Patent Publication Number: US-6210399-B1

Title: Noncontact laser microsurgical method

Description:
This application is a continuation, of application Ser. No. 08/302,382, filed Sep. 8, 1994, now abandoned which is a continuation application of Ser. No. 08/132,352, filed Oct. 6, 1993, now abandoned which is a divisional application of Ser. No. 07/788,513, filed Nov. 6, 1991, now U.S. Pat. No. 5,281,211, which is a divisional application of Ser. No. 07/363,174, filed Jun. 7, 1989, now U.S. Pat No. 5,152,759. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a microsurgical apparatus and, more particularly, a noncontact laser microsurgical apparatus adapted for use in cornea transplant surgery, keratoplasty, keratotomy, and other corneal surgery techniques. 
     2. Description of the Related Art 
     Despite advances in corneal preservation and transplantation techniques, postoperative astigmatism remains the most important complication limiting visual acuity after a corneal transplantation. 
     In order to reduce such postoperative astigmatism, U.S. patent application Ser. No. 07/056,711 filed Jun. 2, 1987 entitled “A Cornea Laser-Cutting Apparatus”, assigned to the same assignee as the present application, discloses that trephination of either a donor cornea or a recipient cornea may be performed utilizing a laser cutting technique. 
     During penetrating keratoplasty, it is further necessary for a surgeon to align the circumferences of the donor corneal button and recipient cornea. To this end, there have been recently developed mechanical marking apparatuses such as those described in Pflugfelder et al. “A Suction Trephine Block for Marking Donor Corneal buttons,” Arch. Ophthalmol., Vol. 106, Feb. 1988, and Gilbard et al. “A New Donor Cornea Marker and Punch for Penetrating Keratoplasty,” Ophthalmic Surgery, Vol. 18, No. 12, December 1987. 
     However, such mechanical marking apparatuses directly contact and distort the cornea such that the marking process is not always precisely accomplished and sometimes results in porstkeratoplasty astigmatism. 
     In radial keratotomy, mechanical contact type surgical utensils as shown in U.S. Pat. No. 4,417,579 have been used to radially incise the cornea of a patient&#39;s eye. This surgical method is apt to cause strain and/or deformation of the cornea, and also results in postoperative astigmatism. 
     Noncontact microsurgery of the cornea would minimize distortion of the cornea tissue, as occurs in contact-type techniques, and would decrease the likelihood of producing postoperative astigmatism. The use of lasers provides the potential for such noncontact microsurgery. 
     Excimer lasers have been investigated in the past to produce linear corneal incisions or excisions. The argon fluoride excimer laser emitting at 193 nm has been shown to produce sharp, smooth-walled corneal cuts. More recently, the hydrogen fluoride, Q-switched Er:YAG, and Raman-shifted Nd:YAG lasers emitting at about 2.9 um (micro meters), which corresponds to the peak absorption wavelength of water, have been experimentally used to produce linear corneal incisions or excisions. 
     Industrial laser cutting by focusing the beam into a ring has been proposed as a method for drilling large diameter holes The axicon, a diverging prismatic lens, has been used for such industrial purposes. An axicon system has been used by Beckman &amp; Associates to study corneal trephination with a carbon dioxide laser. This experimentation is described in an article entitled “Limbectomies, Keratectomies and Keratectomies Performed With a Rapid-Pulsed Carbon Dioxide Laser,” American Journal of Ophthalmology, Vol. 71, No. 6, (June 1971). In this article, Beckman et al. describe the use of an axicon lens in combination with a focusing lens to form an “optical trephine” and perform various corneal experiments with animal&#39;s. The diameter of the trephine was governed by the focal length of the focusing lens in these experiments. Therefore, to vary the diameter of the annular beam it was necessary to change the focusing lens which acted to change the width of the annular ring and, thus, varied the amount of tissue incised or excised by the laser. Moreover, changing the focusing lens requires a time consuming process for each patient or donor. In addition, the optical system proposed in the Beckman et al. article requires the use of multiple focusing lenses of different focal length. 
     Accordingly, it is an object of the present invention to provide a noncontact laser microsurgical apparatus and method of using the same which substantially eliminates strain and/or deformation on a cornea during and after trephination. 
