Some time ago we described how material
index can influence the distance and near powers of multifocal GP
lenses (Choose Your Material Wisely for Aspheric GP Multifocals,
Contact Lens Case Reports, August 2005).
At the time of that publication, the concept was more theoretical
than concrete. To cement this in our minds, we undertook the
Materials and Methods
ordered a series of matched, single-vision contact lenses
manufactured in two GP lens materials, one with a high index of
refraction of 1.490, the other with a low index of 1.415. The
control material in this study was the high-index material, which
happened to be PMMA.
the lenses were manufactured with the same posterior radius of
curvature, 7.89mm or 43.00D. We varied the anterior (power) radius
so that the control (PMMA) material resulted in powers of plano,
-3.00D, -6.00D, +3.00D and +6.00D (Table
TABLE 1 Parameters for Study Lenses
identical lenses are manufactured (anterior curve, posterior curve
and center thickness), the lens should result in the same effective
power in the lensometer; however, if the lenses aren't identical,
the difference in lens power is a function of material index.
different observers verified the powers of all the study contact
lenses in the same calibrated lensometer. The results of our pilot
study showed that the high-index PMMA material consistently resulted
in higher power readings than did the low-index material. You may
not find this surprising because increased index should cause
increased power (Figure 1).
However, this example illustrates two important factors: that modern
lathe cutting manufacturing is very precise; and that GP multifocal
lenses manufactured in high-index materials should yield more add
power on the eye.
Figure 1. The higher index material (1.490) consistently resulted in
increased add power for both myopic and hyperopic contact lenses.