Wavefront's Effect on the Future of LASIK
The hottest research and clinical developments in refractive surgery these days have to do with advanced eye measurement systems using wavefront and ray tracing technology. The ultimate goals of these technologies are to quantitatively and qualitatively measure and treat those illusive cases with irregular astigmatism or higher-order aberrations that clinicians could only previously treat with rigid contact lenses. Currently, approved lasers can correct regular spherocylinders because they ablate only symmetrical patterns on either side of the vertical or horizontal meridians of the visual axis. Irregular astigmatism or higher-order aberrations that directly affect not only the quantity of vision but also importantly the quality of vision, are not presently taken into account during symmetric ablations. Higher-order aberrations make it difficult for refractive surgeons to plan procedures, let alone adequately achieve an acceptable outcome. In this article, we will hear from top refractive surgeons who believe that the wavefront and ray tracing technologies will change the way physicians and patients approach and feel about refractive surgery, respectively, and we will also hear from those clinicians who are not as optimistic, stating that wavefront has not and most likely cannot live up to the hype.
THE MECHANICS
Wavefront analyzers work by passing light into the eye and then obtaining measurements of the light reflected back out through the cornea. (Figure 1A and 1B). Robert Snyder, MD, Professor and Department Head at the University of Arizona College of Medicine in Tucson, AZ, presents an analogy describing the use of a penlight. “If you had a little penlight somewhere out in the universe, and somebody turned on that penlight, the light that came out of that penlight would be emitted in a wave form, just like somebody throwing a pebble into a lake.” When the eye refracts light waves, the light waves should ideally remain in a plane so that they are focused directly onto the fovea. Any deviation from that flat plate configuration in the wavefront analysis is called an aberration (Figure 2A and 2B). A diagram of a “perfect,” undisturbed wavefront emerging from the eye would look like a sheet of paper standing on end. However, irregularities in the visual system can change the wavefront pattern into a contorted three-dimensional shape.
Jeffery Machat, MD, the National Medical Director for TLC Laser Eye Centers in Toronto, Canada, tells us that second-order aberrations refer to defocus and regular astigmatism. Defocus simply means the eye is either nearsighted or farsighted (Figure 3). A second-order astigmatism is simple, regular astigmatism. The next level of aberration is third-order, and includes vertical and horizontal coma as well as third-order trefoil. Coma means a localized area that is not as even as the other areas and appears much like an inhomogeneous ablation from topography with one half of the circular ablation blue and the other half red. The next, or fourth-order, is spherical aberration which means that the peripheral part of the curve does not focus light rays correctly onto the center of your vision.
Data from the wavefront maps are used to describe the three-dimensional surface of the wavefront. This is completed mathematically by a polynomial series; the Zernike method is used most often. The surface description becomes more complicated the further one goes out in the polynomial expansion.
BENEFITS
As laser refractive surgery has developed throughout the last decade, refined techniques and technology continue to improve capabilities, thus there are expected improvements in outcomes and decreasing side effects. Dr. Machat states, “In the past, patients were satisfied with 20/30 vision, and now 20/20 is the standard. At this time, we're looking at trying to get people not only beyond 20/20, but we are trying to get people to have a better quality of vision.” Commander Steven Schallhorn, MD, the Director of Cornea and Refractive Surgery at the Naval Medical Center in San Diego, CA, says that the potential of wavefront is for greater efficacy of intended results and reduced induction of negative “quality of vision” symptoms. “There is also a possibility of improving vision better than is achievable with glasses in some patients,” he says. Commander Schallhorn feels that wavefront's potential is powerful, but unproven as yet.
Identifying Optical Problems in the Eye
Prior to wavefront/ray tracing technology, surgeons were limited in their ability to identify, let alone quantify, sources of higher-order errors within the eye. They could identify the presence of higher-order errors with corneal topography, but were still unable to quantify the amount or degree of error. Presently, wavefront technology allows surgeons to identify and measure these aberrations, and design treatments for them, whether they are for corneal ablations or for developing contact lenses and intraocular lenses, explains Dr. Snyder. Scott MacRae, MD, Professor of Opthalmology and Visual Sciences from the Department of Ophthalmology, University of Rochester Medical Center, Rochester, NY, adds, “It is critical to know the pupil size in the evaluation of higher-order aberrations. The larger the pupil size, the greater the amount of higher-order aberrations.”
