Abstract:
An economical computer-controlled non-invasive laser apparatus and method to perform anterior segment surgery in an eye are disclosed. The laser source may include a pumping laser, a Nd:YAG laser cavity gain media, a stimulated Raman converter crystal, intracavity beam diameter-reducing optics, and an intracavity Q-switching crystal. The laser pulses have a selected wavelength for anterior segment surgery. A laser pulse delivery and treatment control mechanism and method for the practicing surgeon are also provided. The laser pulses and delivery system may be used in anterior segment surgery for cataracts, where the laser pulses may be used to form the capsulotomy, to form the corneal incision or to disintegrate contents of the capsule before removal. The laser and delivery system may also the used to treat a capsule and lens for correcting or preventing presbyopia and to treat a cornea to correct visual deficiencies in an eye.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates to an anterior segment surgical apparatus in ocular surgery, and more particularly to a laser apparatus having application in cataract surgery for capsulorhexis, i.e. the cutting of a capsulotomy in the anterior capsule of a lens of an eye, and for other procedures in cataract surgery. The apparatus may also be used in surgical procedures for the cornea and posterior capsule. 
     2. Description of Related Art 
     The standard procedure for correcting vision loss due to cataracts is to remove the natural lens and to replace it with a prosthetic lens. In typical cataract surgery, an initial incision is made into the cornea of the eye and then the surgeon creates a circular opening in the anterior lens capsule. This is referred to as a capsulotomy. Producing the circular opening is often a critically demanding procedure. It is also known as an anterior lens capsulectomy once the circular fragment of the lens capsule is removed. The latter name is in reference to the analogous posterior lens capsulotomy, which has come to be well known in surgery practice and which involves the use of a pulsed YAG laser. Once the anterior capsulotomy is completed, the natural lens cortex is extracted by breaking it up into small pieces, which are drawn through the initial opening. As this procedure and pseudophakic materials have improved over time, the initial incision has become smaller, such that it is now normally less than 3 mm. Reducing the initial incision has improved the surgical outcome but has placed higher and higher demands on the surgeon and the surgical instruments that pass through the initial incision. 
     The anterior capsulotomy portion of cataract surgery as defined above preferably results in a circular opening of a selected diameter and without radial tears. Ideally, it has smooth edges. The usual instruments used by the surgeon are the cystotome or forceps, which are used to basically puncture and tear the capsule tissue to produce the opening. The results are at best imperfect circles, and sometimes radial tears or other adverse events occur. For advanced lens technologies, it is particularly important to consistently produce a smooth, intact and round capsulotomy without radial tears in the capsule. 
     Laser techniques that have been attempted include the application of the well established posterior capsulotomy YAG laser operating with a 1064 nm wavelength. In 1981, Aron-Rosa reported on laser opening of the anterior capsule from 1 to 24 hours before extra-capsular cataract surgery. (Am Intra-ocular Implant Soc J, Vol 7, p. 332, 1981) It was shown that by depositing one laser pulse at a time aimed at the anterior lens capsule, a rudimentary capsulectomy could be produced, but with complications in some cases. The complications included high intraocular pressures caused by the laser pulse shock wave, edge roughness and irregular shaped capsulectomies. Unintended exposure of the retina to hazardous levels of laser radiation can occur if the pulse is not blocked by a necessary plasma breakdown process at the focal point at the lens anterior capsule. Other disadvantages include the tedium of depositing a few hundred pulses, one pulse at a time, a situation that can contribute to operator error. For these reasons, such a technique has not been accepted by surgeons. 
     In 1982, Horn et al reported on the use of a “cool” laser operating at a 1220 nm wavelength. (Am Intra-ocular Implant Soc J, Vol 8, 1982) The intended objective of moving from the 1064 nm wavelength to the 1220 nm wavelength was to cut power requirements 100-fold and avoid jeopardizing the retina when doing anterior chamber surgical treatments. Horn et al used a very elaborate laser system to achieve the preferred wavelength: a Nd:YAG pump laser source was converted to 532 nm, which was then used to pump a 600 nm dye laser, and finally converted to a 1220 nm laser source by means of a high-pressure hydrogen gas cell. The work was done on rabbit subjects. No report of follow-on work was found. 
