Contact lens discomfort and contact lens dropout are highly prevalent conditions, remaining around 15-20%, even with advanced contact lens materials [1-7]. One property that contributes to contact lens dropout is the lens surface. Tribology is defined as the science of two interacting surfaces in relative motion and encompasses friction, wear, lubrication and related design aspects . When referring to contact lenses, surface characteristics include friction, wettability, and surface water contact. Retrospective studies suggest that lubricity (the friction between the lens surface and lid margin) is the key contributor to contact lens comfort [9-12]. The word lubricity or slipperiness, originates from the Old French lubricité, from Medieval Latin lubricitās and from Latin lūbricus. The coefficient of friction (CoF) refers to the lubricity of the contact lens surface. A lower CoF give lenses a silky, smooth feel. Microtribometry is the measurement used to measure CoF quantitatively. CoF is a “system property,” not an intrinsic material property (such as modulus or Dk), that depends upon many factors of the measurement system, including the counter-surface substrate, pressure, movement speed and lubricating fluid, amongst other factors . The literature suggests that there is some evidence that lenses with a lower CoF may be associated with improved end of day comfort and comfort after two hours of lens wear . Currently, no peer-reviewed studies have been published and this area necessitating further investigation.
Contact lens material surfaces have been altered or new lens surfaces have been engineered to reduce friction by increasing surface lubricity . Commercially available contact lenses used in lubricity studies typically differ in material and design. To negate these differences, one study evaluated the effect of contact lens surface modification. In a randomized double-masked crossover study of nineteen habitual contact lens wearers who wore formofilcon B soft monthly disposable contact lenses, an ultra-thin coating was applied to the lens surface. Wearers wore each lens for one month . Lens comfort was significantly greater with the coated than the uncoated soft contact lenses, similar to levels reported previously in a general population without lens wear . Thus, changing the physical properties (lubricity) of the contact lens surface does impact subjectively rated comfort, which should result in contact lens retention.
Another option is the addition and slow elution of wetting agents from the material. The nefilcon A (DAILIES® AquaComfort PLUS®, DACP) daily disposable contact lens material has polyvinyl alcohol incorporated within the matrix, which may either slowly elute or be released from the surface on blinking . A recent study measured the release of PEG (polyethylene glycol), and HPMC (hydroxypropyl methylcellulose) from DACP over 24 hours using an in vitro eye model (OcuFlow). Liquid chromatography-mass spectrometry was used to analyze the elution of PEG and HPMC from DACP lenses. Wetting agents were released within the first 1.5 hours (P < 0.05), classified as a burst release pattern. The total amount of PEG and HPMC released was less than 1% of the amount in the blister pack solution and more PEG was released than HPMC (P < 0.05).
The inclusion of internal wetting agents that are not released is another method to increase contact lens lubricity. The delefilcon A (DAILIES TOTAL1®) silicone hydrogel daily disposable contact lens has a water content gradient with different surface and bulk water characteristics, with more than 80% water content at the surface to mimic the corneal surface. When compared to nelficon A contact lenses using interferometry, the pre–lens tear film quality was improved . In a crossover clinical trial of three silicone hydrogel daily disposable contact lenses, noninvasive tear breakup time was improved with the delefilcon A lens . In both studies, end of day comfort scores were indistinguishable between the lenses [19,20].
These studies suggest that changing the lubricity of the surface of a contact lens, adding eluting wetting agents or including internal wetting agents can improve contact lens comfort. Future studies that improve contact lens lubricity, evaluate the relationship between the contact lens surface friction and the eye and strategies to improve on eye performance and comfort will potentially improve contact lens retention and impede contact lens dropout.
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