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BACKGROUND OF THE INVENTION
This invention relates to an apparatus and method employing a laser,
especially a pulsed laser, for shaping surfaces, especially surfaces of
organic material. In particular, the invention relates to an apparatus and
method for shaping biological tissue, including 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,
U.S. Pat. No. 3,769,963 to Goldman et al.). 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 DrF 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 European Patent Application No. 0151869 of
L'Esperance that controlled ablative photodecomposition 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 that the surface can be eroded by different
amounts depending on the number of times they are scanned by the spot.
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 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 compression of the excimer laser beam to a small
spot will increase the beam energy density, which will tend to exacerbate
these problems. It is not clear that L'Esperance has found a suitable
scanning system, since in one embodiment he attempts to control the laser
beam by a magnetic field.
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
According to one aspect of the present invention, there is provided a laser
system for reprofiling a surface comprising a laser means and a masking
means disposed between the laser means and the surface for providing a
predefined profile of resistance to erosion by laser radiation, and
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 further comprise a rigid structure which is affixed
to the surface, in particular to the sclera of an eye, and a masking lens
connected to the support structure and disposed above the cornea. The
masking lens can be directly intergrated with the support structure or,
preferably, a transparent stage can be formed as part of the support
structure to support and position the masking lens.
The masking lenses 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 lens material. When the
thickness of the lens is varied, and dependent on the nature of the
erosion of the object which is required and the form of the transparent
stage, the lens may be convexo-concave, plano-convex, plano-concave,
convexo-convex or concavo-concave, and it may also be aspheric or
torroidal at least on one surface. In special cases such as the removal of
ulcers the surface shape may be irregular.
Conveniently, the lens material has similar ablation characteristics to the
object material. 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 pulsed UV excimer laser radiation.
According to another aspect of the invention, there is provide a method of
reprofiling a surface comprising (a) locating a laser means relative to an
optical axis of a 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 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 surface in accordance with the mask profile to
selectively erode the surface.
The methods of the present invention are particularly well suited for
controlled reprofiling of the cornea, particularly the collagen sub-layer
thereof which lies immediately below the uniform, extremely thin,
epithelial layer of the cornea, which is very rapidly ablated on exposure
to the laser light. The extremely thin surface layer heals and eventually
reforms following the reshaping operation. In surgical applications, the
laser light is of a wavelength obtainable from a UV Argon Fluoride laser,
typically about 193 nanometers, 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 per second, preferably between 1 and 100 pulses per second.
Suitable irradiation intensities vary depending on the wavelength of the
laser, and the nature of the irradiated object. For any 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 over which increasing energy densities give increasing depths of
erosion, until a saturation value 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 from wavelength to
wavelength of laser energy and from material to material of the surface to
be eroded. However, for any particular laser and any particular material,
the values can be found readily by experiment. For example, in the case of
eroding a mask and the underlying corneal stroma (collagen sub-layer) 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, and 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 erodible mask and cornea pulses of an energy
density 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 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 applied to the remedy of
stigmatisms, 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, for example,
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 diagramatic 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 erodible mask suitable for use
in the apparatus of FIG. 1.
FIG. 3 illustrates diagramatically the method of the present invention in
reducing the curvature of an object.
FIG. 4 shows a laser apparatus for measurement and reprofiling.
DETAILED DESCRIPTION
In FIG. 1, a laser 10 provides a radiation output 12 to an erodible 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 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 erodible mask 14 of FIG. 1 is shown in
more detail. As illustrated, the erodible mask 14 includes a suction cup
30 which provides a support structure having rigid vertical walls and a
horizontal surface 32. At least a portion of the horizontal surface 32 is
formed by a transparent stage 34. Preferably, the remainder of surface 32
is opaque to laser radiation. Disposed upon the transparent stage 34 is
masking lens 36.
The entire structure can be placed upon the surface of the object, i.e.,
the sclera of an eye, leaving the corneal surface 18 unobstructed. A
flexible tube 38 supplies vacuum suction to the cup, so as to clamp it to
the eye with a force sufficient to hold it in place but not distort the
shape of the cornea.
The erodible mask 14 can be rigidly connected to the laser or otherwise
optically aligned therewith such that radiation from the laser can be
selectively transmitted through the mask to produce the desired erosion of
the surface by pulses of laser energy.
The selected lens material is a material which is erodible by laser
radiation and preferably has ablation characteristics substantially
identical to the object material. For example, the erodible masks of the
present invention can be formed from plastic material such as poly(methyl
pethacrylate) (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 varied from about 10.sup.3 to about 10.sup.6
cm.sup.-1. Other organic polymers exhibiting suitable ablation
characteristics can also be be employed in the manufacture of erodible
masks. Preferably, the polymeric material has an absorption characteristic
of micron or submicron etch depths per pulse similar to those of the
cornea. For further details on organic polymers suitable for construction
of masks, see Cole et al., "Dependence of Photo-etching Rates of Polymers
at 193 nm on Optical Absorption Coefficients", Vol. 48 Applied Physics
letters, pp. 76-77 (1986), herein incorporated by reference.
