Abstract:
A method for preparing an endothelial corneal graft is provided. The method comprises: providing a donor cornea; irradiating the donor cornea from an endothelial side thereof with laser radiation to cause a photodisruption in tissue of the donor cornea at a focal point of the radiation; and moving the focal point of the radiation to form an endothelial graft in the donor cornea. By irradiating the donor cornea from the endothelial side thereof, instead of the epithelial side, to cut the endothelial graft, optical inhomogeneities which may develop after death in stromal tissue of the donor cornea leave the laser cutting process substantially unaffected.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure generally relates to a technique for preparing an endothelial corneal graft. More specifically, and without limitation, a technique for separating at least a Descemet membrane from a donor cornea including corneal stroma is disclosed. 
       BACKGROUND 
       [0002]    Aside the traditional full-thickness Penetrating Keratoplasty (PKP), eyes with permanent endothelial compromise resulting in suboptimal vision can be treated by replacing diseased parts of the cornea while retaining the healthy anterior parts of the cornea. 
         [0003]    Posterior Lamellar Keratoplasty (PLK) was proposed by José I. Barraquer and Charles Tillett in the 1950s. The demand for precisely shaped corneal grafts increased in the late 1990s, when it was shown that a surgically resected corneal disc including diseased endothelium can be replaced with a similar partial-thickness donor disc and that the disc can be attached to the inner corneal surface of the patient&#39;s cornea without any sutures. This development led to similar surgical procedures based on differently prepared grafts. 
         [0004]    For example, Descemet Stripping Automated Endothelial Keratoplasty (DSAEK) uses an endothelial graft, which has been prepared by mounting the donor cornea at an artificial anterior chamber and removing the anterior corneal stroma using a femto-second laser or a microkeratome. For DSAEK, precut eye bank-prepared endothelial grafts are available, which eliminates the need for preparation of the donor corneal tissue in the operating room. However, deep anterior cuts by means of the femto-second laser or the microkeratome can result in irregular sections, if the donor cornea is not properly mounted at the artificial anterior chamber. Furthermore, when using the femtosecond laser, the conventional preparation of the endothelial graft may be affected by optical inhomogeneities in the donor cornea, e.g., since typically more than twelve hours have passed post mortem at the time of graft preparation. E.g., the inhomogeneities can affect a focusing precision for the deep cut. 
         [0005]    For Descemet Membrane Endothelial Keratoplasty (DMEK), the donor Descemet membrane is scored, partially detached under fluid and trephined from the endothelial side. The Descemet membrane with the healthy donor corneal endothelium is removed as a single donor disc without any donor corneal stroma. Hence, there is no need for an artificial anterior chamber or a microkeratome in the preparation of the endothelial graft. However, manually removing the Descemet membrane and the endothelium by means of mechanical trephination as well as subsequent membrane peeling for DMEK can easily cause irreparable damage to the endothelium, which does not regenerate. 
       SUMMARY 
       [0006]    Accordingly, there is a need for a technique that provides an endothelial graft, which is later applicable for Descemet Stripping Endothelial Keratoplasty, from a donor cornea without impairing the Descemet membrane or the endothelium. 
         [0007]    According to one aspect, a method of preparing an endothelial corneal graft using laser radiation comprises: providing a donor cornea isolated from a dead body; irradiating the donor cornea from an endothelial side thereof with laser radiation to cause a photodisruption in tissue of the donor cornea at a focal point of the radiation; and moving the focal point of the radiation to form an endothelial graft in the donor cornea. 
         [0008]    The donor cornea may be provided from a dead human body. Alternatively, the donor cornea may be provided isolated from a living body, e.g., from a living human body. 
         [0009]    Providing the donor cornea may include deforming the donor cornea to have a curvature opposite to a natural curvature of the donor cornea. Providing the donor cornea may include placing an endothelium of the donor cornea in contact with a reference contact surface of a laser apparatus generating the laser radiation. 
