Abstract:
A conventional keratomileusis procedure is modified to the extent that a controlled tissue-ablating laser radiation is applied solely to the freshly cut part of the cornea that is left after severing the lenticle, the radiation being so controlled and characterized as to effect a volumetric removal of exclusively stromal tissue that, upon replacement of the lenticle over the thus-sculptured remainder of the cornea, a new and optically corrected curvature results.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to procedures and apparatus for ophthalmic surgery upon the cornea to correct one or more optical deficiencies of an eye. More particularly, the invention is concerned with controlled use of tissue-ablating laser-beam irradiation of stromal tissue within the optically used central area of the cornea, to effect improved optical performance at the anterior surface of the cornea. And the invention is specifically concerned with the class of surgical procedures which includes keratomileusis and thus a temporary partial or total surgical removal of the lenticle from the cornea. 
     Origin of keratomileusis and related procedures is attributed to Dr. Jose I. Barraquer, of Bogota, Colombia. His procedures and apparatus are described and illustrated by Dr. Louis J. Girard, in his 1981 authoritative text, &#34;Corneal Surgery&#34;, Volume Two, Chapter 6 (&#34;Refractive Keratoplasty&#34;), pp. 143 to 172; Dr. Girard&#39;s Chapter 6 concludes with a full bibliography which accounts for the contributions of Dr. Barraquer and others in the art. 
     It suffices for present purposes to briefly describe Dr. Barraquer&#39;s keratomileusis procedure, which is visually employed for surgically correcting a severely myopic eye, although it can also be used to correct for a hyperopia condition. At the outset, a special microtome (a &#34;microkeratome&#34;) is applied to the cornea and operates to shear off, as a lenticle, namely, a central front truncation of predetermined size, from the central region of the cornea, the diameter of the truncation being in the order of 7.5 mm and thus fully embracing the optically used region of the cornea. Having selected a holding disc for its concavity adapted to the front-surface curvature of the lenticle, the lenticle is fitted to the concavity, and is then frozen, with the freshly cut posterior surface of the lenticle exposed for precision lathing to predetermined contour. After lathing and thawing, the lenticle is reapplied to the eye, and is then sutured in stroma-to-stroma contact with the cornea from which it was removed. 
     A keratomileusis procedure thus contemplates full preservation of the anterior surface of the cornea, and a lathe-sculpturing removal of stromal tissue only from the posterior surface of the severed lenticle. If the lathing is to a characterized predetermined depth of cut which is greatest at the optical axis and which reduces to zero at the outer radius of the optically used part of the cornea, then upon applying the lathed lenticle to the eye, a myopia-correcting result is obtained. If, on the other hand, the lathing depth of cut is greatest at the outer radius of the optically used part of the cornea, with reducing depth of cut as a function of reducing radius, then a hyperopia-correcting result is obtained. 
     We know of no reported efforts to apply laser surgery in any updated performance of a keratomileusis operation. However, Trokel, et al. in their paper entitled &#34;Excimer Laser Surgery of the Cornea&#34;, Am. J. Ophthal. 96: 710-715, December 1983, do suggest the possibility of such an operation. 
     Meanwhile, various L&#39;Esperance, Jr. patents have issued, describing use of tissue-ablating laser radiation, directed to the optically used central region of the anterior surface of the cornea, to effect optical correction by tissue-ablating action, with volumetric removal of corneal tissue. These patents illustratively include those numbered 4,665,913, 4,669,466, 4,732,148, 4,729,372, and 4,770,172, and their disclosures, as well as Dr. Girard&#39;s said Chapter 6, are hereby incorporated by reference. 
     BRIEF STATEMENT OF THE INVENTION 
     It is an object of the invention to provide a new and improved technique and apparatus for performing a keratomileusis or the like operation. 
     Another specific object is to provide a new and improved technique and apparatus for surgically modifying optical properties of an eye without surgically invading the central optically used region of the anterior surface of the cornea. 
     A specific object is to achieve the above objects through laser-ablating removal of only stromal tissue from the optically used central region of the cornea. 
     Another specific object is to provide a technique and apparatus for performing a keratomileusis operation without surgically invading any optically used part of the lenticle that is severed at commencement of the operation. 
