1. Field of Invention
This invention relates to the field of polymeric materials suitable for use in forming contact lenses. More particularly, it relates to a polymeric material that is relatively rigid (a "hard" lens material), has increased gas permeability, has enhanced surface wettability, and is comfortable to wear. 2. Description of the Prior Art
In recent years, corneal contact lenses have become more and more popular in the United States and throughout the world.
The great popularity of contact lenses is easily understood. One important reason is that such lenses provide perhaps the best possible manner of achieving optical correction for the eyes. The lenses fit directly over the eye, and when properly fitted, are easily retained in place. Problems common with spectacles, such as interference with peripheral vision, moving about on the head, discomfort, and the possibility of improper interpupilary distance, are easily overcome. Contact lenses provide cosmetic advantages and afford convenience and increased safety when used in diverse pursuits, particularly sporting events.
Contact lenses, which were originally made from glass, were gradually improved as improved lens materials became available. Now most commonly used contact lenses are generally subdivided into two types, so-called "hard" contact lenses, and "soft" contact lenses. Each type of lens has its own advantages, but each also includes certain disadvantages.
Referring first to the advantages of "hard" contact lenses, these lenses provide dimensional stability, so that the characteristics of an optical prescription will remain unchanged while the lens is in use in the eye. In some cases, the eye will actually conform to the contour of the lens over a period of time so as to improve the vision of the wearer. Moreover, "hard" contact lenses are relatively durable in relation to "soft" lenses.
While "hard" contact lenses have the above and other advantages, some patients find such lenses somewhat uncomfortable in use, and prefer the so-called "soft" contact lens. These lenses fall generally into three categories, namely (1) lenses made from silicone rubber or like materials, (2) lenses made from "HEMA" (hydroxyethylmethacrylate) or so-called "hydrogel" lenses, and (3) lenses of the methyl methacrylate base type, modified by the addition of polymers such as cellulose acetate butyrate ("CAB"). "Soft" lenses readily conform to the eye and are quite comfortable in short term use. They are extremely thin as well as soft and pliable. However, "soft" lenses are not as durable as "hard" lens and generally require greater care.
One of the major disadvantages to most prior art contact lens materials is the fact that they are not gas permeable.
"Hard" lenses of polymethylmethacrylate (PMMA) resins have long been used for the manufacture of contact lenses because of their excellent optical properties and machining and molding characteristics. A major disadvantage of PMMA resins is their very low permeability to gases such as oxygen present in the air and carbon dioxide that is a metabolic waste product produced by the eye. Since the cornea needs a continuous supply of oxygen from the air to provide for ongoing metabolic processes, the low gas permeability of the PMMA resins has necessitated lens designs which ameliorate this problem to some degree. Design changes have included reducing the diameter of the lenses in order to decrease the amount of corneal area covered by the impermeable material and shaping the back surface of the PMMA contact lens to provide for pumping action and concomitant tear flow under the lens, the tears containing dissolved oxygen from the air.
While such designs have made possible the wearing of contact lenses, significant problems and limitations remain, both because of the inadequacy of oxygen supply to the cornea and because the designs may produce discomfort and undesirable physiological symptoms to the wearer, frequently to a degree which makes wearing of the contact lens possible for only short periods of time or not at all.
Continued oxygen deprivation of the cornea results in edema or swelling of the cornea which may result in corneal damage. In addition, while oxygen must be supplied to the cornea for its metabolic processes, carbon dioxide, a waste product of these processes must be removed. The same principles apply for providing a route for removal of carbon dioxide from the cornea as for the transport of oxygen to the cornea, when a contact lens covers the cornea.
In view of the fact that the lacrimal fluid ("tears") supplies some dissolved oxygen to the epithelium of the cornea, it was hoped that "soft" lenses would solve this problem. Researchers experimented with lenses made of hydrophilic polymers, mainly comprising polyhydroxyethyl methacrylate (known in the art as "HEMA"). These hydrogel soft lenses are an improvement over the hard lenses but the materials themselves are not gas permeable. However, these materials absorb water and swell until equilibrium is attained and therefore possess a high degree of hydration which is directly related to the mode of oxygen transport. The highly hydrated lenses are able to obtain satisfactory oxygen transport levels but suffer from several resulting problems. First, since the soft lenses are used in the swollen state, the molecular materials of their composition are markedly reduced in mechanical strength and are extremely fragile. Due to this fragileness, the thickness of the lens must be increased and therefore these prior art soft lenses are ill-suited for the preparation of ultra-thin corneal lenses. By increasing the thickness of the lens, the gas permeability of the lens is thereby decreased forming a vicious cycle between gas permeability and strength.
In making an ultra-thin lens, the greater the strength and the greater the refractive index of the material used, the better the resulting thin lens.
A second problem associated with the prior art soft lenses is that since they are always worn in the wet and swollen state, they are easily contaminated with bacteria. Therefore, they need to be sterilized once a day by boiling. This boiling treatment is not only troublesome, but often causes decomposition and breakdown of the lens material. Thus, the prior art soft lenses are very short lived.
The present invention overcomes the problems of the prior art contact lens materials by providing a contact lens material that is relatively "hard", comfortable to wear due to its enhanced surface wettability, and gas permeable.