RU2528649C1 - Corneal segment for treating keratectasias of various origins - Google Patents

Abstract

FIELD: medicine.

SUBSTANCE: invention can be used for treating keratectasias of various origins by polymer keratoplasty with corneal segment (CS) implantation. That is ensured by making an integral CS of an optically transparent biocompatible material. The segment is ring-shaped with end openings. The CS length makes 90 to 220 angular degrees with an outer diameter of 7.1 mm and an inner diameter of 6.0 mm, with oval points. A CS cross-section is convex-convex for form outer and inner ribs in a junction of the two convex surfaces. A line connecting the ribs matches with a long axis of the CS cross-section. An anterior surface of the segment has a constant radius curve. An apex of the above curve is found on a cross axis of the segment. An inclination of the long section in relation to the CS plane makes 25-30 degrees. An apex of an anterior surface of the segment drawn from the apex of the curve to the middle of the long axis makes 150 to 350 mcm at a pitch of 50 mcm. An opposite convex portion of the CS facing a posterior surface of the cornea has a constant radius curve or reduces to an inner rib of the segment. An apex of the posterior surface of the segment is 50 mcm and drawn from the apex of the curve to the middle to the long axis or at 35 mcm from the inner rib respectively.

EFFECT: reducing high order aberration, stabilised progression of the disease, enlarged intact central optical area, enhancement and wider indications for photorefraction keratectomy as a method for residual ametropy correction.

3 dwg

Description

The invention relates to medicine, namely to ophthalmology, and can be used for the treatment of keratectasis of various origins by the method of polymer keratoplasty with implantation of corneal segments (PC).

One of the methods of treating progressive keratectasias is polymer keratoplasty with implantation of corneal segments, the result of which is to stabilize the process of disease progression by implementing bandage function, affecting the cornea bioarchitectonics and increasing visual functions, which ultimately leads to improved social adaptation of patients. 80% of which are people under 40 years old.

The closest analogue of the proposed PC model is the model proposed by Paolo Ferrara in 1995, made of a transparent biocompatible material, having the form of a ring segment with holes at the ends with a circumference of 150 angular degrees, with an inner radius of 5.4 mm and an outer radius of 6.4 mm , with a cross-sectional shape in the form of a trapezoid, with a large base facing the posterior surface of the cornea, and a height of 150 μm to 350 μm, respectively, varying in increments of 50 μm. During PC implantation, one or two corneal tunnels are formed (depending on the type of ectasia) concentrically limb, each in the form of a half ring, at a depth equal to 80% of the thickness of the cornea, in an annular zone with an outer diameter of 7.0 mm and an inner diameter of 5, 0 mm followed by the introduction of one or two corneal segments, respectively (Ferrara de A, Cunha P. Tecnica cirurgica para correcao de miopia; Anel corneano intra-estromal. Rev. Bras. Oftalmol. 54, 577-588 (1995)).

The disadvantages of the closest analogue of PC are: the shape of the cross section of the segment in the form of a trapezoid, the larger base of which is perpendicular to the plane of the segment, is directed towards the posterior surface of the cornea and forms an edge with each side surface of the segment, and the size of the inner diameter of the segment. The presence of a flat base PC leads to an uneven distribution of the tension force in the posterior layers of the corneal stroma, which creates a peak in tension on the rib of the PC and, in turn, can lead to the eruption of a segment in the postoperative period. The internal radius of the PC is 5.4 mm, which is less than the width of the pupil with mydriasis, this leads to the appearance of higher-order aberrations, manifested by patients with complaints of such optical phenomena as halo, coma, light, especially in the twilight time of the day. The small internal radius of the PC reduces the area of the intact central optical zone, which in turn complicates the implementation of personalized photorefractive keratectomy (PRK), as a method for the correction of residual ametropia in patients with keratectasia of various origins during their complex staged treatment.

The objective of the invention is to create a new model of corneal segments for polymer keratoplasty in the treatment of keratectasia of various origins.

The technical result of the invention on PC is to reduce higher order aberrations, stabilize the disease progression processes, increase the intact central optical zone and, therefore, expand the technical capabilities and indications for PRK as a way of correcting residual ametropias.

