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Surgeons Opinions on Wavefront: The Demise of Conventional LASIK? PDF Print E-mail
Wednesday, 05 July 2006 07:38

Will customized LASIK procedures replace standard treatments?

http://www.birkacre.freeserve.co.uk/new_page_7.htm

BY William I. Bond, MD; Jack T. Holladay, MD, MSEE, FACS; Steven J. Dell, MD; James Schumer, MD; and Sam Omar, MD

Customized LASIK procedures, with their promise of tailored corneal laser treatments, will no doubt generate a lot of interest this year. In November 2002, the FDA approved the LADARVision system with the CustomCornea indication (Alcon Laboratories, Inc., Fort Worth, TX) for use in customized myopic laser procedures, and other companies expect similar approvals in 2003. Not only does customized LASIK look promising for those LASIK candidates who have been waiting for refractive technology to improve, but it is also exciting surgeons with its potential for re-treating patients who were disgruntled with their initial refractive surgeries. Cataract & Refractive Surgery Today asked a group of surgeons whether they thought this new-and-improved procedure would unseat conventional LASIK as king of the refractive marketplace.

William I. Bond, MD

I don’t think that customized LASIK will replace standard LASIK for at least the next 2 or 3 years. The first reason is that the technology simply isn’t there yet. We talk about customized LASIK technology a lot, but I don’t feel that it is accomplishing what it promises. We are only beginning to be able to measure higher-order aberrations with time-consuming and cumbersome technology, and we are unable to measure aberrations with any real sense of confidence or consistency.

The desire for surgeons to be able to say that they possess “customized” or “wavefront” technology pervades the refractive industry and currently far outstrips our ability to deliver concrete results to patients. There are so many treatments currently available that we refer to as “customized,” “wavefront,” “tailor-made,” or “individualized.” Abraham Lincoln said, “If you call a tail a leg, how many legs does a dog have? Well, the answer is four, because calling a tail a leg doesn’t make it a leg.”
More importantly, however, I feel that we are unsure of what we are trying to accomplish in correcting aberrations. I remember believing that a cornea free of aberrations was a desirable state of affairs, but that was just an assumption. Now, we are finding that 20/10 and 20/8 vision can exist with fairly large aberrations, and we aren’t certain what to make of that. Until we determine what the desired refractive state really is, the great hope for customized LASIK is to be able to address previously induced refractive surgical problems. This indication could be a godsend for long-suffering patients and their long-suffering surgeons.

Jack T. Holladay, MD, MSEE, FACS

In order to perform customized ablations effectively, the laser must be able to do what we ask it to do. Specifically, there is not a single laser on the US market today that delivers the appropriate overall energy for the ablations we perform. The reason is that the lasers are calibrated on a flat surface, so their energy is always delivered perpendicular to the treatment site. Because the cornea is not flat, the lasers today only deliver the appropriate amount of energy to the central point of the treatment site, where it is perpendicular. As the beam moves farther out, regardless of the type of beam it is, it strikes the periphery of the treatment zone obliquely, so its energy diminishes from what is necessary for the proper ablation. Our data show that the lasers are actually undertreating in the periphery—at 6.0 mm, it is about 25% reduced from its designated calculation—and in doing so, they make the cornea more oblate, rather than preserve its natural prolate shape.

I recognized this problem with excimer lasers about 3 years ago, and I have since added a new algorithm to the software of the LaserScan LSX excimer laser (LaserSight Technologies, Inc., Winter Park, FL) that increases the amount of laser energy to compensate for hitting the cornea obliquely. Of the 20 patients I have treated with this new software, all have postoperative corneas that are shaped exactly like virgin corneas. They do not have a shrinking optical zone such as those induced by current standard treatments, and these patients’ contrast sensitivity and wavefront measurements are as good as those of patients who have never undergone surgery. Moreover, these treatments were standard—not what we would normally refer to as a customized ablation.1 Correcting the systematic calibration errors in the lasers will produce better results than the wavefront-guided ablations performed today.
From my perspective, there will not be significant differences between the outcomes of customized over conventional ablations, but there will be a noticeable improvement if we calibrate all laser systems to compensate for their error when they hit the cornea obliquely. I believe that correcting the calibration error will account for approximately 99% of the improvement that patients will then receive from corneal laser surgery, and the other 1% will be attributable to customized wavefront technology. The aberrations that wavefront technology corrects are minute compared with the spherical aberrations induced by the calibration error of the lasers. I also feel that we currently understand too little about wavefront technology to significantly benefit patients who see well with glasses and contact lenses; I think that patients with more difficult visual problems will benefit most from customized ablations.

