Wavefront analysis: providing more information for refractive surgery patients

Originally developed by astrophysicists for telescopes, wavefront analysis now holds the promise of revolutionizing refractive surgery, with the ideal of “super vision” more attainable than ever. By taking a complete “fingerprint” of the eye, practitioners are now able to measure, then correct higher-order aberrations that were previously undetectable.

“It’s generating a lot of excitement, because it routinely has the potential to get our patients better than 20/20,” said Marguerite McDonald, MD, clinical professor of ophthalmology at Tulane University School of Medicine, New Orleans. “As a matter of fact, retinal neurobiologists say that the human eye actually has the potential to see between 20/5 and 20/8. That’s seeing a golf ball three to five times further away than we do now, for instance.” Dr. McDonald was the first clinician to participate in a Food and Drug Administration (FDA) clinical trial for custom ablation, according to Alcon’s director of corporate communications, Mary Dulle. Dr. McDonald used the LADARVision and LADARWave systems from Alcon Summit Autonomous (Orlando, Fla.) in the trial.

Although wavefront analysis is not yet mainstream, it has yielded positive results in early clinical trials.

The Visx WaveScan (Santa Clara, Calif.) uses optics that project light into the eye and analyze the returning wavefront using a sensor containing thousands of tiny lenses, or “lenslets.” The result is a map of the eye’s refractive errors.

This device received FDA approval in May 2000, and according to Visx marketing director Thomas McKay, Visx began shipping the WaveScan system to doctors in August. “We have about 20 systems installed,” McKay said. “They were received, installed and are currently being used by doctors.”

The CustomCornea measurement device from Alcon Summit Autonomous, also approved by the FDA in May 2000, works similarly, and links the eye map to its LADARVision excimer laser to produce what is considered a customized ablation pattern.

Some doctors have received the system and are currently working with it both within and independent of the clinical trials, said Ms. Dulle. The majority of customers will receive equipment later this year.

Advantages over topography

---Visx: The WaveScan uses optics that project light into the eye and analyze the returning wavefront using a sensor.

One of the most notable advantages of wavefront analysis is that it measures the entire visual system, rather than just the corneal surface, as in corneal topography.

“The key advantage that the wavefront sensor has over corneal topography is that a topographer generally tells you about one surface of the optics of the eye,” said David Williams, PhD, director of the Center for Visual Science in Rochester, N.Y. “The wavefront sensor gives you the complete effect of all the optical surfaces, and ultimately that’s what determines the quality of the retinal image. That determines how well you see.”

Wavefront technology is able to detect third- and fourth-order aberrations, such as coma, trefoil and spherical aberration. Scott MacRae, MD, professor of ophthalmology and visual sciences at the University of Rochester, said these are the three most common higher-order aberrations experienced after refractive surgery.

Dr. MacRae described the wavefront appearances of the third-order aberrations. “Vertical coma looks like a comet with the tail going up and down,” he said, “and horizontal coma looks like a comet where the tail goes from side to side.” Dr. MacRae described trefoil as looking like a “three-lobed Mercedes-Benz type symbol,” when viewed two-dimensionally.

Of the fourth-order aberrations, Dr. MacRae said spherical aberration is the most significant. He said the wavefront form of spherical aberration looks like a “sombrero hat.”

“Before we had wavefront sensors, we couldn’t even detect those,” said Dr. MacRae. “We didn’t even know they existed. We would just measure sphere and cylinder, and that’s all we could treat. Now, we can detect higher-order aberrations, and we are starting to be able to reduce them.”

Dr. McDonald said in many cases, higher-order aberrations are all that stand in the way of super-vision. “Most of us have enough coma and spherical aberration that if you could get rid of them, you could see better than 20/20. So, the technology holds the promise that we will, on a routine basis, be able to correct people to better than 20/20.”

Studying post-LASIK pilots with COAS

---WaveFront: The COAS uses a Shack-Hartmann sensor to provide an analysis of the aberrations of the entire eye.