         [0010]    The method may comprise: determining a thickness of a predetermined structure of the donor cornea, the predetermined structure formed by one of an endothelium, a Descemet membrane and a Dua layer of the donor cornea or a combination thereof; and based on the determined thickness of the predetermined structure, determining a desired thickness of the endothelial graft; wherein moving the focal point of the radiation includes moving the focal point across a surface that is a distance corresponding to the desired thickness of the endothelial graft from the reference contact surface. The desired thickness of the endothelial graft may be determined to be in a range from 30 to 120 μm, e.g., 90 μm. The surface may extend at least substantially parallel to the reference contact surface. 
         [0011]    The term Descemet membrane, as used herein, may also encompass the Dua layer. The Descemet membrane and the Dua layer may be considered as two manifestations of one common structure, which may be commonly referred to as the Descemet membrane. E.g., the Dua layer may be considered as a crossover from the stroma to the Descemet membrane. 
         [0012]    Providing the donor cornea may include positioning the donor cornea on a support unit in opposition to a reference contact surface of a laser apparatus generating the laser radiation. 
         [0013]    Irradiating the donor cornea may include focusing the laser radiation in stromal tissue of the donor cornea, wherein moving the focal point of the radiation includes moving the focal point through the stromal tissue to create a stromal bed cut for the graft. At least a major part of the bed cut may extend substantially parallel to a Descemet membrane of the donor cornea. A desired distance between the bed cut and a Descemet membrane may be selected in the range from 5 μm to 100 μm, e.g., 50 μm, and the focal point of the radiation is moved through the stromal tissue at the selected distance from the Descemet membrane. The desired distance may be selected differently for different wavelengths of the radiation. 
         [0014]    The focal point of the radiation may be moved so as to leave undisrupted a connection structure connecting the endothelial graft with surrounding tissue of the donor cornea. The connection structure may be formed entirely by stromal tissue of the donor cornea. 
         [0015]    According to another aspect, a method of configuring a control unit of an ophthalmic laser apparatus is provided. The control unit is adapted to control movement of a focal point of a laser beam relative to a reference contact surface of the laser apparatus, the reference contact surface having a known position in a coordinate system of the laser apparatus. The method comprises: determining a thickness of a predetermined structure of a donor cornea isolated from a dead body, the predetermined structure being formed by one of an endothelium, a Descemet membrane and a Dua layer of the donor cornea or a combination thereof; based on the determined thickness of the predetermined structure, determining a desired thickness of an endothelial graft to be prepared from the donor cornea; and configuring the control unit to move the focal point across a surface that is a distance corresponding to the desired thickness of the endothelial graft from the reference contact surface of the laser apparatus. When the predetermined structure is formed by the Descemet membrane, the predetermined structure may also comprise a structure that is referred to as Dua layer in literature. 
         [0016]    The donor cornea may be provided from a dead human body. Alternatively, the donor cornea may be provided isolated from a living body, e.g., from a living human body. 
         [0017]    Yet another aspect provides the use of an ophthalmic laser apparatus for preparing an endothelial graft in a donor cornea isolated from a dead body, the apparatus comprising: a source of laser radiation; a reference contact surface having a known position in a coordinate system of the laser apparatus, the reference contact surface being transmissive for the laser radiation; and a control unit configured to control movement of the focal point of the laser radiation relative to the reference contact surface; wherein the use comprises: mounting the donor cornea on a support unit with an endothelium of the donor cornea contacting the reference contact surface; irradiating the donor cornea with the laser radiation through the reference contact surface; and causing the control unit to move the focal point of the laser radiation through the donor cornea to create the endothelial graft. In the use, the control unit may be cause to move the focal point of the laser radiation across a surface that is no more than 90 μm or 80 μm or 70 μm or 60 μm or 50 μm or 40 μm or 30 μm from the reference contact surface to thereby create a stromal bed cut for the endothelial graft. 
         [0018]    The donor cornea may be provided from a dead human body. Alternatively, the donor cornea may be provided isolated from a living body, e.g., from a living human body. 
         [0019]    According to still another aspect, a method of preparing an endothelial graft from a donor cornea isolated from a dead body is provided, the endothelial graft including a Descemet membrane, the donor cornea including stromal tissue. The method comprises the steps of providing a support unit for supporting the donor cornea; providing a cutting laser apparatus including a cutting laser interface configured for directing radiation towards the support unit; mounting the donor cornea at the support unit so that at least the Descemet membrane is arranged between the corneal stroma and the cutting laser interface; and controlling a focus of the radiation according to a control program including instructions that cause the radiation to pass through the Descemet membrane and to focus within the corneal stroma at focal points defining a bed cut within the corneal stroma that is at least substantially parallel to the Descemet membrane. 
