Excimer laser correction of refraction anomalies

Under the influence of radiation from an excimer laser, a lens of a given optical force is formed from the corneal matter.

S. Trokel et al. (1983) proved the possibility of dosed evaporation of the cornea with micron accuracy using an excimer laser.

Priority in the conduct of excimer laser operations for the correction of refractive errors in Russia belongs to the ophthalmological school of Academician Svyatoslav Fedorov (1984), and abroad - T. Seiler (Germany, 1985) and L'Esperance (USA, 1987).

Laser radiation with a wavelength of 193 nm breaks the interatomic and intermolecular bonds in the surface layers of the cornea to within a tenth of a micron. Clinically, this phenomenon manifests itself in the layered evaporation of the cornea - photoablation.

Operations performed on individual programs, created on the basis of complex mathematical calculations. The construction and implementation of a program to change the refraction of the cornea is carried out using a computer. The operation does not adversely affect other structures of the eye - the lens, vitreous, retina.

Each excimer laser equipment includes an excimer laser (an ultraviolet source) forming an optical system whose purpose is to transform the structure of the laser beam and deliver it to the surface of the cornea; operating computer, operating microscope, surgeon's chair and operating table for the patient.

Depending on the type of the forming system that determines the possibilities and features of the technology of evaporation of the cornea, all installations are divided into homogeneous (diaphragm and mask), scanning, semi-scanning and spatial. Thus, when using the principle of laser diaphragm radiation emits a wide beam on the diaphragm or a system of diaphragms gradually opening or closing with each new impulse. In this case, a thicker layer of tissue evaporates in the center of the cornea than at its edges, as a result of which it becomes less convex and the refraction decreases. In other installations, radiation hits the cornea through a special mask of uneven thickness. Through a thinner layer in the center, evaporation occurs faster than at the periphery.

In scanning systems, the surface of the cornea is treated with a laser beam of small diameter - the "flying spot" technology, with the beam moving along such a path that a lens of a given optical force is formed on the surface of the cornea.

The "Profile" system, developed by SN Fedorov, belongs to the lasers of the spatial type. The basic idea of the spatial distribution of laser energy in the "Profile-500" system is that the radiation hits the cornea with a wide beam with a Gaussian (i.e., parabolic) laser energy distribution profile. Consequently, for the same unit of time, in places at which the energy of higher density acts, the tissues evaporate to a greater depth, and in places where the energy density is lower, the tissue is reduced.

The main refractive excimer laser operations are photorefractive keratectomy (PRK) and laser intrastromal keratomileusis ("Lasik").

Indications for the implementation of refractive excimer laser operations are primarily intolerance of contact and spectacle correction, myopia, hypermetropia and astigmatism of varying severity, as well as professional and social needs of patients no younger than 18 years.

Contraindications to the conduct of photorefractive keratectomy are glaucoma, retinal conditions preceding detachment, or detachment, chronic uveitis, eye tumors, keratoconus, decreased corneal sensitivity, dry eye syndrome, diabetic retinopathy, pupil ectopy, marked allergic status, autoimmune pathology and collagenoses, severe physical and mental illness. In the presence of cataract, photorefractive keratectomy is not advisable, as immediately after cataract extraction, the refraction of the eye can be corrected with the help of an artificial lens.

Photorefractive keratectomy is performed on an outpatient basis under local anesthesia. The technique of performing the operation at foreign facilities includes two steps: removal of the epithelium and evaporation of the stroma of the cornea. At the first stage, the epithelium is scraped in the central zone of the cornea mechanically, chemically or laser. The duration of this stage of the operation depends on the type of laser and can vary from 20 seconds to several minutes, after which the corneal stroma is evaporation.

Within 1 day may be noted pain syndrome, lacrimation, photophobia. From the 1st day after the operation, the patient is prescribed instillation of the antibiotic solution before the complete epithelialization of the cornea (48-72 hours). Then, a course of therapy with corticosteroids is carried out according to the scheme lasting 1-2 months. In order to prevent steroid hypertension, beta-blockers are used at the same time 1-2 times a day.