Various techniques can be employed to manufacture the lenses used in the
present invention from PMMA or PS. These techniques included injection
molding, casting, machining and spin casting. Manufacture by laser
machining can also be employed. In one typical technique, a solution of
PMMA or PS is prepared in toluene and spin cast in a suitably-shaped cup
to obtain a smooth, uniform lens having a pre-defined profile thickness.
Depending upon the concentration of PS in PMMA, a suitable absorption
coefficient is obtained. The films can then be removed from the spin cup
and vacuumed baked to remove residual solvent.
Alternatively, the erodible mask can be made of a material having a
variable composition such that pre-defined regions of the mask selectively
absorb greater amounts of laser radiation even though the entire mask has
a uniform thickness. Again, materials such as PMMA and PS can be employed
in varying concentrations in the erodible mask to achieve the variable
composition of the mask.
FIG. 3 illustrates the principle involved in eroding a surface to effect
reprofiling thereof in accordance with the present invention. Although the
transparent stage shown in the figures is substantially horizontal, it
should be clear that it can also take other shapes (e.g., concave or
convex spherical forms) and can further include a cup-shaped rim to
support a liquid or semi-liquid masking lens.
In FIG. 3, the reference 18 denotes the object such as the cornea of an eye
to be reprofiled and, reference 36 denotes a masking lens disposed over
the area thereof to be treated. Also as indicated in FIG. 3, the lens 36
is uniformly irradiated with a beam of radiation 12 obtained from a pulsed
UV laser source.
During the irradiation, the lens 36 is gradually ablated, and an increasing
area of the object 18 becomes exposed to erosion. As indicated in FIG. 3
at the moment when the lens has been wholly ablated, the surface of the
object has been eroded as indicate at 46, to the extent necessary to
complete reprofiling over the area of the lens. As shown in FIG. 3, the
maximum thickness t.sub.1 of the lens 36 exceeds the minimum thickness
t.sub.2 by an amount equal to the maximum depth (d) of the object erosion
desired.
The present invention is especially suited to the treatment of cornea of an
eye and provides a less dramatic means of effecting reprofiling of the
cornea, for example, as a remedy for certain forms of refractive errors.
FIGS. 2 and 3 illustrate the methods of the present invention in
connection with the treatment of myopia (nearsightedness). Similar lenses
of appropriate shape can, of course, be employed to remedy other forms of
reflective errors, such as hyperopia and astigmatism.
FIG. 4 illustrates an apparatus for performing a method of the present
invention for reprofiling the cornea of a human eye. A laser and
associated control circuitry is contained in a housing 52. The
beam-forming optics, for providing a beam of desired shape and size, can
also be contained within the housing 52 together with the laser power
supply control circuits. An optical wave guide 66, which can be flexible
or rigid and includes suitable mirrors, prisms and lenses, is provided to
transmit the laser beam output from the housing 52 to the patient 60. The
patient 60 is lying face-upwards on an operating table 54. The operating
table 54 will support the patient's head against vertical movement. If
desired, side supports 56 may also be provided to restrain sideways
movement of the patient's head.
The erodible mask of the present invention is disposed within an eyepiece
50A adapted to fit over the patient's eye. The eyepiece 58 includes
suction means for providing suction to clamp the eyepiece over the
patient's eye. The eyepiece can include a cup of resiliently deformed
flexible material such as rubber or plastic, which when placed over the
eyeball will clamp thereto upon being evacuated. Also disposed within the
eyepiece are suitable optical elements for transmitting the laser
radiation to the surface of the eye, and the erodible mask similar in
structure to the erodible mask shown in FIG. 2 and FIG. 3 above. The
erodible mask is manufactured as described above based on measurements of
the patient's eye and has an profile which will impart the desired
refraction correction upon erosion.
During the operation, the eye can be observed using a surgical microscope
64 which is supported above the patient by any convenient means. The
surgical microscope 64 may be connected to the eyepiece 58, but will more
normally be separated therefrom and supported by an arm (not shown) from
the ceiling or by a cantilever (not shown).
A measuring device 62 can also be employed in conjunction with the present
apparatus to measure the changes in the curvature of the cornea following
operation. Such a measuring device 62 can also be employed to monitor the
degree of erosion of the mask during treatment. The measuring device can
take the form of a commercially-available keratometer or other suitable
device and can be connected, as shown in FIG. 5, directly to the laser
optical path or may be movable when needed to occupy the position shown
for the surgical microscope 64, the operator moving the measuring device
62 or the microscope 64 into position as required.
The measuring device 62 can further provide th | | |
