RU2054908C1 - Method for removing corneal tissue - Google Patents

Abstract

FIELD: medicine. SUBSTANCE: method involves exposing cornea to ultraviolet laser radiation pulses with wavelength less than 250 nm, pulse durations not longer than 1•10^-6 s, mean radiation power density being in the range from 0,01-50 Wt/cm² and peak power density being 10⁴-10⁹ Wt/cm². EFFECT: reduced traumatic effects. 2 dwg

Description

 The invention relates to medicine, namely to ophthalmology, and can be used for surgical changes in eye refraction.

 A known method of removing corneal tissue by irradiation of an excimer laser.

The disadvantages of this method include high invasiveness due to thermal and mechanical damage to the biological tissue of the cornea, which are expressed in a sufficiently large width of the zone of coagulated tissue that borders the zone of action. This also occurs as a result of the long operation time of this method for about 3 minutes or more and the need to work at high radiation energy densities, namely with an average radiation power density of at least 3 W / cm ² and a peak radiation power density of at least 10 ⁸ W / cm ² .

 The purpose of the invention is the reduction of morbidity.

The goal is achieved by the fact that in the method of removing corneal tissue by irradiating an excimer laser, the cornea is irradiated with a pulse sequence with a wavelength of less than 250 nm, a pulse duration of no more than 1 x 10 ^-6 s, an average power density of 0.01-50 W / cm ² peak power density 10 ⁴ 10 ⁹ W / cm ² .

The interval of the average radiation power density and peak radiation power density are selected experimentally and interconnected. Lower limits An average radiation power density of 0.01 W / cm ² and a peak radiation power density of 10 ⁴ W / cm ² initiate selective removal of the surface layer of the cornea, called ablation.

At average values of the radiation power density and peak radiation power density, respectively, below 0.01 W / cm ² and 10 ⁴ W / cm ^2, the ablation effect is not observed.

The upper limits mean radiation power density of 50 W / cm ² and peak radiation power density of 10 ⁹ W / cm ² correspond to saturation in the thickness of the vaporized layer and the onset of thermal and mechanical damage to biological tissue. At values exceeding these values, pronounced thermal and mechanical damage to the cornea of the eye occurs.

Temporary range is not more than 1 x 10 ^-6 s is selected from the consideration that the laser pulse duration τ must be shorter than the thermal relaxation time t, determined from the ratio tl ^2/2 κ, where l is the optical depth of penetration at a given wavelength; κ coefficient of thermal diffusivity, determined by the density, heat capacity and thermal conductivity of the corneal tissue.

When the duration of the exposure pulses is greater than 1 · 10 ^-6 s on the corneal tissue, a pronounced coagulation effect will be observed.

 As you know, the surface of the cornea of the normal eye can be described by the equation of the paraboloid of rotation. In pathologies of the cornea of the eye, its surface is also described by the equation of the paraboloid of rotation. So, for example, with myopia, the surface of the cornea of the eye is described by a paraboloid of revolution with a radius of curvature at the apex smaller than for the normal eye. For the treatment of myopia, it is necessary to remove the layer bounded by two parabolic surfaces of different curvature from the cornea of the shaded area in Fig. 1. In case of hyperopia, when the surface of the cornea of the eye is described by a paraboloid of revolution with a radius of curvature at the apex greater than for the case of a normal eye, it is also necessary to remove the shaded area from the cornea of the eye in Fig. 2.

 The effect of far UV radiation on biological tissues leads to ablation of the latter, and in a certain range of energy characteristics of radiation, the thickness of the removed layer is proportional to the energy density of the laser radiation.

 The experiments showed that laser ablation is characterized by thresholds and saturation along the thickness of the vaporized layer per pulse, therefore, for radiation treatment of pathologies of the cornea of the eye, it is necessary to construct the dependence of the depth of the layer removed per pulse on the energy density to find the ablation threshold and determine the operating modes (energy density and pulse repetition rate) at which there are no thermal and mechanical damage to biological tissue.

The following basic parameters of an excimer laser ophthalmic surgical unit operating at a wavelength of 193 nm were experimentally determined: pulse duration 15 ns; the lower limit of the energy density should be above the ablation threshold; the upper limit should be less than 250 J / cm ² , since the damage to the endothelial cells is still insignificant; to reduce thermal and mechanical damage, the pulse repetition rate should be no more than 20 Hz.

Before the operation, patients' eyes are examined: ophthalmometry, clinical refractometry, visual acuity is checked. Then, the operation is performed by irradiating the patient’s cornea with a sequence of laser pulses with parameters that are determined by the calculation of the operation and are within the following limits: radiation wavelength less than 250 nm; pulse duration not more than 10 ^-6 s; average radiation power density of 0.01 to 50 W / cm ² ; peak power density of radiation 10 ⁴ 10 ⁹ W / cm ² .

 The intensity profile of the exposure beam is determined by the specific type of operation.

 PRI me R 1. Patient P. born in 1963 examined for high myopia, clouding of the cornea of both eyes.

