Abstract:
A method for correcting higher order aberrations in an eye requires Laser Induced Optical Breakdown (LIOB) of stromal tissue. In detail, the method identifies at least one volume of stromal tissue in the eye, with each volume defining a central axis parallel to the visual axis of the eye. Thereafter, a pulsed laser beam is focused to a focal spot in each volume of stromal tissue to cause LIOB of stromal tissue at the focal spot. Further, the focal spot is moved through the volume of stromal tissue to create a plurality of incisions centered about the respective central axis of the volume. As a result, a predetermined selective weakening of the stroma is caused for correction of the higher order aberration.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention pertains generally to methods for performing intrastromal ophthalmic laser surgery. More particularly, the present invention pertains to laser surgery to correct higher order aberrations in an eye. The present invention is particularly, but not exclusively, useful as a method for correcting higher order aberrations in an eye wherein incisions centered about a plurality of axes parallel to the visual axis cause a predetermined selective weakening of the stroma via changes in intrastromal biomechanical stress distributions. 
       BACKGROUND OF THE INVENTION 
       [0002]    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. 
         [0003]    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. 
         [0004]    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. 
         [0005]    It is well known that all of the different tissues of the cornea are susceptible to 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. 
         [0006]    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. 
         [0007]    As implied above, reshaping of the cornea by weakening tissue in the stroma can be an effective way to provide refractive corrections that will improve a vision defect. Not all vision defects, however, are caused by aberrations that are symmetrical with respect to the visual axis. Indeed, the higher order aberrations are typically asymmetrical. Accordingly, it may be necessary to weaken tissue in volumes that are offset from the visual axis. With all of this in mind, and as intended for the present invention, refractive surgery is accomplished by making incisions in the stroma centered about axes parallel to the visual axis to induce a redistribution of bio-mechanical forces that will reshape the cornea. 
         [0008]    In light of the above, it is an object of the present invention to provide methods for correcting higher order aberrations through changes in intrastromal biomechanical stress distributions for improvement of a patient&#39;s vision. Another object of the present invention is to provide methods for correcting higher order aberrations that require minimal LIOB of stromal tissue. Still another object of the present invention is to provide methods for performing ophthalmic laser surgery that create incisions having a same pattern at selected locations about the visual axis. Yet another object of the present invention is to provide methods for correcting higher order aberrations via ophthalmic laser surgery that are relatively easy to implement and comparatively cost effective. 
       SUMMARY OF THE INVENTION 
       [0009]    In accordance with the present invention, methods for correcting higher order aberrations in an eye via intrastromal ophthalmic laser surgery are provided that cause the cornea to be reshaped under the influence of intrastromal bio-mechanical stress distributions. Importantly, for these methods, at least one volume of stromal tissue is identified for operation. Structurally, each operational volume extends posteriorly from about ten microns below Bowman&#39;s membrane to a substantial depth into the stroma that is about 150 microns from the epithelium. Further, each operational volume defines a central axis that is parallel to and located at a distance from the visual axis of the eye. 
         [0010]    In general, the method of the present invention requires the use of a laser unit that is capable of generating a so-called pulsed, femtosecond laser beam. Stated differently, the duration of each pulse in the beam will approximately be less than one picosecond. When generated, this beam is focused onto a focal spot in the volume of stromal tissue. The well-known result of this is a Laser Induced Optical Breakdown (LIOB) of stromal tissue at the focal spot. In particular, and as intended for the present invention, movement of the focal spot within each volume of stromal tissue creates a plurality of incisions that are centered about the respective central axis of the volume. The purpose here is to cause a predetermined selective weakening of the stroma for correction of the higher order aberration. Preferably, each incision has a same pattern. For purposes of the present invention, “incision” may refer to a location of weakened or eliminated tissue along the path of the focal point. 
         [0011]    In certain embodiments, various volumes of stromal tissue with corresponding central axes are identified. For each embodiment, the central axes are arranged equidistant from the visual axis. Geometrically, the respective incisions may form concentric cylinders that are centered on the respective central axis. Other incision shapes may, however, be used. For example, the incisions may be concentric cylinder sections centered on the central axis, or they may be rectangular cylinders centered on the central axis, or they may be crosses that are centered on the central axis. In certain embodiments, the incisions will each have a thickness of about two microns. 
