lens case reports
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.
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.
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).
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