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Advanced Optical and Technology Considerations Pertinent to Laser Vision Care

by David A. Wallace MD

This section presents an advanced and technical discussion of some of the challenging optical issues pertinent to vision and vision correction care. It is intended for the sophisticated and very informed consumer seeking in-depth information about the technology, the treatment, and the frontiers of research in vision correction. The topics include:

• What are Higher Order Aberrations? These are subtle optical imperfections other than defocus (near- or far-sightedness) and astigmatism.
• Refractive "Drift" over time, which is what causes a person's optical prescription to change (minimally) every couple of years;
• "Optical Smear" created by some surgeons and some lasers with older laser systems (and why pupil tracking is essential and is employed in ALL laser surgeries I perform);
• Theoretic and design limitations of current wavefront systems; and
• Theoretical limitations applicable to corneal and other refractive surgical procedures, including LASIK

1. Refractive "Drift" over time.
   "Will my prescription change after laser treatment?" is one of the commonest questions I or any laser vision specialist (and his or her staff) is asked. It's a good question, as it goes to the heart of the issues of stability and permanence of the laser corrective effect.
   
   Common sense and your own experience will confirm that your eyeglass and/or contact lens prescription changes by small amounts every few years. The year-to-year changes may be very small, relative to the existing prescription; I call this the slow rate of "drift" of the refraction over time. Eye care professionals have only recently begun to understand the factors that contribute to drift. We used to say that the eyes 'grow slowly' through life, but that's not quite true (if it were, then senior citizens would have bigger eyes than younger people, which is silly!). What does grow through life is the lens inside the eye. This contributes in mid-life to presbyopia and in later life to cataract development.
   
   The lens grows in a fashion similar to a tree trunk: Layers of protein are laid down on the outer surface of the lens like the rings in the tree trunk. This causes the lens to grow in size, and to change in curvature and optical density ("index of refraction" is the more precise term). Changes in lens size, curvature and/or index of refraction would each cause changes in the refraction of the eye. So lens growth through life is likely what contributes most to refractive drift.
   
   Since laser correction involves corneal contouring, and does not affect the lens at all, laser surgery does not change the rate of drift of the refraction. The good news is that if your prescription was reasonably stable before any corrective care, it will likely remain reasonably stable after such care. If there's bad news it is that nobody can guarantee absolute refractive stability of any corneal surgery for many years. In my opinion that should not be a major issue, because (a) any drift that occurs will be around the "zero" point, and (b) enhancement or 'touch-up' care can be performed quite easily even many years after the original treatment.
2. Optical "Smear" produced by certain techniques and/or laser systems.
   One method of manufacturing optical lenses involves lathe-cutting the material. Here, two things are of critical importance: (1) Rotating the material (glass or plastic) or the cutting tool about a central axis, and (2) Securely holding and stabilizing the material and cutting tool relative to each other during cutting. If either is allowed to wobble or move even slightly during lathing, an optically imperfect surface will result. Such a surface will generate a distorted image. In performing laser vision sculpting, the same rules apply. Problems arise, however, in maintaining alignment of the laser beam to the target tissue. 'First generation' laser systems all relied upon the ability of the patient to fixate upon a blinking-light target. This sounds easy, but in practice can be rather challenging, given that (a) patients are unfamiliar with the experience; (b) there are other distractions during the treatment; (c) there are other lights present besides the fixation light; and (d) The fixation target can appear to fade or even disappear during the treatment, due to photoreceptor fatigue.
   
   It is obvious that laser treatment should be as perfectly centered as technically and humanly possible. If the eye receiving treatment is allowed to wobble, or move even slightly, that will smear the treatment over a larger surface than intended. This 'optical smear' is one of the major reasons why some patients complain about glare, reduced night vision, and other visual problems after LASIK.
   
   To reduce optical smear, modern vision-correcting lasers have incorporated pupil-tracking systems into their later-generation machines. This allows the laser to follow and adjust for even microscopic eye movements, maintaining perfect centration of the laser treatment. The Allegretto laser calls this their "Perfect Pulse" technology; while Alcon has invented the term LADAR as a contraction of two words, "laser" and "radar" to describe their pupil-tracking system.

3. Design Limitations of Current Wavefront-Guided Laser Systems
   
   Wavefront technology sounds like a very good thing. Customized treatments capable of optimizing vision based upon individual variations of each eye seems almost too good to be true. In my opinion, it behooves us as surgeons to separate marketing hype from scientific reality. Wavefront technology may be 'the next wave' in treatment technology, but it may not happen as fast as we all might wish. One very significant problem relates to how all current wavefront imaging devices interpret and display their readings.
   
