Abstract:
A device for machining the human cornea with focused pulsed femtosecond laser radiation comprises scanner components for local setting of the beam focus, a control computer for controlling the scanner components, and a control program for the control computer. The control program contains instructions that upon execution by the control computer are designed to bring about the generation of an incision figure in the cornea encompassing a flap incision ( 38, 40 ). In accordance with the invention the incision figure further encompasses an auxiliary incision ( 50 ) connected with the flap incision and leading locally, preferentially directly, away from the latter as far as the surface of the cornea. The auxiliary incision is expediently generated temporally ahead of the flap incision and forms a discharge channel through which gases can escape that may arise in the course of the cutting of the flap incision.

Description:
TECHNICAL FIELD 
     The invention is concerned with the generation of incisions in the human cornea by means of focused laser radiation. In particular, the invention is concerned with the preparation of a LASIK flap by means of such laser radiation. 
     BACKGROUND 
     A frequently employed technique for eliminating visual defects of the human eye—such as, for example, myopia or hyperopia or astigmatism—is so-called LASIK. LASIK stands for laser in-situ keratomileusis and designates a technique in which firstly a small cover disc in the cornea is cut free which is folded aside in order to expose the underlying tissue regions of the cornea. These exposed tissue regions are then treated in ablating manner by means of focused UV laser radiation—i.e. corneal material is removed in accordance with an ablation profile ascertained individually for the patient. The small cover disc is usually designated in specialist circles as a flap and is not severed completely from the remaining tissue of the cornea but is still connected to the rest of corneal tissue in a hinge region which in specialist circles is generally designated as a hinge. This enables a simple folding-away of the flap and, above all, a simple folding-back of the flap after the ablation. On account of the removal of material, a changed shape of the anterior surface of the cornea arises after the flap has been folded back. The associated result of this is a different refractive behaviour of the cornea and consequently of the overall system constituted by the eye. By suitable definition of the ablation profile it can be ensured that the visual defect is at least distinctly attenuated and, at best, is almost completely eliminated. 
     Various procedures for the preparation of the flap are known in the state of the art. One procedure uses a mechanical microkeratome—i.e. a microsurgical scalpel which cuts into the cornea with a cutting blade which is ordinarily driven in oscillating manner. Another procedure, which will be considered in more detail within the scope of the invention, uses focused short-pulse laser radiation for the purpose of preparing the flap. In this case, laser radiation with pulse durations within the femtosecond range, for example within the low three-digit femtosecond range, is ordinarily employed. In addition, the laser radiation usually has a wavelength above about 300 nm, in order to enable a coupling of the radiant energy deep into the corneal tissue. LASIK treatments in which the flap is prepared by means of such short-pulse laser radiation are often designated as fs LASIK. 
     For the generation of incisions by means of focused laser radiation in transparent material (transparent to the laser radiation), the so-called laser-induced optical breakthrough is utilised as a physical effect. This breakthrough ultimately results in a photodisruption of the irradiated tissue in the region of the focus. The laser radiation that is beamed in brings about a local vaporisation of the irradiated material at the focal point. In the process, gases arise which—to the extent that they are not conducted away to the outside—collect in internal cavities or are absorbed by the adjoining material. It has been found that in the course of LASIK treatments of the human eye a residence in the cornea of the gases arising in the course of preparation of the flap can lead to problems in the course of the subsequent laser ablation. In particular, it has been found that these gases can render difficult a precise tracking of the eye by means of an eye-tracker. Laser systems that are employed for the ablation of corneal tissue frequently possess such an eye-tracker, in order to register eye movements during the laser treatment and to reposition the laser radiation correspondingly. As a rule, the eye-trackers are constructed from a camera and suitable image-evaluation software which evaluates the images recorded by the camera and detects changes in the position of the eye. Frequently the image-evaluation software evaluates characteristic features of the eye—for instance, defined points of the iris or/and the pupillary centre or/and the corneal apex or/and the limbus. It has been shown that accumulations of gas remaining in the cornea, which have arisen in the course of preparation of the flap, can impede the acquisition of such characteristic features of the eye. It goes without saying that for the success of the operation a precise functioning of the eye-tracker is absolutely essential. 