     The invention achieves the foregoing objects by modifying conventional keratomileusis procedure to the extent that controlled tissue-ablating laser radiation is applied solely to the freshly cut part of the cornea that is left after severing the lenticle, the radiation being so controlled and characterized as to effect a volumetric removal of exclusively stromal tissue that, upon replacement of the lenticle over the thus-sculptured remainder of the cornea, a new and optically corrected curvature results. 
    
    
     DETAILED DESCRIPTION 
     The invention will be described in detail in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a simplified sectional view of a cornea, with annular eye-retaining and microtome structure in locating relation with the limbus of the cornea; 
     FIG. 2 is a schematic diagram of a laser and control means for operating upon the retained cornea of FIG. 1; 
     FIG. 3 is a sectional view of a cornea, to enlarged scale, in aid of discuss±on of curvature-decreasing optical corrections achievable in use of the invention; 
     FIG. 4 is a view similar to FIG. 3, applicable to curvature-increasing optical corrections; 
     FIG. 5 is a sectional view of a cornea, to illustrate a modified technique of the invention; 
     FIG. 6 is a sectional view of apparatus usable in performing the technique of FIG. 5, shown applied to the cornea of FIG. 5; and 
     FIG. 7 is a simplified sectional diagram, to illustrate an articulated employment of apparatus of FIG. 6, in performing the technique of FIG. 5. 
    
    
     In FIG. 1, annular eye-retaining ring structure 10 is shown in locating engagement with a cornea 11 having a dilated iris 12, and said structure includes a flange 13 which establishes a horizontal plane of reference for the cutting blade 14 of a microtome. The central bulbous contour of the anterior surface of the cornea projects beyond this horizontal plane, and a dashed line 15 indicates the line of cornea-truncating cut by the microtome blade when guided by flange 13 in the direction of arrow 16, transverse to the optical axis of the eye. This truncating cut is at such maximum depth (on the axis of the eye) that a lenticle 17 (FIG. 2) is severed from the cornea, and the diameter D 1  of the lenticle (and thus also the freshly cut portion of the remaining body of the cornea) substantially exceeds the diameter D 2  of the optically used central area of the cornea. Preferably, the depth of this truncating cut is such that the cut area of diameter D 2  is entirely within stroma tissue of the cornea, i.e., it is beneath any involvement with Bowman&#39;s membrane in the area defined by diameter D 2 . And a lateral stabilizing-arm part 18 of structure 10 is adapted for detachably fixed connection to laser apparatus 20 (FIG. 2). 
     The ring structure 10 and microtome blade 14 will be understood to have been simplified because precision equipment, designed by Dr. Barraquer for presently described purposes, exists and is commercially available, for example, as the Microkeratome Set marketed by Steinway Instrument Company, Inc., of San Diego, Calif. The Steinway Microkeratome Set is complete, with an assortment of eye-retaining rings 10 of various sizes, and with an assortment of plates (flange 13) for determining various depths of cut, as well as accessories to facilitate use for production of lenticle 17. The Steinway promotional bulletin, &#34;Series I--The Barraquer Cryo-Lathe and Microkeratome Set&#34;, as well as Dr. Girard&#39;s said Chapter 6 provide detail as to the nature and use of the involved cutting apparatus, so that such detail is not needed in the present description. 
     Having made the cut which produced lenticle 17, it is preferred to retain a memory of the angular orientation of the lenticle with respect to the remaining body of the cornea. To this end, FIG. 2 shows lenticle 17 lying on the flat upper surface of blade 14, the same having been brought to this position by tweezer grasp of the leading edge of the lenticle as the lenticle is being cut, and by drawing this edge over the blade surface while making the cut, thus non-invasively setting aside and retaining the lenticle in known orientation with respect to the remaining body of the cornea from which it has been severed, so that the lenticle can later be placed in its correct original orientation with respect to said body. A drop of isotonic solution applied to the upper surface of blade 14, prior to making the cut, will facilitate this tweezer manipulation and also provides a degree of protection for the cut posterior surface of the lenticle. 
     The patient, who is preferably lying on his back, as related in said L&#39;Esperance, Jr. patents, is now ready for correctly aligned positioning with respect to the laser apparatus 20, the same being secured via means 21 detachably connecting arm 18 to a bracket 22 or other fixture of the laser apparatus. 
