1OA-116 073 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 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 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 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 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. 5 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 o the eye. 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 be underestimated. By virtue of the photorefractive intervention the biomechanical 5 structure of the eye is changed and therefore the eye can become deformed also 1UA-llb U/i -2 after the operation, depending on the altered biomechanical structure. The intraocu lar pressure deforms the cornea more considerably where it is more weakened. 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 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. US patent application 2003/0055497 Al describes an improved method for surgically inserting and positioning a soft hydrogel ceratoplastic in the cornea of a patient. A flap incision is performed at about 25 to 75% of the thickness of the sclera with a certain circumferential length such that a hinge remains connecting the flap with the cornea wherein within the stromal bed, by means of a laser or a microceratom a half circular undercut is generated (an intrastromal pocket). The rim of the semi-circular undercut has a radial distance of 0.5 to 3 mm from the limbus. The undercut is symmetrical with regard to an opening and the ceratoplastic is positioned symmetri cally thereon with a core region and a rim region. US patent application 2003/0212387 Al describes a method and an apparatus for 5 treating material by means of pulsed laser radiation to generate photodisruptions within the material, e.g. in surgical ophthalmology when removing outer layers of the cornea. A pulsed laser beam with pulse length in the range of nanoseconds and femtoseconds is directed onto the eye of a patient and focused and guided in a three-dimensional structure within the cornea to generate a flap incision having an 0o anterior and a posterior surface in the stroma of the cornea connected in the hinge region. To allow a removal of remaining material from the area of the flap incision and to collect the material in an area remote from the flap incision, a reservoir (pocket or undercut respectively) is cut at the circumference of the flap incision in the form of a ring segment having a radial extension of about 300 to 400 pm and an 1OA-116 073 -3 angle of 120* with regard to a central point of the eye at a depth of about 160 pm in the cornea. The reservoir is preferably generated before the actual flap incision. In any case, the reservoir and the undercut, respectively, are symmetrically arranged about a central axis of the eye. US patent application 2008/0212623 Al discloses a device and a method for treating material by means of laser radiation, in particular a surgical laser to correct refraction in the cornea of an eye of a patient by means of a focus positioned within the cor nea. The laser having high energy and being pulsed, wherein plasma bubbles are generated in the focus. If the focus and the generation of plasma bubbles is guided in lateral direction, a cut is generated in the cornea material. This cut can be ex tended laterally to a ring shape or into an annular area in form of an opening to obtain a peripheral opening of the cornea. Through which the plasma material gen erated during laser cutting is removed. The such extended cut is in all embodiments ring-shaped and extends symmetrically over 360* around the central axis of the eye. This does not show any flap cut or an undercut in a hinge area within the meaning of the present invention. None of the above-cited documents describes a flap cut that is offset with regard to obtain a central axis of the eye. According to the present invention, there is provided an apparatus for cutting a flap in the cornea of an eye, with - a laser radiation source for generating laser radiation, 5 - means including a computer adapted for shaping and guiding the laser radiation in relation to the cornea in such a manner that - in the cornea an incision arises, whereby - an epithelium portion of the incision leaves a hinge region in the epithelium of the cornea, via which the flap remains connected to the cornea and which enables 0 a folding of the flap upwards from the cornea, wherein - a stromal portion of the incision has a circular profile within the stroma of the cornea such that a section of the curricular stromal portion of the incision extends under the hinge region to define an undercut, 5 wherein - the computer is programed such that the focuses of the laser radiation 1OA-116 073 -4 and the corresponding incision are such that the perimeter of the incision is off set from the centre of the pupil of the eye. Embodiments of the invention may make available an apparatus for cutting a flap in the cornea of an eye, wherein the risk of undesirable post-operative deformations of the cornea is diminished. 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 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 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 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 scribed is preferably configured so as to be substantially circular in top view of the eye. 5 An exemplary embodiment of the invention will be described in more detail in the following on the basis of the drawing. Shown are: o Figure 1 schematically, an apparatus for cutting a flap in the cornea of an eye; Figure 2 an axial top view of the cornea of an eye with incision guidances according to the state of the art; 5 Figure 3 a section along line I-II in Figure 2; 1OA-116 073 -5 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. 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 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 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 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 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 s in known manner, via which the flap 26 remains connected to the cornea 20. The 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 o known manner to the surface 20c of the cornea. 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 s regions in which the incision 28 is guided. Figure 3 show schematically the epithelial region 20a and the stromal region 20b of the cornea 20. The epithelium of the cor- 1OA-116 073 -6 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 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 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, 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. 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. 5 But, otherwise than in the state of the art that has been described, the undercut 30 of the photodisruptive incision leads to a considerably more symmetrical surgery in relation to a central axis A of the eye, with the advantages described above. o Figure 4 shows with reference symbol 32a a modification of the exemplary embodi 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, leads to a restriction of the region of the cornea s that is available for the ablation. The hinge region cannot be used for the laser abla tion. Therefore, according to this variant of the invention, the perimeter of the flap is 1OA-116 073 -7 chosen to be not concentric in relation to the pupil of the eye. For the purpose of obtaining a maximal ablation zone, the perimeter of the flap incision is being offset in relation to the pupillary centre, specifically away from the edge 24 of the hinge re gion 22. This is indicated in Figure 4 by the dashed line 32a, which indicates sche matically a flap perimeter that has been offset in such a manner. The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.