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Eximer lasers in refractive eye surgery (LASIC & FRK).

Urgency of a subject.

For good vision the sharp image of a an examined subject on a retina is necessary, first of all. This map turns out as a result of passing rays through optical system of an eye, disturbance of which any constituent leads to to obtaining of the indistinct map. As of today there is a plenty of methods of liquidation of such disturbances, including surgical (usage of the most thin diamond knife for exercise of cuts on a cornea). But in the big number of cases the surgical intervention gives by-effects (damage of nearby tissues, small accuracy of made cuts, etc.). Creation and perfecting of the lasers radiating in a ultra-violet part of a spectrum, and opening of process of a photo ablation have framed premises for new forms of laser surgery of an eye.

Since 1982, ability of short-wave Eximer lasers to creation of very exact (submicron) sectional views in various polymeric materials, and then and possibility of level-by-level excision of a biological tissue with minimum effect on enclosing substance has repeatedly been shown.

Physical bases of operation of Eximer lasers.

Eximer lasers are a group of lasers in which typical active medium is the admixture of inert and halogen gases. The term the Eximer - an abbreviation of an English word-combination exited dimmers, that means a stable, existing only in the excited electron status of dimmers of these gases. At transition of Eximer molecules in a main status high energetic photons of UF-light are let out. At various combinations of inert and halogen gases short nanosecond impulses of light on various lengths of waves of UF-area of a spectrum can radiate the EL: fluorine - 157 nanometers, argon - fluorine - 193 nanometers, krypton - chlorine - 222 nanometers, krypton - fluorine - 248 nanometers, xenon - chlorine - 308 nanometers, xenon - fluorine - 351 nanometers. Duration of impulse - 10-16 nanoseconds. Depth of effect on a living tissue - up to 60 microns.

The lasers based on the given principle, have been created in 70th years, are sources of UF-radiation and are used in many branches of science.

Since 1982, ability of the most short-wave eximer lasers to creation of very exact (submicronic) sectional views in various polymeric materials, and then and possibility of level-by-level excision of a biological tissue with minimum thermal effect on enclosing and stayed substance has repeatedly been shown. For an explanation of given phenomenon R. Srininasan has offered the theory of the so-called gear of a photo ablation. It is supposed, that photons of UFFF-light it is enough energetically (for example, in a case of 193 nanometers - 6,4 eV) for direct break of intermolecular chemical links, and the rest of the absorbed energy is spent for evaporation of constituents of molecules from a material. This feature can explain observable minimum damage of near laying irradiated tissues, especially at a wavelength less 220nm. Depth of absorption of radiation argon - fluorine of the laser (193 nanometers) is measured by microns and, thus, effecting energy is arranged in the extremely circumscribed size of a tissue. Besides because of high porosity of impulses of eximer lasers diffusion of heat from a field of incidence in enclosing tissues is minimum. Also it has repeatedly been shown, that the thermal effect amplifies with augmentation of a wavelength.

The first report on usage of eximer lasers on a wavelength of 193 nanometers for obtaining on a cornea of not punching sectional views has been made in 1982. In experiment in vitro exact dependence between quantity of energy and depth of the remote tissue fixed: for execution of a cut by depth 1 micron is required density of energy of 1 Joule/sm2. At histological probe in a light microscope it was not defined tags of thermal damage of nearby tissues to a sectional view, edges of laser sectional views were parallel on all an extent without disorganization of stromal slices or epithelial edge. After that reports have followed operations of various authors on learning effect the EL on various structures of an eye. Simultaneously in other branches of medicine (the vascular surgery, dermatology, neurosurgery, etc.) were spent similar operations on learning effect of laser UV-radiation on various biological structures.

