Contact Lens Design & Materials
Soft Lenses for Keratoconus
BY RONALD K. WATANABE, OD, FAAO
When presented with a new keratoconus case, most practitioners start thinking about rigid lens options such as corneal and scleral GP designs. For most keratoconus patients, these are suitable and logical choices because they create a lacrimal lens that neutralizes most of the corneal irregularity.
However, more and more soft lens options for keratoconus are being introduced and are becoming more popular.
Applications of Soft Keratoconus Designs
Conventional soft lens designs tend not to work well for keratoconus unless corneal irregularity is mild and best-corrected spectacle acuity is good. This is because soft lenses tend to drape the cornea, taking on the shape of its distorted front surface. This does not adequately correct the corneal irregularity and results in poor visual acuity.
However, many patients cannot adapt to corneal GP lenses, and scleral GP lenses may not be an option due to handling difficulties or cost. Soft lenses that can improve acuity for patients who have irregular corneas are attractive to them because such lenses may be relatively easy to adapt to.
Three Soft Keratoconus Lenses
A few of the many available options in this category are Kerasoft IC (Bausch + Lomb), NovaKone (Alden Optical), and Soft K (Advanced Vision Technologies).
Kerasoft IC was developed by UltraVision CLPL in the United Kingdom and was introduced in the United States two years ago. Its design is able to drape and conform to the irregular cornea while providing stable vision via custom lens powers and a modifiable peripheral system. It does not “mask” corneal irregularity.
The front surface is aspheric and is available in toric powers. The peripheral system is designed to fit different corneal shapes to provide even draping and stability. It features nine levels of peripheral curve steepness. In addition, the Sector Management Control feature allows fitters to independently adjust the curvature of different sectors to create a more even fitting relationship for low cones, pellucid marginal degeneration, or very irregular post-graft corneas.
NovaKone was introduced in 2011. Its back surface has a central base curve that is intended to match the average K reading of the cone for an even central fitting relationship. A paracentral fitting curve serves a purpose similar to that of the base curve of traditional soft contact lenses and is responsible for providing the overall stability and movement of the lens.
NovaKone also has fully customizable lens powers for any level of spherical and toric correction. The optic zone is aspheric to minimize spherical aberration. The IT (index of thickness) factor allows fitters to increase the center thickness of the lens to correct increasing amounts of corneal irregularity.
Soft K incorporates two vent holes (fenestrations) to equalize pressure between the front and back lens surfaces, which allows the lens to fully settle on the eye for good acuity and comfort. The lens also has a thicker optic zone that provides greater modulus and is able to mask corneal astigmatism and irregularity. Lens mass is decreased in the periphery via double lenticulation.
Soft K is available in Contamac’s Definitive silicone hydrogel material and in toric powers up to –7.00D, though it is recommended that patients wear a spherical design for two weeks before determining the need for a toric lens power.
When GPs Are Not an Option
Though GP lenses may still be the first choice for keratoconus, these and other soft lens options are becoming increasingly popular for patients who can’t adapt to GP lenses or who have occupations or activities that make soft lenses a better choice. 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.