WAVEFRONT in the
While this technology is still in its infancy, a thorough understanding of wavefront will help optometrists optimize the visual quality of their patients.
BY PAUL M. KARPECKI, O.D., F.A.A.O.,
Kansas City, Mo.
It was only a matter of time before the application of wavefront technology extended beyond excimer laser ablations. This technology is in its infancy, but it has the potential for use in everything from viewing retinal ganglion cells to increasing best corrected vision with contact lenses and spectacles.
Relatively few advances have been made to phoropters since the first was introduced in 1880. During the past 125 years, we've only measured two aberrations:
1. Defocus, which includes hyperopia and myopia
In the upcoming decade, we'll likely measure other aberrations such as coma and spherical aberration. These may require correction to optimize vision in many patients.
We can measure wavefront in a number of ways, including the Shack-Hartman method as well as ray tracing and even holographic imaging. The excimer wavefront laser platforms including the Bausch & Lomb, Alcon and VISX systems use Shack-Hartman methods to measure a
In the Shack-Hartman System of measuring wavefront aberrations, an 800 nm infrared pulse of light passes through the eye and a charge-coupled device
(CCD) captures the wavefront pattern. When the infrared light source passes through the cornea, lens and media, the aberrations refract the light and create a specific wavefront pattern. This wavefront passes through a lenslet array or a series of lenses, which refract the image onto a charge-coupled device camera or video sensor.
Once the CCD (or in some systems, a video sensor) captures this measurement, a computer extrapolates a mathematical representation known as a Zernike coefficient, based on direction and magnitude. The Zernike coefficients are described in the x, y and z axis. We can determine the local refractive correction by calculating the local radii of the curvature for each area of the entrance pupil. Sources suggest that the human optical system may only be able to perceive up to the 5th radial orders. Some aberrations that are important to be aware of include the 3rd-order aberration coma and the 4th-order aberration spherical aberration. (Just for reference, myopia, hyperopia and astigmatism are 2nd-order aberrations and still most directly influence the optical quality of the image.)
Following defocus and astigmatism, the middle aberrations of Zernike tree degrade the optical image more than the peripheral aberrations. Patients, practitioners and scientists alike usually describe coma as an image with a tail or comet-look to it. Spherical aberration, the primary 4th order aberration is more often described as a halo around a light source. This aberration occurs because of the eye's natural provisions including the fact that it's flatter in the periphery, the peripheral cortex of the lens is less dense and that the iris acts as a diaphragm in photopic conditions. Even conventional LASIK introduces spherical aberration at the edge of the treatment because of increased curvature peripherally.
The first application of
wavefront-guided technology in eye care was excimer laser treatment but other possibilities exist.
Excimer laser treatment. With the flying spot lasers, we have for the first time a net decrease in higher-order aberration before surgery. In the FDA clinical trials for approval, the Bausch & Lomb Zyoptix system and the Alcon Custom Cornea reduced higher-order aberration by about 38%. This result translated into an increase in best spectacle-corrected visual acuity of 37% with Alcon's Custom Cornea System and 60% with Bausch & Lomb's
Also, more than 50% and 75% of all patients in the Custom Cornea and Zyoptix FDA trials respectively had an uncorrected visual acuity that was the same or better than the preoperative best-corrected spectacle visual acuity.
In terms of patient symptoms, an improvement in glare and in night-driving symptoms confirmed these data. Although these data are compelling, we must educate patients that this is certainly not "super vision." Tell patients that wavefront ablation will give them the best chance of 20/20 or better vision as well as an improvement in quality, but that it's not a guarantee nor is it hyper-acuity. Some of the reasons why we don't see super vision are because there are numerous variables involved including accommodation, tear film quality, pupil size and even cortical interpretation. Even though the clinical trials showed a 38% reduction, 20% of patients had an increase in higher-order aberrations induced by the procedure. Studies show that simple accommodation while acquiring the wavefront data can induce negative spherical aberration and result in inaccurate measurements.
Researchers may someday develop intra-operative wavefront monitoring systems that allow subtle treatments at the time of each laser pulse or each energy placement, as in the case of CK example, or ongoing excimer treatment. Even spectacles created from holographic wavefront imaging may be a possibility in the future.
A recent paper that Perry S. Binder, M.D., presented at the American Academy of Ophthalmology discussed the application of wavefront to spectacle lenses. In this placebo-controlled, masked study, investigators randomized 30 emmetropic patients with uncorrected visual acuity of 20/25 or better between receiving a placebo lens in one eye and the Z-lens wavefront neutralized lens in the other or two placebo plano lenses. The investigators had
wavefront-customized spectacle lenses manufactured to correct high-order aberrations (3rd-6th order).
They tested patients for best spectacle-corrected visual acuity, low contrast visual acuity and contrast sensitivity and also noted patient symptoms. The investigators noted statistically significant differences between the Z-lens and the placebo lens for visual acuity (p= 0.048), low contrast visual acuity (p=0.002) and contrast sensitivity (p=0.009). Patient response was extremely positive and this was in an emmetropic population.
Although the technology will certainly apply to myopic and hyperopic or astigmatic patients, it may also benefit emmetropic patients.
Currently, aspheric intraocular lenses
(IOLs) are available and many companies are developing and improving on designs. These IOLs are free of spherical aberrations and allow cataract surgeons to address higher-order optical aberrations. Wavefront science suggests that the young crystalline lens compensate for aberrations in the cornea. As the lens ages and becomes larger, the spherical aberration balance changes. Aspheric IOLs may restore the youthful balance and improve contrast sensitivity in neutralizing the wavefront aberrations.
Another exciting application of wavefront technology may have a place in observing living retinal cone cells and measuring the detailed visual function of the central nervous system by eliminating higher-order aberrations during the examination by adaptive optics. Adaptive optics is the concept of intentionally designing the optics of the observatory system to compensate for the measured aberrations of the subject. Examples of adaptive optics include a fundus camera with adaptive optics allowing detection of dot and blot hemes (hemorrhages) or microanuerysms in a diabetic much earlier than would otherwise be possible. Even beyond this, wavefront allows us to observe and study individual cones in the retinal mosaic for clinical diagnosis and the treatment of retinal disease.
Make way for the future
With regard to ophthalmic applications, wavefront science is certainly still in its infancy, but the possible applications are exciting nonetheless. Understanding the basics of wavefront measurement and the terminology is important. Having numerous applications from excimer laser treatment to spectacles will allow us to better treat many patients and to optimize their visual quality.
It's important to have a solid understanding of these things to better educate our patients about their choices because this information continues to play an important role in the ways that we'll practice eye care in the future.
is the director of
Research for Moyes
Eye Center. He has no
financial interest in any of the products mentioned, but serves as a researcher and a paid consultant for Bausch & Lomb and
Optometric Management, Issue: June 2004