Ophthalmologists performing custom corneal ablation are up against the classic struggle between the theory of physics and the reality of 21st-century technology. In theory, a laser beam size can be made small enough to treat higher-order aberrations to the 8th, 9th or even 12th order.
However, there’s one problem: The procedure would take a very long time. “So long, in fact, that you could serve your patients lunch while they were getting the laser treatment,” said Steven E. Wilson, MD, director of corneal research at the the Cleveland Clinic Foundation’s Cole Eye Institute.
Consequently, researchers and manufacturers are searching for a compromise between the best possible custom ablation and what is truly practical. Two independent studies have reached similar conclusions regarding this compromise: That is, a 1-millimeter laser beam is small enough to produce a customized ablation for typical human eyes up to the 5th order.
Yet despite these findings, heated competition continues among manufacturers to create a laser with the smallest spot size imaginable to treat the highest order of aberration possible given the laws of physics.
How We Got Here
There was a time when laser spot size was not even a consideration, noted Stephen D. Klyce, PhD, professor of ophthalmology at Louisiana State University. Early lasers were designed to correct only the spherical power by changing the central curvature of the cornea in a fashion that modeled a spherical lens, he said.
Several approaches were then made to start correcting cylinder as well as sphere, giving surgeons the tools to mimic the corrections made possible by glasses and contact lenses.
“When custom corneal ablation came along,” Dr. Klyce said, “the big challenge was how to measure the changes needed to be effective. Wavefront technology allowed for the measurement of aberrations in the human eye using data on a point-wise basis to determine a spatially resolved refraction—in other words, a refraction made at many points within the pupil.”
Dr. Klyce added, “Zernike polynomials were used to fit wavefront data and extract very accurately the aberrations that needed to be corrected to provide normal or better-than-normal vision.”
Researchers also discovered that higher-order aberrations required much finer adjustments on the corneal surface, prompting laser companies to produce ever-smaller laser beams. “The companies who are manufacturing these lasers are fighting physics to a certain degree,” Dr. Klyce noted. “There is a limitation on how small you can make any point of light because of diffraction phenomena.”
A laser with a 0.9-mm beam originally broke the barrier, but some companies have found ways to make the laser beam spot size even smaller. However, there are trade-offs, and other factors begin to weigh in. For instance, the effect of wound healing limits the extent of ablation, as does movement during treatment. “The slight amount of rotation or displacement during the procedure reduces the efficacy of transferring a very precise pattern on the corneal surface,” Dr. Klyce said. “And while the newer eyetrackers have greatly reduced the untoward effect of movement, it hasn’t been eliminated.
“In my humble opinion,” he continued, “we are getting to the point where the laser beam spot size is adequate to produce the kinds of optimization to treat the higher-order aberrations on the corneal surface.”
In March 2002, David Huang, MD, PhD, and Muhammad Arif, PhD, published a report concluding that current scanning lasers with a beam diameter of 1 mm or less were theoretically capable of eliminating most higher-order aberrations.1 This was confirmed in January 2003 by Scott M. MacRae, MD, and his colleagues, who demonstrated that a beam size of 1 mm or less was sufficient to treat wavefront aberrations of “typical human eyes.” Larger spot sizes, on the other hand, were useful for treating such refractive errors as sphere and cylinder.2
Dr. Huang, associate staff in ophthalmology at the Cleveland Clinic Foundation, began his theoretical analysis about three years ago. “A number of laser systems were being used to develop wavefront-guided treatments for the cornea,” he recalled, “and I was dubious whether some of them could really be suitable based on the spot size.” Working with his postdoctoral research associates, Dr. Huang simulated corrections of wavefront aberrations of Zernike modes from the 2nd to the 8th order to determine the effect of laser spot size on
the outcome of aberration correction. Guassian and top-hat beams of 0.6- to 2-mm full-width-half-maximum diameters were modeled.
