Friction in the Contact Lens Community

It’s not only the tension on the surface. This is leading edge friction. And it applies more particularly to silicone Hydrogels; at least for some more than others.

Firstly I would commend the recent excellent paper by Brian Tighe on this website “Trends and Developments of Silicone Hydrogels”, September 2006, as background.

Having the experiences of silicone Hydrogels since 1998, when we were advised of the necessity to surface enhance the materials in order to prevent deposits, we are now being presented with a new generation of Silicone Hydrogel materials without surface treatment which apart from a quite low deposit rate, are promoted to us in the basis of lower material modulus, with numerous papers published in support.

My experience shows that the modulus as a material property may be less significant to us and our patients than the surface friction encountered by the leading edge of the upper lid, the lid margin muscle and its epithelium, followed by the upper tarsal plate area, across the surface of the lens and pre-lens tear film modulus may be the only element quantifiable.

I believe that the friction encountered by those tissues is responsible for both some of the adverse reactions of the earlier generation Silicone Hydrogels and for the relative success in the face of those events, of their newer generation counterparts. You can feel the difference on the finger, and the patients report that awareness. That applies to the stiffness, but more importantly to the slipperiness.

There is a difference in the pre-lens tear film composition as seen with a tearscope, a different size of surface deposits and a significant difference in tarsal plate inflammatory response and papillae development with Biofinity material, even though these are early days of exposure to that product.

With the expansion of product range to include toric and multifocal options in silicone hydrogel we have an expanding potential patient base. In the presbyopic category there are enthusiastic candidates many of whom already present with compromised tear profiles.

One recent case was a 70-year-old female who had already been under tear hygiene therapy and had tried conventional Hydrogels with very limited success. I approached this lady’s Silicone Hydrogel multifocal with more than a hint of hesitation, and conservative wearing start up. I prescribe the non-dominant eye with more plus (0.50 to 1.00) to boost the reading effect. Within two weeks she had passed her follow-up examination, although I still needed some convincing. After overnight wear she was not only thrilled with the freedom offered but remained all clear physiologically. She now has 6 nights extended wear, and I retain monitoring of her tear film monthly. As an artist she reports a freer expressive flair with contact lenses.

Perhaps the compression beneath the Silicone Hydrogel spreads the mucin layer more evenly. Although I am unable to convince myself of a satisfactory explanation, I cite this case as an opportunity we might otherwise dismiss as a potential candidate, and maintain a "proceed with caution" recommendation.