Ординатура / Офтальмология / Английские материалы / LASIK and Beyond LASIK Wavefront Analysis and Customized Ablation_Boyd_2001

CORNEAL TOPOGRAPHY AND WAVEFRONT ANALYSIS

How Useful Is the Information

They Provide?

Both corneal topography and wave front analysis provide useful information about the anterior corneal surface. Some manufacturers of the many topography instruments available claim their instruments also provide information about the posterior corneal surface as well. From a combination of anterior and posterior measurements, ophthalmologists could then deduce the nature of the intervening tissues. In my experience, the anterior surface information provided by corneal topography is excellent, but posterior measurements are not as reliable. Still, specific changes on the anterior surface cannot be measured as precisely as we would desire for performing custom ablation. Both corneal topography and wave front technology need to be improved over time.

Wavefront analysis is a technology that can provide detailed information about the overall refractive status of the eye, including the cornea, but also the lens, the shape of the eye and changes that occur with pupil dilation (See Figs. 29-1, 29-2, 29-3, 29-4, 29-5, Chapter 29). It provides information about all the aberrations that exist within the eye. Information gained through

both technologies (corneal topography and wavefront analysis) can be integrated to provide a fuller picture of the refraction and higher-or- der aberrations that can occur in normal eyes, and that certainly occur in eyes with poor outcomes after refractive surgery. Corneal topography by itelf cannot provide a complete assessment of severe higher order aberrations.

Is Wavefront Technology by Itself Sufficient to Perform Custom Ablation?

Some ophthalmologists predict that we will be able to use wave front technology exclusively to perform custom ablation (See Figs. 29-6, 29-7, 29-8, Chapter 29). This may be true in treating normal eyes because general assumptions can be made about the shape of the normal cornea. With patients who have some kind of corneal abnormality, we will not be able to make these assumptions. We will not know the relation between higher order aberrations and corneal shape, or how to change the shape of the cornea to eliminate these aberrations. To approach these challenges, both corneal topography and wave front technology are needed.

Eventually, the instrumentation for both technologies will be integrated into the same system; some manufacturers are already exploring these possibilities.

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Variability of Wave Front Mapping

Many factors contribute to the complexity of interpreting wave front maps. The wave front map is affected by changes in the eye like the development of cataracts or even by pupil dilation. The analysis will differ at different stages of pupil dilation from very bright light to fainter light. On which reading should the surgeon base the refractive ablation? The answer to that question may depend on what the patient does most of the time. If the patient drives and works at night, perhaps the more dilated pupil wave front information should be used. On the other hand, perhaps the small pupil reading should be used if the patient works primarily outside during the day. All these considerations need to be better understood in order to make optimal corrections for patients.

Current Status of Custom Ablation

Custom ablation ideally means that the dimensions of the ablation will change precisely according to the particular aberrations of an individual eye in order to give that optimal function under particular conditions. Excimer laser surgeons currently perform minimal customizations by providing some variation in the diameter of the ablation. For instance, it is now very common to use a 6.5mm diameter ablation in a lower correction patient with larger pupils.

Subtle variations in corneal shape cause the same diagnostic label to reflect different conditions in different eyes. Yet current customization is really a one-size-fits-all approach. For instance, all patients who need a –9 diopter ablation get the same ablation, even if one has 1/2 diopter of assymetric bow-tie astigmatism. With true custom ablation, the corneal surface and wave front mapping will be integrated to determine a very

may reverse these small changes. Many companies are now pouring considerable resources into the actual analysis of the wave front and the application in the ablation, but almost nothing into what happens afterwards. Unless they realize that we must look at how to maintain correction, we will end up with great analysis software and applications but an unpredictable result when the technology is applied.

Only 6 years ago, we discovered at our laboratory the concept of keratocyte apoptosis. Since that time, much more knowledge is available about the proliferation of keratocytes, the migration of inflammatory cells and their evolution into different cell types that manufacture collagen. Progress is being made toward understanding how all these contributions are associated with wound healing, and how the wound healing process can be controlled. During the next 5 years we predict that pharmacological manipulation of the corneal wound healing response will become a standard part of refractive surgery.

The familiar objective of blocking wound healing reflects a misguided approach. We encourage ophthalmologists to focus on normalizing the wound healing process. The greatest challenge to understanding and manipulating wound healing is the variability in the wound healing process: one patient may have a much lower healing response than another along a continuum of possible responses. Our goal should be to make the healing process consistent—not to prevent healing, because these wounds must be healed, but to prevent an aggressive healing response that tends to produce regression, haze and other complications like irregular astigmatism. A very low response, resulting in a large overcorrection, is also an undesirable outcome. Somehow we need to ensure that patients will heal in the middle

PROMISING NEW TECHNOLOGY: WAVEFRONT ANALYSIS

Attaining Bionic or Super Vision

Recent technological advances now allow us to measure and treat lower-order aberrations of the eye, problems of the sphere and the cylinder. Experts in neural biology now tell us that in the future human vision will be pushed to the limits of the spacing of the retinal photoreceptors, which is 20/8 or 20/5. Some people call this bionic vision or super vision. For purposes of comparison, the visual acuity of hawks is about 20/6 and of eagles is 20/4. Humans will be able to see not quite as well as eagles but as well as hawks if higher-order aberrations can be corrected.

