Method and system for laser treatment of refractive error using an offset image of a rotatable mask

An ophthalmological surgery system and method for performing ablative photodecomposition of the corneal surface by offset image scanning. A mask having opaque and transparent portions corresponding to a desired type of correction, such as a hyperopic correction, intercepts a laser beam to provide a profiled beam. The mask is mounted for rotation about an axis and the image of the mask is offset from an intended center of rotation corresponding to an ablation center by an imaging lens which is radially offset from the center of rotation. The mask and lens rotate in unison to scan the image over the desired portion of the corneal surface. The invention enables wide area treatment with a laser having a narrower beam and makes optional the use of rotating mirrors and prisms.

BACKGROUND OF THE INVENTION 
This invention relates to ophthalmological surgery techniques which employ 
a laser to effect ablative photodecomposition of the anterior surface of 
the cornea in order to correct vision defects. 
Ultraviolet laser based systems and methods are known for enabling 
ophthalmological surgery on the surface of the cornea in order to correct 
vision defects by the technique known as ablative photodecomposition. In 
such systems and methods, the irradiated flux density and exposure time of 
the cornea to the ultraviolet laser radiation are so controlled as to 
provide a surface sculpting of the cornea to achieve a desired ultimate 
surface change in the cornea, all in order to correct an optical defect. 
Such systems and methods are disclosed in the following U.S. patents and 
patent applications, the disclosures of which are hereby incorporated by 
reference: U.S. Pat. No. 4,665,913 issued May 19, 1987 for "METHOD FOR 
OPHTHALMOLOGICAL SURGERY"; U.S. Pat. No. 4,669,466 issued Jun. 2, 1987 for 
"METHOD AND APATUS FOR ANALYSIS AND CORRECTION OF ABNORMAL REFRACTIVE 
ERRORS OF THE EYE"; U.S. Pat. No. 4,732,148 issued Mar. 22, 1988 for 
"METHOD FOR PERFORMING OPHTHALMIC LASER SURGERY"; U.S. Pat. No. 4,770,172 
issued Sep. 13, 1988 for "METHOD OF LASER-SCULPTURE 0F THE OPTICALLY USED 
PORTION OF THE CORNEA"; U.S. Pat. No. 4,773,414 issued Sep. 27, 1988 for 
"METHOD OF LASER-SCULPTURE OF THE OPTICALLY USED PORTION OF THE CORNEA"; 
U.S. patent application Ser. No. 109,812 filed Oct. 16, 1987 for "LASER 
SURGERY METHOD AND APATUS"; and U.S. Pat. No. 5,163,934 issued Nov. 17, 
1992 for "PHOT

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning now to the drawings, FIG. 1 illustrates a block diagram of an 
ophthalmological surgery system for incorporating the invention. As seen 
in this Fig., a personal computer (PC) work station 10 is coupled to a 
single board computer 21 of a laser surgery unit 20 by means of a first 
bus connection 11. PC work station 10 and the subcomponents of laser 
surgery unit 20 are known components and preferably comprise the elements 
of the VISX TWENTY/TWENTY EXCIMER LASER SYSTEM available from Visx, 
Incorporated of Santa Clara, Calif. Thus, the laser surgery system 20 
includes a plurality of sensors generally designated with reference 
numeral 22 which produce feedback signals from the movable mechanical and 
optical components in the laser optical system, such as the elements 
driven by an iris motor 23, an image rotator 24, an astigmatism motor 25 
and an astigmatism angle motor 26. The feedback signals from sensors 22 
are provided via appropriate signal conductors to the single board 
computer 21, which is preferably an STD bus compatible single board 
computer using a type 8031 microprocessor. The single board computer 21 
controls the operation of the motor drivers generally designated with 
reference numeral 27 for operating the elements 23-26. In addition, single 
board computer 21 controls the operation of the Excimer laser 28, which is 
preferably an argon-fluorine laser with a 193 nanometer wavelength output 
designed to provide feedback stabilized fluence of 160 mJoules per 
cm.sup.2 at the cornea of the patient's eye 30 via the delivery system 
optics generally designated with reference numeral 29 and shown in FIG. 5. 
Other ancillary components of the laser surgery system 20 which are not 
necessary to an understanding of the invention, such as a high resolution 
microscope, a video monitor for the microscope, a patient eye retention 
system, and an ablation effluent evacuator/filter, as well as the gas 
delivery system, have been omitted to avoid prolixity. Similarly, the 
keyboard, display, and conventional PC subsystem components (e.g., 
flexible and hard disk drives, memory boards and the like) have been 
omitted from the depiction of the PC work station 10. 
The system of FIG. 1 is used according to the invention to effect hyperopic 
and other error corrections to the anterior or other surface of the 
cornea, and to provide a smooth transition zone between the outer edge of 
the optical zone and the untreated surface of the cornea. The principle of 
the invention is illustrated in FIG. 2. As seen in this Fig., an imaging 
lens 51 is laterally offset from an image axis 52 by a predetermined 
radial distance D. Lens 51 preferably comprises the existing imaging lens 
found in the delivery system optics 29 of the FIG. 1 system (see also FIG. 
