Surgery – Radioactive substance applied to body for therapy – Radioactive substance placed within body
Reexamination Certificate
1998-10-21
2001-07-17
Nasser, Robert L. (Department: 3736)
Surgery
Radioactive substance applied to body for therapy
Radioactive substance placed within body
C600S004000
Reexamination Certificate
active
06261220
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to ophthalmic laser surgery, and more particularly to a method and system for arranging a pattern of laser shots to erode a shaped volume of corneal tissue in accordance with the treatment of a specified eye condition.
BACKGROUND OF THE INVENTION
Photorefractive keratectomy (PRK) is a procedure for laser correction of focusing deficiencies of the eye by modification of corneal curvature. PRK is distinct from the use of laser-based devices for more traditional ophthalmic surgical purposes, such as tissue cutting or thermal coagulation. PRK is generally accomplished by use of a 193 nanometer wavelength excimer laser beam that ablates away corneal tissue in a photo decomposition process. Most clinical work to this point has been done with a laser operating at a fluence level of 120-195 mJ/cm
2
and a pulse-repetition rate of approximately 5-10 Hz. The procedure has been referred to as “corneal sculpting.”
Before sculpting of the cornea takes place, the epithelium or outer layer of the cornea is mechanically removed to expose Bowman's membrane on the anterior surface of the stroma. At this point, laser ablation at Bowman's layer can begin. An excimer laser beam is preferred for this procedure. The beam may be variably masked during the ablation to remove corneal tissue to varying depths as necessary for recontouring the anterior stroma. Afterward, the epithelium rapidly regrows and resurfaces the contoured area, resulting in an optically correct (or much more nearly so) cornea.
For ablation to occur, the energy density of the laser beam must be above some threshold value, which is currently accepted as being approximately 60 mJ/cm
2
. Such energy densities can be produced by a wide variety of commercially available lasers. For example, a laser could be used that is capable of generating a laser beam of diameter large enough to cover the surface to be ablated, i.e., on the order of 4.5-7.0 millimeters in diameter. However, such laser beams are typically not regular in intensity thereby causing a rough surface ablation. Further, lasers capable of producing such laser beams are typically, large, expensive and prone to failure.
Alternatively, a laser could be used that produces a much smaller diameter laser beam, i.e., on the order of 0.5-1.0 millimeters in diameter. There are several advantages afforded by the smaller diameter laser beam. They can be generated to meet the above noted threshold requirement with a lower energy pulse than that of the larger diameter beam. Further, such smaller diameter laser beams can be produced with a regular intensity while minimizing the variance in pulse-to-pulse energy levels. Finally, lasers producing the smaller diameter laser beam are physically smaller, less expensive and, frequently, more reliable. However, this requires that the position of the small pulses be precisely controlled so that the resulting ablated surface is smoother than that which is produced by the larger laser beam.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method and system of laser sculpting suitable for the recontouring of corneal tissue.
Another object of the present invention is to provide a method and system for arranging a pattern of small diameter, regular intensity laser pulses or shots to erode or ablate a shaped volume of corneal tissue in accordance with the treatment of a specific eye condition.
Still another object of the present invention is to provide a method and system of laser sculpting that is designed to use small inexpensive lasers.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a method and system are provided for eroding or ablating a shaped volume of an eye's corneal tissue in accordance with the treatment of a specified eye condition. A plurality of laser beam shots of uniform intensity are selected to form a uniform shot pattern of uniform shot density. If the laser beam shots were applied in accordance with the uniform shot pattern of uniform shot density, they would be capable of eroding a volume of the corneal tissue of uniform height. The volume of uniform height is approximately equivalent to that of the shaped volume. The laser beam shots are actually applied to the corneal tissue in a spatially distributed pattern spread over an area approximately equivalent to the surface area of the shaped volume to be eroded. The spatially distributed pattern is obtained by distorting the uniform shot pattern in a fixed manner from a reference position on the shaped volume representative of the shaped volume's axis of symmetry. Shot density for the laser beam shots changes in correspondence with distance from the reference position. The particular spatial distribution and change in shot density is adjusted to treat the eye conditions of myopia, hyperopia and astigmatism.
This patent application is copending with related patent applications entitled “Laser Beam Delivery and Eye Tracking System” filed on the same date and owned by a common assignee as subject patent application. The disclosures of these two applications are incorporated herein by reference.
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patent: 4729372 (1988-03-01), L'Esperance et al.
patent: 4941093 (1990-07-01), Marshall et al.
patent: 5207668 (1993-05-01), L'Esperance
patent: 5336217 (1994-08-01), Buys et al.
patent: 0 151 869 (1985-08-01), None
patent: 93/08877 (1983-05-01), None
patent: WO 87/06478 (1987-11-01), None
Burkhalter James H.
Frey Rudolph W.
Gray Gary P.
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Autonomous Technologies Corporation
Nasser Robert L.
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