Astygmatism correction device using corneal surface photo-ablation

Abstract: The invention relates to equipment for the correction of astygmatism by modelling of the corneal surface by means of photo-ablation. A laser beam produced by an excimer laser is used, which is masked by a special mask (1) in such a way that, at each application, a pre-established surface of the cornea (3) is struck, from which cornea (3) a very thin uniform layer of tissue is removed. Two symmetically sliding mask elements (21) from a variable polygonal aperture. A central variable masked area may be created using a pivotable central mask element.

Description: The invention relates to equipment for the correction of astygmatism by remodelling the corneal surface by means of photo-ablation. The prior art teaches laser apparatus, known as excimer lasers, able to generate beams of radiation to vaporize, very exactly and regularly, very thin, microscopic layers (measurable in micrometres) of the eye corneal tissue. Such apparatus is used to remodel the external part of the cornea with the aim of eliminating various refraction defects. The prior art teaches the use of masks and/or diaphragms which, correctly applied, allow the laser beam to reach and ablate the corneal surface in pre-established areas. By means of the use of such masks or diaphragms, whose aperture can be controlled and co-ordinated together with programmed laser-beam application times, it is thus possible to realise step-like sculptures in the cornea through a succession of photo-ablation removal operations of single uniform layers, of different-sized areas of the cornea itself. EP-A 0207648 discloses an apparatus for the correction of astigmatism by remodelling the corneal surface by means of photo-ablation which uses an excimer laser beam able to remove, at every impulse, a thin layer of tissue from the area struck and a mask, to mask the laser such as to expose, at each impulse of the laser beam, a prestablished surface of the cornea; wherein the mask comprises a diaphragm mounted on a frame having a variable aperture geometrically defined by two principal sides opposite to each other. EP-A 0224322 discloses a laser apparatus for eroding a surface comprising means for selecting and controlling the shape and size of the area excised by each pulse of laser without varying the energy density of the beam. By means for beam dimension control the size of the excised area can be varied so that some regions of the surface may be remodelled. The main aim of the present invention is to perfect equipment which, using excimer laser techniques, permits of effectively correcting astygmatism. More in particular, the problem which the invention proposes to resolve is that of correcting corneal surface curvature defects, and the example is of the difference of curvature between the meridians passing through a point, a defect which is at the origin of ametropia or astygmatism. The invention, as it is characterised in the claims that follow, solves the problem through the use, in combination with an excimer laser, of a mask structured to allow special modelling by photo-ablation, which modelling is differentiated according to the various meridians, of a part of the corneal surface arranged in front of the pupillar aperture. Characteristic advantages of the invention are represented by its constructional and functional simplicity and by its intrinsic ability to adapt to various situations, thus making possible very fine regulation. Further advantages and characteristics of the present invention will better emerge from the detailed description that follows, made with reference to the accompanying drawings, which represent a preferred embodiment here illustrated in the form of a non-limiting example, and in which: Figure 1 shows a schematic frontal view of a first embodiment; Figure 2 shows a schematic section made according to line I-I of figure 1; Figure 3 shows, in enlarged scale, a particular of figure 2 in which the part of the corneal surface removed at the end of treatment is evidenced; Figure 4 shows, in enlarged scale, the part of the cornea removed at the end of treatment, from the section, according to line II-II of figure 1, perpendicular to the section performed according the line I-I; Figure 5 shows a schematic, partial perspective view; Figure 6 shows in enlarged scale an embodiment of the particular of figure 1 relative to the conformation of the aperture (25) of the mask (20); Figure 7 shows a schematic perspective view of a further embodiment aimed at the correction of hypermetropic astygmatism; Figure 8 shows a schematic frontal view of figure 7; Figure 9 shows, in enlarged scale, in the same frontal view as in figure 8, a further embodiment; Figure 10 shows, in enlarged scale and schematically, the result of a remodelling operation for the correction of hypermetropic astygmatism, according to a section made along the most curved meridian plane perpendicular to the line X-X. With reference to the figures, 1 denotes in its entirety a mask 1 applied to an eyeball 2 on a surface zone external to the cornea 3 situated in front of the pupillar aperture 4. The mask 1 comprises a structure or frame 11, cylindrical in shape and predisposed to rest directly on the eyeball 2. On the frame 11 is arranged an organ having the function of a diaphragm, denoted in its entirety by 20, which is essentially constituted by two identical flat plates 21 one alongside the other and sliding along the same direction, but in opposite senses, in relation to the frame 11. The two plates 21 are able to move independently of each other (thus allowing operations for the correction of irregular astygmatism to be performed) or they can be reciprocally constrained by means of a mechanism of the self-centring type, through which the contemporary symmetrical movement of the plates 21 with respect to the plate frame is produced, with reciprocal approaching or distancing movements which are symmetrical to a median reference plane X-X of the diaphragm 20 arranged perpendicularly to the plates 21 themselves. Each of the two plates 21 exhibits a recess which edge is constituted by a straight central part or side 22 to the extremities of which two connection parts or sides 23 are connected. In particular, in the illustrated embodiment, the said edge is constituted by a sort of broken line in which there is a straight