Preview lenses can show your patients what aberration-free vision would be like.
In reality, however, we know that the cornea is hardly inert. It is living tissue that dynamically responds to ablation with healing and biomechanical changes that can mask fine ablation detail. While this realization may have deflated previous expectations somewhat, wavefront technology continues to hold tremendous promise.
Wavefront analysis is a method of objectively quantifying all the aberrations of the eye. It was originally applied to telescopes used by astronomers in the 1960s to help resolve distant stars. Refractive surgery was the catalyst that helped wavefront move into eye care.
Now, wavefront analysis allows you to measure higher-order aberrations, which represent about 20% of a typical eyes total optical aberrations. But, in some patients, such as those with keratoconus, higher-order aberrations can represent the majority of optical aberrations. Wavefront analysis opens the door for measuring these formerly inaccessible elements that impact vision so that we can apply new corrective treatments.
Every major excimer laser manufacturer in the international market now has a wavefront analyzer for refractive surgery. With wavefront-guided treatments entering the U.S. market, we can expect improved postoperative visual quality.
Even so, wavefront is not a perfect measurement. Many factorseye fixation, pupil size, accommo- dation, tear film and natural aging changes influence it.
Moreover, wavefront aberrometers only measure monochromatic aberrations, yet our eyes process polychromatic information. This raises the question: Does the discrepancy between the measured monochromatic wavefront and actual polychromatic wavefront overwhelm the amount of higher-order aberrations we are trying to correct? Researchers and clinicians are still working on an answer. This will go far in determining the future clinical role of wavefront, much like what happened when corneal topography was introduced. The hope is that wavefront-driven treatments will significantly improve contrast sensitivity and night vision over standard methods.
This hope is turning into reality. Already, wavefront technology is finding its way into these systems:
LADARVision. In 2002, Alcon became the first company to receive FDA approval for customized LASIK with a wavefront measurement device and an excimer laser. The LADARVision 4000 excimer laser, coupled with the LADAR-Wave wavefront measuring device, uses an integrated system approach to customized laser eye surgery.
The LADARWave aberrometer captures higher- and lower-order aberrations that are unique to each patient. It then transfers this information to the laser, where it is computer-matched to create the precis- ion ablation required in customized laser eye surgery.
Preview lenses. Preview lenses are custom-made, plastic lenses that subjectively demonstrate what a patients vision would be like when all ocular aberrations, including the higher-order components, are corrected. These lenses are comparable to the potential acuity meters (PAM) used to show the best-case scenario for vision after cataract surgery. A preview lens demonstrates what vision might be after refractive surgery. Preview lenses can also help us verify the appropriate correction when there are significant discrepancies between the manifest and wavefront refractions.
Heres how preview lenses work: First, a wavefront aberrometer measures the patients aberrations to create an ablation profile. Then, an excimer laser ablates the profile onto a preview lens to show what vision would be like.
Julian D. Stevens, M.D., of Moorfields Eye Hospital in London, is pioneering the use of preview lenses for patients with vision problems from previous refractive surgeries. Dr. Stevens has reported data on over 100 eyes; after wavefront-guided enhancement almost all gained significant lines of previously lost acuity.1
Some clinicians believe that optimal visual quality requires the complete elimination of higher-order aberrations. However, some patients may actually prefer some higher-order aberrations, just as some presbyopic patients prefer small amounts of myopia. The existence of simultaneous-vision multifocal contact lenses testifies that some individuals prefer multifocality to aberration-free vision.
Preview lenses can help identify preference among higher- order aberrations.
Of course, preview lenses do not demonstrate the likely range of surgical outcomes. What a patient sees through a well-positioned preview lens is not the guaranteed vision after wavefront-guided surgery. One software program, the CTView (Sarver & Associates Inc.) can simulate visual images with varying degrees of wavefront error, corresponding with differing visual outcomes. Another limitation of preview lenses is that they are time consuming to make and demonstrate.
Given the development of preview lens systems, could wavefront revolutionize spectacle lens fabrication? Imagine that you could have your patients wavefront measurement recorded and transmitted to an excimer laser in just seconds. The laser would ablate the correction onto a plastic lens blank. Your patient could go home with new glasses that could provide him or her vision at the limit of optical resolution, in minutes.
Unfortunately, this scenario is challenging. Simply, spectacle applications are severely limited, making them impractical, because higher-order wavefront corrections need to move with the eye, says Raymond Applegate, O.D., Ph.D., director of the Visual Optics Institute and Borish Chair professor at the University of Houston College of Optometry.
Indeed, alignment of the preview lens is absolutely critical for best vision. Similar to pinhole glasses, the preview lens must have the appropriate vertex distance and translational position, making it difficult or impossible for some patients to preview an ideal correction.
Wavefront may, however, advance spectacle lens design in a broader sense. Spectacle lenses are generally designed using front- and back-surface curvatures to minimize marginal astigmatism for up to 25-30 degrees of eye rotation.
Wavefront technology could assist in designing better aspheric lenses for the individual, although the resulting improvement in vision is likely to be small, says Ian Bailey, O.D., M.S., professor at the University of California Berkeley School of Optometry.
Contact Lenses and IOLs
Unlike with spectacles, there is probably greater potential for wavefront-designed contact lenses and IOLs to provide aberration-free vision.
In 1997, CLAAS (Contact Lenses with Aspherical and Asymmetrical Surfaces) began as a collaborative project among researchers in Europe. CLAAS investigates custom-made soft and gas-permeable contacts to correct ocular wavefront aberrations. In addition to the technical difficulties in manufacturing these lenses, the researchers found that the lenses must have translational and rotational stability on the eye. And, because each patient has a different combination and amount of higher-order aberrations, it may not be practical to manufacture these lenses on a large scale.
With intraocular lenses, surgeons are treating more aberrations than just axial myopia or hyperopia. STAAR Surgical received FDA approval in 1998 for its Toric IOL, which allows surgeons to attempt to correct corneal astigmatism from within the eye. Now, Pharmacia researchers have developed the Tecnis Z9000 negative spherical IOL to compensate for the corneas natural, positive spherical aberration.
Move Over Auto-refractor
Today wavefront technology is used almost exclusively in refractive surgery centers and academic institutions. But just like corneal topographers, look for wavefront aberrometers to move into the mainstream with further refinements and cost reductions.
Corneal topography was introduced into clinical practice about 15 years ago. Today topographers are replacing manual keratometers because topography provides more information and reads a larger area of the corneal surface. In an analogous fashion, wavefront aberrometers provide refraction data for a larger area of the pupil than auto-refractors do. Wavefront measures refraction as a function of pupil size, a dy-namically changing characteristic.
Although wavefront aberrometers probably wont replace subjective refraction, their accuracy may significantly speed up and refine the refracting process. At the very least, wavefront could help you diagnose subtle forms of keratoconus and determine why some post-refractive surgery patients complain of poor vision despite good Snellen acuity.
The art of refraction has not changed much over the last century, despite continual changes in our profession. Indeed, wavefront technology promises to advance the very element of our profession that has stayed stagnant for so long.
Dr. Chou is in group practice at Carmel Mountain Vision Care in San Diego. He has no financial interests in the products mentioned in this article.
1. Hidalgo-Simon A. Wavefront emerges as powerful tool for night vision. Eurotimes 2002 Nov.:12. http://www.escrs.org/eurotimes/November2002/wavefrntemerge.asp