Milestones in LASIK
In the short span of 20 years, refractive surgery has advanced to a level even the most optimistic surgeons could not have predicted.
Thanksgiving 1981: Pondering the skeletal remains of the traditional feast, Rangaswamy Srinivasan, PhD, decided to take the turkey bones to work. There, Dr. Srinivasan and his coworkers at IBM's T.J. Watson Research Center in York-town Heights, N.Y., tested a theory they'd been considering. They irradiated the cartilage on the end of the bones with an ultraviolet excimer laser—a tool used to etch circuit boards at IBM—and a conventional green laser. When they saw that the excimer laser produced a clean incision, while the green laser charred the tissue, the scientists realized they had uncovered a new phenomenon, which they named ablative photodecomposition.1 Ophthalmology would never be the same.
Tracing the evolution of LASIK would tax any meticulous historian. Many would begin with the groundbreaking corneal reshaping work by José I. Barraquer, MD, who designed the cryolathe and the microkeratome and developed keratomileusis and keratophakia. Indeed, this was the first instance of a part of the human body being removed, reshaped for better function with the use of a computer, and reattached. Then there was Stephen L. Trokel, MD, who, along with Dr. Srinivasan, was instrumental in adapting the excimer laser for vision correction, and Gholam Peyman, MD, a retina specialist who patented the LASIK procedure in 1989. No history would be complete without a section on Lucio Buratto, MD, of Italy, Marguerite McDonald, MD, of the United States, and Ioannis Pallikaris, MD, of Crete, whose experiments with excimer laser were invaluable in the development of LASIK.
In addition to these innovators, scores of others around the world—physicians, scientists, engineers and manufacturers—contributed to the advancement of LASIK and continue to adapt and refine refractive surgery technology, techniques and treatments in the quest for optimal outcomes. A comprehensive history is beyond the scope of this article, however, we asked several surgeons who have been in practice throughout these years of LASIK innovation to share their personal experiences and to discuss what they consider milestones in LASIK development.
LASIK Forerunners
Stephen F. Brint, MD, Metairie, La., is a self-proclaimed walking history book on refractive surgery. In the early days, Dr. Brint performed radial keratotomy and freeze keratomileusis and then participated in FDA clinical trials to examine the utility of the excimer laser.
“I was one of the first in vestigators for the Summit excimer laser,” he recalls. “We started out doing photorefractive keratectomy (PRK). This was before any of us even knew about Dr. Buratto or Dr. Pallikaris. There wasn't a lot of communication internationally like there is now, where information travels at the speed of the Internet, so we didn't know what the Europeans were doing, or even if they had access to excimer lasers.”
Around the same time, Stephen G. Slade, MD, of Houston was engaged in performing and teaching the Barraquer procedure of myopic keratomileusis (MKM). He, too, became an investigator for Summit. “Dr. Brint invited me to join the Summit excimer laser MKM trial,” Dr. Slade recalls. “It seemed natural to combine our skills with excimer lasers and keratomileusis and keratomes.”
Meanwhile, Eric D. Donnenfeld, MD, Fairfield, Conn., was working as a primary investigator of the VISX excimer laser at Manhattan Eye, Ear & Throat Institute (MEETI) in New York. “I was a resident when I first heard about the excimer laser in the mid-1980s,” he says. “Steve Trokel came to us with this new technology that he was using to create laser keratotomies, and he asked, ‘What else can we do with this?' I had the opportunity to use the VISX laser during its infancy. At first, we performed only PRK, which at that time, was a very rudimentary procedure. The excimer laser created a small optical zone without a blend. There was no tracking device, no registration and no pain control.”
Dr. Brint also recalls the challenges surgeons and patients faced with PRK in the 1990s. “In the early days of PRK, we didn't use bandage contact lenses, and we didn't understand the healing process as well as we do today,” he says. “We used antibiotics, of course, and steroid drops, but the patients were in extreme pain. This was before NSAIDs were available for ophthalmology, so we also had to deal with epithelial healing issues. Corneal haze was a problem for some high myopes. So we had a different set of issues than we have today, but still, all in all, a month later, most patients were seeing well.”
