Epi-LASIK favored over tPRK for myopia treatment
Better predictability, less induced corneal spherical aberration occur in findings
By Cheryl Guttman Reviewed by Osamu Hieda, MD
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 The results showed the type of surgery was a significant predictor for refractive outcome with overcorrection more likely in the tPRK eyes. With both techniques, there was a significant correlation between attempted spherical equivalent reduction and induced fourth-order corneal aberration, such that higher corrections were associated with greater increases in spherical aberration. However, the multiple regression analysis showed type of surgery was also a significant predictor of induced corneal spherical aberration with tPRK resulting in an approximate 0.2 im greater increase at all levels of correction compared with epi-LASIK.
"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. Eor 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 pm), A 9-mm epithelial flap was raised in the epi-LASIK procedures. The tPRK procedure used a 45-im deep, 8- to 8.5-mm epithelial laser scrape. All surgeries were performed using the same treatment nomogram (93% X 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 sixthorder Zernike polynomials. To illustrate the differential effect of the two surgeries. Dr. Hieda presented wavefront maps from both eyes of a single patient who underwent tPRK in the left eye and epi-LASIK contralaterally for identical preoperative refractions. At 6 months after the surgery, ocular higher-order aberrations were 0.31 pm in the epi-LASIK eye and 1 im in the tPRK eye (Figure 1). "The difference could be explained in part by the fact that epi-LASIK induced less corneal spherical aberration," Dr. Hieda said
By Cheryl Guttman Reviewed by Osamu Hieda, MD
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 The results showed the type of surgery was a significant predictor for refractive outcome with overcorrection more likely in the tPRK eyes. With both techniques, there was a significant correlation between attempted spherical equivalent reduction and induced fourth-order corneal aberration, such that higher corrections were associated with greater increases in spherical aberration. However, the multiple regression analysis showed type of surgery was also a significant predictor of induced corneal spherical aberration with tPRK resulting in an approximate 0.2 im greater increase at all levels of correction compared with epi-LASIK.
"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. Eor 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 pm), A 9-mm epithelial flap was raised in the epi-LASIK procedures. The tPRK procedure used a 45-im deep, 8- to 8.5-mm epithelial laser scrape. All surgeries were performed using the same treatment nomogram (93% X 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 sixthorder Zernike polynomials. To illustrate the differential effect of the two surgeries. Dr. Hieda presented wavefront maps from both eyes of a single patient who underwent tPRK in the left eye and epi-LASIK contralaterally for identical preoperative refractions. At 6 months after the surgery, ocular higher-order aberrations were 0.31 pm in the epi-LASIK eye and 1 im in the tPRK eye (Figure 1). "The difference could be explained in part by the fact that epi-LASIK induced less corneal spherical aberration," Dr. Hieda said
Ophthalmology Times, 9/15/2006
posted by Helen at 10:02 PM
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