Abstract:
A system and method for altering the shape of a lamina of transparent material (e.g. the cornea of an eye), as it is being subjected to a transverse pressure differential, requires a computer controlled laser unit. In accordance with specified input parameters, the computer directs the laser unit to perform LIOB over predetermined surfaces within the lamina. This weakens the material for a desired reshaping of the lamina in response to the pressure differential. With respect to a perpendicular axis that is defined by the lamina, surfaces parallel to the axis (e.g. cylindrical surfaces) are separated from each other by about two hundred microns. For surfaces perpendicular to the axis, the separation is about ten microns. In each instance, the cuts that result from LIOB are only about two microns thick.

Description:
FIELD OF THE INVENTION 
     The present invention pertains generally to systems and methods for reshaping a transparent material that is being subjected to a transverse pressure differential. More particularly, the present invention pertains to systems and methods for performing cuts on predetermined surfaces inside the material, to thereby weaken the material and allow it to be reshaped in response to the pressure differential. The present invention is particularly, but not exclusively, useful for systems and methods that correct the vision of patients by weakening stromal tissue in the cornea of an eye, to allow intraocular pressure in the eye to reshape the cornea under the influence of bio-mechanical forces. 
     BACKGROUND OF THE INVENTION 
     The cornea of an eye has five (5) different identifiable layers of tissue. Proceeding in a posterior direction from the anterior surface of the cornea, these layers are: the epithelium; Bowman&#39;s capsule (membrane); the stroma; Descemet&#39;s membrane; and the endothelium. Behind the cornea is an aqueous-containing space called the anterior chamber. Importantly, pressure from the aqueous in the anterior chamber acts on the cornea with bio-mechanical consequences. Specifically, the aqueous in the anterior chamber of the eye exerts an intraocular pressure against the cornea. This creates stresses and strains that place the cornea under tension. 
     Structurally, the cornea of the eye has a thickness (T), that extends between the epithelium and the endothelium. Typically, “T” is approximately five hundred microns (T=500 μm). From a bio-mechanical perspective, Bowman&#39;s capsule and the stroma are the most important layers of the cornea. Within the cornea, Bowman&#39;s capsule is a relatively thin layer (e.g. 20 to 30 μm) that is located below the epithelium, within the anterior one hundred microns of the cornea. The stroma then comprises almost all of the remaining four hundred microns in the cornea. Further, the tissue of Bowman&#39;s capsule creates a relatively strong, elastic membrane that effectively resists forces in tension. On the other hand, the stroma comprises relatively weak connective tissue. 
     Bio-mechanically, Bowman&#39;s capsule and the stroma are both significantly influenced by the intraocular pressure that is exerted against the cornea by aqueous in the anterior chamber. In particular, this pressure is transferred from the anterior chamber, and through the stroma, to Bowman&#39;s membrane. It is known that how these forces are transmitted through the stroma will affect the shape of the cornea. Thus, by disrupting forces between interconnective tissue in the stroma, the overall force distribution in the cornea can be altered. Consequently, this altered force distribution will then act against Bowman&#39;s capsule. In response, the shape of Bowman&#39;s capsule is changed, and due to the elasticity and strength of Bowman&#39;s capsule, this change will directly influence the shape of the cornea. With this in mind, and as intended for the present invention, refractive surgery is accomplished by making cuts on predetermined surfaces in the stroma to induce a redistribution of bio-mechanical forces that will reshape the cornea. 
     It is well known that all of the different tissues of the cornea are susceptible to Laser Induced Optical Breakdown (LIOB). Further, it is known that different tissues will respond differently to a laser beam, and that the orientation of tissue being subjected to LIOB may also affect how the tissue reacts to LIOB. With this in mind, the stroma needs to be specifically considered. 
     The stroma essentially comprises many lamellae that extend substantially parallel to the anterior surface of the eye. In the stroma, the lamellae are bonded together by a glue-like tissue that is inherently weaker than the lamellae themselves. Consequently, LIOB over layers parallel to the lamellae can be performed with less energy (e.g. 0.8 μJ) than the energy required for the LIOB over cuts that are oriented perpendicular to the lamellae (e.g. 1.2 μJ). It will be appreciated by the skilled artisan, however, that these energy levels are only exemplary. If tighter focusing optics can be used, the required energy levels will be appropriately lower. In any event, depending on the desired result, it may be desirable to make only cuts in the stroma. On the other hand, for some procedures it may be more desirable to make a combination of cuts and layers. 
