The present invention relates to a corneal implant to be inserted into the optical centre of the cornea of the human eye for the purpose of correcting impaired vision, in particular presbyopia in otherwise emmetropic eyes (eyes with normal vision) as well as presbyopia in combination with hypermetropia (farsightedness) or myopia (nearsightedness).
The present invention furthermore relates to a procedure for correcting impaired vision in the human eye, in particular for correcting presbyopia, presbyopia in combination with hypermetropia, presbyopia in combination with myopia, and presbyopia in combination with astigmatism, by inserting a corneal implant into the optical centre of the cornea.
The optical apparatus of the human eye that generates an optical image of the environment basically comprises the cornea and the lens, which is positioned behind the iris. This optical apparatus of the eye has a total refractive power of approximately 60 dioptres, with the interface between the cornea and the air—i.e. the outer boundary of the eye—with approximately 40 dioptres accounting for most of the refractive power. This refractive power of the cornea is in general indirectly proportionate to the radius of the corneal surface (interface between cornea and air). A change in the radius of the curvature of the cornea also leads to a change in the refractive power of the eye.
In the case of myopia or nearsightedness, the eyeball is too long and the refractive power of the cornea thus inadequate to assure that the light rays are focussed on the retina; these are focussed in front of the retina instead.
In the case of hypermetropia or farsightedness, the eyeball is too short and the refractive power of the cornea thus insufficient to assure that the light rays are correctly focussed on the retina; they are focussed behind the retina instead.
Presbyopia is a dissociation of the refractive power of the eye in that for accurate far vision a different correction of dioptres than for accurate near vision is required.
Different options for correcting these refractive errors are available. In addition to the classical methods of vision correction via glasses or contact lenses, also surgical methods are known where implants are inserted into the cornea of the human eye with the aim to either modify the curvature of the cornea and thus correct the refractive power of the latter accordingly, or to alter the optical properties of the cornea through the optical properties of the implant.
By enlarging the radius of curvature of the cornea the refractive power is reduced, which allows to correct a myopic condition. To be able to correct a hyperopic condition, the corneal radius needs to be reduced, i.e. the curvature needs to be increased.
To be able to correct presbyopia by surgical intervention, it is necessary to impart a certain degree of bi-focality or multi-focality to the refractive power of the cornea. This means that the refractive power of the cornea is designed in a way that the light rays entering the eye from different distances (near or far away), depending on their point of entry, are simultaneously focussed on the retina, or more precisely in the central area of the retina (=the macula, the area where accurate vision occurs). This implies that one or several images from a far distance and one or several images from a near distance are simultaneously focussed in the macula. The brain then selects the appropriate image. To allow this selection to take place, the far-away image and the near image must have about the same intensity. The use of contact lenses and intraocular lenses, which are inserted after cataract surgery, are based on this principle.
WO 93/05731 reports the insertion of an optical lens into the optical centre of the cornea, the dimensions of which are smaller than those of the optical zone being limited by the diameter of the pupil.
The optical centre of the cornea is that part of the cornea along which the optical axis of the eye passes through the cornea. The optical axis is the axis of projection of the optical system of the eye. The ophthalmologist determines the optical centre by using specific assessment methods. The ophthalmologist may choose from a wide variety of different methods. The methods for determining the optical centre of the cornea described hereinafter represent only a small portion of the many different methods commonly applied, and are not exhaustive. Many systems, in particular excimer laser systems with active eye tracking, use the centre of the pupil or its projection on the corneal surface or around a point at an individually defined distance as the optical centre of the cornea. Other common systems are aimed at the area where the curvature of the cornea is most pronounced. Especially in the case of high-degree myopia, in fact, an angular deflection of the optical axis from the anatomical axis is to be noticed, which is defined as the “kappa angle”. Another method relates to the so-called “Purkinje reflexes”. These are reflexes on the corneal front and back faces as well as on the lens front and back faces, which occur when the patient focuses on a preferrably point-shaped light source. While these reflexes ideally overlap, most of the time this is not the case; the eye specialist then chooses one of these reflexes as the optical centre. It is also quite common to choose the middle position of all four reflexes, or the middle between this middle position and the centre of the pupil etc. Eventually, it is left to the personal discretion, individual experience and preference of the eye specialist how he determines the optical centre of the cornea. Generally speaking, the various methods used for determining the optical centre of the cornea tend to render quite similar results.
