Ординатура / Офтальмология / Английские материалы / Jaypee Gold Standard mini Atlas Series Lasik_Aragawal, Jacob_2009

LWGLF LASERS/ 57

Topical anesthesia is applied for a few minutes before starting the procedure and some more drops are applied to the speculum. The Shahinian well (E Janach-Como, Italy) is applied and filled with alcohol solution (20% in distilled water) previously warmed to 32° (Celsius). After 20 seconds, the content is dried with a cotton sponge and rinsed with diclofenac sodium (preservative free). The surface is now rinsed with BSS (balanced salt solution) and the epi-keratome applied. One can move the instrument nasally to avoid creating the hinge in the photoablation area. If the diameter of the flap is checked with a caliper, and is determined to be too small, it is possible to enlarge it by scraping the periphery with a Hockey spatula. At this point the photoablation is performed, after which some drops of BSS is applied. It is important to remove excess liquid from under the flap, leaving only the amount of fluid necessary to allow the flap to slide over the surface. This reduces flap mobility. Flap mobility is often a cause of postoperative pain due to the stimulation of nerves from the flap itself during lid movements. The surface can now be dried for 30 seconds to increase stability. A soft lens is fitted and an applanator is used to squeeze all fluid from under the flap. The surface should now show a clear aspect.

LWGLF LASERS/ 59

Optical aberration customization can be corneal topography guided which measures the ocular aberrations detected by corneal topography and treats the irregularities as an integrated part of the laser treatment plan. The second method of optical aberration customization measures the wavefront errors of the entire eye and treats based on these measurements. Wavefront analysis can be done either using Howland’s aberroscope or a Hartmann-Shack wavefront sensor (Figure 2.3). These techniques measure all the eye’s aberrations including second-order (sphere and cylindrical), third-order (coma– like), fourth-order (spherical), and higher order wavefront aberrations. Based on this information an ideal ablation plan can be formulated which treats lower order as well as higher-order aberrations.

ZyoptixTM (Bausch and Lomb) is a system for Personalized Vision Solutions, which incorporates ZywaveTM Hartmann Shack aberrometer coupled with OrbscanTM IIz multi-dimensional device, which generates the individual ablation profiles to be used with the Z 100 Excimer Laser System. Thus, this system utilizes combination of wavefront analysis and corneal topography for optical aberration customization. ZywaveTM is based on Hartmann–Shack aberrometry in which a laser diode

LWGLF LASERS/ 61

and a Zyopitx treatment card is inserted. A standard LASIK procedure is then performed with a superiorly hinged flap. A HansatomeTM microkeratome is used to create a flap. Flap thickness varied from 160 to 200 µm. A residual stromal bed of 250 µm or more is left in all eyes. Optical zone varied from 6 to 7 μm depending upon the pupil size and ablation required. Eye tracker is kept on during laser ablation. Once ablation is completed the stromal bed and flap are cleaned and the flap replaced back.

LWGLF LASERS/ 65

cases of so called “amblyopia” may actually be aberropia and these patients have the potential to gain significantly in their visual acuity on correction of aberropia.

Astrophysicists have to be able to measure and correct the imperfect higher-order aberrations (HOA) or wavefront distortions that enter their telescopic lens system from the galaxy for perfect imaging. To achieve this purpose, adaptive optics are used wherein deformable mirrors reform the distorted wavefront to allow clear visualization of celestial objects. Extrapolating these same principles to the human eye raises the question of whether removal or alteration of the wavefront aberrations of the eye might result in a significant improvement in the preoperative best corrected visual acuity.

Prior to the advent of wavefront guided LASIK, the only parameters that could be modified to obtain optical correction for a given patients refractive error were the sphere and cylinder. This would often not give the ideal optical correction, many a times resulting in poor visual quality in an otherwise 20/20 post-refractive surgery patient and in some patients, even resulting in a decrease in best spectacle corrected visual acuity (BSCVA). This situation is usually because of either the persistence or induction of significant amounts of higher order aberrations after LASIK.