contact lens design and materials
Silicone Hydrogel Lenses: Where Do We Stand Today?
BY NEIL PENCE, OD, FAAO
On several occasions recently, it has been mentioned that for the past few years the contact lens “Materials” column has been dominated by discussions relating to silicone hydrogel lenses. Does this mean that there are no new material developments in the hydrogel category? Does this indicate that silicone hydrogel has taken over? I am tempted to answer “yes” to both questions, but a more accurate response might be “pretty much” for each.
There have been some developments and lens introductions in hydrogel materials. Most of these in the last few years involve daily disposable hydrogel lenses. By far most lens introductions from major manufacturers, however, have been silicone hydrogel designs. Silicone hydrogels have become the dominant material, accounting for most contact lenses fit in the United States since passing the 50 percent mark sometime in mid-2007.
It would appear that companies and practitioners have concluded that silicone hydrogels are the lens material of choice. Obviously there is a perceived patient value, and it may be useful to examine what that value is and where we stand with regard to silicone hydrogel materials.
The Good News
Silicone hydrogel materials have had a significant impact on reducing and in many cases eliminating hypoxic complications with contact lenses. The high levels of oxygen transmission compared to hydrogels have largely solved such problems as corneal microcysts, neovascularization, and stromal striae secondary to lens wear. This is most beneficial for those who sleep in lenses, but also aids patients who need thicker designs such as higher-powered prescriptions and toric lenses. The higher oxygen has also reduced the engorgement of limbal blood vessels, lessening the limbal injection present with lens wear.
The Not-as-Good News
Unfortunately, significantly higher levels of oxygen transmission have not lessened the risk of the most serious complication—microbial keratitis. Mainly associated with overnight lens wear, the risk of infectious corneal ulcers for hydrogels and silicone hydrogels has been found to be comparable. Perhaps this will improve in the future with lenses that contain antimicrobial properties or lens surfaces that better inhibit bacterial binding. Until then, patients' actions (sleeping in lenses) and habits (hygiene and lens care procedures) are still the prime factors in lens-related infections. While materials have not lessened the risk of infection, perhaps our continued emphasis on good hygiene and lens care habits can. Patient education on handwashing, lens case care, not topping-off disinfection solutions, and firmly capping solution bottles is very important. Cautioning patients to not wear lenses overnight when they're not feeling their best, when they have been exposed to water that may have gotten in the eye, or when they have been in smoke-filled or irritating environments may also help to lessen risk of infection.
More on Silicone Hydrogels
Without a doubt, the higher-oxygen transmitting silicone hydrogel lenses have been a success. The speed at which they have come to dominate the market is remarkable, and their ability to nearly eliminate a host of hypoxic complications has been exceptional. Despite the best of hopes, they have not reduced the risk of microbial keratitis associated with overnight wear of lenses.
In a future “Design and Materials” column, I will continue this review of silicone hydrogels. Stay tuned. CLS
Dr. Pence is director of the Contact Lens Research Clinic, Indiana University School of Optometry in Bloomington, Indiana. He is a consultant or advisor to B+L, Ciba Vision, and Vistakon, and has received research funding from AMO.