     SUMMARY 
     The object of the invention is to demonstrate a way in which, in the case of LASIK treatments, an impairment of the success of the operation by troublesome accumulations of gas which arise in the course of a preparation of the LASIK flap by laser can be avoided. 
     With a view to achieving this object, the invention proposes a device for cutting the human cornea with focused pulsed laser radiation. The device includes controllable components for local setting of the beam focus, a control computer for controlling these components, and a control program for the control computer. The control program contains instructions that upon execution by the control computer are designed to bring about the generation of an incision figure in the cornea encompassing the flap incision, whereby according to the invention the incision figure further encompasses an auxiliary incision connected with the flap incision and leading locally as far as the surface of the cornea. The auxiliary incision establishes a connection between the flap incision and the surface of the cornea. In this way, gases arising surgically are able to escape to the surface of the cornea and hence out of the corneal tissue. A penetration of these gases into critical tissue regions of the eye can be avoided in this way. Any impairments of an eye-tracker can also be avoided better in the course of the following laser ablation. 
     The above reference to a local progression of the auxiliary incision is intended to serve to avoid confusions with the temporal progression in the course of generation of the auxiliary incision. A statement about a defined local progression of an incision or of a part of an incision is not intended here generally to imply any assertion whatsoever about the temporal sequence in the course of generation of the incision. Accordingly, an incision for which the assertion is made that it proceeds locally from a defined first location to a defined second location may easily be generated temporally in the direction from the second location to the first location. 
     In a preferred configuration the auxiliary incision begins locally directly from the flap incision and proceeds locally away from the latter as far as the surface of the cornea. Though it is also conceivable that one or more gas pockets or gas cavities within the corneal tissue adjoining the first incision are firstly generated surgically, and that the auxiliary incision leads from these gas pockets or gas cavities locally to the surface of the cornea. 
     The auxiliary incision preferably forms a planar and, if desired, substantially flat channel, in which connection the length, width and gradient angle of the channel can be chosen variably. The channel may have substantially constant width over its length; though generating a channel of varying width is not to be excluded. For the generation of the auxiliary incision, it may be sufficient to produce photodisruptions alongside one another in a single plane only. Though producing such photodisruptions also in two or more planes above one another is not to be excluded if a larger cross-section of the channel is desired. 
     In a preferred configuration the auxiliary incision is connected with the flap incision in a hinge region of the flap formed by the flap incision, and extends on the other side of the flap locally to the surface of the cornea. If desired, in this case the auxiliary incision is narrower than the hinge region. 
     For an optimal removal of photodisruption gases arising (i.e. gases that arise in the course of and as a consequence of the photodisruption), it is advisable that the auxiliary incision has greatest corneal depth in a region in which it is connected with the flap incision, and proceeds locally, beginning from this region, with increasingly smaller corneal depth as far as the surface of the cornea. 
     It is favourable if the instructions of the control program are designed to generate the auxiliary incision before the flap incision is produced. This creates the prerequisite for a gas-removal channel to be available right from the start, via which the photodisruption gases arising in the course of preparation of the flap can be conducted away. 
     In advantageous manner the instructions of the control program are designed to generate at least a predominant part and preferably the largest part of the auxiliary incision in a direction from the surface of the cornea to the flap incision. The formation of photodisruption gases has to be reckoned with also in the course of generation of the auxiliary incision. Insofar as the auxiliary incision is generated in the direction away from the surface of the cornea, it is possible to conduct these photodisruption gases away optimally. For example, the instructions of the control program may be designed to generate at least a predominant part of the auxiliary incision with line scans of the beam focus that progress, line by line, along a direction of extension of the auxiliary incision that proceeds from the surface of the cornea to the flap incision. Though it is just as possible that the instructions of the control program are designed to generate at least a predominant part of the auxiliary incision with line scans of the beam focus that progress, line by line, transverse to a direction of extension of the auxiliary incision that proceeds from the surface of the cornea to the flap incision. 
