EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/91447
November 2012 Russian researchers practice two- stage correction of high myopia by A.I. Myagkikh, Ph.D., E.V. Makurin, E.A. Subbotin, M.A. Samoylova T he main factors that influ- ence the kind of refractive surgery you choose are the initial corneal thickness and probability of compli- cations. The cardinal problem of PRK is late post-op haze occurrence. Early on, researchers described significant positive differences of cornea rehabilitation after trans-PRK was performed.1 Up to this time, no reasons or explanations were given about the differences. Let's try to examine this situation from the viewpoint of the physical differences of excimer laser influence on the cornea. Excimer laser radiation (193 nm) creates two general effects: the ablation (cold evaporation) of the cornea layer and energy absorption by the cornea material that was not ablated. Beyond doubt, the second effect is negative. It was associated with raising the cornea temperature during the operation and other problems.2 As the ris- cornea ablation if the value of energy density is below this "threshold." Geometry and edge effects Looking at the laser beam's cross sec- tion from the viewpoint of geome- try, one can say that the main active ablation zone is located in the cen- ter. The areas of potential energy absorbing are the edges of the beam when energy density changes from rating value to zero. As a result of non-ideal focusing and diaphrag- ming, this change takes a non-zero space, named "d" (Figure 1). This distance must be minimized— the ray must have a distinguished boundary. Let us suppose the value of "d" and the low of energy density change are the same for all lasers. The laser with scanning beam However, direct measure- ments of cornea temperature during the operation indicated the absence of any serious problems.3 ing of the cornea temperature is the final stage of any energy dissipation and absorption processes, we can make a supposition about another manifestation of "pre-ablation" laser energy absorption by the cornea. Suggested estimation of the neg- ative radiation impact on the cornea is based only on the fact of ablation threshold existing—there is no has covered and treated the opera- tion zone by scanning elements, which can have different forms and proportions. Let us imagine the case of ablation of a flat, thin (one-pulse) layer with its square S by excimer laser beam having energy density W and the square of scanning element –s. Unsophisticated calculations re- sulted in fact: the square of negative zone (energy absorption zone) is proportional to √(S/s). There are three ablation modes of laser vision correction: wide beam (full aperture), half-scanning (scan- ning elements are spots and sleets), and flying spot. Let us propose the operation zone takes about 30 sq. continued on page 34 Su periorResult s Superior Results Visit Us at the AAO Booth 865 Epith lia l Xlin king , PRK Xlinking, PRK Epitheliale removal in Corneal & Ad ance SurffaceAblation.. Remove the ov Epithe iu m in 5 to 7 seconds. Promotes Consistent Results! Consiste nt Results! + Minimize total procedure tim + Essential in re-treatments + Unifo orm epithelium removal + No need for subsequent scraping e time Epitheliuml seconds & Advancedd Surface Ablation Figure 1. Profile of radial energy density distribution of the laser beam. On the right, initial forms, on the left, diaphragming on threshold of effective ablation. The red line is the wide Gaussian ray. The green line is the sharp super-Gaussian (n=2) ray. Zone of edge effect is marked "d." The scales are provisional T. 800.461.1200 61.1200 | www.inno vativexcimer .com