Steven J. Dell, MD

With the widespread use of customized ablation just around the corner, are we about to witness the technological obsolescence of standard LASIK? The answer is more complex than it may seem on the surface. The concept of customized treatment is very appealing, both to patients and surgeons. We obtain a unique map of each individual eye and custom tailor the treatment accordingly. This obviously will be better than an “off-the-rack” treatment, right?

Some early studies with wavefront treatments have demonstrated better UCVA and BCVA as compared with standard treatments, with fewer induced higher-order aberrations. In many cases, wavefront treatments have reduced spherical aberration in particular and improved night vision. These are remarkable achievements, but which specific aberrations should we eliminate? In a very interesting study presented at the 2001 AAO meeting in Orlando, Florida, Steven Schallhorn, MD, examined aviators at the Navy’s Top Gun school in Nevada. He looked at higher-order aberrations in individuals who had not undergone any type of refractive surgery. Surprisingly, he found that individuals with the very best UCVA had more higher-order aberrations than those with worse UCVA. Should we aim to leave some higher-order aberrations on the cornea, and if so, which ones? This question obviously warrants further study.

Additionally, it has been shown that creating a LASIK flap induces aberrations that are unpredictable. Does this fact steer us more in the direction of surface ablation for customized work? Perhaps, but the epithelial remodeling that occurs for months following surface ablation creates its own constellation of aberrations. Even after LASIK, we see substantial epithelial changes for many months. It will also be interesting to see how lenticular changes affect the situation. In my practice, the average patient requesting refractive surgery is 41 years old, and many individuals are in their 50s. How will these patients fare in the long term, and how will we deal with their residual aberrations when we extract a substantial component of the aberration equation at the time of cataract surgery? Pupil size is another factor that dramatically affects the wavefront profile of any given eye. As this variable changes from moment to moment and in general shrinks with time, how will this influence matter?

Customized ablations hold huge potential in refractive surgery. There are many patients with irregular corneas resulting from problems with prior refractive surgeries who may benefit substantially from this technology. Applying customized ablation to the mainstream refractive surgery patients will require careful consideration of all these issues. The analogy of a “made-to-measure” suit versus an “off-the-rack” suit certainly applies, but we should bear in mind that, if we gain or lose 5 pounds in a few years, or styles change, we can simply buy another suit. Customized ablation is for the duration.

James Schumer, MD

Wavefront technology and customized ablations are in their infancy. However, the tremendous impact these advances will have on our surgical strategies and treatment options is quickly becoming clear. In order to answer the question of whether customized treatments will replace standard LASIK procedures, we first have to agree on the definition of customized ablation. Does it mean simply wavefront-guided ablations, or does it mean reducing the higher-order aberrations of an eye using wavefront technology? This distinction is not a subtle one and is very important to understand for the following reasons. Alcon’s LADARVision 4000 is the first excimer laser in the US approved for customized ablation. However, the pre- and postoperative wavefronts taken of the participants in the study show that their average higher-order aberrations increased after treatment. Although the LADARVision’s customized-ablation approval will allow surgeons to treat eyes using wavefront technology, we are not yet able to reduce pre-existing higher-order aberrations. In fact, we are still increasing higher-order aberrations with our current treatments.

Clinical diagnostic wavefront analysis is teaching us that visually significant higher-order aberrations measured preoperatively are not the norm in our refractive surgery population. In other words, completely correcting sphere and cylinder surgically, without inducing visually significant higher-order aberrations, will delight nearly 100% of our refractive surgery patients. However, it is the atypical refractive surgery patient who complains of higher-order aberrations preoperatively, while most postoperative complaints are due to surgically induced higher-order aberrations (ie, spherical aberration).

Therefore, I feel that wavefront-guided customized ablations will eventually supplant the phoropter-guided LASIK procedures surgeons currently perform. The phoropter, manifest, and cycloplegic refractions will become safety checks included in preoperative evaluations, but they will no longer be the driving parameters of the excimer laser treatment. However, customized ablation (ie, treating the higher-order aberrations of an eye) will not be the typical treatment objective in the refractive surgery population, due to the ocular demographics that show that the number of lower-order aberrations far exceed higher-order aberrations in terms of visual significance. Formulating treatment parameters that eliminate and prevent the induction of higher-order aberrations remains elusive, but the journey will be stimulating and provide us with true customized-ablation (ie, treatment of higher-order aberrations) potential.