     The apparatus 20 is schematically shown to include a laser source 24 producing an output beam 25 of radiation at a tissue-ablating intensity and wavelength, such as the pulsed 193 nm wavelength ultraviolet beam produced by an excimer laser operating with argon fluoride (ArF), all as described in commercial literature of Lambda-Physik, of Gottingen, West Germany and available in the United States from the Lambda-Physik facility in Acton, Mass.; refererence to and performance of a particular Lambda-Physik excimer laser is found in various L&#39;Esperance, Jr. patents, including Pat. No. 4,665,913, and therefore no elaboration thereof is needed for purposes of the present description. 
     The apparatus 20 is further shown to include beam-homogenizing means 26, followed by beam-characterizing means 27, before folding at a mirror 28 for vertically downward delivery on axis 25&#39; to the center of the stromal tissue exposed upon lenticle 17 removal. A dimensional indication at diameter D 3  illustrates the maximum cross section diameter of the beam at impact with cornea 11. For purposes of achieving a desired optical correction, this maximum diameter D 3  of laser-beam exposure to the cornea may be less than the maximum diameter D 2  accommodated by a dilated iris 12, as will later become clear. 
     Reference is made to pending Telfair, et al. application Ser. No. 009,724, filed Feb. 2, 1987, for detailed description of suitable beam-homogenizing means 26, and for present purposes it is sufficient to indicate that the action of means 26 is such as to provide an output of laser-beam delivery to the succeeding means 27 wherein the laser beam is of substantially uniform flux density over the sectional area of the thus-delivered beam. Reference is made to L&#39;Esperance, Jr. Pat. No. 4,732,148, and to pending Yoder patent application Ser. No. 204,504, filed June 9, 1988 for descriptions of suitable beam-characterizing means 27; and it is sufficient to note for present purposes that beam delivery afforded by the disclosure of these latter references is of the variety wherein the size and shape of the laser-beam section is varied by suitable control means 29 in the course of a given laser-surgery exposure. Depending upon the shape which is thus dimensionally varied, the ablation of corneal tissue results in sculpture to a desired new profile of the exposed surface area, the 
     diameter D 3  of which is illustratively 4 to 6 mm for myopia, hyperopia and/or astigmatism correction. We sometimes refer to the technique indicated for beam-characterizing by means 27 as the varying spot-size technique. 
     FIG. 3 illustrates the important step of the invention, using the apparatus of FIG. 2 to perform a myopia-reducing laser-ablating sculpture of the central area (diameter D 3 ) within the freshly cut region 11&#39; of the cornea, after microtome removal of lenticle 17. Within this central area, laser spot size is controlled by means 27, 29 to be a circle centered on the optical axis (beam axis 25&#39;), and the diameter of the circle is caused to vary in the course of a predetermined ablation-producing exposure. Thus, if the diameter is near-zero at commencement of the exposure, and if the diameter expands in the course of the exposure, to the point of achieving diameter D 3  at the end of the exposure, the resulting curvature of depth penetration into the freshly cut region 11&#39; is a concavity having a profile 30 of circular symmetry about axis 25&#39;. And when lenticle 17 is thereafter returned into concentric registry and stroma-to-stroma contact with surface 11&#39; (and its newly sculpted concavity 30), the net effect is a curvature reduction and thus a myopia-reducing change in the anterior-surface curvature of the cornea, it being noted that lenticle 17 is sufficiently pliant to retain its new curvature, as dictated by the stroma-to-stroma contact. 
     What has been said as to the spherical myopia-reducing change effected by the sculpture step of FIG. 3 can also be said for an astigmatism-reducing change, also illustrated by FIG. 3, if the sculpted curvature profile 30 is viewed as being the section of a cylindrical arc, which section extends diametrically across the area of diameter D 3 . To produce such cylindrically arcuate sculpture, i.e., from surface 11&#39; to the diametrically extending constant cylindrically arcuate contour 30, the controlled varying spot size of tissue-ablating radiation delivered on axis 25&#39; is a diametrically extending line or stripe of varying width, which at beginning of the exposure may be most narrow, but which expands in width, and with lateral symmetry both sides of said diametrically extending line, in the course of the predetermined full exposure. If the angular orientation of the said diametrically extending line is set in accordance with the cylindrical axis of astigmatism to be corrected, then upon returning lenticle 17 into concentric registry and stroma-to-stroma contact with surface 11&#39; (and its newly sculpted concavity 30), the net effect is a cylindrical curvature reduction in the anterior surface of the cornea, which curvature reduction is correctly oriented for astigmatism correction of the involved eye. 
