English translation of PCT application 1OA-116 073 Device for cutting a flap in the cornea of an eye 5 The invention relates to an apparatus for cutting a flap in the cornea of an eye. In ophthalmological surgery, particularly in the known LASIK process, the term 'flap' has gained acceptance, also in German, for designating a small cover that is cut from o the side in an anterior region of the cornea. The flap can then be folded aside, so that in known manner the cornea can be reshaped with laser radiation by ablation for the purpose of eliminating imaging defects. Currently available for the cutting of such a flap are, on the one hand, the so-called s mechanical microkeratome, in which an oscillating blade produces the incision in the cornea, or, on the other hand, so-called femtosecond LASIK, in which a laser is em ployed having radiation pulses that are set to be so short that the power density of the radiation in the case of focusing in the interior of the cornea brings about so called photodisruptions therein. By virtue of a control of these femtosecond pulses in o space and time an incision in the cornea can then be produced by means of a plural ity of such photodisruptions. Nowadays this is a widely known technique. Both the conventional mechanical microkeratome and the flap incision with femto second laser pulses that has been described have the fact in common that a hinge 5 remains at the edge of the flap incision, the term 'hinge' having also been adopted in German. Via such a hinge region, in which the cornea is not severed by the incision, the flap remains connected to the cornea, so that it can be folded back again after implementation of the ablation in the stroma of the cornea. 0 In the state of the art, such a hinge brings about an asymmetry of the intervention into the cornea in relation to, for example, the optical axis of the eye or another (imaginary) axis perpendicular to the surface of the cornea. The flap incision is not rotationally symmetrical and, in particular, not circular in relation to a central axis of the eye. 35 After implementation of a LASIK operation, the intraocular pressure of the eye plays a role in the healing process and also in the formation of a corneal shape that cannot English translation of PCT application 1OA-116 073 -2 be underestimated. By virtue of the photorefractive intervention the biomechanical structure of the eye is changed and therefore the eye can become deformed also after the operation, depending on the altered biomechanical structure. The intraocu lar pressure deforms the cornea more considerably where it is more weakened. 5 The asymmetrical guidance of the incision, described above, for the purpose of pro ducing the hinge in the state of the art brings about an asymmetrical configuration of the biomechanical structure of the cornea. After the operation the intraocular pres sure can then also bring about an asymmetrical deformation of the cornea by reason o of the asymmetrical flap incision, progressing as far as an induced cylinder aberration or aberrations of higher order. In other words: in the state of the art the asymmetri cal guidance of the incision in the course of producing the flap can bring about by force an undesirable corneal shape post-operatively by reason of the intraocular pressure. 5 The object underlying the invention is to make available an apparatus for cutting a flap in the cornea of an eye, wherein the risk of undesirable post-operative deforma tions of the cornea is diminished. o For this purpose the invention provides an apparatus for cutting a flap in the cornea of an eye, with a laser radiation source for generating laser radiation, means for shaping and guiding the laser radiation in relation to the cornea in such a manner that an incision arises in the cornea, whereby the incision leaves a hinge region in the cornea, via which the flap remains connected to the cornea and which enables a s folding of the flap upwards from the cornea, the incision extending under the hinge region with an undercut. The incision is preferably made substantially symmetrically in such a way that the biomechanical structure of the cornea after the operation is also substantially sym 0 metrical, so that, as far as possible, the intraocular pressure brings about no undesir able 'bulge' on one side of the eye (in relation to the optical axis). In this connection the symmetry is relative to an axis that is perpendicular to the surface of the cornea, in particular - but not necessarily - the optical axis or the visual axis of the eye. If, for the purpose of obtaining an ablation field that is as large as possible, the flap 5 incision is made somewhat asymmetrically in relation to, for example, one of the aforementioned axes (i.e. the spacing of the hinge from the axis is somewhat in creased, in order to obtain an ablation field that is as large as possible), the symme- English translation of PCT application 1OA-116 073 -3 try observations that have been made here relate to such an imaginary axis which is slightly offset in relation to the optical axis or the visual axis. The flap incision including the undercut under the hinge region that has been de 5 scribed is preferably configured so as to be substantially circular in top view of the eye. An exemplary embodiment of the invention will be described in more detail in the following on the basis of the drawing. 