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Corneal asphericity vital in preventing postop aberrations
New Nidek program designs ablations with minimal variations in corneal eccentricity.
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ROME — Corneal asphericity is the most important aspect to be considered for the successful outcome of refractive surgery, according to Paolo Vinciguerra, MD.
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“So far, we have assumed that good visual results needed large optical zones and that large optical zones were only obtainable through the ablation of large amounts of tissue,” he said here at Rome 2001.
“At present, I am able to demonstrate that large optical zones can be obtained with an effective tissue-sparing strategy, by simply controlling the gradient of curvature of the cornea. We have developed a new Nidek program of customized ablation, which functions on these principles and can be successfully applied to all types and methods of refractive correction. Postoperative aberrometric maps demonstrate that optical aberrations are effectively corrected — and prevented — by this method.”
A system of analysis
Dr. Vinciguerra explained that the cornea has an uneven gradient of curvature, varying from the center to the periphery. The normal, prolate cornea has a more pronounced curvature in the center and flattens towards the periphery. The eccentricity index (e) expresses the measurement of corneal asphericity. The normal cornea has an asphericity index of e = +0.5, which is the ratio between the two perpendicular rays of curvature (figure 1).
He included the eccentricity index (e) in a program for the topographic analysis of postop corneas, which also includes astigmatism at 3 and 5 mm, mean pupillary power (P), spherical aberration (LSA), curvature irregularity (SQM) and asymmetry index (A and SAI).
“These indexes are strictly correlated to aberrometry. They are in fact a less sophisticated and less expensive way of gaining the same type of information,” he said.
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He showed the topographic analysis applied on his own eyes (figure 2), which are a good example of regular cornea with an asphericity of e = 0.37.
“This means that my cornea doesn’t increase its curvature so much from the periphery to the center. I have a positive spherical aberration (LSA) of 0.66 D, which compensates the physiologically negative LSA of the lens,” he said.
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The LSA index evaluates only the surface of the cornea, but the two aberrometric examinations revealed that results were comparable: both spherical aberration and coma were = 0, meaning that there was no asymmetry of curvature.
Asphericity index is crucial
“With this method, I was able to demonstrate that there was a strict correlation between the poor results of some cases of hyperopic correction and too-high asphericity values. This was equally true for PRK and LASIK,” Dr. Vinciguerra said.
The apical scarring that is so typical of hyperopic correction, and badly affects visual acuity, is visible on keratoscopic maps as a distortion of the normal pattern of curvature (figures 3 and 4), which is quite similar to those produced by keratoconus ectasia (figure 5).
“Looking at the topographic maps of such cases of hyperopic correction (figures 6 to 8), we could see that in all cases we had changed the gradient of curvature and took it too far, making the cornea unnaturally prolate,” he said.
“We could also observe that spherical aberration was directly proportional to the asphericity index and clearly visible in the change of color between the center and the periphery. Whenever the eccentricity was higher than e = 1, we could observe on the maps a small optical zone, high dispersion, high spherical aberration and all other indexes increased. A similar situation was shown by keratoconus maps (figure 9): the optical zone looked larger, but there was a high index of spherical aberration and asphericity.”
Reduce spherical aberrations
These considerations led Dr. Vinciguerra to develop a new program of hyperopic correction, which preserves the physiological curvature of the cornea. Maps of a patient treated with a Chiron laser (figure 10 top) showed a much lower asphericity index and a large optical zone. Even better results were obtained with a Nidek program; eccentricity was even lower, and spherical aberration was reduced by almost four times (figure 10 bottom).
“These data were referred to the surface, but aberrometric maps (figure 11) confirmed that we had eliminated spherical aberration. There was even a slight shift toward negative values,” he said.
As a further step, Dr. Vinciguerra verified that the same principles of topographic analysis could be applied to myopia. It could clearly be observed on the maps that when asphericity values increased, the characteristic red ring of myopic cornea got larger, and spherical aberration became higher. Conversely, aberrometric maps showed that low asphericity values corresponded to low spherical aberration.
“The new Nidek program applied to myopic correction has the potential to produce the results we have always wanted,” he said. A topographic map showed a myopic correction of –7 D performed with a minimal variation of the eccentricity.
“Spherical aberration is that of a perfectly normal cornea; curvature irregularity and asymmetry are better than physiological standards. You no longer need to measure the pupil diameter, because you no longer need to ablate on a large optical zone. By just working on the eccentricity, you can obtain a 10- to 11-mm refractive area, which has both surface and aberrometric regularity,” he said.
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Topographic maps after hyperopic treatment show that the gradient of curvature was changed too much, making the cornea unnaturally prolate. |
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A topographic map of keratoconus shows a similar pattern to the hyperopic treatment maps above. |
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Topography of patients treated with a Chiron (top) and a Nidek (bottom) laser. Spherical aberration is greatly reduced. |
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Aberrometric maps confirm the elimination of aberration. |
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For Your Information:
- Paolo Vinciguerra, MD, can be reached at the Istituto Clinico Humanitas, Milan, Italy; (39) 025-521-1388; fax: (39) 025-741-0355; e-mail: vincieye@tin.it. Dr. Vinciguerra has a direct financial interest in the product mentioned in this article. He is a paid consultant for Nidek.
- Rome 2001, the Rome Symposium on Cataract, Glaucoma and Refractive Surgery, was co-sponsored by Ocular Surgery News Europe/Asia-Pacific Edition, The Italian Association of Cataract and Refractive Surgery and the International Society of Refractive Surgery. For information on future meetings, contact Meeting Registration at (856) 848-1000.