After studying with Dr. Buratto, Dr. Brint and Dr. Slade returned to the United States to begin performing MKM in the Summit trial. Dr. Slade assisted Dr. Brint on the first excimer MKM cases in New Orleans. After performing several cap MKM procedures himself in Houston, Dr. Slade tried applying the laser ablation directly on the bed of the keratectomy. These cases, called “in situ excimer MKM” at the time, were the first LASIK cases done in the United States. The year was 1991.
In 1992, Dr. Buratto reported on intrastromal corneal excimer laser photoablation following lamellar corneal keratectomy with a microkeratome,2 and in 1993, he presented a new nomogram for excimer MKM.3
“Excimer MKM was an intermediate step between PRK and LASIK,” Dr. Brint said. “We used a microkeratome to remove a 300-micron cap of tissue. We would place the cap in a special holder, epithelial side down, and then center the helium-neon laser beam on the previously marked visual axis. We would program the laser with our first-generation nomogram for the amount of tissue we wanted to remove to correct the myopia, aim the beam and ablate the cap. Then we would replace the cap and suture it in place using an anti-torque suture, just as we had done for the freeze keratomileusis procedure. The sutures had to remain in place for about 2 months, because corneal recovery from MKM was slow.” According to Dr. Brint, freeze MKM had been used for approximately 15 years, but only by a handful of surgeons worldwide. Excimer MKM had a relatively short 1-year lifespan before the cap evolved into the flap.
In 1995, Summit Autonomous, Inc., received FDA approval for its excimer laser system for the correction of mild to moderate myopia, and in 1999, the company became the first commercial excimer laser manufacturer to receive FDA approval for the LASIK procedure. In the ensuing years, LASIK has emerged as the go-to procedure for most refractive surgeons.
Advances, some of them incremental and others game-changers, have come rapidly in three key categories: the excimer laser ablation profile, the flap formation and the pharmaceuticals.
Ablation Profiles Evolve
According to Dr. Brint, the first Summit and VISX excimer lasers were similar, in that they were broad-beam lasers that delivered a beam of energy approximately 6 mm in diameter. An iris diaphragm shutter controlled the size and shape of the beam. As Dr. Donnenfeld notes, surgeons were limited to a “one-size-fits-all” ablation profile.
FDA approval of the first computer-driven, small-beam, spot-scanning (a.k.a. flying-spot) excimer laser in 2000 was a major milestone in LASIK history, according to Dr. Brint. With this technology, surgeons could program exactly how much tissue they wanted to remove, allowing them to create more precise ablation zones with a 1-mm beam while minimizing the risk of collateral damage.
Over a period of a dozen or so years, incremental advancements, such as eye tracking and registration—limbal and then iris—were introduced, bringing associated improvements in outcomes. “The Autonomous laser was the first to have a registration system,” Dr. Donnenfeld says. “That was supplanted by the VISX S4 iris registration laser, which not only allowed for registration but also for cyclotorsion.”
Dr. Brint notes, “Registration and eye tracking were huge advancements. We had the ability to adjust for cyclotorsion prior to starting the ablation, but we couldn't track it during the ablation. Now we can. In addition, the laser tracks even minor saccadic movements of the eye for precise treatment. This is particularly important when your goal is a custom pattern.”
The development of technology to identify and measure eye anomalies, specifically higher-order aberrations (HOAs) contributed substantially to the evolution of LASIK. Whereas the goal once was to avoid inducing HOAs, refractive surgeons today have the means to correct for them. In addition, intraoperative aberrometry is now a reality.
“We went from a one-size-fits-all conventional oblate ablation profile to an aspheric prolate custom ablation profile that treats higher-order aberrations,” Dr. Donnenfeld says. “We also improved our standard ablation with an optimized ablation that produces a more prolate appearance.”
Custom wavefront LASIK is now the state of the art in refractive surgery. According to data compiled by Richard J. Duffey, MD, and David Leaming, MD, and presented at the 2010 meeting of the American Society of Cataract and Refractive Surgery, 82% of respondents perform customized ablations.4
Refining Flap Formation
Early mechanical keratomes used in refractive surgery had some inherent limitations, such as the potential for variable pressures exerted by the surgeon's hand on the instrument and the speed with which the surgeon advanced the blade. Irregular pressure or speed could produce varying sizes or thicknesses of tissue and induce astigmatism. When surgeons started transitioning from caps to flaps, they had to modify their keratomes to stop short of making a cap by placing a small pin in the track, Dr. Brint recalls.