         [0020]    The donor cornea may be provided from a dead human body. Alternatively, the donor cornea may be provided isolated from a living body, e.g., from a living human body. 
         [0021]    The donor cornea may be a heterograft, e.g., a human donor cornea, or a xenograft. The cornea may be removed from a dead body. The graft and the donor cornea may include any one or any combination of a corneal endothelium, the Descemet membrane and a Dua layer. Herein, the Descemet membrane may be considered as a layer distinct from the Dua layer or as a structure that also comprises the Dua layer. The graft can be subjected to cryogenic storage and/or later be used for endothelial keratoplasty, e.g., in cases of Fuchs dystrophy, bullous keratopathy, iridocorneal endothelial (ICE) syndrome, or other endothelial disorder. 
         [0022]    A conventional cutting laser apparatus and/or a conventional support unit may be used. The donor cornea may be inversely mounted at the support unit, i.e. with the endothelium of the donor cornea facing toward the cutting laser interface and the epithelium of the donor cornea facing away from the cutting laser interface. The cutting laser interface may provide a reference contact surface for an object to be laser-treated by means of the laser apparatus. Upstream of the focal point, the radiation may pass through at least one of the Dua layer, the Descemet membrane and the corneal endothelium of the donor cornea. The radiation may cross the Descemet membrane once. The inverse orientation of the donor cornea relative to the cutting laser interface can, for at least certain donor corneas, improve the precision, e.g., a shape and/or a position, of a focal point for performing the laser cut, reduce attenuation of the radiation and/or reduce scattering of the radiation, e.g., compared to a conventional preparation suffering from attenuation and/or scattering due to inhomogeneities in the corneal stroma. 
         [0023]    The radiation may be focused at a sequence of points in a cutting plane (surface) for the bed cut. Subsequent points of the sequence may be spaced apart to allow dissipation of locally introduced heat. 
         [0024]    The bed cut may be spaced apart from the Descemet membrane by any value in the range from 5 μm to 50 μm. The distance between the Descemet membrane and the bed cut may be selected depending on a wavelength of the radiation. The distance may be selected to be anywhere in the range from 10 μm to 50 μm for infrared radiation. The distance may be selected to be anywhere in the range from 5 μm to 20 μm for ultraviolet radiation. Herein, the Descemet membrane may be considered as a layer distinct from the Dua layer or as a structure that also comprises the Dua layer. 
         [0025]    An energy density of the radiation may be below a cutting threshold in any one of the Dua layer, the Descemet membrane (optionally including a structure that is also referred to as Dua layer) and the corneal endothelium. The energy density of the radiation may exceed the cutting threshold inside the corneal stroma. 
         [0026]    The mounting may include inverting a natural curvature of the donor cornea. The natural shape of a human cornea may be that of a bowl, with the epithelium on the outer side of the bowl and the endothelium on the inner side. The natural curvature may be inverted due to a shape of the support unit and/or manually or by other means prior to mounting the donor cornea at the support unit. The mounting may include clamping a marginal area of the donor cornea. 
         [0027]    The cutting laser interface may include a transparent contact surface facing towards the support unit. The contact surface may be planar or concave. The bed cut may extend at least substantially parallel to the contact surface. A distance between the contact surface of the cutting laser interface and the focal points defining the bed cut may be in the range of 20 μm to 100 μm, preferably 30 μm to 90 μm. 
         [0028]    The cutting laser interface and the support unit may be arranged on an optical axis. The optical axis may intersect the Descemet membrane substantially perpendicularly. The bed cut may extend parallel to at least one of the Dua layer, the Descemet membrane and the corneal endothelium. 
         [0029]    The instructions may further control a circumferential cut through the donor cornea. The circumferential cut may be at least substantially perpendicular to the contact surface. The circumferential cut may range from one side of the corneal graft facing the contact surface to at least the bed cut, e.g., to the other side of the corneal graft facing the support unit. 
