Patent Publication Number: US-2023148378-A1

Title: Method for determining the position of a flap cut on a cornea and of the hinge belonging to the flap, and device for carrying out a method of this type

Description:
RELATED APPLICATION DATA 
     This application is a U.S. national stage of and claims priority to prior filed international application no. PCT/EP2021/057495 filed Mar. 21, 2021 and which claims priority to a German national application no. 10 2020 110 041.8 filed Apr. 9, 2020. The entire contents of these prior filed applications are hereby incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field of the Disclosure 
     The present disclosure relates to a method for determining the position of a flap incision on a cornea and of the hinge associated with the flap, and to a device for carrying out a method of this type. 
     2. Description of the State of the Art 
     In refractive surgery of the eye, the overall refractive power of the eye is to be changed in s such a way that conventional corrective devices, such as spectacles or contact lenses, can be dispensed with thereafter. Among various intervention techniques, the lamellar methods, and among those, in particular, the LASIK method (“Laser in situ Keratomileusis”) have been established as widely-used methods. To achieve the desired change in the curvature of the cornea, tissue is removed from the inside of the cornea. 
     Access to the tissue to be ablated is enabled by an incision in the cornea. To do this, a cornea lamella, which comprises the external epithelial layer and the underlying Bowman&#39;s membrane are detached by an approximately circular to slightly elliptical incision not quite closed in its circumference. Since the incision on the cornea is not quite closed along its circumference, the lamella, then referred to as a “flap”, remains attached to the cornea and can be folded back to one side. The underlying stroma is then exposed to be treated with the laser to create the desired reshaping. 
     After treatment, the flap can be folded back into place, covering the stroma again. The diameter of the flap is chosen such that it comprises the optical zone of the cornea and is usually about 8 mm-9.5 mm in diameter. The thickness of the lamella is about 90-160 μm. 
     The position at which the flap remains attached to the cornea, is called a “hinge”. The position of the hinge is usually chosen to be superior, i. e., at the top, or nasal. Depending on the choice of the position of the hinge, the position at which the folded-back flap comes to lie during the subsequent laser ablation also changes. 
     Since it remains common to create the flap by means of a microkeratoma or by means of a laser different from the ablation laser used for reshaping the stroma surface, the ablation laser is ignorant of the position of the hinge or the flap. To avoid damaging the hinge and the flap during the ablation process, the hinge and the flap are usually covered by a material impenetrable to the ablation laser. This can be, for example, a sponge or a metal spatula. 
     This approach has several drawbacks: additional instruments come into contact with the eye unnecessarily, and the risk of infection is increased. The surgeon can be imprecise in their work when covering the flap, or the eye moves relative to the cover during the procedure. This can lead to the flap being undesirably ablated if it is insufficiently covered, which can disadvantageously affect the ultimate visus. When too much of the flap is covered, part of the otherwise exposed stroma is undesirably covered and is not reached by the ablation treatment. This can also disadvantageously affect the ultimate visus. 
     SUMMARY 
     It is an object of the present disclosure to avoid or at least mitigate the above-mentioned drawbacks. In particular, it would be desirable if the protection of the hinge and of the flap would not require the surgeon&#39;s attention and thus could be carried out in an easily reproducible manner. 
     The object is achieved by a method according to the independent method claim. 
     The method according to the present disclosure serves to determine the position of a flap incision on a cornea and of the hinge associated with the flap and comprises the steps of: capturing an image of a cornea, the image at least comprising the limbus; determining a region comprising the outer circumference of the flap incision and determining the position of the flap incision; or/and determining a straight region comprising the hinge and determining the position of the hinge. Thus, either only the position of the flap incision, only the position of the hinge or both the position of the flap incision and the position of the hinge can be determined. 
     By capturing one or more images, digital images for example, the image contents can be processed. This usually requires a lot of computing power and can therefore not take place at the required speed, with which the position of the eye can change. Therefore, according to the present disclosure, the outer circumference of the flap incision is first determined within the captured image. The computationally demanding further processing of the digital image data can subsequently be carried out based on the determined circumference. This enables a substantial reduction of the data to be processed and thus a substantial increase in speed. 
