The Clinical Relevance of Contact Lens Lubricity
Using science to provide better comfort for contact lens wearers.
|Dr. Fonn is Founding Director of the Centre for Contact Lens Research and Distinguished Professor Emeritus at the School of Optometry. He is a graduate of the School of Optometry in Johannesburg, South Africa and the University of New South Wales in Sydney, Australia. He is the immediate Past President of the International Society for Contact Lens Research and a founding member of the International Association of Contact Lens Educators in which he served as Vice President for 15 years. He is a paid consultant of Alcon and CooperVision.|
By Desmond Fonn, MOptom, FAAO
Despite advances in contact lens materials, designs and lens care products, discomfort, especially end-of-day discomfort and dryness, continues to be the predominant reason for discontinuation of lens wear.1,2 Although much effort has been put into development of effective multifocal soft contact lenses, contact lenses to control myopic progression and attempts to decrease corneal infection rates, the most effective way to increase the number of wearers is a corneo-mimetic contact lens surface that provides outstanding end-of-day comfort and therefore significantly reduce the number of dropouts from contact lens wear.
Most soft lenses are fully hydrated and comfortable upon insertion. We know however that for many lens wearers comfort decreases or dryness worsens during the day3-5 and the maximal fluid state of the ocular environment and conventional hydrogel and silicone hydrogel lenses can change6-8 after insertion. So the challenge for polymer scientists, mechanical engineers and everyone else associated with material and lens development, is how to prevent loss of water from the lens surface and therefore the bulk, or more appropriately how to effectively make the surfaces truly biocompatible. Some manufacturers have claimed to achieve some of these objectives described further in this article, but before that it is worth describing some of physical and mechanical terms associated with lens surfaces and their interaction with ocular tissue.
Tribology, a domain of mechanical engineering, is the science of friction, lubrication and wear (not as in contact lens wear, but deterioration). It is the study of interacting surfaces in relative motion. Subbaraman and Jones9 have described how friction and lubricity can be measured.
Friction can be defined as the “resistance developed between contacting surfaces when one of the bodies moves, or tends to move, over the other”10 The Coefficient of Friction (COF) is a scaled value which describes the ratio of the force of friction between two surfaces and the force pressing them together. The value depends in part on the materials.Two useful examples are: ice on steel, which would have a low COF, while rubber on asphalt would have a high coefficient of friction. There are several types of friction / lubrication in the context of contact lens wear. Hydrodynamic lubrication is where a fluid film completely separates two solid surfaces. Boundary lubrication occurs when the two surfaces come into direct contact. Two contrasting contact lens examples are the lid traversing a soaked hydrogel lens on the eye at high speed (hydrodynamic lubrication) which would have a relatively low COF but if the tear film is completely dehydrated or the movement speeds are low the COF would be considerably higher (boundary lubrication).
Lubricity can be thought of as the reciprocal of friction. That is when friction is low, lubricity is high. A hydrogel contact lens surface in its dehydrated state is not particularly lubricious, however, introducing a fluid even with very low viscosity or soaking the lens in solution will improve the lubricity and reduce friction when the eyelid slides across the lens surface. The ability of the surface to retain moisture will affect the lubricity.
This is a more commonly used term in the contact lens world and is used to characterize the adherence of fluid to the lens surface. Wettability can be an invitro (or ex-vivo) measure of the contact angle (CA) or a clinical in-vivo assessment of pre-lens tear break up time (TBUT) measurement. Although these are frequently quoted values, there are other key factors that may correlate better with comfort of contact lenses. Maldonado-Codina and Efron11 suggested that the in-vivo interaction of tears and the lens surface cannot be predicted by CA measurements. Hysteresis (difference between the advancing and receding contact angles) seems to be a better way of expressing the laboratory wettability of lenses and Tighe has shown that hysteresis values of uncoated silicone hydrogels launched in recent years have decreased significantly, in part due to the markedly greater water content.12
Clinical Consequences of High Coefficient of Friction
It is fair to say that the advancements made in contact lens science and technology have eliminated or minimised many of the complications that pre-dated disposable lenses. Most of those complications were associated with deposition of denatured protein and other lens contaminants, giving rise to auto-immune reactions, mechanical irritation of the tarsal conjunctiva and other ocular tissue, and in some cases, considerable discomfort. Loss of lubricity and increased friction as a result of lens surface contamination were likely responsible for these adverse ocular surface reactions. As lens technology continues to advance it is important to measure the friction not only of fresh lenses but to determine how lens friction changes in response to wearing time and surface deposits.
One of the more dramatic ocular complications that typifies the above description is giant papillary conjunctivitis (GPC) or what is now termed contact lens papillary conjunctivitis (CLPC). GPC was first reported by Thomas Spring in 1974 as an inflammatory reaction of the palpebral conjunctiva more often observed on the upper lid.13 Large (or “Giant”) papillae, palpebral hyperaemia and mucus secretion are characteristic of the condition.14 Symptoms include itching, ocular discomfort and poor, variable vision which frequently led to discontinuation of wear. It was not uncommon to observe an apparently dehydrated contaminated lens move 4 or 5 mm with a normal blink, obviously due the loss of lubricity between the lens and palpebral surfaces. With current lenses and good habits of frequent replacement these findings are quite rarely encountered.
