Abstract:
A system and method for altering the configuration of a transparent material (e.g. the cornea of an eye) requires identifying local stress distribution patterns inside the material. These patterns are then used to define boundary (interface) surfaces between volumes within the material. In operation, a laser unit performs Laser Induced Optical Breakdown (LIOB) along selected boundary surfaces to disrupt stress distribution patterns between volumes of the material that are separated from each other by the boundary surface. This LIOB allows an externally applied force to thereby alter the configuration of the material.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention pertains generally to systems and methods for performing ophthalmic laser surgery. More particularly, the present invention pertains to laser systems that weaken corneal tissue over selected surfaces inside the cornea. The present invention is particularly, but not exclusively, useful as a system and method for weakening corneal tissue on selected boundary surfaces between tissue volumes, where the surfaces have been identified by abnormal deviations in stress distributions. 
       BACKGROUND OF THE INVENTION 
       [0002]    From a mechanical perspective, the cornea of an eye includes a Bowman&#39;s membrane that has exceptionally good tensile strength. Anatomically, Bowman&#39;s membrane is a relatively thin layer of tissue that is located just under the epithelium on the anterior surface of the cornea. More specifically, Bowman&#39;s membrane extends across the cornea, and its peripheral edge connects with the sclera. Most corneal tissue, however, is not in Bowman&#39;s membrane. Instead, it is in the stroma, which is tissue that lies immediately under (posterior) Bowman&#39;s membrane. In comparison with Bowman&#39;s membrane, although the stroma has significantly more tissue, it has substantially less structural strength. 
         [0003]    In the eye, behind (posterior) the cornea is the aqueous humor. Aqueous humor is a clear fluid that fills the space between the lens and the cornea. Importantly, the aqueous humor exerts an intraocular pressure (IOP) against the posterior surface of the cornea. Reactive forces against this IOP are provided by both Bowman&#39;s membrane and the stroma. 
         [0004]    It can happen for any of various reasons that, during the physical development of an eyeball, the anterior surface of the cornea will sometimes be formed with superficial irregularities, such as topographical depressions or bulges. Moreover, these irregularities persist under the influence of biomechanical forces that develop mostly in the stroma. In more detail, the biomechanical forces that naturally result in the stroma, in reaction to IOP, develop stress distribution patterns that maintain the topography of the eye&#39;s anterior surface, with or without irregularities. When irregularities are present, however, the consequences are the creation of optical aberrations. As is well known, these aberrations can be corrected (eliminated or minimized) by returning the anterior surface of the cornea to a normal, substantially spherical shape. 
         [0005]    In light of the above, it is an object of the present invention to provide a system and method wherein existing biomechanical forces in the stroma are weakened to disrupt their stress distribution patterns, and thereby allow IOP to reshape the eye&#39;s anterior surface. Another object of the present invention is to provide a system and method wherein the location of stress distribution patterns in the stroma are determined and targeted for disruption with reference to deviations in the topography of the eye&#39;s anterior surface. Still another object of the present invention is to provide a system and method wherein topographical deviations from a reference datum identify tissue volumes under the deviation, and Laser Induced Optical Breakdown (LIOB) is performed on boundary surfaces of the underlying volume to disrupt stress distribution patterns. Yet another object of the present invention is to provide a system and method for altering a configuration of a transparent material (e.g. a cornea) that is easy to use, is simple to implement and is comparatively cost effective. 
       SUMMARY OF THE INVENTION 
       [0006]    In accordance with the present invention, a system and method for altering the configuration of a transparent material (e.g. the cornea of an eye) requires disrupting stress distribution patterns inside the material. In response to these disruptions, the material reacts to an externally applied force (e.g. IOP) for reconfiguration of the material. Preferably, the required disruptions of stress distribution patterns result from the Laser Induced Optical Breakdown (LIOB) of the material (e.g. stromal tissue in the cornea). 
         [0007]    For ophthalmic surgery, it is known that stress distribution patterns inside the cornea of an eye can be located by measuring the topography of the cornea&#39;s anterior surface. For this purpose, corneal topography can be measured using a diagnostic device, such as a topography sensor. The measured topography can then be compared with a reference datum to identify deviations between the topography and the reference datum. In turn, the deviations are used to locate the stress distribution patterns. Typically, deviations will be manifested as depressions or bulges that form on the cornea&#39;s anterior surface. In any case, a deviation will be an indicator of an underlying abnormal stress distribution. 
