CA2013954A1

CA2013954A1 - Method for reshaping the cornea

Info

Publication number: CA2013954A1
Application number: CA002013954A
Authority: CA
Inventors: Josef F. Bille
Original assignee: Josef F. Bille; Escalon Medical Corp.
Current assignee: Escalon Medical Corp
Priority date: 1989-05-26
Filing date: 1990-04-05
Publication date: 1990-11-26
Also published as: US4988348A; JPH037152A

Abstract

ABSTRACT

A method for reshaping the cornea comprises an initial step of determining the precise volume of corneal tissue which must be removed in order to attain the desired vision correction. A pulsed laser beam, having a pulse energy density sufficient to cause photoablation of corneal tissue near the threshold of the plasma regime, is directed onto the eye for removal of relatively large portions of tissue from the precisely determined volume to establish a corrected surface.
A pulsed laser beam, having a pulse energy density sufficient to cause photoablation of corneal tissue substantially below the threshold of the plasma regime, is then directed onto the corrected surface to take away relatively small portions of corneal tissue and thereby smooth the corrected surface.

Description

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l~IELD OF THE INVENTION
2The present invention pe~tains to ophthalmic surgical 3 procedures for reshaping the cornea of the eye in order to 4 correct vision deficiencies. More particularly, the present invention pertains to ophthalmic surgical procedures which 6 incorporate use of a pulsed laser beam for the photoablation 7 and removal of corneal tissue. The present invention is 8 particularly, but not exclusively, use~ul for reshaping the 9 cornea to attain the desired vision correction by photoablating a predetermined volume of corneal tissue.

12BACKGROUND OF T~E INVENTION
13It is well known that defective vision can be corrected by reshaping the cornea of the eye. Further, it is known that reshaping of the cornea can be accomplished in several ways.

16For example, the well known radial keratotomy procedure is used 17to establish weakened areas in the cornea which respond to 18internal pressure in the eye to move the cornea in its optical 19relationship with the retina. Another way in which vision can 20be corrected is by procedures which actually remove portions of e the cornea to alter its optical properties. For the category 22of procedures wherein portions of the cornea are removed in 23order to directly alter its optical properties, there ls an 24increased appreciation that lasers may be efficacious as a 25surgical tool. U.S. Patent No~ 4,732,148 and ~.S. Patent No.
4,773,4l4 which issued to L'Esperance Jr. for inventions : -1-- , :
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1 entitled "Methods for Performing Ophthalmic Laser Surgery" and 2 "Method of Laser-Sculpture of the Optically Used Portion of the 3 Cornea", respectively, are both exemplary of efforts to ~se 4 laser beams for ophthalmic surgery on the cornea. These s procedures, however, require there be some initial mechanical () removal of portions of the cornea as preparation for the 7 subsequent removal of corneal tissue by photoablation. As will be readily appreciated by the skilled artisan, such a 9 reguirement necessitates the use of different surgical tools in the same operation. It is, of course, preferable if the same surgical tool can be used throughout the procedure. The 12 present invention recognizes that a laser beam can be such a 13 tool for the purpose of ophthalmic surgery which reshapes the 14 cornea. In order to be effective as a surgical tool, however, laser beams must be precisely controlled. Thus, their 16 operative characteristics must be carefully selected and these 17 characteristics must be variable to meet the particular needs 18 of the particular procedure.
19 In light of the above, it is an object of the present invention to provide a method for reshaping the cornea of the 21 eye in which the pulse energy density or the wavelength of a 22 pulsed laser beam can be varied to precisely control the photoablation of corneal tissue~ Another object of the present 24 invention is to provide a method for reshaping the cornea of the eye in which the removal of a precisely predetermined 26 volume of corneal tissue is accomplished by a two-stage , ': ' . ' ' . :' ' .' ', . . : ' ',. : . . ', , ' .: :, . :

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1 photoablation procedure that fi~st takes away relatively large 2 portions of corneal tissue and subsequently takes away relatively small portions of corneal tissue. Still another object of the present invention is to provide a method for , reshaping the cornea of the eye using a pulsed laser beam in 6 which the pulse energy density is relatively low. Yet another object of the present invention is to provide a method for reshaping the cornea of the eye which minimizes or avoids the (3 adverse side effects caused when photoablation of corneal tissue is accomplished using a pulsed laser beam with pulse ll energy densities in the plasma regime. Another object of the 12 present invention is to provide a method for reshaping the 13 cornea of the eye which does not involve or require the mechanical removal of corneal tissue. Still another object of the present invention is to provide a method for reshaping the 16 cornea of the eye which is an essentially continuous 17 operation. Yet another object of the present invention is to 18 provide a method for reshaping the cornea of the eye which is simple to accomplish and which is relatively cost effective.