The technology that will make it possible to correct these higher-order aberrations is wavefront analysis. The wavefront is a like a wrinkled sheet of light that emerges from the eye (See Figs. 29-1, 29-2, 29-3, 29-4, 29-5, Chapter 29). Some people find this image confusing, as they imagine light traveling in rays like arrows. The wavefront is perpendicular to the ray—indeed, the two images are two different ways of describing how light travels.

The first surgeon in the world to perform excimer laser surgery based entirely on wavefront mapping or analysis instead of phoropter refraction was Dr. Teo Seiler in March 1999. Our team in New Orleans under my direction was the first to perform this surgery in the U.S (See Figs. 29-6, 29-7, 29-8, Chapter 29). We accomplished this wavefront-based

laser surgery in October 1999, 7 months after Dr. Seiler’s first procedure. There are patients in different regions of the world who see 20/8 after wavefront surgery. The expanding number of patients treated by research teams around the globe who see 20/8 after wavefront surgery make it clear that we will be able to improve on Mother Nature.

Generating the Wavefront Map

This is how the technology works (See Fig.29-3, Chapter 29). When the eye is dilated, a probe beam of light is focused in the eye, and a 905 wavelength diode laser is sent in. It hits the retina and bounces back out of the dilated pupil. As it bounces back, it picks up the optical characteristics of the eye. When it emerges from the cornea, it hits a lens slit array or slit groupings comprised of many dozens of small lenses. Then the point image of each lens slit is captured by a CCD camera. A color-coded wavefront map is generated from the point image of each lens slit. A mathematical equation called the Zernecki polynomials is applied to these raw data.

The information obtained from wave front analysis is actually integrated into the excimer laser, making it possible to utilize the diagnostic information in the therapeutic mode. The map is then used to generate the shot pattern. This information is written on a floppy disk placed inside the laser to drive the ablation. The wave front sensing device is a separate machine located close to the laser, but not attached to the laser. It is a diagnostic device that is easy to use and painless for the patient.

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Chapter 31

Wavefront Analysis and Corneal

Topography

Wavefront maps are in color and look somewhat like corneal topography. Although they are sometimes used in conjunction with corneal topography, they are actually quite different. Corneal topography will never be supplanted by wavefront analysis, as it is important to know the shape of the cornea. Although one manufacturer, Nidek, uses both a colored wavefront and a colored topography map together to generate the treatment pattern, the two maps are separate.

Marguerite McDonald, M.D.

Director

Southern Vision Institute

New Orleans, Louisiana (USA)

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Promising Achievements

Refractive surgery has always dealt with the correction of spectacle refractive error, or problems with the sphere and the cylinder. Now, when performing a standard LASIK technique with the majority of laser systems the surgeon can achieve correction within half a diopter of the desired refraction for about 75% to 80% of patients with myopia of 10 diopters or less. But what about the other 20% of patients? Our goal is to correct 90% of patients who wear glasses or contact lenses to 20/10 by the year 2010. The technology that promises to achieve even better refractive correction is called wavefront analysis. Whereas correcting the sphere and the cylinder takes care of 80% to 90% of the visual problems both of patients with myopia and hyperopia, wavefront analysis and wave front correction will take care of the additional 10% to 20%.

Principle of Wavefront Analysis

The basic principle behind wavefront analysis is different from the principle behind an ordinary refraction. An ordinary refraction results in an average correction over the entrance of the pupil of the eye. Wavefront analysis, on the other hand, assesses the correction at each point measured over the pupil (See Figs. 29-1, 29-2, 29-3, 29-4, 29-5, Chapter 29). The refraction over the pupil is not uniform—it may be –3 diopters in the center but –4 diopters at the edge of the pupil. Wavefront analysis makes it possible to detect the refractive error at each point (See

Figs. 29-6, 29-7, 29-8, Chapter 29). The resulting spatially resolved fraction serves as the basis for using a 1-mm flying spot excimer laser to refine the refraction by making a different correction at each spot over the pupil. In addition to eliminating the sphere and the cylinder of the average correction, wave front analysis also eliminates the finer points such as the spherical aberration and coma aberration. With these precise corrections the patient’s vision can go from 20/20 down to 20/10.

This technology has been used for many years in the field of astronomy. Stars at a distance look blurry through a telescope because of the aberrations in the telescope. Special optical techniques have been shown to decrease the aberrations in the telescope, allowing astronomers to see an individual star very clearly. This technology is now being transferred from astronomy and optical science to clinical ophthalmology to help patients see not blurry stars, as it were, but very fine and discrete stars.

Wavefront guided optical correction of the eye will have broad application across many aspects of refractive surgery. It will decrease the problem of aberrations that result from surgery. LASIK treatment to induce a standard myopic or hyperopic correction actually increases the amount of spherical aberration and coma aberration. High resolution central acuity is improved, but night vision becomes worse. Wavefront analysis and wave front guided treatments make it possible to reduce spherical and coma aberration and the other finer distortions that occur in vision.