     Yet another object of the present invention is to provide a noncontact laser microsurgical apparatus and surgical method which is capable of marking a recipient cornea and a donor corneal button with a suture track during keratoplasty, and which incises or excises selected portions of a cornea radially and/or paraxially during keratotomy. 
     Still another object of the present invention is to provide a noncontact laser microsurgical apparatus and surgical method which is capable of performing thermokeratoplasty for curing corneal refractive error and/or astigmatism of a patient&#39;s eye. 
     It is still another object of the present invention to provide a noncontact laser microsurgical apparatus and method capable of surgically “welding” donor tissue or synthetic material and recipient corneal tissue together thereby eliminating the necessity of suturing the donor and recipient parts to one another in penetrating and epikeratoplasty procedures. 
     It is still a further object of the present invention to provide a noncontact laser microsurgical apparatus and method wherein selected areas of the cornea may be caused to shrink so as to change the curvature of the natural lens thereby curing or alleviating corneal refractive error and/or asigmatism. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
     SUMMARY OF THE INVENTION 
     To achieve the foregoing objects, and in accordance with the purposes of the invention as embodied and broadly described herein, the noncontact laser microsurgical apparatus of the present invention comprises means for generating laser beams; and means for projecting the laser beams onto a cornea. The projection means defines an optical axis and includes means for converging the laser beams. The projecting means further includes axicon optical means for forming the projected beams into a plurality of paraxially distributed spots on the cornea, and means for varying the radial position of the spots. 
     Preferably, the converging means includes a focusing lens and the axicon optical means includes at least one multiple-facet prismatic (“MFP”) axicon lens mounted for movement along the optical axis of the projecting means. 
     The generating means may comprise an infrared pulse laser beam generator with a preferred wavelength of about 1.3-3.3 um. Also, an ultra-violet laser source may be used such as an Argon fluoride laser emitting at 193 nm. 
     The projecting means preferably includes beam expander means for enlarging the radius of the laser beam emerging from the generating means. 
     The apparatus may also include aiming means for projecting visible laser beams onto the cornea substantially coincident with the positions at which the laser beams projected through the axicon means impinge the cornea. The optical axis of the aiming means preferably overlaps with at least a portion of the optic:al axis of the projecting means. Preferably, the aiming means includes a visible laser beam source, and a mirror obliquely interposed between the beam expander means and the converging means for reflecting the visible laser beams and allowing the laser beams from the generating means to pass therethrough. 
     The apparatus of the present invention may also include mask means disposed in the optical axis for selectively blocking portions of the projected laser beams while transmitting the remaining portions of the projected laser beams therethrough. In this manner incisions or excisions in the corneal tissue may be made only in selected areas of the cornea corresponding to the transmitting portions of the mask means. 
     The present invention also provides a microsurgical method for ablating the cornea in selected areas by appropriate use of the disclosed apparatus. Moreover, by careful selection of the laser generating means the corneal tissue may be heated only sufficiently to cause shrinkage of the tissue in selected areas to alleviate astigmatism and/or corneal refractive error. Furthermore, by appropriate control of the selected laser generating means donor and recipient corneal tissue may be heated in abutting areas to cause the disparate tissue to adhere to one another in the manner of a surgical weld to thereby eliminate the need for sutures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments and methods of the invention and, together with the general description given above and the detailed description of the preferred embodiments and method given below, serve to explain the principles of the invention. In the drawings: 
     FIG. 1 illustrates an optical system arrangement of a non-contact laser microsurgical apparatus incorporating the teachings of the present invention; 
     FIG. 2A is a plan view of a concave MFP axicon lens; 
     FIG. 2B is a cross-sectional view of the MFP axicon lens shown in FIG. 2A; 
     FIG. 3A is a plan view of a convex MFP axicon lens; 
     FIG. 3B is a side view of the MFP axicon lens shown in FIG. 3A; 
     FIG. 4 illustrates a donor cornea holding device which may be used with the apparatus and method of the present invention; 
     FIG. 5 is a plan view of a donor cornea marked by the apparatus of the present invention with spots defining a suture track; 
     FIG. 6 is a plan view of an alignment of a donor corneal button with a recipient cornea with spot marks defining a suture track in each; 