   Aberrations of any optical system can be described in mathematical terms first developed about 100 years ago by an astronomer named Zernike. In the early 1900's, lathes were first widely used to mass-produce lenses (see 'lathe-cutting' described above). One of the features of a lathe-cut lens is that it has virtually perfect radial symmetry (which the human cornea typically does not have). Zernike coefficients are the mathematical representations that describe 'lower-order' and 'higher-order' aberrations of an optical system such as a lathe-cut lens or combination of lenses. However, wavefront imaging alone is not sufficient to identify and treat certain obvious corneal imperfections of the human cornea including keratoconus, decentered prior laser treatment, or asymmetric astigmatism.
   
   Wavefront analysis measures the unique optical characteristics including subtle optical aberrations of the whole eye. This information can then be used to guide customized laser treatment to optimize visual performance for each eye. Think of a tailor or dress-maker custom-tailoring a garment to the particular measurements of each client, compared to buying something 'off the rack' and you will appreciate the difference between wavefront-guided treatment and methods that preceded this capability.
   
   Wavefront guided treatment does cost somewhat more than conventional treatment, much as custom-tailored garments typically cost more than items off the rack. You might ask, "If clothes off the rack fit well, why pay extra for custom tailoring?" While it is true that not everyone needs customized, wavefront-guided treatment, it is also true that quality of vision (overall sharpness, better night vision, reduced or absent glare, halo, better contrast sensitivity) is incrementally better in people that receive wavefront-guided treatment as compared to people with similar prescriptions that receive conventional (non-wavefront) treatment. Also, we tend to be very demanding, particular and finicky about having sharp vision; and perhaps the criteria here are tighter or more exacting than for a garment such as a shirt or a dress. At LA Sight, we recognize this and employ wavefront mapping to guide our recommendations.

   Most of us involved in vision correction, including manufacturers and the surgeons who use their lasers, would like nothing more than to offer the best technology to our patients; and have everyone achieve great visual outcomes. We must be careful, however, not to over-sell present or future capabilities. We need to be candid about advantages and limitations of the technology we use. If imaging systems based on Zernike mathematics are potentially imperfect, we need to acknowledge that and find a better way to solve the problem. Optical engineers and laser companies know this already, and are working hard to develop better imaging devices and better mathematical analysis tools, but the best and latest current systems still fall short of being able to render optically perfect treatments in every single case.

   The Visx wavefront imaging system (referred to as "WaveScan") measures wavefront information and treats without pupil dilation. This instrument uses data only from the central 4.5mm diameter of the cornea, so cannot correct aberrations outside this zone. The Alcon LADAR wavefront analyzer (called LADARWave) is intended to capture wavefront data after dilation of the pupil with drops, so measures typically with a pupil size of ~8.0mm. Therefore, it captures much more data than the Visx WaveScan, and can apply the treatment to a larger treatment diameter. This is one of the main reasons why Dr. Wallace chose to transition from using the Visx laser (which we had in our office until late '03) to using Alcon's LADARVision laser thereafter.
   
   In all vision-correcting laser systems, wavefront-guided treatment sometimes requires sculpting to a greater depth than standard treatment. Treating to this extra depth is not always safe or advisable if the corneal thickness is not adequate. Dr. Wallace will review the particulars of treatment depth (also called "ablation depth") with you if appropriate at the time of your consultation.

   Another important limitation inherent in every corneal laser system currently available has become known as the "cosine correction problem."  Ophthalmic lasers are all calibrated by measuring energy absorption on a flat plastic test surface. When excimer laser energy strikes the (perpendicular) test plastic, 100% of the energy is absorbed. The real cornea is curved, however, not flat. In treating a real cornea, the energy delivered everywhere but the very top of the corneal dome will strike on a down-sloping angle, away from the perpendicular. The energy absorbed is proportional to the angle of the downslope, hence the "cosine" reference. Most laser systems are not able to measure, anticipate and correct for this appropriately, therefore some treatment error can be introduced, and this error cannot be corrected even by fancy "wavefront" systems. One of the main reasons for bringing the Allegretto laser into our practice is that this laser is the only one engineered to anticipate and correct for the cosine-offset effect. In eliminating this (small but non-zero) additional source of treatment error, the Allegretto system is able to achieve better treatment accuracy.

