Optics: eye examining – vision testing and correcting – Spectacles and eyeglasses – Ophthalmic lenses or blanks
Patent
1988-04-14
1990-08-14
Sugarman, Scott J.
Optics: eye examining, vision testing and correcting
Spectacles and eyeglasses
Ophthalmic lenses or blanks
G02C 704
Patent
active
049482459
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to contact lenses and more particularly to soft contact lenses and the design and construction thereof. This invention further relates to lower minus power hydrogel contact lenses.
Contact lenses for correction of ammetropic vision are presently available in a variety of types, styles and materials adapted for correction of most ammetropic conditions. The use of these lenses has presented a number of problems however, as they are worn in intimate contact with the delicate tissues of the cornea of the eye. The special nature of the corneal tissue requires corresponding special considerations in the design, prescription and fitting of these lenses for correction of vision.
The cornea is a nonvascular tissue, because a supply of blood vessels would interfere with the transparency required for proper vision. Yet it is a living tissue and must be provided with oxygen and nutrients, and waste products of metabolism must be removed therefrom. To this end the cornea is bathed in a continuously renewed flow of tear fluid which supplies nutrients and removes metabolic wastes. Oxygen dissolves in the tear fluid directly from the atmosphere to which the eye is exposed, and then diffuses from the tears into the corneal tissue. When the eye is closed for long periods, as during sleep, the amount of oxygen reaching the cornea is reduced, but, even then, enough oxygen ordinarily reaches the cornea through the tears to provide for its metabolic needs.
When a contact lens is fitted to the cornea for correcting vision a thin film of tear fluid remains between the lens and the cornea. However, this thin layer of tears is no longer exposed to the atmosphere for direct absorption of oxygen. Hence some provision must be made for supplying oxygen to the cornea beneath the contact lens either by exchange of the layer between the lens and cornea with fresh, oxygenated fluid or by making the lens permeable to oxygen. Similarly, removal of waste products requires exchange of the tear layer between the lens and the cornea with fresh tear fluid. In practice, the cornea can survive for some time without adequate oxygen supply or waste removal. Accordingly, hard, oxygen-impermeable contact lenses made, such as of poly(methyl methacrylate), typically can be used provided they are worn for periods associated with normal waking hours. Although the cornea can recover partially during the period in which the lens is not worn, short term and long term chronic changes have been reported. Furthermore, some exchange of tear fluid is possible with such lenses due to the generally random movement of the lens over the surface of the cornea as it is pushed to and fro by the eyelids during blinking. This mechanism for the exchange of tear fluid is effective especially when lenses of relatively small diameters are fitted which cover only the central portion of the cornea.
In order to increase patient acceptance, oxygen-permeable hydrogel contact lenses have been developed. These lenses, which are made of a water-swollen synthetic resin, allow at least some oxygen to diffuse through the body of the lens to oxygenate the tear film underneath and thereby to supply oxygen to the cornea upon which they rest. However, thicker hydrogel lenses containing modest amounts of water, such as 25% to 59% by weight of the total weight of the hydrated lens, do not transmit enough oxygen to provide all the needs of the cornea, especially when the eyelids are closed. Accordingly, these lenses also must be removed during periods of sleep. The soft lenses also do not permit significant diffusion of metabolic products, and therefore the removal of these products depends on the exchange of tear fluid produced by movement of the lens. Since soft lenses are designed to cover the entire cornea and since they tend to fit the corneal contour rather snugly, some consideration must be given to the shape of the lens if it is to have adequate movement to produce an adequate exchange of tear fluid.
In order to permit essential
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Mutti Donald O.
Seger Ronald G.
CooperVision, Inc.
Sugarman Scott J.
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