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Wavefront-guided technology aids in new findings, techniques
Longer-term studies are aiding the evolution of new refractive techniques.
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Research of corneal amblyopia, astigmatism and spherical aberration has been aided with use of wavefront and laser technologies, according to several researchers.
‘Corblyopia’ is part of amblyopia
Corneal amblyopia, or corblyopia, exists as a subset of anisometropic amblyopia, according to Cyrus K. Mehta, MD.
Dr. Mehta, and colleagues found that children and adults who have amblyopia, as shown by corneal wavefront analysis, have significantly aberrated corneas. By treating corneas with corneal wavefront-linked treatment, both children and adults demonstrate an improvement in their uncorrected visual acuity and best corrected visual acuity.
“Ophthalmologists will perform corneal wavefront analysis to see if aberrations exist,” Dr. Mehta said in an e-mail interview with Ocular Surgery News. “Higher-order aberrations are known to significantly degrade vision, so possibly improving these can lead to a corresponding increase in vision.”
Dr. Mehta and colleagues tested 60 cases of unilateral anisometropic amblyopia using the Keratron Scout corneal topographer wavefront analyzer (Optikon). Patients ranged in age from 7 to 25 years. The surgeons performed LASIK using a 200-Hz flying spot laser with a pupil-based tracker incorporating a proprietary corneal wavefront-linking software (ORK-CAM, Schwind eye-tech-solutions).
In the first week, no patching, penalization or retinal stimulation was used.
At 1 week postop, the researchers found that nine eyes (15%) improved by one line of BCVA, 47 eyes (78%) improved by two lines of BCVA, and two eyes (3%) each improved by three and four lines of BCVA.
“In view of these astounding results, we must conclude that all cases of anisometropic amblyopia, pediatric or adult, should undergo corneal wavefront analysis and will benefit from corneal wavefront-linked LASIK,” Dr. Mehta and colleagues wrote in the study abstract.
Higher-order aberrations should be considered with wavefront algorithm
Putting preoperative higher-order aberration interactions into a wavefront-guided ablation algorithm can result in better refractive outcomes, according to Scott M. MacRae, MD. He said hyperopic overcorrection and myopic undercorrection in wavefront-guided ablation are most likely because of the interaction between positive and negative spherical aberration and defocus.
Dr. MacRae looked at the wavefront aberration and manifest refraction of 340 myopic eyes treated with Zyoptix customized LASIK (Bausch & Lomb). The mean spherical equivalent was +0.17 ± 0.51 D. Postop, 91.5% of patients attained UCVA of 20/20 or better and 75.9% were within ±0.5 D of target spherical equivalent.
He found that eyes with larger amounts of preoperative positive spherical aberration had smaller increases in positive spherical aberration and larger hyperopic overcorrection. He said the smaller induced spherical aberration was statistically significantly correlated with the degree of postoperative hyperopia because positive spherical aberration causes a myopic shift, at –0.23 D per 0.1 µm spherical aberration for a 6-mm pupil.
Conventional ablation algorithms can overcorrect myopia to compensate for the myopic shift. Customized ablation algorithms can reduce the positive spherical aberration, resulting in postoperative hyperopia.
Optimized prolate ablation has good visual results
A new ablation algorithm called optimized prolate ablation provides good visual results in patients who undergo LASIK for myopia with astigmatism, a study found.
Jack T. Holladay, MD, MSEE, FACS, conducted a contralateral, randomized, prospective study of nine patients who were treated with either optimized prolate ablations or conventional LASIK with the NAVEX EC-5000 platform (Nidek). The preop manifest refractive spherical equivalent was –5.50 ± 1.68 D, with a range of 8.09 D to –3 D, and astigmatism up to –1.50 D.
“What this combination allows you to do is to see what the aberrations in the whole eye are and where they are in the cornea or the crystalline lens. The bottom line is, you treat all the aberrations that you find in the cornea, where you are putting the treatment, but you don’t treat things above coma and above spherical aberration where they are in the crystalline lens because ultimately those are going to be changing,” Dr. Holladay told OSN.
Three months after surgery, Dr. Holladay found that the mean manifest refractive spherical equivalent was –0.02 ± 0.44 D, with a range of –0.5 D to 0.88 D, for the optimized prolate ablation group and –0.04 ± 0.20 D for the conventional group. In the optimized prolate ablation group, seven eyes (78%) had BCVA of 20/20. Likewise, in the conventional group, seven eyes had BCVA of 20/20.
Postoperatively, corneal higher-order aberration was 0.56 µm for the optimized prolate ablation group and 0.62 µm for the conventional group, according to Dr. Holladay. Corneal spherical aberration postoperatively was 0.29 µm for the optimized prolate ablation group and 0.50 µm for the conventional group. Average corneal asphericity postop was –0.78 in the optimized prolate ablation eyes and 0.23 in the conventionally treated eyes.
Eye registration and pupil shift should be measured
Compensation for eye registration and pupil shift is clinically important, a study found.
Frank J. Goes, MD, studied data from 100 eyes and found there was no clinically significant change of values for pupil centroid shift and cyclotorsion between the start and end of treatment.
“I have demonstrated that, once installed on the surgery bed, the torsion and offset don’t change during the whole procedure,” he said in an interview with OSN.
He said OcuLign registration with the MEL 80 laser (Carl Zeiss Meditec) grants exact positioning of customized treatment of the eye.
Pupil center shift and torsion were measured in 100 consecutive eyes. In another 20 LASIK-operated eyes, four measurements were taken during the procedure: before the cut, before lifting the flap, after lifting the flap and at the end of the treatment.
Pupil offset extreme values were –0.41 mm on the X axis and 0.3 mm on the Y axis.
According to Dr. Goes, offset was more than 0.25 mm in 28% of eyes on the X axis and more than 0.25 mm in 6% for the Y axis.
Torsion extreme values were –9.5° and 13.5°, surpassing 5° in 22% and 10° in 7% of eyes.
During treatment, maximal changes were 0 mm, 12 mm and 0.08 mm for the X axis and 0 mm, 13 mm and 0.05 mm for Y offset, exceeding 0.1 mm in only two out of 20 of the eyes.
Dr. Goes said that torsion changed maximally 2.9° between the start and end of treatment; the mean change was 1.6·, or more than 2° in only four out of 20 eyes.
For more information:
- Frank J. Goes, MD, can be reached at W.Klooslaan 6, B2050 Antwerp, Belgium; +32-321-939-25; fax: +32-321-966-67; e-mail: frank@goes.be.
- Jack T. Holladay, MD, MSEE, FACS, can be reached at Holladay LASIK Institute, Bellaire Triangle Building, 6802 Mapleridge, Suite 200, Bellaire, TX 77401; +1-713-668-7337; fax: +1-713-668-7336; e-mail: holladay@docholladay.com.
- Scott M. MacRae, MD, can be reached at the University of Rochester, Strong Vision, 100 Meridian Centre, Suite 125, Rochester, NY 14618; +1-585-341-7817; fax: +1-585-756-1975; e-mail: scott_macrae@urmc.rochester.edu. Dr. MacRae is a consultant for Bausch & Lomb.
- Cyres Mehta, MD, can be reached at Mehta International Eye Institute Sea side, 147 Colaba Road, Mumbai, India 400005; +91-9819850971; e-mail: cyresmehta@yahoo.com.
- Erin L. Boyle is an OSN Staff Writer who covers all aspects of ophthalmology.