Sam Omar, MD

"New-and-improved" is a marketing tactic employed by companies in order to boost their corporate bottom line. Physicans need to cautiously evaluate any revolutionary or evolutionary refractive technology. The refractive surgery industry is extremely motivated to promote new-and-improved technologies, particularly in the form of wavefront-guided refractive surgery, which may supplant current, “conventional” LASIK surgery. Unfortunately for the industry, wavefront technology is still in its infancy, and conventional treatments now feature improved ablation algorithms, smoother ablations, blend zones, and optimized optical zones, which all eliminate many differences between conventional and wavefront-guided LASIK treatments. This comparison has been well demonstrated in countries that are 12 to 18 months ahead of the US in developing refractive technology, and thus far, customized LASIK procedures have not displaced conventional LASIK in these advanced, foreign settings.

Until our understanding of wavefront technology improves, refined conventional refractive treatments may actually outpace wavefront treatments for consistent, effective, and stable refractive results. The potential of conventional treatments can be seen when comparing the postoperative results from specific excimer platforms with the wavefront-guided postoperative results of competing excimer laser platforms. Prior to achieving “supervision” for virgin eyes, the refractive industry needs to develop additional treatments to re-treat postrefractive surgery patients who manifest suboptimal outcomes with decreased BSCVA and decreased-quality mesopic/scotopic vision. Once refractive surgeons develop more effective methods for treating irregular astigmatism and safer microkeratome technology, refractive surgery’s penetration into the general population will tremendously enhance the industry's perceived and actual safety rate.

In order for wavefront-driven excimer treatments to displace conventional LASIK, a number of philosophic and technological hurdles must be overcome. Due to the pioneering work of Cynthia Roberts, PhD, of Columbus, Ohio, and Dan Reinstein, MD, of Cambridge, England, wavefront researchers have begun to comprehend the effect of biomechanical changes in the cornea induced during the lamellar surgical portion of LASIK. The fact that so few wavefront investigations include standardized microkeratome variability partially demonstrates the limits of current refractive surgery knowledge. Because wavefront treatments require micron and submicron resolution, a greater working knowledge of the factors involved with epithelial and stromal wound-healing responses will be critical to maximizing successful customized ablations. Elevation-based topography data must be incorporated into these treatments in order to provide the highest-probability “best fit” for a customized ablation. Current excimer laser beam delivery and tracking technology is rapidly improving, but it still lags behind what is theoretically required to perform wavefront-driven customized ablations. Additionally, adaptive optics, which introduce virtually any desired aberration profile into a subject's eye, must be refined to evaluate a patient’s vision for each controlled aberration profile. This “wavefront phoropter” or “visual simulator” would allow surgeons to determine the exact relationships between specific aberrations and visual quality.

Despite the best designs of current wavefront investigational trials, until the issues described previously are rigorously developed and applied, any significant visual improvement via customized wavefront ablations will be entirely accidental and difficult to reproduce. Therefore, it is unlikely that in the near future customized LASIK will unseat conventional LASIK as king of the refractive marketplace.

William I. Bond, MD, is Director of Bond Eye Associates in Pekin, Illinois. He holds no financial interest in any technology discussed herein. Dr. Bond may be reached at (309) 353-6660; bondeye@bondeye.com.

Jack T. Holladay, MD, MSEE, FACS, is Clinical Professor of Ophthalmology at Baylor College of Medicine in Houston, as well as Medical Director of LaserSight Technologies, Inc., in Winter Park, Florida. Dr. Holladay may be reached at (713) 668-7337; docholladay@docholladay.com.

Steven J. Dell, MD, is Director, Refractive and Corneal Surgery at Texan Eye Care in Austin, Texas. He holds no financial interest in any product or technology mentioned herein. Dr. Dell may be reached at (512) 327-7000; sdell@austin.rr.com.

James Schumer, MD, is in private practice at Eye Surgery Consultants in Mansfield, Ohio. He holds no financial interest in any product or technology mentioned herein. Dr. Schumer may be reached at (419) 525-3737; schumer@revisioneyes.com.

Sam Omar, MD, is from Advanced Vision Institute in Orlando, Florida. He holds no financial interest in any product or technology mentioned herein. Dr. Omar may be reached at (407) 389-0800; omar_eye@yahoo.com.

1. Holladay JT, Jane, JA. Topographic changes in corneal asphericity and effective optical zone size following LASIK. J Cataract Refract Surg. 2002;28:942-947.