Improve Outcomes and Minimize Complications
Fewer over- and under-corrections. Refractive surgery will not be expected to correct all aberrations, so that everyone will achieve “super vision,” but it will shift the number of patients achieving these visual acuities to a higher percentage. Ronald Krueger, MD, Medical Director of the Department of Refractive Surgery, Cole Eye Institute, Cleveland Clinic Foundation, Cleveland, OH, states, “The outcomes of refractive surgery will be improved by increasing the number of patients who will achieve 20/20, 20/16, 20/12.5, and 20/10 vision. He explains that complication rates related to microkeratome blades, infection, and dry eyes would be no different, but hopefully, there would be fewer over-and under-corrections and fewer halos seen with night driving, diplopia, and other aberrations.
Induce fewer aberrations. Each time a surgeon performs refractive surgery, aberrations are induced in the eye—this is unavoidable. However, do those aberrations produce side effects to the patient? “What we're doing right now with wavefront is not reducing the amount of spherical aberrations compared to what they were preoperatively, but inducing far fewer than conventional LASIK for myopia. We are going to need to have very fast lasers that use very small spot sizes with extremely accurate eye tracking systems. Virtually every company is moving in that direction with the flying spot technology now,” offers Dr. Machat.
Who Will Benefit?
According to Dr. Snyder, patients who have spherical aberrations, as well as those who have relatively flat corneas and higher amounts of myopia, stand to benefit from wavefront technology. Jim Schwiegerling, PhD, Assistant Professor in the Department of Optical Sciences at the University of Arizona, in Tucson, adds that patients who need enhancements due to previous refractive surgeries, as well as patients who have a fair amount of irregular astigmatism, will also benefit from this new technology, as long as the problem is optical and not retinal.
Prescreen
Patients whose preoperative corneal asphericity will lead to very oblate corneas following surgery with conventional ablation patterns, or patients with irregularities or asymmetries in their wavefront readings, will not do well with the standard non–custom-type ablation according to Dr. Snyder. These patients may end up with induced spherical aberrations or longitudinal spherical aberrations. Dr. Snyder explains that prescreening patients with a wavefront analyzer can identify those on whom surgery should not be performed. The surgeon can mimic the surgery based on wavefront readings and knowledge of ablation shape by testing the theoretical wavefront after surgery. He or she can see if the patient is likely to have a positive outcome. In the future, Dr. Snyder expects that surgeons will be able to combine wavefront technology and corneal topography to design ablation patterns to minimize induced spherical aberration in order to optimize patient outcomes.
Custom Fit
Dr. Snyder believes that wavefront will be extremely valuable in designing custom ablation patterns, and compares the process to fitting a pair of shoes. The more sizes available, the more flexibility there is with fitting. Ultimately, every eye as an optical system is different and the goal is to customize the ablation to optimize the visual performance for the given optical system.
HURDLES
At the present time, there is a tremendous number of limitations with wavefront technology, especially in terms of acquiring data. Dr. Schallhorn explains that the theory of wavefront has its foundation solidly in physical optics. The correction of higher-order aberrations has been used successfully to improve telescope imaging for many years, but its application to the eye has only recently been introduced. It holds promise, but the field is in its infancy. “I don't believe there is any compelling evidence yet that correcting higher-order aberrations in normal eyes improves visual function,” says Dr. Schallhorn. “The problem is that the hype has gotten in front of the science. The promise of what wavefront can do is way ahead of its evaluation, which will take years.”
Data Interpretation Difficulties
Variables. Dr. Machat agrees that there are issues with wavefront that are not yet realized. These concerns include problems acquiring data, how to interpret the data obtained, and various methods of analyzing the data. The wavefront map is not only pupil-dependent and accommodation-dependent, but also tear film-dependent—a patient can blink and his or her wavefront can change. An individual's wavefront is also age-dependent, meaning that the wavefront map tends to be fairly constant in terms of higher-order aberrations until about age 40, and then the aberrations begin to vary due to the natural aging process of the eye. Another problem with customization is that it can potentially exacerbate presbyopia. For example, a patient may be corrected to 20/8 at distance, but may lose more acuity at near and intermediate distances.
