Relative Performance of a Polymer Developed for Thin Lens Designs

BY DAVID GEFFEN, O.D., & ROGER ALBRIGHT, M.B.A., N.C.L.C.
MAY 1996

Boston ES material, manufactured by Polymer Technology, has a Dk of 31 and was developed to retain its stability and fracture-resistance even in very thin designs (Table 1). The results reported here come from one of numerous studies the manufacturer conducted to compare this material to others and challenge it against patient demand.

We recruited 198 RGP patients from 10 contact lens practices. The inclusion criteria were: a desire and ability to wear RGPs; good ocular health; and a willingness to participate in a six-month clinical study.

We tested the Boston ES against Polycon II, a low Dk silicone acrylate material, and FluoroPerm 30, a daily wear fluorosilicone acrylate material. We used Boston ES in a standard thickness design and a thin lens design. The type of lens to be dispensed was randomized and masked to the patients and the practitioners. We dispensed lenses in one of the four types with appropriate care solutions and instructed patients to follow normally accepted wear and care guidelines. The number of patients discontinued from the study was as expected for a study of this type, and the distribution of those discontinuing was about equal among all four groups.

We detected little significant difference in the incidence of slit lamp findings among the groups (Fig. 1), although we noted a tendency toward more staining for those wearing FluoroPerm 30. There were no significant differences in subjective symptoms, problems or complaints reported among the four groups. Lens comfort was high, with all four groups reporting "very good" to "excellent" comfort.

We also evaluated the frequency and reason for lens replacement. Only the FluoroPerm 30 lenses needed to be replaced due to damage. Replacement for other reasons was evenly distributed among the groups in terms of rate and circumstance.

As shown by this study, Boston ES provides a higher Dk with greater rigidity than Polycon II while offering the benefits of fluorosilicone acrylate chemistry. The overall performance of both the thin and the standard design Boston ES was as good as or better than that of standard thickness Fluoro-Perm 30 lenses. This study also proved that Boston ES is stable and fracture-resistant when compared to currently available materials.

The stability and rigidity of a lens material affects the performance of the lenses from which it is made. Thin lenses can reduce the mass of plus or low-riding lens, improve centration and increase oxygen transmissibility. Thin lenses are also believed to be more comfortable for extremely sensitive patients, allowing the lid to slip over the lens more easily during the blink. Excessive center and edge thickness have been implicated in a number of disorders, including 3 o'clock and 9 o'clock staining. In some circumstances, very thin lenses are intentionally prescribed to correct residual cylinder using lens flexure.

Whether an RGP contact lens is manufactured in standard, thin or ultrathin design, it's important that the material be stable and fracture-resistant. The stability and rigidity of Boston ES make it ideal for the manufacture of specialty lens designs which may be prone to flexing or which need added stiffness to maintain their parameter stability. CLS

Dr. Geffen is in group practice in San Diego. Mr.
Albright is the senior product manager-materials for Polymer Technology.

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