     The flap incision may encompass a bed incision completely situated deep within the corneal material, preferentially at substantially constant depth, as well as a lateral incision adjoining the bed incision and guided out locally to the surface of the cornea. The bed incision is so designated because it defines the stromal bed for the flap. Given prior applanation (levelling) of the surface of the cornea by abutment against a suitable applanation face, the bed incision may be realised, for example, by a flat surface incision which is inserted at constant depth of the cornea. In principle, it is conceivable to generate the bed incision with line scans or with spiral scans of the radiation focus. However, for an optimal removal of the photodisruption gases arising in the course of generation of the bed incision it is proposed that the instructions of the control program are designed to generate the bed incision with line scans of the radiation focus that progress increasingly, line by line, in the direction away from a hinge region of the flap. At the same time, the instructions of the control program are expediently designed to generate the lateral incision temporally after the bed incision. Since the lateral incision leads locally out to the surface of the cornea, it is advisable to generate the lateral incision, beginning from its most low-lying regions, in a direction towards the surface of the cornea. Of course, an opposite direction in the course of generation of the lateral incision is equally conceivable. 
     For a good transition, sufficiently permeable to the photodisruption gases, between the auxiliary incision and the flap incision, it is advisable that the instructions of the control program are designed to bring about, in a transition region between auxiliary incision and flap incision, line scans of the beam focus that progress, line by line, transverse to the transition direction. Equally, for a good opening of the auxiliary incision outwards, it is advisable that the instructions of the control program are designed to bring about, in the surface-side end region of the auxiliary incision (by ‘end region’ here a local end region is meant; the surface-side end region may perfectly well be the starting region in connection with the generation of the auxiliary incision), line scans of the radiation focus that progress, line by line, transverse to the direction of entry of the auxiliary incision into the cornea. 
     In a further development of the invention the device may further include a contact element that is transparent to the laser radiation, with a contact face intended for abutment against the eye, the instructions of the control program being designed to generate the auxiliary incision in such a way that its end opening out on the surface of the cornea lies in a region of the cornea in which the latter bears against the contact face of the contact element. 
     In a first configuration the contact face may exhibit a plane surface portion for levelling a part of the surface of the cornea, the instructions of the control program being designed to generate the auxiliary incision in such a way that its end opening out on the surface of the cornea lies in a region of the cornea in which the latter bears against the plane surface portion of the contact face. 
     In an alternative configuration the contact face may exhibit a plane surface portion for levelling a part of the surface of the cornea, as well as a surface portion adjoining the plane surface portion and proceeding obliquely relative to the latter in the direction towards the side of the contact element facing away from the eye, the instructions of the control program being designed to generate the auxiliary incision in such a way that its end opening out on the surface of the cornea lies in a region of the cornea in which the latter bears against the obliquely proceeding surface portion of the contact face. The obliquely proceeding surface portion is preferentially of rounded construction and adjoins the plane surface portion in kink-free manner. 
     According to another further embodiment the device may include a contact element that is transparent to the laser radiation, with a contact face intended for abutment against the eye, the instructions of the control program being designed to generate the auxiliary incision in such a way that its end opening out on the surface of the cornea lies outside a region of the cornea in which the latter bears against the contact face of the contact element. 
     In this further embodiment, for the purpose of adapting the refractive index a chamber is preferentially provided on the side of the contact element facing towards the eye, which via a filling-channel arrangement is capable of being filled with a liquid or with another suitable flowable medium which reduces the jump in refractive index from the contact element to the cornea and expediently possesses optically homogeneous properties. Instead of being present in liquid form, this medium may also be present, for example, in gel-like form. Also not excluded is the use of a gaseous medium. The instructions of the control program are in this case designed to generate the auxiliary incision in such a way that it leads into the liquid-filled chamber (or, generally, into the chamber filled with the medium). 
     In the case of formation of the flap incision with a flat bed incision, the instructions of the control program are preferentially designed to generate the auxiliary incision opening out into the chamber and the bed incision in a common plane. This relates to the flattened case—that is to say, the state in which the eye is bearing against the contact element. 