     Various reports of the use of lasers in cataract surgery have appeared in more recent years. In 2009, a LenSx femtosecond laser received approval from the U.S. Food and Drug Administration for creation of the capsulorhexis during cataract surgery. (Rev of Ophthalmology, October 2009, p. 29) A recent patent application by the same company discussed the use of a pulsed laser for: photodisruption of a portion of a targeted region in the lens of an eye, for making an incision in the capsule of the lens and for making an incision in the cornea of the eye (WO 2009/039302 A2). These lasers normally emit at wavelengths shorter than 1000 nm, which raises their potential of affecting the sensitive retina. The femtosecond lasers are also expensive and require substantial maintenance. 
     What is needed is a pulsed laser system that can be used to form the capsulorhexis during cataract surgery that is effective and economical, so that it can be made widely available for use by surgeons, and that employs a wavelength having preferred absorption properties in the tissue of an eye. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention substantially eliminates the above mentioned problems associated with the practice of the prior art and provides an economical, automated non-invasive capsulectomy instrument and method. The invention makes novel use of a selected wavelength in the laser spectrum to reduce the required pulse energy and to produce a fine excision on the lens anterior capsule. To accomplish the generation of effective laser pulses, a laser cavity is disclosed, which may be pumped by two solid state laser diodes, that contains a gain medium, a Raman crystal, either Q-switching or a mode-locking device to generate a train of selected wavelength near-IR laser pulses and necessary optics. Also disclosed is a compact and economical laser pulse delivery system, automated to produce reliable pre-programmed capsulectomies or other anterior segment surgical treatments using computer-controlled pattern generation, which may be used to designate the treatment loci and deliver the laser pulses to the capsule, cornea, or lens of an eye. In some embodiments, computer-controlled beam focus, through-the-optical-axis computer-controlled azimuth angle articulator, computer-controlled elevation angle articulator, dichroic beam splitter and treatment beam director with automated eye tracking, contact lens, operator viewing microscope and operator programmable control computer may be provided. Method for use of the apparatus by a surgeon to form a capsulectomy, which may be formed before an incision in the cornea is made (i.e., be non-invasive), to photodisrupt a lens prior to its removal, to make an incision in a cornea and to treat a cornea to improve vision are also disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more fully understood from the following detailed description of representative embodiments thereof read in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a schematic of the component parts of the laser system of the invention; and 
         FIG. 2  is a schematic in partial axial section of a delivery system suitable for use in one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings,  FIG. 1  shows a schematic layout of laser system  109  of the invention. The function of the laser system is to produce a train of pulses emanating as  166  with sufficient energy to incise capsular tissue. The wavelength of the laser pulse generated by the laser system  109  is selected by considering the absorptive properties of the ocular region and the target tissue. Since most of the ocular media is water, the ideal wavelength of the laser source for ocular capsulectomy is one where water has a selected absorption characteristic; that is, the absorption is not too high or too low. This occurs toward the UV end of the spectrum as well as toward the IR end. The UV end is complicated by sharp absorption discontinuities from corneal regions to aqueous regions. The IR region, on the other hand, progresses smoothly to high absorption, becoming opaque at 1400 nm. The optimum absorption lies between 1064 nm and 1400 nm. To derive an estimate of optimum absorption, Beer&#39;s Law for light absorption may be used:
 
