Material Solutions for Gas Permeable Questions
BY THOMAS G. QUINN, OD, MS
While much can be said about gas permeable lens design, I find many providers want guidance when selecting a lens material. How often do you order a material simply because it's familiar? Here are some ideas on choosing the material for the lens at hand.
The Major Players
The three heavy hitters in lens material performance characteristics are oxygen permeability, lens stability and surface wetting. Emphasizing one may compromise the other areas. For example, maximizing stability may compromise oxygen supply and jeopardize physiological response.
What material should you choose if designing a lens for an 18-year-old 3.00D myope with 1.50D of with-the-rule astigmatism fully attributable to corneal toricity?
First decide which lens material characteristic is most vital to achieving success.
Oxygen Transmissibility: The myopic correction required is moderate, so this lens will be fairly thin. The cornea will get adequate oxygen even with a low oxygen permeable material.
Lens Stability: Harris and Chu established that lens flexure can occur anytime there is one diopter or more corneal toricity. This patient has 1.50D of cylinder, so flexural resistance will help ensure good vision.
Surface Wetting: This teen has good hygiene, good lid physiology and tension and an ample tear system. Lens wetting is not likely to be an issue in this case.
Obviously, flexural resistance is this patient's top priority. With silicone-acrylate materials, flexural resistance and Dk have a fairly clear relationship: the chance of flexure increases with higher Dk. If using a silicone acrylate material, choose one with low to moderate Dk for this patient.
The relationship between Dk and flexure with fluorosilicone acrylate materials is not as clear. Newer generation FSA lenses offer revolutionary chemical architecture utilizing highly stable cross-linking agents and oxygen permeable backbones. This increases oxygen supply without significantly compromising stability and flexural resistance. A Dk around 30 will meet this patient's corneal oxygen needs and minimize flexure or wetting problems.
What about a +6.00D patient with 1.50D of astigmatism? This thicker lens will reduce flexure, but we must make sure the cornea gets enough oxygen. A Dk material of 60 or so is a good choice.
For extended wear, Dk becomes the "top dog". Choose one of the newer, high Dk fluorosilicone acrylate materials with flexural resistance.
Sometimes flexure is beneficial. Anytime a patient has moderate to high corneal toricity with low spectacle astigmatism, lens flexure minimizes the creation of an astigmatic tear lens. In such cases, thin, high Dk lenses fit slightly steep work best.
For elderly or dry eye patients who require good wetting, fluorosilicone acrylate materials are preferred. Use the lowest Dk possible that won't compromise physiology. In rare cases of lipid accumulation, a silicone acrylate material may work better.
Achieve lens fitting success by finding the lens design and material that offers the best balance of features for your patient.
Lens material selection requires good balance
Dr. Quinn is in group practice in Athens, Ohio, and has served as a faculty member at The Ohio State University College of Optometry.