Riding the Wave

Experts discuss how wavefront aberrometry has transformed refractive correction and consider some of its future applications.

BY LESLIE GOLDBERG, ASSOCIATE EDITOR

When the excimer laser debuted in the early 1990s, ophthalmic surgeons and their patients were wowed by its ability to etch corneal tissue in 0.25-micron increments. Such precision opened up entirely new frontiers in refractive correction. Unfortunately, early ablation profiles didn't fully exploit the laser's extreme precision. It wasn't until wavefront analyzers were paired with the excimer that outcomes finally matched the laser's potential. Providing customized ablations that take into account each patient's unique aberration data was a game-changer, with some patients achieving 20/15 or better vision postoperatively.

Its effects are still being felt today. LASIK nomograms are continually being refined thanks to further advancements in wavefront analyzers, and the technology recently made the leap from refractive to cataract applications as a means of fine-tuning toric IOL positioning and assisting with limbal relaxing incisions. With other applications ranging from diagnostic screenings to custom-made "high definition" corrective lenses, wavefront aberrometry is becoming ever more ubiquitous in ophthalmology. In this article, experts discuss the continued growth of the technology, what's hot in development and what they are finding in their studies.

Wave of the Future

"Doctors will continue to be more interested in wavefront technology as it permeates everyday practice in a number of different areas," says Scott MacRae, M.D., professor of ophthalmology at the University of Rochester School of Medicine. "The biggest change has been in the area of cataracts." WaveTec's recent introduction of the ORange intraoperative wavefront aberrometer allows surgeons to perform a virtual refraction midprocedure, with the patient still on the table. "It is useful for confirming your astigmatism treatment when doing toric IOLs," says Dr. MacRae.

Miami's William Trattler, M.D., who was part of the clinical trials for ORange and assisted in developing the technology, currently uses the ORange in three situations:

► Toric IOL orientation. He says it is great at helping to orient the toric IOL into the correct position to maximally reduce astigmatism.

► LRIs. "It provides feedback on the table to determine whether the initial LRIs have corrected all the astigmatism. If ORange provides feedback that there is still significant residual astigmatism, I can either deepen or lengthen the LRIs before leaving the operating room," says Dr. Trattler.

► Final refractive outcome. "I can perform a measurement at the end of the case, with the IOL in its final position, and determine whether the patient is close to my final refractive target. This is especially helpful in patients who have previously undergone LASIK. If the patient's intraoperative refraction is way off, then I can decide whether I should exchange the IOL prior to leaving the OR," he concludes.

Dr. MacRae believes that cataract surgeons will soon start seeing more customized aspheric lens designs based on wavefront measurement of the cornea. "You can see where this would be helpful eventually when you have wavefront-customized IOLs. You get instant feedback."

Wavefront measurements are also helpful in predicting whether someone will do well or poorly with a multifocal IOL. "If there is a lot of corneal higher-order aberration, it bodes poorly for putting in a multifocal IOL," says Dr. MacRae. "Clinicians are learning the subtleties of wavefront in the cataract world."

A Higher Calling

The original LASIK protocols of the 1990s were a mixed blessing, eliminating the patient's lower-order aberrations (namely, sphere and cylinder) but adding to their higher-order ones (spherical aberration, coma and trefoil). Trading spherical error for glare and halos was a thin bargain.

Refractive surgeons ushered in the present decade with a burst of activity devoted to eliminating surgically induced higher-order aberrations; as the decade concludes, HOAs have been not merely tamed but mastered. Wavefront-optimized (WFO) procedures spare patients the burden of additional treatment-induced spherical aberration, while wavefront-guided (WFG) protocols take into account whole eye higher-order aberrations and provide a corneal correction that offsets it.

Still, challenges persist. "Ideally, surgery should reduce HOAs, but that this is not always the case for a variety of reasons, such as problems with imperfect registration, eye rotation during the ablation, limits to the fidelity and dynamic range of current wavefront sensors, wound healing and anatomic location of the aberrations (lenticular versus corneal)," says Jay S. Pepose, M.D., Ph.D., director of Pepose Vision Institute in St. Louis.

"In a highly aberrated eye, there would be more impact from a wavefront-guided treatment (i.e., more benefit) than in an eye with little aberration, says Dr. Pepose. "However, if I were to have LASIK, I would want the treatment that reduced aberrations the most and induces them the least — a wavefront-guided treatment."

Dr. Pepose believes that WFO treatments are popular because they do not require acquiring the wavefront aberrometry measurement and incorporating it into the protocol. Nomogram development may be more straightforward as there are fewer variables, and there may be financial aspects of the WFO procedure that are more attractive to doctors/patients. For more on studies of WFG and WFO, see the box below.

The Shape of Things to Come

Even with wavefront analysis designed to detect refractive error and aberrations of the eye, it is still necessary to have detailed corneal topographic information to understand the contribution the cornea makes to vision.

"Wavefront analysis is very good for figuring out refraction and determining the quality of a patient's visual pathway. But if patients have cataracts, dry eye and/or other corneal problems, it may be challenging to obtain a good wavefront image," says Dr. Trattler.

He emphasizes the importance of running other tests in order to make an accurate diagnosis. "I perform a corneal topography and macular OCT prior to all of my cataract surgeries," says Dr. Trattler, "as I feel it is essential to identify abnormalities such as irregular astigmatism on topography or epiretinal membranes on OCT that may impact the final visual results of cataract surgery."