Lt. Col. Corina van de Pol, OD, PhD, director of Aircrew Health and Performance Division for the U.S. Army Aeromedical Research Laboratory in Fort Rucker, Ala., has been using the Complete Optical Analysis System (COAS) (WaveFront Sciences, Albuquerque, N.M.) aberrometer as part of a research project studying pilot applicants who have had refractive surgery. The COAS uses a Shack-Hartmann sensor to provide an analysis of the aberrations of the entire eye. The sensor has the capacity to analyze aberrations in many different forms, including displays of cylinder, higher-order aberrations, Zernike polynomials, pupil size and high-resolution wavefront maps.

“We’re interested in having an objective measure of the optics of their eyes, so we’re getting the wavefront measure,” she said. “We are also using the OrbScan (Bausch & Lomb, Rochester, N.Y.) for corneal measurement. In addition, we’re doing a series of high-contrast, low-contrast, low-light and glare tests on all of these pilot applicants.”

A second study being conducted at Fort Rucker concerns standard pilot applicants who have virtually no refractive error. “We’re measuring them, a number of times, and looking at the repeatability of the measure, as far as its ability to measure not only sphere and cylinder and axis, but also higher-order aberration. We’re finding that so far, the numbers are pretty good.”

Dr. van de Pol said she considers the COAS aberrometer to be effective in measuring higher-order aberrations, but questions the extent to which this information is needed. “It’s always interesting to be able to measure higher-order aberrations,” Dr. van de Pol said. “But the question is, what is the impact of these higher-order aberrations?”

Flight performance considered

The pilot applicants who have had refractive surgery arrive at Fort Rucker at least nine months postoperatively, Dr. van de Pol said. She and her colleagues then measure the standard refractive error of the patients, as well as higher-order aberrations.

“We are looking for the correlation between the level of higher-order aberrations and performance on a battery of visual performance tests,” Dr. van de Pol said. “And then we take it a step further, because we are in a situation here where we can actually get information about flight performance.”

While the subjects are in flight school, Dr. van de Pol said, their instructor pilots provide information about their ability to perform a series of visually dependent flight maneuvers. “The theory is, if you have an increase in higher-order aberrations, you may have a decrease in contrast sensitivity. It would then follow that you would potentially have more trouble flying under less-than-optimal visual conditions.”

Dr. van de Pol stressed that while the COAS is an effective device for measuring higher-order aberrations, it is also crucial to know where to draw the line. “You can go out to the tenth-order, twelfth-order Zernikes or higher,” she said. “We’re now able to measure higher-order aberrations and make that technological leap to correction mode. But I think our mission as researchers is to know when that additional accuracy is no longer producing additional benefits.”

Thomas O. Salmon, OD, PhD, assistant professor of optometry at Northeastern State University College of Optometry, has also utilized the COAS in research projects. “The instrument is used in the same way you would use an autorefractor,” he said. “You simply focus the eye in the video monitor and press a button, and it captures the data.”

The aberrometer then reports the data on sphere, cylinder and axis, Dr. Salmon said, as well as listing higher-order aberrations.

The importance of measuring higher-order aberrations depends upon the circumstances of the particular patient. “If someone has a normal eye and has not had refractive surgery, the aberrations are probably insignificant,” he said. “But conventional refractive surgery can significantly increase the aberrations, so it’s probably a good idea to do the wavefront measurement pre- and postop.”

Aberrations with conventional LASIK

---Alcon Summit Autonomous: The Custom Cornea device links the eye map to its LADARVision excimer laser to produce what is considered a customized ablation pattern.

Dr. MacRae also noted that in conventional refractive surgery, it has been found that higher-order aberrations actually increase. “The first reports are really showing us that with conventional treatment, we’re actually increasing the levels of spherical aberration, coma and trefoil,” he said. “We did a study of conventional LASIK in more than 20 eyes and found that spherical aberration actually doubles on average after the treatment.”

