IT'S EASY TO BELIEVE THAT WAVEfront-guided ablation is superior to standard ablation. After all, today's wavefront-guided procedures have consistently produced better outcomes than traditional procedures--at least in the United States.
According to Jack Holladay, MD, MSEE, FACS, clinical professor of ophthalmology at Baylor College in Houston, however, the improvement in outcomes may be the result of three advances in technology that have nothing to do with customized wavefront. In fact, he says, these same advances can be made part of standard software at a much lower cost to doctor and patient, producing outcomes at least as good. (Dr. Holladay says this has already happened internationally, especially in Europe.)
"Three changes have been made to the software in wavefront-guided systems that haven't been made to standard system software, at least in the United States," he says. "As a result, wavefront-guided ablations do three things standard ablations do not: They keep the cornea prolate, produce larger astigmatic corrections, and reduce the formation of central islands. I believe these three changes account for the improvement in performance seen in comparative studies."
In fact, Dr. Holladay says that the whole idea of correcting higher-order aberrations by ablating the cornea is fundamentally flawed. "The Food and Drug Administration won't allow manufacturers to claim that these systems correct higher-order aberrations, because manufacturer's studies for FDA approval haven't provided any scientific evidence that they do."
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Jack T. Holladay, MD, MSEE, FACS |
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Houston surgeon Jack Holladay cites this example in which the wavefront treatment area is almost 25 percent larger than the standard treatment area based on the same prescription. |
Keeping the Cornea Prolate
Laser ablations tend to produce an oblate cornea, says Dr. Holladay, because many of these lasers were first calibrated using a flat surface, not a dome-shaped surface like the cornea. "Any electromagnetic energy striking a surface, including excimer laser energy, becomes more spread out and less effective when the surface is tilted relative to the light source," he notes. "The laser's effective energy drops off as the beam moves away from the center of the cornea, by 16 percent at 1 mm; 20 percent at 2 mm; and 28 percent at 3 mm. [See graphic, page 65.] This is the reason these lasers produce an oblate cornea; they're not ablating as effectively away from the center of the cornea. All that's necessary to correct this is to embed an energy compensation curve into the software."
Dr. Holladay says correcting this problem is important for two reasons. First, making the cornea oblate increases spherical aberration. "The surface of the cornea in front of the scotopic pupil must be prolate in order for the rays in the periphery not to bend too strongly, causing blurred focus," he says. "Making the cornea oblate increases the total spherical aberration of the eye, producing nighttime halos and glare complaints."
Second, an oblate ablation shrinks the optical zone. Three years ago, Dr. Holladay demonstrated that if you make the cornea more oblate, a greater optical correction produces a smaller functional optical zone.1 "For example," he says, "we did two -12 D treatments using the LaserSight laser. Our intention was to create a 6.5-mm optical zone. A standard treatment with the resulting oblate cornea produced an effective 5.5-mm optical zone. By doing nothing different for the second patient except compensating for the drop-off in energy toward the periphery, we produced an 8.5-mm optical zone." He has shown that a -5 D treatment without prolate compensation shrinks the optical zone by about 10 percent; a -10 D treatment shrinks it 25 percent.
Dr. Holladay adds that any excimer laser system can upgrade its software to make this correction and produce a prolate cornea. "With one exception—Wavelight's Allegretto—laser manufacturers haven't added this feature to their standard laser software in this country, only to their wavefront-guided software," he says. "Not surprisingly, the WaveLight Allegretto gets the best results of any standard laser ablation system in America today. Its results are equivalent to the wavefront-guided treatments of Visx, LADARVision and Technolas." 2 He notes that this feature is part of the standard software in Europe, and says that surgeons he has spoken with in Europe are favoring topography-customized ablations or standard prolate software instead of wavefront-guided systems.
"This prolate compensation is the major reason for the improved outcomes seen with wavefront-guided systems," he adds, "not customized wavefront measurements."
Sizing with the Smaller Axis
The second significant change made in wavefront-guided laser software, according to Dr. Holladay, involves the size of the astigmatic ablation zone. An astigmatic correction takes the form of an elliptical or oval shape which can be defined by two axes, one short (minor) and one long (major). Previously, when a laser was set to create a 6-mm optical zone, the software produced an astigmatic correction with a major axis that was 6 mm wide. This made the short axis less than 6 mm wide.
Dr. Holladay says a standard plano -4.00 x 90 degree treatment set for a 6-mm optical zone actually ends up being 4.5 by 6.0 mm. "About three years ago I explained to the FDA that when a 6.0 mm optical zone is specified it must be at least 6.0 mm in both the major and minor axes to be correct and avoid potential halos at night," he says. "They agreed, and as a result a plano -4.00 x 90 degree wavefront ablation is now 6.0 by 7.5 mm. This makes the wavefront treatment area almost 25 percent larger than the standard treatment area based on the same prescription.