     FIG. 7 is a cross-sectional view of the alignment shown in FIG. 6; 
     FIG. 8 is a cross-sectional view of a cornea of a patient&#39;s eye marked with incisions formed during radial keratotomy; 
     FIG. 9 illustrates a plan view of the eye shown in FIG. 8; 
     FIG. 10 illustrates a plan view of a patient&#39;s eye which has been subjected to curved keratotomy; 
     FIG. 11 illustrates one embodiment of a mask means which may be used with the present invention; 
     FIG. 12 illustrates the angular orientation of the apertures defined by the masked means of FIG. 11; 
     FIG. 13 illustrates a cutaway side view of a portion of the cornea wherein portions of the stroma of the cornea have been treated by thermokeratoplasty; 
     FIG. 14 illustrates a front view of an eye which has been treated in selected areas utilizing the method and apparatus of the present invention with mask means; 
     FIG. 15 illustrates another embodiment of the mask means of the present invention; 
     FIG. 16 illustrates still another embodiment of the mask means of the present invention; 
     FIG. 17 is a cutaway side view of a portion of a cornea ablated by using the method and apparatus of the present invention and which illustrates the configuration of the cuts in the cornea which may be made utilizing the mask means of FIGS. 15 and 16; 
     FIG. 18 illustrates still another embodiment of the mask means of the present invention; 
     FIG. 19 illustrates the fully open position of the mask means of FIG. 18; 
     FIG. 20 illustrates the fully closed position of the mask means of FIG. 18; and 
     FIG. 21 illustrates the relationship between the incised or excised depth of cuts made in the stroma and the circumferential position of annularly shaped laser beams projected by the mask means of FIG.  18 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS AND METHOD 
     Reference will now be made in detail to the presently preferred embodiments and method of the invention as illustrated in the accompanying drawings in which like reference characters designate like or corresponding parts throughout the several drawings. 
     An optical delivery system of a noncontact laser microsurgical apparatus incorporating the teachings of the present invention includes means for generating laser beams. 
     As illustrated in FIG.  1  and embodied herein, the generating means of apparatus  10  comprises a laser source  11  which generates pulsed laser beams that are capable of ablating the tissue of a living organ, i.e., a cornea. Lasers which meet the requirements described above may include HF (Hydrogen Fluoride) lasers and Er-YAG (Erbium-Yttrium Aluminum Garnet) lasers which emit infrared pulses having wavelengths of about 2.0 to about 3.0 um, and preferably about 2.9 um, and which have a pulse duration of less than 200 ns and an energy flux of greater than about 250 mj/cm 2 . Also operable with the present invention are ArF (Argon Fluoride) lasers which produce an ultra-violet laser beam having wavelengths of less than about 200 nm with a pulse duration of about 10-23 ns and an energy flux of about 70 mj/cm 2 . Laser source  11  is connected to a radiation control switch lia. When the control switch is moved to the “on” position laser source  11  generates infrared pulsed beams. Radiation control switch  11   a  is preferably capable of controlling the energy of the pulse beams by insertion of a neutral density filter (not shown) in a transmitting path thereof. 
     In instances where it is desirable to heat the corneal tissue without causing ablation, the laser source selected may be an C.W.HF, or Holmium, or Nd:YAG laser emitting at a wavelength of 1.3-3.3 um, a pulse duration of greater than 200 n sec, and an energy flux of about 250 mJ/cm 2 . 
     In accordance with the present invention, the apparatus includes means for projecting the laser beams along an optical axis onto the cornea. As embodied herein, the projecting means includes a beam expander means, generally referred to as  13 , fc,r expanding the laser beam generated by laser source  11 . Beam expander means  13  includes a concave lens  14  and a convex lens  15 . Laser beams emerging from convex lens  15  are formed in parallel and are in turn projected along optical axis O 1 . The beam expander means may also comprise a variable diverging beam expander comprised of a conventional zooming optical system or a pair of movable convex-concave axicon lenses described in the above-mentioned U.S. patent application Ser. No. 056,711. 
     In accordance with the present invention, the apparatus includes means for converging the projected laser beams onto the cornea. As embodied herein, the converging means includes a condensing and focusing lens  17  which functions to condense and focus light passing therethrough onto a focal plane. The position of the focal plane is determined in accordance with the geometry of the lens as will be well understood by those skilled in the art. Beam expander  13  also acts to increase the laser beam diameter entering lens  17 , thereby increasing the numerical aperture of the optical system. Consequently, the focal spot of the beam at the focal plane of lens  17  is reduced. 
