After several discussions with many thoughtful keratorefractive surgeons, I would like to tackle some very important and difficult questions concerning the concepts of customized corneal ablation, wavefront-guided laser ablation, and the treatment of corneal irregular astigmatism. Certainly, many of my colleagues will disagree with my point of view, but a healthy debate that is based on clinical reality and free from marketing considerations should be useful.
VISUAL FUNCTION FACTS, FOR MEMacular Visual Function
Peripheral to a 6.5-mm diameter optical zone, the refractive surface of the cornea does not effectively contribute to macular visual function. In other words, the corneal topography outside a diameter of 6.5 mm does not clinically affect the results of keratorefractive surgery. Moreover, the central 3 to 4 mm of the cornea is a much more important determinant of good visual function than the midperipheral cornea. Any optical irregularity on the corneal surface, however, that is within the 6.5-mm optical zone can cause an optical aberration that may be perceived by the patient.
The Role of Faceting
The epithelium smoothes the corneal surface by faceting (a facet is an area of epithelium that thickens in order to fill in an irregularity of the surface underlying the epithelium). The epithelium can “facet in” after keratorefractive surgery up to a depth of approximately 10 µm. Without this faceting and smoothing, no laser keratorefractive surgery would be possible because the laser leaves the corneal surface optically irregular. For example, when an excimer laser creates refractive power on a glass slide, the mires of a lensometer that measure this area are optically of very poor quality, even when the irregularity of the surface is in the range of 0.25 µm. The same phenomenon occurs in the stroma after ablation and requires the smoothing effect of the epithelium to transform the corneal surface into one that is optically pure.
After myopic LASIK, the central area of Bowman's membrane is distorted (which the epithelium follows), and, again, the epithelium must facet in and smooth the surface. After myopic LASIK, the surface area of the bed is smaller than the surface area of the flap. Replacing the flap onto the bed creates central folds in Bowman's membrane that produce corneal irregular astigmatism, which the faceting of the epithelium masks.
Automated Topography
It is widely known that the accuracy of automated topography is +0.50 D at best and often +1.00 D in the midperiphery of the cornea. After a myopic LASIK procedure that has corrected 10.00 D of refractive error, the keratometry and topography of the cornea will demonstrate only approximately 6.00 D of change. This is a large degree of artifact. Automated topography is excellent for showing general trends in corneal contour, but it is not particularly helpful for determining a patient's corneal power after keratorefractive surgery or the presence of subtle corneal irregular astigmatism.
Ablation Centration
It is my clinical impression that the average decentration of a laser ablation from the intended target is 0.35 mm (350 µm). Marguerite McDonald, MD, of New Orleans reported that 25% of patients treated with the Autonomous excimer laser had a decentration of greater than 550 µm.1 Some noted keratorefractive surgeons consider an ablation to be decentered only when there is more than a 1-mm (1,000-µm) disparity from the intended target.
The corneal ablation pattern should be centered on the middle of the pupil in order to minimize asymmetrical optical aberrations. Accurate ablation centration is of far greater importance than customized-ablation software. Put another way, customized-ablation software is potentially of little value without excellent ablation centration.
Mydriasis
Excimer lasers that rely on a mydriatic pupil (completely unphysiologic) for ablation centration cannot be as accurate as those that rely on a constricted pupil. If the goal is to create an ablation that is centered about the physiologically normal pupil, then it must be less accurate to place a reference point of an unphysiologic pupil into the equation.
Peripheral Blending
The concept of peripheral customized corneal ablation involves reducing the spherical aberration that is created where the flattened, central ablation meets the peripheral cornea. Flattening this shoulder decreases the diameter of the optical zone (Figure 1). If the central ablation zone is larger than 6.5 mm, then more corneal tissue must be removed in order to correct the ametropia. As a result, the procedure is limited to correcting the midranges of myopia and cannot be used in the high ranges of refractive error. Flattening the corneal “knee” at the 7- to 9-mm optical zone during myopic corneal ablations, therefore, does not significantly alter the outcome because any ablation beyond the 6.5-mm optical zone provides virtually no contribution to the foveolar image.
There is no evidence of which I am aware that a corneal defocus of -3.00 D is any better than a defocus of -6.00 D outside the effective optical zone. The blinking process will not allow a corneal edge to remain abrupt but will instead, in time, smooth the peripheral cornea to a new curve. This change in the periphery can often be seen on topography as a slightly reduced optical zone size, when a 6-mm ablation for either PRK or LASIK measured at 1, 3, 12, and 24 months on average will show a decreasing dimension.
Peripheral blending is in no way detrimental as long as the optical zone remains 6 mm in size and centration is optimal. Making a 10-mm diameter LASIK flap in order to accommodate a 9-mm myopic ablation increases the likelihood of a poor flap and, in my opinion, will not improve the surgical result.
WAVEFRONT-GUIDED ABLATIONAberrations
Only one corneal power (corneal curvature) can create emmetropia without aberration. In the central 6 mm of the cornea, this three-dimensional curve is a dome. Having a 3- or 4-mm pupil diameter as the optical stop of the eye negates the advantage of an aspheric optical surface of any diameter designed to correct for spherical and chromatic aberration.
Minor aberrations in the midperiphery of the cornea do not affect visual acuity or function. Attempting to correct these aberrations would create a multicurved cornea, which is often called irregular astigmatism. In other words, the cornea does not function well with two different curves that meet within the visual axis, a situation that creates a curved line and causes glare. With a bifocal cornea (two different radii of curvature within the visual axis), a patient may obtain near vision but only at the expense of reduced-quality distance visual function.