5, described below). Axis 52 is the axis corresponding to the center of 
rotation of lens 51. Lens 51 is displaced by translating the lens in a 
radial direction off the axis 52 (which may or may not correspond to the 
laser beam axis), which displaces the image of aperture 53 in a related 
manner from an initial position 54 to an offset position 55. By also 
rotating lens 51 about the axis 52 in an eccentric fashion, as illustrated 
in FIG. 3, the displaced image of aperture 53 can be scanned about axis 52 
along a preselected path, which in the embodiment described below is an 
annular path about the axis 52. In FIG. 3, the path described by the lens 
center is designated with reference numeral 56. By using a rotatable mask 
in combination with the off axis translation of lens 51 and eccentric 
rotation of lens 51 about axis 52, various types of large area ablation 
corrections can be effected, including hyperopic error corrections, and 
other vision error corrections, along with simultaneous edge contouring to 
form a smooth transition zone. 
FIG. 4 illustrates an embodiment of a mask 60 used to effect hyperopic 
corrections when used in combination with the imaging lens 51. The mask 
shown in FIG. 4 has an opaque area 61 and a transparent area 62. The 
boundary between the two areas has the required mathematical shape to 
ablate the appropriate surface contour in the cornea to effect a 
predetermined hyperopic error correction. In addition, the boundary shape 
can also incorporate a compensation factor to accommodate for variations 
in the energy profile of the laser beam. Portions 63, 64 of the boundary 
are shaped to provide a smooth transition zone between the outer edge of 
the intended optical zone of the corneal surface and the untreated area of 
the corneal surface. By changing this shape, different transition zone 
curvatures can be obtained. Further, the portion of mask 60 comprising the 
center of ablation can be shaped so that the corneal surface is left 
unablated at the center and over a limited region extending a 
predetermined distance radially outwardly, typically on the order of 
one-half to one mm. The image of the mask is initially offset with the 
lens 51 so that the intended center of the ablation of the mask is imaged 
over the intended ablation center on the cornea. Thereafter, lens 51 is 
synchronously rotated with mask 60 over an angular range of 360.degree. or 
multiples of 360.degree. while pulsing laser 28 to simultaneously effect 
hyperopic correction with edge contouring to provide a gentle transition 
zone extending outwardly to the untreated corneal surface. 
To align the mask to the desired offset position, the optical center of 
lens 51 is first determined using a mask with a pinpoint aperture, and 
rotating lens 51 while adjusting the lateral position of a reference axis 
until the image of the aperture is essentially invariant over a 
360.degree. rotation of lens 51. Thereafter, the pinhole alignment mask is 
removed and replaced by mask 60, and the ablation center of mask 60 is 
aligned to the former position of the aperture center image by radially 
displacing lens 51. Thus aligned, the image of the profiled beam passing 
through mask 60 can be rotated about the ablation center in the eccentric 
manner described above. 
FIG. 5 is a schematic view of the delivery system optics in the preferred 
embodiment. As seen in this Fig. the beam from laser 28 is reflected by a 
first mirror 71 and a second mirror 72, and enters a spatial integrator 
73, where the beam is modified in cross-section. The modified beam exiting 
from spatial integrator 73 is reflected by mirrors 74 and 75 and passed 
through a dove prism 76 to the mask rotation mechanism 78 on which the 
mask 60 is mounted. The profiled beam exiting from the mechanism 78 is 
reflected by a mirror 79 and enters the image offset control unit 80 which 
contains imaging lens 51. Unit 80 is illustrated schematically in FIGS. 6 
and 7. The offset profiled image exiting from unit 80 is reflected from a 
mirror 82 onto the patient's eye. 
FIGS. 6 and 7 illustrate the image offset control unit 80. As seen in these 
Figs., imaging lens 51 is contained in a fixture 81, which is mounted for 
pivotal motion about a first pivot post 83. Post 83 is carried by a first 
mounting member 84, which in turn is mounted by means of bearings 85 (or 
other suitable mounting mechanisms) for rotation about the longitudinal 
axis of member 84. Bearings 85 are mounted in the internal recess of a 
fixture housing 87. A first drive motor 89 is mounted on a flange portion 
90 of housing 87 and has an output shaft 91 for driving a first drive belt 
92 which is coupled to the lower portion of member 84. A second pivot post 
93 is received in a second pivot aperture 94 formed in fixture 81. Second 
post 93 is secured to an annular upper portion 95 of a second rotatable 
member 96. A second drive motor 97 is mounted on a second flange portion 
98 of fixture housing 87 and has an output shaft 99 for driving a second 
drive belt 101. Second drive belt 101 is arranged in driving engagement 
with the lower collar portion 103 of member 96. 