     In a manner analogous to the spherical and/or cylindrical curvature reduction procedures described in connection with FIG. 3, FIG. 4 illustrates the case for curvature increase, to effect correction of a hyperopia condition. In FIG. 4, the laser-sculpted intrusion into the freshly cut surface 11&#39; establishes a convex profile 31 of circular symmetry about axis 25&#39;, over the area (of diameter D 3 ) afforded by the maximum beam diameter used in the cases of FIG. 3, it being recalled that said maximum beam diameter had been selected to accord with the optically used area of the cornea to be surgically improved, i.e., not necessarily the maximum diameter D 2  afforded by a fully dilated iris 12. But to limit the hyperopia-correcting convex profile 31 to the diameter D 3  desired for optical correction is to present the disadvantage of not achieving hyperopia correction to the full desired extent of diameter D 3 . This is because the convex profile 31 alone would mean a sharp outer circular edge of maximum depth penetration into the cut surface 11&#39;, and application of lenticle 17 to such a profile could degrade optical performance of the resulting anterior-surface profile, due to the difficulty of assuring full stroma-to-stroma seating of the lenticle on the sculpted surface 11&#39;, particularly at and near the outer circular locus of greatest ablated-depth penetration into surface 11&#39;. To offset this difficulty, the profile 31 is flared or beveled, from maximum-depth penetration to zero penetration, in an annular zone 31&#39; (of outer diameter D 4 ) contiguous to and radially outside the optically correcting convex profile 31. As explained in detail in L&#39;Esperance, Jr. Pat. No. 4,732,148, the laser-sculpting production to the combined optically correcting and flaring profiles (31, 31&#39;) is obtained by such varying beam-section control by means 27, 29 as to project an annulus of laser beam impingement wherein the annulus is at all times during the exposure based on the circular locus of diameter D 3 , and wherein the inner radius of the projected annulus varies progressively in the radially inward direction to the point of zero or near-zero exposure on axis 25&#39;, while at the same time the outer radius of the projected annulus varies progressively in the radially outward direction to the point of zero or near-zero exposure at diameter D 4 . Having performed the profiling operation (31, 31&#39;) on the freshly cut surface 11&#39;, lenticle 17 is concentrically applied thereto with stroma-to-stroma contact over all cut and sculptured surfaces, whereby the anterior surface of the cornea pliantly assumes the desired new circumferentially beveled hyperopia-correcting curvature. 
     FIGS. 5, 6 and 7 are directed to a modification of technique and apparatus, whereby accuracy of registration and orientation can be assured upon lenticle replacement over the laser-ablated stroma at the freshly cut surface 11&#39;. This is made possible by so making the microtome cut, on alignment 15&#39; (FIG. 5), as to terminate a distance d short of totally severing the lenticle 17&#39; from the rest of the cornea. The lenticle 17&#39; is then treated as a hinged flap which can be engaged and articulated about a fixed axis 35 by the hinged holding device 36 shown in FIG. 6. 
     The holding device 36 comprises an inverted cup-shaped enclosure having a flange 37 for supporting engagement with the upper rim edge of the eye-retaining device 10. The otherwise-open bottom wall of the enclosure is closed by a perforated or foraminous wall 38 selected for contour compatability with the pre-surgery profile of the anterior surface of the patient&#39;s cornea. A valve 39 is selectively operable into three positions, in the first of which the interior volume or plenum of enclosure 36 is connected to a vacuum line, in the second of which the interior volume of enclosure 36 is connected to a source of air pressure, and in the third of which the vacuum and pressure-source connections are both cut off. The hinge designation 35 in FIG. 6 will be understood to suggest selectively removable hinging attachment of the holding device 36 to hinge-connection means 35&#39; forming a part of the eye-retaining structure 10 of FIG. 2. 