     As will be appreciated by the skilled artisan, transparent materials that can be altered by LIOB are susceptible to being weakened by the process. Further, if the material is formed as a lamina (i.e. it is essentially a layer of material), and if the material is subjected to a transverse pressure differential, the lamina can be reshaped when it is weakened by LIOB. In particular, the lamina will be influenced by a change in the force distribution that results from an alteration of the transverse pressure differential that is caused by selective LIOB. Under this influence, the lamina is reshaped. Thus, in a manner similar to the situation disclosed above for a reshaping of the cornea, a lamina, or layer, of transparent material can be similarly reshaped. 
     In light of the above, it is an object of the present invention to provide systems and methods for reshaping a layer of transparent material when the material is being subjected to a transverse pressure differential. Another object of the present invention is to provide computer-controlled methods for performing laser procedures on transparent material that require minimal destruction of the material. Yet another object of the present invention is to provide computer-controlled methods for altering the shape of a lamina of transparent material that are relatively easy to implement and comparatively cost effective. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a system and method for altering the shape of a transparent lamina (e.g. the cornea of an eye) requires a computer-controlled laser unit. More specifically, reshaping of the lamina is accomplished by causing Laser Induced Optical Breakdown (LIOB) on predetermined surfaces within the material, while the lamina is being subjected to a transverse pressure differential. In response to this weakening, the consequent rearrangement of the force distribution within the material will then reshape the lamina. In the specific case of ophthalmic laser surgery in the cornea, the transverse pressure differential is created by intraocular pressure from aqueous in the anterior chamber of the eye. 
     For a preferred embodiment of the present invention, a computer is electronically connected to a laser unit. With this connection, the system first identifies an axis that is substantially perpendicular to the lamina. For ophthalmic laser surgery, this axis will be the visual axis of the eye. In any event, identification of the axis is important for the purpose of establishing a reference datum that can be used to direct the laser beam that is generated by the laser unit along predetermined paths in the transparent material (cornea). 
     In operation, the laser beam is focused to a focal spot in the lamina, and the focal spot is then moved in accordance with a predetermined computer program. The purpose here is to perform Laser Induced Optical Breakdown (LIOB) on a defined surface inside the material. For one type of operation, the surface will be oriented substantially parallel to the axis. For another, the surface will be created substantially perpendicular to the axis. In the former case (i.e. when the surface is parallel to the axis) the cuts that result from LIOB may be made either on a curved cylindrical surface (i.e. cylindrical cuts), or on a flat radial surface (i.e. radial cuts). The exact nature and extent of these cuts will, of course, depend on the particular cut parameters that are input to the computer. In the latter case (i.e. when the surface is perpendicular to the axis) LIOB will create so-called “layer cuts”. Thus, in overview, the present invention envisions cylindrical cuts, radial cuts and layer cuts. 
     For cylindrical cuts (circular or oval), and for radial cuts, the cut parameters that are input to the computer include a location for a distal end of the surface (Z distal ) n . In the notation “(Z distal ) n ”, the letter “n” represents a number from 1 to “n” that identifies the particular surface. In addition to (Z distal ) n , the cut parameters also include a location for a proximal end of the surface (Z proximal ) n , a radius “r n ” measured from the axis, and an azimuthal angle “θ” measured around the axis from a base line. 
     Using the cut parameters, radial cuts result from the specific case wherein the azimuthal angle “θ” is constant. The radius “r n ” can then be varied through a range of approximately three millimeters. On the other hand, for cylindrical cuts the radius “r n ” can either be constant (to create circular cylindrical cuts), or varied along an elliptical path (to create oval cylindrical cuts). Importantly, with both the cylindrical cuts and the radial cuts a plurality of surfaces may be specified. Further, it is very important that each cylindrical surface be centered on the axis, with respective cylindrical surfaces preferably separated from each other by approximately two hundred microns. For both cylindrical and radial cuts, each cut preferably has a thickness of approximately two microns. 
     As indicated above, the present invention also envisions creating layer cuts that are oriented substantially perpendicular to the axis. Like the cylindrical and radial cuts, layer cuts are created by selectively moving the focal spot in accordance with the predetermined computer program. As is done for the cut parameters, layer parameters that define portions of the layer for LIOB need to be input to the computer. 
