Surgery – Instruments – Light application
Reexamination Certificate
2000-03-28
2002-12-10
Gibson, Roy D. (Department: 3739)
Surgery
Instruments
Light application
C606S002000, C606S107000, C606S166000
Reexamination Certificate
active
06491686
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to medical systems and methods. More particularly, the present invention relates to the use of diffractive optics for generating successive patterns of light energy for ablating corneal or epithelial tissue.
Photorefractive keratectomy (PRK) and phototherapeutic keratectomy (PTK) employ optical beam delivery systems for directing laser energy to a patient's eye in order to selectively ablate corneal tissue to reform the shape of the cornea and improve vision. All present commercial systems employ excimer lasers, where the beams from the lasers are spatially and temporally integrated in order to form a beam having uniform characteristics. In particular, the beams are integrated in order to display a flat intensity profile over a circular target region, often referred to as a “top hat” profile.
Once such uniformly integrated beams are achieved, they may be used in different ways in order to effect corneal ablation. In a first type of system, the beam has a width which generally corresponds to the desired target area on the cornea. The beam intensity is manipulated using an iris or other exposure control mechanism, and the desired corneal reshaping can be achieved by properly controlling the exposure. While highly effective and relatively easy to control, the need to employ a laser beam having a width equal to the treatment area (typically on the order of 5.0 mm to 10.0 mm) requires the use of large excimer lasers. Not only are such large lasers expensive, they also occupy a relatively large area, requiring significant space to house them.
As an alternative to such large beam diameter systems, laser “scanning” systems are also employed for corneal ablation. Such scanning systems employ a much smaller beam width, minimizing energy required from the laser. The smaller lasers are both more economic and require less space to house them. The use of a small beam width, however, complicates certain aspects of the treatment protocols. As most of the small treatment beams have a circular diameter, it will be appreciated that it is difficult to control exposure of the cornea. In particular, the beams overlap in non-uniform patterns as they are scanned over the cornea, making it very difficult to achieve properly controlled exposure over the entire target region. While elaborate control and exposure algorithms have been developed to minimize detrimental variations in exposure, none are entirely adequate.
For these reasons, it would be desirable to provide improved methods and systems for the scanning of light beams over corneal tissue in order to selectively ablate the tissue to treat vision disorders. In particular, it would be desirable to utilize small beam geometries with low power requirements while achieving an even energy distribution free from small regions of overexposure and underexposure. Moreover, it would be desirable to simplify the control schemes and systems required to scan small width light beams for corneal treatment. It would further be desired to provide treatment protocols and algorithms which are particularly suitable for accommodating the circular geometry of an ablation zone on the cornea. It would be still further desirable if the methods and systems could be used for the ablation of the epithelial tissue over the cornea prior to corneal treatment. At least some of these objectives will be met by the invention described hereinafter.
2. Description of the Background Art
Scanning systems for performing photorefractive keratectomy (PRK) and phototherapeutic keratectomy (PTK) are described in a number of patents, including U.S. Pat. No. 5,391,165. A laser surgical system employing a diffractive optical element adapted to an individual patient is described in U.S. Pat. No. 5,571,107. A laser scanning system employing unique reflective optics is described in U.S. Pat. No. 5,546,214. A temporal and spatial beam integrated for a PRK/PTK laser system is described in U.S. Pat. No. 5,646,791. The full disclosures of each of the above-cited U.S. patents are incorporated herein by reference.
Use of a diffractive optical element for integrating an excimer laser beam for use in PRK/PTK procedures is described in a co-pending application entitled LASER SYSTEM AND METHOD WITH DIFFRACTIVE OPTIC, U.S. patent application Ser. No. 09/015,841, filed on Jan. 29, 1998, the full disclosure of which is incorporated herein by reference.
SUMMARY OF THE INVENTION
The present invention provides improved methods, systems, and other apparatus for performing scanning-type laser ablation.
The invention is particularly useful for performing corneal ablation in PRK and PTK procedures but will also be useful for removing an epithelial layer prior to corneal ablation in such procedures. For convenience, the following discussion will be directed at corneal ablation, but the teachings are also useful for removing epithelial tissue. The use of laser energy for removing epithelial tissue is described in co-pending U.S. patent application Ser. No. 09/022,774, filed on Feb. 12, 1998, the full disclosure of which is incorporated herein by reference.
By “scanning,” it is mean that an ablation light beam is aimed or “scanned” to successive, discrete locations on the corneal surface, and that those locations are then exposed to a predetermined amount or dosage of the light energy. Usually, the laser system will be operated in a pulsed manner, and the exposure at any particular location will result from a number of pulses which occur over a very short time period. The total area of the cornea to be treated, referred to hereinafter as the “ablation zone,” is eventually treated as the ablative light beam scanned over the zone. As discussed above, however, prior systems which employ light beams having circular cross-sections result in an uneven treatment profile since adjacent circular geometries overlap in an uneven manner. The present invention significantly improves the uniformity of treatment, in some embodiments by employing beam geometries which are selected to cover the entire ablation zone without substantial overlap between adjacent beam patterns. In this way, the entire ablation zone can be treated with each segment or portion of the zone receiving the desired dosage of ablative energy.
The present invention provides a number of specific improvements over such prior corneal ablation methods and systems. First, the present invention provides methods and systems for treating an ablation zone with ablative light beams having annular or ring-shaped geometries. The ablation zone will usually have a circular geometry with a diameter in the range from 0.1 mm to 10.0 mm, usually from 1.0 mm to 6.0 mm. By employing successive ablative light beams having concentric, annular geometries, the entire ablation zone can be treated without substantial overlap between adjacent annular beams. That is, by utilizing adjacent annular light beams where the outer diameter of one beam is substantially equal to the inner diameter of the adjacent beam, each annular segment of the ablation zone will be treated only once. The use of annular beam geometries is particularly preferred since it facilitates dosage control over the ablation zone. That is, it will be relatively easy to expose the radially outward segments and radially inward segments to different energy dosages by properly selecting the beam intensity and dosage time of each of the radially positioned annular light beams. As described below, these annular beam geometries may also treat an ablation zone using substantial overlap between successive beams.
While the use of annular beam geometries will usually be preferred, the present invention could also utilize other beam geometries which are selected to cover individual treatment regions or segments over the total ablation zone without substantial overlap. In addition to the annular geometries described above, the treatment patterns could be pie-shaped, polygonal, irregular, or combinations thereof. In this aspect of the inven
Gibson Roy D.
Townsend and Townsend / and Crew LLP
VISX Incorporated
Vrettakos Pete J
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