 I=I   0   e   −α·x ,  (Eq. 1)
 
where I is the intensity, power or energy in a beam of initial intensity I 0  at depth x into a media having an absorption coefficient α. The energy deposited E d  in a short distance x 1  to x 2  at a depth x 1  into the media can be expressed as
 
 E   d   =E   0   e   −α·x     1   (1− e   −α·x     2   )  (Eq. 2)
 
There is an absorption coefficient where E d  is at its maximum, which may be determined by solving
 
                         ⅆ     E   d         ⅆ   α       =         ⅆ     ⅆ   α       ⁡     [       ⅇ       -   α     ·     x   1         ⁡     (     1   -     ⅇ       -   α     ·     x   2           )       ]       =   0       ,           (     Eq   .           ⁢   3     )               
which yields
 
                   α   =       1     x   2       ⁢     ln   ⁡     (       x   1         x   1     +     x   2         )                 (     Eq   .           ⁢   4     )               
For lens surgery at the anterior capsule, x 1  can be chosen to be 0.36 cm and x 2  approximately 0.37 cm. This results in
 
α=−1.873 cm −1  
 
A similar value for the target plane being the posterior capsule yields
 
α=0.912 cm −1  
 
     A preferable absorption coefficient should, therefore, lie between these two values. To determine what wavelengths produce these absorption coefficients in water (which is the major portion of the clear ocular tissue) reference is made to published measurements, for example: K. F. Palmer and D. William, “Optical properties of water in the near infrared,” J. Opt. Soc. Am., 1107-1110, 1974. The primary Nd:YAG 1064 nm wavelength, for example, has an absorption coefficient of 0.154 cm −1  in water. This is definitely under-absorbing for targets at the lens capsule, posterior and anterior. The corresponding wavelengths for the above derived absorption coefficients in water based on the data by Palmer and William are 1300 nm for the anterior capsule and about 1150 nm for the posterior capsule. If such a laser source can be found that operates at a wavelength somewhat above 1150 nm and preferably below 1300 nm, the disclosed laser instrument can be advantageously applied to capsulectomies in the anterior and posterior capsule. Since in the present invention the beam is not focused anterior to the lens capsule no harm will be done to anterior tissues and the process can be referred to as being non-invasive. There is limited concern with retinal exposure because the aqueous absorption at 1197 nm is much higher than it is at 800 nm to 1100 nm. Further, since the composition of the lens capsule is high in lipids, it would be advantageous to target wavelengths close to 1200 nm, which is close to a peak lipid absorption band. 
     We have invented an efficient embodiment of such a laser source, and it is obtained by a stimulated Raman-shift process acting on a laser gain crystal, such as a Nd:YAG crystal or a crystal that may be in the same family, such as Nd:VNO 4  (neodymium vanadate). The laser gain crystal will be referred to as Nd:YAG in this discussion. The laser gain crystal preferably has a gain greater than 1%, and more preferably greater than 50%. Referring to  FIG. 1 , we disclose an optical arrangement including Raman crystal  158 , which may be barium nitrate, serving as the wavelength converter embedded in a laser cavity. A barium nitrate crystal, when pumped by 1064 nm Q-switched or mode-locked laser pulses, will generate 1197 nm wavelength Raman-shifted laser pulses, which are at a wavelength in the range of preferred wavelengths for capsulectomies and other surgical procedures in the anterior segment of an eye. Other Raman crystals may be used, being selected to produce a wavelength in the preferred range of wavelengths by Raman shifting the wavelength produced by the laser gain crystal, and having a gain preferably greater than 1% and more preferably having a gain greater than 50%. 
