This is an edited version of an article by Dr Desmond Fonn, published in Contact Lens Spectrum.
Advances in contact lens science and technology have eliminated or minimised many of the complications that pre-dated disposable lenses.
New clinical conditions have been reported coincidently with the development of silicone hydrogel lenses. 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 contact lens wearers who were symptomatic of dryness had a significantly higher percentage of lid-wiper staining compared to asymptomatic wearers.1
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.2 It appears 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 aetiology 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 that these more subtle contact lens related conditions appear to have in common with contact lens papillary conjunctivitis is 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.3 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 per cent water content in the core to approximately 80 per cent at the surface and approaching 100 per cent at the outer surface.4,5
Gel layers that are minimally cross-linked (5-6 µm thick) are graded on the surfaces of silicone hydrogel contact lenses.5 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.6,7 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.
* Based on in vitro measurement of unworn lenses
- 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.
- 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.
- Keir N, Jones L. Wettability and silicone hydrogel lenses: A review. Eye Contact Lens 2013; 39: 1: 100-108.
- 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.
- Sawyer WG. Lubricity in high water content surface gel layers. Optom Vis Sci 2012; 89. E-abstract 125089.
- Brennan NA. Contact lens-based correlates of soft lens wearing comfort. Optom Vis Sci 2009; 86. E-abstract 90957.
- Coles C. Coefficient of friction and contact lens comfort. Optom Vis Sci 2012; 89. E-abstract 125603.
Disclaimer: Reprinted with permission Contact Lens Spectrum, Special Edition, published June 2013. Contact Lens Spectrum is published monthly by PentaVision LLC © 2013 All rights Reserved. PentaVision is located at 321 Norristown Road, Suite 150, Ambler, PA 19002 (USA). Visit www.contactlensspectrum.com for more information.