     A process for treating the human eye includes the following steps: providing first pulsed laser radiation, the laser radiation having a radiation focus, directing the first laser radiation onto a human cornea to be treated, controlling the radiation focus of the first laser radiation for the purpose of generating a flap incision in the cornea forming a flap, as well as an auxiliary incision connected with the flap incision and leading locally as far as the surface of the cornea. 
     The process may further include the following steps: folding the flap away, in order thereby to expose underlying corneal tissue, providing second pulsed laser radiation, directing the second laser radiation onto the exposed corneal tissue, and ablating the exposed corneal tissue with the second laser radiation in accordance with a predetermined ablation profile. 
     The first laser radiation preferentially has pulse durations within the femtosecond range and has a wavelength above 300 nm. The second laser radiation has a wavelength within the UV range and may, for example, be generated by an excimer laser, for instance by an ArF excimer laser radiating at 193 nm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be elucidated further in the following on the basis of the appended drawings. Represented are: 
         FIG. 1  in schematic block representation, an exemplary embodiment of a laser arrangement for inserting intracorneal incisions, 
         FIGS. 2   a  and  2   b  two variants of a corneal incision figure for preparing a LASIK flap, 
         FIG. 3  the incision figure shown in  FIG. 2   a , in a cross-sectional view, 
         FIG. 4  an exemplary scan pattern of a laser beam that is used for generating the incision figure shown in  FIG. 2   a,    
         FIGS. 5 and 6  two variants for generating a flap incision, supplemented by an auxiliary incision, in a flattened cornea. 
     
    
    
     DETAILED DESCRIPTION 
     The laser arrangement shown in FIG.  1 —denoted generally by  10 —includes a laser source  12  which generates a laser beam  14  with pulse durations within the femtosecond range. In the beam path of the laser beam  14  a series of components are arranged, inter alia a scanner  16 , indicated here schematically as a unitary functional block, an immovable deflecting mirror  17  and also a focusing objective  18 . The scanner  16  serves for transverse and longitudinal local control of the focal point of the laser beam  14 . ‘Transverse’ here designates a direction at right angles to the direction of propagation of the laser beam  14 ; ‘longitudinal’ corresponds to the direction of beam propagation. In conventional notation the transverse plane is designated as the x-y plane, whereas the longitudinal direction is designated as the z-direction. For the purpose of transverse deflection of the laser beam  14  (i.e. in the x-y plane) the scanner  16  may, for example, include a pair of galvanometrically actuated scanner mirrors which are capable of being tilted about mutually perpendicular axes. Alternatively, for example, a transverse deflection by means of an electro-optical crystal is conceivable. For the z-control of the position of the focus the scanner  16  may contain, for example, a longitudinally adjustable lens or a lens of variable refractive power or a deformable mirror, with which the divergence of the laser beam  14  and consequently the z-position of the beam focus can be influenced. It will be understood that the components of the scanner  16  serving for the transverse focus control and for the longitudinal focus control may be arranged distributed along the beam path of the laser beam  14  and, in particular, apportioned to different modular units. For example, the function of the z-focus control may be fulfilled by a lens arranged in beam-expanding optics (beam expander, e.g. Galilean telescope), whereas the components serving for the transverse focus control may be accommodated in a separate modular unit between the beam-expanding optics and the focusing objective  18 . The representation of the scanner  16  as a unitary functional block in  FIG. 1  serves merely for better clarity of layout. 
     The focusing objective  18  is preferably an f-theta objective and is preferentially separably coupled on its beam-emergence side with a patient adapter  20  which forms an abutment interface for the cornea of an eye  22  to be treated. For this purpose the patient adapter  20  exhibits a contact element  24  that is transparent to the laser radiation and that on its underside facing towards the eye exhibits a abutment face (contact face)  26  for the cornea. In the exemplary case that is shown, the abutment face  26  is constructed as a plane face and serves for levelling the cornea, by the contact element  24  being pressed against the eye  22  with appropriate pressure or by the cornea being aspirated onto the contact face  26  by reduced pressure. The contact element  24  (in the case of plane-parallel construction, ordinarily designated as the applanation plate) is fitted at the narrower end of a conically widening carrier sleeve  28 . The connection between the contact element  24  and the carrier sleeve  28  may be inseparable, for example by virtue of adhesion, or it may be separable, for instance by virtue of a screwed joint. At its wider sleeve end the carrier sleeve  28  has, in a manner not represented in any detail, suitable coupling structures for coupling onto the focusing objective  18 . 