     For layer cuts, the layer parameters include an axial location for each layer Z m  wherein “m” identifies the particular layer. The layer parameters also include an inner diameter (d i ) m , an outer diameter (d o ) m , and an azimuthal angle θ measured around the axis from a base line. The result in this case is the LIOB of material on a plurality of annular shaped layers within the lamina (cornea). Note: when the inner diameter is zero (i.e. (d i ) m =0) the layer cut will actually be disk shaped. Importantly, like cylindrical cuts, each layer is centered on the axis. Similar to the cylindrical and radial cuts, the LIOB of the material for layer cuts results in a layer having a thickness of approximately two microns. Unlike cylindrical cuts, however, when a plurality of layers is created, adjacent layers are only about ten microns distant from each other. 
     When the system and methods of the present invention are used for ophthalmic laser surgery, it is important that the cylindrical cuts, radial cuts and layer cuts, if made, need to be confined within an operational volume. Specifically, this operational volume is confined within the stroma and extends from just below Bowman&#39;s capsule (e.g. approximately 8 microns below Bowman&#39;s) to a depth equal to about ninety percent of the cornea (e.g. to about four hundred and fifty microns below the anterior surface of the eye). Further, the operational volume extends in the stroma through a radial distance of about four millimeters. It will be appreciated that the actual boundaries of the operational volume may vary slightly. Importantly, however, LIOB should not occur in Bowman&#39;s capsule, nor should LIOB extend into the anterior chamber of the eye. 
    
    
     
       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 presentation of the system of the present invention shown in relation with the cornea of an eye; 
         FIG. 2  is a cross sectional view of the cornea of an eye; 
         FIG. 3  is a logic chart showing a relationship of the steps in a methodology for use with the present invention; 
         FIG. 4  is a schematic presentation of an operational volume in accordance with the present invention showing parameters for the creation of cylindrical cuts; 
         FIG. 5  is a schematic presentation showing parameters for the creation of radial cuts; and 
         FIG. 6  is a schematic presentation of an operational volume in accordance with the present invention showing parameters for the creation of layers. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to  FIG. 1 , an ophthalmic laser system in accordance with the present invention is shown, and is generally designated  10 . As shown, the system  10  includes a computer  12  that is electronically connected to a laser unit  14 . For the present invention, the laser unit  14  is intended to direct a laser beam along a beam path  16 , for focus of the laser beam at focal points inside the cornea  18  of an eye of a patient (not shown). It is envisioned that the laser beam will be a so-called “femtosecond” laser, and that the laser unit  14  will be capable of generating a sequence of laser pulses, wherein each pulse in the sequence has a duration that is less than approximately one picosecond. Further, it is envisioned that the laser unit  14  includes optics that will focus the “femtosecond” laser to focal spots in the cornea  18  for Laser Induced Optical Breakdown (LIOB) of tissue in the cornea  18 . According to the present invention, the computer  12  is used to control operation of the laser unit  14 , and this operation will be consistent with specified input parameters  20 . 
     Referring now to  FIG. 2 , a cross-section of a cornea  18  is shown with a representative visual axis  22 . Although the visual axis  22  will be unique for each cornea  18 , it can, nevertheless, be accurately identified. Importantly, for ophthalmic laser surgery, the operation of system  10  must be conducted with reference to the visual axis  22 . On the other hand, for a lamina of transparent material (i.e. material that is not a cornea  18 ) an axis similar to the visual axis  22  can be identified and defined for operational purposes. 
     As shown in  FIG. 2 , the present invention contemplates the identification of an operational volume  24  that is located completely within the stroma  26  of cornea  18 . In general, the operational volume  24  extends from a predetermined distance below Bowman&#39;s capsule  28  (e.g. 8 microns) to a depth in the stroma  26  that is about 90% of the distance between the anterior surface  30  and the posterior surface  32  of the cornea  18  (e.g. approximately 450 microns). Further, the operational volume  24  extends through a radial distance  34  from the visual axis  22  that is equal to about four millimeters. As indicated above, it is important for purposes of ophthalmic laser surgery that the operational volume  24  be confined to tissue within the stroma  26 . As will be appreciated by the skilled artisan, the operational volume  24  in the cornea  18  is influenced by pressure exerted against the cornea  18  by aqueous fluid in the anterior chamber  36 . 
     Operation 
     For the operation of the system  10  of the present invention, the action block  38  in  FIG. 3  indicates that the first task to be performed is the location of the axis  22 . Specifically, in the case of ophthalmic laser surgery, the axis  22  will be a visual axis. On the other hand, for a lamina of transparent material (i.e. not tissue), the axis  22  can be defined as required. Typically, however, the axis  22  will be generally perpendicular to the lamina and, therefore, similar to the orientation of a visual axis  22  relative to a cornea  18 . 
