Despite its complexity, this technology is the wave of the future.
Karl G. Stonecipher, MD Greensboro, N.C.
Originally developed as part of the Star Wars anti-missile defense program and used by astronomers to enhance the images found through ground-based telescopes, wavefront technology is now positioned as the hottest new thing in refractive surgery. The technology's complexity, however, and the overwhelming din surrounding it have made some ophthalmologists question whether or not any real value exists beneath the hype.
I think it does. In this article, I'll explain why we need wavefront technology and how manufacturers are working to bring it into our practices.
Figure 1. Wavefront systems project light rays into the
eye. As the wavefront (pictured as a yellow disc above) travels
toward the back of the eye, the patient's optical system distorts
it. Wavefront technology can calculate how the corneal surface
should be changed in order to create a crisply focused image on
the retina.
Autonomous Technologies Corp.
Why Wavefront?
One common misconception right now is that wavefront technology
is supposed to supercede and replace corneal topography. Rather,
the measurements that wavefront systems provide are used in addition
to those yielded by corneal topography and refraction. I like
to think of wavefront technology as another piece of the puzzle,
with the puzzle's being the physics of vision.
Corneal topography provides us with invaluable information, but only about the anterior surface of the cornea. Wavefront, in turn, allows us to measure more of the optical system in order to determine what exactly distorts a ray of light during its journey from the front to the back of the eye. With more parameters to enter into the laser, we should be able to achieve better outcomes.
When a ray of light enters the eye, whatever is between the origin of the light and the retina (e.g., cornea, lens, aqueous, vitreous) distorts it. Wavefront technology can detect these so-called higher-order aberrations.
First, wavefront systems project light rays into the eye. Next, the technology measures how much the patient's optical system distorted this wavefront by comparing it to the original wavefront, which was perfect before it hit the eye. The system then determines what adjustments must be made to the subject's corneal surface in order to produce a crisply focused image on his or her retina.
The arguments. One of the main debates right now is over who needs wavefront. One critic, Houston surgeon Jack Holladay, has asserted that only 38 percent of the population has corneal topographic irregularities. He says that wavefront technology may be helpful in these cases, but he stipulates that the asymmetry will likely be in the cornea only in younger patients; in patients over 40 years of age, it's more likely in the lens.
Reservations about the technology change, however, when the topic of corneal shape arises. Critics like Dr. Holladay, who prefers the term spatial refraction to wavefront, agree that wavefront will benefit 100 percent of patients if it can enable surgeons to maintain a prolate corneal shape postoperatively. The reason they favor a prolate shape, of course, is that the cornea's natural design is what helps to focus peripheral light rays on the center of the retina. Right now, refractive procedures entail the flattening of the central cornea, which creates an oblate shape.
So, will wavefront technology help us to maintain prolate corneas? It should, when used in conjunction with the new breed of excimer lasers, which we're just starting to see. A big objection to wavefront has been that, although it will give us very fine measurements of the optical system, our lasers will not be able to produce such delicate changes. Newer guided lasers, however, like Autonomous's LADARVision, Wavelight's Allegretto laser and Lasersight's LaserScan LSX can make these smaller incremental alterations and change the radius of curvature. They also feature an eye tracker, which can compensate for saccadic movements by signaling back to the laser when and by how much the eye has moved. In other words, new lasers will ensure that we ablate the right corneal tissue according to our wavefront-generated calculations.
Additionally, wavefront technology should enable us to fix our previous mistakes: namely, all of our patients with oblate corneas who complain about glare and halos. Wavefront should let us determine which parts of these patients' optical systems are distorted and help us to reduce those aberrations, perhaps even to create prolate corneas in these cases.
The Manufacturers
Some of the original applications of wavefront technology appear
as early as 1619, when Christoph Scheiner developed the Scheiner
disc. In 1894, Marius Tschernig published his initial work on
the aberroscope, a principle upon which J. Hartmann expanded in
the early 1900s. Howard C. Howland and Bradford Howland then applied
both principles to adaptive optics, and, in 1971, Roland Shack,
PhD, and Benjamin Platt, PhD, were the first to publish work related
to the foundation of wavefront technology. By 1976, the principle
of adaptive optics was being applied to the field of astrophysics.
In 1978, Professor Josef Bille, director of the Institute for Applied Physics at the University of Heidelberg, developed a system to measure wavefront distortions that occurred when light traveling through the atmosphere entered a telescopic lens. He was awarded a U.S. patent in 1986 and founded 20/10 Perfect Vision in 1999. He is credited with reducing the size of the apparatus to the point where the benefits of large-scale deformable mirrors are captured on a microchip, making the system more compact and affordable.