Through the development phase of silicone hydrogel lenses, CLPC has been reported to occur more frequently (especially with continuous wear) than with conventional hydrogel lenses and less with newer silicone hydrogels than first generation.15 In addition to the design and modulus changes, one would have to suspect that increased water content and methods to retain surface moisture has helped to decrease the incidence of CLPC.
Coincident with the development of silicone hydrogel lenses, new clinical conditions have been reported. In 2002, Korb and colleagues described a condition called lid-wiper epitheliopathy (LWE) which is a band of affected tissue of the marginal conjunctiva of the upper eyelid that wipes the ocular surface. The condition is diagnosed by staining with fluorescein, rose bengal or lissamine green. They found that contact lens wearers who were symptomatic of dryness had a significantly higher percentage of lid-wiper staining compared to asymptomatic wearers.16
Pult and colleagues described a condition called lid-parallel conjunctival folds (LIPCOF) which are sub-clinical folds of the bulbar conjunctiva above and parallel to the lower lid margin.17 It appears that this condition is also more prevalent in symptomatic contact lens wearers but can also be detected in patients with dry eye who do not wear contact lenses.
The suspected etiology of both LWE and LIPCOF is mechanical and in the case of LWE, the tarsal conjunctiva is subjected to increased frictional force or reduced lubricity of the contact lens surface causing micro trauma to epithelial cells. This could be aggravated by a lack of lubrication from tears. The factor these more subtle contact lens related conditions appear to have in common with GPC appears to be reduced lubricity, which probably provokes the symptoms of discomfort and dryness.
Efforts to Increase Lubricity
A number of attempts to increase the wettability of the lens or to retain its moisture during the wearing period appear to have been somewhat successful. Examples of incorporating wetting agents into lens materials are polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP) and hyaluronic acid. These substances have also been used in artificial tears and contact lens rewetting drops. PVP acts as a hydrophilic layer thereby shielding the hydrophobic properties of silicone hydrogel lenses. Other humectants (substances that help to retain water) such as hydroxypropyl methylcellulose (HPMC) and polyethylene glycol (PEG) have been shown to improve wettability of silicone hydrogel lenses. Keir and Jones have eloquently and more extensively described this topic.18 However it is unknown whether these wetting agents have had a lasting effect on lubricity.
The most recent development in daily disposable silicone hydrogel technology is termed a water gradient lens*, ranging from 33% water content in the core to approximately 80% at the surface.19,20 Gel layers that are minimally crosslinked (5-6 μm thick) are graded on the surfaces of silicone hydrogel contact lenses.20 Sawyer concluded that these gel layers provide a lubricious surface with very low friction coefficients (below μ = 0.01).
Finally, the most compelling evidence of a measureable lens variable that correlates with end-of-day comfort is the coefficient of friction, demonstrated by Brennan and Coles in two separate studies.21,22 The efforts of measuring and modifying lens surfaces that truly retain moisture and lubricity throughout the day could be the most important development since the Wichterle soft lens, perhaps even surpassing the discovery of the silicone hydrogel material. CLS
*Based on in-vitro measurement of unworn lenses.
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10. Malhotra M, Subramanian R, Gahlot P, Rathore B. Textbook in Applied Mechanics. New Delhi: New Age International; 1994.
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13. Spring TF. Reaction to hydrophilic lenses. Med J Aust 1974;1(12):449-450.
14. Allansmith MR, Korb DR, Greiner JV, Henriquez AS, Simon MA, Finnemore VM. Giant papillary conjunctivitis in contact lens wearers. Am J Ophthalmol 1977;83(5):697-708.
15. Dumbleton K. Noninflammatory silicone hydrogel contact lens complications. Eye Cont Lens 2003;29(1 Suppl):S186-189;discussion S190-191, S192-194.
16. Korb DR, Greiner JV, Herman JP, et al. Lid-wiper epitheliopathy and dry-eye symptoms in contact lens wearers. CLAO J 2002;28(4):211-216.
17. Pult H, Purslow C, Berry M, Murphy PJ. Clinical tests for successful contact lens wear: relationship and predictive potential. Optom Vis Sci 2008;85(10):E924-929.
18. Keir N, Jones L. Wettability and silicone hydrogel lenses: A review. Eye Contact Lens 2013;39(1)100-108.
19. Pruitt J, Qiu Y, Thekveli S et al. Surface characterization of a water gradient silicone hydrogel contact lens (delefilcon A). Invest Ophthal Vis Sci 2012; 53. E-abstract 6107.
20. Sawyer WG. Lubricity in high water content surface gel layers. Optom Vis Sci 2012;89. E-abstract 125089.
21. Brennan, N.A., Contact Lens-based correlates of soft lens wearing comfort. Optom Vis Sci 2009;86. E-abstract 90957.
22. Coles C. Coefficient of friction and contact lens comfort. Optom Vis Sci 2012;89. E-abstract 125603.