In the study: myopia of the right eye 0.8 D: left 8.0 D. Ophthalmometry of the right eye: 0 ^about - 38.0; 90 ^about - 43.25; left eye: 0 ^about - 40.25; 90 ^about - 44.5. The thickness of the cornea of both eyes in the center is 520 μm, at a distance of 6 mm from the center of 540 μm. A keratotomy of the right eye was performed using a UV laser with a wavelength of 193 nm using this technique. After anesthesia with a 1% solution of dicain and fixation of the eyeball with a specially designed lens.

The central zone of the cornea was irradiated with UV laser radiation with a wavelength of 190 nm, with an average power density of 0.01 W / cm ² . The total depth of exposure in the center was 82.4 microns. The diameter of the impact zone is 6.0 mm. When exposed to a visually marked appearance in the center of the cornea of a circular defect with gradually diome deepening towards the center. The exposure time was 120 s. The patient did not experience any pain during the intervention.

On examination, on the first day after the operation, a weakly expressed mixed injection of the eyeball, corneal edema along the edge of the impact. Epithelization on the 3rd day. The eye is calm. Ophthalmometry of the right eye 2 ^about - 31.25. 93 ^about - 36.2. Visual acuity without corr. 0.6 (due to turbidity in the deep layers). There are no complaints.

 A week after the operation of the right eye, a keratectomy of the left eye is performed: under local anesthesia with a 1% solution of dicain, 85.2 microns were removed using the same technology. The parameters and technology of exposure are the same. The exposure time is 120 s. When viewed on the first day, a round cut of the cornea is visible, deepening in the center. On the third day, epithelization.

 Visual acuity without corr. 0.7 (due to opacities in the deep layers of the cornea). There are no complaints. The effect persists for 1 year.

 PRI me R 2. Patient S. born in 1948 Examined for stationary myopia of the highest degree.

During the examination: myopia of the right eye 7.0 D, left 8.0 D. Ophthalmometry of the right eye 0 ^about 41.0; 90 ^o 44.0; left eye 0 ^about 41.5 to 90 ^about 44.0.

 The thickness of the cornea in the center of the right eye is 494 μm, the left eye is 498 μm.

The operation was performed UV laser keratectomy according to the proposed method, with an average power density of 0.15 W / cm ² . The total exposure depth was 79.5 μm. Exposure time 40 s. The wavelength is 193 nm.

The next day, a mixed injection of the eyeball. In the center of the cornea, a rounded defect with swelling along the edge. Epithelization on the third day. Ophthalmometry: 0 ^o 34.5, 90 ^o 37.25. After 1 week, visual acuity without corr. 1,0. After 10 days, a similar operation was performed on the left eye. The production technology of the operation is the same. Epithelization on the third day. Ophthalmometry: 2 ^o 34.0. 89 ^about 36.25. Visual acuity without corr. 1,0. The effect is steadily divided for 6 months. Thinning of the cornea to the removed tissue thickness is maintained.

 PRI me R 3. Patient Ch. Born in 1945 Examined for a high degree of myopia, anisometropia, amblyopia of the right eye, central dystrophy of the left eye retina.

Visual acuity of the right eye of 0.01 with Sph-22,0 0,03; left eye 0.01 Sph 14.0 0.1. Low visual acuity due to a change in the fundus. The thickness of the cornea of the right eye is 542 microns. In order to ensure the same refraction, a UV laser keratectomy of the right eye was performed according to the proposed method at an average power density of 0.83 W / cm ² and a wavelength of 193 nm.

In the center of the cornea, tissue was removed to a depth of 82.5 microns. The diameter of the impact zone is 6.0 mm. Ophthalmometry of the right eye before surgery 31 ^about 43.0; 120 ^about 44.7.

The technology of operations is similar to the previous ones. Epithelization on day 3. Ophthalmometry after surgery 25 ^about 36.0; 120 ^about 37.2. Visual acuity of the right eye with corr. 14.5 D 0.08-0.1. The thickness of the cornea is 460.0 microns. With follow-up periods of up to three months, the effect of the operation is preserved.

 Indications for the operation are superficial (up to 1/3 of the thickness) opacities of the cornea; dystrophic diseases of the cornea (keratitis, ulcers, etc.); refractive errors (myopia, hyperopia, astigmatism).

 Contraindications to the operation are clouding of the cornea more than 1/3 of the thickness of the refractive errors with a thin cornea.

 The method has been clinically tested at the ISTC "Eye Microsurgery" from August 1988 to June 1989 with 62 eyes.

 The application of this method allows to reduce the injuries of the operation by about 6 times due to the reduction of thermal and mechanical effects due to the optimization of the operation time of not more than 1-2 minutes and the possibility of working at the lowest radiation energy densities.

Claims (1)

METHOD FOR REMOVING THE CORNERAL TISSUE by irradiation with an excimer laser, characterized in that, in order to reduce trauma, the cornea is irradiated with a sequence of pulses with a wavelength of less than 250 nm, a pulse duration of not more than 1 • 10 ^{- 6} , and an average power density of 0.01 - 50.0 W / cm ² peak power density of 10 ⁴ - 10 ⁹ W / cm ² .

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