Contact Lens Design & Materials
Still Fitting Corneal GP Lenses?
BY RONALD K. WATANABE, OD, FAAO
One of my best friends, who is also a specialty contact lens fitter, recently told me that he no longer fits corneal GP lenses. All of his new GP fits and refits are sclerals. After all, sclerals have many wonderful advantages, including great comfort, stable vision, and no direct mechanical effects on the cornea. They have allowed many people to improve quality of life significantly just by wearing a small piece of plastic.
I enjoy fitting scleral lenses, but I also still fit corneal GP designs for various purposes, including keratoconus, post-corneal surgery, high myopia and astigmatism, presbyopia, and orthokeratology. Some people just can’t get those large lenses onto their eyes due to small apertures or poor dexterity. Some people just do better with smaller lenses.
For keratoconus, post-surgical, and other irregular cornea applications, special designs are a must. Typically, a design with a small diameter and optic zone with a rapidly flattening periphery is used for keratoconus. Small corneal designs are much better suited to small, central nipple cones, though they can be used for decentered, larger oval cones as well. However, when the corneal lens decenters too much, vision and comfort are compromised, so a scleral design may work better in these cases.
Post-surgical corneas, such as penetrating keratoplasty and refractive surgery, typically result in oblate corneas that require reverse geometry GP designs. GP laboratories are capable of generating very high amounts of reverse geometry; no matter how flat the central cornea has become relative to the midperiphery, a corneal lens can be designed to fit.
High Refractive Errors and Presbyopia
High myopes and astigmats may not do well with soft lenses even though their parameter ranges have greatly expanded over the years. If vision with soft lenses is not satisfactory, a corneal GP lens will almost always improve how they see. High astigmats, in particular, will benefit from the high-quality GP optics.
The best vision for presbyopes is still provided by multifocal corneal GP lenses. Segmented/translating GP designs have the cleanest optics for distance and near (and, in some cases, intermediate too), but even aspheric designs provide better clarity compared to soft multifocal designs. They may also be superior to multifocal scleral designs because they can benefit from a small amount of translation when looking down to read. Scleral lenses are stationary and, therefore, rely on simultaneous vision.
The myopia “epidemic” has prompted many parents to bring in their children for myopia control treatment. The most common is orthokeratology with corneal GP lenses. This treatment is very effective, and though the kids may have some uncomfortable nights early on, they adapt very quickly. If and when scleral orthokeratology designs become available, that may be a better way to ensure centration of treatment. Until then, orthokeratology will continue to do well with the corneal designs available.
A Good Fit Breeds Success
Small GP lenses have more direct impact on the corneal surface, so take care to optimize the fitting relationship to minimize corneal changes. If you do so, your patients will be successful in their lenses for many years. CLS
Dr. Watanabe is an associate professor of optometry at the New England College of Optometry. He is a Diplomate in the American Academy of Optometry’s Section on Cornea and Contact Lenses and Refractive Technologies and is in private practice in Andover, Mass. You can reach him at firstname.lastname@example.org.