         [0008]    As envisioned for the present invention, the reference datum represents a desired corneal configuration that will give the desired vision correction. In most cases, the reference datum will be a substantially spherical surface. For the specific case of ophthalmic surgery, deviations from the reference datum will identify areas on the anterior surface of the cornea where superficial changes in the cornea are required. Also, and importantly for the present invention, deviations can be used to identify an underlying volume of material (e.g. stromal tissue). Further, this underlying volume of material will define a boundary (interface) surface that separates the underlying volume from adjacent volumes of material. 
         [0009]    For the present invention, a laser unit is used to cut material (stromal tissue) on the boundary (interface) surface of the underlying volume. The extent and scope of this cut will be determined by the extent and scope of the deviation that is used to identify the underlying volume. As for the shape of the cut, depending on the particular reconfiguration that is desired, the cut may be a planar cut or a cylindrical cut. The cut may also be otherwise customized for the particular requirements of the procedure. For example, a predictive model as disclosed in U.S. application Ser. No. 12/016,857 for an invention entitled “Finite Element Modeling of the Cornea,” which is assigned to the same assignee as the present invention, can be used for this purpose. In any event, as noted above, the cuts are intended to disrupt the stress distribution on the boundary (interface) surface between material in the underlying volume and adjacent material. More specifically, the cuts may be made on only portions of a tissue volume boundary and may be made on the boundaries of more than one volume. The consequence is that the external force (e.g. intraocular pressure “IOP”) will then alter the configuration of the transparent material in response to the weakening of the material that has been cut. 
         [0010]    In an alternate embodiment of the present invention, the internal stress distributions can be identified by any of various devices known in the pertinent art. In each case, however, it is important to identify boundary (interface) surfaces that separate volumes in the material from each other. LIOB can then be performed on the boundary surfaces, or portions of the boundary surfaces, as indicated above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    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: 
           [0012]      FIG. 1  is a schematic presentation of a system in accordance with the present invention, with the system shown in its intended operational relationship with the anterior portion of an eye; 
           [0013]      FIG. 2  is a cross section view of a cornea of an eye; 
           [0014]      FIG. 3  is a top plan view of a cornea of an eye showing a symmetrical aberration substantially centered on the visual axis of the eye; 
           [0015]      FIG. 4A  is a cross section view of the cornea as seen along the line  4 - 4  in  FIG. 3 ; 
           [0016]      FIG. 4B  is a view of the cornea shown in  FIG. 4A , with superposed exemplary iso-stress lines; and 
           [0017]      FIG. 4C  is a cross section view of the cornea shown in  FIG. 4A  after corrective surgery in accordance with the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    Referring initially to  FIG. 1 , a system in accordance with the present invention is shown and is generally designated  10 . As indicated and shown, the system  10  includes a laser unit  12 , that is electronically connected to a computer  14 , and to a topography sensor  16 . For purposes of the present invention, the laser unit  12  is preferably of a type that can generate a laser beam  18  that is characterized by femtosecond pulses. Importantly, the laser beam  18  needs to be capable of altering transparent material, such as the cornea  20  of an eye, by a process known as Laser Induced Optical Breakdown (LIOB). Further, the topography sensor  16  can be a corneal topographer of any type well known in the pertinent art, that is capable of detecting aberrations in the cornea  20 . 
         [0019]    Still referring to  FIG. 1 , the anatomy of the anterior portion of an eye is shown to include the cornea  20  and a lens  22 . The aqueous humor  24  is a clear fluid filing the space between the lens  22  and the cornea  20 . Importantly, the aqueous humor  24  exerts an intraocular pressure (IOP), represented by the arrows  26 , against the posterior surface  28  of the cornea  20 . 
         [0020]    The cornea  20 , as best seen in  FIG. 2 , includes a number of various layers. Going in a posterior direction from the anterior surface  30  of the cornea  20  toward the posterior surface  28 , these various layers are: epithelium  32 , Bowman&#39;s membrane  34 , stroma  36 , Descemet&#39;s membrane  38  and endothelium  40 . Of these, the strongest tissues are Bowman&#39;s membrane  34  and the stroma  36 . Bowman&#39;s membrane  34  is the strongest. The stroma  36 , however, is the most responsive to the IOP  26 . 