In operation, when member 84 is driven by motor 89 and belt 92, the lens 
housing 81 pivots about post 93. Similarly, when outer member 96 is driven 
by motor 97 and belt 101, housing 81 is pivoted about post 83. This latter 
motion is suggested in FIG. 6, in which two different positions of the 
housing 81 are illustrated: one in full lines and the other in broken 
lines. By operating motors 89, 97 simultaneously, compound motion of the 
housing 81 in a plane about both pivot posts 83, 93 can be effected so 
that both translational and rotational motion can be imparted to the lens 
51. Motors 89 and 97 are driven by the on-board computer 21, which is in 
turn driven by the p.c. workstation 10. By properly programming 
workstation 10, the desired motion can be imparted to imaging lens 51 and 
mask rotation mechanism 78 in order to scan the aperture image over the 
desired ablation region of the corneal surface. 
Referring back to FIG. 4, in order to determine the desired motion for the 
aperture image scanning, the depth of the required ablation is first 
selected. Next, the number of pulses required to effect ablation to that 
depth is determined from the equation: 
EQU n=t.pi./dx.theta..sub.max 
where t is the desired ablation depth at the edge of the optical zone, d is 
the ablation depth per laser pulse (or a scaling factor thereof) and 
.theta. max is the angle subtended between a line passing through the 
ablation center and the aperture center and a line passing from the 
ablation center to the point on the mask profile corresponding to the 
outer edge of the desired optical zone. Once the number of pulses has been 
determined, the angle of rotation between laser pulses can be chosen 
taking into consideration the length of time required by the mechanical 
elements in the system to reposition the lens 51 and the mask 60 (i.e., 
the minimum time), the amount of successive image overlap which can be 
tolerated, and the size of the optical zone desired. 
The invention offers the advantage of relatively wide area coverage without 
requiring a laser beam of size approximately equal to the treatment area. 
As a consequence, for hyperopic error corrections the transition zone can 
be fully formed using a controlled laser beam having a beam area 
substantially smaller than those required in prior art systems. This is 
highly advantageous since it requires substantially less energy than a 
larger beam generating laser, and avoids premature failure of optical 
components which are subject to deterioration due to high energy levels. 
The laser beam size should be large enough to span from the center of 
rotation to the outer boundary of the desired transition zone. For most 
human eyes, the largest treatment area is approximately 10 mm. 
Consequently, a laser having a beam diameter of about 5 mm will provide 
satisfactory ablations according to the invention. In the preferred 
embodiment the laser has a beam with a 6 mm maximum width. Further, the 
invention can be implemented in existing laser surgery systems by merely 
modifying the delivery system optics to enable the imaging lens 51 to be 
offset from the beam axis by selected amounts and to rotate with the mask 
60. The design and construction of such modifications will be readily 
apparent to those of ordinary skill in the art of optomechanical design. 
Because the invention obviates the need for rotating mirrors and prisms, 
the difficulties noted above encountered with the use of such optical 
elements can be completely eliminated, when desired. 
While the invention has been described above with reference to ablation of 
the anterior corneal surface, other portions of the cornea may also be 
treated using the invention. For example, the epithelium may be 
mechanically removed by scraping, as is typically done in photorefractive 
keratectomy, and the exposed surface may be ablated. Further, the 
invention can also be used for laser keratomileusis of corneal lamella 
removed from the cornea. This procedure is described in U.S. Pat. No. 
4,903,695 issued Feb. 27, 1990 for "Method and Apparatus For Performing A 
Keratomileusis Or The Like Operation". In applying the invention to this 
procedure, a flap of corneal tissue is physically removed from the cornea, 
the size of the removed portion typically lying in the range from about 8 
to 10 mm wide and a variable thickness up to 400 microns. This flap of 
tissue is typically removed using a microkeratome. Next, the flap is 
placed in a suitable fixture - typically an element having a concave 
surface - with the anterior surface face down. Thereafter, the required 
ablation is performed on the reverse exposed surface of the flap, after 
which the ablated flap is repositioned on the cornea and reattached by 
suturing. Alternatively, after the flap is removed from the cornea, the 
exposed stromal tissue of the eye can be ablated according to the 
invention, after which the flap is re-attached over the freshly ablated 
stromal tissue. 
While the above provides a full and complete disclosure of the preferred 
embodiments of the invention, various modifications, alternate 
constructions and equivalents may be employed as desired. For example, 
while the invention has been disclosed and described with respect to a 
mask 60 suitable for use in performing hyperopic corrections, mask 
profiles designed to achieve other types of corrections may be employed, 
as desired. Also, while the invention has been disclosed and described 
with reference to an imaging lens offset mechanism which is both rotatable 
and translatable by selected amounts, a simpler mechanism providing a 
fixed amount of offset for the image of the mask may be employed, as 
desired. Therefore, the above description and illustrations should not be 
construed as limiting the invention, which is defined by the appended 
claims.