Abstract:
A method for photodisrupting a preselected subsurface volume of corneal tissue to alter a cornea&#39;s refractive properties is disclosed. Specifically, at least one stromal volume having a substantially conical shaped surface is photodisrupted. For this purpose, a laser device having a laser source, laser scanner and one or more optical elements is typically used. In one embodiment, a plurality of stromal volumes, with each stromal volume having a substantially conical shaped surface, is sequentially photodisrupted to form a contiguous stromal cavity. In a particular implementation, each conical shaped surface defines a cone axis that is aligned to be co-linear with a reference axis that passes through the anterior surface of the eye and may be aligned orthogonally to the anterior surface of the eye.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention pertains generally to ophthalmic laser surgery techniques. More particularly, the present invention pertains to devices and methods for photodisrupting a preselected, subsurface volume of corneal tissue. The present invention is particularly, but not exclusively, useful for removing a volume of stromal tissue from a cornea to correct the cornea&#39;s refractive properties.  
       BACKGROUND OF THE INVENTION  
       [0002]     In the widely performed Laser In-Situ Keratomeleusis (LASIK) procedure, a microkeratome is used to incise the cornea of a patient and create a flap. The flap is then lifted to expose a bed of stromal tissue which is subsequently ablated using an Excimer laser. After ablation, the flap is replaced and allowed to heal. This process, although being somewhat successful in correcting vision deficiencies, has several drawbacks. For example, the creation of a suitable flap for a LASIK procedure is labor intensive and relies heavily on the skill and eye-hand coordination of the surgeon. In addition, the use of a microkeratome often produces an irregular incision which can create vision defects when the irregular, inner flap surface is replaced over a relatively smooth bed of ablated tissue.  
         [0003]     As an alternative to corneal reshaping using LASIK, a train of laser pulses having relatively short pulse durations can be directed to a focal point at a predetermined subsurface location within a patient&#39;s cornea. This focal point can then be used to photodisrupt tissue at the focal point with precision and accuracy. For example, infrared pulses can be passed through corneal tissue with minimal energy loss to a subsurface focal point. An example of a procedure that uses a pulsed laser beam that is focused to a predetermined, subsurface location within a patient&#39;s cornea is disclosed in U.S. Pat. No. 4,907,586, which issued to Bille et al. for an invention entitled “Method for Reshaping the Eye”.  
         [0004]     In greater detail, the photodisruption of tissue by a pulsed laser results from a process termed “laser induced optical breakdown” (LIOB). Specifically, in the LIOB process, tissue breakdown occurs in the laser focus due to the extremely high, local electrical field that is generated. This high electric field exceeds the electron binding energy of the tissue atoms, and results in the generation of a microplasma, shockwaves and a cavitation bubble. Typically, the vaporized tissue diffuses out of the cornea within about 30-60 minutes. Importantly, the cavitation bubble created at each focal point collapses under intraocular pressure. As a consequence, this process can be used to effectively reshape the cornea.  
         [0005]     When considering the use of subsurface photoablation for corneal reshaping, a general knowledge of the anatomy of the cornea is helpful. In detail, the cornea consists of several layers of tissue which are structurally distinguishable. In order, going in a posterior direction from outside the eye toward the inside of the eye, the various layers of a cornea are: an epithelial layer, Bowman&#39;s membrane, the stroma, Descemet&#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. For this reason, stromal tissue is generally selected for removal in a refractive correction procedure.  
         [0006]     Considering the stroma in further detail, it is generally comprised of around two hundred identifiable and distinguishable layers of lamellae. Each of these layers of lamellae in the stroma is somewhat dome-shaped, like the cornea itself, and they each extend across a circular area having a diameter of about nine millimeters. Each layer includes several lamellae. Unlike the entire 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). Finally, it is to be noted that, in a direction perpendicular to the layer, each individual lamella is only about two microns thick.  
         [0007]     Within the general structure described above, it is to be appreciated that the stroma is considerably anisotropic. 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 lamella together. Due to this relationship between strength and direction in the stroma, it is more efficient to photodisrupt tissue in volumes that extend orthogonally to the lamella layers than it is to photodisrupt tissue in volumes which extend along the lamella layers.  
         [0008]     In addition to the considerations described above, another factor that can affect the efficiency and accuracy of a photodisruption procedure is the optical path that the laser takes to reach a focal point at a targeted location. In this regard, it can be appreciated that if the laser must pass through a previously photodisrupted location, the beam can become distorted. This unwanted distortion can affect both the location and size of the focal point and lead to inaccurate results.  
