contact lens materials
Advocating a New Lens Group
BY LORETTA B. SZCZOTKA-FLYNN, OD, MS, FAAO
In changing this column from Continuous Wear Colloquy to Contact Lens Materials, we've recognized that silicone hydrogels aren't just for continuous wear (CW). The obvious change when moving moving from CW to daily wear with silicone hydrogel lenses is the introduction of lens care systems. Recently, Serina Stretton, BSc, PhD, from Sydney, Australia, wrote an article on this topic suggesting that vast differences between silicone hydrogels and conventional hydrogels require a new FDA lens group that better describes the unique characteristics of silicone hydrogels.
This argument has been brewing in our industry for some time, and I took the opportunity to research the topic by interviewing Ralph Stone, PhD, vice president of Consumer Products R & D at Alcon Laboratories. In 1984, working at Bausch & Lomb at the time, he developed the theory behind the four FDA Lens Groupings that we still adhere to today. Following are his responses to my questions.
What prompted the development of the four FDA Lens Groups? The initiative came from the FDA Device Branch in recognition that with the growing number of new lens brands coming to the market, it wasn't feasible to list all compatible lens brands on care product labels. We designed the lens groupings to test care products by lens group, rather than testing each individual lens brand for compatibility with each care system. The four groups streamlined the testing and approval process by allowing manufacturers to test only one lens in each of the four groups; the FDA quickly streamlined the process further by allowing testing of Group 1 and Group 4 only.
In the early 1980s, we were increasingly aware of certain lens and solution incompatibilities. For example, the Hydrocurve 55 lens (baufilcon A, 55 percent water) behaved differently from the Hydrocurve 45 percent water material: Sorbic acid-preserved saline and heat discolored the worn 55 percent water material, yet not the 45 percent water material. This provided an early hint that differences in lens materials were responsible for different solution interactions.
What are the key properties of silicone hydrogel materials that make them unique in terms of solution interactions? Silicone changes the total properties of the lens. The base siloxane material is uncharged, non-wettable and hydrophobic. The water loving domains have channels that bind water differently than do hydrogels. We determined the current FDA Lens Groupings in part by how water gets incorporated into a material and how tightly the water is bound. The chemistry puts a silicone hydrogel lens somewhere between a traditional hydrogel and GP lens material.
How are the four FDA Lens Groupings used today?
We generally assume that contact lenses within the same FDA group will behave similarly and that contact lens solutions approved for use with one FDA group are suitable for all lenses within that group. We've expanded them, somewhat unintentionally, to clinical care guidelines that predict on-eye lens performance and estimate the compatibility between lenses and patients' ocular responses.
Where do we go from here? Silicone hydrogels differ markedly in chemical composition from conventional hydrogels. To date, the industry has "forced" them into one of the current FDA groupings. If practitioners use these guidelines when prescribing solution systems with this new class of contact lenses, then unexpected lens/solution incompatibilities may result. In fact, a few silicone hydrogel/lens care system incompatibilities are already documented.
A new FDA classification system for silicone hydrogel lenses is the first step in identifying the unique characteristics that silicone and other monomers impart to lens performance and solution compatibility.
Dr. Szczotka-Flynn is an associate professor at Case Western Reserve University Dept. of Ophthalmology and is director of the Contact Lens
Service at University Hospitals of Cleveland.
Contact Lens Spectrum, Issue: February 2005