Eyecare Center of Maryland – Dr. Norman Shedlo O.D.

Eyecare Center of Maryland

4701 Randolph Rd Suite G02 Rockville MD 20852   301-779-2424    Email:  info@drshedlo.com

 

Soft Contact Lenses

The first soft contact lenses were invented by Czech chemists Otto Wichterle and Drahoslav Lím, who published their work "Hydrophilic gels for biological use" in the journal Nature in 1959. In 1965, National Patent Development Corporation (NPDC) bought the American rights to produce the lenses and then sublicensed the rights to Bausch & Lomb, which started to manufacture them in the United States. The Czech scientists' work led to the launch of the first soft (hydrogel) contact lenses in some countries in the 1960s and the first approval of the Soflens material by the US Food and Drug Administration (FDA) in 1971.

These soft lenses were soon prescribed more often than rigid contact lenses, due to the immediate and much greater eye comfort. Polymers from which soft lenses are manufactured improved over the next 25 years, primarily in terms of increasing oxygen permeability. Contact lens induced corneal hypoxic adverse events have always presented a great challenge for eye doctors. Insufficient oxygen to the cornea creates both short and long term physiological changes to the cornea which lead to increasing discomfort and serious eye infections.

In 1972, British optometrist Rishi Agarwal was the first to suggest disposable soft contact lenses. In 1998, the first silicone hydrogel contact lenses were released by Ciba Vision in Mexico. These new materials encapsulated the benefits of silicone which has extremely high oxygen permeability—with the comfort and clinical performance of the conventional hydrogels that had been used for the previous 30 years. These contact lenses were initially advocated primarily for extended (overnight) wear, although more recently, daily wear silicone hydrogels have been launched and marketed very successfully.

In a slightly modified molecule, a polar group is added without changing the structure of the silicone hydrogel. This is referred to as the Tanaka monomer because it was invented and patented by Kyoichi Tanaka of Menicon Co. of Japan in 1979. Second-generation silicone hydrogels, such as galyfilcon A (Acuvue Advance, Vistakon) and senofilcon A (Acuvue Oasys, Vistakon), use the Tanaka monomer.

Vistakon improved the Tanaka monomer even further and added other molecules, which serve as an internal wetting agent.

Comfilcon A (Biofinity, CooperVision) was the first third-generation polymer. Its patent claims that the material uses two siloxy macromers of diverse sizes that, when used in combination, produce very high oxygen permeability (for a given water content). Enfilcon A (Avaira, CooperVision) is another third-generation material that is naturally wettable with a water content is 46%.

Soft lenses are more flexible than rigid lenses, and can be gently rolled or folded without damaging the lens. While rigid lenses require a period of adaptation before comfort is achieved, new soft lens wearers typically report mild lens awareness rather than eye discomfort.

Older hydrogel lenses rely on their water content to transmit oxygen through the lens to the cornea. As a result, higher water content lenses allowed more oxygen to the cornea. In 1998, silicone hydrogel, or Si-hy lenses became available. These materials have both the extremely high oxygen permeability of silicone and the comfort and clinical performance of the conventional hydrogels. Because silicone allows more oxygen permeability than water, oxygen permeability of silicone hydrogels is not tied to the lenses' water content.

Lenses have now been developed with so much oxygen permeability that they are approved for overnight wear (extended wear). Most eye doctors, however, discourage sleeping in contact lenses. Lenses approved for daily wear are also available in silicone hydrogel materials.

Disadvantages of silicone hydrogels are that they are slightly stiffer and the lens surface can be hydrophobic, which means they have a tendency to repel water making them less "wettable". This can influence how comfortable the lenses are. New manufacturing techniques and changes to multipurpose solutions have minimized these effects.

A surface modification process called plasma coating alters the lens surface's hydrophobic nature. Another technique incorporates internal rewetting agents to make the lens surface hydrophilic, making it have the ability to attract water rather than repel it.

A third process uses longer backbone polymer chains that results in less cross linking and increased wetting without surface alterations or additive agents.

Current brands of soft lenses are either traditional hydrogel or silicone hydrogel. Because of drastic differences in oxygen permeability, replacement schedule, and other design characteristics, it is very important to follow the instructions of your eye doctor prescribing the contact lenses to prevent eye infections.