Contact Lens Case Reports
The Role of Index of Refraction in Multifocal GP Lenses
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 following study.
Materials and Methods
We 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.
All of 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 1).
TABLE 1 Parameters for Study Lenses
If two 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.
Three 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.