Surgery – Instruments – Light application
Reexamination Certificate
2001-10-12
2003-08-26
Shay, David M. (Department: 3739)
Surgery
Instruments
Light application
C606S010000, C606S012000
Reexamination Certificate
active
06610051
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains generally to ophthalmic laser surgery procedures. More particularly, the present invention pertains to laser surgical procedures which are performed to reshape or restructure the cornea of an eye by using photodisruption techniques to remove stromal tissue. The present invention is particularly, but not exclusively, useful as a method and system for focusing laser energy inside a stromal lamella to photodisrupt stromal tissue.
BACKGROUND OF THE INVENTION
It is well known that the refractive properties of the cornea can be altered by the selective removal of corneal tissue. For example, a myopic condition of the eye can be corrected by selectively removing corneal tissue from the central portion of the cornea. Similarly, a hyperoptic condition can be corrected by selectively removing corneal tissue within a peripheral ring surrounding the central portion of the cornea.
A general knowledge of the anatomy of the cornea of an eye is helpful for appreciating the problems that must be confronted during refractive corrections of the cornea. In detail, the cornea comprises various layers of tissue which are structurally distinct. In order, going in a posterior direction from outside the eye toward the inside of the eye, the various layers in a cornea are: an epithelial layer, Bowman's membrane, the stroma, Decimet's membrane, and an endothelial layer. Of these various structures, the stroma is the most extensive and is generally around four hundred microns thick. Additionally, the healing response of the stromal tissue is generally quicker than the other corneal layers. For these reasons, stromal tissue is generally selected for removal in refractive correction procedures.
In detail, the stroma of the eye is comprised of around two hundred identifiable and distinguishable layers of lamellae. Each of these layers of lamellae in the stroma is generally dome-shaped, like the cornea itself, and they each extend across a circular area having a diameter of approximately six millimeters. Unlike the layer that a particular lamella is in, each lamella in the layer extends through a shorter distance of only about one tenth of a millimeter (0.1 mm) to one and one half millimeters (1.5 mm). Thus, each layer includes several lamellae. Importantly, each lamella includes many fibrils which, within the lamella, are substantially parallel to each other. The fibrils in one lamella, however, are not generally parallel to the fibrils in other lamellae. This is so between lamellae in the same layer, as well as between lamellae in different layers. Finally, it is to be noted that, in a direction perpendicular to the layer, each individual lamella is only about two microns thick.
Within the general structure described above, there are at least three important factors concerning the stroma that are of interest insofar as the photodisruption of stromal tissue to effect a refractive change is concerned. The first of these factors is structural, and it is of interest here because there is a significant anisotropy in the stroma. Specifically, the strength of tissue within a lamella is approximately fifty times the strength that is provided by the adhesive tissue that holds the layers of lamellae together. Thus, much less energy is required to separate one layer of lamellae from another layer (i.e. peel them apart), than is required to cut through a lamella. The second factor is somewhat related to the first, and involves the response of the stromal tissue to photodisruption. Specifically, for a given energy level in a photodisruptive laser beam, the bubble that is created by photodisruption in the stronger lamella tissue will be noticeably smaller than a bubble created between layers of lamellae. This is important because the creation of large bubbles tends to cloud the cornea, and thereby reducing the effectiveness of wavefront analysis during the procedure. Additionally, at a given laser energy, much more tissue is photodisrupted when the laser beam is focused inside a lamella than when the laser beam is focused between layers of lamellae.
After the photodisruption of stromal tissue, water resorption occurs, lessening the effect of the photodisruption. For some tissues, up to 80% of the water vapor produced by photodisruption is resorbed. Thus, the present invention recognizes that photodisruption is more effective on some types of stromal tissue than others. It is also preferable to create small bubbles inside the stromal lamellae to effect a refractive change in the cornea by photodisruption. The third factor concerning the stroma is optical, and it is of interest here because there is a change in the refractive index of the stroma between successive layers of lamellae. This is due to differences in the orientations of fibrils in the respective lamella.
Somewhat related to the present invention, a method for finding an interface between layers of lamellae for photodisrupting using a wavefront analyzer and an ellipsometer was disclosed in co-pending U.S. patent application Ser. No. 09/783,665, filed on Feb. 14, 2001 by Bille and entitled “A Method for Separating Lamellae”. As such, the contents of co-pending application Ser. No. 09/783,665 are hereby incorporated herein by reference. In co-pending application Ser. No. 09/783,665, a procedure for creating a corneal flap for a LASIK type procedure was presented. Unlike the present invention, the goal in the creation of a corneal flap is to minimize the total amount of tissue that is photodisrupted while establishing a continuous cut of stromal tissue.
In light of the above, it is an object of the present invention to provide a device and method for positioning the focal point of a laser beam inside a stromal lamellae and maintaining the focal point at locations inside the stromal lamellae to photodisrupt stromal tissue and alter the refractive properties of the eye. Another object of the present invention is to provide a method for using a laser beam to photodisrupt relatively large amounts of stromal tissue with a laser beam of relatively low energy. Still another object of the present invention is to provide a method for photodisruption of stromal tissue that avoids the large bubbles and associated clouding that occurs when the laser beam is focused on tissue lying on an interface between layers of lamellae. Another object of the present invention is to provide a device and method for tracking the progress of the photodisruption procedure, providing information that can be used to update the amounts and locations of stromal tissue that must be removed to obtain the desired refractive correction. Yet another object of the present invention is to provide a method for altering the refractive properties of the cornea that is easy to perform and is comparatively cost effective.
SUMMARY OF THE PREFERRED EMBODIMENTS
In accordance with the present invention, a method for altering the refractive properties of the cornea involves photodisrupting tissue at selected locations within the stroma of the cornea. Specifically, each photodisruption location is selected to preferably be inside a stromal lamella rather than at a location between lamellae. By photodisrupting a plurality of stromal lamellae in this manner, the refractive properties of the cornea can be altered at relatively low laser energies and with minimal clouding of the cornea. To photodisrupt a location inside a stromal lamella, the focal point of the laser, and consequently the laser energy, is focused inside a stromal lamella.
For the present invention, a wavefront detector can be used during the photodisruption procedure to track the progress of the corrective procedure. Using the wavefront detector, continuously updated information concerning the refractive properties of the cornea is provided to the surgeon during the course of the procedure. This continually changing information allows the surgeon to select the amounts and locations of stromal tissue that must be subsequently altered to obtain the desired shape for the cornea.
To locate t
20/10 Perfect Vision Optische Geraete GmbH
Nydegger & Associates
Shay David M.
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