         [0021]    During the growth development of an eye, it will often happen that the cornea  20  will become somehow misshapen. This, unfortunately, will cause a person to experience vision defects that result from optical aberrations introduced by the cornea  20 . For example,  FIG. 3  shows a cornea  20  having an aberration (irregularity)  42  that is symmetrically oriented on the visual axis  44 . With cross reference to  FIG. 4A , it will be appreciated this aberration (irregularity)  42  manifests itself in the topography of the cornea  20  as a generally flat portion of the anterior surface  30 . This is in contrast with a more normal, spherical shape for the topography of the anterior surface  30 . A consequence of the aberration (irregularity)  42 , is an annular bulge  46  that surrounds the depression (irregularity)  42  on the anterior surface  30  (i.e. the bulges  46   a  and  46   b  in cross section). In accordance with well known techniques, the aberration (irregularity)  42  can be easily identified by the topography sensor  16 . 
         [0022]    Referring now to  FIG. 4B , it is to be appreciated that in order to dimensionally evaluate the topography of the anterior surface  30  of cornea  20 , a reference datum  48  needs to be defined for the present invention. Specifically, this reference datum  48  represents the desired configuration for the anterior surface  30 ; after the aberration (irregularity)  42  has been corrected. In  FIG. 4B  it has been indicated and shown that, for the purpose of vision correction, the reference datum  48  will preferably be a substantially spherical shaped surface. Due to the aberration (irregularity)  42 , however,  FIG. 4B  also indicates that before the aberration (irregularity)  42  has been corrected, there will be a deviation  50  between the actual configuration of anterior surface  30 , and the reference datum  48 . 
         [0023]    As envisioned for the present invention, after the aberration (irregularity)  42  has been located (such as by use of topography sensor  16 ), a volume of stromal tissue  52  that lies under the aberration (irregularity)  42  can be identified. An example of such an underlying volume  52  of tissue is shown bounded by the dotted line in  FIG. 4B . Further, and still referring to  FIG. 4B , it will be appreciated that the underlying volume  52  can be identified as having a peripheral boundary surface  54  that, in actuality, is a portion of a cylindrical surface  56 . With cross reference to  FIG. 3 , it can be appreciated that the cylindrical surface  56  (and therefore boundary surface  54 ) is centered on the axis  44  and can be generally determined by the periphery of aberration (irregularity)  42 . 
         [0024]    As depicted in  FIG. 4B , tissue in the stroma  36  of cornea  20  will naturally develop iso-stress lines  58  that are characteristic of stress distribution patterns. As is well known by the skilled artisan, these stress distribution patterns result from the biomechanical forces that are generated in the stroma  36 . In this case, these biomechanical forces result directly from the IOP  26 , and they are the reactive forces provided by the stroma  36  and Bowman&#39;s membrane  34  in response to the IOP  26 . Importantly, when the cornea  20  is formed with an aberration (irregularity)  42  that is manifested by a deviation  50 , the iso-stress lines  58  in the stroma  36  are distinctively different from what they would normally be. The detection of these distinctions by the topography sensor  16 , or by any other well known means for determining stress distribution patterns in the stroma  36 , can then be used to locate appropriate boundary surfaces  54 . 
       Operation 
       [0025]    In the operation of the system  10  of the present invention, a device (e.g. topography sensor  16 ) is used to measure the topography of the anterior surface  30  of the cornea  20 . Based on this measurement, irregularities in the anterior surface  30  (e.g. aberration (irregularity)  42 ) are observed and located. The aberration (irregularity)  42  is then compared with the reference datum  48  by the computer  14 , and the deviation  50  that results from this comparison is identified. In turn, the deviation  50  is used to identify an underlying volume  52  of tissue in the stroma  36 . Most importantly, depending on the dimensions and location of the deviation  50  (recall, the deviation  50  shown in the drawings is only exemplary), the boundary (interface) surface  54  is also identified. The laser unit  12  can then be employed for the LIOB of stromal tissue over the boundary surface  54 , or portions of the boundary surface  54 . Further, additional volumes of tissue may also be targeted. In any event, this LIOB effectively disrupts the stress distribution patterns over the boundary surface  54  and results in a significant weakening of tissue in the stroma  36  on the boundary surface  54 . Stated differently, this weakening of tissue occurs between tissue in the underlying volume  52 , and tissue in the stroma  36  that is not in the underlying volume  52 . In response, the IOP  26  against the posterior surface  28  of the cornea  20  causes a reconfiguration of the cornea  20 . Specifically, as envisioned by the present invention and shown in  FIG. 4C , this reconfiguration results in a shape for the anterior surface  30  of the cornea  20  that conforms with the reference datum  48  (i.e. a substantially spherical shape). As intended, this provides the vision correction that is required. 
         [0026]    While the particular System and Method for Altering Internal Stress Distributions to Reshape a Material 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.