Intraocular lenses of the type described herein are used primarily to treat problems of vision in a human eye.
The image which would normally be directed to a damaged or diseased area of a retina is refocussed to another area of a retina so that vision may be improved.
A normal eye has two lenses with four refracting surfaces. Both lenses are convergent lenses which tend to focus light rays inward toward an axis of the eye. Greatest refraction takes place at the air-cornea interface and lesser refraction occurs at the cornea-aqueous humour interface. Refraction again occurs at the aqueous humour-crystalline lens interface and again at the crystalline lens-vitreous humour interface. The aqueous humour and vitreous humour have approximately the same refractive index which is slightly below the refractive indices of the cornea and crystalline lens. The crystalline lens is composed of the capsule, the cortex and the nucleus, all of these surfaces having different indices of refraction allowing for color correction of incoming wavelengths of light.
When a crystalline lens is removed because of occlusion, for example, with cataracts, its function may be replaced by a relatively thick converging lens worn outside of the eye. Preferably in removing the crystalline lens the capsule is held intact to maintain separation between the vitreous humour and the aqueous humour.
In recent times an intraocular lens is surgically positioned in the posterior chamber of the eye behind the iris and in front of the capsule. The introacular lens, called a pseudophakos, is held in place with a haptic which extends outward from the lens and engages the outer wall of the posterior chamber or the sulcus. In another mounting method the lens may be attached directly to the iris or the chamber angle or within the cornea by any known means.
The pseudophakos has the advantage of eliminating the requirement for thick eyeglasses and increasing the field of vision as compared with thick eyeglasses which may be used to replace a removed crystalline lens.
Techniques are known for anchoring a lens within the eye.
When a normal eye gazes upon a distant object the cornea and crystalline lens focus an object upon a part of the retina which is in optical alignment with those lenses and which is called the macula which joins the retina with the optic nerve and which is most sensitive to light.
A common problem which may develop especially in elderly persons, is loss of color correction on the surface of the retina due to the inability of current lenses to correct the color wavelengths of light adequately. The achromatic lens will improve the color correction problem and thus improve overall image quality.
Current intraocular lenses are made of glass, plastics, silicone or hydrogel.
The use of a single element lens will allow for a focus of some wavelengths of light, but will cause a blur for other wavelengths.
The construction of achromatic lenses is a well-known process. Fine controlled forming, molding and grinding, matching spherical and nonspherical concave and convex or planar surfaces on lenses and joining exact opposite surfaces together with bonding material is known in the art.
Glass, such as combined lenses of flint and crown glass chosen for their different refractive indices may be joined together as an achromatic lens or plastics having different indices of refraction may be joined together or may be combined with glass of the desired index of refraction. One plastic well known for use in intraocular lenses is methyl methacrylate. Other suitable materials are known such as polycarbonate silicone, hydrogel, or glass. One basic plastic may be used and impregnated with materials which provide differing indices of refraction in separate lens elements of the achromatic lens. The use of an achromatic lens consisting of two materials of different refractive indices will allow for better color correction than lenses currently described.