Abstract:
A system and method are provided for an ophthalmic surgical procedure to provide a refractive correction for an eye. Specifically, the procedure is indicated when the desired refractive correction “d reqd ” exceeds the capability of a correction achievable when corneal tissue is only ablated. In accordance with the present invention, an optimized refractive correction “d 1 ” is accomplished by the ablation of corneal tissue (e.g. by a PRK or LASIK procedure). The optimized correction is then followed by a complementary refractive correction “d 2 ” wherein stromal tissue is weakened with Laser Induced Optical Breakdown (LIOB). Together, the optimized refractive correction (ablation) and the complementary refractive correction (LIOB) equal the desire refractive correction (d reqd =d 1 +d 2 ).

Description:
FIELD OF THE INVENTION 
     The present invention pertains to systems and methods for ophthalmic laser surgical procedures. More particularly, the present invention pertains to laser surgery wherein a desired refractive correction requires removal of more corneal tissue than can be efficaciously removed by a single procedure. The present invention is particularly, but not exclusively, useful for surgery wherein a removal of corneal tissue by ablation is complemented by a weakening of stromal tissue by Laser Induced Optical Breakdown (LIOB) to collectively achieve a required refractive correction for an eye. 
     BACKGROUND OF THE INVENTION 
     Presently there are several different types of procedures that can be used for performing refractive surgery on the cornea of an eye to correct vision defects. In general, these procedures can be categorized according to the kind of instrument that is used to perform the surgery (e.g. a “mechanical means” or a “laser means”). Further, within the “laser means” category, there are essentially two methodologies for altering stromal tissue that differ from each other depending on whether tissue is actually removed by photoablation, or is merely weakened by a phenomenon commonly referred to as Laser Induced Optical Breakdown (LIOB). 
     A removal of tissue from the cornea of an eye, when using a laser means to correct a vision defect, is typically accomplished by the photoablation of exposed corneal tissue. For example, the well-known procedures of LASIK and PRK (Photo Refractive Keratectomy) both rely on the removal of exposed tissue by photoablation. On the other hand, it is also known that tissue inside the stroma can be merely weakened to correct a vision defect. For example, the weakening of intrastromal tissue for the purpose of correcting a vision defect is disclosed and claimed in U.S. patent application Ser. No. 11/958,202 for an invention entitled “Method for Intrastromal Refractive Surgery” which is assigned to the same assignee as the present invention. More specifically, the result of weakening tissue is a redistribution of biomechanical stresses in the stroma that responds to Intra-Ocular Pressure (IOP) to reshape the cornea. It can happen, however, that neither the removal of corneal tissue, nor the weakening of stromal tissue, when performed alone, may be able to achieve the desired refractive result. Much here depends on the cause of the vision defect that needs to be corrected. Thus, there are limits to the extent of an effective refractive correction when either a weakening of stromal tissue or an actual removal of tissue are considered separately. 
     The inherent limitations of tissue removal procedures are primarily a result of the amount of corneal tissue that can be removed. Specifically, for the rather common visual defects of myopia, hyperopia and astigmatism, it is known that going beyond the limits set forth below can cause unwanted instabilities in the cornea. In general, the practical limits for effective refractive corrections to be achieved for the more common visual defects, by tissue removal alone, are: 
                                                     Myopia   PRK   −6 diopter limit               LASIK   −8 diopter limit           Hyperopia   PRK   +3 diopter limit               LASIK   +5 diopter limit           Astigmatism   PRK    3 diopter limit               LASIK    5 diopter limit                        
Also, it is known that presbyopia, in combination with the visual defects considered above, may require a refractive correction that goes beyond the limits set forth above.
 
     On the other hand, when stromal tissue is weakened, rather than removed, refractive corrections are limited to approximately 2 or 2.5 diopter. Further, for safety reasons, any stromal tissue that is to be weakened should not be within the one hundred microns immediately anterior to Descemet&#39;s membrane and the endothelium. 
     With the above in mind, it is an object of the present invention to provide a system and method for making myopic/hyperopic/astigmatic corrections requiring refractive changes that go beyond the limits provided by only removing corneal tissue. Another object of the present invention is to provide a system and method for combining corneal tissue removal with an intrastromal redistribution of biomechanical stresses to achieve a predetermined refractive correction for an eye. Still another object of the present invention is to provide a system and method for both removing and weakening corneal tissue, in combination, to provide refractive corrections for an eye that are relatively easy to implement and comparatively cost effective. 