Though it's not a wavefront system, the Orbscan from Bausch
& Lomb/Orbtek provides diagnostic information about the cornea,
iris and lens. Orbscan II acquires over 9,000 data points in 1.5
seconds to map 11 mm of corneal surface. Among its capabilities
is analysis of elevation and curvature measurements on both the
anterior and posterior surfaces of the cornea. The system is part
of B&L's laser refractive package that now includes the Technolas
217 excimer laser. (See "Bausch & Lomb Excimer Approved,"
page 7.)
B&L/Orbtek
Currently, four manufacturers are working on wavefront analyzers for use in refractive surgery. Only two of them, Wavelight and Autonomous Technologies, have treated patients, with significant follow up, using their wavefront analyzer linked to their laser system.
* The aberroscope. Drs. Michael Mrochen, Maik Kaemmere, Mierdel, Krinke and Theo Seiler developed the Dresden Wavefront Analyzer based on Tschernig's aberroscope. The system uses a frequency-doubled Nd:YAG laser, emitting at a wavelength of 532 nm, and a mask system for creating 128 equidistant and parallel light rays, which are shone through the cornea. Via optical imaging, the system focuses these rays on the retina. A computerized, low-light, closed-circuit-device (CCD) camera then uses indirect ophthalmoscopy to measure the deviation of the spots as they appear on the retina. The system reconstructs the wavefront using Zernike polynomials.
The Dresden Wavefront Analyzer is linked to Wavelight's Allegretto laser. In Germany, Dr. Seiler has shown improved outcomes when using the wavefront-guided laser vs. conventional laser systems with and without eye trackers. He treated his first patient in June 1999. Myopic sphere ranged from -1.5 D to -6.5 D, with less than 1.0 D of astigmatism. Preoperative best corrected visual acuities ranged from 20/25 to 20/8 for all patients. Postoperative uncorrected visual acuities ranged from 20/20 to 20/8, with an observable reduction in higher-order aberrations. Postoperatively, 26 percent of patients had the same best corrected vision, 61 percent experienced an improvement of one or two lines, and 13 percent gained three or more lines. Scotopic vision either remained unchanged or improved. Dr. Seiler found that reductions in wavefront aberrations correlated with improved uncorrected visual acuities.
Figure 2. The preoperative (left) and one-week postoperative
wavefront profiles for a patient enrolled in Autonomous' U.S.
FDA feasibility trial. Postop uncorrected visual acuity was 20/16.
Autonomous Technologies Corp.
* Hartmann-Shack. Visx's and Autonomous Technologies' analyzers rely on a Hartmann-Shack wavefront sensor to detect optical aberrations. The systems direct an eye-safe probe laser beam into the patient's eye to illuminate a small spot on the retina. A fraction of the probe light reemerges from the eye but is distorted by the aberrations in the optical system. The system's measurement devices contain optics, which convey the wavefront to the Hartmann Shack sensor's entrance face, which in turn contains an array of microscopic optical lenses or "lenslets." These lenslets then divide the wavefront into a number of subapertures. Next, the analyzer focuses each part of the wavefront onto a CCD chip. It reconstructs the wavefront and measures the shift in these points caused by aberrations, again using Zernike polynomials.
Visx has teamed up with Dr. Bille to incorporate wavefront analysis into a Visx laser system. The 20/10 Perfect Vision system features a closed-loop system, which allows patients actually to experience what it would be like to see after undergoing wavefront-based correction. It does so based on the principle of adaptive optics, which enables the system to adjust the distorted wavefront emerging from the patient's eye back into perfect alignment. It is this perfected image that the patient then sees. At present, Visx is at work on its own version of a scanning spot laser with an automated eye tracker to be used in conjunction with its wavefront technology.
At Autonomous, Drs. George Pettit, John Campin and colleagues have designed the CustomCornea Accessory Device. Their preclinical studies confirmed that wavefront analysis is an effective and repeatable measure of visual optics. Company data presented at the 1999 meeting of the Association for Research in Vision and Ophthalmology (ARVO) showed that wavefront measurements of standard sphero-cylindrical errors were consistent with those obtained with a phoropter. It also demonstrated that these measurements can reveal significant aberrations not seen with conventional phoropter measurements.
