This article is from March 2005 and may contain outdated material.
A decade of success in refractive surgery for myopia has produced an inevitable outcome: exceedingly high expectations. Indeed, 20/20 is no longer good enough, noted Ronald R. Krueger, MD.
Night vision symptoms—glare, halos, loss of BCVA and double vision—can turn a patient from “20/ happy to 20/ unhappy,” added Scott M. MacRae, MD, professor of ophthalmology at the University of Rochester in New York.
Before wavefront-driven custom ablation, outcomes of plano and 20/20 were the industry standard, and “there was nothing we could do or measure if our patients weren’t happy with that,” said Dr. Krueger, medical director of refractive surgery at the Cleveland Clinic’s Cole Eye Institute. “However, we now can identify any higher-order aberrations that a LASIK procedure may have induced. Now the question becomes whether we can treat these aberrations.”
The Culprit: Conventional LASIK
Traditional LASIK may be most responsible for induced aberrations. According to Dr. Krueger, “spherical aberration can increase by more than 100 percent —and coma and other aberrations by more than 50 percent—after conventional techniques.”
With conventional myopic refractive surgery, just the act of making the cornea flatter in the center creates spherical aberrations, he said. Additionally, with traditional techniques, inadequate tracking and registration can lead to subclinical decentration and ablation nonuniformity, inducing coma.
In a study by Mrochen et al., researchers looked at the clinical and theoretical effects of subclinical decentrations on the optical performance of the eye after PRK. They found that PRK-induced aberrations were significantly greater than preoperative aberrations, with subclinical decentration (< 1 millimeter) a major factor in increased coma- and spherical-like aberrations after laser surgery. To minimize these higher-order optical errors, the authors recommend eye-tracking systems that consider the visual axis.¹
Additionally, said Dr. Krueger, laser vision correction induces spherical aberration due to small optical zones, peripheral pulse reflectivity, nonoptimized aspheric profiles and biomechanical thickening in the periphery.
The result? Diminished visual quality and contrast sensitivity.
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Upping the Ante. Conventional LASIK can lead to a host of problems, including subclinical decentration and ablation nonuniformity.
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Can Aberrations Be Treated?
Given today’s technology, it would appear that induced aberrations could be treated. Yet “it is important to note that none of the custom platforms are approved for therapeutic use. Currently, treatment of induced aberrations is an off-label, evolving art that has not undergone rigorous trials,” stressed John A. Vukich, MD, assistant clinical professor of ophthalmology at the University of Wisconsin, Madison, and surgical director of the Davis Duehr Dean Center for Refractive Surgery.
Several factors must be taken into consideration when deciding whether an induced aberration can be safely treated, said Dr. Krueger. The decision depends on the custom ablation platform, patient selection and technique. Since this is an off-label technique, the surgeon must decide if it is safe to proceed with a “custom upgrade.”
Capturing the data. Dr. Krueger said, “The first step is successful diagnosis of the aberration.” However, Dr. Vukich pointed out that capturing the wavefront measurement can be a challenge, especially in highly aberrated eyes that experienced a flap complication, trauma or other problem.
“The Hartmann-Shack centroid data, used on most custom platforms, have a relatively narrow depth of focus,” Dr. Vukich said. “Often it is not possible to capture an image in eyes with keratoconus, button holes or flap amputations. So we are limited in that regard.”
Interpreting the data. “Hartmann-Shack data are nothing more than a series of centroids—a single beam of collimated light—that reflects back to a collecting device to calculate the wavefront,” Dr. Vukich said. “The method by which the centroid data are interpreted influences the fidelity of the derived wavefront shape.”
Zernicke polynomials increase their resolution through a layering effect: the higher the number of orders, the higher the level of analysis, the greater the resolution. But “there are functional limits,” Dr. Vukich warned. “In theory, we can apply an infinite number of polynomials to achieve the desired resolution. In practice, however, analysis of the raw data beyond 10 orders leads to the introduction of more noise than information, and we actually lose fidelity.”