In a study conducted at the University of Rochester by Dr. MacRae and associates (presented at Association for Research in Vision and Ophthalmology in May 2001), a Shack-Hartmann wavefront sensor was used to measure the eye’s wave aberration in three populations: 194 preoperative patients, a group of 20 who had undergone conventional LASIK and considered their visual outcome to be generally successful, and six patients who had undergone conventional LASIK that resulted in reduced best-corrected vision, as well as weakened vision under low lighting conditions.

Procedures were performed by a variety of clinicians and laser types. The wave aberration was calculated as a Zernike polynomial expansion up through fifth order for 5.7-mm diameter pupils.

The study concluded that conventional LASIK surgery increased higher-order wavefront aberrations while correcting defocus and astigmatism.

Clinical trials of the Alcon CustomCornea platform were consistent with these findings, showing less inducement of higher-order error in custom-treated eyes than in conventionally treated eyes. The study, which treated one eye of each patient conventionally and one with custom ablation, showed that postoperative error was less in the custom eye in 79% of the patients.

Within the context of Alcon’s CustomCornea FDA clinical study performed at Brint Vision, Metairie, La., 100% of patients achieved 20/20 vision while 95% achieved 20/16 or better, said Ms. Dulle. The treatments were performed on the Alcon LADARVision platform using higher- and lower-order aberration data from the LADARWave wavefront system.

In a clinical study performed at Kraff Eye Institute, Chicago, in April 2000 under a U.S. FDA IDE, 100% of patients who had a wavefront-driven custom ablation achieved 20/20 vision, while 81% achieved 20/16 or better and 31% saw 20/12 or better. The treatments were performed with the Visx Star S3 ActiveTrak laser using data from the WaveScan.

With wavefront analysis, doctors are better able to identify patients whose optical aberrations would be worsened by normal ablation patterns. “The preliminary results are starting to show that we can minimize that increase in higher-order aberrations,” Dr. MacRae said. “We hope that with future generations of custom-guided ablations we will actually be able to reduce it.”

LASIK or PRK?

One dilemma regarding wavefront technology, Dr. McDonald said, is whether it would be best used with LASIK procedures or PRK. She cited data collected by herself and G. Brock Magruder, MD, in Orlando, Fla.

Of 13 PRK patients in which wavefront analysis was used, 46% achieved 20/12.5 best-corrected vision at 6 months, with 8% seeing 20/10. Of 20 LASIK patients in which wavefront was used, 25% saw at 20/12.5 best-corrected vision, and 5% saw at 20/10.

“A lot of people wonder whether we should do this with PRK or with LASIK, and the jury is still out,” Dr. McDonald said. “The early results with PRK are slightly better than the results with LASIK. They’re both good, but PRK seems to be better.”

Dr. McDonald said this difference could be attributed to the presence or absence of a flap. “LASIK induce many biomechanical changes that have their own effect,” she said. “Making a flap has a profound effect on the biomechanical properties of the cornea and induces its own higher-order aberrations.”

For this reason, Dr. McDonald said, wavefront technology could mean a return to PRK. “It may be that we have to go back to the future, back to PRK because there’s no flap,” she said. “In other words, we’re trying a lot of things. We’re trying LASEK [laser epithelial keratoplasty], which is a more sophisticated version of PRK. We’re trying to get that up and running.”

In whatever way it is applied, Dr. McDonald emphasized, wavefront analysis has the potential to achieve unsurpassed visual acuity. “Things are looking good,” she said. “We’re getting awfully close to the Holy Grail.”

The pursuit of perfect vision

While wavefront-guided ablation has the potential to exceed nature’s optics, some clinicians argue that “super-vision” should not be the primary objective.

“The greatest benefits are in treating people with many higher-order aberrations, not necessarily in driving all of the eye’s aberrations down to a miniscule level so that people have ‘perfect vision’,” said Dr. MacRae. “Realistically, it will be difficult to get a perfect correction, but we should be able to improve higher-order aberrations more than we do with conventional ablations.”

