Contact Lens Design & Materials
A Decade of Silicone Hydrogels
BY RONALD K. WATANABE, OD, FAAO
It is hard to believe that silicone hydrogel (SiHy) lenses have been available in the United States for more than 10 years. Today, SiHy lenses dominate the market, and there is a growing list of designs using these materials. The January 2013 Eye & Contact Lens commemorates SiHy lenses with a series of excellent articles that review the evolution of SiHy lenses to their current state. Here are a few points to keep in mind about SiHy lenses from that issue.
Oxygen permeability is the hallmark of SiHy materials. Dk values range from 55 to 140, higher than any HEMA-based hydrogel material. Because of the high Dk values, hypoxia is a non-issue for most wearers (Sweeney, 2013), and it has allowed specialty designs with thicker profiles to be healthier for patients.
There are many more designs today than 10 years ago. In the U.S. market there is one SiHy daily disposable with more coming soon. Standard spheres, torics, and multifocals are readily available. Custom torics, multifocals, and keratoconus and postsurgical designs are offered by a growing list of custom labs. The newest hybrid lenses utilize a SiHy soft skirt.
Protein deposition is generally lower for SiHy materials than for HEMA, which may benefit some patients. Technology advancements also have resulted in materials that are more wettable, more compatible with the ocular environment, and lower in modulus.
The Not-So Good
Despite material advances, dryness, wettability, and lipid deposition continue to be challenges for some SiHy wearers. Because of the mobile nature of the siliconoxygen bond in SiHy polymers, these hydrophobic chains tend to migrate to the lens surface, causing it to lose its hydrophilicity (Tighe, 2013). Manufacturers have devised strategies to make SiHy surfaces hydrophilic, but they can lose wettability over time. This also limits comfortable wearing time, and though many enjoy comfort throughout the day, comfort with SiHy lenses has not been shown to be significantly different than that with HEMA lenses (Guillon, 2013).
Other factors can influence comfort such as lens care solution usage, compliance with cleaning regimens and lens replacement schedules, lens design, surface properties, and fitting characteristics. In short, comfort is a complex issue with no easy answers.
There have been discussions about incompatibilities with multipurpose solutions and about infiltrative events. Several studies have investigated increases in corneal staining and corneal infiltrates, and there appears to be a higher rate of solution-related problems with SiHy lenses in general (Willcox, 2013; Jones and Powell 2013; Carnt and Stapleton, 2013). As a result, manufacturers have developed new care solution formulations that are more compatible with SiHys, and the FDA is developing new guidelines for lens disinfection and material categories.
Despite greater Dk of SiHys, it has been shown that infiltrative events have increased two-fold (Szczotka-Flynn, 2013; Chalmers and Gleason, 2013), and microbial keratitis incidence is unchanged (Stapleton et al, 2013). This may be due to an increased tendency for bacteria to adhere to these materials (Willcox, 2013). There is also an increase in bacterial adherence to the corneal epithelium when multipurpose solutions are used with some SiHy materials (Robertson, 2013).
More Improvements Ahead
SiHy materials have significantly changed the contact lens field. Current SiHy materials have many benefits, and we can look forward to continued improvement in material technology. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #208.
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.