SUMMARY OF THE INVENTION

22 The present invention pertains to a method for reshaping 23 the cornea of an eye using pho~oablation techniques. More ~e~-o~ 24 specifically, in accordance with the present invention, the methods for reshaping the cornea employ lasers which can be 26 conteollably varied in wavelength, pulse energy density and ., .. .. ,... ~ ~, . . .
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1 foçuc;ed ~p~t size to ef~:3tiveïy phot~abl,~e t~ v;arious 2 tis~es in ~he! s~ro-na whlch rq~l~ir~ cemoval7 3 U6ing wel:l known techn~ 3ue~, t:he ~reci~ vollllne of corn~al 4 ti~ue whi~h mu~ be ~emoved 11l order to at~ n ~he dR~ired S YiSi~n corrc!c~ion caln ~e pr~cl~terJ~lin~9d. One ~3uch tec:hniclue 6 assumes that a one ~1) diopter cor.re~tion will be real:i;5~c~ by 7 the :~emov~l of: ~:orn~al ti~.~ue ~hi¢~ COJ ~SPVrId~I ~0 ~1- ex~e~nt of g approximateli~ ei.cJht ~8) mi~ron5 in depl:h z.long ~h~e eye'~ vis~al 9 axi~l In oId~r ~o do It:hi~, the r~e~hods o:e 'che pr esen~
lo invention ccln/:e~npla~3! ren~vval of t~ by p~ao~o~blation. from 11 the el?ithelium, ~owman'~ me~brane a:nd t.he ~trom~ Fwr h~!r, t}~e 12 pre~ent inven.~.i.on contempla~ hiæ photoal~l~tion o~ b~
13 ~ompli~hed in two ~tage~ First, there 1s the r~oval or 14 grinding o r~latively l~r~e por~ion~ o~ ue fr~n~ ~be p~edet:ermined volume i:c~ e tablic~h i~ correc~ed ~u~ Thl~ t~
16 d~ne ~llrou~h the E)ho~oAbl~lt;ion o~ ti~s~le by pul~e~ o~ ïa~ser 17 energy able ~o rli!mov~ port:lon~ o:lE 'ci~~ue ~hi~b ~re 18 approxim~tel5~ one hundred ( lOO ) mi~on~ in diailn~r ;3nd in the 19 ran~e o~ one ~o eight ~ l-B ~ mic~on~ ~n depth~ l?r~era~:lyt ~n ~0 ~ is ~irst s'~age~ a pulæed l.~r be~m l~ u~ed that h~ a pu18~
f 21 ene~gy ~ density whl~ll wlll c.. ;~use photoa~la~tic3n ~ar ~he 22 thre~hold of ~he pla~lha re~im~ he ~orne~l t~s~ue l~eirlg 23 r~mov~d~ Next~ th~e i~ th~ ~:moothincf or ~ol~æ}~ing o the o~ 7~, corre~ted ~ur~ace in which 'che~ pho~o.1b~tion~ o~ ti~s~ i8 ~5 a~complished by pulse~ o la~ ne~qy ~ h ~emo~i portiorl~ of 26 tissu~ tha~ are ~pproxlmate3~y one (13 micron in d~ t~r ~Inl~d on ,~