     SUMMARY OF THE INVENTION 
     A system and method are provided for performing an ophthalmic laser surgical procedure, wherein a required (desired) refractive correction for an eye necessitates removal of more corneal tissue than can be efficaciously removed by photoablation alone. It follows that the required refractive correction also necessitates a greater weakening of stromal tissue than is possible by LIOB alone. The present invention, however, recognizes that the removal of an optimal amount of corneal tissue, by ablation, can be complemented by a weakening of stromal tissue, by LIOB, to achieve a required overall refractive correction for an eye. 
     To begin, a required diopter refractive correction for an eye “d reqd ” is established. Typically, this is done diagnostically. Next, the maximum permissible diopter refractive correction by tissue removal (i.e. ablation) and the maximum permissible diopter refractive correction by weakening stromal tissue (i.e. LIOB) are respectively determined. Stated differently, a first maximum diopter correction for the eye “d 1max ” is clinically determined. Specifically, d 1max  is the refractive correction that is achievable by removing corneal tissue from the eye. Also, a second maximum diopter correction for the eye “d 2max ” is clinically determined. In this case, d 2max  is the refractive correction that is achievable by changing a biomechanical stress distribution in the stroma of the eye (i.e. a weakening of the stroma). As envisioned for the present invention, d reqd  will be greater than d 1max,.  And, d reqd  will also be greater than d 2max . 
     In operation, corneal tissue is first ablated to achieve a first actual (i.e. an optimized) diopter correction “d 1actual ”. Preferably, this is done using an excimer laser to remove corneal tissue by either a LASIK or a PRK procedure. In either case, d 1actual  (i.e. the optimized refractive correction by ablation) is equal to or less than d 1max . Then, a complementary refractive correction is performed using a pulsed femtosecond laser to weaken stromal tissue. This is done by causing a Laser Induced Optical Breakdown (LIOB) of the tissue. In this case, LIOB achieves a second actual diopter correction “d 2actual ”. For the present invention, d 2actual  (i.e. the complementary refractive correction) is less than d 2max . Importantly, d 1actual +d 2actual  equals d reqd  (d reqd =d 1actual +d 2actual ). 
     As envisioned for the present invention, the photoablation of corneal tissue and the weakening of stromal tissue can be accomplished substantially simultaneously, in a same procedure. It may be preferable, however, to accomplish photoablation and weakening of tissue in separate procedures that are separated from each other by a time interval that may be as much as several weeks (e.g. three weeks). In such a case, it is most likely that the photoablation (PRK or LASIK) will be accomplished first. 
    
    
     
       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 a system in accordance with the present invention shown in its operational relationship to the cornea of an eye, with the cornea shown in cross section; 
         FIG. 2  is a logic flow chart for implementation of the present invention; 
         FIG. 3A  is a cross section view of a cornea when the present invention involves a LASIK procedure; and 
         FIG. 3B  is a cross section view of a cornea when the present invention involves a PRK procedure. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to  FIG. 1  a system for use with the present invention is shown and is generally designated  10 . As shown, the system  10  includes a first laser unit  12  for generating and directing a laser beam along a beam path  14  toward the cornea  16  of an eye. The system  10  also includes a second laser unit  18  for generating and directing a laser beam along a beam path  20  toward the cornea  16 . For purposes of the present invention, the first laser unit  12  is of a type well known in the pertinent art that is capable of generating a laser beam for photoablating tissue of the cornea  16 , such as an excimer laser. On the other hand, the second laser unit  18  is of a type that is better suited to cause the Laser Induced Optical Breakdown (LIOB) of tissue in the cornea  16 . Thus, the second laser unit  18  is preferably of a type that will generate a so-called, pulsed femtosecond laser beam. 
     Anatomically, the cornea  16  of an eye is shown in  FIG. 1  to include, in order from its anterior surface  22  to its posterior surface  24 , an epithelium  26 , a Bowman&#39;s membrane  28 , a stroma  30 , a Descemet&#39;s membrane  32 , and an endothelium  34 . As also shown in  FIG. 1 , an intra-ocular pressure (IOP) is directed against the cornea  16  from inside the eye (this IOP is represented by the arrow  36 ). In accordance with the present invention, the refractive properties of the cornea  16  are changed to make a desired vision correction. This is done by altering tissue in the cornea  16  through the concerted exploitation of two different mechanisms. They are: 1] actual tissue removal, and 2] a weakening of tissue. To do this, the first laser unit  12  is used for the removal of tissue from the cornea  16  by photoablation, and the second laser unit  18  is used to weaken tissue in the stroma  30  by LIOB. A consequence of both mechanisms is that the refractive power of the cornea  16  (measured in diopters) is changed. As envisioned for the present invention, a removal of tissue (photoablation), together with a weakening of the remaining tissue (LIOB), will provide a greater, more efficacious, and more beneficial change in the refractive power of the cornea  16  than can be accomplished with either mechanism alone. 