         [0030]    The bed and circumferential cuts may separate the graft from at least a part of the corneal stroma. Alternatively, the bed and circumferential cuts may substantially separate the graft from the corneal stroma except for a tearable connection with the corneal stroma. The tearable connection may include one, two or three laterally separated connections. Each of the laterally separated connections may have a cross section of 100 μm×100 μm or less, or may have any other geometrical shape (e.g., an ellipse or circle) of corresponding size. Alternatively or in combination, the corneal stroma may remain connected with the graft along a circumferential line, preferably a circle. The circumferential line may be closed or perforated. A thickness of the circumferential line, e.g., in a radial direction, may be less than 50 μm, preferably less than 20 μm. 
         [0031]    The support unit may include a fluid-filled chamber supporting the corneal stroma. The fluid-filled chamber may be an artificial anterior chamber. The chamber may include a recess, e.g., in an at least partially planar surface of the support unit facing towards the cutting laser interface. The recess may be covered by the mounted donor cornea. E.g., the recess and the mounted donor cornea may enclose the chamber. The chamber may be formed by the recess and the mounted donor cornea. A boundary region of the donor cornea may circumferentially abut against the surface, e.g., so as to seal the chamber. 
         [0032]    The recess may include one or more openings for injecting air, a balanced salt solution and/or a viscoelastic. The chamber may be pressurized using a fluid, e.g., air. The pressurized donor cornea may abut against the contact surface. The pressure may define a contact pressure of the graft abutting against the contact surface. 
         [0033]    A method of generating a control program for preparing an endothelial graft including a Descemet membrane from a donor cornea, e.g., isolated form a dead body, and having stromal tissue is also provided. The method comprises the steps of determining a thickness of any one or any combination of a corneal endothelium, the Descemet membrane and a Dua layer of the donor cornea; and generating a control program for a control unit configured for controlling a focus of radiation according to the control program, the control program including instructions that depend on the determined thickness and, when executed by the control unit, cause the radiation to pass through the Descemet membrane and to focus within the corneal stroma at focal points defining a bed cut within the corneal stroma that is at least substantially parallel to the Descemet membrane. 
         [0034]    The donor cornea may be provided from a dead human body. Alternatively, the donor cornea may be provided isolated from a living body, e.g., from a living human body. 
         [0035]    Herein, the Descemet membrane may be considered as a layer distinct from the Dua layer or as a structure that also comprises the Dua layer. 
         [0036]    The thickness may be determined, e.g., by means of Optical Coherence Tomography (OCT) and/or by means Optical Low Coherence Reflectometry (OLCR). 
         [0037]    According to yet another aspect, a computer program product is provided. The computer program comprises program code for carrying out one or more of the method steps disclosed herein, when the computer program product is executed on one or more computing devices. Moreover, a computer readable recording medium is provided that stores the computer program product. The computer program product may also be provided for download to such a recording medium, e.g., via the Internet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]    In the following, the invention will be described in more detail with reference to exemplary embodiments illustrated in the drawings, wherein 
           [0039]      FIG. 1  schematically illustrates a laser system for preparing an endothelial corneal graft; 
           [0040]      FIG. 2  shows a schematic cross-sectional view including details of the laser system of  FIG. 1 ; 
           [0041]      FIG. 3  shows a flowchart for a method embodiment that can be performed by the laser system of  FIG. 1 ; 
           [0042]      FIGS. 4A ,  4 B, and  4 C schematically illustrate focal point sets for performing a bed cut and a circumferential cut in the donor graft; 
           [0043]      FIGS. 5A ,  5 B,  5 C and  5 D schematically illustrate variants of the focal point sets for preparing one or more stromal connections; and 
           [0044]      FIGS. 6 and 7  show cross-sectional views of examples of a donor cornea in an inverted orientation after creation of a bed cut. 
       
    
    
     DETAILED DESCRIPTION 
       [0045]      FIG. 1  schematically illustrates a laser system  100  equipped to allow a surgeon to prepare an endothelial graft from a donor cornea removed from a dead body. The laser system  100  comprises a cutting laser apparatus  102  and a support unit  116 . The cutting laser apparatus  102  includes a source  104  of laser radiation, focusing and deflecting optics  106 , and a cutting laser interface  108  (sometimes referred to as a patient interface, or adapter, when the cutting laser apparatus  102  is used for laser-treatment). The cutting laser apparatus  102  further comprises a control unit  110  including a control program  112 . 