     The cavity of laser system  109  is formed by high-reflectivity mirrors  110 ,  119  and  134 , which reflect both 1064 nm and 1197 nm wavelengths, and mirror  161 , which partially transmits the 1197 nm wavelength but highly reflects at the 1064 nm wavelength of the YAG crystal. The folded arrangement for the laser cavity of laser system  109  allows for the efficient optical pumping of Nd:YAG laser crystal  152  by coupling the emission of pump lasers, which may be two solid state diode lasers emitting at 808 nm. Pump lasers  128  and  146  with corresponding power supplies (not shown) may be coupled to the cavity by fiber optic couplings  131  and  143 , coupling lenses  125  and  140  and coupling lenses  122  and  137 , respectively. Fiber optic couplings  143  and  131  convey the pump radiation to coupling lenses  140 ,  137  and  125 ,  122 . These optical elements couple the pump 808 nm optical radiation through cavity reflectors  134  and  119  to the respective ends of Nd:YAG laser gain crystal  152 . Preferably, two pump lasers are used, although one may be used. Cavity laser reflectors  134  and  119 , in addition to having high reflectivity at 1064 nm and 1197 nm, have high transmissivity at the 808 nm pump wavelength, which allows the pump light to pass through. The cavity of laser system  109  may be further refined to enhance the efficiency of the Raman conversion process by giving end reflector  110  a concave curvature and adding lens  155  so as to form a contracting or beam-reducing telescope, reducing the beam diameter passing through the Raman crystal  158  to a selected distribution and thereby increasing the power density and, as a result, increasing the conversion efficiency. The laser cavity of laser system  109  can be modulated to produce Q-switched or mode locked pulses by incorporating a saturable absorber or an acousto-optic modulator as element  113  in the optical cavity, or both a saturable absorber and an acousto-optic modulator may be used. The saturable absorber may be a Cr +4 :YAG crystal. The operation of the modulator may be controlled by an electronic or computer-generated clock signal. Such electronic clock-controlled modulators are readily available. The resultant pulses, which may range in pulse width from 0.1 psec to 10 nsec, raise the pulse power and consequently the conversion process efficiency. The thus-generated 1197 nm laser optical pulses emanate as beam  164  at the output of cavity mirror  161 . This beam is expanded by telescope  165 , producing the larger beam  166  which passes on to the delivery system  211 , schematically represented in  FIG. 2 . Still referring to  FIG. 1 , computer system  167 , taking programming instructions input  173  from an operator (not indicated), issues control signals  170  and  171 . Signal  171  of the control computer enters the pump diode lasers  146  and  128  as signals  149  and  129 . These control signals adjust the laser parameters, for example power, pulse rate and start- and stop-times. Signal  170  goes on to simultaneously control the laser pulse delivery system  211  of  FIG. 2 . Computer  167  also receives signal  174  from pulse delivery system  211  which aids in the control of the treatment process as will be described in the following. 
     Referring now to  FIG. 2 , laser pulse delivery system  211  is comprised of an aiming low-power visible laser beam source  213 , which may be a HeNe 633 nm source, dynamic focusing system  220 , eye tracking sensor system  232 , dichroic reflectors  210 ,  227  and  258 , beam focusing lenses  219  and  225 , rapid two-dimensional beam articulating system  239  and surgery viewing microscope system  267 . The operating surgeon may view the target to be treated, as indicated by the eye symbol  264 , by means of microscope  267  through dichroic beam splitter  258 , which passes the image of the treatment site (the eye of the patient  276  and, in particular, the anterior capsule  273  or posterior capsule, not indicated) of natural lens  282  of eye  276 . Optional contact lens  270  may be employed to assist stabilizing and focusing of the delivered treatment beam  166 . Element  258  is preferably a dichroic reflector that preferentially reflects 1197 nm laser treatment light pulses and 800 nm eye tracking illumination light and essentially blocks off any laser and near infrared light from reflecting back to the surgeon&#39;s eye(s), while passing a clear visible image of the treatment site. 
     With continued reference to  FIG. 2 , the function of the delivery system is detailed in terms of its actual use by a surgeon as follows: The first step of the function is initiated while the treatment laser beam  166  is maintained in the off-state. This is the aiming and treatment adjustment step. Aiming beam  216  from alignment laser  213  is co-aligned with the path of the treatment beam  166  by means of dichroic reflector  210  which passes for example 1197 nm but reflects 633 nm. The aiming beam generally passes through the entire optical path and projects a focused point on the target capsule layer. By means of the control signal  173  of  FIG. 1 , the surgeon adjusts the focus of the aiming beam by means of signal  170  of  FIG. 1 , part of which is the control signal  209  in  FIG. 2 , acting on lens positioning mechanism  222 , moving lens  219  along dimension  223 . The thus moved lens in conjunction with lens  225  establishes a focused point  279  on the lens capsule of eye  276 . 