     In accordance with a first embodiment of the present invention, the apparatus includes axicon optical means for forming the converged laser beams into a plurality of paraxially distributed beam spots on the cornea. As embodied herein the axicon optical means may comprise a multiple-facet prismatic (“MFP”) lens  18 . 
     As shown in FIGS. 2A and 2B, axicon lens  18  may be configured with concave multiple-facet prisms (eight-facet prisms, for example) whose outer edges (prism bases) 100 are larger in width than the optical axis portion  102  thereof. Axicon lens  18  may also be configured with convex multiple-facet prismatic lenses as shown in FIGS. 3A and 3B whose outer edges (prism bases) 100 are larger in width than optical axis portions  102  thereof. With reference to FIG. 6, the multiple-facet prism function of axicon lens  18  causes the converging laser beams emerging from lens  18  to be formed into a plurality of paraxially distributed beam spots  61  on cornea  31  of eye  30 . The spots  61  are radially spaced from the apex of cornea  31  and from optical axis O 1 . Preferably, the laser beams are converged onto cornea  31  of eye  30  (a patient&#39;s eye or donor&#39;s eye or tissue held in the holding device shown in FIG. 4) after reflection by a dichroic mirror  19  as shown in FIG.  1 . As a result, cornea  31  is marked, excised, or incised by the laser beam energy at spots  61 . 
     It is necessary to vary the cornea marking, excising, or incising diameters in keratoplasty and keratotomy. To this end, the present invention may include means for moving the axicon optical means along the optical axis O 1 . Furthermore, in order to carry out curved keratotomy in accordance with the present invention, means may be provided to rotate the axicon optical means about the optical axis. Furthermore, to perform radial keratotomy, means may be provided for moving the converging means along the optical axis. 
     As embodied herein, movement of the axicon optical means comprised of MFP lens  18 , and movement of the converging means comprised of focusing lens  17  along optical axis O 1 , and rotation of MFP axicon lens  18  about optical axis O 1 , may be carried out by a known electromechanical device (not shown) which may comprise a combination of stepping motors, for example, controlled by a microprocessor or minicomputer (not shown). By way of example and not limitation, model no. SPH-35AB-006 stepping motors manufactured by Tokyo Electronic Co., Ltd. may be used to move MFP lens  18  and focusing lens  17 . 
     In accordance with the present invention, the microsurgical apparatus incorporating the teachings of the present invention may include aiming means for projecting visible light beams onto the cornea substantially coincident with the positions at which the laser beams are to impinge the cornea. As embodied herein, the aiming means comprises an aiming system  20  which includes a He-Ne laser source  21  for generating visible light beams, beam expander means  22 , and dichrotic beam combiner  16 . Beam expander means  22  may comprise a concave lens  23  and a convex lens  24 . Dichrotic beam combiner  16  is positioned in optical axis O 1  and is selected such that it functions to reflect incident He-Ne laser beams, while laser beams from source  11  pass therethrough. 
     He-Ne laser beams from laser source  21  are enlarged in diameter by expander means  22 , whose output laser beams, in turn, are projected onto condensing and focusing lens  17  after reflection by cold mirror  16 . Thus, laser beams from source  21 , which are reflected by mirror  16 , are coincident with a portion of optical axis O 1.    
     For observing the cornea and the He-Ne laser beams projected thereon, the noncontact laser microsurgical apparatus may also include viewing means comprised of an operation microscope  2 , indicated by phantom lines in FIG.  1 . The configuration and function of operation microscopes is well known in the ophthalmology field, therefore, its detailed description is omitted. By way of example and not limitation, a model no. OMS-600 microscope manufactured by TOPCON CORPORATION may be used. 
     An optical axis O 2  of operation microscope  2  is disposed to be coincident with a portion of optical axis O 1  of the noncontact laser microsurgical apparatus  10 . In this configuration, mirror  19  functions as a half mirror for the laser beams from source  21 , but as a complete mirror for the laser beams from source  11 . An operator can determine an optimum diameter size of the laser beams projected onto the cornea by observing the laser beams from source  21  through operation microscope  2 . 
