Contact Lens Mechanics
KELLY K. NICHOLS, OD, MPH, PHD
One of the things I enjoy most in writing this
column is the response
I get on occasion from readers. Yesterday I received a
copy of the book, The
Mechanics of Corneal Contact Lenses, by Vernon R. Lamb,
in response to my August column on how to win a staring contest. This
isn't the average
book on contact lenses. Written in 1966, it's dedicated to contact lens wearers and is
intended to be a teaching tool about the importance of appropriate blinking in contact
lens wear. The
book comes with
a record, which patients listen to while
reading the book.
Illustrations cover ocular anatomy and important concepts in lens wear, in particular
the ergonomics of vision and lens wear.
The book was ahead of its time, and in both the letter accompanying
the book and in the book itself, Lamb's dedication to the contact lens profession (in
which he embarked in 1956) and his patients is clearly evident.
In a Blink
We blink every day without thinking. Can teaching a
blinking, squinting, discomfort, misuses of the eye, and indicators of overwear help
with succeeding in contact lens wear? How much time do we take to discuss contact lens
expectations with our patients prior to contact lens wear? In today's contact lens
era, we're quick to make conclusions about wear habits, and even quicker to make lens
and solution changes.
The Mechanics of Symptoms
Inserting a contact lens into the ocular environment has
impact on the eye. Some patients experience no sensation of a "foreign body" while
others report anything from minor discomfort to severe dryness and irritation. Prior
to silicone hydrogel lenses, I was an unsuccessful contact lens patient. I took out
my lenses if they bothered me too badly, regardless of where I was. Being a low myope,
removing my lenses didn't have a big impact on my daily activities (some would say
I am a visual slob).
Why do some patients not remove their lenses, even with extreme discomfort?
Patients can be extremely motivated to tolerate lens wear for many reasons, but also
because removing lenses creates a burden. Spending time to discuss lens wear motivation
with new lens patients and evaluating potential burdens, like glasses or eye drops,
can help establish appropriate expectations of lens wear from the onset.
For existing patients, inquiring whether they have back-up glasses
can be enlightening in several ways. If they don't have back-up glasses, they may
be successful lens wearers free of discomfort (no need for glasses, according to the patient),
or they have a high tolerance for lens discomfort. Of course, I'm avoiding the "too
cheap to pay for glasses" scenario. Regardless, asking patients about the difference
in time between comfortable lens wear and total wear time can help in determining
if a symptomatic patient is just tolerating lens wear. As a general rule, a difference
of no greater than three hours can be considered normal (ex. comfortable wear time =
12 hours, total wear time = 14 hours.)
The Mechanics of the Lens
There have been studies on the mechanics of lens wear. Researchers
have evaluated the hydrodynamic squeeze pressure in the fluid thin film beneath hydrogel
contact lenses on a model eye and in vivo (Martin and Holden, 1986; Martin et al,
1989; Monticelli et al 2005). Lid force during a blink in conjunction with curvature
of the eye, the post lens tear film and perhaps lid innervation, and lens paramaters,
such as modulus, most likely have an effect on lens fit and lens comfort.
Further studies of patient motivation/tolerance in conjunction to
the mechanics of lens fitting are needed to better understand the impact of both on
successful lens wear. In the meantime, some quality Vernon Lamb-like chair time can
aid in the patient management aspects of lens wear.
For references, please visit
and click on document #131.
Dr. Nichols is an associate
at The Ohio State University College of Optometry in the area of dry eye research.
Contact Lens Spectrum, Issue: October 2006