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At Issue: Wavefront technology and surface procedures
At Issue asked a panel of experts the following question: “What role do you think wavefront technology will play in surface ablation procedures such as PRK and LASEK?”
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LASIK favored for now
Noel Alpins, MD: Any future role played by wavefront technology in surface ablation is likely to emulate the same principles that are applied in LASIK surgery. This is likely to be directed to the advantages achieved in reduction of spherical aberrations. The treatment of astigmatism and the reduction of consequent higher-order aberrations such as third-order coma and trefoil will be limited without closer alignment of the maximum ablation to the principal corneal meridia than is currently performed with wavefront technology.
Epithelial healing factors are a major influence on the very small treatments applied for the correction of higher-order aberrations. This is relevant to photorefractive keratectomy and laser epithelial keratomileusis, as well as the greater changes induced by LASIK flaps. The biomechanical changes that occur with these less invasive procedures have the advantage of less corneal disruption and less “background noise” to enable wavefront-guided treatment to be appreciated. There is some additional advantage gained by the reduced spherical aberration existing in the absence of a lamellar flap.
Other impediments to an optimal outcome such as excess remaining corneal astigmatism, both regular and irregular, may prevail to reduce any benefit derived from these aberration corrections attempted by wavefront-guided treatments. Because of its popularity and general acceptance, LASIK may remain the favored technique to PRK and LASEK for the application of wavefront technology in the foreseeable future.
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Bright future for wavefront
Jean-Louis Douenne, MD: Since wavefront technology was introduced as an instrument with the potential to boost human eye performance, many conceptions have changed. We now speak of approaching more physiological vision quality.
There is still much to be done to determine whether the method is better applied to intrastromal or surface laser refractive procedures, and whether the variations of optical aberration induced when creating a flap can be anticipated.
Until haze problems can be resolved, we shall continue to prefer PRK and LASEK for small refractive errors, for extreme corneal profiles or thinner corneas and in suspected cases of basal membrane dystrophy.
In such cases, the machine we use, the WaveLight Allegretto, allows “wavefront-adjusted” treatments, treating only symmetrical errors, spherocylindrical ametropia and the average spherical aberration. Such treatments aim to restore a prolate profile, to correct the tendency to induce an oblate shape with standard spherocylindrical central correction of ametropia. The purpose of obtaining better results in first procedures is already achieved: less night glare and improvement in visual acuity. In such cases, wavefront-adjusted ablation appears to give impressive good results in terms of visual quality and patients’ satisfaction.
In the near future, coupled to a Tscherning-type aberrometer, the WaveLight will permit us to address numerous cases that today have practically no way to gain better vision quality, such as asymmetrical refractive errors and some higher-order aberrations. We plan to use it in second procedures, decentrations mainly, in irregular astigmatism, and also early keratoconus. It promises also to avoid decentration in hyperopic treatments and to provide more precise correction of astigmatism. Another field will be the enhancement of visual performance in patients having undergone lens extraction for whatever reason, or phakic lens implantation to correct aberrations induced by the IOL while adjusting any remaining ametropia.
As a refinement of our current procedures, the widespread use of wavefront technology appears to promise a bright future. It will allow us to more precisely provide our patients a safe way to discard spectacles with fewer side effects.
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Wavefront will gain importance
Theo Seiler, MD, PhD: The main advantages of wavefront-guided ablation compared to standard profiles — or even worse, multizone treatments — are to reduce existing optical aberrations and to avoid inducing optical aberrations with the operation. Both problems can be addressed with intrastromal ablation (LASIK) as well as surface ablation (PRK or LASEK). However, the immediate result will be modulated by the healing response of the cornea.
In our experience, this healing response was considered to be stronger after surface ablation than after intrastromal ablation and, therefore, wavefront-guided ablation was expected to be less effective in PRK or LASEK than in LASIK. We were surprised when Marguerite McDonald, MD, reported her results after wavefront-guided PRK to be better or at least not worse than after wavefront-guided LASIK. However, we meanwhile learned that the lamellar keratotomy during LASIK itself creates optical aberrations dependent upon the position of the hinge and flap thickness.
