Contact Lens Design & Materials
All Sclerals Are Not the Same
BY RONALD K. WATANABE, OD, FAAO
Scleral lenses are the current hot topic in the contact lens design world. They have numerous advantages and benefits for both “normal” and “irregular” cornea patients. At a basic level, almost all scleral lens designs are intended to entirely vault the cornea and limbus while resting evenly on the sclera. In this way, corneal irregularities are bypassed, a fluid layer is retained over the corneal surface, and the delicate limbus is not affected.
The number of available scleral lens designs is growing, and although they may have the same basic fitting goals, each one is unique in its approach to the fitting relationship. Here are some examples of unique scleral lens design features.
Sag Versus Curvature
This is not really a design feature, but the way in which labs specify back-surface architecture. Most use radii of curvature to specify the base curve and peripheral curves, while others specify lens sagittal depth or sag. Using curvature may be more familiar for fitters, and it allows you to specify virtually any curvature for each lens zone. Also, it is easier to calculate lens power changes as the base curve is adjusted. On the other hand, using sag makes more intuitive sense when trying to determine the ideal amount of corneal vault. If the lens is 200 microns too deep for a patient’s eye, simply selecting a lens with 200 microns less sagittal depth is very straightforward.
In the end, it really doesn’t matter which you use as long as you can make the appropriate changes when needed.
Limbal and Scleral Zones
This is where the rubber meets the road. A scleral lens must minimally vault the limbus and land gently and evenly on the sclera to prevent long-term discomfort and complications. The new NormalEyes 15.5 (Paragon Vision Sciences [Paragon]) lens uses Proximity Control technology found on Paragon CRT lenses to provide limbal vault and tangential landing on the sclera. The MSD (Viscon/Blanchard) lens has various midperipheral profiles and reverse geometry elements to accomplish a good fit over this area. The ICD 16.5 (Paragon) lens has a planar limbal clearance zone that is intended to vault the limbus and can be adjusted by tangent angle.
More traditional scleral designs have radii of curvature that can be altered to optimally fit each patient’s eyes. Some, like the new Atlantis (X-Cel) lens, simplify this by specifying the limbal and scleral zones in “steps.”
The sclera often has significant toricity or curvature irregularity that cannot always be acceptably fitted with a symmetrical scleral lens design. In these cases, the lens may cause compression or impingement in one or more regions of the sclera, and a nonrotationally symmetrical peripheral design may be needed. The Jupiter (Visionary Optics, Essilor) lens is available with a toric scleral zone that can better align with toric scleras. NormalEyes 15.5 features Dual Axis technology as in the CRT. The DigiForm (TruForm Optics) and Dyna Scleral (Lens Dynamics) lenses are available with quadrant-specific technology that allows you to specify different curves in each of four scleral quadrants.
As already stated, some scleral designs utilize reverse geometry elements in their designs. But for oblate corneas such as post-refractive surgery, a more conventional reverse geometry design may be needed. The Jupiter and DigiForm lenses are available with reverse geometry back surfaces to better fit these types of corneas. Reverse geometry is also useful for pellucid marginal degeneration to maintain appropriate central clearance while vaulting the very low cone. 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.