         [0009]     In light of the above, it is an object of the present invention to provide devices and methods for photodisrupting stromal volumes having shapes which extend generally normal to the direction of the lamella layers. It is another object of the present invention to provide devices and methods for photodisrupting preselected stromal volumes which avoids placing the surgical laser on a beam path that passes through a previously photodisrupted location to reach a targeted location. Yet another object of the present invention is to provide devices and methods for correcting the refractive properties of a cornea which are easy to use, relatively simple to implement, and comparatively cost effective.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention is directed to devices and methods for photodisrupting a preselected volume of corneal tissue. In a particular embodiment, a volume of stromal tissue is vaporized using a pulsed, infrared laser beam that is focused to a subsurface location in the stroma. In one application of the invention, a vision deficiency (e.g. myopia, hyperopia, etc.) can be corrected by vaporizing a preselected volume of corneal tissue to create a contiguous cavity within the stroma. When the cavity collapses under intraocular pressure, the cornea assumes a new shape and curvature.  
         [0011]     In one aspect of the invention, a method is disclosed which begins by identifying the coordinates for each of a plurality of stromal locations. Specifically, the stromal locations are selected such that photodisruption at each location results in the photodisruption of a stromal volume that is bounded by two substantially parallel conical shaped surfaces that mutually define a cone axis. With these coordinates identified, the pulsed laser beam is brought to a focus at an initial, stromal location to photodisrupt tissue there. Next, the focal point is scanned along a predetermined path from one identified location to the next until photodisruption has occurred at all identified locations. For this purpose, a laser device having a laser source, laser scanner and one or more optical elements is typically used. In one exemplary setup, the laser device can include, in order, a laser source, a laser scanner for scanning in three dimensions, a plurality of lenses arranged as a telescope, a mirror, and a cutting lens. For some procedures, a contact lens can be used to stabilize the cornea relative to the laser source and conform the anterior surface of the eye to a preselected radius of curvature, R.  
         [0012]     In one implementation of the invention, a plurality of stromal volumes are sequentially photodisrupted to form a contiguous stromal cavity. For this implementation, each stromal volume is bounded by two substantially parallel conical shaped surfaces, respectively. For example, each stromal volume can be photodisrupted having substantially parallel surfaces that are each a portion of a respective cone. Moreover, each conical surface may only extend partially around the cone&#39;s axis. Stated another way, each cone surface may subtend an azimuthal angle, α, about the cone axis that is less than three hundred sixty degrees (α&lt;360 degrees). Alternatively, the conical surface can extend completely around the cone&#39;s axis (α=360 degrees).  
         [0013]     In some cases, the surface includes the vertex of the cone and in other cases a surface of the stromal volume consists of a cone portion that is distanced from the cone&#39;s vertex. In each case, the surface generated can be selected as desired by the operator. In one particular implementation, each conical shaped surface defines a right circular cone having a cone axis that is aligned to be co-linear with a reference axis that passes through the anterior surface of the eye and is aligned orthogonally to the anterior surface.  
         [0014]     As indicated above, photodisruption occurs along a predetermined path. In one implementation of the present invention, the predetermined path includes focal point displacements that can be characterized by displacement vectors having a nonzero azimuthal component relative to the reference axis. For this implementation, photodisruption proceeds generally from an initial focal point location to a final focal point location with the initial focal point location being positioned further from the anterior surface of the eye than the final focal point location. In some cases, this technique can be used to ensure that the surgical laser is not placed on a beam path that passes through a previously photodisrupted location to reach a targeted location. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     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:  
         [0016]      FIG. 1  is a schematic view showing the primary optical components of a system for photodisrupting a preselected volume of corneal tissue;  
         [0017]      FIG. 2  is an enlarged cross-sectional view of a cornea that has been photodisrupted to create a lenticular shaped stromal cavity to alter the refractive properties of the cornea;  
         [0018]      FIG. 3  is a schematic diagram illustrating a focal point path through the stroma;  
         [0019]      FIG. 4  is a perspective view of a photodisrupted stromal cavity in partial cross section to show the conical shaped surfaces created by the present methods;  
         [0020]      FIG. 5  is a perspective view of another photodisrupted stromal cavity in partial cross section to show the conical shaped surfaces created by the present methods;  
         [0021]      FIG. 6  is a top view of the photodisrupted stromal cavity shown in  FIG. 5 ; and  
         [0022]      FIG. 7  is a top view of the photodisrupted stromal cavity in which conical surfaces are created that subtend an azimuthal angle about a cone axis of less than one-hundred eighty degrees. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     Referring initially to  FIG. 1 , an ophthalmic laser system for intrastromal subsurface photodisruption (by the effect of laser induced optical breakdown (LIOB)) is shown and generally designated  10 . As shown in  FIG. 1 , the system  10  includes a laser source  12  for producing a pulsed laser beam and directing the laser beam along a first beam path  14 . A typical embodiment of the laser source  12  includes an all solid-state, femtosecond (fs) laser with a pulse duration of 1 fs to about 100 ps, an operating wavelength in the near infrared and repetition rate in the range of 1 kHz to 1 Mhz.  