0 Shown are: Figure 1 schematically, an apparatus for cutting a flap in the cornea of an eye; 5 Figure 2 an axial top view of the cornea of an eye with incision guidances according to the state of the art; Figure 3 a section along line I-II in Figure 2; 0 Figure 4 an axial top view of the cornea of an eye with a flap-incision guidance according to the invention; and Figure 5 a section along line III-IV in Figure 4. s Figure 1 shows schematically the essential components of an apparatus for cutting a flap in the cornea of an eye 10. This apparatus is well-known in principle and there fore does not need to be described further here in all details. As a matter of princi ple, recourse may be had to a known apparatus of femtosecond LASIK (fs LASIK), and in accordance with the invention a reprogramming of the computer control of the 30 foci of the fs pulses in the cornea is undertaken in novel manner. The apparatus exhibits a laser radiation source 12 which generates femtosecond laser radiation pulses 14. Means 16 serve for optical shaping and guidance of the laser radiation 14 in relation to the cornea 20 of the eye 10. The optical means re 3s quired for this purpose and, in particular, the means for controlling the radiation in time and space (scanner etc.) are well-known. A computer controller 18 controls English translation of PCT application 1OA-116 073 -4 both the laser radiation source 12 and the means 16 for the shaping and guidance of the radiation. Figures 2 and 3 illustrate the problems in the state of the art. Figure 2 shows a top 5 view of the cornea 20 in the axial direction. Figure 3 shows the incision along line I-II in Figure 2. In the case of the photodisruptively produced incision 28 a hinge region 22 remains in known manner, via which the flap 26 remains connected to the cornea 20. The o edge of the hinge region 22 is denoted in the Figures by 24. In the state of the art the flap 26 is accordingly folded upwards at this edge 24. In the state of the art the incision 28 terminates at the edge 24 - that is to say, it does not extend under the hinge region 22. On the opposite side the marginal incision 32 is guided upwards in known manner to the surface 20c of the cornea. 5 Since in Figures 2 and 3 the incision terminates at the edge 24, an asymmetrical configuration of the biomechanical properties of the cornea arises in relation to a central axis A. In the hinge region 22 the cornea is weakened less than in those regions in which the incision 28 is guided. Figure 3 show schematically the epithelial o region 20a and the stromal region 20b of the cornea 20. The epithelium of the cor nea has a microstructure differing from that of the stroma. The latter exhibits so called lamellae which extend parallel to the corneal surface. These lamellae substan tially establish the biomechanical stability of the cornea. An incision through the lamellae consequently constitutes a considerable intervention into the biomechanical s structure and symmetry of the formal structure constituted by the eye. Therefore after implementation of an operation with asymmetrical incision guidance according to Figures 2 and 3 the radially acting intraocular pressure may bring about post operatively a deformation of the cornea that is not precisely foreseeable, particularly when the ablative intervention extends relatively far and weakens the stroma. Such a risk of an undesirable post-operative deformation of the cornea is avoided with the incision guidance illustrated in Figures 4 and 5. In the Figures, parts and components that are functionally similar to one another have been provided with the same reference symbols. Figure 5 is a section along line III-IV in Figure 4. Accord 3s ing to Figures 4 and 5, the incision 28 generated photodisruptively with femtosecond pulses does not terminate at the edge 24 of the hinge region 22 but is guided below the hinge region 22 in the form of an undercut 30 beyond the edge 24. Accordingly, English translation of PCT application 1OA-116 073 -5 no incision takes place at the edge 24, the dotted representation is only an imaginary line (similarly in Figure 3). At the two end points 24a, 24b the connection between the flap 26 and the cornea 30 is consequently preserved, and these two end points 24a, 24b consequently define a hinge line along which the flap 26 is folded upwards. 5 With the flap 26 folded upwards, the undercut 30 accordingly remains below the hinge region 22 which has remained intact. Stroma for the subsequent ablative treatment is exposed, just as in the state of the art illustrated above. But, otherwise than in the state of the art that has been described, the undercut 30 0 of the photodisruptive incision brings about a substantially more symmetrical inter vention in relation to a central axis A of the eye, with the advantages described above. Figure 4 shows with reference symbol 32a a modification of the exemplary embodi 5 ment described above. In this modification the flap 26 is not precisely concentric in relation to the centre of an eye feature such as, for example, a pupil. According to this modification, it is taken into account that the hinge region, via which the flap remains connected to the cornea, gives rise to a restriction of the region of the cor nea that is available for the ablation. The hinge region cannot be used for the laser 0 ablation. Therefore, according to this variant of the invention, the perimeter of the flap is not chosen to be concentric in relation to the pupil of the eye. For the pur pose of obtaining a maximal ablation zone, the perimeter of the flap incision is offset in relation to the pupillary centre, specifically away from the edge 24 of the hinge region 22. This is indicated in Figure 4 by the dashed line 32a, which indicates 5 schematically a flap perimeter that has been offset in such a manner.