     The laser source  12  and the scanner  16  are controlled by a control computer  30  which operates in accordance with a control program  34  stored in a memory  32 . The control program  34  contains instructions (program code) that upon execution by the control computer  30  bring about such a local control of the beam focus of the laser beam  14  that a LASIK flap arises in the cornea of the eye  22  bearing against the contact element  24 . The incision figure generated in the cornea in this regard encompasses not only a flap incision forming the actual flap but additionally an auxiliary incision, through which photodisruption gases arising are able to escape from the cornea to the outside. 
       FIGS. 2   a  and  2   b  show two variants of such an incision figure. In both cases a dashed circular line  36  denotes the levelling region in which the cornea is levelled as a consequence of its abutment against the contact element  24 . It will be understood that, in reality, the levelling region  36  does not have to be exactly circular. In particular, in view of the ordinarily differing radii of curvature in the principal meridional directions of the surface of the cornea an outline of the levelling region  36  deviating from a circular shape may arise. 
     In the exemplary cases that are shown, the flap incision forming the flap is composed of two partial incisions. A first partial incision is a so-called bed incision, which severs the flap from the stromal bed and is realised as a flat surface incision parallel to the contact face  26 . The bed incision is denoted by  38  in  FIGS. 2   a  and  2   b . Said bed incision is produced at a depth of the cornea corresponding to the desired thickness of the flap. Whereas in  FIG. 2   b  it extends over a complete circular area, in  FIG. 2   a  it is shortened by a segment of a circle and terminates at a chord of a circle. It will be understood that, depending on the desired shape of the flap, the bed incision  38  may have a non-circular outline, for example an elliptical outline. In any case, the bed incision  38  is complemented by a lateral incision  40  which proceeds along a partial periphery of the bed incision  38  and—considered locally—extends to the surface of the cornea, beginning from the bed incision  38 . The lateral incision  38  is also generated in the levelled state of the cornea, i.e. with the eye  22  bearing against the contact face  26 , and proceeds obliquely outwards locally from the bed incision  38 . Alternatively, the lateral incision  40  may proceed obliquely inwards locally from the bed incision  38 . 
     The flap is formed by the bed incision  38  and the lateral incision  40  together. Said flap is denoted by  42  in  FIGS. 2   a  and  2   b  and also in  FIG. 3 . In the part of the periphery of the bed incision  38  not encompassed by the lateral incision  40  the flap  42  is still connected to the remaining corneal tissue (apart from the region of an auxiliary incision yet to be elucidated). The transition region between the flap  42  and the remaining corneal tissue forms a hinge which permits the flap to be folded away in order to expose the underling tissue for an ablating laser treatment. The hinge line is, at least in sufficient approximation, rectilinear and is denoted by  44  in  FIGS. 2   a  and  2   b . In the case of  FIG. 2   a , it is situated approximately overlapping the straight edge of a segment of a circle at which the bed incision  38  terminates; in  FIG. 2   b  it proceeds, at least when considered in the top view of this Figure—transversely beyond the bed incision  38  from one peripheral end of the lateral incision  40  to the other. 
       FIG. 3  illustrates the two partial incisions (bed incision, lateral incision) of the flap  42  for the case of  FIG. 2   a  with the cornea relaxed, i.e. after removal of the eye  22  from the contact element  24 . The curved line  48  which has been drawn with dashes designates the anterior surface of the cornea. 