         [0024]     Continuing with reference to  FIG. 1 , it can be seen that the laser beam generated at the laser source  12  is directed along the beam path  14  where it is reflected using a forty-five degree mirror  16 . From the mirror  16 , the beam is directed into a plurality of lenses that are arranged as a Galilei telescope  18 . In the telescope  18 , the laser beam is expanded and forwarded to the master shutter  20 . The master shutter  20  acts as a safety element. After passing the shutter  20 , the beam enters a scanning unit  22 .  
         [0025]     The scanning unit  22  includes means for three dimensional scanning of the laser beam. A more thorough description of a suitable scanning unit  22  and its operation is provided in co-pending, co-owned U.S. patent application Ser. No. 10/821,402, titled “Beam Steering System for Corneal Laser Surgery” and is hereby incorporated by reference in its entirety herein.  
         [0026]     For the system  10 , control signals are routed to a unit processor (not shown) where they are processed by, for example, a real-time operating system and evaluated by adequate hardware tools. If an error in laser output or positioning occurs during a procedure or calibration, the master shutter  20  is activated to block the beam to prevent any detrimental radiation from reaching the patient&#39;s eye.  
         [0027]     Continuing with  FIG. 1 , it can be seen that after leaving the scanning unit  22 , the beam is relayed to a cutting lens  24 . Specifically, as shown, the beam passes first through a plurality of lenses  26  that are arranged as a telescope and is then reflected by a forty-five degree dichroic mirror  28 . The dichroic mirror  28  permits the observation of the patient&#39;s eye  30  via a microscope (not shown) through the mirror  28  and cutting lens  24 . It can also be seen that the system  10  includes a contact lens  34 , which is typically made of transparent PMMA, to stabilize the cornea  32  relative to the cutting lens  24 . Moreover, as shown, the contact lens  34  is positioned in contact with the cornea  32  to conform the anterior surface  36  of the cornea  32  with the contact lens  34 . Typically, the conformed cornea  32  has a radius of curvature, R, in a range of between approximately 7.5 mm and approximately 11.0 mm. In most cases, a radius of curvature, R, of approximately 8.8 mm (which is close to the natural curvature of the cornea&#39;s exterior surface) is used.  
         [0028]     The fixation and alignment of the patient&#39;s eye  30  is typically accomplished using the contact lens  34  and an alignment device (not shown). For this purpose, the contact lens  34  is applied to and held against the eye  30  using an eye stabilizing element (not shown) that is attached to, or is integral with, the contact lens  34 . Once centered on the eye  30 , the eye stabilizing element is fixed by applying a vacuum. Next, the alignment device is placed between the beam exit of the cutting lens  24  and the alignment device. Using a motorized patient chair, the eye  30  and the eye stabilizing element are moved towards the alignment device. The connection between the eye stabilizing element and the alignment device is self-centering to maintain a proper x-y alignment. Furthermore, the setup provides the correct “z” distance between the patient&#39;s eye  30  and the cutting lens  24 . A pressure sensor (not shown) is used to measure the pressure on the eye  30  when the contact between the alignment device and the eye stabilizing element is established. A more thorough description of the fixation and alignment system and its operation is provided in co-pending, co-owned U.S. patent application Ser. No. 10/790,625, titled “System and Method for Positioning a Patient for Laser Surgery” and is hereby incorporated by reference in its entirety herein.  