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l the order of one (1) mic~on in depth. Yor this subsequent 2 stage, a pulsed laser beam is used which has a pulse energy 3 density that is substantially below the threshold of the plasma ,~ regime but which will still cause photoablation of corneal ~ tissue.
6 According to one procedural operation, as contempla'ed by _ the present invention, each stage of the procedure is accompllshed using pulsed laser beams of different 9 wavelengths. During the removal or grinding stage the wavelength of the laser is selected according to its efficacy for removing the particular tissue. Generally, it is preferred 12 that a 0.527 micron wavelength (green) laser which is generated 13 by a Nd:YLF crystal be used in this stage for the relatively 4 rapid removal of selected portions of epithelial tissue, Bowman's membrane, and stroma. This removal or grinding stage is continued until substantially all of the predetermined 17 volume of corneal tissue is removed and a corrected surface is exposed. Once the predetermined volume has been rernoved, the corrected surface which has been exposed is then smoothed or polished. This so-called second stage is accomplished by c21 scanning the entire corrected surface with a laser generated by 22 an erbium crystal having the longer 2~94 micron wavelength.
O~ 23 Alternatively, tissue removal and tissue smoothing can be ~ 24 accomplished using a single crystal and, hence, a single ~-_~ wavelength. For example, in addition to effectively removing relatively large portions of corneal tissue to expose the '' ' .. ' ' " ' " ... ' , ~ . ' .

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2 crystal may also be used to smooth the corrected surface.
3 Smoothing this surface with the Nd:YLF, however, requires additional elements in the beam generator which are able to compress the pulses. Addition of these elements may be () undesirable~ Similarly, a pulsed laser beam generated by an 7 erbium crystal can be used both for removing corneal tissue to 8 expose a corrected surface and for smoothing this surface.
9 However, in order to generate a pulse energy density for this beam which is sufficient to remove relatively large portions of Il corneal tissue within a realistic time period, it is necessary l2 to improve the efficiency of the erbium laser. This increase 13 in efficiency can be accomplished in several ways, perhaps most l4 typically by including equipment which will refrigerate or cool 1~ the erbium crystal. Again, the addition of elements may be 16 undesirable.
l7 As a final step, and regardless whether a sin~le or dual 18 crystal procedure is used, the smoothed or polished surface can 19 be sealed. For example, the corrected surface may be heat treated by semiliquification after it has been smoothed.
21 Further, it may be possible to chemically treat and seal the 22 corrected surface after the cornea has been reshaped.
~e~ ^ 23 The novel features of this invention, as well as the ~n~O~ 2~ invent}on itself, both as to its structure and its operation, ~ 25 will be best understood from the accompanying drawings, taken Z

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I 1 conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in 3 which:

`.sBRIEF DESCRIPTIO~ OF THE DRAWIN~S
: fiFigure l is a theoretical curve relating tissue ablation 7 depth to the pulse energy density of a pulsed laser beam;

8~igure 2 is an empirical curve relating tissue ablation 9 depth to pulse energy densities for various pulsed laser beams of selected wavelength;
Figure 3 is a cross-sectional view of the cornea of the 12 eye;
13Figure 4 is a cross-sectional view of the cornea shown in 14 Figure 3 with portions of a predetermined volume of corneal tissue removed by photoablation; and 16Figure 5 is an enlarged cross-sectional view of a portion of the corrected surface shown in Figure 4.

DESCRIPTION OF THE PREFERRED METHOD
1~
20Before consi~ering a specific application for the ~ 21photoablation of living tissue, it is first helpful -to E ~ ~go appreciate and understand some general notions about the ~^~ 23 reaction of living tissue to the photoablation process. For 24 this purpose Figure l shows a curve l0 which indicates the ~ theoretical relationship between the pulse energy density (I~
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--l tissue. Specifically, ablation depth (d) (measured in microns) 2 is indicated along ordinate 12 and the pulse energy density ~3 (measured in joules per square centimeter) is indicated along 4 abscissa 14. As shown, curve lO identifies several points and `s regions of particular interest. For instance, there is some 6 initial pulse energy density (Imin) which is required before 7 the pulsed laser beam has any affect on the tissue.
8 Theoret1cally, Imin will be approximately one (l.0) J/cm2. At 9 a slightly higher pulse energy density designated Iph, tissue lo begins to photoablate. As shown on curve lO photoablation ll begins to occur when Iph is equal to approximately one and one 12 half (1.5) J/cm2. Figure l also indicates there is a l3 substantially linear region 16 on curve lO which extends from l4 Iph through higher pulse energy densities until the photoablation process begins to create a plasma at Ipl. At 16 pulse energy densities above Ipl it is generally accepted that 17 curve lO will begin to flatten out in accordance with a logarithmic relationship. The focus of the present invention, 19 however, is not on elevated pulse energy densities. Indeed, such elevated energy pulse densities should be avoided in order 21 to minimize the adverse side effects caused by the formation of 22 plasma, i.e., heat and mechanical shock. Rather, the present 23 invention is concerned with the resultant ablation depth of 24 tissue for pulse energy densities which are substantially between Iph and a value slightly greater than Ipl-Importantly, there i5 a general linear relationship between .'~ .