Figure 3. The eye refraction maps above were created with
Tracy Technology's retinal ray-tracing system. The eye refraction
information map, depicted in the upper right-hand corner, will
provide surgeons with information about spherocylindrical spatial
refraction and higher-order aberrations.
Tracy Technology, Drs. Joe Wakil and Ionis Pillikaris
George Pettit, MD, PhD, chief scientist for Autonomous, outlined a U.S. Food and Drug Administration feasibility trial, which was begun in conjunction with Marguerite McDonald, MD, in October 1999. Subjects included 20 patients undergoing bilateral LASIK surgery and 20 patients undergoing bilateral PRK surgery. Myopic sphere ranged from -2.00 D to -3.75 D, and cylinder ranged from 0 D to -1.25 D. Hyperopic patients with sphere between +0.25 D and +4.25 D and cylinder ranging from 0 D and -3.25 D of astigmatism are also being followed.
Figure 4. Actual wavefront-generated maps depicting myopia
(top), astigmatism and emmetropia (bottom).
Autonomous Technologies Corp.
In the study, investigators randomly selected one eye for CustomCornea and the other for conventional treatment with the LADARVision surgery system. Although one-week postoperative data revealed no statistically significant difference between the two groups, all patients had visual acuities between 20/16 and 20/32. Dr. Pettit notes that these are initial patients and early results--before nomograms could be refined. As refinements to the nomogram have been implemented at each progressive stage of the study, investigators have found improved visual outcomes for the CustomCornea patients (especially the myopes) over those receiving conventional techniques.
* Ray-tracing refractometer. Drs. Joe Wakil and Ionis Pillikaris are currently working with the Tracy Ray-tracing Refractometer. Rather than projecting a grid pattern of light rays on the eye, the refractometer measures the eye's refractive power on a point-by-point basis. For this reason, the system will not be subject to the potential problem in a highly aberrated eye of crisscrossing data points, as the three other wavefront systems could be. The refractometer rapidly fires a series of very small parallel light beams one at a time through the entrance pupil of the eye in an infinite selection of software-selectable patterns. Unlike other wavefront technologies, the Tracy system, therefore, is capable of probing particular areas of the eye's aperture, not just the entire aperture at once.
The system features semiconductor photodetectors, which can detect where each light ray strikes the retina and provide raw data that actually measures the (x-y) error distance from the ideal conjugate focal point. The refractometer functions with exceptional speed, which will compensate for saccadic movements, unlike other systems, says to the manufacturer.
Despite our technological advances, questions remain. For instance, will refractive surgery on a 40-year-old cause problems in the future when his cataractous lens is removed? While we cannot know the future, the prospect of improved surgical outcomes seems to be within our grasp. Certainly, we can hope that wavefront technology will allow us to offer better vision to our patients.
Dr. Stonecipher has performed over 12,000 refractive procedures and is director of refractive surgery at Southeastern Laser and Refractive Center in Greensboro, N.C. He serves as clinical advisor to several laser companies and centers.
Glossary of Common Terms
* ABERROSCOPE. This device measures the spatial refraction of an optical system, such as the eye, and can thereby tell the spherocylindrical refraction and optical aberrations of the system.
* ADAPTIVE OPTICS. Wavefront systems using adaptive optics feature a flexible, membrane mirror, on the back of which are mounted electrodes. When a wavefront, distorted by the patient's optical system, emerges from the eye, it hits this mirror. The electrodes move the mirror in order to adjust the wavefront back into perfect alignment.
* HIGHER-ORDER ABERRATIONS. When a light ray enters the eye, it can be bent and distorted by the optical system before it strikes the retina. Higher-order aberrations include irregularities to the anterior and posterior corneal surface, lenticular changes and retinal imperfections.
* WAVEFRONT. A beam of light is commonly thought of as a bundle of light rays. These rays are perpendicular to the normally bowl-shaped wavefront. The concept of a wavefront can be easily understood by picturing ocean waves approaching the shore. The direction each part of the wave is moving is analogous to a light ray. The crest, or trough, of the wave is the wavefront. Light travels in a procession of wavefronts, or flat sheets. A wavefront is an isochronic surface, meaning it's of equal time.
After light has left a point source, the rays must converge simultaneously on the retina to form a perfect image. Wavefront sensing is a new way of mapping the spatial refraction (spherocylindrical error) and aberration profile (spherical aberrations, coma aberrations, higher-order astigmatic aberrations and more) of the eye.