"This means that when we compare overall results involving astigmatic correction done by wavefront-guided systems with results achieved using traditional system software, we're comparing different size optical zones that are labeled as being the same! Is that a fair comparison? Absolutely not." He notes that a substantial number of patients in those comparative studies had astigmatism of 3 to 4 D, more than enough to have a profound impact on the resulting quality of vision.
Cleaning Up the Islands
The third change made in wavefront-guided systems is improved software designed to reduce islands in the center of the treated zone. Dr. Holladay says the specific change is proprietary and hasn't been published by any of the companies, but like the other two improvements, in America this has only been incorporated into the software used in the wavefront-guided lasers.
He notes that several European laser systems currently provide all three upgrades as part of their standard laser package, including Carl Zeiss Meditec, Mel 80, Schwind and Wavelight. "However," he adds, "only Wavelight was willing to spend the time and money to get FDA approval to add these three features to its traditional laser system software in the United States."
A Problematic Premise
In spite of the improvement caused by the addition of these features, custom wavefront guidance leads to ablations that often increase higher-order aberrations, Dr. Holladay says. The exception: spherical aberration, which isn't increased because the wavefront-guided systems keep the cornea prolate.
The reason, according to Dr. Holladay, is that most of the higher-order aberrations (excluding spherical aberration) are located in the crystalline lens; ablating the cornea won't address that. To correct this is problematic if four ways:
- Correcting higher-order aberrations on the cornea creates an on-axis system. "The eye is a two-lens system," says Dr. Holladay. "Correcting a tiny bump on the crystalline lens with a tiny ablated divot on the cornea can only cause limited on-axis improvement for a single point at distance, such as a star. Points that are just a little off-axis may actually be made worse. If you're looking at a printed word, the center letter will be clear, but the letters further away will get progressively less clear.
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| Surface tilt undercuts the impact of laser energy away from the center. |
"Most surgeons know intuitively that it's inappropriate to correct problems on one surface by changing another surface," he adds. "That's why many surgeons are moving to refractive lensectomy when the problem is in the crystalline lens."
Changing the cornea changes the path of the light rays. Dr. Holladay notes that making a change in the cornea to correct a problem in the crystalline lens assumes that the "corrected" light rays will follow the same path through the crystalline lens. In fact, he says, changing the cornea assures that the light will take a different path. "The crystalline lens isn't symmetrical; it's aspheric with a gradient index of refraction," he says. "So, the 'correction' can't be effective, and it may even make vision worse. This could account for some of the increase in higher-order aberration noted after customized wavefront treatment." In addition, Dr. Holladay observes that even if you could line up the correction and the flaw perfectly, the alignment wouldn't last; the optics of the crystalline lens change as we age.
Laser systems don't compensate for pupil/cornea misalignment. "The average person's pupil is nasally displaced about 0.4 mm from the geometric center of the cornea," Dr. Holladay points out, "putting the line of sight about 5 degrees away from the optical axis of the eye. So, when we center the laser over the pupil, we're centering the treatment over the nasal side of the cornea, not the top of the cornea. Unless the software compensates for this, we end up treating asymmetrically, inducing coma and irregular astigmatism."
Releasing the stretch on the LASIK flap induces aberration. Dr. Holladay notes that when tissue is stretched, irregularities are smoothed out. This is why a fully inflated rubber balloon is shiny, producing a clear reflection, but the surface loses its optical smoothness and doesn't produce a clear reflected image as it deflates. He notes that the same principle applies to corneal tissue. "Before a LASIK flap is cut, the corneal collagen fibers are under tension created by intraocular pressure. Once we cut the flap, the corneal fibers in the flap are no longer on stretch and optical smoothness is reduced."
The Topography-Guided Alternative
Because higher-order aberrations are primarily in the crystalline lens, Dr. Holladay says the only truly effective way to correct them is to treat the aberrations directly with surgery or replacement of the crystalline lens. However, he acknowledges that altering the cornea can certainly improve vision. "Treatments such as conductive keratoplasty or Intacs often exaggerate the prolate shape of the cornea, compensating for positive spherical aberration in the aging lens," he notes.
"Also, it is possible for the cornea to be the source of some higher-order aberration besides spherical aberration. When that's the case, correcting it by altering the corneal surface makes sense, but topography is much more precise for this measurement than wavefront." He adds that many doctors he has spoken to outside of the United States are foregoing custom wavefront and using custom topography-guided systems to correct the problems located on the surface of the cornea, with excellent results.
"I believe that in the future wavefront measurement will still be used to determine where problems are located and how much spherical aberration needs correction. But we'll use asymmetrical, topography-guided treatments instead of wavefront-guided, and we'll get better results."
From Myth to Reality
Given Dr. Holladay's observations, it seems clear that the only way to judge the true capabilities of custom wavefront is to conduct a comparison between a custom wavefront system and a traditional system that share the same software advantages.