         [0046]    The source  104  generates pulsed laser radiation  113  propagating along an optical axis  114 . The pulse duration of the laser pulses is in the alto-, femto-, pico- or nanosecond range. The energy density (i.e. fluence) and intensity of the pulses is suitably selected to achieve a photodisruption (which includes a laser-induced optical breakdown) in human corneal tissue as a result of the irradiation of a particular tissue location with a pre-defined number of pulses (one or more). The focusing and deflecting optics  106  includes a scanning device (including e.g., one or more galvanometer mirrors or adaptive mirrors) for laterally deflecting the radiation  113 , i.e. in a direction transverse to the optical axis  114 . The focusing and deflecting optics  106  is further configured to focus the radiation at positions closely below the cutting laser interface  108 . The focusing and deflecting optics  106  may be electrically connected, e.g., by wire or wirelessly, to the control unit  110 , which controls the focus position and the deflection of the radiation  113  according to instructions of the control program  112 . 
         [0047]    The radiation  113  is directed through the cutting laser interface  108  towards the support unit  116 , which is configured for inversely supporting a donor cornea  118 . The support unit  116  is arranged on a basis  120 , which is height-adjustable (as represented by a double-arrow  122 ). A line  124  provides a fluid connection into an artificial anterior chamber  126 . The chamber  126  supports the inversely mounted donor cornea so that an endothelium of the donor cornea  118  faces the cutting laser interface  108 . 
         [0048]      FIG. 2  schematically illustrates a cross-sectional view  200  of the cutting laser interface  108 , the mounted donor cornea  118  and the support unit  116  in a section including the optical axis  114 . 
         [0049]    The donor cornea  118  is inversely arranged at the support unit  116 . The pressure in the chamber  126  causes the donor cornea  118  to planarly abut with its posterior side (i.e. endothelial side) against a reference contact surface  109  of the cutting laser interface  108 . The reference contact surface  109  may be formed, in a manner generally known per se, from a contact member (e.g., applanation plate, contact lens) which is transparent to the laser radiation  113 . Optionally, an enclosure  111  circumferentially encloses the contact surface  109 . One end of the enclosure  111  is mounted at a conical surface of the cutting laser interface  108 . Another end of the enclosure  111  is resilient and contacts a flat upper surface of the support unit  116 . A vacuum is applied to the volume enclosed by the enclosure  111  to further facilitate the planar contact between the donor cornea  118  and the contact surface  109 . 
         [0050]    The support unit  116  includes a clamping ring  210  and a socket  212 . The socket  212  includes a recess  214  in an upper surface of the socket  212 , e.g., a surface of the socket  212  closest to the contact surface  109 . The line  124  fluidly connects an inlet opening at or outside the socket  212  with an opening in the recess  214 . The inlet opening is external to the chamber  126 . The opening in the recess is inside the chamber  126 . A circumferential edge  216  in the upper surface of the socket  212  is configured for receiving the clamping ring  210 . The donor cornea  118  is mounted at the support unit  116  by clamping a boundary area of the donor cornea  118  between the clamping ring  210  and the socket  212 . 
         [0051]    The donor cornea includes at least a Descemet membrane  204  and corneal stroma  208 . The endothelial graft to be prepared includes at least the Descemet membrane  204 . 
         [0052]      FIG. 3  shows a flowchart of a method  300  for preparing an endothelial graft including at least a Descemet membrane from a donor cornea that further includes at least corneal stroma. In a step  310  of the method  300 , a support unit is provided for supporting a donor cornea that includes the Descemet membrane and the corneal stroma. In a step  320 , a cutting laser apparatus is further provided including a cutting laser interface for directing radiation towards the support unit. The donor cornea is mounted at the support unit in a step  330 . The donor cornea is inversely mounted, i.e., in an orientation so that at least the Descemet membrane is arranged between the corneal stroma and the cutting laser interface. In a step  340 , a laser cut within the corneal stroma is performed using the cutting laser interface  108 . 
