Refractive laser ablation through topography

Details Refractive laser ablation through topography Refractive laser ablation through topography

1999-04-13

2003-04-22

Dvorak, Linda C. M. (Department: 3739)

Surgery

Instruments

Light application

C606S004000, C606S010000, C606S012000, C128S898000, C351S212000, C623S005110

Reexamination Certificate

active

06551306

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the field of ocular surgery, and more particularly to the use of excimer lasers for corneal refractive and therapeutic surgery.
2. Description of the Related Art
The use of excimer lasers in ocular surgery is today well known for performing corneal ablations with a high degree of accuracy. Applications of the excimer laser in refractive surgery include corrections of myopia, hyperopia, astigmatism, and presbyopia through the ablation of tissue on or within a patient's cornea. Thus, in photorefractive keratectomy (PRK), the aim is to flatten or steepen the central cornea to eliminate myopic, hyperopic, or presbyopic refractive errors and to correct corneal astigmatism. In phototherapeutic keratectomy, on the other hand, the intent is to smooth irregular corneal surfaces or remove opaque corneal tissue. A normal cornea is shown on the anterior portion of the eye, also known as the ocular globe, in FIG.
1
.
The cornea includes five distinct layers, as seen in FIG.
2
. Outermost layer
1
is the Epithelium, which is an anterior surface layer that promotes constant and active cellular reproduction. Second layer
2
, known as the Bowman has a structural function of serving as an attachment means for the corneal epithelium. The Bowman layer adheres anteriorly to the epithelium and posteriorly binds to third layer
3
, which is called the stroma. The stroma is the thickest layer, which gives body to the cornea. Fourth layer
4
, called the Descemet, posteriorly bounds the stroma. The innermost and fifth layer
5
is known as the Endothelium, and includes a layer of immunological cells whose function is to pump-dehydrate the cornea and keep it transparent.
The excimer laser used for ocular surgery is typically a 193 nanometer Argon Fluoride surgical laser system, such as the Schwind Keratom System.
FIG. 3
illustrates a schematic of laser beam delivery system
14
for the Schwind Keratom System. The Argon Fluoride excimer laser beam first passes through several optical components
18
where it is collimated and aligned to the eye to be treated. These optical components, which must be transparent for conducting the ultraviolet radiation of the beam, are made from a synthetic quartz such as Suprasil II.
Mechanical shutter system
16
controls the emission of the excimer laser beam. The shutter system consists of two independently actuated shutter blades that block the laser beam if the shutters are not energized. When the shutters are energized to permit passage of the laser beam, which is originally rectangular and exhibits a cross-section of approximately 8×24 mm, the beam is directed by 90-degree bending mirror
20
onto a specially designed optical component called an integrator, referenced as
22
. The integrator homogenizes the laser beam energy over its cross-section.
The uniform and homogenous rectangular laser beam is then passed through a series of beam-stops
24
on moving steel band
26
to create the intended diameter of the laser beam. After passing the beam-stops, the laser beam is bent downwardly by dicroitic beam-splitter
28
for presenting the beam to the fixation target, the patient's cornea
10
. The beam-splitter also permits the patient's eye to be observed via video camera
30
, as indicated in FIG.
3
.
After exiting beam splitter
28
, the laser beam passes through another lens system
32
before hitting the patient's cornea. The lens system consists of two large-diameter lenses that produce an image of the beam-stop onto the patient's cornea. These lenses thus act like a zoom projection system.
The operation of integrator
22
, steel band
26
with beam-stops
24
, and zoom projection system
32
is computer controlled for precise variation of the corneal ablation diameter, typically over a range as wide as 0.6 to 8.0 mm. The corneal ablation results from the energy and wavelength of the laser beam, which disrupts the bond between molecules in the cornea and destroys corneal tissue in a controlled manner.
The system does not work in the same way for phototherapeutic keratectomy and photorefractive keratectomy. For phototherapeutic keratectomy, the surgeon selects a certain diameter of the ablation zone. This means that only one aperture of the steel band is used, and that the two large-diameter lenses do not move during the ablation process.
In the photorefractive keratectomy mode, however, the computer-controlled system changes the laser fluence during the treatment, depending on the selected steel band aperture and the position of the zoom optic. The zoom range and the available diameters of the beam-stops are designed so that a continuous variation of the ablation diameter of the cornea should be possible. For example, a simple myopic ablation of the patient's cornea may be achieved by a computer controlled combination of changes of different beam-stops and movements of the two large-diameter lenses.
Surgical lasers, such as the Schwind Keratom system, are controlled with the aid of nomograms that are based upon numerous studies and data bases. Such nomograms allow for appropriate correction of refractive defects in most patients, once the patient's particular gradation of myopia, hyperopia, astigmatism, or presbyopia has been identified through keratometric and subjective examination.
Refractive corneal surgery with an excimer laser is presently conducted in one of two ways: photorefractive keratectomy (PRK), shown in
FIG. 4
, and Laser In situ Keratomileusis (LASIK), shown in FIG.
5
.
In the PRK technique, laser beam
40
is applied directly to the patient's corneal surface
10
according to the particular refractive defect. The laser system thus destroys an anterior portion of the cornea according to the machine's nomogram, leaving the stroma uncovered and resulting in the change shown at
42
in FIG.
4
A. The stroma will later be covered with new Epithelial cells during the healing process, which takes a few days. One of the shortcomings of the PRK technique is that the Bowman layer or membrane is destroyed by the direct corneal application of the laser beam. The destruction of the Bowman layer is a major concern due to that layer's role in maintaining corneal transparency while serving as point of adhesion for the epithelium.
The LASIK technique involves the use of a microkeratome (not shown), which makes an access cut across the anterior portion of the cornea. More specifically, the microkeratome makes a lamellar resection of the cornea to create a corneal flap and hinge, as seen in FIG.
5
. Flap
44
, also known as a “pediculado,” includes corneal tissue from the Epithelium, Bowman, and anterior stromal layers. The corneal flap is typically circular, having a diameter between 7 and 9 mm, and averages about 160 microns in thickness. A presently preferred microkeratome is described in pending U.S. application Ser. No. 09/002,515, the entire contents of which are incorporated herein by reference.
The corneal flap allows corneal stroma
3
to be exposed for ablation by laser beam
40
that is appropriate to correct the patient's refractive defect, as indicated in FIG.
5
A. This results in altered stromal region
46
shown in FIG.
5
B. Following the laser application, corneal flap
44
is returned to its initial position, as shown in
FIG. 5C
, and, since no ablation has been performed at the corneal surface, the patient suffers no destruction of the Bowman layer and recovers very rapidly.
A technique similar to the present-day LASIK procedure was described as early as Jun. 20, 1989 in U.S. Pat. No. 4,840,175 to Gholam A. Peyman. The '175 patent teaches the use of an excimer laser to modify the curvature of a patient's cornea. A thin layer of the cornea is removed “by cutting” (no details are provided), and a laser beam is then applied to either the thin layer or the exposed corneal surface. A variable diaphragm is used to form the laser beam into a desired predetermined pattern for ablat

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