     As a consequence of the vaporisation of corneal tissue in the course of the cutting of the flap  42 , gases arise which are able to diffuse out of the cut surface into the adjoining tissue regions. The residence of such gases in the eye may, on the one hand, be dangerous if the gases penetrate into particularly sensitive regions of the eye; on the other hand, it may impair the functionality of an eye-tracker in the course of the subsequent laser ablation of the stromal bed. Therefore the incision figure generated in the cornea is not only limited to the flap incision but exhibits an additional auxiliary incision  50  which enables an escape from the eye of the gases arising in the course of preparation of the flap. In the exemplary cases of  FIGS. 2   a ,  2   b  which are shown, the auxiliary incision  50  directly adjoins the bed incision  38 , to be specific in the hinge region of the flap  42 —that is to say, where the lateral incision  40  leaves free a part of the periphery of the bed incision  38 . Starting from the bed incision  38 , the auxiliary incision  50  extends locally away from the flap  42  in the direction towards the corneal surface  48 , i.e. it proceeds on the other side of the flap  42 . In this case the auxiliary incision  50  proceeds locally at increasingly smaller depth within the cornea; in particular, it ascends steadily to higher corneal layers until it reaches the surface of the cornea. In this way it forms a channel (tunnel), by which the bed incision  38  is connected with the environment outside the eye, so that gases that arise in the course of the cutting of the bed incision  38  are able to escape outwards through the channel. 
     As can be discerned in  FIGS. 2   a ,  2   b , in the exemplary cases that are shown the channel formed by the auxiliary incision  50  has a constant width over its length, being narrower than the hinge region of the flap  42  and, relative to the direction of the hinge axis  44 , situated approximately centrally in the hinge region. It will be understood that the auxiliary incision  50  may also be as wide as the hinge region or even wider than the latter. Restrictions in this regard are not intended within the scope of the invention. 
     In  FIG. 3  the auxiliary incision  50  is represented as a rectilinear incision. It will be understood that the auxiliary incision  50  may alternatively rise in a curved path locally from the bed incision  38  to the surface of the cornea. The intensity of the ascent of the auxiliary incision  50  may also be defined differently. Comparable remarks apply to the width of the auxiliary incision  50 ; said width may vary over the length of the auxiliary channel; for example, it may become larger in the direction towards the surface of the cornea. 
     The point at which the auxiliary incision  50  reaches the surface of the cornea may lie outside the levelling region  36  of the cornea, as indicated in  FIGS. 2   a ,  2   b  where the auxiliary incision  50  extends outwards beyond the levelling region  36 . Nevertheless, it is just as possible that the auxiliary incision  50  is guided locally up to the surface of the cornea within the levelling region  36  or at the edge of the levelling region. 
     For the purpose of elucidating the temporal sequence in which the auxiliary incision  50  and the bed incision  38  are inserted, and the scan patterns that are used in the process for the laser beam  14 , reference will now additionally be made to  FIG. 4 . The bed incision  38  and the auxiliary incision  50  are shown therein; the lateral incision  40  has been omitted for the sake of clarity of layout; it is usually generated only after the bed incision  38 , to be specific starting from the bed incision  38  in the direction towards the surface of the cornea. 
     The auxiliary incision  50 , on the other hand, is generated before the bed incision  38  is inserted. This guarantees that gases are able to escape outwards via the auxiliary incision  50  already at the start of the preparation of the bed incision  38 . The auxiliary incision  50  is generated from the surface of the cornea—that is to say, in the direction towards deeper corneal layers. This is indicated by an arrow  52  which has been drawn at the top in  FIG. 4 . After generation of the auxiliary incision  50 , the bed incision  38  is generated, to be specific beginning in the hinge region  44 —that is to say, where the auxiliary incision  50  terminates. Beginning from the hinge region  44 , the bed incision  38  is gradually generated in the direction towards the end that is remote from the hinge. This direction of generation of the bed incision  38  is indicated at the top in  FIG. 4  by an arrow  54 . 
     On its largest part the auxiliary incision  50  is generated by line scans of the laser beam  14  that follow one another, line by line, in the arrow direction  52 , i.e. in the direction from the surface of the cornea towards the bed incision  38 . The individual scan lines of this line scan are denoted by  56 . The bed incision  38  is also generated with a scan pattern consisting of line scans, the individual scan lines following one another in the direction from the hinge region  44  towards the end of the bed incision  38  that is remote from the hinge, i.e. in the arrow direction  54 . The scan lines of the bed incision  38  are denoted by  58  in  FIG. 4 . 
