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BACKGROUND OF THE INVENTION
The technical field of this invention is laser ablation of surfaces,
especially surfaces of biological materials. In particular, the invention
relates to systems and methods for reprofiling the cornea of the eye.
It is known to employ laser sources to erode surfaces of workpieces and the
like. Such apparatus is in general relatively complex and demands highly
skilled use. It is an object of the present invention to provide improved
and simplified apparatus and method for eroding surfaces.
It is also an object of the present invention to provide an improvement
whereby laser techniques can be applied to sensitive surfaces and, in
particular, to objects in which it would be undesirable to affect
underlying layers.
In the field of medicine, a known technique for the treatment of certain
forms of myopia is surgically to remove a segment of the collagen
sub-surface layer of the eye, to reshape the removed segment as by
surgical grinding, and to restore the reshaped segment in the eye. The eye
heals by reformation of the outer cellular layer over the reshaped
collagen layer. Alternatively, a layer of the cornea is opened up as a
flap, an artificial or donor lenticular implant is inserted under the
flap, and the flap is sutured up again.
It is a further object of this invention to provide an improved and less
traumatic method and apparatus for reshaping the cornea of the eye.
Various other surgical techniques for reprofiling of the corneal surface
have also been proposed. One increasingly common technique is radial
keratotomy, in which a set of radial incisions, i.e., resembling the
spokes of a wheel, are made in the eye to remedy refractive errors such as
myopia (nearsightedness). As the incisions heal, the curvature of the eye
is flattened, thereby increasing the ocular focal distance. The operation
is not particularly suitable for correction of hyperopia (farsightedness)
and can pose problems if the surgical incisions are uneven or too deep.
The use of a laser beam as a surgical tool for cutting incisions, a
so-called laser scalpel, has been known for some time (see, for example,
Goldman et al. U.S. Pat. No. 3,769,963). In 1980, a study was made of the
damage which might be inflicted on the corneal epithelium by exposure to
the recently developed excimer laser (see Taboada et al., "Response of the
Corneal Epithelium to ArF excimer laser pulses" Health Physics 1981,
Volume 40, pp. 677-683). At that period, surgical operations on the cornea
were commonly carried out using diamond or steel knives or razor, and
further, such techniques were still being studied (see, for example,
Binder et al., "Refractive Keratoplasty" Arch. Ophthalmol. May 1982, Vol.
100, p. 802). The use of a physical cutting tool in corneal operations,
and the insertion of an implant under a flap, continue to be widely
practiced up to the present day (see for example "Refractive Keratoplasty
improves with Polysulfone, Pocket Incision" Ophthalmology Times. July 1,
1986).
It has been suggested in U.S. Pat. No. 4,665,913 issued to L'Esperance that
controlled ablative photo-decomposition of one or more selected regions of
a cornea can be performed using a scanning action on the cornea with a
beam from an excimer laser. Because of the scanning action, it is
necessary for L'Esperance to bring his laser beam to a small spot,
typically a rounded-square dot of size 0.5 mm by 0.5 mm.
L'Esperance suggests that myopic and hyperopic conditions can be reduced by
altering the curvature of the outer surface of the cornea by repeatedly
scanning the cornea with an excimer laser beam having this standard, small
spot size but varying the field which is scanned during successive scans,
so that some areas of the cornea are scanned more often than others. In
this way, it is claimed, the surface can be eroded by different amounts
depending on the number of times the spot scans the surface. Additionally,
he suggests that certain severe myopic and hyperopic conditions may be
treated with a reduced removal of tissue by providing the outer surface of
the cornea with a new shape having Fresnel-type steps between areas of the
desired curvature.
In practice, complex apparatus is required to cause a pulsed laser beam to
scan with the precision required if the eroded surface is to be smooth.
Thus, if successive sweeps of a scan overlap, there will be excessive
erosion in the overlap area, whereas if they fail to meet, a ridge will be
left between the sweeps. The pulsed nature of excimer laser radiation also
tends to exacerbate this problem. Additionally, the scanning method is
inherently time-consuming even with highly refined techniques and
apparatus, since the laser beam is only eroding a very small part of the
total area to be treated at any given moment. Furthermore, such a scanning
system can cause rippling effects on relatively soft materials such as
corneal tissue.
It is therefore a further object of the present invention to provide a
method and apparatus for eroding a surface using a laser which does not
require scanning of the area of the surface to be eroded.
Another technique for corneal reshaping involves the use of a laser
photoablation apparatus in which the size of the area on the surface, to
which the pulses of laser energy are applied, is varied to control the
reprofiling operation. In one preferred embodiment, a beam-shaping stop or
window is moved axially along the beam to increase or decrease the region
of cornea on which the laser radiation is incident. By progressively
varying the size of the exposed region, a desired photoablation profile is
established in the surface. For further details on this technique see
also, Marshall et al., "Photo-ablative Reprofiling of the Cornea Using an
Excimer Laser: Photorefractive Keratoctomy", Vol. 1, Lasers in
Ophthalmology, pp. 21-48 (1986) herein incorporated by reference.