principal side 22 and two contiguous connection sides 23. The straight principal side 22 is arranged perpendicularly to the relative sliding direction of the two plates 21, indicated by the arrows 5. The two connection sides 23 are also straight or slightly curved and describe, in the plane of the single plate 21, obtuse "internal" angles with the contiguous part or straight central side 22, with which they are connected. On the whole the two plates 21 are arranged one in front of the other so as to create a variable aperture 25 which, in a generic position, can be geometrically described as a polygon-shaped edge constituted by six sides; the two straight principal sides 22, which are opposite, reciprocally parallel and symmetrically facing each other, and the contiguous inclined side pairs 23. The variation of the aperture 25 of the diaphragm 20 is made by the reciprocally-distancing movement of the two plates 21 in the direction indicated by the arrows 5. These movements occur in relation to the said median plane of the mask 1, which is indicated by line X-X of figure 1, and which constitutes the median reference plane for the positioning of the mask itself with respect to the principal meridians or axes of the corneal surface zone involved in the treatment. The relative movement of the two plates 21 modifies the shape, hexagonal on the whole, which describes the variable aperture 25, simply by causing the approaching and the distancing of the two straight principal sides 22 with consequent "shortening or lengthening" of the contiguous connection sides 23. The length of the straight principal sides 22 is about 8 millimetres. The maximum distance obtainable between the two said straight principal sides 22, which corresponds to the maximum work aperture, is also about 8 millimetres. These dimensions are intended to be indicative, and substantially depend on the corneal surface area which must be involved in the treatment with reference to the diameter of the pupillar aperture. The mask is predisposed to be rested directly on the inferior part of its frame 11 on the eyeball 2 in such a way that the plates 21 are arranged parallel to a plane tangential to the corneal surface at the central point of the area to be treated. The mask 1 is predisposed to direct, through its variable aperture 25, a laser beam 6 on to an area 7 of the corneal surface 3, situated in front of the pupillar aperture 4. In order for the treatment of correction of myopic astygmatism to be performed, the mask 1 must be correctly oriented and positioned on the cornea 3. More precisely, the median reference plane of the mask 1, X-X, must be brought to coincide with the cornea 3 meridian plane external surface identifying the least curved meridian 31, that is the one characterised by the largest curvature degree. With such an orientation the two straight principal sides 22 of the mask 1 are perpendicularly located with respect to the meridian plane, and their intersection with the external surface of the cornea 3 identifies the most curved meridian 30, that is the meridian having the smallest degree of curvature, which in its turn is perpendicular to the above-mentioned least curved meridian 31. Once the above-described reference is established, the correction operation is proceeded to according to a succession of operative phases which first see the two plates 21 reciprocally positioned at the minimum (or maximum) reciprocal distance so as to identify the minimum (or maximum) pre-established value of the variable aperture 25 of the diaphragm 20. At the minimum value of the variable aperture 25 the external surface of the cornea 3 struck by the laser beam 6 is constituted by an extremely thin strip of a length which practically coincides with the length of the opposite straight principal sides 22. The influence of the contiguous connection sides 23 is negligible. Following this, a progressive reciprocal distancing of the two plates 21 is performed, according to a pre-established sequence of very small and predetermined relative movements according to the direction of the arrows 5, up until the maximum envisaged value of the variable aperture 25 is reached. According to and in synchrony with the subsequent and pre-established values imposed on the variable aperture 25, the laser beam 6, produced by an excimer laser, is brought to act. At each aperture value, the laser beam strikes a predetermined surface of the cornea 3 which internally comprises all the surface (only a part in cases where the initial conformation of the diaphragm 20 is at its maximum) identified by the aperture value immediately preceding and thus already struck by the previous application of the laser beam. Thus a modelling comprising tiny steps is obtained of the corneal zone struck by the successive single applications of the laser beam. At each application, as is well-known, a very thin layer of predetermined and uniform thickness is removed from the surface illuminated by the laser beam. When the operation is over, in the example herein provided, when the variable aperture 25 has reached its maximum value, the curvature of the interested zone is completely remodelled; indeed, at the meridian which was initially most curved 30, a sickle-shaped thickness of corneal tissue has been removed, evidenced in figure 3. Instead, at the meridian initially least curved, the total thickness removed is constant over all of the central area 7 equal in length to the straight principal sides 22. Starting from the ends of the area 7, however, the thickness removed diminishes progressively and gradually, so as to identify two zones 8 having a connecting function with the least curved meridian 31 of the non-treated part. The size and the gradualness of the connection depend on the angle comprised between each side 23 and the straight principal sides 22 contiguous to it. The final result of the treatment consists in a remodelling of the corneal surface which does not modify the initial curvature of the least curved meridian while modifying, by diminishing it, the curvature of the most curved meridian. By correlating the values of the single apertures to be effected at each excimer laser beam 6 application with the thickness removed at each application, it is possible to generate on the plane of the most curved meridian (figure 3) a profile of any shape and in particular a profile having a constant curvature