     In  FIG. 2  a methodology for implementation of the present invention is shown as a logic flow chart and is generally designated  38 . There it will be seen that the initial step (represented by block  40 ) involves establishing certain operational parameters. Specifically, it is necessary to diagnostically determine the desired refractive correction “d reqd ” for the cornea  16 . It is also necessary to ascertain the maximum permissible photoablation for removal of tissue from the cornea  16  “d 1max ”, and the maximum permissible weakening of the stroma  30  by LIOB “d 2max ”. Once these parameters have been established, inquiry block  42  indicates that the desired diopter correction “d reqd ” is to be compared with the maximum permissible photoablation correction “d 1max ”. If d reqd  is less than d 1max  the procedure moves to block  44  for reevaluation. Specifically, the methodology  38  of the present invention is properly indicated when the desired (i.e. required) correction “d reqd ” is greater than could be achieved if only photoablation were used. On the other hand, as indicated by reevaluation block  44 , when the maximum photoablation is sufficient (i.e. d 1max &gt;d reqd ) a weakening of the stromal tissue by LIOB with the second laser unit  18  may not be necessary. 
     When the desired (required) refractive correction for the cornea  16  is greater than the maximum effective photoablation (i.e. d reqd &gt;d 1max ), methodology  38  indicates that a subsequent inquiry is needed at inquiry block  46 . Specifically, the question at this point (i.e. inquiry block  46 ) concerns whether the desired (required) refractive correction is less than the sum of the maximum permissible photoablation and the maximum permissible LIOB (i.e. is d reqd &lt;d 1max +d 2max  ?). If so, (i.e. if d reqd &lt;d 1max +d 2max ), block  48  indicates that an effective ablation “d 1actual ” and an effective LIOB “d 2actual ” for refractive corrections need to be respectively determined. Block  50  then shows that the procedure is performed using “d 1actual ” and “d 2actual ”. On the other hand, if d reqd &gt;d 1max +d 2max , reevaluation block  44  would indicate that a procedure should, most likely, not be performed. 
       FIGS. 3A and 3B  conceptually show the combination of the two refractive correcting mechanisms employed by the system  10  of the present invention. Specifically,  FIG. 3A  shows a LASIK-LIOB procedure wherein a flap  52  has been created to expose underlying tissue of the stroma  30 . Once the flap  52  has been lifted, the first laser unit  12  can then be used for the photoablation (i.e. removal) of a region  54  of stromal tissue. The consequence of this is the refractive correction “d 1actual ”. Next, the second laser unit  18  can be used to perform LIOB that will create a series of incisions (i.e. cuts)  56  in the stroma  30  to thereby weaken the tissue. Although the incisions  56  can be made in a variety of patterns, the incisions  56  shown in  FIG. 3A  are considered here to be cylindrical cuts that are centered on the visual axis  58 . Further, it is important that a safety distance  60  be preserved between the incisions  56  and the posterior surface  24  of the cornea  16 . Preferably, this safety distance  60  is about one hundred microns. Thus, there is a weakening of tissue that will cause a redistribution of biomechanical stresses in the stroma  30 , with a consequent reshaping of the cornea  16  under the influence of IOP (arrow  36 ). The result is the refractive correction “d 2actual ”, with the overall objective being that the sum of the individual refractive corrections will provide the desired (required) correction (d 1actual +d 2actual =d reqd ). Similarly,  FIG. 3B  shows a PRK-LIOB procedure wherein a region  62  of superficial tissue in the cornea  16  is removed by photoablation for the same purpose discussed above with reference to  FIG. 3A . Again, the over all objective is that the sum of the individual refractive corrections will provide the desired (required) correction (d 1actual +d 2actual =d reqd ). 
     While the particular System and Method for Refractive Surgery with Augmentation by Intrastromal Corrective Procedures 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.