Although this technique for varying the size of the exposed region is a
substantial improvement over physical shaping (i.e., scalpel) techniques
and laser spot scanning protocols, a considerable number of optical
elements and control systems still are required for precise operation,
particularly on human corneal tissue. There exists a need for better and
simpler procedures for shaping surfaces, particularly the surfaces of
biological tissues, such as corneal tissue.
SUMMARY OF THE INVENTION
A laser system and masking apparatus are disclosed for reprofiling material
surfaces. The system comprises a laser means and a masking means disposed
between the laser means and the target surface. The laser means is
collimated to provide a uniform beam of radiation to the masking means.
The masking means provides a predefined profile of resistance to erosion
by laser radiation, and includes a control means for controlling the laser
such that upon irradiation of the masking means, a portion of the laser
radiation is selectively absorbed and another portion is transmitted to
the surface in accordance with the mask profile to selectively erode the
surface.
The masking means can comprise a mask and a support structure, preferably
affixed to the laser or otherwise optically aligned therewith, such that
the laser beam selectively passes through the masking means and onto the
target surface. The masking means can further comprise a transparent
stage, which is attached to the support structure. The masking means may
be independently fixed between the laser and surface, or it may be
directly attached to the surface.
The masks of the present invention provide a predefined profile of
resistance to erosion by laser radiation. Such profiles can be provided by
varying the thickness or composition of the mask material. When the
thickness of the mask is varied, the mask may be convex-concave,
plano-convex, plano-concave, convex-convex or concave-concave, depending
upon the nature of the desired erosion of the target surface. In addition,
the masking lens may be aspheric or torroidal at least on one surface, or
for special cases, such as the removal of ulcers, the surface shape may be
irregular.
Conveniently, the mask material has similar ablation characteristics to the
target surface. Various polymeric materials can be employed including, for
example, poly(methyl methacrylate), poly(methyl styrene) and mixtures
thereof. For corneal reprofiling, the ablation characteristics of the
masking material can range from about 10.sup.3 to about 10.sup.6
cm.sup.-1. Preferably, the masking material has an absorption
characteristic of micron or submicron etch depths per pulse similar to
those of the cornea when it is exposed to pulsed UV excimer laser
radiation.
Alternately, the mask may be of uniform thickness but vary in composition
to provide the desired profile of resistance to radiation.
The invention may further comprise any combination of mirrors, lenses and
prisms, located either upstream or downstream of the masking means, or
both, for imaging, focusing and redirecting the laser beam. Such
configurations allow for the use of an oversized or undersized mask for
greater convenience. Depending upon the application, the configuration of
the optical elements may include focusing lenses, divergent lenses, and
collimating lenses, in various combinations and in a variety of shapes
well known to those skilled in the art.
According to another aspect of the invention, there is provided a method of
reprofiling a surface comprising (a) optically aligning a laser means with
a target surface, the laser means being operable to deliver laser
radiation to the surface; and (b) disposing a masking means between the
laser means and the target surface, the masking means having a predefined
profile of resistance to erosion by laser radiation such that upon
irradiation a portion of the radiation is selectively absorbed and another
portion is transmitted to the target surface in accordance with the mask
profile to selectively erode the target surface.
The methods of the present invention are particularly well suited for
controlled reprofiling of the cornea, particularly a region known as
Bowman's membrane, which lies immediately below the uniform, extremely
thin, epithelial layer of the cornea. The epithelial layer is very rapidly
ablated on exposure to the laser light, and heals and eventually reforms
following the reshaping operation. In surgical applications, the laser
source is preferably an excimer laser, such as a UV Argon Fluoride laser
operating at about 193 manometers, which does not penetrate through the
cornea. A minimum laser irradiance level is essential for ablation, but it
is preferred not greatly to exceed this minimum threshold.
The pulse repetition rate for the laser may be chosen to meet the needs of
each particular application. Normally, the rate will be between 1 and 500
pulses/sec., preferably between 1 and 100 pulses/sec.
Suitable irradiation intensities vary depending on the wavelength of the
laser and the nature of the irradiated object. For a given wavelength of
laser energy applied to any given material, there will typically be a
threshold value of the energy density below which significant erosion does
not occur. Above the threshold density, there will be a range of energy
density above which increasing energy densities give increasing depths of
erosion, until a saturation level is reached. For increases in energy
density above the saturation value, no significant increase in erosion
occurs.
The threshold value and the saturation value will vary between wavelengths
of laser energy and between target surface materials. However, for any
particular laser wavelength and any particular material, the values can be
found readily by experiment. For example, in ablation of the Bowman's
membrane of the cornea alone or the membrane and the underlying corneal
stroma by energy of wavelength 193 nm (the wavelength obtained from an ArF
excimer laser), the threshold value is about 50 mJ per cm.sup.2 per pulse,
and the saturation value is about 250 mJ per cm.sup.2 per pulse. There
appears to be little benefit in exceeding the saturation value by more
than a small factor, and suitable energy densities at the corneal surface
are 50 mJ per cm.sup.2 to one J per cm.sup.2 per pulse for a wavelength of
193 nm.