     For marking a donor corneal button, the noncontact laser microsurgical apparatus may be coupled to a donor cornea holding device  40  as shown in FIG.  4 . Donor cornea holding device  40  is provided with a housing cover  41  and a receiving pedestal  42 . Housing cover  41  includes an “O”-ring  43  disposed on an inner wall  44 , and an annular magnet member  45 . Receiving pedestal  42  includes a convex portion  46 , a gas tube  47  extending from the center of the convex portion  46  to a pressure pump  491  a pressure gauge  48 , and a magnet member  50  which is disposed opposite to and attracts the magnet member  45  of housing  41 . In operation, a donor cornea or corneascleral tissue  51  is mounted on the receiving pedastal  42  and the housing cover  41  is placed over button  51 . Due to attractive forces between magnet members  45  and  50 , “O”-ring  43  presses the corneal button tightly to the housing pedestal  42 . Pressurized gas or fluid is supplied to the underside of the corneal tissue  51  through the tube  47  so that the donor tissue is maintained with a constant underside pressure which may be controlled to correspond to the intraocular pressure of the live eye, about 15 mmHg-20 mmHg, by monitoring pressure gauge  48 . 
     The steps of the method of the present invention for marking a cornea in transplanting surgery or keratoplasty can be carried out by using the noncontact laser microsurgical apparatus as hereinbelow described. 
     (1) A donor cornea or corneascleral tissue  51  cut out from a donor eye is mounted on the cornea holding device  40 . The donor cornea is pressurized at normal intraocular pressure. (15-20 mmHg) on the underside thereof by the fluid supplied from pump  49  through tube  47 . 
     (2) The cornea holding device  40  is coupled to the laser microsurgical apparatus shown in FIG. 1 by appropriate mechanical means (not shown). 
     (3) Visible aiming laser light beams from source  21  are projected onto the cornea of a patient or onto a donor cornea through beam expander means  22 , cold mirror  16 , condensing lens  17 , MFP axicon lens  18 , and dichroic mirror  19 . 
     (4) The visible aiming laser beams projected onto the cornea are observed through operation microscope  2 . The diameter and position of the projected aiming lasier beams are adjusted by moving the noncontact laser microsurgical apparatus and the operation microscope in tandem along optical axis O 2 . 
     (5) Next, axicon lens  18  is moved along optical axis O 1  so that the diameter or radial positions of paraxially distributed aiming laser beam spots from source  21  i.s adjusted to the desired size. 
     (6) After the diameter or radial positions of aiming laser beam spots has been set at an optimum size or radial position, control switch  11   a  is turned on and laser source  11  generates the infrared or ultraviolet pulsed laser beams. 
     (7) The pulsed laser beams are projected onto the cornea through the beam expander  13 , cold mirror  16 , condensing lens  17 , MFP axicon lens  18  and the dichroic mirror  19 , respectively, to mark the donor cornea or corneascleral tissue with spots  61  as shown in FIGS. 5 and 6. When irradiating laser energy is properly controlled, light-point marking in accordance with this particular embodiment of the method of the present invention can be carried out on the epithelium of the donor cornea  51 . 
     (8) The donor cornea may then be trimmed or cut about its periphery so as to match the diameter of a recipient hole in the recipient cornea and is then preserved, is noncontact laser microsurgical cutting apparatus as disclosed in U.S. patent application Ser. No. 056,711, commonly assigned with this application and incorporated herein by reference, may be used to cut a donor corneal button and a recipient cornea. 
     (9) After.the cornea holding device  40  has been removed from the noncontact laser microsurgical apparatus, a recipient eye is set up as shown in FIG.  1 . The recipient cornea is also subject to marking and cutting as stated in the above steps (3)-(8), provided, however, that the diameter and radial positions of the paraxially distributed beam spots  61  on the recipient cornea is made slightly larger than the diameter and radial positions of the beam spots  61  on the donor corneal button. 
     (10) Next, the donor corneal button  51  is aligned with the recipient cornea  52  by matching the paraxially distributed spots  61  as shown in FIG.  6 . The cornea has epithelium  71 , stroma  72 , and endothelium  73  as shown in FIG.  7 . The paraxially distributed spots  61  on the donor corneal button  51  and recipient cornea  31  may be marked only on the epithelium  71 , or through the epithelium, the Bowman&#39;s layer and to a predetermined depth (for example 100 um) into the stroma, to create suture tracks. 