Clinical trials have compared treatment with the standard excimer laser sphere and cylinder correction to wave front guided correction. Wavefront

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guided correction is proving superior in small increments in a stepwise process of clinical refinement of this technology. The ultimate goal, optimal vision for each patient, means that each excimer laser treatment of the cornea will be unique. Each eye truly has a unique refractive error and a unique visual system. The goal of wavefront guided surgery is to create an individual correction appropriate for the individual eye.

Availability of Technology

A more practical question concerns the availability of the technology for clinical use. In the year 2001 it is at the experimental level. Approximately six laser systems have coupled with them a wavefront analysis unit and the software to convert the results of the analysis into customized laser treatment of the eye. (Consult Chapter 29 for more specific information on the equipment being made available).

Custom Intraocular Lens

But the story does not stop there. Besides the excimer laser, the other major force in refractive surgery today is the phakic intraocular lens. This lens offers the potential of using wave front guided optics to create a custom intraocular lens. The patient’s eye would be analyzed. The manufacturer would make a custom intraocular lens. The implanted custom lens would correct not only the sphere and the cylinder, as today’s lenses do, but would also correct the higher order aberrations, giving the patient 20/10 vision.

Cataract surgery today is truly refractive surgery. Removing the opaque lens is now standard procedure and fairly easy for a competent ophthalmologist. The current challenge is to enable the cataract patient to see 20/10 uncorrected if the condition of the macula makes this possible.

Goal in Mind

With this new goal in mind, ophthalmologists will be performing more and more combined procedures. The cataract will be removed. An aphakic intraocular lens will be placed in the eye. LASIK will then be done with wave front guidance to create

20/10 vision.Similarly, when phakic intraocular lenses are implanted, the majority of correction will be done with the phakic intraocular lens, possibly even with a custom lens. A refinement will then be done in the cornea to fine-tune the refraction in order to attain the best possible vision for the patient.

George Waring, M.D.

Professor of Ophthalmology

Emory University;

Co-Founder

Emory Vision Correction Center

Atlanta, Georgia (USA)

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General Considerations

The correction of the conventional refractive errors present in the eye by modifying the curvature of the anterior surface of the cornea using the excimer laser is already an accomplished technological fact. The next step is to significantly improve this technology and have it perform closer to perfection in the following manner:

1)Correct the optical aberrations that naturally exist in all eyes and which people are born with, some more, some less.

2)Prevent the optical aberrations that may be induced by LASIK surgery such as when we treat myopia, astigmatism and hypermetropia with LASIK. If, on the contrary, we create surface irregularities or opacities of the cornea, these will increase the quantity and the quality of already existing aberrations and consequently reduce the benefits that we are trying to attain through emmetropia.

In order to accomplish this, we need to be able to measure these aberrations during our diagnostic analysis, a procedure that we call "aberrometry". The second step following diagnosis and identification of the existing aberrations is to correct those aberrations, a procedure that we might call "aberrocorrection", even though this term does

not yet exist.

ray of light that is refracted over a perfect spherical surface follows a direct and straight path. But at the eye level, this does not happen because the corneal surface is not perfect since it generally presents aberrations. This results in that the light rays will follow these small variations transforming themselves into rays of light that are not straight but irregular (Figure 33-1).

Among the different aberrations, the most important ones are six geometrical aberrations (prismatic, astigmatic, coma, spherical, field curvature and distortion) and two chromatic aberrations (axial and lateral). In the human eye these optical aberrations occur mainly in the anterior and posterior

tions or irregularities. The superficial corneal surface is not smooth as we all have been accustomed to think but is more like the skin of an orange, which has multiple irregularities. In the eye the irregularities of the anterior surface of the cornea are diminished upon being covered by the lacrimal film, which has a very smooth anterior surface.

The phenomena of aberrations are repeated in all the optical surfaces of the eye that constitute the media such as the posterior surface of the cornea, the anterior and the posterior surface of the crystalline lens and the anterior surface of the retina. To these surface irregularities, we must add those related to the imperfect position of the optical surfaces of the eye. Neither the cornea nor the crystalline lens are centered over the visual axis so the wavefronts cannot have an oblique direction or tilting over them.

We also must take into consideration the irregularities in the transparency and the index of refraction of the ocular media (Figure 1). They exist in the cornea, in the crystalline lens, and in the vitreous body and they are greater as the patient gets older. These different irregularities or aberrations occur as characteristics of each particular individual but many of them are increased when the patient undergoes refractive surgery.

How Do Different Aberrations Affect

Vision in Humans?

The two main types of aberrations: the "perfect" spheric lenses and the "imperfect" spheric lenses do affect vision in humans. A ray of light that is following a path parallel to the eye's optical axis and is refracted on the anterior surface of the cornea theoretically should converge and be focused at one single point located in the photoreceptors of the retina. This does not occur because the personal characteristics of each person vary in relation to the form, position and transparency of the ocular refractive elements. The latter will deviate the different components of the wavefront toward different focal points in the retina (Figure 33-1). This results in defocusing of the images that are manifested as a