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1 ablation depth and the p~se energy density I ~or values of I
2 in this range.
:3 As should be expected, it is necessary to also consider ~l the wavelength of the particular laser beam that is to be used `5 for the photoablation of living tissue. For this purpose, , Figure 2 shows several ablation curves which generally indicate the respective causal relationships between ablation depth (d) ~3 and pulse energy density (I) for several laser beams of '3 differing wavelengths. Although it is recognized that other 1() laser mediums which are well known to the skilled artisan can be used to generate pulsed laser beams that may also be 12 effec~ive for the purposes of the present invention, the 13 consideration here will be on only two such mediums.
14 Specifically, the present invention is concerned with the well known Nd:YLF laser crystal which emits laser light with a 16 wavelength of approximately 0.527 microns. Also, the present 17 invention is concerned with the well known erbium laser crystal 1~ which generates laser light at a wavelength of 2.94 microns.
19 Both mediums have certain beneficial characteristics~
2~ Simply stated, the general objectives of the method and 21 procedure according to the present invention are to remove a 22 predetermined volume of corneal tissue which will effectively 23 reshape the cornea in order to obtain a desired vision 24 correction. To accomplish this it is necessary to first determine the volume of corneal tissue which must be removed.

26 Several methods for de~ermining this volume are well known in 2 ~

the pertinent art and all of these methods need not be disclosed here in detail~ One in particular, however, helps to 3 more fully understand the method of the present invention.
According to this particular calculation, it is known that ~i removal of an eight (8) micron thick layer of stroma tissue fi along the visual axis will result in an approximately one (1) _ diopter correction for the eye. To more fully appreciate what u this means, consider Figure 3.
9 Figure 3 depicts a cross section of the cornea of an eye, ~enerally designated 18. As shown, cornea 18 comprises an ll epithelium 20, Bowman's membrane 22, stroma 24, Decimet's 12 membrane 26, and an endothelium 28. Behind the cornea 18, and 13 inside the eye, is the aqueous humor 30. The eye's visual axis l~ 32 is shown in Figure 3 as being along a line which extends in the direction of sight and which is substantially normal to the 16 external surface 34 of cornea 18. As indicated above, certain 17 vision deficiencies may be corrected by removal of tissue from 18 stroma 24. Specifically, the amount of correction will depend l9 on how much tissue is removed and feom where. More ~n specifically, it is known that a distance 36 measured along 21 visual axis 32 in stroma 24 can be used to calculate the amount ` ~7~o~ 22 of tissue to be removed for a desired diopter correction.
u,u,~ Thus, for approximately each eight (8) microns of distance 36, C~OO 2~ a one (1) diopter correction will result. To realize such a 2.5 coreection, however, it is necessary to remove a predetermined 26 volume of corneal tissue approximately equal to a dome having a ' -10 . .
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I rad~us oE corvature s~bstantia~ly eguivalent to that of cornea 2 18 and a height equal to the sum of distance 36, the depth of ~ epithelium 20 and sowman's membrane 22. It is to be 4 appreciated that this is but one of several possible solutions ~ to the problem. The exact predetermined volume of tissue to be 6 removed is a matter of choice which can be determined on a _ case-by-case basis without consequence to the remainder of the procedure. In any eventt the result after removal of the 9 predetemined volume of corneal tissue is the exposure of a corrected sur~ace 38~
Il As contempla~ed by the present invention, reshaping of the 12 cornea is accomplished by a two-stage pho~oablation l~ procedure. After the volume of corneal tiss~e to be removed 14 has been determined, relatively large portions of this volume are removed in the first stage by photoablation. Ideally, 16 portions of tissue in the size of up to one thousand (1,000) 1~ cubic microns are ~emoved during this stage with each pulse of l8 the laser beam. This removal or grinding step can be l9 controlled by a device such as the one disclosed in a co-2~ pending application for an in~ention entitled "3-Dimensional 21 Laser Beam Guidance System," U.S. Patent No. 4,901,718 issued ~ ~80 February 20, 1990, which is assigned to the same assignee as the 23 present invention. After this removal of tissue, howeverl the ~'u 24 corrected surface 38 of the reshaped cornea 18 remains somewhat uneven and irregular. Specifically, corrected surface 38 is 26 characterized by ridges 40 and depressions 42, as generally ~11-.