     (11) Suturing is then carried out through the paraxially distributed dots as indicated by dotted line  74  in FIG.  7 . 
     By carrying out the steps of the first embodiment of the method of the present invention as described above, eight dots are marked on each of the donor corneal button  51  and the recipient cornea  31  as shown in FIG.  6 . 
     When sixteen spots are required to be marked, after the first marking steps are carried out as described above, axicon lens  18  may be rotated by a predetermined angle about optical axis O 1 , for example 22.5°, to mark an additional 8 spots. Similarly, an additional eight suture tracks are marked on the recipient cornea  31  so that sixteen symmetrical spots are marked on both the donor corneal button and the recipient cornea. 
     The noncontact laser microsurgical apparatus of the present invention may also be utilized in keratotomy, as will be described hereinbelow. 
     The diameter or radial positions of the distributed beam spots can be adjusted by moving MFP axicon lens  18  along optical axis O 1  as already explained hereinabove in connection with keratoplasty. Moreover, in radial keratotomy where the radial incisions or excisions made in the cornea extend outwardly from the apex of the cornea, the focal plane of the laser beams must. be varied since the curvature of the cornea causes the radial incision or excisions in the cornea to be formed at varying distances from the laser source as the incision or excision moves progressively outward from the apex of the eye. Thus, to insure that the laser beams are brought to focus in the correct focal plane corresponding to the varying depths or distances at different radial positions on the cornea, the focal plane of the apparatus of the present invention is adjusted by moving condensing lens  17  along optical axis O 1 . 
     In radial keratotomy, wherein incisions or excisions are produced in the stroma and are oriented radially from the center of the cornea to the outer edge of the cornea, the curvature of a cornea is measured in advance by a keratometer so as to predict the distance of the focal plane of the laser beams at all radial positions on the cornea. With reference to FIG. 8, after the initial focal points “C” have been set in the upper portion of stroma  72 , MFP axicon lens  18  is moved axially and preferably continuously within a predetermined range along optical axis O 1  so that radial incisions or excisions in stroma  72  may be made along the curvature of the cornea from points “C” toward points “D” as shown in FIGS. 8 and 9, FIG. 8 being a cutaway side view of FIG. 9 along lines  8 - 8 . Simultaneously, condensing lens  17  is moved axially and preferably continuously along optical axis O 1  to adjust the focal plane of the laser beams to correspond to the radial position of the cornea at which the incision or excision is being made. 
     Laser energy from source  11  may be set to a sufficient energy to ablate and incise or excise the tissue of stroma  72  to a desired depth “Δ”. 
     In curved keratotomy, wherein the excisions or incisions in stroma  72  are oriented as curved portions, i.e., as circumferential arcs spaced a predetermined and substantially constant radial distance from the apex or center of the cornea, MFP axicon lens  18  may be comprised of at least one two-facet prismatic lens. After focal points “C” have been set in stroma  72 , MFP axicon lens  18  is rotated about optical axis O 1  within a predetermined rotating angle while laser source  11  is emitting beams of sufficient energy to ablate the stroma, thereby producing incisions or excisions in the stroma as illustrated in FIG.  10 . 
     In another embodiment of curved keratotomy utilizing the apparatus and method of the present invention, MFP axicon lens  18  may be composed of at least one MFP lens having more than two facets, eight facets for example, which is rotatable about the optical axis O 1 , Furthermore, apparatus  10  may include mask means, disposed between focusing and condensing lens  17  and cold mirror  16 , for selectively defining at least one open aperture in optical axis O 1 . In the present preferred embodiment, the mask means includes mask  25  having three mask units  26 ,  27 , and  28  as shown in FIG.  11 . Mask units  26  and  27  each include a pair of transparent portions  26   a ,  27   a  configured as 120° fan-shaped aperture angles  110  for transmitting laser beams emerging from cold mirror  16  therethrough, and a-pair of opaque portions  26   b ,  27   b  configured as 60° fan-shaped aperture angles  112  for blocking the laser beams emerging from cold mirror  16 . 
