Surgical correction of ametropia

By changing the optical power of the two main optical elements of the eye - the cornea and the lens, it is possible to form the clinical refraction of the eye and thus correct myopia, hyperopia, and astigmatism.

Surgical correction of refractive errors of the eye is called "refractive surgery".

Depending on the location of the surgical intervention area, a distinction is made between corneal, or corneal, and crystalline lens surgery.

The cornea is the most accessible biological lens in the optical system of the eye. When its refraction decreases or increases, the refraction of the eye as a whole changes significantly. In addition, the cornea is a convenient structure of the eye for performing surgical intervention. A healthy cornea has no vessels, quickly epithelializes, maintaining transparency. Refractive corneal surgery does not require opening the eyeball and allows for precise dosing of the refractive effect.

The first refractive operations on a transparent cornea were performed by the Colombian ophthalmologist H. Barraquer in 1949. In recent years, there has been a rapid increase in the number of operations performed: up to 1.5 million operations are performed worldwide every year.

The goal of the operation for myopia is to "weaken" the excessively strong refractive power of the eye focusing the image in front of the retina. This is achieved by weakening the refraction of the cornea in the center from 40.0-43.0 to 32.0-40.0 diopters depending on the degree of myopia. The parameters of the operation (its plan) are calculated using special computer programs. The anatomical and optical parameters of the eye measured before the operation and its refraction data are entered into the computer. The effectiveness of refractive surgery largely depends on the accuracy of the measurement of the anatomical and optical parameters of the eye, computer calculations of the operation plan and its implementation by the surgeon, compliance with all the requirements of refractive surgery.

To correct myopia, the following are used:

• anterior radial keratotomy;
• myopic keratomileusis;
• insertion of intracorneal rings and lenses.

Anterior radial keratotomy, developed by S. N. Fedorov in 1974, is used to correct myopia of 0.5-6.0 D. The technique of the operation consists of making non-penetrating deep (90% of the thickness) radial incisions of the cornea on the periphery using a dosed diamond knife. The peripheral part of the cornea weakened by the incisions bulges under the action of intraocular pressure, and the central section flattens.

The diameter of the central optical zone of the cornea, which remains without incisions (3.2-4 mm), the number of incisions (4-12) and their depth are selected by the surgeon using a computer program depending on the parameters of the eye and the age of the patient.

To correct myopic astigmatism, special operations have been developed that allow the corneal refraction to be reduced to 4.0 D along the meridian corresponding to the astigmatism axis using dosed incisions made perpendicular or parallel to the highly refractive axis - tangential or longitudinal keratotomy.

The technique of performing myopic keratomileusis, developed by H. Barraquer in 1964, has changed significantly at present. Special microkeratomes allow making a precise cut of the superficial layers of the cornea to a depth of 130-150 microns (with its thickness of 550 microns) and forming a "lid". After performing a second, deeper cut, the excised inner layers are removed, and the "lid" is put back in place. The thickness of the removed corneal stroma "doses" the degree of flattening of the center of the cornea and the effect of the operation. Myopic keratomileusis is used for myopia over 6.0 diopters.

Currently, mechanical excision of the corneal stroma has been replaced by its evaporation using an excimer laser, and this operation is called "LASIK".

The introduction of plastic rings and intracorneal lenses into the peripheral layers of the cornea is ineffective, so this method has not become widespread in clinical practice.

The goal of corneal refractive surgery for farsightedness is to "strengthen" the weak optical apparatus of the eye, focusing the image behind the retina. To achieve this goal, an operation developed in 1981 by S. N. Fedorov is performed - corneal thermokeratocoagulation.

In farsightedness, it is necessary to increase the refractive power of the cornea from 40.0-43.0 to 42.0-50.0 D depending on the degree of hyperopia. This is achieved by exposing the peripheral part of the cornea to infrared (thermal) energy, under the influence of which the collagen of the corneal stroma liquefies, the ring of the peripheral part of the cornea contracts, and the central optical zone "bulges", while the refraction of the cornea increases.

Thermal exposure is carried out using a special thin needle (electrode), which is automatically pulled out to a given depth and at the moment of the corneal puncture is heated to 700-1000 "C, therefore tissue contraction occurs throughout the entire thickness of the cornea. The number of punctures and the pattern of their location are calculated using a special computer program depending on the parameters of the patient's eye. The operation allows correcting farsightedness from 0.75-5.0 D and farsighted astigmatism (when acting on one of the main meridians of the astigmatic eye) up to 4.0 D.

Currently, thanks to the use of a solid-state laser, thermal energy has been replaced by laser energy, resulting in a reduction in the trauma of the operation.

Lens refractive surgery includes several methods of influencing the refraction of the eye:

• removal of the transparent lens - refractive lensectomy with or without the introduction of an artificial lens;
• insertion of an additional negative or positive intraocular lens into the eye.

Removal of the transparent lens for the purpose of correcting myopia was proposed by Fukala back in 1890, but it was not widely used due to severe complications. Currently, thanks to the use of modern microsurgical technology, the risk of complications has been reduced, but the method can be used for myopia no higher than 20.0 diopters.

In order to correct high-degree farsightedness, an operation is performed to replace the transparent lens with a stronger intraocular lens of 30-48 diopters, depending on the anatomical and optical parameters of the eye.

Currently, to correct high degrees of ametropia, a method of introducing an additional correcting lens into the eye is used - "glasses inside the eye". A super-thin elastic lens is inserted into the posterior chamber of the eye through a minimal incision and placed in front of the transparent lens, which is why it is called an intraocular contact lens. A negative intraocular lens allows correcting myopia up to -20.0-25.0 D, a positive lens - hyperopia up to +12.0-15.0 D. Modern methods of refractive eye surgery are very effective, provide high-quality stable vision and successfully replace glasses and contact lenses.

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