NOV 2012

EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.

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Page 36 of 82

34 34 EW REFRACTIVE SURGERY Russian continued from page 33 mm, and typical squares of scanning elements are 2-4 sq. mm for the half-scanning machine and 0.4-1.0 sq. mm for the flying spot machine. That square of absorption zone for the half-scanning machine will be 3-4 times greater than for wide beam. The same parameter for the flying spot machine is 6-10 times. The total energy impact for layer ablation will be the same for all ablation modes. But the absorbed energy will be nearly a product of effective ablation threshold and square of absorption zone. With estimation of geometry parameter d=0.05 mm and diameter of flying spot –0.5 mm, we have the square of absorption zone for every pulse: π *0.05 * 0.5 = 0.0785 sq. mm. If the effective ablation threshold is about 100 mJ/sq. cm, then the value of en- ergy absorption may reach 78.5 µJ for every laser pulse. This energy seems microscopic, but the laser pulse width is about 40 ns, and it resulted in up to 2 kW of absorbed pulse power for every pulse of scan- ning laser. Therefore, we must look for the manifestation of effect of absorption laser energy and its potential biochemical consequences in the form of imprints of strong but very short energy impact on the mi- croscale of the corneal surface. For example, in the form of microscopic coagulation seats in the treated cornea surface layer the thickness is about laser wavelength, i.e., 0.2 µm. Using the wide beam stands apart from another ablation modes (trans-PRK method) because of rela- tive minimality of radiation harm and the fact that "negative absorp- tion zone" is located at the edge of the operation zone, or beyond the optical zone. So in the case of trans- PRK, the geometric factor of laser energy absorption should not exert influence on the regeneration processes in the optical zone. This conclusion finds confirmation in serious post-op increasing corneal thickness without regression of refraction.4 effect of trans-PRK with maximal physical perfection of the ablation process. The application of effect of corneal thickness growth The appearance of individual cornea thickness increasing effect is waiting for its own investigator. As far as we know, this effect occurs after PRK.4,5,6 It is very important that in most cases the post-op regeneration of corneal thickness does not entail the regress of refraction. From the view- point of physical optics, it is possible only in the case of uniform growth across the whole operation zone. There is no doubt about the real benefit of revealing effect. Laser Initial myopia Quantity of eyes Corneal thickness UDVA/CDVA (data span) Corneal thickness UDVA/CDVA (average) Corneal thickness 0.78 From 10-14 D 371 0.2-0.7 From 14-18 D 60 500 +/–40 µm 0.2-0.5 410 +/–32 µm 0.79 375 +/–30 µm Table 1. Parameter "corneal thickness" in this table is described in format: "arithmetical mean" plus/minus "standard deviation" for general data array correction of high and very high myopia becomes possible.7 It is at- tained by breakdown of a correction procedure into two stages, each of which is the safest complete action with a predicted refractive effect. At the first stage, 70-80% of initial my- opia was removed. Cornea tissue considerably restored its thickness 8- 12 months later, but with lower my- opia. Thus, the second stage of the operation in respect to its basic parameters becomes similar to the correction of weak (and moderate) myopia, which is a more simple procedure with a good, predictable result. Russia.8 This technique is patented in It may be directly used on the wide Gaussian beam excimer laser installations. As an example of efficacy of high myopia correction, we will share some results from our re- search. For review, 443 eyes were taken, whose initial myopia was over 10 D, and for whom the second stage was performed. The number of eyes with initial myopia from 10-14 D was 371, from 14-18 D was 60, and over 18 D there were 12 eyes. Correction was performed stage by stage: 1.Initially, not more than 80% of initial myopia was removed by trans-PRK, which was about 10 D and determined by the initial thickness of the cornea. We coupled this positive 2.Standard treatment with corticos- teroids within 2 months was per- formed. 