Difficulty with readings. Not all units can provide accurate readings in irregular or post-refractive eyes. “With the Hartmann-Shack device, I cannot take a reading, nor am I able to get a reproducible reading. With the Tracey (Tracey Technologies, Inc; Bellaire, TX) I can,” explains Dr. Machat. The Tracey measures one point at a time at the entrance pupil, rather than the entire entrance pupil at once to avoid the possibility of data points criss-crossing in the case of eyes with severe higher-order aberrations. He continues, “We can put hundreds of light rays through the entrance pupil, but because we are putting one light ray through at a time, we know exactly where that light ray ends up on the back of the eye.”
Registration
Dr. Machat tells us that when lasering the eye, to take the wavefront map and create an ablation map or profile. “We go ahead and put the algorithm into the laser, we'd laser the patient, but we would have to make sure that he or she is lined up in exactly the same fashion that we took the wavefront reading, what we call registration.” If the patient is not registered exactly the same, then he or she might move and each point would be lasered improperly.
Changing Aberrations
There is a chance that the effect of any refractive correction procedure will regress. The surgeon can correct a patient to 20/10 and then 2 years later, the patient could end up with 20/20 vision because of changing aberrations. Dr. Machat provides an example: “Suppose we take a patient who is between 50 and 60 years old, and do a wavefront-guided ablation and treat other higher-order aberrations to give him or her extremely good vision. If that patient developed a cataract, and if we removed the cataract, this would create a reverse template of the patient's aberrations.” The patient's vision after cataract surgery would have a negative impact because of reverse aberrations being induced in the cornea. The patient would need either a custom IOL or another custom corneal ablation.
FUTURE APPLICATIONS
We have not even begun to tap the potential of future surgical applications. Dr. Machat notes that lasers are continuing to improve, but not every single patient will end up with 20/10 vision, or 20/8—there is too much variability with the biomechanical effects of the cornea. “The jump from PRK to LASIK was a very significant jump, and now what we are seeing is that the jump from LASIK to custom-LASIK will be as significant. So this is going to revolutionize the industry all over again,” says Dr. Machat.
Dr. Snyder states that designing treatments such as LASIK ablations, PRK ablations, as well as phakic and pseudo phakic IOLs will certainly optimize visual performance. Wavefront defines the desired shape of the cornea, or the shape of an IOL which will provide perfect vision. Dr. Snyder comments, “There is probably going to be a utility with wavefront in determining if there is a cataract, something in the visual pathway, or something altering the waveform.” Dr. Krueger adds, “Our retina colleagues will use wavefront for creating adaptive optics of lenses to see the retina and photoreceptors with greater resolution than conventional optics.”
Dr. Schwiegerling says that additional applications of wavefront technology include optical design, customized contact lenses, binoculars, telescopes, and microscopes (Figure 4). He states, “We will be able to take a look at people with visual deficits and determine whether it is an optical or neural effect. For instance, if you measure with the system and the optics look perfect, but if the patient still sees poorly, it's a problem in the visual pathway.”
WAVEFRONT'S AFFECT ON THE FUTURE OF LASIK
Despite its current limitations, wavefront will continue to improve. Dr. Machat offers, “Microkeratomes have improved dramatically over the past 3 years, and now that we are going for an all-laser solution with Intralase's femtosecond laser technology, it is going to evolve. As we introduce wavefront, it is going to get even better because we are going to be able to customize it for a patient, actually eliminating a lot of the aberrations that we are inducing, as well as the naturally occurring aberrations, to obtain much better visual performance for patients.”
Marketplace Increase
Dr. Machat feels that wavefront technology will have a dramatic impact on the refractive marketplace. He explains that using wavefront technology will give greater value to the patient and help maintain pricing, which he feels is going to be important.
“It is also going to make a lot of the people who had been contemplating the procedure, but who have a very high fear factor far more comfortable, and more likely to have the procedure,” says Dr. Machat. A higher safety profile also contributes to an increase in the penetration rate, which will be positive for patients as well as doctors.
Higher Safety Profile
Dr. Machat explains that with wavefront technology, the surgeon will be able to enter extremely accurate data which will result in improved postoperative outcomes. The better surgeons can define the refractive map of the cornea and the eye preoperatively, the better he or she will be able to treat potential postoperative problems. Dr. Machat clarifies, “It's almost like insurance. We are taking a template of the refractive profile of the eye and if the patient develops irregularities, we can look at the postoperative eye, look at the preoperative eye, subtract the difference, utilizing what the theoretically calculated curvature should have been, and determine the appropriate treatment.”
