Abstract:
In accordance with the present invention, a system and method are provided for removing the corneal epithelium from a patient&#39;s eye while monitoring the autofluorescent response that results during laser photoablation of the epithelial tissue. Structurally, the system includes a computer-controlled laser generating unit. Also, connected to the computer are a sensor for receiving the autofluorescent response, and an imaging unit for monitoring changes in the topography during a procedure. By monitoring both the autofluorescent response and changes in epithelial topography, the computer controls the laser unit. When there is no longer an autofluorescent response, the procedure has been completed and the system is shut down.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention pertains generally to systems and methods for removing corneal epithelium tissue. More particularly, the present invention pertains to the photoablation of corneal epithelium tissue using a laser beam. The present invention is particularly, but not exclusively, useful as a system and method for corneal epithelium removal which monitors fluorescence epithelial tissue to ensure that the correct amount of tissue is removed. 
       BACKGROUND OF THE INVENTION 
       [0002]    The corneal epithelium forms a protective tissue layer on the front surface of the cornea. Structurally, the tissue of the epithelium layer is relatively soft and it is in contact with the tear film of the eye. Upon removal from the eye, the epithelium can completely replace itself from limbal stem cells within a few days with little or no loss of clarity. 
         [0003]    During some surgical procedures it is often desirable to remove the corneal epithelium or portions thereof. For example, the removal of the epithelium is a necessary step in several corneal procedures, including (but not limited to) corneal opacity or scar removal, photorefractive keratectomy for treatment of refractive errors (PRK) and the treatment of anterior basement membrane corneal dystrophy (ABMD). 
         [0004]    Heretofore, several methods have been used to remove the corneal epithelium. In general, these methods rely on chemical and/or mechanical processes. For example, the application of ethyl alcohol is often used to loosen or sever the connections that join the epithelium to the underlying Bowman&#39;s membrane and corneal stroma. Alternatively, rotating brushes, surgical knives, and other instruments have been used to remove the epithelium using mechanical means. Usually this is done by hand, and under an operating microscope. These methods are typically performed under topical anesthesia. 
         [0005]    The use of lasers to remove the corneal epithelium has several advantages over the chemical/mechanical techniques described above. These include improved patient comfort and decreased trauma to the underlying cornea. In addition, as compared with chemical/mechanical techniques, laser removal often results in decreased liberation of cellular contents and their associated inflammatory components. Also, laser removal provides a more exact matching of the desired and actual zone of epithelial removal. Further, laser removal typically results in a shorter time for re-epithelialization after surgery, and avoids risks associated with using toxic chemicals, such as ethyl alcohol, on the ocular surface. 
         [0006]    On the other hand, there are certain challenges associated with the use of lasers to remove the corneal epithelium. In particular, it is sometimes difficult to accurately determine during a laser procedure, when the epithelium has been completely removed. Two factors contribute to this difficulty. For one, the thickness of the corneal epithelium varies over the corneal surface, and varies inconsistently from eye to eye. For another, the thickness of the epithelial layer is not consistent from patient to patient. Plus, the corneal epithelium is difficult to visualize, making determination of the endpoint of removal difficult to ascertain. 
         [0007]    Because of the inherent epithelial thickness variations described above, a somewhat complex laser treatment is typically employed to remove the entire epithelial layer without disturbing the underlying tissue layers (i.e. Bowman&#39;s Membrane or the stroma). In most cases, substantial negative consequences arise if the entire epithelium layer is not removed or if the underlying tissue layers are disturbed. In particular, if too little epithelium is removed, residual epithelial tissue that is left behind can interfere with subsequent procedures. For example, if the goal is to remove all of the epithelium to create new epithelial attachments to Bowman&#39;s layer (such as with treatment of ABMD), residual epithelial tissue can cause a treatment failure. As another example, if the goal is to remove the epithelium as a component of laser refractive surgery in a PRK procedure, the residual epithelium can result in unpredictable and irregular ablation patterns, with adverse visual consequences. This is made worse by the irregular thickness of the epithelial layer, such that the subsequent refractive treatment may cause elevations and depressions in the cornea that may not be amenable to correction with current technology. 
         [0008]    Adverse consequences can also occur when tissue is inadvertently removed beyond the epithelium (i.e. in Bowman&#39;s membrane or the stroma). For example, if the removal is performed to treat ABMD, then removing the underlying layer (called Bowman&#39;s layer) can interfere with the treatment success. On the other hand, if the removal is performed as a step in a laser refractive correction (such as PRK) then an incomplete removal of the epithelial tissue can alter the refractive correction in the underlying corneal tissue and impair the visual outcome, which may require additional treatment. 
         [0009]    In light of the above it is an object of the present invention to provide a system and method for corneal epithelium removal which monitors epithelium tissue removal to ensure that the correct amount of epithelium tissue is removed. Another object of the present invention is to remove a corneal epithelium without leaving residual epithelial tissue that can interfere with subsequent procedures. Still another object of the present invention is to safely remove a corneal epithelium without disturbing underlying tissue layers such as Bowman&#39;s Membrane and the stroma. Yet another object of the present invention is to provide a system and method for removing a corneal epithelium that are easy to use and comparatively cost effective. 