         [0012]    In accordance with the present invention, various procedures can be customized to treat identifiable refractive imperfections. Specifically, in addition to specific incisions alone, the present invention contemplates using combinations of various types of incisions. In each instance, the selection of incisions will depend on how the cornea needs to be reshaped. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    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: 
           [0014]      FIG. 1  is a cross-sectional view of the cornea of an eye shown in relationship to a schematically depicted laser unit; 
           [0015]      FIG. 2  is a cross-sectional view of the cornea showing a defined operational volume in accordance with the present invention; 
           [0016]      FIG. 3  is a front view of a stroma centered on the visual axis and illustrating a plurality of operational volumes, with each operational volume having a plurality of incisions. 
           [0017]      FIG. 4  is a perspective view of a plurality of cylindrical surfaces where laser incisions can be made by LIOB; 
           [0018]      FIG. 5  is a cross-sectional view of incisions on the plurality of cylindrical surfaces, as seen along the line  5 - 5  in  FIG. 4 , with the incisions shown for a typical treatment of presbyopia; 
           [0019]      FIG. 6A  is a cross-sectional view of the plurality of cylindrical surfaces as seen along the line  6 - 6  in  FIG. 4  when complete incisions have been made on the cylindrical surfaces; 
           [0020]      FIG. 6B  is a cross-sectional view of the plurality of cylindrical surfaces as seen along the line  6 - 6  in  FIG. 4  when partial incisions have been made along arc segments on the cylindrical surfaces for the treatment of astigmatism; 
           [0021]      FIG. 6C  is a cross-sectional view of an alternate embodiment for incisions made similar to those shown in  FIG. 6B  and for the same purpose; 
           [0022]      FIG. 6D  is a cross-sectional view of an alternate embodiment for incisions; 
           [0023]      FIG. 6E  is a cross-sectional view of an alternate embodiment for incisions; and 
           [0024]      FIG. 7  is a cross-sectional view of a cornea showing the bio-mechanical consequence of making incisions in the cornea in accordance with the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    Referring initially to  FIG. 1 , it will be seen that the present invention includes a laser unit  10  for generating a laser beam  12 . More specifically, the laser beam  12  is preferably a pulsed laser beam, and the laser unit  10  generates pulses for the beam  12  that are less than one picosecond in duration (i.e. they are femtosecond pulses). In  FIG. 1 , the laser beam  12  is shown being directed along the visual axis  14  and onto the cornea  16  of the eye. Also shown in  FIG. 1  is the anterior chamber  18  of the eye that is located immediately posterior to the cornea  16 . There is also a lens  20  that is located posterior to both the anterior chamber  18  and the sclera  22 . 
         [0026]    In  FIG. 2 , five (5) different anatomical tissues of the cornea  16  are shown. The first of these, the epithelium  24  defines the anterior surface of the cornea  16 . Behind the epithelium  24 , and ordered in a posterior direction along the visual axis  14 , are Bowman&#39;s capsule (membrane)  26 , the stroma  28 , Descemet&#39;s membrane  30  and the endothelium  32 . Of these tissues, Bowman&#39;s capsule  26  and the stroma  28  are the most important for the present invention. Specifically, Bowman&#39;s capsule  26  is important because it is very elastic and has superior tensile strength. It therefore, contributes significantly to maintaining the general integrity of the cornea  16 . 
         [0027]    For the methods of the present invention, Bowman&#39;s capsule  26  must not be compromised (i.e. weakened). On the other hand, the stroma  28  is intentionally weakened. In this case, the stroma  28  is important because it transfers intraocular pressure from the aqueous in the anterior chamber  18  to Bowman&#39;s membrane  26 . Any selective weakening of the stroma  28  will therefore alter the force distribution in the stroma  28 . Thus, as envisioned by the present invention, LIOB in the stroma  28  can be effectively used to alter the force distribution that is transferred through the stroma  28 , with a consequent reshaping of the cornea  16 . Bowman&#39;s capsule  26  will then provide structure for maintaining a reshaped cornea  16  that will effectively correct refractive imperfections. 
         [0028]    While referring now to  FIG. 2 , it is to be appreciated that an important aspect of the present invention is the identification of operational volumes  34  which are defined in the stroma  28 . Although the operational volumes  34  are shown in cross-section in  FIG. 2 , they are actually three-dimensional, and extends from an anterior surface  36  that is located at a distance  38  below Bowman&#39;s capsule  26 , to a posterior surface  40  that is located at a distance  41  from the endothelium  32 . Both the anterior surface  36  and the posterior surface  40  essentially conform to the curvature of the stroma  28 . For a more exact location of the anterior surface  36  of the operational volumes, the distance  38  will be about ten microns. For the posterior surfaces  40 , the distance  41  will be about one-hundred-fifty microns. 
