Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Eye prosthesis – Corneal implant
Reexamination Certificate
1997-11-17
2001-05-08
Isabella, David J. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Eye prosthesis
Corneal implant
Reexamination Certificate
active
06228114
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method and apparatus for adjusting corneal curvature and, more particularly, to an implantable device adapted for insertion into the peripheral cornea of an eye and which may be modified in the amount of corneal volume it displaces at the time of insertion and at post-operative times to correct refractive error by adjusting or removing solid material from the implanted device or augmenting said device with solid material.
Ametropia, an undesirable refractive condition of the eye, has three main subdivisions: myopia, hyperopia, and astigmatism. In myopia, by far the most common type of ametropia, the parallel light rays
1
which enter the eye as shown in
FIG. 1
come to a focus Fl in front of the retina
2
as shown in FIG.
2
. In hyperopia, the rays of light
1
come to a focus F
2
behind the retina
2
as shown in FIG.
3
. When the rays of light converge to not one, but several foci, it is referred to as astigmatism, in which condition the various foci may all lie before the retina; all lie behind the retina; or partly before and partly behind the retina.
Ametropia is usually corrected by glasses or contact lenses. However, these refractive disorders may also be corrected by surgery. Refractive eye surgery is defined as that surgery on the eye which acts to change the light-bending qualities of the eye. More common current refractive procedures include radial keratotomy, as described in U.S. Pat. Nos. 4,815,463 and 4,688,570 and also laser ablation of corneal stroma, described in U.S. Pat. No. 4,941,093. Various other surgical methods for the correction of refractive disorders have been tried including thermokeratoplasty for the treatment of hyperopia, epikeratoplasty to correct severe hyperopia, and keratomileusis which can steepen or flatten the central cornea. Keratomileusis was introduced by Barraquer of Colombia in 1961 and essentially involves grinding a corneal button into an appropriate shape to correct the refractive error and replacing the reshaped corneal button. Some of the more common keratorefractive procedures are discussed below, none of which have currently shown itself to have all the characteristics of an ideal keratorefractive procedure. The disadvantages of corneal refractive surgery include limited predictability, lack of reversibility, corneal destabilization, optical zone fibrosis, post-operative discomfort, and visual symptoms such as glare, halos, and starbursts.
The goal of the ideal keratorefractive procedure is to allow all the advantages of eyeglasses or contact lenses, namely, being able to correct a wide range of refractive errors, accuracy or predictability, to allow reversibility in the event that the refractive state of the eye changes and it becomes necessary to adjust the correction again, to yield minimal complications and be associated with technical simplicity, low cost, and being aesthetically acceptable to the patient.
For well over a century, ophthalmologists have searched for a surgical method to permanently correct refractive errors. At least 15 different techniques have been developed and considerable experience accumulated in both animal and human models. Laser photorefractive keratectomy has come the closest to gaining widespread acceptance in our profession but the difficulty in gaining acceptance for keratorefractive procedures are because of the unsolved problems with poorly predictable and unstable refractive outcomes, adverse effects on the quality of vision, lack of adjustability, and irreversibility.
Poor predictability looms as the largest unsolved problem with refractive corneal surgery. The two major factors that contribute to poor predictability are: (1) the variability and inaccuracy inherent with manual surgical techniques, and (2) the variable influence of corneal wound healing in determining the refractive outcome. Until these two deficiencies are corrected, it is unlikely that a refractive surgical procedure will predictably correct ametropia to within a half diopter, the margin which can be achieved routinely with glasses or contact lenses. Photorefractive Keratectomy (PRK) offers the possibility of solving one of the major causes of poor predictability by reducing the surgical variability of the procedure. A major unresolved issue is how the second nemesis that causes poor predictability, corneal wound healing, will affect the results of PRK.
Reasons for a lack of perfectly predictable outcomes in any keratorefractive procedure, in a particular individual patient, include variations in individual surgical technique, the difficulty in repetitively performing manual microsurgery to submicrosurgical tolerances, and idiosyncracies of individual patients' wound healing.
In radial keratotomy (RK) multiple peripheral radially directed incisions are made into the cornea at 90-95% depth in an attempt to flatten the central cornea and thus correct myopia. The problem of unpredictability of result was tackled by multiple extensive retrospective analyses of the patients in whom surgery had already been performed. These studies revealed certain factors that seemed to control the outcome of the surgery, such as the size of the optical zone, the initial keratometric readings, corneal diameter, corneal rigidity, number of incisions, incision depth, intra-ocular pressure, thickness of the cornea, and degree of astigmatism. Age and sex are also factors that are taken into consideration in most of the nomograms which have been devised to predict what effect to expect for a certain surgery. At one point, many experts in the field considered it nearly impossible to fully and accurately correct patients in one surgery and felt that RK should be considered a two-stage surgery, with the initial surgery to achieve the “ball-park” correction, followed by an enhancement procedure to adjust or titrate the result near the desired outcome for an individual eye. It was felt that because of individual variability which may lead to an under or over-correction in the individual different from that predicted by the nomogram, attempting to fully correct the refractive error in one surgery could lead to over-correction in a not insignificant percent of the surgeries, resulting in hyperopia which is much more difficult to correct. Unfortunately, the second-stage surgery is even less predictable than the initial procedure. No one has yet devised a formula to take into account the profound changes which occur in the cornea after the initial RK, especially when weeks or months have passed. Most studies quote only 50-60% of eyes achieving 20/20 or better visual acuity following RK. Patients who are accustomed to 20/20 or better corrected visual acuity before surgery are not typically satisfied with less than 20/25 or 20/30 uncorrected post-operative visual acuity.
In addition, a gradual hyperopic shift is a major concern after RK. Refractive stability is critical for all refractive procedures but all corneal refractive procedures show significant degrees of instability. To date, there have been no clear explanation of why the cornea is destabilized by RK. A recent report on the long-term results of RK stressed the “natural” hyperopic refractive progression of “normal” eyes as a function of age. It is possible that patients are initially overcorrected and the over-correction masked by the patient's accommodative powers. With time and loss of accommodation, the hyperopia may be gradually unmasked with the hyperopia becoming visually symptomatic. At the time of surgery, a patient may be corrected with resultant slight hyperopia and yet have 20/20 vision because of the ability of the lens to accommodate. There is a range of residual correction within which the patient can have 20/20 uncorrected vision. This range varies depending on the individual but probably spans two to three diopters. Even with this range, the percentage achieving 20/20 is only 50-60%. This reflects poorly on the precision of the technique. It is important to note that this range diminishes with presbyopia, or loss
Isabella David J.
McCutchen Doyle Brown & Enersen LLP
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