in the treated zone and equal to the curvature in the same zone for the least curved meridian (figure 4). In this way, in the treated zone a spherical crown-shaped portion of surface is generated. In intermediate meridians, owing to the effect of the straightness and the reciprocal parallelism of the straight principal sides 22, intermediate corrections are obtained with respect to the corrections of the said two principal meridians. For the correction of hypermetropic astygmatism, that is in order to modify the initial curvature of the least curved meridian so as to increase it, at least in a restricted zone corresponding to the pupillar aperture, with the aim of making it equal to that of the most curved meridian, the mask 1 can be alternatively equipped with a thin plate 40 which is used in addition to the diaphragm 20 this new embodiment is denoted by the reference numeral 20'. The Thin plate 40 is pivoted about an axis which is on the said median plane X-X and is parallel to the two principal plane 22. The thin plate 40 comprises two external sides 42 which are straight and parallel to the principal sides 22. The external sides 42 and the principal sides 22 identified two semiapertures 25' which are located in opposite position with respect to the median plane X-X. The plate 40 is pivoted by means of two coaxial pivots 44. The length of the plate 40, which two sides 42 are located on opposite sides with respect to the axis of the coaxial pivots 44, is predisposed in such a way as to guarantee that the projection of te said sides 42 on one same plane (perpendicular to the direction of the laser beam), in reality coinciding with the plane containing the straight principal sides 22, is about the same as or at least not inferior to the corresponding projection on the same plane as the straight principal sides 22. In the embodiment shown in figure 9, at both its opposite ends, the plate 40 is equipped with connecting sides 43 destined to constitute, together with the connecting sides 23, the remaining sides of the closed edges of the two semiapertures 25'. According to the angular position of the plate 40 about the relative coaxial pivots 44 with respect to the direction of the excimer laser beam 6 a strip of shadow is determined, of pre-established breadth according to the most curved meridian on which the X-X plane of the mask 1 is brought to coincide. The strip of shadow, which breadth is variable through the rotation of the plate 40, is thus aligned on the most curved meridian and is thus orthogonal to the least curved meridian which must be treated. By correlating the reciprocal distance values of the straight principal sides 22 with the inclination values of the plate 40 with respect to the direction of the incident excimer laser beam, at each application of the laser beam it is possible to generate any profile on the plane of the least curved meridian, and in particular a profile tract having, as can be seen in figure 10, a greater curvature than the initial curvature and equal to the curvature to be found in the same zone for the meridian which was initially most curved. The presence of connection sides 43 on the plate 40 can be envisaged, with the function of performing a gradual connection of the zones placed at the extremity of the part to be treated. Normally, however, the plate 40 is not equipped with connection sides 43, but exhibits a substantially rectangular shape in which the sides 42 must be of length which is at least equal to the size of the maximum aperture of the mask evaluated in the direction of plane X-X.

Claims: Equipment for the correction of astygmatism by remodelling the corneal surface by means of photo-ablation, comprising: an excimer laser beam able to remove, at every impulse or application, a thin layer of tissue from the area struck; a mask (1) to mask the laser so as to expose, at each impulse or application of the said laser beam, a pre-established surface of the cornea; said mask (1) comprising, thereon a frame (11), a diaphragm (20) having a variable aperture (25) geometrically defined by a closed edge in which diaphragm (20) at least two principal sides (22) are identified, centrally opposite to each other; said two principle sides (22) being straight, parallel and constrained to move in reciprocal approaching or distancing with respect to a median plane of reference (X-X) of the said diaphragm (20); said median plane (X-X) being arranged as a reference plane for a correct positioning of said diaphragm (20) with respect to corneal surface principal meridians; characterised in that the remaining parts of the variable aperture (25) edge identify; two connecting sides (23) fixedly arranged at both ends of each said principal side (22); the "internal" angles between each connection side (23) and the contiguous principal side (22) being greater than 90 degrees and less than 180 degrees. Equipment as in claim 1, characterised in that the said diaphragm (20) comprises two flat sheets (21) able to slide one sheet (21) on the other sheet (21) along the same direction, but in opposite senses, with respect to said median reference plane X-X; each of said two sheets (21) being shaped with a recess which recess edge is constituted by the said straight principal side (22) and by the two contiguous connection sides (23); the direction of the relative sliding of the said said sheets (21) being perpendicular to the said principal sides (22). Equipment as in claim 1, characterised in that the said mask (1) comprises a thin plate (40) pivoted about an axis and which is on the said median plane (X-X) and is parallel to the plane identified by the two principal sides (22); said thin plate (40) comprising two external sides (42) which are straight and parallel to the said principal sides (22); said external sides (42) and said principal sides (22) identifying two semiapertures (25') situated in opposite position with respect to the said median plane (X-X). Equipment as in claim 3 characterised in that the length of the said plate (40) is not inferior to that of the said principal sides (22). Equipment as in claim 4 characterised in that said external sides (42) are located on opposite sides with respect to the said plate (40) rotation axis.

Inventors: NIZZOLA GUIDO MARIA, NIZZOLA, GUIDO MARIA
Applicants: NIBATEC SA, NIBATEC S.A.
Application year: 1992
Main classification: A61F9
IPC codes: A61F9