     In accordance with one embodiment, when a plurality of images is captured, the captured images can be averaged. Alternatively, or additionally, when there is a plurality of images, a change in the reflexes on the edges of the incision from one image to another can be considered. 
     In an embodiment of the present disclosure, it is provided that the step of determining a region including the outer circumference of the flap comprises determining an approximately circular or annular region as the region including the outer circumference of the flap incision. This is based on the assumption that the flap incision is essentially circular. 
     When determining the position of the flap incision, or the position of the outer circumference of the flap incision, a circular or annular region can thus already be assumed. It may be preferred, in particular, to assume neither a circle nor an ellipse for the region including the outer circumference of the flap incision, but a learned-in, specific approximately circular shape. This substantially reduces the number of image regions to be considered and thus substantially increases the processing speed. 
     In a further development of the inventive idea, the step of determining a straight region including the hinge comprises limiting the possible position of the straight region to a region within the circular region. The hinge—thus the region of the cornea in which the flap is moveably attached to the rest of the cornea—can only be situated within the already determined circular region. Limiting the possible position of the hinge to a region within the already found circular incision region in turn reduces the amount of data to be processed and increases the processing speed. 
     In an embodiment, it can be provided that the step of determining the position of the hinge comprises comparing the currently determined position to the previous position of the hinge. Once the position of the hinge has been determined, when the position of the hinge is determined again at an immediately following point in time, this position to be newly found can only be situated within a determined region which extends around the previous position of the hinge. In other words, the position of the hinge cannot change in an abrupt fashion. 
     In particular, when determining the position of the hinge, the time can be considered which has elapsed between the previous position and the currently determined position of the hinge. Depending on the time period between the position of the hinge currently to be determined and the last previously determined position, the region to be considered for determining the new position can be larger or smaller. 
     In one embodiment, when determining the position of the hinge the eye movements performed between the previous and the currently determined position of the hinge or still being carried out, are considered. Particularly, the above-mentioned framework conditions, thus, for example the time, the previous position and/or the eye movements, are combined. 
     In one embodiment, it can be provided that the determination or detection of the hinge is newly performed for each new image. Alternatively, it can be provided that the hinge is detected a single time prior to the start of the treatment and then the position of the hinge is indicated based on the translatory and rotatory eye movements, for example of the pupil, the iris and/or the limbus. 
     In one embodiment, it can also be provided to newly determine the position of the hinge only after expiry of a predetermined period of time or after capturing a predetermined number of images and meanwhile to determine the position of the hinge based on the tracked eye movements. 
     In a further development of the present disclosure, when determining the position of the hinge, the ratio of the areas is considered, in which the straight region—representing the position of the hinge—subdivides the circular region in the manner of a secant. The division ratio reflects the provided hinge-flap geometry which remains equal with the same cutting technique and, in particular, is also independent of the actual size of the flap and the associated hinge. The absolute size of the flap or of the hinge can vary depending on the size of the optical zone of the cornea, with the division ratio remaining about equal, however. 
     The object is also achieved by a device according to the independent apparatus claim. It relates to a device for determining the position of a flap incision on a cornea and of the hinge associated with the flap, the device comprising a camera for capturing an image of a cornea, an image processing unit for real-time analysis of the image, and a control unit, which is adapted to perform one or all of the afore-mentioned methods. The camera can additionally comprise an illumination unit. 
     Overall, the method according to the present disclosure and the device according to the present disclosure provide the following advantages: 
     By avoiding any other instruments touching the eye, the risk of infection is reduced. The treatment result is improved because the position of the flap/hinge is detected and thus only the relevant region is treated. 
     Imprecise covering of the region by the surgeon by means of the instrument is avoided: because, if too little is covered by means of manual covering, there is a risk of the flap being at least partially ablated. This leads to the postoperative visus being negatively affected. However, if too much is covered—thus also part of the surface to be treated—a region of the optical zone is only insufficiently treated. This also leads to the postoperative visus being negatively affected. 
     Overall, a smaller number of pulses are needed due to the specifically tailored treatment so that the treatment can be carried out more quickly. Among other things, this reduces the issues involved with the eye drying out during the operation. 
     Furthermore, the surgeon will be less burdened and can therefore better focus on the procedure and better care for the patient since they no longer need to cover the flap with the aid of an instrument. 