The technical result is achieved by the fact that the corneal segment for intralamellar polymer keratoplasty, made in the form of a ring segment with holes at the ends as a single unit of an optically transparent biocompatible material, according to the invention, the PC length is from 90 to 220 angular degrees, with an external radius of 7.1 mm and an inner radius of 6.0 mm, with rounded ends, with a biconvex cross-section, with the formation of the outer and inner ribs at the points of transition of one convex surface to another, the line connecting the ribs coincides with the longitudinal axis of the PC section; the part of the segment facing the front surface of the cornea (the front surface of the segment) has a constant radius radius, the highest point of which is on the transverse axis of the section of the segment, the angle of inclination of the longitudinal axis of the section relative to the PC plane is 25-30 degrees, and the height of the front surface of the segment, drawn from the highest point of curvature to the middle of the longitudinal axis of the cross section, is from 150 to 350 microns, in increments of 50 microns, and the opposite convex part of the PC facing the rear surface of the horn of the ovum (the posterior surface of the segment), has a constant radius rounding or decreases to the inner edge of the segment, while the height of the posterior surface of the segment is 50 μm and drawn from the highest point of the rounding to the middle of the longitudinal axis or at a distance of 35 μm from the inner edge of the section, respectively.

This cross-sectional shape was chosen taking into account the histological structure of the cornea: a denser arrangement of collagen fibers in the anterior layers, in contrast to the posterior layers, where they are loosened, which affects the modulus of elasticity of the tissue depending on the depth of the layer. In this regard, for the deformation of the anterior surface of the cornea, implantation of a segment with a steeper profile is necessary, and for the deformation of the posterior surface, a more “flat” one. To reduce the dissecting action on the stroma, and hence reduce the likelihood of teething, the presence of smoothed surfaces (increase in streamlining of the PC) is necessary, which, in addition, will contribute to a more uniform distribution of the tension force in the front and rear directions. The performed mathematical calculations helped to conclude: the part of the PC facing the front surface of the cornea should be more “cool”, i.e. have a radius of curvature less than that of the part of the segment facing the posterior surface of the cornea. Based on the foregoing, we can conclude: stabilization of the disease progression processes occurs due to the direct impact of the corneal segment on both the anterior and posterior layers of the stroma of the cornea, and the corneal segment performs bandage function when it is para-limbal in the stroma of the cornea.

The length of the corneal segment is from 90 to 220 angular degrees and depends on the type of keratectasia determined using keratotopograms, while corneal segments corresponding to the area of corneal ectasia must be used.

To calculate the height of the PC surface facing the posterior surface of the cornea, the equation of the geometric ratio of Barraquer (1967) for the normal cornea was applied:

$$D=337.5\left(\mathrm{one}/R_H-\mathrm{one}/R_K\right)$$

; $$\Delta R_K=\Delta R_H-h$$

;

; $$\Delta R_H=R_H-\sqrt{R_H^2-{\left(\frac{d}{2}\right)}^2}$$

;

Where

D is the diopter effect of PC, D implantation;

R _H is the average radius of curvature of the anterior surface of the cornea prior to implantation, mm;

R _K is the radius of curvature of the anterior surface of the cornea after implantation, mm;

ΔR _H is the difference in the radius of curvature of the posterior surface of the cornea before and after PC implantation;

ΔR _K is the difference in the radius of curvature of the anterior surface of the cornea before and after PC implantation;

h - segment height, mm;

; $$h=a+\frac{{\mathrm{<figure-callout\; id="0"\; label="h"\; filenames="00000006.png"\; state="\{\{state\}\}">h</figure-callout>}}_0}{2}$$

;

a is the height of the part of the segment facing the front surface of the cornea, mm;

h ₀ - the height of the part of the segment facing the posterior surface of the cornea, mm.

It is known that the boundary between two media with equal refractive indices does not change the course of the optical beam, therefore, the posterior surface of the cornea, which is a special case of a centered optical system, has little effect on the refraction of the optical system of the eye, which is confirmed by the experience of previous studies. Calculations carried out confirmed the absence of significant refractive difference with a change in the height of the back surface of the PC. Therefore, the height value of the rear surface of the PC is a constant value.

Given the physiological characteristics of refraction (fluctuation of the spherical index ± 0.5D, cylindrical ± ± 0.75D), the minimum effective refractive effect was taken equal to 0.75D. The calculated height of the back surface of the PC was 50 μm.

Due to the fact that the cornea is not an absolutely spherical optical medium, the calculation of the inclination angle of the longitudinal axis of the PC section relative to the segment plane was carried out taking into account the radii of curvature of the front and back surfaces of the cornea.

To optimize the location of the corneal segment, the longitudinal axis of the PC section should be perpendicular to the radius of the eyeball at the segment location point.

sin γ = R / d, where

γ is the angle of inclination of the longitudinal axis of the section of the segment relative to the plane of the segment;

R is the inner radius of the corneal segment;

d is the radius of curvature of the anterior surface of the cornea.

