contact
lens materials
Uncovering the Best Two Materials for a Patient
BY
LORETTA B. SZCZOTKA-FLYNN, OD, MS, FAAO
It's hard enough to decide on one lens material
for a compromised cornea, let alone two. In piggyback lens systems, how should we
select the two best materials for a given patient?
Piggyback
lens systems are almost always used to visually rehabilitate compromised corneas
such as post-penetrating keratoplasty, corneal dystrophies, recurrent erosions or
keratoconus, to name a few. Research has shown that all layers of the cornea function
better under higher oxygen conditions. Thus, providing these corneas with the highest
oxygen transmissible system available is a worthy pursuit.
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Prosthetic
iris soft lens carrier beneath a Menicon Z GP.
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Soft Lens Options
Before silicone hydrogel lenses, the oxygen-limiting lens was
usually the soft lens carrier. When functioning in a piggyback system, a large-diameter
low-Dk lens is often enough to tip the scale in favor of corneal edema even when
using a thin, low powered design. Silicone hydrogel lenses have largely overcome
this concern. However, they don't function well as a piggyback carrier in a few
circumstances:
Steep Corneas Very steep corneas from advanced keratoconus or
proud grafts require steeper base curves than are available in today's silicone
hydrogel lens options. Because of the stiffer modulus of these lenses, they don't
conform as well to the corneal contour as does a low-Dk hydrogel. A key sign the
lens is too loose is lens buckling in the inferior nasal or temporal quadrant. You
should avoid this type of fit to prevent mechanical complications of lens wear.
You can almost always find a suitable low-Dk hydrogel material to contour even the
steepest of corneas, even if it requires a custom order.
For
example, I have fit custom order soft lenses from Alden Optical in a 7.7mm base
curve/ 14.0mm diameter design in a very advanced keratoconic patient with success.
This patient failed with all other disposable soft lenses including all silicone
hydrogel and all daily disposable lens options. In these tough-to-fit steep corneas,
I often have success with CIBA Vision's Focus 1-2 week lens as the piggyback carrier,
but even this lens buckled nasally.
Prosthetic Needs It's not uncommon to find a patient who requires
a prosthetic iris lens as part of the piggyback system. For example, a common finding
in specialty lens practices are patients who present with total or partial aniridia,
coupled with corneal scars or transplants, who would benefit from a soft prosthetic
iris lens carrier beneath a GP lens (Figure 1). Currently, silicone hydrogel lenses
can't be tinted for such needs. Either polymacon or hioxifilcon materials are commonly
used as the soft lens material to be tinted. I usually opt for hioxifilcon for greater
oxygen permeability.
Gas Permeable Options
Undoubtedly, the highest oxygen transmissible system incorporates
a silicone hydrogel carrier with a hyper-Dk GP lens on top. Most of the recent reports
using this theory utilize the Menicon Z material (tisilfocon A) in their GP design
of choice because it significantly allows maximum transmissibility of a given piggyback
system, even when conventional hydrogels are involved.
Dr. Szczotka-Flynn is an
associate professor at Case Western Reserve University Dept. of Ophthalmology and
is director of the Contact Lens Service at University Hospitals of Cleveland.
Contact Lens Spectrum, Issue: April 2006