     When the surgeon&#39;s instruments, hand or arm for covering the flap in the conventional manner interfere with the field of view of cameras or the optical path of the illumination, there is a risk that by shadowing the illumination the eye tracking will be compromised or will fail completely. When it is no longer necessary to cover the flap, this risk is eliminated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, embodiments of the present disclosure are described as examples with reference to the drawings. In the drawings: 
         FIG.  1    schematically shows an eye to be treated; 
         FIG.  2    shows the eye of  FIG.  1    with a flap incision indicated as a dash-dot line; 
         FIG.  3    shows the eye of  FIG.  3    with the flap folded back; 
         FIG.  4    shows the eye of  FIG.  3    including an indicated circular region and a linear region; 
         FIG.  5    shows the eye of  FIG.  4   , including a detected circular incision line and a linear hinge line; and 
         FIG.  6    shows, in strongly schematized fashion, a device suitable for carrying out the method. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG.  1    illustrates, in a schematic representation, an eye  1  to be treated. The illustration of  FIG.  1    shows the upper lid  3 , the lower lid  5  and the eyeball  7 . The sclera  9 , the limbus  11  as a transition to the cornea  13 , and the iris  15  are schematically shown on the eyeball  7 . The pupil  17  is indicated within the iris  15 . 
     For carrying out a LASIK operation, as initially described, the cornea  13  is incision in an approximately circular shape to gain access to the stroma which is to be reshaped in a subsequent step. The situation after the process of cutting is shown in  FIG.  2   . The incision can be performed, for example, mechanically by means of a microkeratoma or optically by means of a femtosecond laser. The circumferential line  19  of the incision is indicated as a dashed line in  FIG.  2   . The incision itself is essentially along a plane of which the normal vector is parallel to the direction of light impingement. 
     As shown in  FIG.  2   , the cornea  13  is not fully sectioned, rather, the incision ends at points  21 ,  23 . There is therefore a rest that remains attached to the cornea  13 . This rest will later form a kind of joint  25  (a “hinge”), at which the lamella  27  formed by the incision—referred to as a “flap”—can be folded back. This will be described with reference to subsequent  FIG.  3   . 
       FIG.  3    illustrates the state in which the cut-away flap  27  (shown here in cross-hatching) has been folded back. Since, in the example shown, the hinge  25  is nasally situated, the flap  27  will also come to lie in a nasal position. The now openly accessible stroma layer  29  (also shown in cross-hatching) can now be treated. 
     Both the stroma  29  and the underside of the flap  27  have a reflectivity different to that of the cornea surface. While the outer cornea surface is relatively smooth and has good reflectivity, the stroma surface is relatively dull. 
     If the flap incision is performed by a microkeratoma or by means of a different laser than the actual ablation laser, the precise position of the flap incision or the exposed stroma  29  is not known during the ablation treatment. The position of the hinge  25  is of particular relevance for the position of the exposed stroma region  29 . As already initially mentioned, the position of the hinge can be nasal or superior, for example. The hinge can also have a slightly varying length with the flap incision radius remaining equal, i. e., its position can vary relative to the flap incision center. 
     To protect the hinge  25  against laser radiation during the ablation process, it is provided according to the present disclosure to detect the position of the hinge  25  based on a camera image and to consider this position during the ablation. 
       FIG.  4    illustrates, in a strongly schematized manner, an image  100  of the eye  1 , which at least shows the limbus  11 . To keep the representation from being overburdened, in the description of the image of  FIG.  4    the structures shown will be given the same reference numerals as the structures themselves. 
     The method according to the present disclosure for determining the position of the flap incision and of the hinge  25  will be described in the following. 
     In a first step, in processing the image  100 , the position of the outer circumferential line  19  is determined. To reduce computing overhead during this step, it is assumed that the circumferential line  19  is situated in an almost annular region. This is indicated in  FIG.  4    by a circular ring  102  in a cross-hatched area. The radial position of the circular ring  102  and the radial extension of the same can be derived from various parameters, such as those that can be derived from the image  100  itself, such as the absolute size of the iris  15 , or the limbus  11 , or those that can be externally provided to the algorithm, such as information on the way in which the flap  27  was created (microkeratoma/laser), the implements used, or, for example, parameters to be input or determinable by the surgeon. 