' '' shown in Fig~re 5~ It i~ ex,pe~ed th~ he ele~ti~ional I

di~f~ren¢~ 44, ~)e~we~n a ridge 40 ~nd a depre~sion 4;~ wi:l 1 he on ~he order of one tl) or two (2) microns~ This diffe~ence 44 can, however~ c~au~e hazy vi~;ion and in order to avoid hazy, S albei~ corre~ ed, vi~ion ~or the pat.ient co~r.ected ~urface 38 need~3 to be ~moo~hedl o~ pol ~;h~ed ~

7 In the ~econa ~3Jc~ge o t.h~; proced~l~e, cor re~ted lurf~c!e 3~ i~ smootlled or pol~shed by rel~oving ~eîativelY~ s~all 9 port:;ion~ of corneal ~ e by photoabl.~tion~ Ideall~ only the ridqes ~0 ar~! removed ~nd ~;h,er~fore, durinq ~hi~ oothinq s~age, it is nec~ss.~ry to use! la~3er pulse~ which photo~ late Il strC~ma ~4 ~ ue l:o ~ depth o;E one 51) micLol~ or le~s~O

Ideally, as ~nvl8ioned by ~ie pre~en~. invenl:ion, the depth o~
13 pllotoabl~ion i.n ~h~ ~mool:hln~ ~tage will be tt~ ble ~ in 1~
the l:arsge o one ( 1~ to one te~th ~ O ~ mi~on .

Re~e~ring to Fi~ure 5, :I.t can l~e appre~late~ ~h~t only selected pocl:iQn~ of corr~¢ted ~;urface 38 ~houl-d be pho~c)a~l~ted dur~ the 5~00thlng ~ac~ eci~ lly 1~
deslred ~o re~ove ~nly the i:idg6~ 4'~ ~o do ~o; ho~ever, ~ r~qui.re3 dete~n~inin~ the preci~e loc,a~t10n of ~h~ ridge,~ 40 O
~Pre~er~bly, the de~er.~ination o~ wh~re ridges ~0 are lo~t;ed on ..... .aorrected ~u~ace 3~ 1~ done by using ~ deviae ~u~h a~ ~be one di3a~0sed in cc~;pe~cling appli~ation f(~r an inV~ntion e,nSi~l~d 24 "Imaging System for Surgical Lasers~', U.S. Patent No.
æ 4,881,808 issued November 21, 1989, which is assigned to the same assignee as the present invention ,l -12-..,. . , , '.

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, ' - : . , sriefly~ this device is able to establish the ~ ~opoclraphy of cor~ec~ed surfal~e 3~ ~y calc~latins~ ~eception 3 re~;pc~n~es of ~pecul~lr ~ef:Lec~ion~ ~rom the correc~ed ~urface 38. t~ f ~ in~<: rmation, the l.oca~lon of rit~ge; 40 c!an be s asce~tained and they c~ then t~ removed by di.rect:ing a p~llsed 6 laser beam ~ ~pe~cif ic charact~!r ist ics dir~cly into the 7 rid~fe 40.
8 As men~ioned above, lt ir, to be appreci~ted ~:ha~ lasers 9 with diffe~ent waveleng~hs may be u8ed f.or ~l~e pres¢nt inv~ntion. Ideally, one wavelerlgth ct~uld ~e used ~or bof;.h the 11 ~irs~ stace ~ , remova:l of rala~i.Yely la~rge portio~ of 7 corneal ti~sue) and 'che seconcl ~a~fe (i.e., .smoothln~f of the 3 correc~ed sllr~e by remov~l ~ ~elal ively ~ 11 E~ortlc~ns of 14 co~rleal ~i~;sue~., This m~ly, however, be ~l~prao~i~al.
N~ve~thcle~&, t~le u~ of ~ingle, w~velength proc~e~ure~ ~hould be 16 ~onsi~ered ~long ~ith the pre~ently pre~erred dual wav~ gth 17 procedure .
18 COT1 ider 1r~t, the u~e o~ only an er;l~ium ~Er~ las~r 19 cry~a~. A~ i~ well known, isn erbium c:~ystal e~ la~er llsht o `.a'c a wi~velenS~ i E~proxir~tely 2.94 micr~n~ Fur~her, i.t h 21 been de~e~m;ined ~ha~. th~ light emitted by an erb~ la~er g~ 22 cry~ l h~s p~lotoabli~ion ch~ra~ter i~ $ whic:h are gerle~lly 23 as depi~ted by ~urv~ 46 in Fi.cJure 20 ~pecif'icall~f, a~ w~h 2~ othe~ pulsed ].asier ~ams, the e! ig a r~gion whi~h ext~nd~ ~rom a pu].~e en~!rS~y den~;L~y o~ al3p~0~simat;ely thi rty 1, 30~ m2 26 upwi~rd to ~i.gh~r pul~ie ene~ g~ den~ies in wh:ich. operation o~

