Contact Lens Design & Materials
Material, Design, and Dry Eye
BY NEIL PENCE, OD, FAAO
For this dry eye issue, let’s briefly review what we know about the effect that contact lens material and design factors have on contact lens-related dry eye (CLRDE).
Water content in hydrogel lenses is a factor in CLRDE (Nichols, 2006). The greater the water content, the higher the demand to combat evaporation. Lower water contents seem preferred in general, but lens thickness must be considered as well. Silicone hydrogels may also benefit from lower water content, but that correlates less in this category.
Dehydration rates, particularly higher rates of water loss, may have some relationship to CLRDE although reports conflict on this point. Omafilcon A (Proclear, CooperVision) lenses show less on-eye dehydration, and hioxifilcon lenses (Extreme H2O and Clarity H2O, both Hydrogel Vision) also have similar properties (Riley, 2005).
While water content and rates of dehydration are both related to CLRDE, how strong a role they play remains in question.
Coefficient of friction (or lubricity) of the lens surface is likely a factor in lens comfort. In the case of reduced tears, a surface with less friction will be less abrasive to the lid blinking across it.
Deposit resistance properties of different materials affect CLRDE. Increased surface deposits can disrupt the tear film, affect lens hydration, and increase friction with the lid—all of which could result in CLRDE.
Oxygen transmission has been postulated as a factor in CLRDE. While higher-Dk/t lenses are believed to help, these lenses have many other different attributes as well. With no studies isolating this single variable, the impact of oxygen remains debatable.
Solution compatibility with materials has been related to corneal staining (Andrasko, 2007). Dry eye is associated with superficial staining, and there may be an association between staining and comfort (Jalbert, 2006). This is tempered, however, by the many patients who have staining but are asymptomatic.
Silicone hydrogel versus hydrogel—silicone hydrogels have been reported to reduce symptoms of CLRDE (Schafer, 2003; Moody, 2009). More specifically, end-of-day discomfort is improved with silicone hydrogels (Chalmers, 2008; Riley, 2006). This may be due to water content, less dehydration, altered surface/lid interactions, increased oxygen, a combination of these— or to yet unexplained factors.
Modality—single-use daily disposable lenses are often prescribed to lessen CLRDE. A lens surface that is free of deposits, contaminants, oils, or disinfection chemicals may be an advantage. Wetting agents in the package and within the lens may also help.
Modulus (lens stiffness) may affect comfort, particularly initial comfort or adaptation. It’s not evident how this relates to CLRDE, unless lens movement is a factor. More movement may increase friction with the lid wiper area. Lack of movement resulting in trapped debris might also trigger various responses such as dryness.
Lens thickness can affect CLRDE. With hydrogels, a prism-ballasted lens with increased lower lens thickness can reduce staining and symptoms in dry eye patients. Thicker lenses may also take longer to show dehydration at the back surface.
Continuing the Search
Knowledge of how materials and design affect CLRDE has in large part been gained via clinical observation. We need better insight into the actual tissue or location responsible for triggering symptoms of end-of-day dryness. We also need to learn more about the role that friction, evaporation, tonicity, temperature, pH, or the presence of inflammatory cells play as triggers. The result will be better lens materials and designs to alleviate CLRDE. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #176.
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. You can reach him at email@example.com.