A total of 49 consecutive patients, 20 men and 29 women, who underwent PRK for myopia or myopic astigmatism between September 2002 and June 2003 was included in this study. Patients were considered eligible for the study if they were at least 21 years old and free of ocular disease, underwent no previous ocular surgery, and had at least 2 years of refractive stability. Patients wearing contact lenses were asked to discontinue their use for at least 4 weeks before surgery. The study followed the tenets of the Declaration of Helsinki. Informed consent was obtained from all patients. Institutional review board approval was not required for this study. Patients were subdivided into three groups according to the preoperative spherical equivalent (SE) cycloplegic refraction and the amount of cylinder component: the low-myopia group (range, −1.25 to −4.00 D) and the high-myopia group (range, −4.10 to −9.00 D), in whom the cylinder component was lower than 1.75 D, and the astigmatism group, in whom the cylinder component ranged between −2.00 and −5.00 D.
Either of two experienced surgeons (ML and SS) performed the procedures. PRK was performed with an excimer laser platform (Technolas 217C; Bausch & Lomb, Dornach, Germany) with an ablation zone diameter of 6.00 mm (transition zone up to 9.00 mm in diameter) in all eyes. The smoothing technique was performed immediately after the procedure, using a viscous 0.25% sodium hyaluronate solution for masking the cornea.
12 With the laser in PTK mode, the ablation depth was set at 10 μm (divided into four intervals for a total of 428 spots), and the maximum diameter of the ablation zone was set at 9.00 mm. A spatula was used to spread the masking fluid onto the corneal surface. The astigmatism was corrected using the cross-cylinder technique to homogenize the treatment across the steepest and flattest corneal meridians. The technique consists of treating half the cylinder component with hyperopic ablation and the other half with SE refraction using myopic ablation. In all cases, a 6.00-mm ablation zone (with a transition zone up to 9.00 mm in diameter) was used.
All patients included in the study underwent complete ocular examination before surgery and 1, 3, and 6 years after surgery. Monocular uncorrected (UCVA) and best-corrected (BCVA) photopic visual acuities were assessed preoperatively and postoperatively. Corneal topography and pupillometry were performed with a topographer (Keratron Scout; Optikon 2000 SpA, Rome, Italy). For each eye, measurements were repeated three times to assess the repeatability of the topography; the best image was then chosen for analysis. The topographer software calculates corneal WA on the corneal elevation with respect to an ideal aspherical corneal shape with eccentricity 1/
n (where
n = 1.3375) and centered on the corneal vertex. The WA was then computed with respect to the line of sight (i.e., the center of the entrance pupil) using the move axis function of the topographer and was obtained from the derivatives using a least squares best-fit procedure to the desired pupil area of analysis and described as a seventh-order Zernike polynomial expansion.
11 Corneal aberration data output were exported and processed into custom software written in technical computing software (MatLab, version 7.0; The MathWorks, Inc., Natick, MA) for analysis. Preoperative and postoperative high-order corneal aberration data were computed over simulated pupils of 3.50-mm and 6.00-mm diameters. Root mean square (RMS) of the high-order corneal WA was computed from the Zernike coefficients, and the more recent recommended notation was used.
13,14 Parameters analyzed included the total RMS-HOA up to the seventh order, the RMS of coma (the square root of the sum of the squared coefficients of
Z 3 −1 and
Z 3 1), and the RMS of the spherical aberration (SA; the square root of the sum of the squared coefficients of
Z 4 0 and
Z 6 0). The MTF and the point spread function (PSF) of the anterior corneal optics were computed from the corneal WA at 555 nm wavelength of light, excluding first- and second-order terms. The mean preoperative and postoperative radial MTFs and the ratio between the preoperative and postoperative MTFs (i.e., the MTF ratio) were used to evaluate the relative optical effect induced on the corneal optics by the surface ablation procedures. Detailed information on the calculation of objective metrics of optical quality can be found in a previous review article.
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