     Whereas the representation shown in  FIG. 4  is an example of a succession of the scan lines  56  in the direction of the longitudinal extent of the auxiliary incision  50  (‘longitudinal extent’ in this connection means an extent from the surface of the cornea to the flap incision, more precisely to the bed incision), it is readily conceivable to generate the main part of the auxiliary incision  50  with a line scan, the scan lines of which follow one another at right angles to the longitudinal extent of the auxiliary incision. The scan lines of such a transverse scan then proceed similarly to the scan lines  60  and  64 . 
     The transition region between the auxiliary incision  50  and the bed incision  38  is, in addition, prepared with line scans that progress, line by line, at right angles to the transition direction. ‘Transition direction’ here means the direction in which the auxiliary incision  50  merges with the bed incision  38 . This direction corresponds to the direction of the arrows  52 ,  54 . By producing scan lines alongside one another at right angles to this direction in the transition region, it is possible to realise a good connection, open for the passage of gas, between the auxiliary incision  50  and the bed incision  38 . The transverse scan lines that have been produced in the transition region are denoted by  60  in  FIG. 4 . The direction of their succession is represented by an arrow  62  (may optionally also be in the opposite arrow direction). 
     Similar transverse scan lines are, furthermore, produced in the entry region of the auxiliary incision  50 —that is to say, where it enters the corneal tissue on the surface of the cornea. The corresponding scan lines are denoted by  64  in  FIG. 4 ; the direction of their succession is indicated by an arrow  66  (may optionally also be in the opposite arrow direction). These scan lines  64  proceeding at right angles to the entry direction of the auxiliary incision are expedient in order to create a clean opening of the auxiliary incision  50  on the surface of the cornea. 
     As far as the temporal sequence is concerned, expediently firstly the scan lines  64  are produced, subsequently the scan lines  56 , thereupon the scan lines  60  and, thereafter, the scan lines  58 . In this manner the generation of the incisions progresses increasingly from the surface in the direction towards deeper layers. The representation in the lower part of  FIG. 4  illustrates this once again. Therein the auxiliary incision  50 , the bed incision  38  and also the entry region (denoted by  68 ) and the transition region between the two incisions (denoted by  70 ) are shown in a view from the side. With the temporal sequence of the scan lines that has been elucidated it is possible that during the generation of the auxiliary incision  50  and also during the generation of the bed incision  38  a tunnel to the outside is already always open, through which gases currently arising are able to escape. 
     In a modification of the above sequence, the generation of the transition region  70  may be temporally favoured and may be undertaken ahead of the entry region  68 . After this, as previously, the remainder (i.e. the main part) of the auxiliary incision  50  and also the bed incision  38  are inserted. In a further modification, firstly the transition region  70  may be generated. Then the main part of the auxiliary incision  50  and, after this, the entry region  68  are generated. After complete generation of the auxiliary incision  50 , the bed incision  38  is inserted. Though it should be pointed out that within the scope of the invention no restriction whatsoever to a defined temporal sequence of the generation of the incisions is intended. 
     In principle, in the entire auxiliary incision  50  (including the entry region  68  and the transition region  70 ) and also in the bed incision  38  the local spacings of the photodisruptions following one another along the scan lines may be substantially the same. The same applies to the mutual spacing of consecutive scan lines. 
     However, it is possible to vary the local spacing of the photodisruptions or/and the mutual line spacing at least in parts of the auxiliary incision  50  or/and of the bed incision  38 . In particular, it is conceivable to choose for the entry region  68  or/and for the transition region  70  a closer local succession of the photodisruptions or/and to choose a closer mutual spacing of the consecutive scan lines than for the main part of the auxiliary incision  50  and for the bed incision  38 . 
