Abstract:
A device and method for altering the refractive properties of the cornea by photodisrupting stromal lamellae involves focusing a laser beam within the stroma. To effectuate tissue alteration with minimal laser energies, the focal point of the laser beam is maintained inside the stromal lamella, rather than on an interface between layers of lamellae. To maintain the focal point inside the lamella, the bubbles that result from the photodisruption are measured using a wavefront detector. When a large bubble is observed, indicating photodisruption on an interface between layers of lamellae, the depth of the focal point, measured from the anterior surface, is adjusted to thereby resume photodisruption inside a lamella. A wavefront detector can be used to track the progress of the photodisruption procedure, providing information that can be used to update the amounts and locations of stromal tissue that must be removed to obtain the desired refractive correction.

Description:
FIELD OF THE INVENTION 
     The present invention pertains generally to ophthalmic laser surgery procedures. More particularly, the present invention pertains to laser surgical procedures which are performed to reshape or restructure the cornea of an eye by using photodisruption techniques to remove stromal tissue. The present invention is particularly, but not exclusively, useful as a method and system for focusing laser energy inside a stromal lamella to photodisrupt stromal tissue. 
     BACKGROUND OF THE INVENTION 
     It is well known that the refractive properties of the cornea can be altered by the selective removal of corneal tissue. For example, a myopic condition of the eye can be corrected by selectively removing corneal tissue from the central portion of the cornea. Similarly, a hyperoptic condition can be corrected by selectively removing corneal tissue within a peripheral ring surrounding the central portion of the cornea. 
     A general knowledge of the anatomy of the cornea of an eye is helpful for appreciating the problems that must be confronted during refractive corrections of the cornea. In detail, the cornea comprises various layers of tissue which are structurally distinct. In order, going in a posterior direction from outside the eye toward the inside of the eye, the various layers in a cornea are: an epithelial layer, Bowman&#39;s membrane, the stroma, Decimet&#39;s membrane, and an endothelial layer. Of these various structures, the stroma is the most extensive and is generally around four hundred microns thick. Additionally, the healing response of the stromal tissue is generally quicker than the other corneal layers. For these reasons, stromal tissue is generally selected for removal in refractive correction procedures. 
     In detail, the stroma of the eye is comprised of around two hundred identifiable and distinguishable layers of lamellae. Each of these layers of lamellae in the stroma is generally dome-shaped, like the cornea itself, and they each extend across a circular area having a diameter of approximately six millimeters. Unlike the layer that a particular lamella is in, each lamella in the layer extends through a shorter distance of only about one tenth of a millimeter (0.1 mm) to one and one half millimeters (1.5 mm). Thus, each layer includes several lamellae. Importantly, each lamella includes many fibrils which, within the lamella, are substantially parallel to each other. The fibrils in one lamella, however, are not generally parallel to the fibrils in other lamellae. This is so between lamellae in the same layer, as well as between lamellae in different layers. Finally, it is to be noted that, in a direction perpendicular to the layer, each individual lamella is only about two microns thick. 
     Within the general structure described above, there are at least three important factors concerning the stroma that are of interest insofar as the photodisruption of stromal tissue to effect a refractive change is concerned. The first of these factors is structural, and it is of interest here because there is a significant anisotropy in the stroma. Specifically, the strength of tissue within a lamella is approximately fifty times the strength that is provided by the adhesive tissue that holds the layers of lamellae together. Thus, much less energy is required to separate one layer of lamellae from another layer (i.e. peel them apart), than is required to cut through a lamella. The second factor is somewhat related to the first, and involves the response of the stromal tissue to photodisruption. Specifically, for a given energy level in a photodisruptive laser beam, the bubble that is created by photodisruption in the stronger lamella tissue will be noticeably smaller than a bubble created between layers of lamellae. This is important because the creation of large bubbles tends to cloud the cornea, and thereby reducing the effectiveness of wavefront analysis during the procedure. Additionally, at a given laser energy, much more tissue is photodisrupted when the laser beam is focused inside a lamella than when the laser beam is focused between layers of lamellae. 