     With continued reference to  FIG. 2 , the second step of the function may be to establish eye tracking to secure accurate delivery of the treatment process. This preliminary step is also accomplished by maintaining the treatment laser in the off-state. The principal components of the eye tracking system  232  are an IR illuminator  215 , image position sensitive detector system  217 , such as are described in the eye tracker disclosed in U.S. Pat. No. 5,345,281, which is hereby incorporated herein by reference, polarizing beam-splitter cube  224  and band pass filter  221 . Other eye tracking systems known in industry or that may become available may also be used. The IR source may be an LED emitting IR light at 800 nm. This light beam designated as  214  is polarized and reflected by beam splitter cube  224 . The polarized illumination light passes through narrow band pass filter  221 , which blocks out any extraneous light, and is reflected by dichroic beam splitter  227 , which is transparent to 1197 nm laser light. The polarized IR illumination light is passed through the 2-D articulating system  239  and generally illuminates the iris and pupil of treatment site eye  276 . IR illumination that is scattered from the iris of eye  276  creates an optical object with a dark circular center representing the angular position of the eye. This scattered light from the eye is imaged back through the system by means of dichroic reflector  258 , lens  225 , dichroic reflector  227 , band pass filter  221  and beam splitter cube  224  to position sensitive detector system  217 . The beam splitter cube  224  on this return path blocks the perpendicular polarized illumination and only passes iris scattered light, thereby enhancing the contrast and creating a dark pupil image on the position sensitive detector  217 . Movement of the dark pupil spot at the position sensitive detector represents movement of the patient&#39;s eye  276 . The position-sensitive detector system produces signal  230 , which is communicated to the controlling computer as part of signal  174  in  FIG. 1 . Signal  230  represents the departure of the pupil from a normal centered fixation position. Based on the values of signal  230 , control computer system  167  in  FIG. 1  automatically computes compensating control signals designated as  170  in  FIG. 1  that are received as signals  229  and  238 , acting on actuating motors  228  and  240  in  FIG. 2 . Motors  228  and  240 , by means of the articulating system  239 , impart azimuth and elevation deflection relative to the optical axis of dichroic mirror  258 , compensating the deviation reported by position sensitive detector system  232 . This establishes the process of eye tracking, where the pupil and consequently the lens capsule remains stationary relative to the delivery optical axis. 
     The detail function of articulating system  239  is as follows. With continued reference to  FIG. 2 , signal  229  causes motor  228  to produce rotary motion  231 , which moves by gear  234  hollow cylinder element  237  about an axis coaxial with the optical axis. This imparts an azimuth motion of the dichroic mirror  258  by means of coupling support element  249 . Elevation motion  261  of element  258  is generated by motor  240  (which is also coupled to and moves with hollow cylinder  237 ) in response to signal  238 . The elevation motion of dichroic mirror  258  results from the rotation of coupled axle gear  255  actuated by worm gear  252  connected to motor  238  by axle  246 . 
     Having established the eye tracking function as detailed above, the surgeon next examines the programmed pattern on the lens capsule. To obtain this pattern the surgeon enters commands to the central computer  167  of  FIG. 1  to execute a preprogrammed procedure that turns on low-power aiming laser  213  in  FIG. 2  by means of the control signal  212 , producing beam  216 , which projects through the articulating system  239  onto patient&#39;s eye  276  as focused point  279  on lens capsule of lens  282  of eye  276 . The surgeon observes a focused point steadily fixed on a given point on the capsule or other target in the anterior segment of an eye as a result of the eye tracking process detailed above. The surgeon next examines the preprogrammed loci of exposure points displayed as a ring, ellipse or any general contour of preprogrammed dimensions on the capsule for a capsulorhexis. This is done by a rapid and cyclical motion of articulating system  239  executing a controlled motion of dichroic mirror  258 , repeatedly yielding the preprogrammed loci of points on capsule  273 . Throughout this process, the eye tracking system may contribute controlling corrections as described above to compensate for eye movement so that the surgeon observes a stable rapidly repeated preprogrammed pattern of the subsequent desired surgical treatment. The surgeon may enter adjustments to the programmed pattern generated by control computer system  167  of  FIG. 1 . 
