Article Date: 7/1/2008

Soft Contact Lenses

Soft Contact Lenses

Hydrogel and Silicone Hydrogel Lens General Considerations

The FDA gives each contact lens material a generic name. In general, all hydrogel and silicone hydrogel lens generic names end in the suffix "filcon" and all nonhydrogel lenses end in "focon."

Hydrogel lenses are categorized into four groupings for purposes of evaluating effects of accessory products upon the lens material. Lenses with less than 50 percent water content are considered to be "low water" and the others are "high water." Less reactive surfaces are termed "nonionic" and more reactive materials are labeled "ionic." Silicone hydrogel lenses are low water.

Disinfection of hydrogel and silicone hydrogel lenses can usually be done safely and effectively by approved chemical or hydrogen peroxide systems. However, some Group 4 ionic polymers may react negatively with potassium sorbate- or sorbic acid-preserved solutions.

In the term Dk, D stands for diffusion and k for solubility. The oxygen permeability of hydrogel lenses is almost entirely the result of solubility and for GP materials almost entirely the result of diffusion. In silicone hydrogel materials, oxygen permeability is a combination of solubility and diffusion, with the larger amount from diffusion. That is, the hydrogel component in the formulation provides solubility and the silicone component provides diffusion. Each generic material below is accompanied by a representative Dk number.

A clinically useful approximation is to consider Dk values for hydrogel lenses in three groups:

H2O     Dk
38±%     9±
55±%     18±
75±%     36±

It is also important to remember that Dk/t (central transmissibility) and Dk/t (average overall transmissibility) are dependent upon contact lens thickness configuration and are more important than Dk with hydrogel lenses. With silicone hydrogel contact lenses, Dk/t and Dk/t are more similar than with hydrogel lenses.

This table should assist you in identifying specific lenses in each category as related to disinfection and Dk; and it will also help you determine an appropriate mix of lens types for your practice.

Contact Lens Spectrum, Issue: July 2008