     After the photodisruption of stromal tissue, water resorption occurs, lessening the effect of the photodisruption. For some tissues, up to 80% of the water vapor produced by photodisruption is resorbed. Thus, the present invention recognizes that photodisruption is more effective on some types of stromal tissue than others. It is also preferable to create small bubbles inside the stromal lamellae to effect a refractive change in the cornea by photodisruption. The third factor concerning the stroma is optical, and it is of interest here because there is a change in the refractive index of the stroma between successive layers of lamellae. This is due to differences in the orientations of fibrils in the respective lamella. 
     Somewhat related to the present invention, a method for finding an interface between layers of lamellae for photodisrupting using a wavefront analyzer and an ellipsometer was disclosed in co-pending U.S. patent application Ser. No. 09/783,665, filed on Feb. 14, 2001 by Bille and entitled “A Method for Separating Lamellae”. As such, the contents of co-pending application Ser. No. 09/783,665 are hereby incorporated herein by reference. In co-pending application Ser. No. 09/783,665, a procedure for creating a corneal flap for a LASIK type procedure was presented. Unlike the present invention, the goal in the creation of a corneal flap is to minimize the total amount of tissue that is photodisrupted while establishing a continuous cut of stromal tissue. 
     In light of the above, it is an object of the present invention to provide a device and method for positioning the focal point of a laser beam inside a stromal lamellae and maintaining the focal point at locations inside the stromal lamellae to photodisrupt stromal tissue and alter the refractive properties of the eye. Another object of the present invention is to provide a method for using a laser beam to photodisrupt relatively large amounts of stromal tissue with a laser beam of relatively low energy. Still another object of the present invention is to provide a method for photodisruption of stromal tissue that avoids the large bubbles and associated clouding that occurs when the laser beam is focused on tissue lying on an interface between layers of lamellae. Another object of the present invention is to provide a device and method for tracking the progress of the photodisruption procedure, providing information that can be used to update the amounts and locations of stromal tissue that must be removed to obtain the desired refractive correction. Yet another object of the present invention is to provide a method for altering the refractive properties of the cornea that is easy to perform and is comparatively cost effective. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     In accordance with the present invention, a method for altering the refractive properties of the cornea involves photodisrupting tissue at selected locations within the stroma of the cornea. Specifically, each photodisruption location is selected to preferably be inside a stromal lamella rather than at a location between lamellae. By photodisrupting a plurality of stromal lamellae in this manner, the refractive properties of the cornea can be altered at relatively low laser energies and with minimal clouding of the cornea. To photodisrupt a location inside a stromal lamella, the focal point of the laser, and consequently the laser energy, is focused inside a stromal lamella. 
     For the present invention, a wavefront detector can be used during the photodisruption procedure to track the progress of the corrective procedure. Using the wavefront detector, continuously updated information concerning the refractive properties of the cornea is provided to the surgeon during the course of the procedure. This continually changing information allows the surgeon to select the amounts and locations of stromal tissue that must be subsequently altered to obtain the desired shape for the cornea. 
     To locate the focal point inside a stromal lamella in accordance with the present invention, the laser beam is initially focused to a start point in the stroma at a depth of approximately one hundred and eighty microns from the anterior surface of the cornea. As contemplated by the present invention, the anterior surface of the cornea can be identified using a wavefront detector. Preferably, the laser beam is set to operate at an energy level that is slightly above the threshold for photodisruption of stromal tissue (i.e. slightly above approximately five microjoules for a ten micron diameter spot size). For example, the initial energy level used for the laser beam may be around six microjoules for a ten micron diameter spot. 
