Surgery – Instruments – Light application
Reexamination Certificate
2001-03-05
2004-03-16
Peffley, Michael (Department: 3739)
Surgery
Instruments
Light application
C606S005000
Reexamination Certificate
active
06706036
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to methods of, and apparatus for, surgery of the cornea, and more particularly to a laser-based method and apparatus for corneal surgery.
2. Related Art
Art Related to the Inventive Method and Apparatus for Surgery
The concept of correcting refractive errors by changing the curvature of the eye was brought forth early on, as illustrated in the notable mechanical methods pioneered by J. Barraquer. These mechanical procedures involve removal of a thin layer of tissue from the cornea by a micro-keratome, freezing the tissue at the temperature of liquid nitrogen, and re-shaping the tissue in a specially designed lathe. The thin layer of tissue is then re-attached to the eye by suture. The drawback of these methods is the lack of reproducibility and hence a poor predictability of surgical results.
With the advent of lasers, various methods for the correction of refractive errors have been attempted, making use of the coherent radiation properties of lasers, and the precision of the laser-tissue interaction. A CO
2
laser was one of the first to be applied in this field. Peyman, et al., in Ophthalmic Surgery, vol. 11, pp. 325-9, 1980, reported laser burns of various intensity, location, and pattern were produced on rabbit corneas. Recently, Horn, et al., in the Journal of Cataract Refractive Surgery, vol. 16, pp. 611-6, 1990, reported that a curvature change in rabbit corneas had been achieved with a Co:MgF
2
laser by applying specific treatment patterns and laser parameters. The ability to produce burns on the cornea by either a CO
2
laser or a Co:MgF
2
laser relies on the absorption in the tissue of the thermal energy emitted by the laser. Histologic studies of the tissue adjacent to burn sites caused by a CO
2
laser reveal extensive damage characterized by a denaturalized zone of 5-10 microns deep and disorganized tissue region extending over 50 microns deep. Such lasers are thus ill-suited to corneal laser surgery.
In U.S. Pat. No. 4,784,135, Blum et al. discloses the use of far-ultraviolet radiation of wavelengths less than 200 nm to selectively remove biological materials. The removal process is claimed to be by photoetching without requiring heat as the etching mechanism. Medical and dental applications for the removal of damaged or unhealthy tissue from bone, removal of skin lesions, and the treatment of decayed teeth are cited. No specific use for cornea surgery is suggested, and the indicated etch depth of 150 microns is too great for most corneal surgery purposes. Further, even though it is suggested in this reference that the minimum energy threshold for ablation of tissue is 10 mJ/cm
2
, clinical studies have indicated that the minimum ablation threshold for excimer lasers at 193 nm for cornea tissue is about 50 mJ/cm
2
.
In U.S. Pat. No. 4,718,418, L'Esperance, Jr. discloses the use of a scanning laser characterized by ultraviolet radiation to achieve controlled ablative photode-composition of one or more selected regions of a cornea. According to the disclosure, the laser beam from an excimer laser is reduced in its cross-sectional area, through a combination of optical elements, to a 0.5 mm by 0.5 mm rounded-square beam spot that is scanned over a target by deflectable mirrors. (L'Esperance has further disclosed in European Patent Application No. 151869 that the means of controlling the beam location are through a device with a magnetic field to diffract the light beam. It is not clear however, how the wave front of the surgical beam can be affected by an applied magnetic to any practical extent as to achieve beam scanning.) To ablate a corneal tissue surface with such an arrangement, each laser pulse would etch out a square patch of tissue. Each such square patch must be placed precisely right next to the next patch; otherwise, any slight displacement of any of the etched squares would result in grooves or pits in the tissue at the locations where the squares overlap and cause excessive erosion, and ridges or bumps of unetched tissue at the locations in the tissue where the squares where not contiguous. The resulting minimum surface roughness therefore will be about 2 times the etch depth per pulse. A larger etch depth of 14 microns per pulse is taught for the illustrated embodiment. This larger etch depth would be expected to result in an increase of the surface roughness.
Because of these limitations of laser corneal surgery systems, it is not surprising that current commercial manufactures of excimer laser surgical systems have adopted a different approach to corneal surgery. In U.S. Pat. No. 4,732,148, L'Esperance, Jr. discloses a method of ablating cornea tissue with an excimer laser beam by changing the size of the area on the cornea exposed by the beam using a series of masks inserted in the beam path. The emitted laser beam cross-sectional area remains unchanged and the beam is stationary. The irradiated flux and the exposure time determines the amount of tissue removed.
A problem with this approach is that surface roughness will result from any local imperfection in the intensity distribution across the entire laser beam cross-section.
Furthermore, the intended curvature correction of the cornea will deviate with the fluctuation of the laser beam energy from pulse to pulse throughout the entire surgical procedure. This approach is also limited to inducing symmetric changes in the curvature of the cornea, due to the radially symmetrical nature of the masks. For asymmetric refractive errors, such as those commonly resulting from cornea transplants, one set of specially designed masks would have to be made for each circumstance.
Variations of the above technique of cornea ablation have also been developed for excimer lasers. In U.S. Pat. No. 4,941,093, Marshall et al. discloses the use of a motorized iris in a laser beam path to control the beam exposure area on the cornea. In U.S. Pat. No. 4,856,513, Muller discloses that re-profiling of a cornea surface can be achieved with an erodible mask, which provides a pre-defined profile of resistance to erosion by laser radiation. This method assumes a fixed etch rate for the tissue to be ablated and for the material of the erodible mask. However, etch characteristics vary significantly, depending on the type of the materials and the local laser energy density. The requirements of uniformity of laser intensity across the beam profile and pulse to pulse intensity stability, as well as limitation of the technique to correct symmetric errors, also apply to the erodible mask method.
Another technique for tissue ablation of the cornea is disclosed in U.S. Pat. No. 4,907,586 to Bille et al. By focusing a laser beam into a small volume of about 25-30 microns in diameter, the peak beam intensity at the laser focal point could reach about 10
12
watts per cm
2
. At such a peak power level, tissue molecules are “pulled” apart under the strong electric field of the laser light, which causes dielectric breakdown of the material. The conditions of dielectric breakdown and its applications in ophthalmic surgery had been described in the book “YAG Laser Ophthalmic Microsurgery” by Trokel. Transmissive wavelengths near 1.06 microns and the frequency-doubled laser wavelength near 530 nm are typically used for the described method. The typical laser medium for such system can be either YAG (yttrium aluminum garnet) or YLF (yttrium lithium fluoride). Bille et al. further discloses that the preferred method of removing tissue is to move the focused point of the surgical beam across the tissue. While this approach could be useful in making tracks of vaporized tissue, the method is not optimal for cornea surface ablation. Near the threshold of the dielectric breakdown, the laser beam energy absorption characteristics of the tissue changes from highly transparent to strongly absorbent. The reaction is very violent, and the effects are widely variable. The amount of tissue removed is a highly non-linear function of the incident beam power. Hence
Gordon & Rees LLP
Peffley Michael
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