       SUMMARY OF THE INVENTION 
       [0010]    In accordance with the present invention, a laser system for removing tissue of a corneal epithelium includes a laser unit for generating a surgical laser beam. Once generated, the laser beam is directed onto target tissue in the corneal epithelium to photoablate the target tissue. During the photoablation procedure, an autofluorescent response from tissue of the epithelium is monitored to determine whether residual epithelial tissue remains. 
         [0011]    In one aspect of the present invention, the autofluorescent response is induced by the surgical laser beam. In another aspect, an external light source, such as a light emitting diode (LED), provides light having a wavelength suitable for creating the autofluorescent response from the corneal epithelium. For the present invention, monitoring of the autofluorescent response can be accomplished by visual observation of the eye, or a sensor can be employed. In some cases, a display for presenting the autofluorescent response as an image can be employed. In any event, the autofluorescent tissue response is monitored during the laser procedure and laser unit output is stopped where an autofluorescent response is not detected or observed. 
         [0012]    When a sensor is used to monitor the autofluorescent response, the system can include a control unit which is operationally connected to the sensor and the laser source. With this arrangement, the control unit receives an input from the sensor and provides a control signal to the laser source. When the sensor input indicates that an autofluorescent response is detected, a control signal is transmitted to the laser source to continue photoablation. Where the sensor input indicates that an autofluorescent response is not detected, a control signal is transmitted to the laser source to discontinue photoablation. 
         [0013]    In a particular laser procedure protocol, an ablation zone in the epithelium is first identified. Next, topographical contour features on an anterior epithelial surface within the ablation zone are determined. For example, the topographical contour features can be determined using an Optical Coherence Tomography (OCT) device. Then, based on the contour features of the epithelial surface, a predetermined pathway for movement of the laser beam&#39;s focal point through tissue of the epithelium is developed. Tissue is then photoablated along the predetermined pathway. In some cases, the protocol can also include the sequential identification of another ablation zone for subsequent conduct of the protocol. Further, during the implementation of a protocol, where no autofluorescent response has been detected by the sensor, the protocol can be modified to indicate such a non-response. 
         [0014]    In one process, the predetermined pathway is layered over the ablation zone using a sequence of an n number of raster patterns. In this process, each raster pattern is positioned in a layer at a respective predetermined elevation e n  from the interface between the epithelium and Bowman&#39;s membrane of the cornea. During the event, photoablation is performed in each respective raster pattern according to contour features of the epithelial surface. Upon completion of a raster pattern at a specified contour elevation, the laser beam&#39;s focal point is selectively advanced through a predetermined contour interval distance, Δe, toward the epithelium/Bowman interface for photoablation along a successive raster pattern at the next (i.e. lower) contour elevation. 
         [0015]    In another process in accordance with the present invention, the predetermined laser point pathway is segmented with a plurality of contiguous segments. For this process, each segment includes an ablation area that is determined by topographical contour features on the epithelial surface in the ablation zone. In more geometric terms, for this process, each segment extends through the epithelium from the ablation area to the interface between the epithelium and Bowman&#39;s membrane of the cornea. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
           [0017]      FIG. 1  is a schematic of the combination of the interactive components for a system in accordance with the present invention; 
           [0018]      FIG. 2  is a top plan view of a topographical contour map of features within an ablation zone on the epithelial surface of the cornea; 
           [0019]      FIG. 3A  is a cross-section view of the epithelium and Bowman&#39;s membrane as seen along the line  3 - 3  in  FIG. 2  depicting a layered technique for performing a photoablation protocol in accordance with the present invention; and 
           [0020]      FIG. 3B  is a cross-section view of the epithelium and Bowman&#39;s membrane as seen along the line  3 - 3  in  FIG. 2  depicting a segmented technique for performing a photoablation protocol in accordance with the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    Referring initially to  FIG. 1 , a system in accordance with the present invention is shown and is generally designated  10 . For a basic embodiment of the present invention, the system  10  includes a controller  12 , and a laser unit  14  which is electronically connected with the controller  12 . Additionally, the system  10  includes an eyepiece  16  which can be used by the system user (not shown) for viewing the epithelium  20  on the stroma  22  of an eye  24 . In this combination, the controller  12  can be used to activate the laser unit  14  for the generation and control of a laser beam  26  as it is directed and focused onto the epithelium  20  of the eye  24 . As intended for the present invention, control of the laser beam  26  by the controller  12  will result in the photoablation of epithelial tissue. For the basic embodiment, the user can view the photoablation process using the eyepiece  16 . 
         [0022]    For a more automated embodiment of the present invention, the system  10  can also include an imaging unit  28  which is electronically connected with the controller  12  for viewing the epithelium  20 . More specifically, in this automated embodiment, a display  30  is also connected with the controller  12  for visually presenting images that are generated by the imaging unit  28 . 