The described technology makes it possible to effectively and safely correct myopia to 6.0 diopters and astigmatism up to 2.5-3.0 diopters. The technology of performing photorefractive keratectomy with a transepithelial approach (without preliminary scarification of the epithelium) at the domestic "Profile-500" allows simultaneous correction of myopia up to 16.0 diopters in combination with complex myopic astigmatism up to 5.0 dpt without any additional interventions.

Patients with hypermetropia and hypermetropic astigmatism of photorefractive keratectomy are less likely to be treated, which is explained by the need for de-epithelialization of the large corneal zone and, consequently, its long-term healing (up to 7-10 days). With hypermetropia greater than 4.0 D, usually perform the operation "Lasik".

The change in refraction depends on the thickness of the evaporated cornea. The residual thickness of the cornea in the thinning zone should not be less than 250-300 μm to prevent postoperative deformation of the cornea. Consequently, the limit of the possibilities of the method is determined by the initial thickness of the cornea.

Early postoperative complications of photorefractive keratectomy include long-term (more than 7 days) non-healing erosion of the cornea; postoperative keratitis (dystrophic, infectious); marked epitheliopathy, accompanied by edema and recurrent erosions; gross subepithelial opacities within the entire zone of evaporation of the cornea.

Complications of the late postoperative period include subepithelial opacities of the cornea; hypercorrection; myopization; incorrect astigmatism; dry eye syndrome.

The formation of subepithelial opacities is usually associated with a large volume of evaporation of the cornea at high degrees of corrected refractive anomalies. As a rule, due to the resorption therapy it is possible to achieve complete disappearance or significant regression of turbidity. In cases of development of persistent irreversible opacities of the cornea, repeated photorefractive keratectomy can be performed.

Operation "Lasik" is a combination of surgical and laser treatment. It consists of three stages: the formation of a microkeratome of the superficial corneal flap (valve) on the foot; evaporation by the laser of deep layers of the cornea under the flap; the valve is put back in its place.

Slightly expressed pain ("mote" in the eye) are noted, as a rule, in the first 3-4 hours after the operation. Lacrimation usually stops after 1.5-2 hours. Drug therapy reduces to instillations of antibiotics and steroids within 14 days after the intervention.

In cases of correction of myopia by performing the operation "Lasik", the maximum refractive effect is determined by the anatomical features of the patient's cornea. So, considering that the thickness of the valve is usually 150-160 microns, and the residual thickness of the cornea in the center after laser ablation should not be less than 250-270 microns, the maximum possible correction of myopia in the operation "Lasik" does not exceed 15 on average, 0-17.0 diopters.

"Lasik" is considered an operation with fairly predictable results for myopia of mild and moderate degree. In more than 80% of cases, the postoperative refractive result is within 0.5 D from the planned one. Visual acuity 1.0 is observed in an average of 50% of patients with myopia to 6.0 D, and a visual acuity of 0.5 and higher in 90%. Stabilization of the refractive result, as a rule, occurs 3 months after the operation "Lasik". At high degrees of myopia (more than 10.0 D), in 10% of cases, there is a need for repeated operations with the aim of correcting the residual myopia, which are usually performed in terms of 3 to 6 months. When the operation is repeated, the corneal valve is lifted without re-cutting with a microkeratome.

When correcting hypermetropia, the refractive result within 0.5 D from the planned one can be obtained only in 60% of patients. Visual acuity 1.0 can be achieved only in 35-37% of patients, visual acuity of 0.5 and above is noted in 80%. The achieved effect in 75% of patients remains unchanged. The frequency of complications in the operation "Lasik" ranges from 1 to 5%, and the most common complications occur during the formation of the valve of the cornea.

It is obvious that technical progress in the near future will lead to the emergence and wide clinical use in medicine, in particular ophthalmology, of new generation lasers that will allow refractive operations to be carried out without contact and without opening the eyeball. Laser energy, focused at one point, can destroy intermolecular bonds and vaporize the corneal tissue at a given depth. Thus, the use of femtosecond systems already at present makes it possible to correct the shape of the cornea without disturbing its surface. Excimer laser refractive surgery is one of the most dynamically developing high-tech directions in ophthalmology.

[1], [2], [3]