         [0029]    In  FIG. 3 , incisions  44   a - 44   f  are made in a plurality of operational volumes  34   a - 34   f  as envisioned for the present invention. Although six different volumes  34   a - 34   f  are shown in  FIG. 3  (also  FIGS. 6D and 6E ) it will be appreciated by the skilled artisan, this is only exemplary and presented here for purposes of disclosure. More specifically, for third order aberrations only three volumes  34  need to be identified. In any event, the exact number of volumes  34 , and their respective radial distances from the visual axis  14  for any specific higher order aberration can be ascertained from the well known Zernike polynomials. As shown, for each operational volume  34   a - 34   f , a plurality of incisions  44 ′,  44 ″ and  44 ′″ are made, though there may be more or fewer incisions  44 , depending on the needs of the particular procedure. With this in mind, and for purposes of this disclosure, the plurality in a selected volume  34  will sometimes be collectively referred to as incisions  44 . Further, as shown in  FIG. 3 , six operational volumes have been identified. However, any number of operational volumes  34  may be used for the present invention. 
         [0030]    As shown in  FIG. 3 , the exemplary incisions  44  for each operational volume  34  are made on respective cylindrical surfaces. Although the incisions  44  are shown as circular cylindrical surfaces, these surfaces may be oval. When the plurality of incisions  44  is made in the stroma  28 , it is absolutely essential that it be confined within the respective operational volume  34 . With this in mind, it is envisioned that incisions  44  will be made by a laser process using the laser unit  10 . And, that this process will result in Laser Induced Optical Breakdown (LIOB). Further, in the illustrated embodiment, it is important these cylindrical surfaces be concentric, and that they are centered on a respective central axis  45   a - 45   f  distanced from and parallel to the visual axis  14 . 
         [0031]    Cross-referencing  FIG. 3  with  FIGS. 4 and 5 , it can be seen that each incision  44  has an anterior end  46  and a posterior end  48 . Further, the incisions  44  (i.e. the circular or oval cylindrical surfaces) have a spacing  50  between adjacent incisions  44 . Preferably, this spacing  50  is equal to approximately two hundred microns.  FIG. 5  also shows that the anterior ends  46  of respective individual incisions  44  can be displaced axially from each other by a distance  52 . Typically, this distance  52  will be around ten microns. Further, the innermost incision  44  (e.g. incision  44 ′″ shown in  FIG. 4 ) will be at a radial distance “r c ” that will be about 1 millimeter from the central axis  45 . From another perspective,  FIG. 6A  shows the incisions  44  centered on the central axis  45  to form a plurality of rings. In this other perspective, the incisions  44  collectively establish an inner radius “r ci ” and an outer radius “r co ”. Preferably, each incision  44  will have a thickness of about two microns, and the energy required to make the incision  44  will be approximately 1.2 microJoules. 
         [0032]    As an alternative to the incisions  44  disclosed above,  FIG. 4  indicates that only arc segments  54  may be used, if desired. Specifically, in all essential respects, the arc segments  54  are identical with the incisions  44 . The exception, however, is that they are confined within diametrically opposed arcs identified in  FIGS. 4 and 6B  by the angle “α”. More specifically, the result is two sets of diametrically opposed arc segments  54 . Preferably, “α” is in a range between five degrees and one hundred and sixty degrees. 
         [0033]    An alternate embodiment for the arc segments  54  are the arc segments  54 ′ shown in  FIG. 6C . There it will be seen that the arc segments  54 ′ like the arc segments  54  are in diametrically opposed sets. The arc segments  54 ′, however, are centered on respective axes (not shown) that are parallel to each other, and equidistant from the central axis  45 . 
         [0034]    As an alternative to the incisions  44  disclosed above,  FIG. 6D  indicates that incisions  44  may be created to form rectangular cylinders centered on the respective central axes  45 . Similarly,  FIG. 6E  indicates that the incisions  44  may be created to form crosses centered on the respective central axes  45 . As shown in  FIGS. 6D and 6E , the rectangular cylinders and crosses are also aligned with the visual axis  14 . 
         [0035]      FIG. 7  provides an overview of the bio-mechanical reaction of the cornea  16  when incisions  44  have been made in the operational volume  34  of the stroma  28 . As stated above, the incisions  44  are intended to weaken the stroma  28 . Consequently, once the incisions  44  have been made, the intraocular pressure (represented by arrow  56 ) causes a change in the force distribution within the stroma  28 . This causes bulges  58   a  and  58   b  that result in a change in shape from the original cornea  16  into a new configuration for cornea  16 ′, represented by the dashed lines. As intended for the present invention, this results in refractive corrections for the cornea  16  that improves vision. 
         [0036]    While the particular System and Method for Correcting Higher Order Aberrations with Changes in Intrastromal Biomechanical Stress Distributions 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.