4. Theoretical Limits of Human Vision and Corneal Refractive Surgery
   Some ads and infomercials would have you believe that everyone having laser vision correction care obtains a “fairy tale” result, with perfect vision, no side effects, and no changes over even a long period of time. At present, a large percentage of patients do achieve extremely good results (and the percentage is higher in practices of more careful surgeons with better lasers, better techniques, etc.). Proper wavefront imaging combined with wavefront-guided laser treatment holds the promise of significantly improving upon the good results available with current technology.
   
   Listed below are some of the theoretical and practical “hard limits” to vision correction by any refractive technique, be it wavefront or any other method.
   
   
   1. Corneal Biomechanical Bulge. The cornea is composed of collagen and therefore is a type of “soft tissue”. Even when laser sculpting is performed well within safe limits, the tissue may bulge or stretch (microscopically) as a result of its’ inherent biomechanical and elastic properties. It is not possible to anticipate in every case how much this will contribute to uncertainty, error or imperfection in surgical outcome.
   2. Corneal Epithelial Changes. The epithelium (the cells lining the outer surface of the cornea) can change as a result of laser treatment, even if the treatment is performed under a flap (as in LASIK). Normally, the epithelial cell layer is about 80 microns thick (but like every biological parameter, there is individual variability). The layer of epithelium can become thinner (down to ~50 microns) or thicker (up to ~110 microns). The epithelial layer can change slightly differently in the center of the cornea than the periphery, which creates a lens effect. Therefore, even if one were able to perfectly measure an eye’s wavefront pattern and aberration profile, and perfectly treat this with laser therapy, and if there were no biomechanical bulge effect, there would still be uncertainty or “noise” introduced in the outcome by epithelial changes.
   3. Flap Considerations. When a corneal flap is created (by whatever method – blade or laser) and is then repositioned, the wavefront aberration profile of the whole eye can be subtly altered. This turns out to be true whether or not any excimer laser treatment is rendered. The reasons relate to sub-microscopic settling of the corneal collagen fibers, and sub-microscopic biomechanical effects including local compression and/or stretch of collagen fibers. Even though laser accuracy may be as precise as 0.25 microns (the amount of tissue removed per laser pulse), flap changes may be significant enough to create relative depressions and elevations on the cornea of up to +/- 1 micron. While the optical consequences of this are extremely minor, it is of concern that the variability introduced by flap creation is itself is slightly greater than the theoretic accuracy of laser treatment.
   4. Positional Considerations. Precision of aberration measurement, and of any laser treatment ("wavefront-guided" or not) assumes that one can measure with perfect certainty the position of the eye and the position of all optically significant sources of aberration perceived by the eye. This assumption is flawed, for two reasons. First, even the best pupil-tracking systems do not have positional accuracy greater than about 20 microns, but optically significant corneal anatomy contributing to aberration may be as small as 5 to 10 microns. Second, while aberrometer instruments can measure aberration profiles of the whole eye , they are unable to discern where (whether on the cornea or within the lens, for instance) the location of any particular aberration resides. If an aberration is lenticular in origin (arising from irregularities in the eye's crystalline lens), it is not necessarily reasonable to conclude that any corneal sculpting treatment, even with the best of systems, could possibly correct this.
   5. Individual Variability in Response to Treatment. In any population, any intervention, for any purpose, by any method will be characterized by some variation in individual response. This is true for every medical therapy including laser treatment.
   6. Retinal and Visual Processing Considerations including photoreceptor density, image processing, and psychophysical factors. Eye care professionals know how to focus images on the retina. We are beginning to understand how to remove subtle aberrations from the image-forming process. But there are inherent limits to how sharp a perceived image can be, based in part upon photoreceptor density in the retina. In addition, numerous factors influence the perception of images sharply focused on the retina. These include the hard-wiring of retinal ganglion cells, the pre-processing of signals and signal-enhancement ‘firmware’ resident in the visual system, and the multiple higher cortical and psychophysical factors that affect “perception” of what we see. In a nutshell, just because light lands sharply on the retina does not mean we will “see” something.
   7. Random Effects. The above discussion presumes that we know today absolutely everything relevant to laser vision correction care and corneal refractive surgery. That is a rather presumptive and perhaps arrogant assumption. We know a lot, to be sure, and can achieve incredible results in many cases based upon this knowledge. But we must strive to identify and understand what we do not yet know. If certain things happen in surgery that we do not understand, we may choose to characterize them as “random effects”. Whether they represent truly random events or systematic consequences of care that have not yet been identified is, for now, a moot point. This is part of the reason that medicine is today still considered by many an “art” as much or more than a “science.”