Other considerations. Other criteria that Dr. Krueger takes into consideration include good consistency/reproducibility of multiple wavefront maps, a reasonable correlation with refraction and topography, adequate corneal thickness and absence of ectasia. Also, he advised against treating excessive higher-order aberrations, for example, those that are individually > 2 micrometers.
Need for Patience
Obtaining accurate pre- and postoperative measurements calls for patience. Dr. Krueger measures every patient preoperatively and one week and three months postoperatively. Occasionally, he will wait an additional three months to achieve a stable refraction before deciding to correct the aberration. “The waiting time may lead to some patient unhappiness, but treating any sooner, before they are stable, is not wise. Sometimes, it takes a little longer for custom retreatments to stabilize.”
Problems to Avoid
Dr. Vukich cautioned that most of the higher-order aberrations require careful registration of both centration and axial alignment. “Coma is a good example. Not only is it positionally sensitive to centration but radially sensitive even as little as five degrees of rotation. The ability to register and track this is very important, and are features that we are now perfecting on our platforms.”
Misaligned registration. Centration, repeatability and registration present critical elements of accurate custom ablation. Dr. Krueger noted that an inadequate method of aligning measured aberrations with their matched location on the cornea can actually result in the induction of new aberrations, especially in highly aberrated eyes.
Overcorrection. This can result from excessive tissue drying following tissue removal to correct the induced aberrations after conventional LASIK, Dr. Krueger said.
Dr. MacRae, who also cautioned against overcorrection, recommended “pulling back” on the spherical correction in eyes with large amounts of cylinder and spherical aberration. “To illustrate, if the patient is a diopter myopic on the re-treatment, correct only 0.5 D of myopia in an eye with 0.5 mm of positive spherical aberration [6 mm pupil]. The same holds true in terms of third-order aberrations such as coma or trefoil. You need to pull back on the sphere you are correcting. When we treat large amounts of astigmatism, coma, trefoil or spherical aberrations, we end up putting more pulses in the peripheral portion of the optical zone and this causes central flattening and hyperopia.”
Deep ablations. These can cause changes in the structure of the cornea and lead to instability and ectasia.
New aberrations. Finally, there is always the possibility of not only failing to treat the previous induced aberrations but actually inducing new aberrations.
Overcoming Limitations
Dr. Krueger has specific suggestions for overcoming current off-label limitations:
- Understand your custom platform’s limitations, as none is FDA-approved for this procedure. You should thoroughly understand the machine’s limitation of accurate and reproducible capture, matching alignment and potential for overcorrective treatment.
- Avoid treating high spherical aberrations, which could lead to overcorrection.
- Avoid deep ablation depth for low refractive errors.
“In addition, in an attempt to leave room for nomogram adjustment, a lesser value than the full wavefront profile can be treated when using a positive offset on highly aberrated eyes,” Dr. Krueger said. “And additional copious irrigation can help avoid excessive tissue drying during high-induced aberration treatments.”
Prevention Is Primary
Dr. Krueger observed that the best treatment of induced aberrations is prevention with a primary customized ablation. Dr. Vukich’s personal data appear to back up this assertion.
“With the traditional platform, approximately 85 percent of our patients achieved 20/20 with complete satisfaction,” Dr. Vukich said. “This has increased to 93 percent to 96 percent with custom ablation.”
He added, “A secondary benefit is that our enhancement rate has dropped. In our practice, 7 percent to 12 percent of our patients underwent enhancements after standard treatment. With custom treatment, this rate has decreased to 3 percent.”
Short of using custom ablation for every procedure, Dr. Krueger concluded that there is a real need to obtain FDA approval of treatment algorithms for previously treated eyes and those with higher-order induced aberrations. “The first company that secures this approval with commercial success will be the dominant platform for future laser vision correction purchases,” he said. “This is a huge challenge, but one that is not insurmountable.”
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1 J Cataract Refract Surg 2001;27:362–369.
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Dr. Krueger is a consultant for Alcon; Dr. MacRae is a consultant for Bausch & Lomb and Dr. Vukich is a consultant for Staar and Visx.