EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/78708
June 2012 EWInternational 53 fractive effect is determined only by the number of pulses. This brings to the forefront the possibility of per- forming the operation in stages. A feature of trans-PRK is that the stan- dard diameter of the working zone is 6-8 mm (and more, when neces- sary). A transitional zone of opera- tion field to the optical visual zone is smooth, which excludes halos and glare. With post-op keratotopograms (Figures 2 and 3), one can vividly see the smoothness of a transitional zone and how the conditions for maintenance and increase of a func- tional optical zone are created. We would like to specifically note the absence of "central islands." One more consequence of the use of the wide ray is that there is no need for an eye tracking system or any eye- fixing mechanisms. The number of impulses is counted in the hun- dreds, and the influence of casual minor eye movements (like sac- cades) for the time of the operation is averaged up to 0. Gaussian radial distribution of energy density Corneal stromal ablation provides a refractive effect with the thickness of a removed layer that is proportional to the logarithm of laser ray energy density.5 Thus, every laser impulse that has a Gaussian distribution of energy density removes the "ideal lens" from the surface—ideal from the optical standpoint because its surfaces are paraboloids of rotation. The initial surface of the cornea has such a form (if there is astigmatism stretched over a weak axis). This means that the action from trans- PRK should not cause considerable distortions in the eye, such as high- order aberrations.6 A consequence of this is "natural, normal" vision, even at night and at dusk. A choice of the parameter of width of Gauss- ian distribution enables the ability to correct both very weak and super high myopia in a single step. Choosing energy density It is possible to reach monofocal and multifocal reprofiling on the cornea.1,2 In the event of a definite choice of energy density in the cen- ter of the laser ray, the obtained op- tical profile is non-homogeneous in optical force. This phenomenon is vividly demonstrated by a keratoto- pogram (Figure 4). A bright distinctive feature of the obtained optical profile of the cornea is the availability of physio- logical asphericity (i.e., monotonous growth of myopia toward the pe- riphery of the optical zone). Such growth of myopia reduces a jump of optical force on the border of the ablation zone and ensures a smooth transitional zone. In addition, the patients with a similar optical profile should experience so-called continued on page 54