     The position of the auxiliary incision  50  described here relative to the bed incision  38  guarantees that the two incisions do not overlap reciprocally. This is because no further underlying plane can be cut through an already cut plane. Since in the ideal case the auxiliary incision  50  should already be present when the cutting of the bed incision  38  is begun, it is advisable to cut the auxiliary incision  50  into the cornea from outside the flap (further remote from the corneal centre) and to allow it to merge with the bed incision in the hinge region of the flap. 
     In  FIGS. 5 and 6 , identical or identically-acting elements are denoted by the same reference symbols as in the preceding Figures, but supplemented by a lower-case letter. To the extent that nothing else results in the following, reference is made to what was stated above for the purpose of elucidating these elements. 
     It has already been explained that the auxiliary incision may open out to the surface of the eye inside or outside the levelled region of the cornea. This assertion may be generalised to the extent that the auxiliary incision may open out to the surface of the cornea at a point that lies inside or outside (or at the edge of) a region in which the cornea bears against the contact element of the patient adapter. Even though within the scope of the invention a very extensive levelling of the surface of the cornea by the contact element is striven for, it is nevertheless not necessary that the cornea bears against the contact face exclusively in flat regions of said contact face. 
     In this regard, reference will now be made to the variant shown in  FIG. 5 . Therein a contact element  24   a  is shown which on its underside facing towards the eye bears a contact face  26   a  which in its main part is of flat construction but in its marginal region is rounded and proceeds there obliquely to the flat main part in the direction away from the eye  22   a . The rounded surface portion forms a ring segment surrounding the flat main part of the contact face  26  and is denoted by  72   a . The flat main part of the contact face  26   a , on the other hand, is denoted by  74   a . The contact between the contact element  24   a  and the eye  22   a  is established in such a way that the cornea rests closely against the contact face  26   a  not only in the main part  74   a  but also in the outer ring segment  72   a.    
     The auxiliary incision  50   a  is generated in such a way that it emerges to the surface of the cornea in the region of the ring segment  72   a  (the term ‘emerge’ is meant here purely locally and implies no statement whatsoever about the temporal sequence in which the individual parts of the auxiliary incision are prepared). This means the auxiliary incision  50   a  opens out at a point on the surface of the cornea where the cornea bears against the rounded ring segment  72   a . This is favourable to the extent that any possible gases which arise in the course of preparation of the auxiliary incision  50   a  and of the bed incision  38   a  and escape to the outside though the auxiliary incision  50   a  can, at least for the most part, reach the surrounding air and are not drawn deeper between the contact element  24   a  and the corneal surface  48   a  by capillary action. In other words, a better degassing of the operative field is possible in this way. 
     The rounded ring segment  72   a  adjoins the flat main part  74   a  of the contact face  26   a  preferentially in kink-free manner. Nevertheless, it is not to be excluded in principle to construct the ring segment  72   a , instead of with a roundish configuration, in the form of a rectilinear oblique surface which is separated from the flat main part  74   a  by a kink. 
     The variant shown in  FIG. 6  illustrates an example in which the auxiliary incision  50   b  opens out at a point on the surface of the cornea where the cornea does not bear against the contact face  26   b  of the contact element  24   b . Instead, it opens out outside the flattened region of the corneal surface  48   b . In concrete terms, in the exemplary case shown in  FIG. 6  the auxiliary incision  50  opens out into an annular chamber  76   b  which is delimited between the contact element  24   b , the corneal surface  48   b  and a sealing component  78   b  which, for example, may be part of a suction ring (not represented in any detail) to be attached onto the eye  22   b . Represented schematically in  FIG. 6  is a filling channel  80   b , via which the annular chamber  76   b  is capable of being filled with a physiological liquid (e.g. solution of common salt). The filling channel  80   b  may be part of the aforementioned suction ring. 
     The bed incision  38   b  and the auxiliary incision  50   b  may in this variant be generated in a common plane, i.e. with constant z-position of the beam focus of the laser radiation that is used for the generation of the incisions. The direction of generation corresponds to that shown in  FIG. 4 , i.e. the auxiliary incision  50   b  is generated before the bed incision  38   b , to be specific expediently in a direction from the surface of the cornea to the bed incision  38   b.