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l an erbium pulsed laser beam will cause fo~mation of a plasma 2 du~ing photoablation. I~portantly, however, the erbium laser 3 also has characteristics in range 50 of pulse energy densities 4 below approximately ten (l0) J/cm2 wherein photoablation will ~s result without any significant formation of a plasma. Equally 6 important are the indications that the shape of curve 46 in the 7 range 50 is moderate and the curve 46 itself is substantially 8 linear. Consequently, an ecbium pulsed laser beam lends itself 9 to being precisely controlled during operation in the range 50. Unfortunately, an erbium laser medium has relatively low 11 efficiencies and generates a great amount of heat during 12 operation at pulse energy densities above approximately ten 13 (l0) J/cm2. Consequently, in order for an erbium laser to be 14 efficient and have greater efficacy at the higher pulse energy densities necessary for the removal of relatively large 16 portions of corneal tissue in the first stage of the process of 17 the present invention, the erbium medium must be cooled.
18 Further, it is known that this requires cooling the erbium laser medium to approximately minus fifty degrees centigrade (-50C) Thus, an erbium laser medium can be used for 21 performing the methods of the present invention if the 22 components necessary to cool the medium to these temperatures 23 in the first stage are provided. On the other hand, for the 24 purposes of the present invention, an Nd:YLF laser medium exhib1ts good operating characteristics at the higher pulse ~14-,, `' :
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l energy densities required for removal of corneal tissue during 2 the first stage.
~ As shown in Figure 2, a standard Nd:YLF laser medium emits 4 light at a wavelength of approximately 0.527 microns in a ` 5 pulsed beam which has photoablation characteristics that are 6 substantially as depicted by curve 52. Importantly, unlike 7 erbium, an Nd:YLF laser medium does not require cooling for 8 operating the higher pulse energy densities required for 9 efficient removal of relatively large portions of corneal tissue. Thus an Nd:YLF laser beam is preferred for operation 11 during the first stage removal of corneal tissue. An 12 unmodified Nd:YLF laser, however, has less desirable operating 13 characteristics at the lower pulse energy densities.
14 Specifically, as shown in Figure 2, the shape of curve 52 is relatively steep in region 54 where operation of the Nd:YLF
16 laser would result in removal of relatively small portions of 17 corneal tissue. Thus, it would be difficult to control the 18 Nd:YLF medium in the second stage of the process of the present invention. Perhaps more importantly, it happens at pulse energy densities below approximately ten (lO) J/cm2, the beam 21 characteristics become somewhat similar to the characteristics r ~og~ 22 of an erbium laser beam if pulses from the Nd:YLF laser medium ~O~ 23 are compressed. To realize this correlation it is necessary to e~-o~~ 24 compress the laser pulses from their relatively easily attained duration o~ approximately thirty pico seconds (30 psec) down to 26 a duration of approximately one pico second (l psec).