Comparative probe of effect on a cornea and a lens of radiation by a wavelength of 193 and 248 nanometers has been lead. Threshold values of an ablation have been defined and fixed, that at usage of laser radiation with a wavelength of 248 nanometers the greater energy consumption is required, than at a wavelength of 193 nanometers, for obtaining similar results, both in a cornea of an eye, and in a lens. At a wavelength of 193 nanometers with the help of a submicroscopy the border zone of damage in width 0,1 - 0,3 microns is detected, further laying stromal structures have not been damaged. At usage a krypton - fluorine of the eximer laser (248nm) the zone of damage was much wider - up to 2,5 microns with disorganization and damage of near laying stromal structures. Absorption metrics of a stromal of a cornea and lens, and by one of the factors explaining a difference in changes, two nearby lengths of waves originating under effect UVED - areas of a spectrum have been measured, there can be a difference in an absorption coefficient of radiation a stromal of a cornea. Radiation with a wavelength of 193 nanometers was successfully used for creation of an inspected zone of an ablation in a lens, the effect of effect reminded those in a cornea. In the further probes by definition of optimal power doses for a choice of effect on a cornea and a lens have been lead. At a wavelength of 193 nanometers the value of an ablation is insignificantly enlarged at oscillations of density of energy since 220 mJoule /sm2 and remains at the achieved level at the further rising density up to 600 - 800 mJoule /sm2. At effect of radiation with a wavelength of 248 nanometers scaling up of quantity of the remote corneal tissue was marked at density of 620 mJoule/sm2 and is higher. At matching tissue specimens it was marked, that in a case of usage of the eximer laser with a wavelength of 248 nanometers not only a zone of damage are wider, but also character of damage (there is a disorganization and damage of near laying stromal structures, changes of collagenic fibers of a stromal) sharply differs.

Chart 1. Graph of amount of the remote substance of a cornea against density of a radiation energy (a wavelength - 248 nanometers)
Chart 1. Graph of amount of the remote substance of a cornea against density of a radiation energy (a wavelength - 248 nanometers)
Chart 2. Graph of amount of deleted substance of a cornea against density of a radiation energy (a wavelength - 193 nanometers)
Chart 2. Graph of amount of deleted substance of a cornea against density of a radiation energy (a wavelength - 193 nanometers)

From mentioned below schedules follows, that at exercise of an ablation by the eximer laser with a wavelength of 248 nanometers there is a greater thermal effect, than the laser with a wavelength of 193 nanometers. As absorption of a ray with a wavelength of 193 nanometers the best also the ablation will be observed more exact.

All contributors, studying effect of radiation of eximer lasers on a cornea, assume the further usage of this method with reference to refractive surgeons. By means of radiation the EL (193 and 248 nanometers) it has been lead a keratectomy on corneas of rabbits and a cornea of the monkey. It is marked, that results of an adhesion, as well as optical results at usage of a wavelength of 193 nanometers, meet the requirements of refractive surgery. H. Kerr-Muir and co-authors have compared results of the keratectomy which has been lead with the help the EL with a wavelength 193 and usual trepan. At a scanning microscopy on walls and bottom surgical a box defined prominences a size more than 10 microns. Laser to a box sharply differed on quality: walls and a bottom smooth, covered with a pseudo membrane.

A. Cotliar and co-authors rendered notches on the Enucleated cadaver eyes, using the EL with a wavelength of 193 nanometers. Rendered on 4 notches in turn, by turning movement of a laser source around of a shaft. The refractive effect on the average was 5 Deportees. E. Schroeder and co-authors described the commercial equipment(installation) created by them, permitting to finish laser radiation the EL to an operative microscope and by means of a special mask to render the radical not punched sectional views. Also clinical experiment on plotting notches for the volunteer to whom operation of an enucleation concerning an intraocular tumour was coming has been lead. 4 perpendicular notches with usage of radiation the EL with a wavelength of 193 nanometers have been put, the density of energy was 370 mJoules/sm2, time of continuation of impulse from 10 up to 16 nanoseconds, sectional views in width 75-80 microns. After procedure the cornea remained transparent, in 4 days there was a complete cuticularization. The eye has been Enucleated for 14-th day when sectional views were hardly appreciable at probe in a slit lamp. At histological probe the good adhesion, absence of tags of an inflammation and immune response have been marked. N. Scharlin and co-authors by means of radiation the EL with a wavelength of 193 and 248 nanometers have generated corneal donor lenses which can be used at operations kerato-, epikeratofagies and a keratomilez. Histological probes have shown, that the wavelength of 193 nanometers induces minimum damage of tissues of a lens (about 10 microns), and also, that is especially important, the survival rate of Keratocytes was defined. R. Ziencerce and co-authors used radiation argon - fluorine the EL for obtaining lenses from a donor material for an epikeratofagy. The received lens had diameter of 8 mm, with a thickening in the middle up to 0,2 - 0,24 mm and narrowed edges. Optical force of a lens +8,0 Deportees. At probe of a lens marked high quality of a surface of a lens, a normal structure of a stroma with alive Keratocytes. The lens remained with the recipient transparent. Later other, more perfect methods of creation of donor lenses have been offered also.