     Once the start point is located, tissue at the start point is photodisrupted by the laser beam to generate a photodisruptive response (i.e. a bubble is created). The size of this bubble is then measured and compared with a reference value to determine whether the bubble was created inside a lamella or between layers of lamellae. This measurement of the bubble is preferably accomplished with a wavefront detector. If it is determined that the initial bubble was created between layers of lamellae, a subsequent bubble is created at a different point in the stroma. In most cases, this subsequent bubble is created at a smaller depth from the anterior surface of the cornea than the initial bubble. The new bubble is then compared to the reference value to determine whether the new bubble was created inside a lamella. This process is continued until a bubble is created having a bubble size indicating that photodisruption is occurring inside a lamella. 
     For the purposes of the present invention, the reference value is chosen to correspond to a hypothetical gas bubble in the stroma that, as a result of photodisruption, would have a diameter of approximately fifteen microns. A condition wherein the measured bubble is less than the reference value is indicative that the photodisruption of tissue is occurring in the stronger tissue that is located on the inside of a lamella, rather than at an interface between layers of lamellae. Accordingly, further photodisruption is accomplished by maintaining the initial depth of the laser and moving its focal point to create the desired photodisruption pattern at locations inside the lamella. On the other hand, when the measured bubble is greater than the reference value, the indication is that the focal point is no longer located inside a lamella. Thus, in accordance with the present invention, the depth of the focal point is altered until the subsequent photodisruption occurs inside a lamella (i.e. until a bubble is produced that is smaller than the reference value). 
     Once a bubble is created indicating that photodisruption has occurred at a location inside a lamella, an ellipsometer can be used to detect a birefringent condition at the location. Specifically, this birefringent condition results from the orientation of fibrils in the lamella. Further, it is known that from layer to layer of lamellae there will be a birefringent change that is manifested as a change in phase of about one half degree. In accordance with the present invention, the detection of the birefringent change can indicate a change from one layer of lamellae to another. Consequently, detection of the birefringent change can be used to establish and maintain a desired focal depth in the stroma. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
     FIG. 1 is a schematic diagram, in a closed-loop feedback control format, showing the operative components of an apparatus that is useful for performing the methods of the present invention; 
     FIG. 2 is a logic flow chart of the sequential steps to be accomplished in accordance with the methods of the present invention; 
     FIG. 3 is a cross sectional view of the cornea of an eye; 
     FIG. 4 is a cross sectional view of two exemplary layers of lamellae in the cornea of an eye; and 
     FIG. 5 is a plan view of the cornea of an eye. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIG. 1, an apparatus for use in performing the methods of the present invention is shown schematically in a control loop format and is generally designated  10 . As shown, the apparatus  10  includes a laser source  12  which, preferably, is capable of generating a continuous train of ultra-short pulses, with each pulse having a pulse duration of approximately one pico-second. Specifically, it is necessary that each pulse have an energy level that is above the threshold necessary for the photodisruption of stromal tissue. Preferably, for the present invention, a laser source  12  having an energy level of approximately six microjoules per ten micron diameter spot size is provided. The apparatus  10  also includes an ellipsometer  14  that is capable of determining the birefringent properties within stromal tissue. For the purposes of the present invention, an ellipsometer of the type disclosed and claimed in U.S. Pat. No. 5,822,035, which issued to Bille for an invention entitled “Ellipsometer,” is suitable. Further, FIG. 1 shows that the apparatus  10  includes a wavefront detector  16 , such as a Hartmann-Shack sensor, which is capable of modeling a wavefront. Additionally, the apparatus  10  includes guidance optics  18  that are capable of steering and focusing a laser beam onto predetermined focal points. A power unit  20  is also provided. In combination, these components cooperate with each other to generate a laser beam  22  that is directed to a focal point in the cornea  24  of an eye  26  with a predetermined energy level. Control over this operation, to include the location of the focal point and its energy level, is made possible by using the ellipsometer  14  and the wavefront detector  16  to monitor reflected light  28  as it is reflected from the cornea  24 . 