3.As desired by the patient, eye- glasses or contact lens correction was selected upon expiration of 2 months. 4. In 8-12 months, the second stage of correction was performed simi- lar to the first one. Parameters of repeated action were selected with some deviations in comparison with the first stage in accordance with residual myopia, corneal thickness, and, of course, refrac- tive results after the first stage. In every specific case, in the course of PRK the designated number of impulses on the stroma was strictly maintained. 5.Standard treatment with corticos- teroids within 2 months was per- formed. The efficacy of correction was determined with the use of an effi- ciency coefficient, which is a ratio of uncorrected visual acuity after oper- ation to maximum corrected visual acuity before operation.9 The results of two-stage high myopia correction are tabulated in Table 1. Conclusions 1.The less a square of the scanning element of an excimer laser machine, the more a relative part of laser energy is absorbed by corneal tissue. At the same time, the absorption seats are distrib- uted evenly at all surfaces of the operation zone. It may be useful for the forecast of PRK-like operation results for the different installations. 2.In the case of the minimization of negative effects of energy absorp- tion within the operation zone (trans-PRK method), it becomes apparent that there is considerable effect of post-op increasing corneal thickness without refractive regress. 3.Corneal thickness growth is weakly dependent on corrected sphero-equivalent value and the patient's age. The array of corneal thickness growth has a consider- able positive trend versus time of examination and negative ten- dency, versus initial corneal thick- ness. 4.The time history of corneal thick- ness growth is realized by weakly increasing early in the post-op period, growth intensifying to 6 months post-op, going top and stabilization at 1 year, and more post-op trans-PRK. 5.The patented method of the two- stage correction of myopia with the use of technology trans-PRK on the installation "Profile 500" is safe and enables us to reach high functional results in initial myopia over 10 D. The trans-PRK applica- tion permits us to declare the practical absence of restrictions on initial stage of corrected myopia, including in cases of thin cornea. EW References 1. Alexander I. Myagkikh, Ph.D., Eugene V. Makurin, Eugene A. Subbotin. Characteristics of trans-PRK performed by the Profile 500 laser. EyeWorld, June 2012. eyeworld.org/ article-characteristics-of-trans-prk-per- formed-by-the-profile-500-laser. 2. Maldonado-Codina, Carole, Morgan, Philip B., Efron, Nathan. Thermal consequences of photorefractive keratectomy. Cornea (2001), 20(5), pg. 509-515. 3. Duryagina M.N., Chuprov A.D., Zamyrov A.A. et al. The temperature dynamics of the cornea during PRK laser ablation. An Actual Ophthal- mology Problem: IV Russian Young Researcher Conference. Collected Science Articles, 2009. www.eyepress.ru/article.aspx?6010. 4. Myagkikh A.I., Makurin E.V., Subbotin E.A., Myagkikh M.A. Organ-Preservation of Eye Cornea in Myopia Correction with Trans-PRK Method. Glaz, 2012, №3 (85), pg. 34-37. 5. James J. Salz, Perry S. Binder. Is There a "Magic Number" to Reduce the Risk of Ectasia after Laser In Situ Keratomileusis and Pho- torefractive Keratectomy? American Journal of Ophthalmology, August 2007, 144(2), pg. 284- 285. 6. Kasparova E.A. Diagnostics and treatment of precocious keratoconus. Glaz, 2001, №2, pg. 35-38. 7. Myagkikh A.I., Subbotin E.A., Makurin E.V. High and Extra-High Myopia Correction. Glaz, 2008, №4 (85), pg. 17-18. 8. RU Patent №2402306. Priority of inven- tion – 13.04.2009. 9. A.I. Myagkikh, Ph.D. A new way to deter- mine refractive operations efficacy. EyeWorld, September 2012. eyeworld.org/article-a-new- way-to-determine-refractive-operations-effi- cacy. Editors' note: The authors have no financial interests related to this article. Contact information ostoptik@mail.ru 0.4 After the II Stage More than 18 D 12 0.1-0.5 After the I Stage November 2012 Total = 443

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