Silicone Hydrogels and Tear Interactions
BY JASON J. NICHOLS, OD, MPH, PHD
As we all know, silicone hydrogel lenses are unique and vastly superior to traditional HEMA-based lenses in terms of oxygen performance. Yet, we hardly understand the relation between a HEMA-based lens and the components of the tear film, let alone the interaction between a silicone hydrogel and the tears. This is because silicone hydrogels are unique polymer formulations, and in that regard, not all silicone hydrogels are alike.
Siloxanes, which comprise the polymer, exhibit an inherently different chemical behavior in and of themselves. Siloxane is very oxygen permeable, but at the same time quite hydrophobic, and this presents somewhat of a dilemma relative to maintaining their wettability and care. Further, improving oxygen permeability isn't likely to improve patient-reported dryness, the most commonly reported symptom of contact lens wearers.
The two strategies to overcome this lack of wettability include treating the surface and incorporating hydrophilic groups in the polymer. These approaches might lead to drastically different inherent material properties (modulus), on-eye performance, interaction with tear film components (deposition), and ultimately, comfort. Lens modulus is a characteristic of several components including water content, polymer cross-linking and silicone content. In a way, these are all related in and of themselves. Stiffer lenses do provide benefits in terms of lens handling, but some evidence suggests that they may be associated with more mechanical complications.
Preliminary unpublished data from our laboratory have shown that lenses with a higher modulus are associated with a reduced post-lens tear film thickness. This is likely the mechanism for the observed mechanical phenomenon (mucin balls and superior epithelial arcuate lesions).
Tear Film Interaction
Another unique property of some silicone hydrogels is their surface treatment, of which two types are currently used on available lenses.
Alternatively, two available lenses (and at least one upcoming lens that hasn't yet been marketed) do not incorporate a surface treatment, but use hydrophilic groups within the polymer to attract and bind water. This typically results in lower modulus values and softer feeling lenses. Their lack of a treatment and/or their lower modulus may also translate into improved patient comfort.
Initial studies have indicated that these lenses benefit symptomatic contact lens patients by improving comfortable wear time and symptoms of dryness and discomfort.
However, relative to an interaction with tear film components, it's unclear what role a surface treatment plays. For instance, initial biochemical studies (that haven't been extensively reported on) indicate that surface-treated lenses tend to be associated with increased levels of lipid deposition instead of the protein deposition of their HEMA-based counterparts. Conversely, the lack of a surface treatment is associated with higher-water-content lenses (albeit nonionic), and a high water content could be associated with an increase in deposition as well. Specifically, high-water-content lenses might attract the more polar tear film molecules given the polarity of water itself. This, too, hasn't been studied extensively.
Unique Lens Care?
As other well-respected columnists have stated, this unique chemistry associated with silicone hydrogel materials might necessitate unique care products and regimens.
Dr. Nichols is an assistant professor of optometry and vision science at The Ohio State University College of Optometry.