Contact Lens Design & Materials
When Sclerals Go Toric
BY RONALD K. WATANABE, OD, FAAO
Scleral lenses are a fantastic option for all types of eyes, but fitting challenges can sometimes crop up. One of the biggest challenges is in achieving alignment on the sclera in all meridians. Even normal eyes can have significant amounts of asymmetry in the different quadrants, resulting in areas of compression and/or edge lift.
Small amounts of asymmetry can be fit satisfactorily with a radially symmetrical scleral lens. It may even be beneficial to have a quadrant or two that has a looser fitting relationship to allow some amount of tear exchange under the lens.
Incorporating a Toric Periphery
When scleral asymmetry is too great, radially asymmetrical designs may be necessary. The most common type of asymmetry is a with-the-rule sclera in which a spherical scleral lens compresses the horizontal meridian and lifts off in the vertical meridian. This will likely result in vessel blanching or impingement at 3 o’clock and 9 o’clock and in greater lid interaction at 12 o’clock and 6 o’clock, causing discomfort.
The solution to this problem is a toric peripheral zone that better aligns the sclera. Virtually all scleral lens designs have toric peripheral curve options. When designed correctly, the lens will align better around the sclera, and the patient will be more comfortable. Each manufacturer has its own way of specifying the amount of toricity, so communicate with the lab consultant to get the desired result. A toric periphery will also allow the lens to settle closer to the cornea, so the corneal vault may have to be increased to compensate.
Going a Step Further
In some cases, scleral toricity is greater than what a toric periphery can handle, such as on an eye that has high corneal and scleral toricity. In these cases, a bitoric or back-toric scleral lens may be needed. Not many manufacturers offer these toric scleral lenses, but they can provide scleral alignment on very toric eyes.
Occasionally, the sclera is asymmetric, but not simply toric. In these cases, patients may need a quadrant-specific periphery. A few labs can generate quadrant-specific surfaces on a scleral lens. In these designs, each of four quadrants can have a different curvature to best align with an asymmetric eye. More critical observation of the lens fit in each quadrant, or using an instrument such as an anterior segment optical coherence tomographer, is needed to determine the best fit for this type of design.
Finally, some patients manifest significant residual astigmatism through their scleral lenses. Though prescribing glasses that correct the astigmatism is a simple solution, many of these patients would rather not wear both glasses and contact lenses.
In these cases, either front-surface or bitoric designs may be needed. Bitoric designs orient rotationally via the back-surface toricity, which should allow for accurate placement of the cylinder. However, front-surface torics that have spherical back surfaces will need either prism ballast or dual thin zones to keep the cylinder at the proper axis. More labs are offering front-toric designs, but each is stabilized differently; find out the stabilization design element used before ordering.
Expanding the Patient Base
With toric design options becoming more common, more patients will be able to enjoy the benefits of scleral GP contact lenses. 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 email@example.com.