Surgery – Instruments – Light application
Reexamination Certificate
2000-09-28
2003-03-11
Cohen, Lee (Department: 3739)
Surgery
Instruments
Light application
C606S004000
Reexamination Certificate
active
06530917
ABSTRACT:
The invention relates to a device for photorefractive surgery on the cornea of the eye for the correction of sight defects of a higher order.
Photorefractive keratectomy is a hitherto widely established procedure for correcting defective vision of a lower order, i.e. for example of myopia, hyperopia, astigmatism, myopic astigmatism and hyperopic astigmatism. The term “photorefractive keratectomy (PRK)” is usually understood to mean that an intervention on the surface of the cornea is only intended after the so-called corneal epithelium has been removed. After removal of the epithelium the Bowman's membrane or the corneal stroma is exposed and can be removed by a laser. The LASIK procedure (laser in situ keratomileusis) is generally distinguished from PRK. In the LASIK procedure an approximately 100 &mgr;m to 200 &mgr;m thick cornea slice (so-called “flap”) with a diameter of 8 to 10 mm is cut down to a small remnant serving as a “hinge” with a so-called microkeratome. This slice (flap) is folded to the side and ablation (removal) of material is then effected by laser radiation directly in the stroma, i.e. not on the surface of the cornea. After laser treatment the lid is folded back to its original position again and healing generally takes place relatively quickly.
The invention described below is suitable both for the above-described PRK as well as in particular the LASIK technique.
In PRK and in LASIK, corneal material is removed. The removal is a function of the lumination of the laser beam striking the cornea (energy per unit of area). Various techniques are known for beam formation and beam positioning thus, for example, the so-called slit scanning, in which the radiation is guided by means of a moved slit over the region to be treated, the so-called scanning-spot, in which a radiation spot with very small dimensions is guided over the area to be removed, and also the so-called full-ablation or wide-field ablation, in which the radiation is directed extensively over the entire area to be removed and wherein the lumination alters across the beam profile in order to achieve the desired removal of cornea. The state of the art includes suitable algorithms for controlling the radiation for the aforementioned beam positioning in each case in order to remove the cornea such that the cornea finally has the desired radius of curvature.
The aforementioned scanning-spot uses a laser beam focused on a relatively small diameter (0.1 to 2 mm), which laser beam is directed by means of a beam positioning device onto various points of the cornea and is moved successively by a so-called scanner such that ultimately the desired removal of cornea is achieved. Removal takes place therefore in accordance with a so-called ablation profile. In PRK and LASIK so-called galvanometric scanners can in particular be used (cf. Essay by G.F. Marshall in LASER FOCUS WORLD, June 1994, page 57). In the meantime other scanning techniques have been disclosed for the positioning of the laser beam.
According to the state of the art, the aforementioned types of defective vision of a lower order (for example myopia, hyperopia, astigmatism) are at present determined according to the so-called refraction data of the patient's eye i.e. the dioptric value measured for the patient's eye determines the ablation profile in accordance with which 5 material is removed (ablated) from the cornea (cf. T. Seiler and J. Wollensak in LASERS AND LIGHT IN OPHTHALMOLOGY, Vol. 5, No. 4, pages 199 to 203, 1993). In accordance with this state of the art, for a given patient's eye with a specific dioptric value the laser radiation is therefore guided over the cornea such that a predetermined ablation profile corresponding, for example, to a parabola in a correction for myopia is removed. In other words: the ablation profile is adapted only in accordance with the dioptric value to the individual eye but not however in accordance with local irregularities of the optical system “eye”.
The essay by J. K. Shimmick, W. B. Telfair et al in JOURNAL OF REFRACTIVE SURGERY, Vol. 13, May/June 1997, pages 235 to 245 also describes the correction of sight defects of a lower order by means of photorefractive keratectomy, wherein the photoablation profiles correspond to theoretical parabolic shapes. Furthermore, it is only proposed there to incorporate some empirical correction factors into the ablation profile, which correction factors take into account the interaction between laser and tissue in order to achieve a paraboloid-shaped removal on the eye as a result.
A particular problem in photorefractive kerotectomy and LASIK is the relative positioning of laser beam and eye. The state of the art knows various processes for this thus, for example, so-called “eye trackers”, i.e. devices which determine the movements of the eye in order to then control (track) the laser beam used for the ablation in accordance with the eye movements. DE 197 02 335 C1 for example, describes the state of the art with regard to this.
As aforementioned above, the procedures for photorefractive cornea surgery of the state of the art for correcting defective vision of a lower order are substantially “all-inclusive procedures” in the sense that the correction takes account of the (all-inclusive) dioptric value of the eye. Such defective vision of a low order can, for example, be corrected with spherical or astigmatic lenses or also with a photorefractive correction of the cornea.
The optical image in the eye is however affected not only by the aforementioned types of defective vision of a lower order but also by so-called image distortions of a higher order. Such image distortions of a higher order occur in particular after operative interventions to the cornea and inside the eye (cataract operations). Such optical aberrations can be the reason why complete visual acuity (visus) is not attained despite a medical correction of a defect of a lower order. In DER OPHTHALMOLOGE, No. 6, 1997, page 441 P. Mierdel, H.-E. Krinke, W. Wigand, M. Kaemmerer and T. Seiler describe a measuring arrangement for determining the aberration of human eyes. With such a measuring arrangement, aberrations (image distortions) for monochromatic light can be measured, more specifically aberrations caused by the cornea as well as image distortions caused by the entire ocular image system of the eye can be measured and this can be done site-dependently, i.e. with a specific resolution for given sites within the pupil of the eye, it can be determined how large the image distortion of the entire optical system of the eye to be corrected is at this point. Such image distortions of the eye are mathematically described in the above-aforementioned work by P. Mierdel et al as a so-called wave-front aberration. Wave-front aberration is understood to mean the spatial course of the distance between the actual light wave-front of a central light point and a reference surface, such as, for example, its ideal, ball-shaped form. Therefore, the ball surface of the ideal wave-front, for example, serves as a spatial reference system. It is also known as such in the state of the art to select a plane as a reference system for the aberration measurement if the ideal wave-front to be measured is flat.
The measuring principle according to the aforementioned work by P. Mierdel, T. Seiler et al is also used as a starting point in the realisation of the present invention. It substantially involves a parallel beam bundle of sufficient diameter being divided by a shadow mask into separated parallel individual beams. These individual beams pass through a convex lens (so-called aberroscope lens) and as a result are focused in the emmetropic eye at a specific distance in front of the retina. The result is clearly visible projections of the mask shadows on the retina. This retinal light point pattern is depicted according to the principle of indirect ophthalmoscopy onto the sensor surface of a CCD video camera. In the aberration-free ideal eye the depicted light point pattern is not distorted and corresponds to
Kaemmerer Maik
Mrochen Michael
Seiler Theo
Browning Clifford W.
Cohen Lee
Johnson, III Henry M.
Wavelight Laser Technologie AG
Woodard, Emhardt, Naughton Moriarty & McNett LLP
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