Epi-LASIK favored over tPRK for myopia treatment

September 15, 2006

San Francisco-Treatment of low-to-high myopia with transepithelial PRK (tPRK) has poorer predictability and results in greater induction of corneal spherical aberration compared with epi-LASIK, reported Osamu Hieda, MD, at the annual meeting of the American Society of Cataract and Refractive Surgery.

San Francisco-Treatment of low-to-high myopia with transepithelial PRK (tPRK) has poorer predictability and results in greater induction of corneal spherical aberration compared with epi-LASIK, reported Osamu Hieda, MD, at the annual meeting of the American Society of Cataract and Refractive Surgery.

Dr. Hieda and his colleague Shigeru Kinoshita, MD, department of ophthalmology, Kyoto Prefectural University of Medicine, Kyoto, Japan, compared the two surface ablation procedures in two groups of 19 eyes each based on data collected at 6 months postoperatively and using multiple linear regression analysis. The tPRK was performed using the EC-5000 Flexscan (Nidek) epithelial laser scraping technique and epi-LASIK was performed using the Epi-Lift (Gebauer) and EC-5000 laser.

Predictor for outcome

"These findings were consistent with the hypotheses we established before undertaking this study," Dr. Hieda said. "In tPRK with the Flex-scan, the cornea is not ablated to an equal depth from the center to the periphery due to inconsistencies in epithelial thickness across the cornea. Flexscan also has the effect of myopic overcorrection. For these reasons, that technique leaves residual epithelial cells in the corneal periphery that lead to a decrease in optical zone diameter, overcorrection, and greater induction of corneal spherical aberration."

The eyes in the two study groups were matched for attempted correction. Mean preoperative sphere and cylinder values in the epi-LASIK eyes were –5.78 and –0.67 D, respectively, and were –6.35 and –0.74 D in the tPRK eyes, respectively.

There were also no significant differences between groups in gender distribution (about 60% female) or laterality. However, the epi-LASIK group was significantly older than the patients in the tPRK group (mean 33 versus 26 years) and had a greater mean corneal thickness (548 versus 517 μm).

A 9-mm epithelial flap was raised in the epi-LASIK procedures. The tPRK procedure used a 45-μm deep, 8- to 8.5-mm epithelial laser scrape. All surgeries were performed using the same treatment nomogram (93% × sphere) and same ablation profile (OATZ Mo. 6; 4.5-mm optical zone, 8-mm transition zone). Corneal wavefront analysis was performed using the OPD-Scan (Nidek) at a 6-mm zone for up to sixth-order Zernike polynomials.

"The difference could be explained in part by the fact that epi-LASIK induced less corneal spherical aberration," Dr. Hieda said.

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