For about a year, Dr. Machat has been working on ray tracing technology with Tracy Technologies to try to smooth any irregularities, but the problem is that not every single irregularity should be removed. “We actually want to leave some of that irregular profile because if there is one little area that is elevated, the standard treatment with TopoLink would smooth it,” illustrates Dr. Machat. He explains that right now, we look at the light rays that travel through that bump and we see where they focus on the back of the eye. If they focus at the fovea, we don't touch that bump, and if they don't focus at the fovea, then we do. The amount that we smooth the bump is directly related to the amount that the light rays deviate from where they should land on the fovea, to where they do land on the fovea. HYPE OR HOPE?
I'm a Believer
Standard of care. Dr. Machat says that even if we don't routinely get people beyond 20/20, what we are seeing is a greater safety profile; far fewer people are losing best-corrected vision compared to noncustomized treatment. “I remember when I talked about PRK and people were still touting RK, and when I was talking about LASIK, people were advocating PRK. Well, I am going to be at the front of the pack, saying that wavefront-guided ablations, custom LASIK, is definitely going to become the gold standard for refractive surgery 2 to 3 years from now,” he proclaims.
Tried and true. Dr. Krueger, who uses the Alcon LADARWave (Fort Worth, TX), affirms that wavefront aberrations have already been correlated to visual acuity in a number of ways. Retinal researchers can image individual photoreceptors in the retina by using adaptive optics generated by wavefront technology. VISX, Inc. (Santa Clara, CA) has developed a Prevue lens which corrects for the wavefront aberration in plastic and then lets the patient see with his or her aberrations corrected. Finally, Alcon has performed side-by-side comparisons of customized ablation (LASIK and PRK) in one eye, and conventional treatment in the contralateral eye, showing initial improvement in best-corrected vision at a higher percentage in the custom cornea eye.
A breakthrough diagnostic tool. Dan Durrie, MD, Clinical Assistant Professor of Ophthalmology at the Kansas University Medical Center in Kansas City, MO, maintains that wavefront is a breakthrough, as it provides diagnostic equipment that will identify which patients should have surgery and why the patient may not be seeing as well as expected. Dr. Durrie states that wavefront is not yet ready to run our lasers. “This needs to be studied in depth. That's why there are FDA clinical studies ongoing with these devices connected with lasers,” he explains. So far, the results of those studies have been very promising. Dr. Durrie is involved in both the Alcon and Bausch & Lomb (Rochester, NY) studies and says that the preliminary results appear promising. He considers wavefront to be a diagnostic tool and expects that eventually, because it is digital data, it will be able to run the software of the laser, bypassing a great deal of the testing that is done right now.
It's Overhyped
New and cool. Robert Kershner, MD, Director of the Orange Grove Center for Corrective Eye Surgery in Tucson, AZ, adds, “There's a lot of money and energy being hoisted into this area simply because it's new and it's cool. It's hot and docs exclaim, ‘Wow, maybe this is the way I should go.' But the bottom line is this—we don't know if neutralizing the optical aberrations within the eye are truly going to result in better vision.” So far, all the results on wavefront technology currently offered by several manufacturers have not shown that it is superior to anything we already have, he concludes.
All sizzle and no steak. Vivek Kadambi, MD, head of the Department of Ophthalmology at the Bangalore Hospital in Bangalore, India, declares that first, it is almost impossible to perfectly assess the refractive status of a patient. Secondly, it is impossible to precisely control all variables that influence laser settings. Dr. Kadambi says that he has seen many marketing gimmicks come and go, such as automated microkeratomes, active and passive tracking devices, erodeable masks, energy stabilizers, and automated gas refills. “I firmly believe that there is no substitute to surgical skill and common sense,” states Dr. Kadambi. “Today, the concept of wavefront is being used solely for the purpose of marketing. This concept is ‘all sizzle and no steak.' “ Dr. Kadambi believes that wavefront will die a “natural death,” but is sure it will leave behind some useful offshoots.