     In a second step, the position of the hinge  25  is determined by means of a linear region  104 . This linear region  104  is also used at first to limit the possible image regions to be processed. The position of the linear region, just like the position of the circular region  102  before, can be made dependent on parameters, such as the preferred positioning of the hinge  25 , the way in which the hinge  25  was created, or other parameters, such as can be or must be input by the surgeon. 
     The position and extension of the linear region  104  can be determined independently from the previous determination of the circular region  102  or of the outer circumferential line  19  of the flap incision. It may, however, be preferred if the determination of the position and/or the extension of the linear region  104  is determined in dependence on the determination of the outer circumferential line  19 . For example, the ratio with which the hinge position subdivides the circle defined by the outer circumferential line  19 , can be used as a criterion. This is shown in the subsequently described  FIG.  5   . 
       FIG.  5    illustrates, in a strongly schematized manner, the result of the determination of the position of the outer circumferential line and of the hinge of a flap incision. In  FIG.  5   , as a result of the determination of the outer circumferential line  19  a dashed circular line  106  is indicated. Furthermore, as a result of the determination of the position of the hinge  25 , a dashed straight line  108  is indicated. As already mentioned, in determining the position of this straight line  108 , the above-mentioned division ratio can be considered. This is illustrated in  FIG.  5    by the distances  110 ,  112  provided with arrows at either end. For the computation of the division ratio, as shown in  FIG.  5   , the length of the two distances  110 ,  112  or the surface areas of the areas crossed by the two distances  110 ,  112  can be used. 
     For computing the length of the hinge  25  and thus, among other things, the position of the straight line  108 , apart from the criteria mentioned above, further criteria can be used to improve the computation result and to reduce the computational overhead. 
     In the current determination of the position of the hinge  25 , it can be considered that the position currently to be determined of the hinge  25  can only deviate from the previously determined position of the hinge  25  within certain limits. The limits can be determined, for example, based on the time elapsed since the previous determination of the position of the hinge  25 . Further parameters to be included in the computation can be, for example, the eye movements having occurred and/or occurring between the time point of the last determination of the position of the hinge  25  and the current point in time. 
     Furthermore, it can be considered that with certain surgical techniques or/and surgeons the hinge  25  always has a certain orientation (nasal, superior etc.) or has a certain division ratio. 
       FIG.  6    illustrates, in a strongly schematized manner, a device  200  for determining the position of a flap incision on a cornea  13  of an eye  1  and of the hinge  25  associated with the flap. The device  200  comprises a camera  202  for capturing an image  100 , as shown, for example, in the  FIGS.  4  and  5    as described above. The camera  202  is integrated into the axial optical path of the treatment unit  206 —for example a treatment laser by means of a beam divider  210 . The treatment unit  206  can be adapted, for example, to carry out a LASIK or similar operation on the eye  1 . 
     The camera  202  can also be supplemented by an illumination unit  212 , which can provide homogeneous illumination and/or can project patterns helping with the detection, such as stripes, onto the cornea. It can be advantageous, for example, for the orientation of the stripes to be perpendicular to an expected orientation of the hinge  25 . The illumination patterns can lead to better detectability of the circular incision and of the hinge  25 . In an embodiment, an illumination wavelength in the IR range is used to provide independence from the illumination settings used for the visual control carried out by the surgeon. 
     The device  200  further comprises an image processing unit  204 , which is schematically shown as separate from the camera  202  in  FIG.  6   . However, this separation only serves to illustrate the functionality. From a structural point of view, the image processing unit  204  could also be integrated in the camera  202  or be part of a larger data processing unit, such as a computer. The image processing unit  204  is adapted to process in real time an image  100  produced by the camera  202 . This means that the images provided by the camera  202  in the necessary speed can be processed and evaluated without delay. 
     Both the image processing unit  204  and the treatment unit  206  are connected to a control unit  208 . The control unit  208  can receive the evaluation parameters provided by the image processing unit  204 , such as the position of the outer circumferential line  19  and/or of the hinge  25 . At the same time, the control unit  208  is further adapted to control the treatment unit  206  by means of the evaluation parameters of the image processing unit  204 . For example, the control unit  208  can delimit the region to be ablated in a LASIK treatment based on the evaluation parameters in such a manner that the hinge  25  is not ablated.