     Once the surgeon is satisfied with the desired treatment pattern he may initiate the exposure with treatment laser beam  166  of  FIG. 2 . This is done by issuing an appropriate command signal to control computer  167  of  FIG. 1 . For the exposure process the control computer turns on the laser system  109  for a single or few cycles of the desired pattern as observed in the preliminary preprogrammed loci detailed above. 
     With the aforementioned computer control signals, highly reliable and reproducible capsulectomies can be accomplished by a reasonably trained surgeon. In other embodiments, an electrical beam-scanning device, such as employing well-known galvanometers, may be used in place of the mechanical beam-scanning device disclosed herein. Either beam-scanning device is preferably adapted to be accommodated in the working distance of the viewing optical system. 
     The invention described above, therefore, provides an improved surgical instrument and methods for the performance of lenticular capsulectomy. 
     The laser system of  FIG. 1  and the delivery system of  FIG. 2  may also be used in other surgical applications in the anterior segment of an eye. In cataract surgery, the laser pulses may be used for photodisintegration of a portion of or all of the interior material of a lens, using methods such as disclosed in WO 2009/039302, which is hereby incorporated by reference herein in its entirety. The wavelength disclosed herein provides safer procedures than provided by the shorter wavelengths of other lasers. Focus depth of the laser beam may be adjusted to cause photodisintegration in a selected pattern in the capsule. The photodisintegration may be used non-invasively. It may be followed by the usual procedure for cataract removal. 
     There is a great need for techniques to correct or prevent presbyopia. By programming patterns of laser pulses to be applied in x, y and z directions, using the apparatus of  FIGS. 1 and 2 , all or part of the nucleus or cortex of a lens capsule may be treated with pulses having a selected intensity, preferred wavelengths provided by the apparatus, pulse frequency and width and pre-programmed treatment loci. Such pulses at a preferred frequency can disrupt the bonds between molecules or aggregates in crystalline lens  282 . The pulses may be delivered over a very wide frequency range and the most effective frequency or frequencies selected. The pulses cause temperature and volume changes, creating sonic pulses. Such disruption can decrease the rigidity of lens  282  and allow greater accommodation of eye  276 . 
     The laser system of  FIG. 1  and the delivery system of  FIG. 2  may also be used by the surgeon to form the incision in a cornea during cataract surgery. This step may be taken before or after the capsulorhexis. Beam articulating system  239  ( FIG. 2 ) may be adjusted with respect to eye  276  to allow a pattern to be formed on the cornea of the eye at a selected location, which may be at an angle greater then 75 degrees off the optic axis of the eye. The selected pattern for such incision, which may be circular or elliptical, for example, may be programmed in computer  167 . The beam may then be focused at a depth to afford formation of the incision. 
     Rather than forming an incision in the cornea, the thickness of the cornea may be modified in selected areas using the apparatus of  FIGS. 1 and 2 . Arcuate cuts may be made in the cornea, for example. Such techniques are well known for correction of astigmatism and other visual deficiencies in an eye. The selected pattern for such treatment of a cornea may be programmed in computer  167 . 
     The laser pulses provided by the apparatus and methods disclosed herein provide several advantages over prior art apparatus and methods. (1) The range of wavelengths is selected to obtain optimum absorption in water and lipids, which means that lower power levels of the laser are required to obtain photo-dielectric breakdown; and (2) the focusing characteristic allows cutting of tissue to occur where the light is focused while using a source that is far below the damage threshold of the retina. All these characteristics are safety mechanisms for use of a laser in anterior segment eye surgery. 
     It is understood that modifications to the invention may be made as might occur to one skilled in the field of the invention within the scope of the appended claims. All embodiments contemplated hereunder which achieve the objects of the invention have not been shown in complete detail. Other embodiments may be developed without departing from the spirit of the invention or from the scope of the appended claims. Although the present invention has been described with respect to specific details, it is not intended that such details should be regarded as limitations on the scope of the invention, except to the extent that they are included in the accompanying claims.