     Referring now to FIG. 2, it will be seen that in the operation of apparatus  10 , the performance of the methods of the present invention begins by establishing a start point (action block  30 ). In FIG. 3 it will be seen that an actual start point  32  is established in the stroma  34  of cornea  24 . Specifically, the start point  32  is established at a distance  36  that is measured from the anterior surface  38  of the cornea  24  in a direction that is substantially perpendicular to the anterior surface  38 . As intended for the apparatus  10 , the exact location of the anterior surface  38  can be determined using the wavefront detector  16 , and the distance  36  can then be arbitrarily chosen to be around about one hundred and eighty microns from the anterior surface  38 . 
     Once a start point  32  has been established in the stroma  34 , action block  40  in FIG. 2 indicates that the next step in the methods of the present invention is to photodisrupt tissue at the start point  32  to create a response (i.e. a bubble in the stromal tissue). As indicated by inquiry block  41 , this response is then compared with a reference (e.g. 15 μm). If the response is greater than the reference, action block  43  indicates that the focal point should be moved from the start point  32  through a distance  42  (FIG.  4 ). This distance  42  will preferably be taken in an anterior direction (indicated by the arrow  44  in FIG. 4) and will, most likely, be less than two microns. It will be appreciated, however, that in some cases this distance  42  may be taken in a posterior direction (indicated by arrow  46  in FIG.  4 ). In either case, as this movement from the start point  32  is being accomplished, the inquiry block  41  in FIG. 2 indicates that when the response becomes less than the reference, reflected light  28  from cornea  24  can be monitored by the ellipsometer  14  to determine a birefringent reference (action block  48 ). It happens that this birefringent reference can be determined due to a variation in the orientation of tissue in the stroma  34  and will, perhaps, be best understood by reference to FIG.  4 . 
     In FIG. 4, a portion of the stroma  34  in the cornea  24  of the eye  26  is shown to include a plurality of lamellae  50 , of which the lamellae  50   a ,  50   b  and  50   c  are only exemplary. Dimensionally, each of the lamellae  50  in the stroma  34  have a depth  52  that is approximately two microns, and a width  54  that is between approximately one tenth and one and one half millimeters. Thus, the lamellae  50  each have a very thin disk shape. Anatomically, the lamellae  50  lie on top of each other in layers that extend across the cornea  24  through a distance  56  that is approximately nine millimeters. As shown in FIG. 4, the individual lamella  50  overlap to some extent and are somewhat randomly arranged. Nevertheless, they create many interface layers that, in general, are substantially parallel to each other and extend all the way across the cornea  24 . The interface  58  shown in FIG. 4 is only exemplary of the many interface layers in the cornea  24 . 
     For the purposes of the present invention, the lamellae  50  and interface layer  58  are important in two aspects. First, the birefringent properties of stromal tissue in the lamellae  50  change at the interface layer  58 . Recall, from the disclosure above, this change in birefringent properties is due to changes in the orientation of fibrils (not shown) in the lamellae  50 . Thus, by measuring the birefringent properties at different locations within the stroma  34 , it can be determined which locations are within the same layer of lamellae  50 . Second, the stromal tissue along the interface layer  58  is weaker than stromal tissue inside the lamellae  50 . Accordingly, by focusing the laser beam  22  at a location inside a lamella  50 , stromal tissue can be effectively photodisrupted at relatively higher rates and at relatively lower energy levels. 
     It happens that whenever stromal tissue is photodisrupted, a bubble is formed in the stroma  34 . For a given type of tissue, the size of the bubble that is formed will be a function of the energy level in the laser beam  22 . In this case, the higher the energy level, the larger the bubble. Further, for a given energy level, the size of the bubble that is formed will be a function of the type of tissue photodisrupted. In this case, with the same energy level, the stronger tissue inside a lamella  50  will yield a smaller bubble and the weaker tissue at an interface  58  will yield a larger bubble. With this in mind, consider the photodisruption of locations  60  and  62  shown in FIG. 4 using a laser beam  22  at the same energy level. In accordance with the discussion above, a larger bubble will result at location  60  in weaker tissue at the interface  58  between the lamellae  50   a  and  50   b . On the other hand, a smaller bubble will result at location  62  due to the stronger tissue inside the lamella  50   b . Fortunately, as used for the present invention, the respective sizes of the bubbles will serve as photodisruptive responses that can be measured by the wavefront detector  16  using relatively well known wavefront techniques. Accordingly, the photodisruptive response of a bubble can be compared with a reference value to determine whether the bubble resulted from photodisruption of a location  60  on an interface  58  or a location  62  inside a lamella  50 . 