Based on these results, we believe that wavefront-guided surface ablation will gain importance, especially in cases of low to mild myopia. Wavefront-guided surface ablation for myopia of more than 6 D seems not to be appropriate for two reasons: theoretical optics predict an inevitable increase in spherical aberration in corrections of more than 6 D, and the incidence of significant haze is correlated with the amount of the correction.
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LASIK to treat wavefront errors
Kimiya Shimizu, MD, PhD: We have treated low and high myopia with wavefront-guided LASIK using the Technolas 217Z system (Bausch & Lomb). Aizawa et al showed excellent results in 2002. We have also treated irregular astigmatism by wavefront LASIK with good results, as shown by Komatsu et al this year.
It has been said that wavefront errors may be too subtle to be treated within the corneal stromal layer as in LASIK. As we know, the post-LASIK ablated effect is stable due to the low degree of stromal reactions such as inflammation; the ablated stromal shape can be preserved as it has been treated. According to our data, wavefront-guided LASIK can sufficiently reflect the attempted wavefront error correction.
In surface ablation procedures such as PRK and LASEK, wavefront error may be reflected more adequately than LASIK because we treat just beneath the epithelial layer in these procedures. In PRK, this might be true just after the ablation, with no epithelial layer in the ablated area; the stromal surface can reflect the wavefront error adequately. But after epithelial recovery, due to the activation of epithelial cells, the stromal keratocytes just beneath the epithelial layer are also activated, resulting in inflammation, haze, tissue proliferation and regression. These were historically some of the main reasons for the conversion from PRK to LASIK.
It is not known whether the epithelial layer of a wavefront-ablated stroma can adequately reflect the ablated shape on the epithelial surface. Even in LASEK, haze is also observed after treatment, suggesting the activation of stromal tissue, which can cause the deformation of the wavefront-ablated shape. It is difficult to completely suppress the stromal activation in surface ablation procedures.
Although there are merits and disadvantages in the procedures, now it seems better to use LASIK to treat wavefront errors. Further estimation of the results of each procedure is needed.
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Aberration-free ablation coming
Ray Jui-Fang Tsai, MD: The ideal goal of super-vision is approaching with the introduction of increasingly sophisticated wavefront analyzers and excimer laser delivery systems into modern refractive surgery.
Perfect vision and higher visual quality, superior to that obtained with treatment of the lower-order aberrations of sphere and cylinder with traditional refractive surgery, may be obtained once higher-order aberrations have been eliminated. This may be accomplished via accurate detection of the eye’s overall aberrations and creation of a customized aberration-free ablation profile using wavefront technology.
If this can be done, it will be impossible to ignore the hinge-related astigmatism, flap-induced higher-order aberrations and the minor changes of biomechanical properties seen in LASIK, all of which may make the ideal of “aberration-free” less possible. Regarding the significant microkeratome-related aberrations, surface ablation procedures such as PRK and LASEK may theoretically be preferable at present to provide precise, predictable results. Wavefront-guided PRK was reported by Nagy in 2002 to be a safe and efficacious technique for the correction of low to moderate myopia. In 2001, Panagopoulou reported less increase of aberrations following wavefront-guided PRK as compared with LASIK (1.3 times increase in RMS value vs. 1.6 times).
Although there are still no reports in the literature, LASEK may theoretically provide an alternative to LASIK and PRK for moderate myopia, through its elimination of microkeratome-related complications and potential for less postoperative haze. In the future, customized surface ablation using a combination of even more precise and sophisticated topography and wavefront analysis, may let the dream of “aberration-free” become reality.
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Reference:
- Alpins NA. Wavefront technology: A new advance that fails to answer old questions on corneal vs. refractive astigmatism correction. J Refract Surg. 2002;18:737-739.