contact lens materials
Contact Lens Polymer Impact On Corneal Staining
BY JASON J. NICHOLS, OD, MPH, PHD
Clinicians have used corneal staining with fluorescein to assess ocular surface integrity for years. Numerous recent studies, anecdotal reports, editorials and lectures have been devoted to the topic of ocular surface staining as it relates to lens care solutions.
Much of this attention has been prompted by the work of Dr. Gary Andrasko detailed on "The Staining Grid" (www.staininggrid.com). Essentially, his work suggests that certain biguanide-based care solutions interact with several specific contact lens polymers to result in moderate-to-heavy corneal staining (two hours after lens application), whereas solutions containing polyquad or hydrogen peroxide showed low levels of corneal staining. The work has evoked varying responses in the industrial, research and clinical communities and has generally emphasized an awareness of the importance of assessing and addressing ocular surface staining in our lens-wearing patients.
Irrespective of contact lens/care solution interactions leading to corneal staining, I think it's also important that we consider material-based differences that may be associated with corneal staining responses. The work discussed about and detailed in The Staining Grid occurred after lenses underwent an overnight soak in a care solution. I'm suggesting that it's important to also consider the role of individual contact lens materials, irrespective of their interaction with contact lens care solutions, in their potential impact on corneal staining. A variety of factors may come into play in this regard.
Modulus One of the factors to consider is the modulus (or stiffness) of a contact lens material. You might expect a stiffer lens to interact in a more mechanical fashion with the ocular surface, although we don't necessarily comprehend this fully. This is particularly true because we really don't understand the dynamics of the post-lens tear film and how reliability and effectively it serves as a lubricant (a boundary lubricant) under varying conditions. We don't have a full understanding of the impact of modulus on the post-lens tear film.
Contact Lens Surfaces A second factor to consider relative to contact lens material characteristics and their interaction with the ocular surface is the lens surface itself. While not completely understood, contact lens surfaces can vary in their coefficients of friction (a measure of the force of friction-associated movement across the lens surface).
This may relate to staining in two regards. Friction associated with the front surface of a lens may be associated with "lid wiper epitheliopathy." Friction associated with the back surface of a contact lens may be associated with corneal or bulbar conjunctival staining.
Dehydration Resistance The ability of contact lens materials to resist dehydration may also cause alterations in the tear film and further ocular surface/frictional interactions. Traditional contact lens materials (non-silicone hydrogels) generally dehydrate up to about 5 percent, with nominally labeled high-water content materials dehydrating to a greater extent than low-water content materials. Some silicone hydrogel lenses show very little dehydration.
Numerous other factors may contribute to ocular surface staining during contact lens wear, many of which we still don't understand. It's important that we continue to develop a complete understanding of factors associated with ocular surface staining during lens wear, but critical that we also not lose focus of the clinical relevance in doing so. CLS
Dr. Nichols is an assistant professor of optometry and vision science at The Ohio State University College of Optometry.