RGP insights

Choose the Right RGP Lens Material for Your Patient

BY EDWARD S. BENNETT, OD, MSED
May 2001

A survey from the Contact Lens Manufacturers Association (CLMA) to member laboratories asks, "What is the most frequent question asked by recent optometry school graduates to your consultant(s)?" The overwhelming response was, "What RGP lens material do you recommend?"

Numerous RGP lens materials are available today. The first successful RGP lens materials were composed of silicone acrylate (S/A). These copolymers contain silicon as siloxane bonded to side branches of the main carbon-carbon chain. S/A materials contain "silicone" methacrylate, wetting agents and cross-linking agents. Higher Dk S/A materials introduced to meet corneal oxygen requirements often caused increased hydrophobicity and flexibility of the lens material.

The dryness experienced with S/A lens materials was reduced with the introduction of fluoro-silicone acrylate (F-S/A) materials. Fluorine, known for its nonstick properties in Teflon-coated cooking materials, increases the deposit resistance of the lens materials.

F-S/A RGP lens materials are currently the gold standard. You can group the materials based on their oxygen permeability (Table 1).

Low Dk (25-50). Most of the lenses I prescribe are in this category. Low Dk lens materials generally are more wettable and stable than higher Dk materials. They also meet or approximate corneal oxygen needs for daily wear myopic patients, who are ideal for these lenses. The differences between these materials are not great. If in doubt, contact your RGP laboratory and ask for its recommended low Dk F-S/A lens material. Low Dk ultrathin designs in minus powers (Paragon Thin, Boston ES, X-Cel Visions) have reduced the incidence of RGP inferior decentration.

High Dk (51-100). High Dk lenses are indicated for all hyperopic and extended wear patients due to corneal oxygen requirements. Also choose these materials for prism ballasted designs due to greater center thickness.

Hyper Dk (>100). Greater than 100 Dk lens materials have been beneficial in extended wear, especially for hyperopic patients. Some practitioners and researchers argue that these materials will be more successful with daily wear patients because of their oxygen permeability. Often they are prescribed in conjunction with a planned replacement schedule due to wettability and stability factors. Prominent in this group is the Menicon Z material. Polymer Technology introduced a new hyper Dk material, Boston XO, this past year.

Dr. Bennett is an associate professor of optometry at the University of Missouri-St. Louis and executive director of the RGP Lens Institute.