Laser correction of vision

Stages of development

Development of engineering of laser correction of vision was preceded with durable period of probes of methods of change of a refraction of a cornea. The first attempts of solution of this problem concern to 1949 at the expense of transplantation of a donor cornea on an apex of a cornea of the patient and its strengthening with the help of seams. The new age starts with 1963 in refractive surgery. Doctor J. Barraquer has designed the first microkeratomas (the instrument for stratifying a cornea). Appearance of such device opened new possibilities in surgery of a cornea - simulation of new refracting force. With the help a microkeratoma cutting from a cornea of a flap by width about 300 microns was spent. Then this flap was frozen and located in the special machine tool, where was ground before giving to him special forms dissipating (at correction of a myopia) or collecting (at correction of a hyperopia) lenses. Further he was defrosted, transferred back on a cornea and became stronger with the help of seams. These operations have received names: Mioptical (at correction of a myopia) and hypermetroptical (for correction of a hyperopia) a keratomilez.

In the further there was a perfecting a keratoma. Width of a cutted flap managed to be reduced up to 160 microns. It has allowed to spend operation on change of a refraction directly on an eye under a flap that has much reduced time of an adhesion of a cornea. This type of operations began to be named as automated lamelar keratoplasty (ALK)

Since 1982 with appearance of eximer lasers the refractive surgery follows the road of the perfection. With implantation of eximer lasers there was possible a correction not only myopias and hyperopias, but also an astigmatism with unique before a degree of accuracy. Correction of vision by the eximer laser is named as a photorefractive keratectomy (FRK). Accuracy and a simplicity of carrying out of operation have led to general addicting for it in all, however, already to the beginning 90 have started to be taped and its disadvantages. It is rather durable and morbid postoperative period, necessity of a durable dig of not indifferent drops for an eye, limitation on the value of corrigated anomaly of a refraction, etc. In 1991 the Greek ophthalmologist Ioannis Pallikaris has found path of elimination of these disadvantages at the expense of overlapping FRK with ALK therefore the new method of change of a refraction of an eye - a laser specialized keratomilez (LASIC) has turned out.

Eximer lasers with the built - in system of a topography and a computer.

LASIC

LASIC

The combined laser-surgical operation which has received the name from cutting-down of English "Laser in Situ Keratomileusis" (word-for-word: a laser keratomilez at-sight). The technology of it is developed for correction of a myopia, a hyperopia and an astigmatism.

How operation passes?

  1. Eyelids are opened with the help of a special blepharostat. The patient ask to look at a white bulb inside the instrument and spend centering eyes in front of the laser. Then on a cornea of an eye the special marking is rendered, permitting to define in the further a relation of the future surface flap and a cornea.
  2. The sticking ring on which the microkeratom will move ahead is approximately superimposed. Further the size of a cutted flap, and the following stage - cutting of a flap by a microkeratom is defined. It is the most important stage of operation on which in many respects its result depends. Eyelids ask to not worry, to not squeeze the patient and to not spin eyes. This stage borrows approximately 15 seconds.
  3. The surface petal turns away on edge of a cornea, centering and laser correction is made.
  4. After laser correction the cornea is cleared, the surface flap of a cornea comes back to a place, the repeated lavage under a flap is made. Further final smoothing a cornea is made, and during 3 - 5 minutes occur its final self-adapting.

Average time of operation for one eye - 12-15 minutes.

Possible complications.

FRK

FRK

Photorefractive keratectomy - a laser method of correction of the vision, based on evaporation of a surface layer of a cornea with the help high-intensive UVED - radiations of the eximer laser. Depending on a dose the form of a cornea varies.

How operation passes?

  1. Anesthetizing drops are instilled, the patient is stacked on an operating table. Eyelids extend with the help of a blepharostat. The eye is centered under the laser head.
  2. In the middle corneas the site where there will pass laser correction is marked. From this site the epithelium, more often with the help of the special surgical instrument is deleted. Sometimes it make, putting to a surface of a cornea the cotton impregnated with alcohol, on some models of lasers vaporize radiation of the laser. Diameter of the prepared site and more time a centration is checked.
  3. The eximer laser on beforehand calculated on the basis of entered by the doctor given a computer program vaporizes a part of a tissue of a cornea and models thus its new surface. The form of a scanning ray can be different - as a wide fascicle, a cleft or a point, - depending on model of the laser (scanning by a point is considered the most modern).
  4. After completion of an ablation the operational zone is cleared, anti-inflammatory drops and drops with an antibiotic are instilled. The eye becomes covered by a bandage.

Average time of operation - 10 minutes, but much more durable and disease process of an adhesion of a cornea as a result of damage of a boumened bellow.

Possible complications.