Corneal Contact Lens Mechanics
BY 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 patient about 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 a definite 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 www.clspectrum.com/references.asp and click on document #131.
Dr. Nichols is an associate professor at The Ohio State University College of Optometry in the area of dry eye research.