Gets Makeover with Advanced Materials
BY JOHN MARK JACKSON, OD, MS, FAAO
I'm always amazed at how advancing technologies can breathe new life into old ideas. A perfect example is the increasing popularity of prescribing piggyback contact lens systems (Figure 1).
Figure 1. High-Dk materials are
In the case of an irregular cornea such as keratoconus, if clinicians can't achieve an acceptable fit with a GP lens, they'll sometimes use a GP lens directly on top of a hydrogel lens. This fitting approach often provides better comfort and centration than using the GP alone and may be the only way to fit the patient.
Piggybacking has one definite drawback: Because oxygen must pass through two lenses, it decreases the supply to the cornea. However, today's silicone hydrogels and high-Dk GPs minimize this problem.
Figure 2. Patient's post-RK topography.
A Post-RK Case
Our 51-year-old patient underwent RK surgery in 1990. She reported that her surgery was initially successful, but over the years her vision worsened. Her entrance acuities were 20/70 OD, 20/400 OS while wearing soft spherical lenses prescribed elsewhere that she'd worn for three years. She also noted monocular diplopia in each eye while wearing the soft lenses. Subjective refraction was +9.50 –2.25 x 045 20/50 OD and +8.25 –1.25 x 140 20/25 OS. Her corneas had prominent RK scars with about 0.25mm of vascularization into the scars 360 degrees; otherwise her ocular health was unremarkable. Figure 2 shows her corneal topography.
We fit her with GP lenses in an attempt to eliminate the diplopia and improve her visual acuity. We tried reverse-geometry lenses because of her oblate cornea. The lenses improved her vision substantially, eliminating the diplopia and providing acuity of 20/20 in each eye. However, the lenses we tried would decenter or eject from the eye, or they would adhere.
A Winning Combination
We decided to try a piggyback lens system with a high-Dk spherical GP lens on top of a silicone hydrogel lens. We first fit the soft lens and took K readings over the lens. We used this reading to select the base curve for the GP lens. Piggyback systems usually use a low-powered soft lens with the main correction in the GP lens. We achieved an acceptable result with +0.50D soft lenses and spherical GP lenses with parameters of 7.50mm base curve, 9.6mm overall diameter and +7.00D power. The patient was successful with this combination and pleased with her vision.
Although today's specialty lens designs (such as reverse geometry
lenses) are better than ever, sometimes they just won't cooperate. Using an old
friend like the piggyback system combined with today's advanced materials can be
a winning combination for you and your patients. CLS
Dr. Jackson is an assistant professor at Southern College of Optometry where he works in the Advanced Contact Lens Service, teaches courses in contact lenses and performs clinical research.