Raymond Applegate, OD, PhD, professor of ophthalmology at the University of Texas, San Antonio, emphasized that the first step in refractive surgery should be preventing new aberrations.

“I argue that if we first induce no new aberrations in refractive surgery, everything else will be icing on the cake,” Dr. Applegate said. “Refractive surgery is just now beginning to address higher-order aberrations. Previously, more higher-order aberrations were being induced with refractive surgery.”

Dr. McDonald said wavefront-guided ablation is also especially effective in “tuning up” patients with irregular astigmatism or other ocular abnormalities.

“If a patient has irregular astigmatism after a penetrating keratoplasty or contact lens warpage, a corneal scar from herpes or a bacterial ulcer, or if he or she had a bad RK or a bad PRK, we can do touch-up procedures with this,” Dr. McDonald said. “We can bring these patients back to 20/20.”

Not a replacement

Will wavefront analysis render corneal topography obsolete? It is unlikely, according to practitioners.

“There are several weaknesses of wavefront-guided ablation,” Dr. MacRae said. “One is that wavefront sensors are only able to measure over the pupil. So, we don’t have a complete picture of the shape changes occurring in the cornea, outside the pupil area. That’s where corneal topographers can help us.”

Corneal topography will always serve a purpose in refractive surgery, Dr. McDonald said. “The corneal topography units will never be obsolete, because we will always need to know information from the cornea,” she said. “As a matter of fact, you can take corneal abnormalities, subtract them from the wavefront map and find out what abnormalities are caused by the lens.”

Dr. Applegate agreed that corneal topography is necessary for refractive surgery for several reasons. “It is very useful in all kinds of corneal pathology,” he said. “It helps us understand the nature of the pathology and its natural history.”

Dr. Williams said he believes wavefront analysis can be effectively used in conjunction with corneal topography. “I think if you really want to do a thorough job in laser refractive surgery, you need to understand a lot of information about the patient,” he said. “The first surface of the cornea is valuable to know about, and you don’t get that from a wavefront sensor. I think the most useful instruments will be those that combine topography and wavefront sensing into a single, convenient device for the clinician.”

For Your Information:

• Marguerite McDonald, MD, can be reached at 2820 Napoleon Ave., New Orleans, LA 70115; (504) 896-1240; fax: (504) 896-1251.
• Mary Dulle is director of corporate communications for Alcon. She can be reached at 6201 South Freeway, Fort Worth, TX 76134-2099; (817) 551-8058; fax: (817) 551-568-7000; e-mail: Mary.Dulle@AlconLabs.com.
• Thomas McKay is director of marketing for Visx Inc. He can be reached at 3400 Central Expressway, Santa Clara, CA 95051; (408) 773-7198; fax: (408) 773-7278.
• David Williams, PhD, can be reached at the Center for Visual Science, University of Rochester, Rochester, NY 14627; (716) 275-2459; fax: (716) 271-3043.
• Scott MacRae, MD, can be reached at 601 Elmwood Drive, Rochester, NY 14642; (716) 273-2020; fax: (716) 756-1975. He has no direct financial interest in the products mentioned in this article. Dr. MacRae is a paid consultant for Bausch & Lomb Inc. and Nidek Inc.
• Lt. Col. Corina van de Pol, OD, PhD, FAAO, can be reached at PO Box 620577, Fort Rucker, AL 36362; (334) 255-6876; fax: (334) 255-6993. Lt. Col. van de Pol has no direct financial interest in the products mentioned in this article, nor is she a paid consultant for any companies mentioned.
• Thomas O. Salmon, OD, PhD, can be reached at 1001 N. Grand Ave., Tahlequah, OK 74464; (918) 456-5511; fax: (918) 458-2104. Dr. Salmon has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
• Raymond Applegate, OD, PhD, can be reached at 7703 Floyd Curl Drive, San Antonio, TX 78229-3900.
• Primary Care Optometry News could not determine whether Drs. Applegate, McDonald or Williams have a direct financial interest in the products mentioned in this article, or if they are paid consultants for any companies mentioned.