Additionally, the instruments measuring the cornea are more inaccurate in the midperiphery than at the center. A half-diopter in refractive change may require a change in curvature of a few microns, something that the epithelium can cover and smooth out postoperatively such that this change is clinically ineffective. Can an ablation that causes such a minimal amount of change be effective under a 140-µm LASIK flap as well?
Centration
How can a wavefront-guided treatment effectively address a local corneal aberration to a depth of 5 to 20 µm if the ablation is routinely 350 µm decentered? This form of treatment still relies on the epithelium to cover up the optical inadequacies.
TREATMENT OF CORNEAL IRREGULAR ASTIGMATISM
For the past 10 years, I have treated corneal irregular astigmatism caused by many etiologies (keratoconus, corneal pellucid marginal degeneration, LASIK, RK, mild ectasia, etc.) with broad-beam ablation (Figure 2). If a patient has a preoperative UCVA of 20/40+ or better, then, generally, his results will be rewarding. The first step of the treatment is performing a myopic PRK to a depth of at least 25 µm. If this step creates hyperopia, I perform a hyperopic PRK 2 to 3 months later (Figure 3).
Clinically, a myopic PRK smoothes the central cornea. This phenomenon is not in doubt because it is clinically observable. My theory on the reason for this smoothing is that the areas of the cornea that are affected more perpendicularly by the laser beam undergo a greater degree of ablation where more heat is generated than the sloping areas, because the absorption of the laser energy is more effective. The “valleys” of the irregular cornea accumulate moisture, which shields the valley floor from effective ablation. It is interesting that the location of the corneal haze following myopic PRK is central, whereas, following hyperopic PRK, it is midperipheral where the degree of ablation and heat generation are the greatest.
Corneal irregular astigmatism covers the entire visual axis. It cannot be corrected by the “pinpoint customization” of every corneal hill and valley due to the decentration issue mentioned earlier, because the treatment profile will be “out-of-phase” with the corneal surface to be treated. For that reason, a broad-beam ablation that treats the entire central cornea and reduces the hill/valley depth to less than 5 µm offers surgeons a much greater advantage when treating corneal irregular astigmatism than does precise treatment centration, which is not possible. The ability to be effective in the presence of decentration is beneficial.
The VISX Custom Contoured Ablation Pattern (Custom-CAP; VISX, Inc., Santa Clara, CA) for repairing grossly decentered optical zones functions well because it attacks a large target (eg, the unablated corneal tissue opposite the direction of the decentration). A decentration of 500 µm does not greatly affect the efficacy of removing larger amounts of tissue in some areas of the cornea than in others or of producing a relatively smooth corneal contour that the epithelium can make even.
CONCLUSIONS
I am in favor of trying to improve keratorefractive surgery and have spent my entire ophthalmic career attempting to do so. Nevertheless, our profession reacts quickly to theory, hype, hope, and the dream of marketing new technology, whereas it responds much more slowly to clinical reality. We need some independent, well-controlled studies that demonstrate the efficacy or fallacies of customized corneal and wavefront-guided ablations. Of course, the results will become apparent without formal studies within 6 to 12 months as patients accumulate more postoperative time and surgeons have an opportunity to compare various lasers and computer programs. A report such as “three out of six patients were improved from 20/16 to 20/12” is not sufficient for me. I truly wonder whether there have been any significant advances in corneal ablation design compared with the original, well-centered, 6-mm broad-beam ablation for the treatment of myopia.
There are limitations regarding the degree to which a myopic LASIK treatment can be customized. Surgeons are constrained by the thinness of the patient's cornea and by the induction of irregular astigmatism that is inherent to the “collapse of the central cornea” and requires epithelial masking. How can customized ablation be accurate on a collapsing surface that is not predictable? These factors make customization a true challenge.
Treating hyperopia at the corneal level remains more difficult than treating myopia. Centration is more critical, and the steeper corneal surface within the visual axis creates a greater amount of aberration than does the central, flatter surface of a myopic ablation.
After performing thousands of myopic keratomileusis cases, Professor José I. Barraquer of Barcelona, Spain, found the 6-mm optical zone to be the best compromise between quality of vision and preserved corneal strength. PRK performed with the earliest models of excimer lasers used 4.5- and 5.0-mm optical zones and yielded very poor results. Most IOL optics are 6 mm in diameter. There is a message here.
I am keeping an open mind and awaiting more meaningful clinical results, not just engineering diagrams. I have examined eight eyes treated outside the US with customized ablation. They all possessed a significant degree of corneal irregular astigmatism, and their visual acuity and function were clinically no better than preoperatively. Currently, ocular surgeons are afraid not to own new technology, but each additional piece of equipment should possess proven merit. If technology were so important, then all the surgeons who use the same devices and systems should have the same results. We know that is not the case. A thoughtful, skilled surgeon is far more important than marginally improved technology in attaining excellent surgical results.
Lee T. Nordan, MD, is the director of Nordan Eye Laser Medical Group in Carlsbad, California. He holds no financial interest in the products and companies mentioned herein. Dr. Nordan may be reached at (760) 930-9696; laserltn@aol.com.1. Coorpender SJ, Klyce SD, McDonald MB, et al. Corneal topography of small-beam tracking excimer laser photorefractive keratectomy. J Cataract Refract Surg. 1999;25:674-684.