     Once the location of the axis  22  has been verified for the system  10  (see inquiry  40  in  FIG. 3 ), it is necessary for the computer  12  to determine whether “cuts” or “layers” are to be created by LIOB. If inquiry  42  indicates that “cuts” are to be made, the computer  12  retrieves the appropriate input parameters  20  in accordance with action block  44 . In this case, the input parameters  20  will include (z distal ) n , (z proximal ) n , radius “r n ” and an azimuthal angle θ. Specifically, (z distal ) n  and (z proximal ) n  are established at different distances from a same datum (see  FIG. 4 ). And, the radius “r n ” is selected at a distance from the axis  22 , while the azimuthal angle θ is measured around the axis  22 . With these input parameters  20 , the system  10  can then perform LIOB on either cylindrical cuts  46  (see  FIG. 4 ) or radial cuts  48  (see  FIG. 5 ). 
     In  FIG. 4 , the cylindrical cuts  46   a  and  46   b  are only exemplary. For these cylindrical cuts  46   a  and  46   b , as with others, each will have its own (z distal ) n , and its own (z proximal ) n . As indicated there can be an “n” number of cylindrical cuts  46 , but all must be centered on the visual axis  22 . Thus, the cuts  46  will be parallel to each other and also parallel to the axis  22 . If the radius “r n ” is constant, the cylindrical cuts  46  will be circular cylindrical cuts  46 . On the other hand, if the radius “r n ” is varied along an oval path, the cylindrical cuts  46  will be elliptical cylindrical cuts  46 . Further, the azimuthal angle θ can extend through a complete 360° arc or be divided into desired segments. As intended for the system  10  of the present invention, the azimuthal angle θ is measured from a common base line  50  (see  FIG. 5 ). 
     With reference to  FIG. 5  it will be appreciated that when a constant azimuthal angle θ is selected and maintained, while the radius “r n ” is allowed to change through a pre-selected range between an inner radius “r i ” and an outer radius “r o ”, radial cuts  48  can be created. Specifically, as shown in  FIG. 5 , the radial cut  48   a  is made at an azimuthal angle θ 2 , and the radial cut  48   b  is made at an azimuthal angle θ 1 . 
     Returning to  FIG. 3  the creation of cylindrical cuts  46  and radial cuts  48  are accomplished individually as indicated by action block  52 . After the creation of each cut  46  or  48 , however, the system  10  determines whether additional cuts  46  or  48  are to be made. To do this, inquiry  54  specifically questions whether all “n” cuts  46  or  48  have been made. If not, action block  56  decrements “n” and action blocks  44  and  52  create an additional cut  46  or  48  in accordance with appropriate remaining input parameters  20 . Preferably, in the case of cylindrical cuts  46 , there will be a separation distance of about two hundred microns between adjacent cuts  46 . 
     After all of the desired cylindrical cuts  46  or radial cuts  48  have been made, inquiry  58  questions whether the system  10  requires the creation of layers  60  (see  FIG. 6 ). If not, operation of the system  10  is ended. On the other hand, if layers  60  are to be created, the operation of the system  10  proceeds to action block  62  where additional input parameters  20  are input to the computer  12 . At this point, it is to be noted that if inquiry  42  had indicated that no cylindrical cuts  46  or radial cuts  48  were to be made, the operation of system  10  would have proceeded directly to action block  62  at that time. In either case, the input parameters  20  for use in the creation of layers  60  include a depth into the operational volume  24  “z m ”, an inner diameter (d i ) m , an outer diameter (d o ) m  and, again, an azimuthal angle θ. 
     In  FIG. 6 , it can be seen that an “m” number of layers  60  can be created. Specifically, with a depth “z m ” individually selected for each layer  60 , the diameters (d i ) m  and (d o ) m  can also be selected to create the layer  60  as an annulus (i.e. d i &gt;0) or as a disk (i.e. d i =0). After the creation of each layer  60 , the system  10  determines whether additional layers  60  are to be made. To do this, inquiry  66  specifically questions whether all “m” layers  60  have been made. If not, action block  68  decrements “n” and action blocks  62  and  64  create an additional layer  60  in accordance with appropriate remaining input parameters  20 . Further, as with the cuts  46  and  48  discussed above, the azimuthal angle θ for layers  60  can be a complete 360° arc, or be in segments. Preferably, the separation distance between adjacent layers  60  will be about ten microns. 
     Once all of the cylindrical cuts  46 , radial cuts  48  and layers  60  have been created as indicated for the present invention, operation of the system  10  is ended. 
     While the particular Computer Control for Bio-Mechanical Alteration of the Cornea 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.