The threshold value can vary very rapidly with wavelength. At 157 nm, which
is the wavelength obtained from a F.sub.2 laser, the threshold is about 5
mJ per cm.sup.2 per pulse. At this wavelength, suitable energy densities
at the corneal surface are 5 mJ per cm.sup.2 to one J per cm.sup.2 per
pulse.
Most preferably, the laser system is used to provide an energy density at
the surface to be eroded of slightly less than the saturation value. Thus,
when eroding the cornea with a wavelength of 193 nm (under which
conditions the saturation value is 250 mJ per cm.sup.2 per pulse), it is
preferable to provide to the erodable mask and cornea pulses of an energy
density of 100 to 200 mJ per cm.sup.2 per pulse. Typically, a single pulse
will erode a depth in the range 0.1 to 1 micrometer of collagen from the
cornea.
The invention will next be described in connection with certain illustrated
embodiments; however, it should be clear that those skilled in the art can
make various modifications, additions and subtractions without departing
from the spirit or scope of the invention. For example, the invention can
be used in connection with corneal transplants or synthetic inlays where a
donor insert is stitched into the patient's eye. Quite often, accidental
over-tightening of the stitches introduces refractive errors in the cornea
following the operation. At present, the transplant operation must be
repeated or relaxing incisions must be made in the cornea. The present
invention can provide an improved and less traumatic method for remedying
such refractive errors.
Additionally, the present invention can be used to treat astigmatisms,
corneal ulcers and keratomic growths which affect vision. In such
instance, specific masks can be designed and constructed to selectively
remove the corneal tissue which interfere with normal refraction.
Moreover, the teaching of the present invention can be applied to other
biological tissues requiring reprofiling, including lenticular implants,
ligaments, cartilage, and bone.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described by way of example with reference to the
accompanying drawings, in which:
FIG. 1 is a schematic illustration of an apparatus for practicing a method
of reprofiling the surface of an object, in accordance with the invention;
FIG. 2 is a more detailed illustration of an erodable mask suitable for use
in the apparatus of FIG. 1;
FIG. 3 illustrates diagrammatically the method of the present invention in
reducing the curvature of an object;
FIG. 4 illustrates another embodiment of an erodable mask suitable for use
in the apparatus of FIG. 1; and
FIG. 5 shows a laser apparatus for measurement and reprofiling.
DETAILED DESCRIPTION
In FIG. 1, a laser 10 provides a radiation output 12 to an erodable mask 14
which provides a predefined profile of resistance to the radiation. A
portion of the laser radiation 16 is selectively transmitted in accordance
with the profile of mask 14 and irradiates the surface 18 of the object
which is to be reprofiled and which as shown may comprise the cornea of an
eye. The system can further include one or more imaging lens elements 15
to image the mask 14 onto the surface 18.
The laser is powered by a power supply unit 20 and control circuit 22 which
can be adjustable to cause the laser to produce pulses of light at a
specific frequency and intensity. To further control the laser, a feedback
device 24 can be provided which receives information from optical or other
inspection of the mask 14 and/or surface 18 while it is exposed to
irradiation by the laser 10. A feedback path 26 communicates with the
control circuit 22 for controlling the laser 10.
In FIG. 2, one embodiment of the erodable mask 14 of FIG. 1 is shown in
more detail. As illustrated, the erodable mask 14 includes a support
structure 30, which may be rigidly connected to the laser device or
otherwise optically aligned such that radiation 12 from the laser (through
collimating means not shown) can be selectively transmitted through the
mask to produce the desired erosion of the surface by pulses of laser
energy.
At least a portion of the horizontal surface 32 is formed by a transparent
stage 34, which allows laser radiation to pass through to the target
surface. Preferably, the remainder of surface 32 is opaque to laser
radiation. Disposed upon the horizontal surface 32 and the transparent
stage 34 is masking lens 36.
In another embodiment, the transparent stage may include a lens system for
focusing the profile of radiation that passes through the masking lens.
This would enable the use of an oversized masking lens relative to the
desired erosion of the target surface. Alternately, the transparent stage
may include a lens system to spread out the profile of radiation that
passes through the masking lens. This would enable the use of an
undersized masking lens relative to the desired erosion of the target
surface.
The selected mask material is erodable by laser radiation and preferably
has ablation characteristics substantially identical to the object
material. For example, the erodable masks of the present invention can be
formed from plastic material such as poly(methyl methacrylate) (PMMA) or
poly(methyl styrene) (PS). These polymers are both bio-compatible and can
be efficiently eroded by laser radiation, i.e., by a pulsed ArF excimer
laser (193 nm). These polymers are mutually soluble in each other, and by
changing the concentration of PS in PMMA, absorption coefficients can be
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