One exciting recent advancement is the combination of corneal topography with wavefront scanning so that both corneal curvature and whole-eye aberrometry data are accounted for in the surgical nomogram. Devices such as the OPD Scan from Nidek, the iTrace Combo from Tracey Technologies, and Topcon's KR-9000PW Wavefront Analyzer integrate corneal mapping with aberrometry, measuring both higher-order aberrations and the lower-order aberrations in an attempt to achieve an ideal correction. All-in-one units also obviate the registration problems that may occur when topography and aberrometry are performed by separate devices.

Surgeons are eager to embrace the new technologies that are already entering the marketplace. These new products have a great deal to offer both refractive and cataract specialists.

"The most important advantage of the iTrace Combo is that this single combination machine functions as a topographer, while also providing high-quality wavefront data that can be rapidly and simultaneously acquired at the same sitting," says David Chang, M.D., of Los Altos, Calif.

The device has the ability to measure total ocular HOAs and can also tell whether the HOAs are coming from the cornea, from the lens, or both, which is important for a cataract surgeon such as himself. "My practice is limited to cataract surgery, but now that all cataract surgeons are essentially refractive surgeons, we can really benefit from technology that was previously only of interest to our refractive colleagues," says Dr. Chang.

Dr. Chang says that to improve patient flow, his practice performs auto-keratometry/refraction and uses both the IOLMaster from Carl Zeiss Meditec and the iTrace (both topography and aberrometry) on every potential cataract patient during their pre-testing.

He states that if his optometrist confirms a cataract, the data are printed out for him, rendering it unnecessary to have a lengthy discussion about multifocal IOLs if the patient has too much regular or irregular astigmatism to qualify.

"Preoperatively, especially for a multifocal IOL candidate, I want to know if there are HOAs coming from the cornea," says Dr. Chang. "These will likely create problems with visual quality that will persist postoperatively and will not be correctable with spectacles, and certainly not with the IOL. An atypical topography pattern raises suspicions, but the iTrace allows us to quantify, characterize and classify the HOA, and distinguish between corneal and lenticular HOA."

Another important use for the iTrace is in the preoperative evaluation of a patient complaining about poor vision (e.g., at night), but who has an unimpressive cataract, and no other visible pathology. "I may see these patients referred for consideration of cataract surgery, because a mild cataract is all that the referring doctor sees. Wavefront aberrometry allows me to see whether HOAs are the cause, and if so — whether they are from the cornea or the lens. If there are significant HOAs from the cornea, the patient probably won't benefit from cataract surgery," says Dr. Chang.

Finally, knowing the fourth-order corneal spherical aberration helps him to decide whether an aspheric IOL would be useful in a given individual. "Postoperatively, wavefront aberrometry is very useful for patients complaining about poor visual quality. They may have been told that the IOL and the retina look fine, and that the ophthalmologist can find nothing wrong," says Dr. Chang. "Until the iTrace, we didn't have an easy way to measure and quantify HOA, and to determine if they were from the cornea or the IOL. This has been of great diagnostic value, particularly for unhappy premium IOL patients referred for consultation."

Dr. MacRae says that AMO's VISX platform will be upgraded with a new system tentatively called iDesign that combines aberrometry and topography. It's expected to debut later this year. "If you can combine this in an operating microscope, it is very powerful," he says. Topographic data is particularly helpful when treating patients who've previously undergone surgery and may have highly aberrated corneas.

iDesign, billed as "high-resolution wavefront," captures 1257 data points for a 7 mm pupil, a fivefold increase over existing wavefront aberrometers, according to Dr. Trattler. The additional dynamic range provided by the increased resolution should allow for better detection of keratoconus and irregular astigmatism, he says.

Although experts in optics have known about HOAs for decades, only in recent years have wavefront aberrometers advanced sufficiently to produce measurements accurate enough to yield clinically-applicable insights, Dr. MacRae says. A number of companies working on more advanced wavefront sensors with much higher resolutions, he points out, which then register those measurements with the actual treatment.

For example, B&L introduced the Advanced Control Eye Tracker (ACE), an active rotational eye tracker just recently available in the U.S. market. B&L's ACE compensates for any movement of the eye and moves the tracker to align with that movement. He says that better registration is one of the major benefits of wavefront technology and that now that systems are aligning, users will get better results in HOA correction.

Something Old, Something New

Wavefront technology can even be used to help improve the oldest of refractive corrections: spectacle lenses. The Z-View Aberrometer from Ophthonix of Vista, Calif., uses wavefront measurements to objectively measure HOAs. The system captures the patient's wavefront measurements and generates a wavefront-guided sphere and cylinder prescription that is then sent to an Ophthonix lab where custom lenses called iZon are produced. In clinical testing, iZon "high definition" lenses delivered significantly better vision than conventional lenses in low and intermediate contrast, glare and low luminance conditions, according to Ophthonix. Additionally, using an FDA-validated night driving simulator, the iZon ML Lens system demonstrated improved reaction by 20 feet at 55 mph, the company says. A similar approach for contact lenses is being pursued by WaveTouch Technologies of San Jose, Calif.

"The bottom line," says Dr. MacRae, "is we are continuing to see the wavefront evolution — not revolution. It is a slow evolution of wavefront technology being integrated into cataract surgery, being refined in terms of corneal refractive surgery and then the combination of both cataract and corneal refractive surgery to use those tools jointly. It pushes our ability to improve outcomes." OM