         [0023]    An important aspect of the present invention is the capability of the system  10  to detect when a predetermined portion of the epithelium  20  has been completely removed without affecting other tissues of the eye  24 . To do this, the present invention relies on an autofluorescent response that will result when the laser beam  26  interacts with tissue of the epithelium  20 . When a sensor  32  is used for detecting the autofluorescent response, the response is transferred by the controller  12  for presentation on the display  30 . 
         [0024]    As disclosed above, the system  10  of the present invention is intended to detect an autofluorescent response that will result when tissue of the epithelium  20  is photoablated by the laser beam  26 . For one embodiment of the present invention, the detection of an autofluorescent response is accomplished by direct visual observation, such as by a user (e.g. attending physician not shown) viewing the epithelium  20  through the eyepiece  16 . In this case, the wavelength of the laser beam  26  must be capable of causing autofluorescent response, as well as performing the required photoablation. Accordingly, a laser beam  26  having a wavelength in the far violet or near ultraviolet wavelengths, will be required. For the other embodiment, the detection is accomplished by the sensor  32 . In this case, it will be necessary to employ a Light Emitting Diode (LED)  34  that is capable of inducing the autofluorescent response. A Wood&#39;s lamp could be used. 
         [0025]    Referring now to  FIG. 2 , a zone  36  of the anterior surface  37  of the epithelium  20  is shown in a top plan view. As shown in  FIG. 2 , the zone  36  includes the exemplary topographical features  38   a,    38   b  and  38   c  which are representative of typical irregularities on the anterior surface  37  of the epithelium  20 . 
         [0026]    With reference to  FIG. 3A  it will be appreciated that different elevations, e n , on the various topographical features  38  can all be referenced to the interface  40  that is located between the epithelium  20  and Bowman&#39;s membrane  42 . As is well known in the pertinent art, a cross-section view of the zone  36  can be provided by the imaging unit  28  using well known imaging techniques, such as Optical Coherence Tomography (OCT). 
         [0027]    By cross-referencing  FIG. 2  with  FIG. 3A  for purposes of disclosure, it will be appreciated that a series of contour lines, c n , with each contour line c n  having a same elevation e n  in the zone  36 , can be used to define the topographical features  38 . Further, a contour interval, Δe, between adjacent contour lines (e.g. c n  and c n-1 ) can be established. Importantly, Δe will depend on the extent to which tissue of the epithelium  20  is photoablated at a focal point of the laser beam  26 . Thus, based on Δe, n will equal the number of horizontal photoablation layers  44 , or vertical photoablation events  46 , that must be performed to remove tissue of the epithelium  20  from the zone  36 . In any event, once the contour interval, Δe, has been determined, the elevation, e n , of different contour lines, c n , can also be determined. Initially, of course, within the zone  36  the highest elevation for tissue in the topography of the epithelium  20  will be e n . For example, within this scheme, the contour line c n-2  will designate an elevation of e n-2  above the interface  40  for the third horizontal photoablation layer  44  or the third vertical photoablation event  46 . 
         [0028]    With the above in mind, and with reference to  FIGS. 3A and 3B , it will be appreciated that an operation of the present invention can be performed essentially in either of two different ways. For one (see  FIG. 3A ), tissue of the epithelium  20  can be removed by photoablating tissue in a sequence of layers  44 . In this case, the first layer  44  will be at the elevation e n  above the interface  40 . As envisioned for the present invention, for a removal of tissue by layers  44 , the pathway  48  for the focal spot of laser beam  26  will be a horizontal straight line that, typically, will be part of a raster pattern. It is to be further appreciated that the pathway  48  will then continue on a subsequent raster pattern over the layer  44 ′ at the elevation e n-1 , and so on. Another way for photoablating tissue in accordance with the present invention is by removing tissue of the epithelium  20  in vertical segments  50  (see  FIG. 3B ). In this case, the pathway  48 ′ for focal points of the laser beam  26  will be a straight, vertical line that is substantially normal to the interface  40 . Further this pathway  48 ′ will extend from the elevation e n  of the anterior surface  37  of the epithelium  20  for a segment  50  to the interface  40 . Another segment  50  can then be identified, and the process repeated, as needed. 
         [0029]    It is to be further appreciated that a combination of the horizontal and vertical photoablation procedures disclosed here can be used together if desired. Regardless how employed, the resultant autofluorescent response is monitored and, whenever there is no such response, the conclusion is that all tissue of the epithelium  20  that was above the interface  40  has been removed. Depending on the absence of an autofluorescent response, or an indication from the imaging unit  28  that epithelial tissue remains in the zone  36 , an operation of the present invention is either stopped, continued as indicated, or it is moved to another zone  36  where epithelial tissue still remains. 
         [0030]    While the particular System and Method for Removing Corneal Epithelium as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.