As surgeons became more experienced and knowledgeable, advances in keratome technology followed. “We learned a lot along the way,” Dr. Brint says. “We found that flatter corneas tended to create smaller flaps and, therefore, the risk of a free cap was greater because the microkeratome travels the same distance whether a cornea is steep or flat. We also learned how to choose appropriately sized suction rings for different corneal curvatures.”
A major advancement in keratome technology was the development of the Hansatome, which, unlike its predecessor, the automated corneal shaper, was configured specifically for LASIK. The Hansatome had a fixed-depth plate, created a superior hinge as opposed to a nasal hinge and improved flap reproducibility. Working with Hansatome developmer, Hans Hellenkamp, Dr. Slade performed the first Hansatome cases, then taught several thousand surgeons how to perform LASIK using the technology.
Further improvements were incorporated into subsequent generations of mechanical microkeratomes. For example, they were preassembled, eliminating the need for on-eye assembly; they could be operated with one hand; and surgeons could vary the flap diameter as well as the hinge placement. Although these incremental improvements were important, another game-changer was on the horizon. In 2002, the FDA approved the femtosecond laser for use in LASIK.
The early work in femtosecond lasers was done by Lee T. Nordan, MD, San Diego, Calif., and Dr. Slade, who has the longest experience in femtosecond lasers for corneal surgery. The femtosecond laser allows the surgeon to customize the flap to fit the individual patient and ablation profile, similar to customizing the ablation. This concept of matching the depth, diameter, shape, edge and so on to the patient was termed sub Bowman's keratomileusis or SBK by Dr. Slade and Daniel S. Durrie, MD, Overland Park, Kan. Today, more than 55% of surgeons are performing all-laser LASIK, according to the 2009 survey by Drs. Duffey and Leaming.4
Dr. Brint uses the femtosecond laser for about 90% of his LASIK cases. The advantages are numerous. “The femtosecond laser creates a planar flap, which has a consistent thickness across the diameter of the flap, as opposed to the mechanical microkeratome's meniscus-shaped flap, which is thicker at the edges and thinner in the middle,” he explains. “There is less chance of epithelial ingrowth with a planar flap. In addition, if you lose suction, instead of having a ragged flap that might not heal well, essentially all you've created is a layer of bubbles. You can either continue with the same layer of bubbles, or you can come back another day. You have many options.
“Any mechanical microkeratome is dependent on the corneal curvature to determine flap size,” Dr. Brint continues. “With the femtosecond laser, flap size is unrelated to corneal curvature, so you can create the exact flap diameter and shape, and you can choose the hinge location you prefer. For example, many surgeons use a temporal hinge for hyperopia, and that is not possible with a mechanical microkeratome. Another consideration is that we can create thinner flaps, down to about 100 microns, with the femtosecond laser, which leaves more residual corneal tissue so that we can either correct more myopia or feel more secure that we will end up with a thick cornea and less chance of corneal ectasia.”
Dr. Donnenfeld says the utility of the femtosecond laser is continuously expanding. “Not only can we make traditional flaps with the most recent generation of femtosecond technology, we can actually invert the side cuts, make oval flaps and customize the flaps to meet our patients' needs to improve outcomes. There have been some dramatic improvements in flap formation.”
Surpassing Expectations
The surgeons we interviewed agree that LASIK has had a profound effect, not only on their patients, but also on their outlooks. “LASIK has allowed me to combine my passion for precision with my love for giving patients quality vision,” Dr. Donnenfeld says. “To me, it's extraordinary that we in ophthalmology have the ability to harness technology to give patients the gift of vision, which is arguably one of the greatest gifts that anyone can possibly have. It has been an exciting 20 years.”