         [0053]      FIG. 4A  schematically illustrates a cross-sectional view of the laser cut performed according to the step  340  by the radiation  113  transmitted through the cutting laser interface  108 . The viewing plane of  FIG. 4A  includes the optical axis  114 . The step  340  is controlled by the control unit  110 . The donor cornea  118  includes an endothelium  202 , the Descemet membrane  204 , a Dua layer  206  and the corneal stroma  208 . The endothelium  202  abuts against the contact surface  109  of the cutting laser interface  108 . 
         [0054]    The radiation  113  is focused at a sequence of exemplary focal points  401  within the corneal stroma  208 . The sequence of focal points defines a bed cut  402  that is, at least substantially, perpendicular to the optical axis  114  and completely inside the corneal stroma  208  of the donor cornea  118 . 
         [0055]    The bed cut  402  is spaced apart from the Descemet membrane  204  (or the Dua layer  206 ) by a safety distance  403 . The safety distance  403  prevents damage caused by cavitation shockwaves due to cavitation bubbles created by the radiation  113  at the focal points  401 . The safety distance  403  depends on the fluence, i.e. the pulse energy used for the radiation  113 . The technique disclosed herein can be realized using, e.g., an infrared (IR) femtosecond laser or an ultraviolet (UV) femtosecond laser. For IR radiation  113 , the safety distance  403  is larger than the safety distance used for UV radiation  113 . For example, the safety distance  403  is in the range of 10 μm to 50 μm for IR radiation  113 . For UV radiation, the safety distance  403  is in the range of 5 μm to 20 μm. 
         [0056]      FIG. 4B  schematically illustrates a cross-section of a graft  400  as a result of the method  300 . The cross-sectional view in  FIG. 4B  shows the graft  400  in the everted configuration used for the step  340 . 
         [0057]    The method  300  may further include performing a circumferential cut  404  from a backside of the donor cornea  118  to a frontside of the donor cornea  118 . The circumferential cut  404  is indicated by a dashed line in  FIGS. 4B and 4C . The circumferential cut  404  is preferably rotationally symmetric with respect to the optical axis  114 . The boundary defined by the circumferential cut  404  may include straight lines and/or kinks. E.g., the circumferential cut  404  can be performed and shaped as described in document WO 2012 076033 A1. The circumferential cut  404  is also referred to as side cuts. In one implementation of the method  300 , a conical circumferential cut  404  is performed. The conical cut  404  may, in a cross-section including the optical axis  114 , include a predefined angle with the optical axis  114 . The angle may be in a range of 5° to 40° with respect to the optical axis  114 . In another implementation of the method  300 , the circumferential cut  404  has Z-shaped line segments in a cross-section including the optical axis  114 . Corresponding Z-shaped lines may be provided in an eye receiving the graft for a positive fit and improved healing. 
         [0058]      FIG. 4C  schematically illustrates the graft  400  in a viewing plane perpendicular to the optical axis  114 . A hatched area represents the cut  402  within its circular border line  402 ′, which is also indicated in the cross-sectional view of  FIG. 4B . 
         [0059]    In one embodiment of the apparatus  100  and the method  300 , the circumferential cut  404  is performed by means of the cutting laser apparatus  102  according to further instructions included in the control program  112 . Alternatively, the circumferential cut  404  is performed by means of a cornea button punch or a radial vacuum trephine. 
         [0060]      FIG. 5A  shows a cross-sectional view of a variant of the bed cut  402  performed in the step  340 . The bed cut  402  is performed similar to the bed cut  402  described with reference to  FIG. 4A  except for one or more connections  406  that remain uncut for connecting the endothelial graft  400  and the stroma  208 . When the circumferential cut  404  crosses the bed cut  402  between the connection  406  and the border line  402 ′ of the bed cut  402 , as shown in  FIG. 5B , the resulting graft  400  remains connected via a minimal bridge connection  406  close to or at the boundary of the graft  400 , as is shown in  FIG. 5C . 
         [0061]      FIG. 5D  schematically illustrates a cross-sectional view of a further variant of the bed cut  402  for a minimal point connection  406 . The point connection  406  can be arranged close to or at the boundary of the graft  400 . 