There Is a Chance
Dr. Nordan maintains that wavefront only has a chance if it can show that the consistency of a problem causes a certain type of visual problem determined by wavefront; if the problem is consistent and reproducible, wavefront could potentially change the cornea to fix that aberration. However, it must be proven that in normal, healthy eyes, wavefront correlates with the visual problem. “Right now, there is no correlation and if there is no correlation, anybody who is saying that wavefront is going to help their surgery, is just hoping it's going to help,” says Dr. Nordan.
Understanding aberrations. Dr. Krueger points out that the first step toward gaining widespread acceptance in diagnosing, understanding, and treating higher-order aberrations, is finding a way for the surgeons to learn how to read the maps. Just as it took some time for corneal topography to gain popularity, wavefront technology will take some time before ophthalmologists will be comfortable in using it as a diagnostic tool. There are many factors that will require control before the bulk of aberrations will be correctable. The biomechanics of the cornea, flap-induced aberrations, wound healing, as well as accurate and reproducible centration, will all take time to develop and refine, and ultimately will lead to more and more complex nomograms.
PARTING REMARKS
Correcting higher-order aberrations is not going to be easy. Although there are still many unknown issues to resolve before customized ablations become a reality, Drs. Machat and Krueger are confident that wavefront will be the standard of care within the next few years. Dr. Kershner states that with LASIK, for instance, customized ablations could potentially neutralize inherent aberrations. Or, it may turn out that neutralizing corneal aberrations could worsen the effect of treatment. Dr. Nordan asserts, “I think it's truly 50/50 whether wavefront will have no value and that it's great for marketing, sounds great for engineers, but has nothing to do with eye surgery. I think it's possible that if consistency and correlation can be shown, that it could then help us achieve better results.” Dr. Machat is more optimistic on the topic, “Although we are in the infancy of this technology, the fact that we can reduce the amount of spherical aberration that we induce with refractive surgery makes it a superior procedure.” Don't get your hopes up for “super vision” at the moment, as wavefront seems to produce as many questions as it answers.
Jeffery J. Machat, MD, FRCSC, DABO, is the co-founder and National Medical Director of TLC Laser Eye Centers, in Toronto, Canada. Dr. Machat has no financial interest in any of the materials mentioned herein. (416) 364-8475: jeff.machat@tlcvision.com
CDR Steven C. Schallhorn, MD, is the Director of Cornea and Refractive Surgery at Naval Medical Center, San Diego, California. Dr. Schallhorn has no financial interests in any of the materials mentioned herein (619) 532-6700; schallhorn@nmcsd.med.navy.mil
Scott M. MacRae, MD, is a Professor of Opthalmology and Visual Sciences at the University of Rochester, Rochester, New York. Dr. MacRae serves as a consultant to the FDA, as well as Bausch & Lomb and Nidek, Inc. (716) 341-7817; Scott_MacRae@urmc.rochester.edu
Ronald R. Krueger, MD, MSE, is the Medical Director of the Department of Refractive Surgery, Cole Eye Institute, Cleveland Clinic Foundation, Cleveland, Ohio. Dr. Krueger is a paid consultant for Alcon Surgical. (216) 444-8158; krueger@ccf.org
Jim Schwiegerling, PhD, is an Assistant Professor of Ophthalmology and Optical Sciences at the University of Arizona, in Tucson. Dr. Schwiegerling has no financial interest in any of the materials mentioned herein. (520) 621-8688; jschwieg@U.Arizona.edu
Dan S. Durrie, MD, is in private practice with the Hunkeler Eye Centers in Overland Park, Kansas, and is Clinical Assistant Professor of Ophthalmology at the Kansas University Medical Center in Kansas City, Missouri. Dr. Durrie is a paid consultant for Nidek, Bausch & Lomb, Alcon, Sunrise, and Allergan. (913) 491-3737; ddurrie@novamed.com
Lee T. Nordan, MD, is the Director of Nordan Laser Eye Medical Group in Carlsbad, California. Dr. Nordan has no financial interests in any of the materials mentioned herein. (760) 930-9696; lasertn@aol.com
Robert M. Kershner, MD, is the Director of the Orange Grove Center for Corrective Eye Surgery in Tucson, Arizona. Dr. Kershner has no financial interests in any of the materials mentioned herein. (520) 797-2020; ogeyes@aol.com
r. Kadambi has no financial interests in any of the materials mentioned herein. + 91-80-6562753; vkadambi@blr.vsnl.net.in
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