"So far, wavefront is much like 'The Emperor's New Clothes' ... a story with no real substance," he says. "Hopefully, the success of Wavelight's Allegretto system with its improved standard software, and a clearer understanding in the United States of what's actually happening, will pressure manufacturers to add the three upgrades to the standard software in all lasers. This will relieve our patients and practices of an unnecessary financial burden and wasted time making unnecessary measurements. It may also force the creative minds in the industry to find ways to really make wavefront-guided ablation do what we've always hoped ... reduce unwanted higher order aberrations. The result of that revolution could be a truly impressive increase in our ability to restore or even improve the quality of vision." RO
1. Holladay JT, Janes JA. Topographic changes in corneal asphericity and effective optical zone size following LASIK. J Cataract Refract Surg 2002; 28: 942-947.
2. Kezirian GM, Stonecipher KG. Subjective assessment of mesopic visual function after laser in situ keratomileusis. Ophthalmol Clin North Am 2004 Jun;17:2:211-24, vii.
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In Defense of Wavefront
Other experts acknowledge that some of Dr. Holladay's charges have merit, but they dispute just as many.
- On the contribution of wavefront. Some of the improved results may be attributable to non-wavefront-related improvements, but custom wavefront plays a significant role, according to Steve C. Schallhorn, MD, director of cornea and refractive surgery at the Naval Medical Center in San Diego. "With current technology, it's difficult to correct small amounts of higher-order aberrations," he says. "Studies at the Naval Medical Center have found that patients who had very little higher-order aberration to start with may end up with an increase. However, most patients who start out with a lot of higher-order aberration end up with less aberration after custom wavefront. [See graph, above.] Custom wavefront works; it just doesn't yet work perfectly in every case."
Regarding the success of the "wavefront-optimized" Allegretto system, Dr. Schallhorn notes that a recent study compared the current Allegretto to a wavefront-guided version of the same system.1 "On average, the wavefront-guided Allegretto produced significantly improved contrast sensitivity one month after LASIK."
On prolateness. An analysis of several hundred eyes from Visx's U.S. investigational device exemption for wavefront-guided treatment of myopia and myopic astigmatism found "no correlation between the prolateness of the cornea and quality of vision as measured by visual acuity or contrast sensitivity at various frequencies," says Carol Harner, PhD, senior vice president of research & development for Visx.
Visx sees the "cosine effect"—the fall-off in energy as a function of corneal curvature—as a separate issue. "Visx lasers have always compensated for the cosine effect," says Dr. Harner. "The correction is the same in our traditional and wavefront-guided systems." She did not speak to whether competitors have the same kind of compensation in their traditional systems.
On astigmatism zones. All parties concede that the ablation zones for astigmatism are larger in the custom wavefront systems, which is clearly a factor in the improved results.
On central islands. Dr. Harner says that both traditional and custom Visx systems correct for central islands, and have done so since it first became an issue.
On creating an on-axis system by correcting crystalline lens aberrations on the cornea. Dr. Schallhorn agrees that this is true, but says that if it were a big problem, visual performance would be worse after surgery. "That's not what the data shows," he notes.
On redirecting light away from the aberration. Dr. Schallhorn says this might be true when aberrations are large, but most patients' quality of vision improves following wavefront treatment. Douglas D. Koch, MD, professor of ophthalmology at Baylor College of Medicine in Houston, agrees: "In a personal communication to me, Julian Stevens described using the Tracey System to compare the wavefront characteristics of every measured point before and after surgery in one of his patients. Every point was improved. That suggests that any deflection is minimal."
Dr. Koch agrees with Dr. Holladay that the crystalline lens will change over time. "But why shouldn't we provide patients with the best quality of vision possible now? If we have to touch them up later, that's fine."
On visual axis misalignment. Dr. Harner says Dr. Holladay's observation is correct, but both Visx systems take this into account. "I believe most companies are in various stages of correcting for it."
On aberration resulting from making the flap. Dr. Schallhorn says this applies to all LASIK. "It's not an argument against wavefront-guided ablation," he adds.
On topography-guided ablation. Dr. Schallhorn is skeptical about the potential of topography-guided ablation. "Basing the ablation on topography is like saying you'll only correct the diopters of myopia that are attributable to the cornea. I want to correct them all," he says.
On adding upgrades to traditional systems. "Most of us have decided that wavefront is the way to go," says Dr. Koch. "Adding upgraded features to the traditional system might require a clinical trial, and I would argue that it's a waste of a company's resources and time."
Brad Fundingsland, director of global professional education at Visx agrees. "We feel firmly that wavefront-driven ablations are the future of refractive technology. We're not planning to retroactively add new features to our conventional ablation system."
—C.K.
1. Kaiserman I, Hazarbassanov R, Varssano D, Grinbaum A. Contrast Sensitivity After Wavefront-Guided LASIK. Ophthalmology 2004;111:454-7. |
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Vol. No: 12:02Issue:
2/15/05
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JULY DIGITAL EDITION
Review of Ophthalmology
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