Caption: The push for ever-smaller laser beams grew out
of the discovery that higher-order aberrations require finer
adjustments on the corneal surface. But “since most aberrations
are 4th order or less, laser beams of 1 mm or less are
sufficient to correct the majority of higher-order aberrations,”
Dr. Huang said. Credit: VISX.
Using a distortion/correction ratio of less than 0.5 as a cutoff for adequate performance, Drs. Huang and Arif found that a 2 mm or smaller beam was adequate for spherocylindrical correction (2nd order), a 1 mm or smaller beam was adequate for correction of up to 4th order modes and a 0.6 mm or smaller beam was adequate for correction of up to 6th order modes.
“These findings show that since most aberrations are 4th order or less, laser beams of 1 mm or less are sufficient to correct the majority of higher-order aberrations,” Dr. Huang said. “Based on this study, it appears that most lasers on the market have small enough beams to accomplish this.”
He added that the Alcon LadarVision system has a beam diameter of 0.8 mm; the Bausch & Lomb Technolas system initially was developed with a 2-mm beam but the company upgraded to include a 1-mm beam to treat higher-order aberrations; the Visx Star S4 system has a variable spot size that ranges from to 0.65 to 6.5 mm; and Wavelight’s Allegretto laser has a 0.95-mm beam.
Dr. MacRae and his colleagues also reached a similar conclusion in their study, which showed that while large diameter laser beams decreased the ability to correct higher-order aberrations, a beam of 1 mm was small enough to produce a customized ablation up to the 5th order.
“You may need to go slightly smaller to treat even more subtle abnormalities, but there are theoretical limits,” said Dr. MacRae, professor of ophthalmology and visual science at the University of Rochester.
Dr. MacRae noted the clinical implications of this research: “For treating sphere and cylinder, actually using a larger treatment zone is more efficient because you can do it quicker—a 1-mm spot takes four times longer to treat than a 2-mm spot size. You want to use the 1-mm or smaller spot when you are treating a subtle aberration.” He added that about 90 percent of aberrations involve sphere and cylinder, and that 5 to 10 percent need to be treated with the smaller spot size.
Ronald R. Krueger, MD, medical director of refractive surgery at the Cleveland Foundation, noted that while laser spot size is a timely issue, it should be placed in context with other requirements for wavefront technology. “To achieve better vision and better outcomes, there are four requirements that must be in place: a small scanning spot, a very fast eyetracker, an accurate wavefront device and a wavefront laser interface. As you can see, the small scanning spot is the first of my four requirements. The criteria are more demanding if you are going to correct at high-level aberrations.”
While the two studies referred to above confirm each other, “they were based on mathematical analysis—and nothing is mentioned about the tracking system. In order to correct higher-order aberrations, you need the smaller beam size and the other elements. They are all intimately related,” Dr. Krueger said.
Can we continue to reduce spot size and achieve correction of even higher-order aberrations? “Wound healing variability and the biomechanics of the cornea will spoil it for us,” Dr. Krueger noted. “If you can come up with drugs and mechanisms to control wound healing and the biomechanical aspects of the procedure, we could go higher.”
While manufacturers continue on their quest to bring ophthalmologists the smallest laser beam possible, Dr. Wilson views the current competition as healthy.
“There is more than one way to skin a cat,” he said. “In my opinion, several lasers with different strategies are currently available that can give a very fine custom corneal ablation. If we only had one brand, with one laser point size, we wouldn’t have the progress that we are experiencing today. It is pushing the field forward.”
1 J Cataract and Refract Surg 2002;28(3): 407–416.
2 Guirao, A. et al. J Refract Surg 2003;19(1): 15–23.
Drs. Huang and Klyce have no financial interests; Dr. Krueger receives travel and research support from Alcon and serves on its medical advisory board; Dr. MacRae is a consultant for Bausch & Lomb; and Dr. Wilson has served on the advisory board for Visx without compensation.