Article Date: 4/1/2005

Measuring Wettability
A researcher discusses how this important factor can affect your patients' contact lens wearing comfort.

By David Meadows, PhD

No doubt, you're aware of ocular stress related to pollutants, allergens, contact lens care products and the formation of deposits on the lenses. At Alcon, during the past 3 to 4 years, we've been researching a relatively new concept — the wettability of the contact lens surface — which can significantly affect the success of your contact lens patient.

In this article, I'll discuss what we've learned thus far and how you can apply our findings to your patients and practice.

Effects of Solutions and Wetting Agents

Once a contact lens is placed on the eye, we all know it accumulates proteins and starts to dewet, destabilizing the tear film. But you may not be fully aware of how these developments are affected by the solution in which the lens soaks before application and by the rewetting agents that are used during lens wear.

Contact angles can range from 0° (complete wetting) to 180° (non-wetting). The lower the angle, the more wettable the surface.

As part of our research, we've observed dry areas develop on Acuvue 2 lenses worn by patients for 8 hours. When we used a water droplet to relubricate the dry areas, we found the water had no effect because hydrophobic domains had developed on the lens surfaces. Water and saline only cool and momentarily lubricate the lens surface once it has dewetted in these areas. Therefore, we must provide some agent that will address these dry areas and, ultimately, early breakup times on the lenses.

The Wettability Factor

Wettability becomes an issue at the end of the day when the wetting agents associated with the lenses are no longer able to do what they're supposed to do. Hydrophilic lenses can actually develop hydrophobic surfaces over the wear day.

The science of wettability is based on contact angles, which measure the area between the non-adhering part of a droplet and the surface on which it sits. Contact angles can range from 0° (complete wetting) to 180° (non-wetting). The lower the angle, the more wettable the surface. See "Contact Angles" for a helpful illustration of this concept.

Even high-water, hydrophilic contact lenses can lose wettability when dehydration draws hydrophobic components to the air interface of the contact lens. If you add tears or water to the lens, the surface continues to become even less wettable, increasing wearer discomfort throughout the day. Only an appropriate lens conditioning system can reorient the hydrophilic components, restoring the lens to its high-water state at the surface.

To avoid this problem, your patients should use overnight lens care solutions that include effective wetting agents. We experimented with two of the more common surface agents — Tetronic1 1304 (used in Opti-Free Express Multi-Purpose Disinfecting Solution) and Tetronic 1107 (used in ReNu Multi-Purpose Solution and several other lens care products). Acuvue 2 lenses were soaked in solutions containing each of these agents for 12 hours. The in vitro contact angles we found at the dewetting plateaus were:

Lens Wear Chronology

 

The lens wear cycle (above) shows the many factors that affect your patients during a 12-hour day. Successful conditioning is required to prepare contact lenses for comfortable wear in the morning and throughout the day. But when a contact lens has been exposed overnight to disinfection products, the lenses release biocides that can lead to significant corneal toxic staining 3 to 6 hours after application. We have to ask ourselves what impact that's having on long-term wear or end-of-day comfort. We really don't know whether this produces immediate or cumulative ocular stress.

—David Meadows, PhD

As you can see, the Tetronic1 1304 solutions produced lower lens surface contact angles and higher wettability. It's important to consider this issue when prescribing a lens care solution. When a hydrophobic surface develops on the contact lens, the tear film will be less able to continue to wet that area. We found this kind of response — a 91° contact angle — in our in-vitro model in the most recent product released, ReNu Moisture Loc.

Another important factor to consider is that the anterior lens surface will become hydrophobic more rapidly than the posterior side, as you would expect. We measured contact angles of 77°+/-4° on the anterior surface and less than 30° on the posterior surface after 3 hours of wear.

Looking at Lens Materials

Lens material also can affect wettability. We recently found that, contrary to what you might expect, hydrophilic lens material can make wettability problems worse. See "Advancing Contact Angle" above for an overview of all these lenses.

Impact of Wettability

Wettability is one more important issue to consider, along with ocular stress from instillation, biocide release and maintaining clean lenses. Surface wettability along with biocide compatibility and clean lenses are all important factors for patient comfort. Changing a patient's lens care system can have a big impact, especially near the end of the day. All of these factors combine to make for a more compatible and comfortable lens for your patient.

Dr. Meadows is Senior Director of Consumer Products Research at Alcon Research Limited in Fort Worth, Texas, where he is responsible for research on new, over-the-counter contact lens care and dry eye products.

References

1. Tetronic® is a registered trademark of BASF.

 


Contact Lens Spectrum, Issue: April 2005