     In accordance with the present invention, means may be provided for rotating mask units  26  and  27  in opposite directions, designated by arrows “a” and “b” in FIG. 11, relative one another about optical axis O 1  to change aperture angles  110   a  and  112   a  within a prescribed range, for example, between 60°-120° as illustrated FIG.  12 . As embodied herein the rotating means may comprise a stepping motor (not shown) controlled by a microprocessor (not shown). One skilled in the art will readily identify such motors and controllers, and, since the configuration of these motors and controllers do not themselves constitute any portion of the present invention, a detailed description is omitted. However, by way of example, a model no, SPH-35AB-006 type motor manufactured by Tokyo Electric Co., Ltd, may be used. Also, any suitable controller such as an IBM PC/Ar may be used to control the motor. 
     The mask means may also include a mask unit  28  having a pair of semicircular portions  28   a ,  28   b . Semicircular portion  28   a  is transparent to transmit laser beams therethrough, and semicircular portion  28   b  is opaque to the laser beams. Means may be provided to insert and remove mask unit  28  within optical axis O 1  to selectively block a portion of the laser beams from source  11 , the blocked portion corresponding to the position of opaque semi-circular portion  28   b . The mask units  26 ,  27  and  28  are constructed to be able to rotate in unison about optical axis O 1  to selectively position the apertures defined by masks  25  and  26  in optical axis O 1 . 
     In this embodiment, after aperture angles  110   a  and  112   a  have been selected by individual opposite rotation of masks  25  and  26 , and after the inclination angle β illustrated in FIG. 12 has been selected by common rotation about axis O 1  of masks  25  and  26  as a unit, the MFP axicon lens  18  rotates continuously in predetermined minute steps about optical axis O 1  for incising or excising the cornea. 
     The noncontact laser microsurgical apparatus and method of the present invention has further applications to thermokeratoplasty as will be described hereinbelow. 
     In thermokeratoplasty, for curing corneal refractive errors, e.g., hyperopia, myopia and/or astigmatism, laser source  11  may comprise an infrared pulse type laser that emits a laser pulse having wavelengths of about 1.3 um to about 3.3 um. Laser source  11 , in this embodiment, may comprise a Ho (Holmium) laser, for example, emitting at a wavelength of about 2.1 um. 
     With reference to FIG. 13, in thermokeratoplasty for curing hyperopia, for example, after the focal points “C” have been set at points “e” with an optimum diameter and an optimum depth “Δ” in the stroma  72  by independently moving focusing and condensing lens  17  and MFP axicon lens  18  along optical axis O 1  as has been described above, the Ho laser source  11  generates and projects the pulse laser beams onto the points “e”. 
     Eight burns are formed instantly and simultaneously at the points “e” and cause shrinkage of the tissue of the stroma in thus vicinity “f” around the points “e. ” The thermal effect of the laser causing shrinkage of the tissue about points “e” leads to a change of the shape of the cornea to alleviate hyperopia. 
     With reference to FIG. 14, in thermokeratoplasty for alleviating astigmatism, mask unit  25  may be used to selectively define the angular position b and the angle a, and to set the number of spots  61  projected onto the points “e”. The thermal effect of the laser and the resulting shrinkage of tissue in vicinity “f” lead to a change of the shape of the cornea to alleviate the astigmatism. 
     In the embodiments of the method and apparatus of the present invention described above, the tissue of the cornea is impinged with laser beams of sufficient energy and for a sufficiently long time that the cornea is ablated by causing the fluids comprising the corneal tissue to go from solid or liquid phases to a gaseous phase. Such ablation is termed photo-vaporization when carried out using a hydrogen-fluoride short pulsed, and photo decomposition when carried out using an argon-fluoride excimer laser. 
     In other embodiments of the surgical method of the present invention, it is desirable not to vaporize or decompose the corneal tissue. Since the corneal tissue is comprised almost entirely of water, this means that under atmospheric pressure the temperature to which the corneal tissue is raised must be less than 100° C. This is best carried out with lasers having longer pulse durations, such as a long-pulsed C.W.HF laser, Nd:YAG laser, or a Holmium laser each having pulse durations greater than 200 ns and an energy flux of about 250 mJ/cm 2 . 
