In 1999, the first silicone hydrogel (SiHy) contact lenses were made available to consumers. Today, reports suggest that SiHy lenses comprise 60% of new fits and refits by eye care practitioners.1 That percentage has nearly doubled since 2005. With current fitting trends favoring SiHy contact lenses, it is imperative to understand the various surface chemistry interactions that occur between the cornea, the contact lens, and solutions containing wetting agents.
Corneal Surface Chemistry
The corneal epithelial surface is intrinsically hydrophobic (water-repelling). The cornea epithelium has microvilli that project outward from its surface. Goblet cells from the conjunctiva produce mucous, which migrates across the epithelial surface. The mucous coats and covalently bonds to the epithelial microvilli, comprising the glycocalyx. Once the mucous layer has dispersed across the surface, the cornea becomes hydrophilic (water-attracting). Aqueous secreted from the lacrimal gland disperses across the hydrophilic surface, which enhances the cornea’s nutritive, bacteriolytic and lubricating properties. And finally, the lipid layer––the outermost entity of the tear film—serves to protect against evaporative dynamics.
Needless to say, the corneal epithelial surface behaves and functions much differently than the contact lens surface.
CL Surface and Wearing Dynamics
Most SiHy contact lens manufacturers have incorporated different manufacturing processes to “hide” the silicone portions of the lens and help maintain a hydrophilic surface. Initially, a fresh SiHy lens has a very hydrophilic surface; however, these surfaces cannot be controlled readily once the lens is placed onto the eye. Shearing forces of the eyelids in conjunction with air exposure begin to create dry spots, or hydrophobic portions, throughout the lens.2 These dry spots on the lens surface become non-wettable, which often leads to patient discomfort and poor visual quality. Combining the proper contact lens solution wetting agent with the lens material is critical to maximizing comfort (especially end-of-day comfort) and maintaining visual quality throughout the entire day.
Which Wetting Agents Work Best?
Multipurpose contact lens solution wetting agents are categorized into two groups: cellulose-based derivatives (polysaccharides, such as HPMC, HEC, CMC and HA) and block copolymers (surface active agents). Cellulose-based derivatives have been shown to enhance the covalent properties of the glycocalyx tear film layer.3 However, because the contact lens surface does not have covalently bonded molecules like the cornea, celluose-based wetting agents may be unable to create a uniform hydrophilic surface that delivers all-day comfort.
OPTI-FREE® RepleniSH® MPDS contains the TearGlyde® Reconditioning System (TETRONIC* 1304 & C9-ED3A) which forms a cross-shaped molecule that demonstrates both hydrophobic and hydrophilic properties. When a patient soaks their lens overnight after a long wearing day, the surface-active agent binds to both the hydrophilic- and hydrophobic-exposed portions of the contact lens surface, leaving the hydrophilic heads exposed on the molecule. Once the contact lens is applied to the eye, the hydrophilic heads of C9-ED3A bind with the tear layer to create a moist lens that lasts up to 14 hours per day.4
So, the next time you upgrade your patients to new contact lenses, prescribe a lens solution that can provide them with end-of-day comfort like OPTI-FREE® RepleniSH® MPDS.
*TETRONIC® is a registered trademark of BASF.
1. Nichols JJ. Contact lenses 2009. Contact Lens Spectrum. 2010;25(1):20.
2. French K. Contact lens material properties. Optician. 2005;230(6022):20-8.
3. Stone R. Research and Review: Innovations in Lens Care Science. Available at: www.visioncareprofessional.com/emails/researchandreview/9/index.asp (Accessed July 14, 2010).
4. Data on file, Alcon Laboratories, Inc.
This column is sponsored by Alcon.