         [0062]    While the exemplary bed cuts  402  shown in  FIG. 5A to 5D  include a boundary cut  402 ′, the connection  406  alternatively coincides with the boundary  404  of the graft  400 . For example, the circumferential cut  404  defining the boundary of the graft  400  passes through the connection  406  without crossing the bed cut  402 . For the benefit of a precise relative arrangement of the bed cut  402  and the circumferential cut  404 , the circumferential cut  404  is also performed by the cutting laser apparatus  102  performing the bed cut  402 , e.g., while the donor cornea  118  stays inversely mounted at the support unit  116 . 
         [0063]    The endothelial graft  400  is a thin lamella, e.g., 30 μm to 90 μm in thickness.  FIGS. 6 and 7  show cross-sectional views of the donor cornea  118  after performing the bed cut  402  according to the step  340 . The viewing plane in  FIGS. 6 and 7  includes the optical axis  114 . In the Optical Coherence Tomography (OCT) images  600  and  700 , the endothelial graft  400  is visible as a curved white line at the upper surface of the cut donor cornea  118 . The curved white line indicating the endothelial graft  400  at the endothelial side of the donor cornea  118  includes the endothelium  202  and the Descemet membrane  204 . 
         [0064]    The OCT image  600  shows the graft  400  separated from the donor cornea  118  along the bed cut  402  performed using IR radiation  113 . The graft  400  shown in the OCT image  700  is prepared using UV radiation  113 . 
         [0065]    The inverted curvature configuration of the cut donor cornea  118  shown in the OCT images  600  and  700  can be seen from the formation of folds at the anterior side (shown as the bottom surface in the OCT images  600  and  700 ) of the corneal stroma  208 . 
         [0066]    Distance values shown in  FIGS. 6 and 7  above the upper surface of the graft  400 , i.e., near the endothelial side of the graft  400 , indicate an arc length from the optical axis  114 . Negative values are used on the left-hand side and positive distance values on the right-hand side relative to the optical axis  114 . 
         [0067]    Below the bottom surface of the donor cornea  118 , pairs of thickness values are shown. The upper smaller value indicates the thickness of the endothelial graft  400  measured at the lateral position indicated by the corresponding straight line. The greater value (shown below the graft thickness value) indicates the thickness of the corneal stroma  208  included in the donor cornea  118  and separated from the endothelial graft by the bed cut  402 . For the exemplary cut  402  shown in the OCT image  600 , a graft thickness of 65 μm and a stroma thickness of 1103 μm have been observed at the lateral position −2.31 mm, a graft thickness of 62 μm and a stroma thickness of 1095 μm have been observed at the lateral position −1.22 mm, and a graft thickness of 65 μm and a stroma thickness of 1193 μm have been observed at the lateral distance +2.50 mm. The separation between stroma  208  and endothelial graft  400  is visible at the lateral position +2.50 mm as a black stripe in the OCT image  600 . 
         [0068]    For the exemplary cut  402  shown in the OCT image  700 , a graft thickness of 39 μm and a stroma thickness of 944 μm have been observed at the lateral position −2.41 mm, a graft thickness of 34 μm and a stroma thickness of 919 μm have been observed at the lateral position +1.10 mm, and a graft thickness of 35 μm and a stroma thickness of 910 μm have been observed at the lateral position +2.88 mm. The separation due to the bed cut  402  is visible as a black stripe in the OCT image  700  at the lateral positions −2.41 mm and +1.10 mm. 
         [0069]    As has become apparent based on above exemplary embodiments of the method  300  performed by the apparatus  100 , an endothelial graft can be prepared by a precise bed cut independently of optical inhomogeneities in the corneal stroma. One or more small connection structures (e.g., connection bridges) at or near the boundary of the graft to the stroma can ensure that the prepared endothelial graft does not accidentally separate from the support unit. Furthermore, one or more connections structures can indicate the orientation of the endothelial graft. E.g., after the preparation of the graft, the connected stroma can indicate simply by visual inspection that the opposite side includes the endothelium of the graft, which is otherwise not visible. 
         [0070]    The technique can also prepare an endothelial graft that is later applicable for Posterior Lamellar Keratoplasty, including Deep Lamellar Endothelial Keratoplasty (DLEK). E.g., the instructions in the control program may specify a thickness of the endothelial graft, so that a corneal stroma layer thicker than the safety distance is included in the endothelial graft.