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cover story Friction a Factor in Contact Lens Comfort Here’s a primer on what doctors should look for in studies that attempt to measure and compare contact lens coefficient of friction. By Kathrine Osborn Lorenz, OD, MS I n a single day, our eyes blink approximately 11,000 times. This means that each year, the eyelids travel the equivalent of a marathon, crossing the smooth, tearlubricated surface of the cornea over and over again. In a contact lens wearer, of course, each blink takes the eyelid (and the sensitive lid wiper area, in particular) across the lens surface, which may or may not be covered by an adequate tear film. Ideally, we optometrists want that contact lens to be moisture loving, lubricious, and smooth so that the lids can glide over it as easily as they do the natural cornea. On the other hand, a thin or unstable tear film may cause dryness symptoms and increase the contact between the lens and the lid-wiper area. It is difficult to measure a subjective sensation such as smoothness or lubricity, but the degree of friction between a lens and another moving surface such as the eyelid, known as dynamic coefficient of friction (dCoF), can be measured. In 2009, independent researchers first reported that coefficient of friction was more closely correlated with end-of-day contact lens comfort than water content, oxygen permeability, modulus, and other factors long thought to be associated with comfort.1 Since then, additional work correlating the results from a different dCoF method to two separate data sources for contact lens comfort has shown that dCoF accounts for approximately 80% of the differences in comfort among contact lens brands.2-4 MEASURING FRICTION Present methods of measuring friction are too invasive to be performed in vivo, so we have to rely on laboratory testing that necessarily falls short of perfectly replicating the human eyelids, cornea, and tear film. Friction is typically measured by rubbing the test surface (the contact lenses) after it has been bathed in some liquid or solution against a counter surface that serves as a stand-in for the eyelid. As one might imagine, differences in the solutions and counter surfaces employed in various studies can have a significant impact on the results. For example, some models of friction have relied on simple saline, a poor substitute for the complex human tear film, which contains several mucins and hundreds of proteins and lipids. The Swiss research firm SuSoS AG tested friction using a tear-mimicking solution that was more complex3 but still lacked a number of important characteristics. At the 2013 American Academy of Optometry meeting, my colleagues and I presented a study in which we used the same Roba methodology,3 but with a refined tear-like fluid (TLF) that contained proteins and lipids to more closely resemble the tear film.5 In another example of how methodology can affect outcome, some models use untreated glass as a counter surface. In addition to the fact that glass is not representative of any surface in the human eye, glass binds with polyvinyl pyrolidone, a molecule embedded in the lens matrix of most of the Acuvue brand contact lenses (Vistakon Division of Johnson & Johnson Vision Care, Inc.). In our models, we currently use mucin-coated glass, which mimics the mucosal surface of the lids.5 Solution osmolality and the speed and pressure used in the study may also affect friction results. At very light pressure, which may not represent the force of the eyelid, for instance, some contact lenses appear january/february 2014 Advanced ocular care 53 cover story to have low friction which is, in reality, a much higher value when tested at eyelid pressures. NEW AREAS FOR FRICTION RESEARCH Recently, using a similar methodology to that of our previous studies, we were able to measure friction in 10 fresh human donor corneas in TLF.6 The average dCoF for the cornea was found to be 0.015 ±0.009. This has never been previously completed in fresh human corneal tissue, so these results are very exciting. It is also interesting to look at more real-world conditions. Most studies of contact lens friction compare lenses straight out of the blister pack. That does not, however, reflect the patient experience throughout the day or wear cycle, when changes in the lenses may affect the lens coefficient of friction. We recently tested this by cycling contact lenses in and out of TLF to simulate all-day wear.7 Over repeated dip cycles, there was little change in the dCoF of some of the lenses with an internal wetting agent, and those with a blink-activated wetting agent experienced an increase in friction. Ongoing refinement of dCoF models will give us further insight into lens materials and performance. Until there are in vivo testing methods and/or recognized American National Standards Institute or International Organization for Standardization standards for measur- ing dCoF in vitro, however, it is impossible to compare results for contact lens friction across studies conducted with different methodologies. That should not stop clinicians from looking for contact lenses with a low coefficient of friction, given that we know friction is linked to contact lens comfort. Rather, one should look for transparency about key elements of the methodology (ie, the solution, counter surface, and pressure) to evaluate the validity of a given study and its relevance to contact lens patients. n Kathrine Osborn Lorenz, OD, MS, is the head of global strategic claims for Johnson & Johnson Vision Care, Inc., in Jacksonville, Florida. Dr. Osborn Lorenz may be reached at (904) 443-1032; [email protected]. 1. Brennan N, Coles-Brennan C. Contact lens-based correlates of soft lens wearing comfort. Optom Vis Sci. 2009:86;E-abstract 90957. 2. Coles-Brennan C, Brennan N. Coefficient of friction and soft contact lens comfort. Optom Vis Sci. 2012:89:Eabstract 125603. 3. Roba M, Duncan EG, Hill GA, et al. Friction measurements on contact lenses in their operating environment. Tribology Letters. 2011:44(3);387-397. 4. Brennan N, Coles-Brennan C. Supporting data linking coefficient of friction and soft lens comfort. Contact Lens Ant Eye. 2013;BCLA abstract. 5. Tosatti S, Aeschlinmann R, Kakkassery J, Osborn-Lorenz, K. Effect of different probe solutions on dynamic coefficient of friction of contact lenses. Optom Vis Sci. 2013;90:E-abstract 135077. 6. Wilson T, Aeschlimann R, Tosatti S, et al. Coefficient of friction of human corneal tissue. Optom Vis Sci. 2013;90:Eabstract 130742. 7. Tosatti S, Aeshlimann R, Kakkassery J. Dynamic coefficient of friction measurements of contact lenses after simulated lens wearing cycle. Optom Vis Sci. 2013;90:E-abstract 135340.