Understanding the Need for Speed |
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Eight-second LASIK isn't about speed merely for the sake of speed. “It's more about accuracy and repeatability,” Dr. Brint says. “The faster the laser, the less time the cornea has to dry out, and maintaining a uniform level of hydration throughout the LASIK procedure is important for achieving optimal outcomes. A dry cornea ablates faster than a wet cornea, so a surgeon could theoretically be removing more corneal tissue at the end of the procedure than at the beginning. Patients also appreciate a faster procedure, which minimizes their fixation time.” In addition to a fast ablation time, rapid flap creation is another consideration. Dr. Brint notes that femtosecond lasers gained wider acceptance after reaching a speed of 60 kHz. They now operate at 150 kHz and higher. |
The LASIK Diagnostic Exam |
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From a diagnostic point of view, in addition to aberrometry, corneal topography—with Orbscan, Pentacam or traditional Placido technology—has an increasingly important role when evaluating refractive surgery candidates, says Stephen F. Brint, MD. “It's important for us to identify cases that could potentially become ectatic and exclude them from LASIK. We understand much more about the anomalies we can see with topography, such as forme fruste keratoconus and irregular astigmatism, and their effects on LASIK outcomes than we did just 5 years ago. Using technology that shows us corneal curvature on the posterior surface can alert us to potential problems that we might not have detected on the anterior surface.” |
REFERENCES |
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1. IBM press release: http://www-03.ibm.com/press/us/en/pressrelease/22042.wss. Accessed Dec. 19, 2010. 2. Buratto L, Ferrari M. Excimer laser intrastromal keratomileusis: case reports. J Cataract Refract Surg. 1992;18:37-41. 3. Buratto L, Ferrari M, Genisi C. Myopic keratomileusis with the excimer laser: one-year follow up. Refract Corneal Surg. 1993;9:12-19. 4. Duffey RJ, Leaming D. Practice Styles and Preferences of U.S. ASCRS Members—2009 Survey. Presented at: American Society of Cataract and Refractive Surgery Annual Meeting; April 7-9, 2010; Boston, MA. |
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Comparing Wavefront Optimized® vs. Wavefront-guided LASIK Procedures
By Stephen F. Brint, MD, FACS
Wavefront Optimized® LASIK is one of five types of laser treatments available on the Allegretto Wave® Eye-Q laser (Alcon, Fort Worth, Texas). It is by far the most frequently used treatment worldwide. While based on the patient's spectacle prescription, it also takes into account corneal curvature and thickness, and applies additional laser energy in a unique fashion in the periphery of the cornea, to maintain the same corneal asphericity as exists preoperatively. The goal is not to induce any new spherical aberration, which is the most important higher-order aberration.
While wavefront-guided (WFG) LASIK may benefit some patients, study results from the FDA trial of the Allegretto Wave® Excimer Laser System, of which I was medical monitor, suggest that in the majority of cases, they offer no advantage over Wavefront Optimized® treatments. The trial was designed to directly compare two approaches using the Allegretto Wave® laser. Wavefront Optimized® LASIK is designed to treat the spherocylinder error without affecting the higher-order aberrations. Similar to conventional LASIK, Wavefront Optimized® LASIK is based on a phoropter refraction. Unique to Wavefront Optimized® LASIK is the ability to treat the refraction without inducing new aberrations.
The study protocol hypothesized that the visual and refractive outcomes between the two treatment groups would be equivalent. This hypothesis was validated by the results. At the 3-month postoperative mark, 94% of Wavefront Optimized® and 95% of WFG eyes in both cohorts saw 20/20 or better without correction, and 69% of Wavefront Optimized® treated eyes attained 20/16 acuity compared to 63% of eyes treated with wavefront-guided technology.
Allegretto Wave® Eye-Q (Alcon) 400 Hz Excimer laser
Comparison of postoperative UCVA vs. preoperative best spectacle-corrected visual acuity (BSCVA) was statistically the same in both groups. We found that 84% of the eyes in the Wavefront Optimized® cohort had postoperative uncorrected acuity as good as or better than their preoperative BSCVA. This rate was 81% in the wavefront-guided group. Manifest refraction spherical equivalent results were also similar, with 94% of WFG patients within 0.5D of target, and 97% of Wavefront Optimized® patients reaching this level.
We did not find any correlation between higher-order aberrations and contrast sensitivity between the Wavefront Optimized® and WFG groups, nor was there was any mean worsening of low contrast acuity in either treatment group.
This study holds significant implications for patient selection for LASIK. The overwhelming majority of patients, about 80%, do not need WFG treatments with a laser that offers a Wavefront Optimized® alternative. Wavefront-guided treatments have significantly contributed to our understanding of aberrations and their effects on vision. We found WFG to be beneficial in the 7% of patients with over 0.4 microns of preexisting HOA.