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1 This relationship is shown in Figu~e 2 by curve extension 2 56. Importantly, compression of the pulses in an Nd:YLF laser beam results in effective operation at the lower energy levels below the level where photoablation results in the formation of , a plasma. This results when using compressed pulses due to the (, more efficient multiple photon absorption processes within the _ tissue molecules and a coherent cumulative effect of the 8 photons on the particular molecule. Thus, with the 9 incorporation of components in a Nd:YLF laser beam generator which will compress the laser pulses to durations of 11 approximately one pico second (l psec) at lower pulse energy 12 densities, a Nd:YLF laser could be effectively used for both fiest and second stage operation of the present invention.
14 Further it is important to note that a system incorporating an Nd:YLF laser medlum would lend itself to operations requiring 16 photoablation internally in the eye, such as might be required 17 for retinal surgery.
18 The photoablation curve 58 for the well known excimer 19 laser is provided in Figure 2 for comparison purposes only. As clearly seen in Figures l and 2, the fact that the excilner ~ 21 laser has little, if any, ability to operate at pulse energy s~ ~,R~ 22 densities in region 16 below the plasma formation threshold ^ 23 makes it unsuitable for the present invention.

cvc~O~ 2~ Due to complications arising from the need to cool an 25 erbium laser medium at the higher pulse energy densities, and 26 the need to compress pulses of an Nd:YLF laser medium at the ,........... : , .. ...
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1 lower pulse energy densities, it is preferred due to the 2 purposes of the present invention to use both mediums and 3 operate them in their respective regions of greatest efficacy.
4 Speciically, it is preferred that an Nd:YLF laser medium be ; used to remove relatively large portions of corneal tissue in 6 the first stage and that an erbium laser medium be used to _ smooth the corrected surface 38 by removing relatively small 8 portions of corneal tissue therefrom. As for the actual 9 generated characteristics of the respective beams it is preferred that the Nd:YLF laser be operated during the method's Il first stage in a regime wherein the resultant beam is pulsed to 12 generate ten thousand pulses per second (10,000 pps) with each 13 pulse having approximately one hundred micro joules (100 ~J) of 14 energy (and being approximately thirty pico seconds (30 psec) ?
in duration. On the other hand, it is preferred that an erbium laser be operated during the method's second stage in a regime 17 wherein the resultant beam is pulsed to generate two thousand 18 pulses per second (2,000 pps) with each pulse having 19 approximately five hundred micro joules (500 ~J) of energy and being approximately one hundred pico seconds (100 psec) in 21 duration. As implied above, it is preferred that operation of 22 the Nd:YLF laser during the irst stage be accomplishea at :~ pulse energy densities generally at or above ten (10) J/cm2 and 4 that operation of the erbium laser during the second stage be accomplished at pulse energy densities generally below ~wo ~2) 26 or three (3) J/cm2.

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1 It is to be appreciated that these particular regimes and 2 ranges are only exemplary and that variations may be made 3 therefrom without dfeparting from the spirit and intent of the present invention.
.S While the particular method for reshaping the cornea as 6 herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely 9 illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details ` 10 11 of construction or design herein shown other than as defined in the appended claims.

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Claims

1. A method for controlled reshaping of the cornea of the eye which comprises the steps of:
removing a specified volume of corneal tissue by photoablation using laser pulses having plasma forming energy densities to establish a predetermined corrected surface; and smoothing said corrected surface by photoablation using laser pulses having energy densities substantially below an effective plasma forming level.

2. A method for controlled reshaping of the eye as recited in claim 1 wherein the laser has an energy and the energy of the laser used for said removing step is variable in a range greater than ten joules per square centimeter (I>10 J/cm2).

3. A method for controlled reshaping of the eye as recited in claim 1 wherein the laser has an energy and the energy of the laser used for said smoothing step is variable in a range of 1-10 joules per square centimeter (1-10 J/cm2).

4. A method for controlled reshaping of the eye as recited in claim 1 wherein said plasma forming energy density is greater than approximately ten joules per square centimeter (10 J/cm2).

5. A method for controlled reshaping of the eye as recited in claim 1 wherein said smoothing is accomplished using laser pulses having energy densities in the range of two to four joules per square centimeter (2-4 J/cm2).

6. A method for controlled reshaping of the eye as recited in claim 1 wherein said removing step is accomplished using a laser spot size which is larger than a laser spot size used for said smoothing step.

7. A method for controlled reshaping of the eye as recited in claim 6 wherein the laser spot size for said removing step is approximately one hundred microns in diameter and the laser spot size for said smoothing step is approximately one micron in diameter.

