Comparing Semi-Scleral RGP Contact Lenses
BY LORETTA B. SZCZOTKA, OD, MS, FAAO
Two additions to specialty contact lens options are semi-scleral lenses made from RGP materials. Specialty Ultravision's Epicon was FDA approved in February 2000 for keratoconus. It is the first moldable, flexible gas-permeable lens to be manufactured from the company's carbosilfocon material. Macrolens from C&H Labs in Dallas is manufactured from standard RGP materials (Boston EO is recommended) and was patented in 1999. (Table 1).
Both lenses are promoted as providing the patient with the comfort of a soft lens and the vision afforded by an RGP lens. However, both lenses are made in RGP materials ranging from 13.9mm to 15.0mm in diameter. The comfort claims arise from utilizing diameters similar to soft lenses which minimize lens movement and lid interactions. In addition, the bearing pressure is distributed on the central and mid-peripheral cornea along with the sclera.
Epicon is fit from a 21-lens diagnostic set that cannot be modified. Each lens has a series of aspheric peripheral curves which are meant to rest on the peripheral cornea and extend to the sclera, with a trans-limbal zone maintaining a lacrimal reservoir (Figure 1). The base curve and peripheral systems function independently, which should be kept in mind when clinically altering lens parameters.
|Figure 1. Fluorescein pattern of Epicon lens on keratoconus.||Figure 2. Macrolens A design on pellucid marginal degeneration.|
Macrolens-A is C&H's primary semi-scleral design, available in a variety of peripheral curvatures, eccentricities (E value) and widths. The recommended starting point is a peripheral system with 0.8 E value, although a 0.6 E value (tighter peripheral fit) or a 1.0 E value (looser peripheral fit) is available. Minimal changes can be made to the overall diameter, and no changes should be made to the optical zone size (Figure 2).
Both lenses are welcome additions to my specialty RGP practice for highly irregular corneas with poorly centering RGPs, frequent dislodging or RGP intolerant patients. I find the Epicon works well for steeper corneas that can be fit easily into one of the 21 trial lenses. The Macro-lens works well for flatter and irregular corneas that require more customized modifications.
Dr. Szczotka is an assistant professor at Case Western Reserve University Dept. of Ophthalmology and Director of the Contact Lens Service at University Hospitals of Cleveland.