     Returning now to FIG. 2, and in light of the above discussion with reference to FIG. 4, it will be appreciated that the combined functions of inquiry block  41  and action blocks  40  and  43  is to find a location  62  inside a lamella  50 . Upon finding a location  62  inside a lamella  50 , the ellipsometer  14  (FIG. 1) can be used to establish a birefringent reference (action block  48 ) for the location  62 . Next, once a location  62  inside a lamella  50  is found, the photodisruption of a pattern designed to correct an optical deficiency can be performed. For a typical optical correction, the pattern generally constitutes a volume of stromal tissue having a thickness of five to fifteen (5-15) lamellae. In accordance with the present invention, during the photodisruption of the pattern the focal point is maintained within a single layer of lamella  50 . Preferably, the focal point is maintained inside the lamella  50  within a single layer of lamellae. In accordance with the present invention, the refractive condition of the cornea  24  can be periodically measured, and adjustments made to the initial pattern design. These steps are shown in closed loop format by the blocks enclosed by dashed line  64  in FIG.  2 . 
     Referring still to FIG. 2, during photodisruption of the pattern, the guidance optics  18  are used to scan the laser beam  22  to a new location within the pattern (action block  66 ). Upon photodisruption at the new location (action block  68 ), the resultant bubble is compared with the reference standard bubble (action block  70 ). Thus, a determination is made whether the new location is on an interface  58  or inside a lamella  50 . For the present invention, the reference standard bubble will correspond to a hypothetical bubble in stromal tissue (not shown) which would have a diameter of approximately fifteen microns. If the resultant bubble in the stroma  34  has a photodisruptive response that is greater than the reference value, it is indicative of the fact that weaker tissue in the interface layer  58  is being photodisrupted. In this case, the focal point should be moved from the start point  32  through a distance  42  (FIG. 4) before photodisrupting the next location within the pattern. 
     Next, as shown in FIG. 2, the birefringent properties at the new location can be measured (action block  72 ) using an ellipsometer  14 , for comparison to the birefringent reference. This measurement (i.e. action block  72 ) can be used to determine whether the new location is in the same layer of lamella  50  as the location measured in action block  48 . It will happen that locations on two different layers of lamellae  50  will result in a birefringent change on the order of plus or minus one half degree. Importantly, maintaining the focal point within a single layer of lamellae  50  will fix a focal depth for the laser beam  22  that will be an approximate combination of the distances  36  and  42 . 
     Referring still to FIG. 2, for the present invention, the refractive properties of the cornea  24  can be continuously or periodically measured (action block  74 ). This measurement can be performed with a wavefront detector  16 , and is preferably performed on a portion of the cornea that is not being photodisrupted. For example, referring to FIG. 5, during corrections for hyperopia, the photodisruption is often performed in a peripheral ring  80  surrounding the central portion  82  of the cornea  24 . In this case, the refractive properties of the central portion  82  of the cornea  24  can be measured with the wavefront detector  16 . If further refractive correction is required (inquiry block  76 ) then the determinations/measurements made in action blocks  70 ,  72  and  74  can be used to update the locations and amounts of stromal tissue that require photodisruption (i.e. the pattern) to achieve the desired refractive correction (action block  78 ). With the updated pattern, the laser beam  22  is then scanned to a new location requiring photodisruption (action block  66 ), and the process enclosed by dashed line  64  is repeated until the desired refractive correction is obtained. 
     While the particular Method for Performing Refractive Surgery 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.