* ZERNIKE POLYNOMIALS. Since the mid-19th century, terms such as astigmatism, coma, spherical aberration and distortion have been used to describe the defects in perfect imagery that occur in optical systems that have an optical axis. In addition to their type, such aberrations are usually assigned an order such as primary and secondary, or sometimes third-order, fifth-order and so on. These so-called higher-order aberrations introduce their own type of defect, such as oblique spherical aberration, elliptical coma and others.
Aberration polynomials express the magnitude of the aberration. In the early 20th century, Fritz Zernike introduced polynomials that describe the properties of an aberrated wavefront. Today, they have a new use in representing aspheric surfaces (either refractive or diffractive), particularly in systems that produce higher-order aberrations.
Beyond Wavefront
Wavefront technology can determine exactly what final shape would produce an eye with 20/10 vision. Yet, Daniel Z. Reinstein, MD, MA, FRCSC, for one, says there is much yet to be learned about the basic biological mechanisms of LASIK surgery and its effect on the cornea before we can accurately achieve that change in the shape of the cornea.
Surface topography only provides a graphical depiction of the presence and location of corneal irregularities, not their underlying anatomical basis. Corneal-thickness measurement by conventional pachymetric methods before and after refractive surgery correlates poorly with the intended amount of tissue removal because there are changes in the epithelial and stromal layers that occur during healing. The variability and relative imprecision of microkeratome cuts make truly precise prediction of flap thickness impossible, and produce an unpredictable final depth of keratectomy (and of residual stromal thickness) that may affect biomechanics.
Top: Four months after LASIK, the epithelium and keratctomy interface are clearly visualized from one end of the cornea scan to the other. Note the keratome entrance point, with evidence of the retraction of the cut end of Bowman's layer under the epithelium (E); a small irregularity in the smoothness of the keratectomy (I); the flap is noted to be thinner (T) nasally than temporally. The keratome track stops abruptly within the nasal stroma to produce the flap hinge (H).
Bottom: VHF ultrasound 3D-pachymetric topography of the epithelial layer before and six months after LASIK in a patient with postop 20/15 uncorrected visual acuity. The epithelial thickness changes are greatest centrally, thus producing a refractive effect on the cornea (reversing the flattening by the laser).
The epithelium appears to possess the ability to remodel itself to compensate for stromal-surface abnormalities, caused by either flap irregularity or irregular stromal resection. A cornea with asymmetric astigmatism thus possesses an irregular epithelial thickness profile. Some compensation will mask the anatomical irregularities and cause miscalculation from topographical and wavefront measurements currently used to plan customized surgery.
Wavefront may address none of these. Dr. Reinstein, a professor of ophthalmology at the University of Paris, an associate professor at Cornell University and national medical director for Lasik Vision Corp. of Vancouver, B.C., has been working with a new digital, very-high frequency ultrasound system. By digital signal processing, the technology is able to clearly resolve the epithelial layer from the stroma, and the LASIK flap from the residual stromal bed (left, top) in three-dimensions (bottom). Dr. Reinstein developed the device with Ronald Silverman, MD, and Jackson Coleman, MD, and has been using it to study corneal refractive surgery for nearly 10 years.
Though the data is preliminary, Dr. Reinstein will be presenting specifics on a two-year study at next month's ARVO meeting. "We discovered dramatic changes in the epithelial refractive power of the cornea after LASIK cases," he says. "We are also showing significant biomechanical changes in regular LASIK cases with 'enough' under the bed. Amazingly, epithelial and biomechanical factors together appear to fully explain the inaccuracy of current LASIK.
"Wavefront gives us an enormous amount of information about the optical aberrations in the eye but no information about the corneal anatomy," he says. "We're unable to accurately put the lower-order aberrations--known as sphere and cylinder--on the finish line, and that's only two numbers. How could we expect that adding another 10 Zernike coefficients would help? The reason why only 60 to 70 percent of low myopes are only 20/20 is not because of extraneous ocular aberrations, it's because there are epithelial and biomechanical changes in LASIK that we are not measuring and, therefore, not controlling for.
"The eye is a balloon. If you thin the wall of a tiny area of a balloon, it makes sense that it's going to bulge. About 15 percent of the refractive-cutting effect of the laser can be lost by this bulging after LASIK. The cornea is part of that balloon, which is under pressure. And the tissue in it models and bulges in different ways in different individuals.
"We have failed to ask some fundamental questions about the stability of refraction in an eye," he concludes.
--Chris Glenn, Editor-in-Chief