         [0029]      FIG. 2  shows a cornea  32  that has been photodisrupted to create a lenticular shaped stromal cavity  38 . Although a lenticular shaped cavity  38  is shown, it is to be appreciated that the lenticular shape is merely exemplary and that the system  10  can be used to photodisrupt cavities having other non-lenticular shapes.  FIG. 2  further illustrates that the cavity  38  is formed by the photoablation of eight stromal volumes  40   a - h . As shown, each stromal volume  40  is formed having a substantially conical shaped surface, of which exemplary conical surface  42 , which corresponds to volume  40   e , has been labeled. In a typical procedure, these volumes  40   a - h  are sequentially photodisrupted to form the contiguous stromal cavity  38 . For the cavity  38  shown, each stromal volume  40   a - h  has a surface that is a portion of a respective substantially circular cone having a cone axis that is co-linear with the optical axis  44  of the eye  30 . It can be further seen that some volumes (e.g. volume  40   g ) include the vertex  46  of the cone while other volumes (e.g. volume  40   b ) consist only of a cone portion that is distanced from the cone&#39;s vertex. To create the cavity  38 , the volumes  40  are typically photodisrupted in sequential order beginning with volume  40   a  and ending with volume  40   h . Although not illustrated by  FIG. 2 , it is to be appreciated that some volumes (e.g. volume  40   a ) will typically be partially or fully collapsed during the photodisruption of volume  40   h.    
         [0030]     As shown in  FIG. 3 , within each volume  40   a - h , photodisruption occurs along a predetermined path. Typically, as shown in  FIG. 3 , the predetermined path can include focal point displacements that can be characterized by displacement vectors having a nonzero azimuthal component relative to the optical axis  44 . For this implementation, photodisruption proceeds generally in the direction of arrow  48  from a relatively posterior focal point  50   a  to a relatively anterior focal point  50   b . This technique can be used to ensure that the surgical laser is not placed on a beam path that passes through a previously photodisrupted location to reach a targeted location.  
         [0031]      FIG. 4  shows another view of a portion of the lenticular shaped stromal cavity  38  that has been photodisrupted. From  FIG. 4 , it can be seen the volume  40   g  is bounded by two substantially parallel conical shaped surfaces  42   a,b . Moreover,  FIG. 4  shows that these conical shaped surfaces  42   a,b  mutually define a cone axis  44 . As further shown, surface  42   a  is part of a right circular cone having a cone angle, θ, that is between about 0 degrees and 50 degrees. It happens that for a treatment diameter of about 12 mm, the angle, θ, will be equal to about forty degrees (θ=40°).  
         [0032]      FIGS. 5 and 6  show another stromal cavity (designated cavity  38 ′) having volumes  40   a ′- 40   c ′. As shown, for the stromal cavity  38 ′, each volume  40   a ′- 40   c ′ is bounded by two substantially parallel conical shaped surfaces. For example, volume  40   a ′ is bounded by conical shaped surfaces  42   a ′ and  42   b ′. It can be further seen that the conical shaped surfaces for each volume  40   a ′- 40   c ′ include the conical vertex for their respective cones. In addition, as best seen in  FIG. 6 , each of the volumes  40   a ′- 40   c ′ extend completely around the cone axis  44 ′. Also, it can be seen in  FIG. 5  that the cavity  38 ′ surrounds a conical shaped, non-ablated volume  52 .  
         [0033]      FIG. 7  illustrates yet another example of an ablation pattern having two stromal cavities (designated cavity  38   a ″ and cavity  38   b ″).  FIG. 7  shows that cavity  38   a ″ includes volumes  40   a ″- 40   c ″ and cavity  38   b ″ includes volumes  40   d ″- 40   f ′. For the stromal cavities  38   a ″ and  38   b ″, each volume  40   a ″- 40   f ′ is bounded by two respective, substantially parallel conical shaped surfaces. For example, volume  40   a ″ is bounded by conical shaped surfaces  42   a ″ and  42   b ″. Unlike the embodiment shown in  FIG. 6 , however, for the stromal cavities  38   a ″ and  38   b ″, each volume  40   a ″- 40   f ′ has a pair of conical shaped surfaces with each conical shaped surface subtending an azimuthal angle, α, about a cone axis  44 ″ that is less than one-hundred eighty degrees, as shown. Thus, each volume  40   a ″- 40   f ′ does not extend completely around the cone axis  44 ″. In one implementation, the entire cavity  38   a ″ is ablated followed by ablation of the cavity  38   b ″. Alternatively, the ablation pattern shown in  FIG. 7  can be ablated with ON-OFF, full circle raster scans. In this manner, the cavities  38   a ″,  38   b ″ are ablated by sequentially ablating volumes in the following order: volume  40   a ″, volume  40   d ″, volume  40   b ″, volume  40   e ″, volume  40   c ″ and then volume  40   f′.    
         [0034]     While the particular Systems and Methods for Intrastromal Scanning Patterns as herein shown and disclosed in detail are fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that they are 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.