8. A method for controlled reshaping of the eye as recited in claim 1 wherein said removing step comprises:
removing an epithelium in said specified volume by photoablation;

removing Bowman's membrane in said specified volume by photoablation; and removing a stroma in said specified volume by photoablation.

9. A method for controlled reshaping of the eye as recited in claim 8 wherein said step of removing the epithelium is accomplished using a shorter wavelength than is used for removing the stroma.

10. A method for controlled reshaping of the eye as recited in claim 9 wherein said wavelength for removing the epithelium is 0.527 microns.

11. A method for controlled reshaping of the eye as recited in claim 9 wherein said wavelength for removing the stroma is 2.94 microns.

12. A method for controlled reshaping of the eye as recited in claim 9 wherein said wavelength for removing the epithelium is 0.527 microns and the wavelength for removing the stroma is 2.94 microns.

13. A method for correcting vision which comprises the steps of:
determining a diopter correction necessary to achieve substantially normal vision;
calculating an extent of corneal tissue corresponding to said diopter correction;
removing a specified volume of corneal tissue from said eye by photoablation to reshape the eye in accordance with said calculated extent using laser pulses having plasma forming energy densities; and smoothing said corrected surface by photoablation using laser pulses having energy densities substantially below an effective plasma forming level.

14. A method for correcting vision as recited in claim 13 wherein said calculating step is accomplished by equating a change of one diopter with a change of approximately eight microns of corneal tissue thickness along the visual axis of the eye.

15. A method for correcting vision as recited in claim 13 wherein said calculated extent is measured into the eye from surface thereof substantially along the visual axis of the eye and said volume of removed corneal tissue is contained between the surface of said eye and a plane substantially perpendicular to said visual axis at said calculated extent.

16. A method for correcting vision as recited in claim 13 wherein removing said volume of corneal tissue creates a corrected surface for the eye and comprises the steps of:
removing an epithelium in said specified volume by photoablation;
removing Bowman's membrane in said specified volume by photoablation; and removing a stroma in said specified volume by photoablation.

17. A method for controlled reshaping of the eye as recited in claim 16 wherein said step of removing the epithelium is accomplished using a shorter wavelength than is used for removing the stroma.

18. A method for controlled reshaping of the eye as recited in claim 11 wherein said removing step is accomplished using a laser spot size which is larger than a laser spot size used for said smoothing step.

19. A method for controlled reshaping of the eye as recited in claim 18 wherein the laser spot size for said removing step is approximately one hundred microns in diameter and the laser spot size for said smoothing step is approximately one micron in diameter.

20. A method for reshaping the cornea of an eye for improved vision which comprises the steps of:
defining a specified volume of corneal tissue to be removed;
aiming a beam of laser pulses at said specified volume;
focusing said pulses to a predetermined spot size;
setting the power in said pulses above a plasma forming level to photoablate corneal tissue with each of said pulses to a depth of approximately eight (8) microns;
scanning said beam through said specified volume to photoablate and remove said corneal tissue therein and expose a corrected surface;
subsequently setting the power in said pulses effectively below said plasma forming level to photoablate corneal tissue with each of said pulses to a depth of approximately one (1) micron; and scanning said beam across said corrected surface to photoablate and smooth corneal tissue at said corrected surface.

21. A method for reshaping the cornea of an eye for improved vision as recited in claim 20 wherein the diameter of said spot size is varied between approximately one hundred (100) microns and one (1) micron.

22. A method for reshaping the cornea of an eye for improved vision as recited in claim 20 by removing an epithelium in said specified volume by photoablation;
removing Bowman's membrane in said specified volume by photoablation; and removing a stroma in said specified volume by photoablation.

23. A method for reshaping the cornea of an eye for improved vision as recited in claim 22 wherein said step of removing the epithelium is accomplished using a shorter wavelength than is used for removing the stroma.

24. A method for reshaping the cornea of an eye for improved vision as recited in claim 20 further comprising the steps of:
determining a diopter correction necessary to achieve substantially normal vision; and calculating an extent of corneal tissue corresponding to said diopter correction and scanning said beam to reshape the eye by said calculated extent.

25. A method for reshaping the cornea of an eye for improved vision as recited in claim 24 wherein the power of said laser pulse used for reshaping the eye is variable in the range of one (1) to ten (10) joules per square centimeter (1-10 J/cm2).