Your Material Wisely With Specialty Contact Lenses
JEFF SCHAFER, OD, MS
Fitting patients who have keratoconus, particularly
in advanced stages of the disease, can be challenging to both novice and experienced
contact lens fitters. Often many diagnostic lenses are meticulously
evaluated, best corrected vision is measured through careful over-refractions and
complicated sag calculations are computed all while considering base curves, optic
zone sizes and peripheral curve designs. Choosing the material for these carefully
designed lenses is often the last, yet very important, step in this complicated
process. The material you choose may determine lens success or failure by impacting
corneal physiology, patient comfort and vision.
A Dramatic Improvement
I still remember the case that taught me this lesson the hard
way. During my fellowship, I was fitting a young man who had keratoconus and had
driven eight hours to be examined at our practice. He'd seen countless practitioners
who were unable to help him. He'd been given a leave of absence from nursing school
to correct his vision because his condition was negatively affecting his ability
to complete his clinical rotations.
He presented already wearing a piggyback design, but the GPs would
frequently decenter and dislodge, and his best-corrected vision was 20/50. After
several attempts with multiple lens designs including piggyback and semi-sclerals,
we achieved success with a piggyback cut-out soft lens carrier with a GP lens on
The young man rattled off the 20/15 line, which brought tears
to his eyes as well as to his family members who had made the trip with him. We
ordered lenses and I saw him two weeks later to dispense his new lenses.
After applying his new lenses, I confidently displayed the chart
only to realize that he couldn't see better than 20/50. He was disappointed, and
I was confused. I verified his new lenses and the diagnostic lenses, but the parameters
were correct. I put the diagnostic lenses back on his eye and he read 20/15
immediately. After some pondering I realized that the only difference between these
identical lenses was the material. The diagnostic lenses were PMMA, and the new
lenses were a high-Dk material. The new lenses were flexing much more than the diagnostic
lenses, which dramatically reduced his vision.
We reordered the patient's lenses in a stiffer, lower-Dk material
and a slightly increased center thickness, which resulted in 20/15 VA.
Carefully selecting a material will maximize your chances of success
when fitting specialty designed lenses. Changing materials with your keratoconus
and irregular cornea patients can affect not only oxygen transmissibility and wettability,
but also the visual outcome by prohibiting, or enhancing, lens flexure.
I typically use low to moderate Dk materials such as Boston ES (Bausch
& Lomb) and Fluoroperm 30 or 60 (Paragon Vision Sciences) with routine
keratoconic patients to provide wettability and comfort and minimize flexure. I
generally piggyback a keratoconic patient when adequate comfort or centration of
the GP lens alone can't be achieved. I almost exclusively choose silicone hydrogel
materials for the soft lens carrier; unless I need a countersunk front surface,
then I use the Flexlens piggyback cut-out by X-Cel in hioxifilcon B material.
When piggybacking and fitting keratoconus patients who are status-post
penetrating keratoplasty, I prefer higher Dk GP materials such as Menicon Z (Menicon)
and Boston XO (B&L). In these cases, maximizing oxygen transmissibility is crucial
to maintain adequate corneal physiology.
Dr. Schafer is a clinical
assistant professor and chief of the contact lens service at The Ohio State University
College of Optometry.
Contact Lens Spectrum, Issue: July 2006