Surface shaping using lasers - Patent Review 4994058

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DESCRIPTION

1. Field of the invention

This invention relates to apparatus and method employing lasers, especially pulsed lasers, for shaping surfaces, especially surfaces of organic material. In particular, the invention relates to apparatus and methods for shaping biological tissue, including the cornea of the eye.

BACKGROUND OF THE INVENTION

It is known to employ a laser source 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 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 practised and techniques further developed up to the present day (see for example "Refractive Keratooplasty improves with Polysulfone, Pocket Incision" Ophthalmology Times, July 1, 1986).

It has been suggested in European Patent Application No. 01518699 of L'Esperance, to perform controlled ablative photodecomposition of one or more selected regions of a cornea 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 by 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, described in British Patent Application No. 8604405 and herein incorporated by reference, 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).

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 a corneal tissue.

THE INVENTION

According to one aspect of the present invention, there is provided, a laser apparatus for reprofiling a surface comprising, a laser means, control means for controlling the laser to project laser radiation towards the surface, and a masking means disposed between the laser means and the surface having a predefined profile of resistance to the laser radiation, so 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 may be formed from material which is ablated by absorption of the laser radiation so that the masking means is progressively destroyed during the surface reprofiling.

Alternatively the masking means may be formed from material which has differing transmission characteristics over the masked area but which is not substantially ablated or otherwise eroded during the surface reprofiling.

The masking means may comprise a lens-like device which is supported by a rigid structure which is affixed to the surface, (for example to the sclera of an eye where the apparatus is to be used in conjunction with corneal surgery), the lens being connected to the support structure and disposed above the surface either in contact with the surface or a small distance thereabove. The lens can be directly integrated with the support structure or, preferably, the support structure may include a transparent stage to support and position the lens.

In another embodiment, the masking means may comprise a contact-type lens device which is disposed upon, and directly affixed to, the surface (e.g. the cornea of an eye in the case of corneal surgery). Typically the contact-type lens is constructed so as to have a first surface contoured to fix to the surface to be eroded and a second surface contoured to provide the desired surface contour following erosion by exposure to laser radiation.

In a further embodiment the masking means may comprise a tray or well of optically transparent material in which a quantity of a selected masking material in the form of a liquid or gel or gas or vapour or volatile material can be contained. The base of the tray or well may be curved so that the underside of the masking material contained therein is either convexly or concavely shaped to define a "lens". By choice of material so the absorption of the laser light by the masking material will cause selective erosion of the surface below the tray or well. The latter may be supported on or above the surface and may be in contact with the surface if desired.

Whichever is selected, a masking lens of the present invention provides a predefined profile or 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 if varied, and dependent on the nature of the erosion of the object which is required, 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 the surface shape may be irregular, as might be required in the case of surgery on a cornea to remove an ulcer.

Conveniently the lens material has similar ablation characteristics to the surface material. Various polymeric materials can be employed including, for example, polymethylmethacrylate, polymethylstyrene 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.

According to another aspect of the invention, there is provided 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 the laser radiation, and

(c) irradiating 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 method may include varying the thickness of the masking means or varying the composition of the masking means, to provide the desired resistance profile.

Typically, the laser is set to operate so that a single pulse erodes a depth in the range 0.1 to 1 micrometer of surface material.

The method may be applied to any ablatable surface including biological tissue such as a ligament or a cartilage in a bone.

The method of the present invention is 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 1 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 an 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 150 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 also lies in a system for reprofiling a surface using laser radiation in which masking means is disposed between the source of laser radiation and the surface for providing a predefined profile of resistance to the said laser radiation, 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 surface which undergoes erosion may be biological tissue, particularly corneal tissue, and may include means to immobilise the surface.

The masking means may include a rigid support structure affixed to the surface with a masking lens connected to the support structure and disposed above the surface. The support structure further may include a transparent stage with the masking lens affixed to the stage. The masking lens may vary in thickness, or may vary in composition to provide the predefined profile of resistance.

The lens may be formed from polymethylmethacrylate, polymethylstyrene, or mixtures thereof.

The masking means may include a masking lens disposed upon, and directly affixed to, the cornea, which as above described may vary in thickness or in composition, to provide the predefined profile of erosion resistance. As before the lens may be formed from polymethylmethacrylate, polymethylstyrene, or mixtures thereof.

The laser source may be a pulsed excimer laser, typically an Argon-Fluoride laser operating at a wavelength of about 913 nanometers.

The invention also lies in masking apparatus for use in laser reprofiling of corneal tissue comprising a rigid support structure adapted for fixation upon a cornea, and a mask connected to the support structure and disposed above the cornea, the mask having a predefined profile of resistance to the laser radiation, whereby upon irradiation of the mask, a portion of the laser radiation is selectively absorbed and another portion is transmitted to the cornea in accordance with the mask profile to selectively erode the tissue.

The support may include a transparent stage adapted to receive the mask.

The mask may comprise a lens which varies in thickness or composition, to provide the profile.

The mask may be formed from polymethylmethacrylate, polymethylstyrene, or mixtures thereof.

The invention also lies in masking apparatus for use in laser reprofiling of corneal tissue comprising a masking lens adapted for direct fixation upon a cornea, the lens having a predefined profile of resistance to erosion by laser radiation, whereby upon irradiation of the lens, a portion of the laser radiation is selectively absorbed and another portion is transmitted to the cornea in accordance with the lens profile to selectively erode the tissue.

The lens may have a diameter in the range of about 3 to 12 millimeters and a maximum thickness of about 2 millimeters or less, and may vary in thickness, or in composition to provide the profile.

The masking means may be secured to the cornea by a suction means and a vacuum pump may be provided to reduce the pressure within the suction means, to fix the suction means in place on the cornea. As before the lens may be formed from polymethylmethacrylate, polymethylstyrene, or mixtures thereof, or the lens may be formed by a mass of material contained in a well or dish, above the cornea, which is optically transparent to the laser radiation.

The well or dish may include a transparent lid or cover, and the well or dish may be a liquid or gel, or gas or a vapour.

The apparatus may be formed at least in part from a material which is ablated or eroded by the laser radiation, said resistance being a measure of the resistance to ablation or erosion by the laser radiation.

The rate of ablation or erosion for the lens material may be substantially the same on the rate of ablation or erosion of the corneal surface.

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 overtightening 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.

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 diagrammatic 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 is an illustration of alternative embodiment of an erodable mask suitable for use in the apparatus of FIG. 1;

FIG. 4A illustrates diagramatically the beginning of a reprofiling operation to reduce the curvature of an object in accordance with the present invention;

FIG. 4B illustrates diagrammatically the completion of the reprofiling operation of FIG. 4A;

FIG. 5 shows a laser apparatus for measurement and reprofiling;

FIG. 6 illustrates a modified version of the apparatus as shown in FIG. 2, capable of retaining a liquid or gel as a convex lens-like mask, and

FIG. 7 illustrates a modification to the arrangement of FIG. 6 in which a concave lens-like mask of liquid or gel can be formed.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

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 wh