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.
Prosthetic iris soft lens carrier beneath a Menicon Z GP.
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.