researcher discusses how this important factor can affect your patients' contact
lens wearing comfort.
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.
angles can range from 0° (complete wetting) to 180° (non-wetting).
The lower the angle, the more wettable the surface.
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.
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.
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
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.
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:
Control lens from the package: 100°
1304 in Unisol: 40°
0.1% Tetronic 1107
in Unisol: 100°
Multi-Purpose Disinfecting Solution: 10°
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
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.
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
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
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.
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.
Tetronic® is a registered trademark of BASF.
Contact Lens Spectrum, Issue: April 2005