We will calculate based on R = 6.5 mm, d ₁ = 7.75 mm, d ₂ = 7.35 mm, d ₃ = 7.1 mm, d ₄ = 7.35 mm, which corresponds to the radius of curvature of the front surface of the cornea normal, at the first, second and third stages of keratectasia, respectively. In the calculation, the average value of the angle γ was 25-30 degrees. With such values of the angle γ, the penetrating effect at the boundary of the segment and stroma of the cornea decreases.

When using corneal segments with an internal radius of 5.0 mm or less for the treatment of keratectasia in the postoperative period, part patients complain about the presence of such optical phenomena as halo, glare, haises, all this significantly reduces the quality of the final vision, and therefore a decision was made an increase in the value of the internal radius of the PC to a value of 6.0 mm, which is more than the average statistical value of physiological mydriasis that occurs in the twilight time of the day.

The diameter of the outer radius is determined by the thickness of the segment and is 7.1 mm.

One way to correct residual ametropia after implantation of the corneal segments is to use a personalized PRK, the effectiveness of which depends on the area of the central optical zone of the cornea, where the maximum ablation is performed, and the size of the transition zone formed in this case between the intact and laser-affected corneal stroma, which queue can also lead to the occurrence of unwanted aberrations in patients with physiological mydriasis.

Using the proposed PC will achieve the claimed technical result, namely, due to the impact on the front and back layers of the corneal stroma, to improve the process of stabilizing the progression of the disease, by increasing the internal radius of the PC to increase the area of the intact central optical zone of the cornea and reduce higher-order aberrations that occur more often all in the twilight time of the day.

The invention is illustrated by the drawings shown in figures 1-3

Figure 1 is a top view.

Figure 2 is a cross section of a PC with a constant radius of curvature of the rear surface.

Figure 3 is a cross-sectional view of a PC with a radius of curvature of the rear surface decreasing toward the inner edge of the segment.

The corneal segment of the proposed model is a single monolithic part 1 with front 2 and rear 3 surfaces having different radius of curvature. When one convex surface passes to another, inner 4 and outer 5 annular ribs PC are formed. The ends 6 of the corneal segment are rounded with holes 7. Position 8 is the longitudinal axis of the cross section of the corneal segment, 9 is the height of the front surface of the segment, 10 is the height of the back surface of the segment with a constant radius of curvature, 11 is the height of the back surface of the segment with a radius of curvature decreasing to the inner edge of the PC.

The proposed PC is implanted under instillation local anesthesia. They mark the cornea, focusing on the graduated scale of the vacuum ring, with a diamond knife perpendicular to the front surface of the cornea, make an incision to a depth of 80% of the thickness of the cornea in this area, then, using delaminators and a circular corneal knife, the corneal tunnel is formed concentrically to the limb and parallel to the front surface of the cornea in the 5-7 mm zone, PC implantation is carried out with tweezers and a spatula. The PC is installed in the corneal tunnel in accordance with the ectasia area using a hook and a pusher, which respectively act on the holes 7 in the rounded ends 6. The PC is located in the stroma of the cornea so that its front surface 2 faces the front surface of the cornea and is parallel to it, and the posterior surface 3 is towards the posterior surface of the cornea, with the inner rib PC 4 facing the center of the cornea, and the outer 5 towards the periphery.

Claims (1)

Corneal segment (PC) for performing intralamellar polymer keratoplasty, made in the form of a ring segment with holes at the ends as a single unit of optically transparent biocompatible material, characterized in that the PC length is from 90 to 220 angular degrees with an external radius of 7.1 mm and 6.0 mm inner radius, with rounded ends, with a biconvex cross-section, with the formation of the outer and inner ribs at the points of transition of one convex surface to another, while the line connecting the ribs coincides with the longitudinal axis of the PC section; the front surface of the segment has a constant radius rounding, the highest point of which is located on the transverse axis of the section of the segment, the angle of inclination of the longitudinal axis of the section relative to the PC plane is 25-30 degrees, and the height of the front surface of the segment drawn from the highest point of rounding to the middle of the longitudinal axis of the section , is from 150 to 350 microns in increments of 50 microns, and the rear surface of the segment has a radial rounding of a constant value or decreasing to the inner edge of the segment, when The height of the posterior surface of the segment is 50 μm and drawn from the highest point of curvature to the middle of the longitudinal axis or at a distance of 35 μm from the inner edge of the section, respectively.

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