     The inventors herein have discovered that above 60° C., the corneal tissue becomes adhesive and will begin to shrink. Thus, in one embodiment of the method of the present invention, selected areas of the cornea are impinged with laser beams having sufficient energy and for a sufficient time to heat the corneal tissue to less than about 100° C. to thereby cause shrinkage of the corneal tissue in those selected areas. In this manner the shape of the cornea and the curvature of the crystalline lens of the eye can be modified to alleviate astigmatism and/or corneal refractive error. Moreover, any of the embodiments of the apparatus of the present invention may be used in the practice of this method for shrinking corneal tissue to thereby control and define the selected areas of the cornea in which shrinkage is desirable. 
     In a further embodiment of the method of the present invention, where, during corneal transplant surgery, the donor cornea button has been placed in the recipient cornea, the abutting edges of the donor button and recipient cornea may be impinged with laser beams of sufficient energy and duration to heat the abutting edges to between about 60° C.-85° C. For this embodiment it is desirable to use a laser source having a pulse duration of greater than about 200 ns, a frequency of about 2.0-3.0 um, and an energy flux less than about 250 mJ/cm 2 . In this manner, the tissue at the edges of the button and recipient hole become adhesive and attach to one another in a type of surgical weld thereby eliminating the need for sutures. This “welding” method may also be employed in epi-keratoplasty to attach a piece of cornea-like tissue over a whole recipient cornea to act in the manner of a contact lens. This cornea-like tissue may be human, animal or synthetic. 
     In a second embodiment of the present invention, a conical axicon lens  18  illustrated with the dot-dash line in FIG. 1 may used in place of MFP axicon lens  18 . In such an embodiment the conical axicon lens need not be rotated about optical axis O 1  to perform curved keratotomy because the conical axicon lens converts the laser beams exiting from condensing and focusing lens  17  into annular beams and projects them toward the patient&#39;s cornea. 
     Each of FIGS. 15 and 16 illustrate alternative embodiments of the mask means. In both alternative embodiments of the mask means, two masks are spaced from one another on optical axis O 1 . Each mask is identical in configuration. Therefore, only one of the masks, mask  130 , is illustrated in FIGS. 15 and 16. 
     By way of example and not limitation, mask  130  may comprise a metal plate having two fan-shaped opaque portions  132  and  134 , and side edges  136  and  138  which are formed in a concave arc as shown in FIG. 15, or in a convex arc as shown in FIG.  16 . Sinces the energy density of arc laser beams emerging through the aperture defined by side edges  136  and  138  decreases gradually along the radial direction of the arcs defined by the side edges  136  and  138 , the depth of the incised or excised cut on the patient&#39;s cornea is shallowed gradually and smoothly at both end portions as shown in FIG. 17 such that delicate or subtle curved keratotomy may be carried out utilizing the method and apparatus of the present invention. 
     FIG. 18 illustrates a still further embodiment of the mask means. Therein, masks  150  and  152  having wedge shaped portions  154  and  156 , respectively, may be moved simultaneously or individually in opposite directions relative one another along arrows  158  and  160 , respectively. In this manner, the aperture angle α defined by the edges of the wedge portions  154  and  156  may be selected in a predetermined manner. Furthermore, masks  150  and  152  may be rotated individually or in unison by any suitable motor or manual means about optical axis O 1  to selectively define the position angle b of the incised or excised cuts on the cornea of the eye. 
     Utilizing the mask means illustrated in FIG. 18, masks  150  and  152  may be moved towards each other along directions  158  and  160 , respectively, during impingement of the cornea with laser beams. In FIG. 19, masks  150  and  152  are in the fully open state such that no portion of the cornea is blocked by wedge portions  154  and  156 . As masks  150  and  152  are moved closer together as illustrated in FIG. 18, wedge portions  154  and  156  selectively block larger portions of the cornea from the laser beams emitted by source  11 . In FIG. 20, wedge portions  154  and  156  overlap each other to fully block the cornea from any laser beams emitted by source  11 . 
     Utilizing the mask means illustrated in FIG. 18 in combination with a conical axicon lens, the annular laser beams P projected onto the cornea are changed in total emitted energy in accordance with the circumferential position d as illustrated in FIG.  19 . Therefore, the incised or excised depth D of the cuts made in the stroma are gradually changed in depth to resemble a sine curve as illustrated in FIG.  21 . In this manner, alleviation of corneal astigmatism may be carried out more delicately and subtly utilizing the teachings of the present invention to cure not only corneal astigmatism, but also corneal spherical power error. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.