| The wavefront aberration, is the sum of individual aberrations including defocus and astigmatism as well as many other aberrations referred to as higher order aberrations. The conventional refraction attempts to specify what is wrong with the eye using only three parameters (sphere, cylinder and axis), whereas the wave aberration specifies the errors in the eye with many more parameters. Advances in optics technology have moved the wavefront aberration from an obscure academic concept to one that is central to efforts to improve vision, especially for higher order aberration. Some reports have showed that any improvement in retina contrast achieved by reducing the eye's higher order aberration will improve contrast sensitivity. Higher order aberration has a significant effect on retinal image quality for the dilated pupil and can influence visual performance as well as the resolution of images of the retina. In addition, outcomes are currently analyzed in terms of sphere and cylinder, which is a measure of central corneal performance. Higher order aberration, on other hand, is produced by a larger region of the cornea. The aim of the refractive surgery is to make the eye optically perfect. Its use in the surgical correction of myopia and astigmatism has been widely reported. Little has been published on higher order aberration and effects following corneal refractive surgery. In this investigation, the aberrations of the human eye were measured preoperatively and compare with those after PRK and LASIK for myopia and myopic astigmatism. Possible correlations between aberrations and some factors were analyzed. Because of easier interpretation, we prefer to represent the results in terms of Zernike Coefficients, which are standard in optical engineering.Method:1,Subjects: Experimental group: This is prospective study. Refractive errors (from -5.0 to -6.0D pre-operatively) for 32 eyes were corrected with PRK (n=21) or with LASIK (n=18). All eyes were ablated with same parameter for optical zone (6mm diameter) and transition zone (7mm diameter); Normal control group: Group A: the preoperative group: 84 eyes with myopia and myopic astigmatism. Group B: Postoperative group: 67 eyes post-PRK and 55 eyes post-LASDC. 2, Instrument: Wave-front aberrations were tested using a ray tracing technique and subjective aberrometer (Su Zhou BriteEye Model WFA 1000) preoperatively and 3 months following PRK and LASIK. Three measurements were obtained for each condition, and root mean squared wavefront erros (RMS) values for overallwavefront aberrations and each order of the Zernike aberrations were analyzed byMadlab software.3, Statistics: F analysis and 2-tailed paired t test.Results:1, Experimental group: At 3 months, the mean RMS value for higher-order (third-order and higher) showed a statistically significant increase after both PRK and LASIK compared with the corresponding preoperative values, p≤0.05. and the fourth order aberrations (spherical -like aberration) were dominant by 2.64 and 2.31 folds respectively. The total wavefront errors for higher order aberration were not significantly different between post-PRK and post-LASIK. Zernike coefficients Z6,Z18,Z19, however, showed significantly different. ,Z18,Z19 was higher after PRK, whereas Z6 was higher after LASIK.2, Normal control group: In myopic eyes, the coma and sperical aberration, which corresponds to zernike mode C-13 and C+13, C04 are the first three largest aberration. The remainder coefficients, except for C33, was close to zero, showed similarity with normal eyes.The aberration for higher order did not correlate with the preoperative spherical equivalent refraction and astigmatism, only weak relation was found with age of the patients.3, Higher order aberration with comeal Haze: RMS of the higher order aberration with comeal Haze was a mean of 0.59 m, which was smaller than ones of the post-PRK (0.81 m). However, no statistically significant directional changes were found in our data. p>0.05. It i... |
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