Contact Lens Design & Materials
GP Material Properties Review
By Ronald K. Watanabe, OD, FAAO
Modern GP materials are available in a wide array of polymer chemistries that result in different material properties. It is sometimes difficult to select a GP material, especially if a patient has ocular surface or tear film issues. Reviewing GP material properties can help you determine the best option for each patient.
GP Material Properties
Oxygen Permeability (Dk) Current methods for measuring GP material Dk are the ISO/ANSI, ISO/Fatt (polarographic), and ISO/coulometric methods, which all yield similar results (although the coulometric method is preferable for Dk values above 100). Current methods also account for the “boundary layer effect” and the “edge effect.” Today's materials have Dk values that range from about 12 to 163. Though it may seem that the highest-Dk material would be preferable, Dk values need to be balanced with other material properties to provide the best overall performance for each patient.
Wetting Angle (WA) There are several methods to measure a material's wetting angle, or surface wettability: sessile drop, captive bubble, and dynamic contact angle. Each method results in different values that are difficult to compare directly. In addition, it has been shown that a material's WA does not necessarily correlate to on-eye surface wetting because it doesn't account for the eye's ability to coat the lens surface with a mucin “biofilm” that helps the lens surface attract and retain water. Despite these inconsistencies, WA can still be an important factor in GP lens success.
Refractive Index Most GP materials have refractive indices ranging from 1.42 to 1.48, and in most cases these variances have little effect on lens design. However, high-index (1.51 to 1.54) materials can keep lens thickness to a minimum for high refractive errors and can create slightly more add effect in aspheric multifocal designs.
Specific Gravity (SG) Specific gravity is the ratio of the mass of a solid to that of an equal volume of water. Therefore, a lens with SG greater than 1.00 will sink in water. Current materials have SGs ranging from 1.07 to 1.27. In most cases this has very little effect on the performance of a lens, but when lenses are thick, such as for high-plus powers or prismballasted lenses, it can cause lens mass to vary by as much as 20 percent. In these cases, a lower SG may help prevent the lens from riding low.
Hardness and Toughness Lens hardness is measured by the Rockwell R and Shore D methods. Rockwell determines a GP button's resistance to compression, while Shore predicts scratch resistance. Most package inserts report the Shore D value, which ranges from 75 to 85 units. Stiffness or modulus is determined by applying force to a lens until it breaks. This value is generally not listed in a package insert, but it can be important in influencing a lens' ability to mask corneal astigmatism and other irregularities.
Ultraviolet Blocking Compounds such as benzotriazole and Uvinul D-49 can be incorporated into a GP lens polymer to block ultraviolet light below 380nm. This translates into roughly 70 percent to 90 percent blockage of UV-A light and up to 99 percent blockage of UV-B light. Though a GP lens does not fully cover the cornea, a UV-blocking lens can be of benefit to patients who spend a lot of time outdoors but do not wear sunglasses.
Help With Material Selection
Selecting a GP material does not always require consideration of all of these factors. But for patients who have high refractive errors, compromised ocular surfaces, poor tear films, or critical visual needs, familiarity with these properties can help you select the best material. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #202.
|Dr. Watanabe is an associate professor of optometry at the New England College of Optometry. He is a Diplomate in the American Academy of Optometry's Section on Cornea and Contact Lenses and Refractive Technologies and is in private practice in Andover, Mass. You can reach him at firstname.lastname@example.org.|