Drug – bio-affecting and body treating compositions – Preparations characterized by special physical form – Matrices
Reexamination Certificate
1998-08-25
2003-07-08
Webman, Edward J. (Department: 3739)
Drug, bio-affecting and body treating compositions
Preparations characterized by special physical form
Matrices
C424S484000, C514S912000, C514S944000
Reexamination Certificate
active
06589558
ABSTRACT:
Excimer laser ablation of part of the cornea has been used extensively in refractive surgery for the treatment of refractive errors (myopia, hyperopia, astigmatism) of the eye (Photorefractive keratectomy). Moreover excimer laser ablation of the superficial layers of the corneal stroma has been employed to remove subepithelial opacities as well as for the smoothing of the corneal surface (Phototherapeutic Keratectomy).
Generally in photorefractive keratectomy (PRK) or in phototherapeutic keratectomy (PTK) an ArF excimer laser (wavelength 193 nm) is used for the controlled ablation of corneal tissue. Typical operating parameters are: Energy fluence: 100-250 mJ/cm
2
, repetition rate: 20 pulses/sec, Pulse duration: 20 nsec. The application of Excimer lasers in photoablation procedures of the cornea has been reported extensively in the scientific literature. The following papers can be mentioned as examples:
Trokel S, Srinivasan R, Braren B. Excimer Laser surgery of the cornea,
Am. J. Ophthalmology
1983; 94:125, Gartry G S, Kerr Muir M G, Marshall J. Photorefractive keratectomy with an argon fluoride excimer laser: a clinical study. J Refractive & Corneal Surgery 1991;7:420-431, Pallikaris I G, Papatzanaki M, Stathi E, Frenschock O, Georgiadis A, Laser in situ keratomileusis. Lasers Surg Med 1990;10:463-468.
The ablation of cornea with the 193 nm excimer laser has excellent precision, and insignificant thermal side effects. In general the tissue beneath the ablation site remains transparent and smooth. The laser irradiation takes place in a series of pulses of predetermined duration, energy and repetition rate which series of pulses is shaped by a series of diaphragms or it is scanned on the corneal surface in order to produce an irradiation pattern capable of changing the shape of the cornea and consequently it's optical performance. The desired dioptric correction determines the total depth of the ablation which is achieved by proper selection of the irradiation parameters (energy, repetition rate total number of pulses) and beam shaping algorithm.
The principal parameter that can modify the refractive outcome in respect to the desired is the change of corneal ablation rate (&mgr;m/pulse) as a result of corneal hydration variations.
In some cases modulators can be used for the creation of the ablation profile. A modulator is a photoablatable element that is placed between the laser source and the corneal surface and during the progress of the ablation process allows the gradual transmission of different portions of the laser beam and finally reproduces an irradiation profile which corresponds to a desired ablation profile.
A polymer modulator which is placed on the optical path of the laser beam delivery system and has the shape of a spherical or toric lens is in wide use for the correction of standard refractive errors (U.S. Pat. No. 4,856,513).
Application of certain collagen solutions on the corneal surface prior to laser irradiation in order to form an in-situ modulator has also been referred (U.S. Pat. No. 5,492,135). This solution while liquid covers the irregularities of the corneal surface while maintaining a relatively smooth anterior surface which is finally “projected” on the corneal surface provided that the collagen gel and the cornea have equal ablation rates.
The case of the remote polymer modulator has the disadvantage of the limited effectiveness in corneas with irregular shapes.
The application of a solution on the corneal surface prior to laser ablation promotes smoothing of the surface but it does not allow a major modification of the corneal shape in a large scale in respect to it's size in order to correct refractive errors such as myopia, hyperopia, regular and irregular astigmatism etc. Therefore, there is a need in the art for modulators that can be used in a single process to simultaneously correct the surface irregularities of the cornea and modify it's shape in a way that optimum optical performance is achieved (
FIGS. 1
,
2
). It is the object of the disclosed invention to provide suitable materials, accurate methods and special devices for the preparation of centered photoablatable modulators on the corneal surface. These modulators have a posterior surface that contacts the cornea and follows it's shape filling all the irregularities, while their anterior surface has a predetermined position and shape in respect to the cornea to be treated.
A material found to be suitable for the preparation of modulators is a gelatin solution with the following properties:
It is prepared at temperatures 1 to 10 degrees below the boiling point of the solution and can be kept liquid at temperatures 50-55° C.
At a temperature of 40-45° C. it forms a rigid gel.
When solid it can be ablated with an ArF Excimer laser at fluences varying between 50 and 200 mJ/cm
2
(20 Hz repetition rate) without melting or deformation.
Under these conditions the ablation rate of the gel practically equals the ablation rate of the normal cornea.
The material can be prepared and stored in gel form at room temperatures in air tight containers.
It is fully thermoreversible and it can be heated above it's solution to gel transition point prior to use in order to form a homogeneous solution.
A mixture of porcine skin gelatin and vegetable gelatin (carrageenan) has been found to be suitable for the preparation of a solution with the above properties. Concentration of porcine skin gelatin can be varied from about 10% to about 25% and concentration of carrageenan can be from about 0.2% to about 5% w/w
A small quantity of a dye (e.g. Sodium Fluorescein) can be added in the gelatin solution in order to produce a colored gel. A colored modulator has the advantage that it can be visually distinguished from the cornea and the ablation process can be terminated when the modulator is removed thoroughly.
Additionally, an anesthetic (e.g. tetracaine or proparacaine) added in the solution may render the molding an ablation process more comfortable for the patient.
A device suitable for the in-situ molding of the modulator comprises the following parts:
a) Lightweight mold constructed of transparent material having at least one molding surface which has optical quality and is being imprinted on the anterior surface of the gel and corresponds to the desired post-ablation shape of the cornea. In a preferred embodiment this mold has the shape of a hard contact lens with a peripheral transition zone to a flat area and a central mark. This mold remains on the surface of the gel until the gel completely solidifies (typically 3-5 min after application). After the lapse of this interval the mold is being removed. In a preferred embodiment the shape of the molding surface is spherical having a radius of curvature ranging from about 7 mm to about 10 mm. In another preferred embodiment the shape of the molding surface is ellipsoid having a base curvature from about 7 mm to 10 mm and an eccentricity ranging from about 0.1 to about 0.5.
b) Reference ring which attaches on an area of the eye outside the optical zone and serves as a guide which enables the precise placement of the mold in respect to the cornea. The plane defined from the circle of contact between the reference ring is perpendicular to the axis of symmetry of the mold. In a preferred embodiment this reference ring may attach in an area of the sclera. In another preferred embodiment it may be designed to fit a peripheral part of the cornea having a diameter from about 8 mm to about 10 mm. In another embodiment the reference ring may be designed to fit the limbus. In another embodiment the reference ring may feature a number of legs in order to ensure stable contact in the case of irregular shape of the eye. These legs may vary in length individually in order to control the orientation of the level of the reference ring in respect to the level defined by the points of contact. The reference ring may incorporate air suction for it's attachment on the eye. The reference ring guides either the mold itself or it's holder.
McGlew and Tuttle PC
Webman Edward J.
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