The invention relates to non-invasive ophthalmogical instrumentation and techniques for determining physical dimensions of an eye and to the informed use of thus-derived dimensional data in the performance of corrective surgery upon the cornea.
The human eye is an extremely powerful focusing device that produces an image on the surface of the retina. The focusing elements of the eye are the cornea and the lens. The cornea accounts for approximately 80 percent of the focusing ability (48 diopters) and the lens approximately 20 percent (12 diopters). In the case of myopia, the eye is assumed to have a longer egg-like shape in which case the light beam focuses to a spot located in front of the retina and therefore is out of focus. In hyperopia, the focusing system is inadequate, and the focusing spot and image are located behind the retina and also out of focus. In the case of astigmatism, a spot or clear image is not created, and the eye basically focuses at two areas behind or in front of the retinal surface. In order to correct myopia, hyperopia, or astigmatism, spectacles or contact lenses are used to place the image directly on the rods and cones of the retina. As an alternative to artificial correction (i.e., spectacles, or contact lenses), it has been dembnstrated, as noted previously, that refractive keratoplasty, surgically altering the shape of the cornea, will achieve the same result.
As alternative techniques of refractive keratoplasty, radial keratotomy and corneal sculpting are the two kinds of corneal surgery which have been receiving increased consideration. In radial keratotomy, some 8 to 32 radial incisions are applied with a knife to the cornea, and it has been shown that the curvature of the cornea is thereby flattened to a degree which places the focus further back in the eye, hopefully near the retinal surface. Such an operation has been demonstrated to improve vision by reducing objective myopia, with a measured improvement of up to 12 diopters. The operation is performed by using a diamond or ruby knife with an adjustable belt or sleeve which controls the depth of incision to fractions of a millimeter.
The extent of myopia correction is determined by the depth of cut, the number of radial incisions, and the proximity of the incisions to the center of the cornea. Various other incisions have been combined with radial incisions to achieve other corrective effects, and by combining circumferential incisions with radial incisions in various portions of the cornea, a characterized flattening of the cornea is possible, whereby a concurrent decrease is achieved in myopia as well as in astigmatism.
Corneal sculpting consists of an advanced procedure, beyond radial keratotomy, which involves the removal of external layers of the cornea, in such a way as to affect the radius of curvature in order to increase or decrease the dioptric power of the front surface of the cornea. By removing various layers of the cornea to the extent of 0.15 to 0.25 millimeters of the 0.60-millimeter thickness of a cornea, up to 12 diopters of myopia or hyperopia can be corrected, along with correction of extremely high degrees of astigmatism (or unevenness of the cornea). By thus sculpting the cornea, in effect, the outer surface of the cornea can have the radius of curvature of a correcting contact lens, as if the contact lens had been inserted over the malformed cornea. It is the outer surface of the cornea with its air/cornea interface that provides the increase or decrease in the focal length (power) of the cornea and therefore alters the refractive state of the eye.
One approach to corneal sculpting has been a procedure termed keratomileusis, whereby the exterior of the cornea is removed as a plano-convex button, frozen, placed on a microlathe, and shaped by the lathe under computer control until a predetermined curvature of the cornea is achieved. The corneal button is then thawed and sewn back onto the patient's eye. In this way, the external corneal curvature is changed by a process of mechanical intervention.
Another approach to corneal sculpting and to radial keratotomy is via laser incision, as described in my original patent application Ser. No. 552,983, filed Nov. 17, 1983 (abandoned, subject to continuing prosecution, eventuating in U.S. Pat. No. 4,665,913). From the aspect of corneal scultping, said patent application describes methods and apparatus for changing the anterior surface of the cornea from an initial curvature having defective optical properties to a subsequent curvature having correctively improved properties, by using ultraviolet laser radiation to selectively ablate the optically used central area of the anterior surface of the cornea by photodecomposition, with penetration into the stroma and volumetric sculpturing removal of corneal tissue to such penetration depth and profile as to characterize the anterior surface of the cornea with said subsequent curvature. As explained in said application, ablation by photodecomposition is photo-chemical, i.e. the direct breaking of intra-molecular bonds, and all damage adjacent the photodecomposed ablation is insignificant; this is in sharp contrast with the effect of laser radiation at greater wavelengths wherein the ablation or incision is thermally achieved, through photocoagulation and/or photovaporization, wherein cells adjacent the ablated or incised margin are charred.
But no matter what the procedure for operating upon the cornea to achieve refractive correction the fact remains that what has been done and is being proposed to be done is largely experimental. There is no fund of experience upon which to draw for an acceptably accurate prediction of ultimate refractive correction in the eye, for a given technique in application to a given category of corneal dimensions. And, particularly in the case of radial keratotomy, the danger is ever present that incision will be made to excessive depth, thus aborting the otherwise non-invasive nature of the operation.