     In use of the procedure and apparatus of FIGS. 5 to 7, a microtome cut is made on alignment 15&#39;, leaving a pliant hinge connection (of extent d) between the lenticle flap 17&#39; and the remaining body of the cornea. The holding device 36 is then hinge-connected at 35-35&#39; to the eye retainer 10, and the device 36 is lowered with the porous wall 38 in conforming relation to the anterior surface of the lenticle flap 17&#39;. Valve 39 is then actuated to provide a vacuum connection to the inner plenum volume of device 36, whereupon the lenticle flap adheres to wall 38, and device 36 is elevated to its up position, shown by phantom outline in FIG. 7; in the raised position of FIG. 7, an arrow 36&#39; is suggestive of position-certifying contact with the plenum structure 36, and means 29, whereby tissue-ablating laser radiation cannot be delivered unless the raised position of FIG. 7 has been certified to the control means. The cut surface 11&#39; to be laser sculpted is then clearly exposed, centered on axis 25&#39;, for such corrective ablating surgery as has been illustratively described in connection with FIGS. 3 and 4. Upon completion of the surgical procedure, device 36 (and the lenticle flap it has been holding) is returned to its down position (FIG. 6) with the lenticle flap in precise register with its original relationship to the rest of the cornea, and further actuation of valve 39 from its vacuum connection to its air-pressure connection is operative not only to release the lenticle flap from wall 38 but also to apply gentle and well-distributed downward force loading of the lenticle flap in stroma-to-stroma contact with the now-sculptured surface 11&#39;; in the process, the anterior surface of the lenticle flap becomes once more the anterior surface of the cornea, but with a newly corrected optically functioning curvature achieved by pliant adaptation to the sculptured profile. 
     It will be seen that the described procedures and apparatus achieve all stated objects, and it will be appreciated that the invention is not limited to particular wavelengths as long as the involved laser radiation is tissue-ablating, i.e., has the inherent ability to photo-chemically (in the ultraviolet), photo-thermally (in the infrared) or otherwise destroy tissue without leaving scar tissue in or adjacent to the region of laser-radiation exposure. 
     It is also to be understood that the invention is not limited to the myopia-correcting, hyperopia-correcting and astigmatism-correcting usages described in connection with FIGS. 3 and 4. For example, the near-vision capability of a hyperopic eye may be improved by making a curvature-increasing profile that is limited essentially to a relatively small central area (e.g., as suggested by dimension D 5  in FIG. 4), and by uniformly ablating the remaining annulus (D 5  -D 3 ) of the visually used area of the cornea. More specifically, the curvature-increasing profile within dimension D 5  will involve greatest ablation-depth penetration (d max .) at diameter D 5 , and by uniformly ablating the remaining annulus (D 5  -D 3 ) to said maximum depth (d max .) one leaves the far-vision capability substantially untouched in the annulus (D 5  -D 3 ). However, at the outer limit D 3  of this annulus, a sharp outer-wall contour may be produced, with degrading effect at diameter D 3  after the lenticle has been replaced; it is therefore again recommended, as in FIG. 4, that a bevel 31&#39; be developed up to diameter D 4 , in the outer annulus (D 4  -D 3 ) which is not relied upon for optical use. 
     Also, by way of example, the near-vision capability and far-vision capability of a particular eye can be enhanced by effecting a first (and far-vision enhancing) curvature profile in the outer annulus (D 3  -D 5 ) of the optically used area of the cornea and a second (and near-vision enhancing) curvature profile in the central circular area defined by the diameter D 5 , with the respective curvature profiles being of circular symmetry about the optical axis and also smoothly continuous at their circle (D 3 ) of contiguous adjacency. 
     And a myopic eye may be given improved far-vision capability without destruction of its natural near-vision capability, provided the myopic error coincides with appropriate near-vision distance, by limiting the area of myopic correction to the area (D 3  -D 5 ) outside the central optically used area D 5  of the cornea. This result is achieved by laser ablation of the outer surface 11&#39; within the area (D 5  -D 3 ) to a concave profile annulus similar in all other respects to profile 31 described in connection with FIG. 4, i.e., with tissue ablating exposure only within the annulus area (D 5  -D 3 ) which would correct the myopic refractive error for distance. Various degrees of tissue-ablating exposure to the small central area (diameter D 5 ) would be necesssary only if the inherent myopic refractive error were greater or less than the appropriate distance for reading or other near-vision capability required by the patient&#39;s work. Again preferably, a bevel sculpture 31&#39; up to diameter D 4 , would be made in the annular area (D 3  -D 4 ) not relied upon for optical use.