Download friction a factor in contact lens comfort

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Friction a Factor in
Contact Lens Comfort
Here’s a primer on what doctors should look for in studies that attempt to measure
and compare contact lens coefficient of friction.
By Kathrine Osborn Lorenz, OD, MS
I
n a single day, our eyes blink approximately 11,000
times. This means that each year, the eyelids travel the
equivalent of a marathon, crossing the smooth, tearlubricated surface of the cornea over and over again.
In a contact lens wearer, of course, each blink takes
the eyelid (and the sensitive lid wiper area, in particular) across the lens surface, which may or may not be
covered by an adequate tear film. Ideally, we optometrists want that contact lens to be moisture loving,
lubricious, and smooth so that the lids can glide over
it as easily as they do the natural cornea. On the other
hand, a thin or unstable tear film may cause dryness
symptoms and increase the contact between the lens
and the lid-wiper area.
It is difficult to measure a subjective sensation such
as smoothness or lubricity, but the degree of friction
between a lens and another moving surface such as the
eyelid, known as dynamic coefficient of friction (dCoF),
can be measured.
In 2009, independent researchers first reported that
coefficient of friction was more closely correlated with
end-of-day contact lens comfort than water content,
oxygen permeability, modulus, and other factors long
thought to be associated with comfort.1 Since then,
additional work correlating the results from a different dCoF method to two separate data sources for
contact lens comfort has shown that dCoF accounts
for approximately 80% of the differences in comfort
among contact lens brands.2-4
MEASURING FRICTION
Present methods of measuring friction are too
invasive to be performed in vivo, so we have to rely
on laboratory testing that necessarily falls short of
perfectly replicating the human eyelids, cornea, and
tear film.
Friction is typically measured by rubbing the test
surface (the contact lenses) after it has been bathed in
some liquid or solution against a counter surface that
serves as a stand-in for the eyelid. As one might imagine, differences in the solutions and counter surfaces
employed in various studies can have a significant
impact on the results.
For example, some models of friction have relied on
simple saline, a poor substitute for the complex human
tear film, which contains several mucins and hundreds
of proteins and lipids. The Swiss research firm SuSoS
AG tested friction using a tear-mimicking solution that
was more complex3 but still lacked a number of important characteristics. At the 2013 American Academy of
Optometry meeting, my colleagues and I presented a
study in which we used the same Roba methodology,3
but with a refined tear-like fluid (TLF) that contained
proteins and lipids to more closely resemble the tear
film.5
In another example of how methodology can affect
outcome, some models use untreated glass as a counter
surface. In addition to the fact that glass is not representative of any surface in the human eye, glass binds
with polyvinyl pyrolidone, a molecule embedded in the
lens matrix of most of the Acuvue brand contact lenses
(Vistakon Division of Johnson & Johnson Vision Care,
Inc.). In our models, we currently use mucin-coated
glass, which mimics the mucosal surface of the lids.5
Solution osmolality and the speed and pressure
used in the study may also affect friction results. At
very light pressure, which may not represent the force
of the eyelid, for instance, some contact lenses appear
january/february 2014 Advanced ocular care 53
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to have low friction which is, in reality, a much higher
value when tested at eyelid pressures.
NEW AREAS FOR FRICTION RESEARCH
Recently, using a similar methodology to that of our
previous studies, we were able to measure friction in 10
fresh human donor corneas in TLF.6 The average dCoF
for the cornea was found to be 0.015 ±0.009. This has
never been previously completed in fresh human corneal tissue, so these results are very exciting.
It is also interesting to look at more real-world conditions. Most studies of contact lens friction compare
lenses straight out of the blister pack. That does not,
however, reflect the patient experience throughout
the day or wear cycle, when changes in the lenses may
affect the lens coefficient of friction. We recently tested
this by cycling contact lenses in and out of TLF to simulate all-day wear.7 Over repeated dip cycles, there was
little change in the dCoF of some of the lenses with an
internal wetting agent, and those with a blink-activated
wetting agent experienced an increase in friction.
Ongoing refinement of dCoF models will give us further insight into lens materials and performance. Until
there are in vivo testing methods and/or recognized
American National Standards Institute or International
Organization for Standardization standards for measur-
ing dCoF in vitro, however, it is impossible to compare
results for contact lens friction across studies conducted with different methodologies.
That should not stop clinicians from looking for
contact lenses with a low coefficient of friction, given
that we know friction is linked to contact lens comfort.
Rather, one should look for transparency about key
elements of the methodology (ie, the solution, counter
surface, and pressure) to evaluate the validity of a given
study and its relevance to contact lens patients. n
Kathrine Osborn Lorenz, OD, MS, is the
head of global strategic claims for Johnson
& Johnson Vision Care, Inc., in Jacksonville,
Florida. Dr. Osborn Lorenz may be reached at
(904) 443-1032; [email protected].
1. Brennan N, Coles-Brennan C. Contact lens-based correlates of soft lens wearing comfort. Optom Vis Sci.
2009:86;E-abstract 90957.
2. Coles-Brennan C, Brennan N. Coefficient of friction and soft contact lens comfort. Optom Vis Sci. 2012:89:Eabstract 125603.
3. Roba M, Duncan EG, Hill GA, et al. Friction measurements on contact lenses in their operating environment.
Tribology Letters. 2011:44(3);387-397.
4. Brennan N, Coles-Brennan C. Supporting data linking coefficient of friction and soft lens comfort. Contact Lens Ant
Eye. 2013;BCLA abstract.
5. Tosatti S, Aeschlinmann R, Kakkassery J, Osborn-Lorenz, K. Effect of different probe solutions on dynamic coefficient of friction of contact lenses. Optom Vis Sci. 2013;90:E-abstract 135077.
6. Wilson T, Aeschlimann R, Tosatti S, et al. Coefficient of friction of human corneal tissue. Optom Vis Sci. 2013;90:Eabstract 130742.
7. Tosatti S, Aeshlimann R, Kakkassery J. Dynamic coefficient of friction measurements of contact lenses after
simulated lens wearing cycle. Optom Vis Sci. 2013;90:E-abstract 135340.