Article Date: 8/1/2005

contact lens case reports
Choose Your Material Wisely for Aspheric GP Multifocals

A popular mode of correction for presbyopes, especially for presbyopic astigmats, is aspheric GP lenses, which incorporate highly aspheric radii on the posterior and/or anterior lens surface to provide both the distance and near optics over the pupil.

How Aspheric GPs Work

Patient DG is a 52-year-old male with a prescription of OU –3.00 –1.50 x180, with a near add of +2.00D. Keratometric readings were 43.00 @ 180/44.00 @ 90 OU. We designed a pair of posterior aspheric lenses with an eccentricity of 0.80 and base curves fit 2.50D steeper than K.

Now for the interesting part: The refractive index of the lens material we select will dramatically influence the amount of effective add power that the lenses can generate. The higher a lens material's refractive index, the greater the potential add power. For example, if we were to manufacture the lenses in a high-index material such as PMMA (index 1.490), then they would have a potential add power of +2.25D. The same lenses manufactured in a low-index material such as Boston XO (Bausch & Lomb) (index 1.415) would have an effective add power of only +1.00D (Figure 1).

It's All Material

Table 1 shows the refractive index and potential add power for a number of common GP materials. It appears that as the fluorine content of a material increases, the refractive index decreases. For example, a non-fluorinated polymer such as Boston IV (Dk 19) has a refractive index of 1.468, and a potential add power of +1.75D. The fluoro-silicone-acrylate Boston ES (Dk 18) has a lower refractive index of 1.433 that results in a lower potential add power of +1.25D.

It's clear that the industry would benefit from the introduction of a high index GP material that's specifically for multifocal lenses.

Figure 1. The potential add power of two identically designed lenses, right lens manufactured in PMMA (+2.25D) and the left lens manufactured in Boston X0 (+1.00D).

Patrick Caroline is an associate professor of optometry at Pacific University and is an assistant professor of ophthalmology at the Oregon Health Sciences University. He is also a consultant to Paragon Vision Sciences and SynergEyes, Inc. Mark André is director of contact lens services at the Oregon Health Sciences University and serves as an assistant professor of optometry at Pacific University. He is also a consultant for Alcon Labs, CooperVision and SynergEyes, Inc.

Contact Lens Spectrum, Issue: August 2005