Surgery – Instruments – Light application
Reexamination Certificate
2001-11-28
2003-11-04
Gibson, Roy D. (Department: 3739)
Surgery
Instruments
Light application
C128S898000
Reexamination Certificate
active
06641577
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains generally to ophthalmic laser surgery procedures. More particularly, the present invention pertains to methods for creating corneal flaps for use in corneal reshaping procedures. The present invention is particularly, but not exclusively, useful as a method for using a pulsed laser beam to efficiently create a corneal flap that can be lifted to expose stromal tissue for photoablation.
BACKGROUND OF THE INVENTION
The cornea provides approximately two thirds of the total focusing power of the eye. Along with the lens, the cornea refracts incoming light and focuses the light on or near the retina. The curvature of the cornea determines where the incoming light will be focused. If the curvature of the cornea is too steep relative to the length of the eye, light from distant sources will be focused in front of the retina, causing a vision impairment known as myopia (near-sightedness). Similarly, if the curvature of the cornea is too flat relative to the length of the eye, light from close sources will be focused behind the retina, causing a vision impairment known as hyperopia (far-sightedness). Finally, when the curvature of the cornea is non-uniform, light from both close and distant sources will fail to properly focus on the retina, resulting in a blurring of vision known as astigmatism.
The refractive errors mentioned above can generally be corrected using eyeglasses or contact lenses. Alternatively, the cornea of the eye can be surgically reshaped to provide the needed optical correction. Currently, the most popular technique for reshaping the cornea is laser-assisted in situ keratomileusis (LASIK). In the widely used LASIK procedure, a microkeratome is used to cut a flap in the cornea. Next, the flap is lifted to expose a bed of stromal tissue. Once exposed, the bed of stromal tissue is vaporized to a prescribed depth using an excimer laser. After laser treatment, the flap is repositioned and allowed to heal. The result is a reshaped cornea. Unfortunately, the creation of a flap using a microkeratome can result in some complications. For example, the effective creation of the flap with the microkeratome often relies on the skill of the surgeon. Complications can result if the flap is cut improperly or completely severed from the cornea. Further, use of the microkeratome requires the eye to be restrained from movement, often causing patient discomfort. Additional drawbacks associated with using a microkeratome to create a flap include the inability to control the shape of the flap and the fact that a relatively large amount of corneal tissue needs to be cut to create the flap.
As an example of another corneal reshaping procedure, U.S. Pat. No. 4,907,586, which issued to Bille et al. for an invention entitled “Method for Reshaping the Eye,” discloses an intrastromal photoablation technique for reshaping the cornea. Importantly for the purposes of the present invention, the above cited Bille patent discloses the use of a pulsed laser beam for photoablation of intrastromal tissue. Unlike the excimer laser, the pulsed laser beam, as disclosed by Bille, penetrates corneal tissue and can be focused at a point below the surface of the cornea to photoablate stromal tissue at the focal point. The ability to reach a subsurface location without necessarily providing a physical pathway allows for volumes of stromal tissue having complex shapes to be accurately disrupted, while minimizing the amount of total tissue disrupted. The present invention uses subsurface photoablation to create a portion of a corneal flap.
When considering the use of subsurface photoablation to create a flap for corneal reshaping, a general knowledge of the anatomy of the cornea of an eye is helpful. 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, Decemet'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 nine 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.
Somewhat related to the present invention, a method for finding an interface between layers of lamellae for photoablation 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 method disclosed in Bille '665 involved using subsurface photoablation to cut the entire inner surface for the flap. The present invention, in contrast, contemplates using subsurface photoablation along an interface solely for the purpose of establishing a periphery for the flap. This periphery, in turn, can be used as a starting point to allow layers of lamellae to be separated from each other along an interface by simply peeling the flap away from the remainder of the cornea.
Within the general structure described above, there are at least three important factors concerning the stroma that are of interest insofar as the creation of a corneal flap 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 lamella together. Thus, much less energy is required to separate one layer of lamella from another layer (i.e. peel them apart), than would be required to cut through a lamella. The second factor is somewhat related to the first, and involves the stromal tissue response to photoablation. Specifically, for a given energy level in a photoablative laser beam, the bubble that is created by photoablation in the stronger lamella tissue will be noticeably smaller than a bubble created between layers of lamellae. The third factor 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. When consideration is given to using a laser beam for the purpose of creating a corneal flap in a LASIK procedure, these factors can be significant.
In light of the above, it is an object of the present invention to provide an efficient surgical method for creating a corneal flap suitable for use in a corneal reshaping procedure. Another object of the present invent
20/10 Perfect Vision Optische Geraete GmbH
Gibson Roy D.
Johnson Henry M.
Nydegger & Associates
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