze and register wavefront data, as well as deliver the ablation in a way manufacturers believe best suits each system. While each of the platforms has some distinguishing characteristics, one principle remains true for all of them: The resulting quality of vision depends on the system's fidelity and the weakest link in the ablation process.

Custom LASIK takes all of those components — capture, analysis, registration and delivery — and develops a chain of events that must occur in the proper sequence with the same level of fidelity to achieve the best possible result. You may have the best possible wavefront analyzer, but if you don't develop the most accurate analysis of collected data, deliver the shape to the right spot, and track ablation precisely to account for eye movement, you'll lose the fidelity of the wavefront and deliver a poor translation of the custom procedure. 

All three platforms that have the components necessary for custom LASIK use the Hartmann-Shack model for the essential "capture" of the wavefront image. The information is then analyzed by Zernike (Alcon and Bausch & Lomb systems) or Fourier expression (VISX system) to bring out the detail and characteristics of the wavefront measurement. These approaches represent legitimate differences in how the wavefront is analyzed, affecting the shape that's generated and the fidelity of reproduction. The Fourier, or sine-wave-derived, transformation of a complex shape is more detailed, providing the equivalent of 64 orders of Zernike polynomials. The resulting shape is more complex and more faithful to the original (see "Data Resolution Summary"). In contrast, you can reproduce a simple shape accurately with a simple algorithm, but Fourier shines when you need a therapeutic application of a complex shape.

System Distinctions

Another distinction among custom ablation systems is how they register the visual axis to the optical axis. The shape produced by analyzed wavefront information must be appropriately registered to the target. The laser can apply a generated contour to the eye, but it needs to be projected precisely onto the right place on the cornea.

Registration, the process by which the pupil, the limbus and the delivery are synced up with the captured wavefront, is an important step in custom LASIK.

Finally, avoidance of fixation error is crucial to ensure that delivery is accurate compared with rotational components of the eye during normal respiration and ocular pulsations.

Incremental Advances

Currently, the steps we're taking to improve treatment accuracy are marginal. All approved platforms already report more than 90% 20/20 or better outcomes. Some systems are even reaching the high 90th percentile of 20/20 outcomes.

At this time, any changes we can make to improve these systems will involve incremental steps. The low-hanging fruit, so to speak, has already been picked. Now, each small step we take will cumulatively create better platforms. Better custom treatment eventually may depend on changes in the laser, instead of the current focus on improving communication among synchronized components.

The next logical step in the evolution of custom LASIK is torsional tracking. More than registration, this process tracks real-time rotational movements. Currently available excimer lasers track for X and Y, but none of them tracks for cyclotorsional movement. Incorporating this feature may be the next step in improving custom LASIK.

This brings us back to the main lesson: Custom LASIK isn't just about the wavefront anymore. It's about the platform. At this point, providing accurate laser vision correction requires a holistic approach. Individually, none of the components will achieve the goal of giving patients the best optical solution.

What Patients Want

We should never lose sight of the fact that, ultimately, our patients want better vision. They don't care how fast the laser tracks, how big the spot size is or what mathematical solution was used to measure their wavefront. Even if they show up in your office spouting sound bites and catch phrases that they've gleaned from Internet research, in the end they don't really care about the mechanics of laser surgery. What they do care about is seeing better without glasses. Our job is to figure out the nuances of LASIK technology so that we're comfortable enough to deliver its full potential.

What's Ahead?

The alphabet soup of acronyms that chronicles the evolution of refractive surgery is all you need to appreciate the progress that has been made in the past quarter of a century, as well as the fleeting nature of each "next best thing." My advice to young practitioners is make an effort to keep their excimer laser systems current, follow new advances and use all available resources to achieve these goals.

John A. Vukich, M.D., is assistant clinical professor at the University of Wisconsin, Madison.

Latest CustomVue Outcomes

Young ophthalmologists embarking on a refractive surgery career today are the lucky recipients of a generation's worth of development and research, the culmination of which is wavefront technology and custom LASIK. A standout among this technology is VISX CustomVue used in tandem with the WaveScan WaveFront System. Recently approved for treating high myopia and mixed astigmatism, the VISX platform offers the widest range of treatment indications for refractive surgery patients. Figure 1. With the CustomVue platform, 84% of eyes achieved uncorrected visual acuity of 20/20 or better, and 65% achieved 20/16 or better. High Myopia and Cylinder The CustomVue system is approved for reduction or elimination of myopia and myopic astigmatism between –6.00D and –11.00D mean radical sphere error (MRSE), with cylinder between 0.00D and –3.00D. Patients must be 21 years old or older with documented evidence of a change in manifest refraction of no more than 1.00D in cylinder and sphere for at least 1 year prior to the date of preoperative examination. I'm very excited about offering custom LASIK to my patients with high myopia. The results from the clinical trial are outstanding. No high-myopia results on any other laser platform, past or present, come close to the results that were achieved in this clinical trial. Clinical Trial Results The FDA clinical trial results illustrate that 6 months after undergoing custom ablation with the CustomVue platform, 84% of eyes achieved uncorrected visual acuity (UCVA) of 20/20 or better and 65% achieved 20/16 or better. (Figure 1) At this same time point, 75% of eyes had the same or better post-op UCVA compared with pre-op best spectacle-corrected visual acuity (BSCVA). (Figure 2) All eyes were targeted for emmetropia; 77% of eyes were within 0.50D and 96% were within 1.00D of the intended correction. No eyes lost more than two or more lines of BSCVA. Based on these results, the CustomVue LASIK procedure appears to be better or significantly better than conventional LASIK treatments. The multicenter study comprised 184 eyes of 94 patients, ranging in age from 23 to 55. The preoperative refractive error range of the cohort was –5.47D to –11.25D of sphere; 0.00D to –5.22D of cylinder; and –6.35D to 11.75D of MRSE. Eyes were treated with a minimum 6-mm optical zone and an 8-mm ablation zone. Figure 2. At 6 months, 75% of patients had the same or better uncorrected visual acuity post-op compared to their pre-op best corrected visual acuity. About half the cohort (52%) had UCVA of 20/16 1 month after surgery, whereas 85% achieved 20/20 UCVA at that interval; 92% had 20/25 UCVA 1-month post-op; and 99% had 20/40 UCVA at the 1-month mark. Preoperatively, 63% of patients were satisfied or very satisfied with their BCVA at night compared with 80% who were satisfied or very satisfied with their uncorrected vision at night 6 months after surgery. Adverse events included diffuse lamellar keratitis in two eyes that resolved with no significant loss of BSCVA. Only four eyes of three subjects, or 2% of the study cohort, needed to be retreated. Although the clinical investigators felt that four re-treatments were not sufficient to derive clinically useful information, they agreed that surgeons should take care that patients' refractions are stable before resorting to re-treatment. Treatment Rate and Registration In addition to good outcomes, the CustomVue platform also affects overall treatment time. In clinical trials, the CustomVue platform treated –6.00D of myopia 10 seconds quicker than conventional LASIK. CustomVue also treated –10.00D of myopia in less time, reducing procedure time by about 5 seconds compared with conventional LASIK. The CustomVue high myopia and mixed astigmatism clinical trial outcomes illustrate the significant progress of custom LASIK over conventional LASIK, but I expect the addition of iris registration technology to bring further improvement. Increasing numbers of refractive surgeons are starting to incorporate iris registration into the CustomVue platform, and I expect to see data supporting the benefits of this addition in the near future. - Colman R. Kraff, M.D.